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LECTURE NOTES ON THE GEOGRAPHY, FORMATION, PROPERTIES AND USE OF THE MAJOR OF THE WORLD

P.M. Driessen & R. Dudal (Eds)

in Q AGRICULTURAL KATHOLIEKE 1 UNIVERSITY UNIVERSITEIT ^.. WAGENINGEN LEUVEN \^

ivr^ - SI I 82 o ) LU- w*y*'tinge n

ökSjuiOTHEEK ÎCANDBOUWUNWERSIIEI^ WAGENINGEN TABLE OF CONTENTS

PREFACE

INTRODUCTION

The FAO-Unesco classificationo f soils 3 Diagnostichorizon s anddiagnosti cpropertie s 7 Key toMajo r SoilGrouping s 11 Correlation 14

SET 1. ORGANIC SOILS

Major SoilGrouping : S 19

SET 2. MINERAL SOILS CONDITIONED BYHUMA N INFLUENCES

Major SoilGrouping : S 35

SET 3. MINERAL SOILS CONDITIONED BYTH E

Major landforms involcani c regions 43 Major SoilGrouping : S 47

Major landforms inregion swit h 55 Major SoilGrouping :ARENOSOL S 59

Major landforms insmectit eregion s 65 Major SoilGrouping : S 67

SET 4. MINERAL SOILS CONDITIONED BYTH E TOPOGRAPHY/PHYSIOGRAPHY

Major landforms inalluvia l lowlands 83 Major SoilGroupings : S 93 (with specialreferenc e toThioni c Soils) 105

Major landforms inerodin gupland s 111 Major SoilGroupings : S 115 119

SET 5. MINERAL SOILS CONDITIONED BYTHEI RLIMITE D AGE

Major SoilGrouping : 125 SET 6. MINERAL SOILS CONDITIONED BYA WE T (SUB)TROPICAL CLIMATE

Major landforrasi ntropica l regions 133 Major SoilGroupings : S 139 FERRALSOLS 147 157 161 167 171

SET 7. MINERAL SOILS CONDITIONED BYA (SEMI-)AR-ID CLIMATE

Major landforms inari d regions 177 Major SoilGroupings : S 181 191 197 203

SET 8. MINERAL SOILS CONDITIONED BYA STEPPIC CLIMATE

Major landforms instepp e regions 211 Major Groupings: S 215 219 227 GREYZEMS 231

SET 9. MINERAL SOILS CONDITIONED BYA (SUB)HUMID TEMPERATE CLIMATE

Major landforms intemperat e regions 237 Major SoilGroupings :LUVISOL S 241 P0DZ0LUVIS0LS 247 253 259

REFERENCES

Literature cited 269

ANNEXES

Annex 1 Definitions ofdiagnosti c horizons 275 Annex 2 Definitions ofdiagnosti c properties 281 Annex 3 Definitions ofphase s 287 Annex 4 Soilhorizo ndesignation s 291 PREFACE

These Lecture Noteswer e co-producedb y theAgricultura l University Wageningen, theNetherlands ,an d theFacult y ofAgricultura l Sciences of the Catholic University ofLeuven ,Belgium , insuppor t of introductory courses on SoilGeograph y and Classification. TheRevise d Legend of the FAO-Unesco SoilMa p of theWorl d (FAO,1988 )i s followed to structure the discussion of the distribution, formation, characteristics,managemen t andus e of themajo r soils of theworld .

The following personshav e contributed to this syllabus:

Blokhuis,W.A . Introduction, Cambisols,Vertisol s Bouma,J . Gleysols, , Regosols Driessen, P.M. Introduction,Arenosols ,Calcisols , Gypsisols, , , Solonetz Dudal, R. Introduction (The FAO-Unesco Clas­ sification of soils) Dijkerman, J.C. Ferralsols,Nitisols , Kroonenberg, S.B. Landforms (volcanic regions,sands , smectite regions, lowlands, eroding uplands,we t tropics,ari dregions , steppe regions,temperat e regions) Legger,D . Acrisols,Alisols ,Ferralsols ,Lixi - sols, Luvisols Miedema, R. ,Greyzems , Podzoluvisols Mulders, M.A. Chernozems, , s Poels,R.H.L . Planosols, Podzols VanMensvoort ,M.E.F . VanReeuwijk , L.P.

The generoushel p received from the International SoilReferenc e and Information Centre (ISRIC),Wageningen ,whic hmad e available reference materials and illustrations, isgratefull y acknowledged. These Lecture Notes are for internalus e only; they are still ina first stage ofdevelopmen t andwil lb eprogressivel y improved.Sugges ­ tions for amendments or additions aremos twelcome .

Wageningen andLeuven ,Jul y 1989

TheEditors ,

P.M. Driessen R. Dudal Dept. of &Geology , Faculty ofAgricultura l Sciences, Agricultural University, Catholic University Leuven, P.O. Box 37 92Kardinaa l Mercierlaan 6700AA Wageningen B-3030 Leuven theNetherland s Belgium INTRODUGTION Introduction 3

INTRODUCTION

Soils are formed through the impact of climate. & fauna, (including Man) and topography on the soil's parent material, over a variable time span. The relative importance of each of these five 'soil forming factors' insoi l formation (or '')varie s amongsites ; this explainswh y there is such agrea tvariet y ofsoils . With few exceptions,soil s are still ina proces s of change; they show in their 'soil profile' signs of differentiation or alteration of the soil material, indicative ofa particula r pedogenetic history.

Traditional soil names often indicate a domination of one soil forming factor over all other factors. Examples are 'desert soils' (the factor climate dominating), 'plaggen soils' (man), 'prairie soils' (vegetation), 'mountain soils' (topography), or 'volcanic ash soils' (parentmaterial) . Alternatively, soilname s refer toa dominan t single soil characteristic, e.g. the colour ('brown soils'), a chemical characteristic ('alkali soils'), ora particula r physical attribute ('hydromorphic soils').

THEFAO-UNESC O CLASSIFICATION OF SOILS

Soils are identified through a system of soilclassification .Man y such systemshav ebee n developed; they differwidel ybecaus e they arebase d on different appreciations of soil formation,an dus e different criteria and different hierarchical subdivisions. The system used inthi s syllabus isth eon edevelope db y FAO (1974,1988 )fo rth eprepara ­ tion of the FAO-Unesco of the World at scale 1:5,000,000. This systemha s ageographi c dimension through theWorl d SoilMap .

In this syllabus, the 28 'Major Soil Groupings' that are distinguished atth ehighes t taxonomieleve l inth eFAO-Unesc o classification,hav ebee n grouped in nine 'Sets' by applying a three-step key which addresses the evolutionary and geographic background of thesoils : First, a separation ismad ebetwee norgani c soils andminera lsoils ; Secondly, mineral soils are clustered according to the dominant soil forming factor or factorswhic h conditioned soil formation; Thirdly, clusters are subdivided intoMajo r SoilGrouping s by considering which combinations of soil forming factors occurwithi n eachSet .

Table 1present s thenin e Sets thatevolve :

SET 1 includes the organic soils, i.e soils with more than a defined quantity of 'organic soilmaterials' .Al lorgani c soilsar eaccomodate d in only oneMajo r SoilGrouping : theHISTOSOLS .

SET 2contain s theman-mad e soils. These soilsvar ywidel y inpropertie s and appearance andca noccu r inan y environment influenced by human intervention. They too have been ac­ comodated inonl y oneMajo r SoilGrouping : theANTHROSOLS . 4 Introduction

TABLE1 . AllMajo r Soil Groupings (MSGs)o f the FAO-Unesco system innin e Sets.

SETS DOMINANT IDENTIFIERS MSGs

SET 1 Organic soils HISTOSOLS

SET2 Mineral soils inwhic h soil formation is ANTHROSOLS conditioned byHuma n influences (notconfine d toan yparticula r region)

SET3 Mineral soils inwhic h soil formation is conditionedb y Parent Material -Soil s developed involcani c material ANDOSOLS -Soil s developed inresidua l and shifting sands ARENOSOLS -Soil s developed inexpandin g clays

SET 4 Mineral soils inwhic h soil formation is conditioned byTopography/Physiograph y -Soil s inlowland s (wetlands')wit h level topography FLUVISOLS GLEYSOLS -Soil s inelevate d regionswit hnon-leve ltopograph y LEPTOSOLS REGOSOLS

SET 5 Mineral soils inwhic h soil formation is CAMBISOLS conditioned by their limitedAg e (not confined toan yparticula r region)

SET 6 Mineral soils inwhic h soil formation is PLINTHOSOLS conditioned by theClimat e (&Vegetation ) FERRALSOLS ofwe t tropical and subtropical regions NITISOLS ACRISOLS ALISOLS LIXISOLS

SET 7 Mineral soils inwhic h soil formation is SOLONCHAKS conditioned by theClimat e (&Vegetation ) SOLONETZ ofari d and semi-arid regions GYPSISOLS CALCISOLS

SET 8 Mineral soils inwhic h soil formationi s KASTANOZEMS conditionedb y theClimat e (&Vegetation ) CHERNOZEMS of steppes and steppic regions PHAEOZEMS GREYZEMS

SET 9 Mineral soils inwhic h soil formationi s LUVISOLS conditioned by theClimat e (&Vegetation ) PODZOLUVISOLS of subhumid forest and grassland regions PLANOSOLS PODZOLS Introduction 5

SET 3 includes the mineral soils whose formation is conditioned by the particular properties of theirparen tmaterial . The Set includes threeMajo r SoilGroupings : (1)th e dark and fluffyANDOSOL So fvolcani cregions , (2)th e sandyARENOSOL S ofdeser t areas,beac h ridges,inlan d dunes, areaswit hhighl yweathere d sandstones,etc. ,an d (3)th e swelling and shrinking heavy clayeyVERTISOL S ofbackswamps , riverbasins ,lak ebottoms ,an d otherperiodicall y wet areaswit h a high content ofexpandin g 2:1 latticeclays .

SET4 i scompose d ofminera l soilswhos eformatio nwa smarkedl y influen­ cedb y their topographic/physiographic setting. Such soils canb e found ina low terrainposition ,associate dwit hrecur ­ rent floods and/or prolonged wetness, but also in elevated and/or acci- dented terrainwher e soilformatio n ishindere db y lowtemperature s and/or . The Sethold s fourMajo r SoilGroupings : (1)I n lowareas . they areyoun g alluvial FLUVISOLS which showstrati ­ ficationo r other evidence ofrecen t sedimentation, and (2)non-stratifie dGLEYSOL S inwaterlogge d areas that dono t receive regular additions ofsediment . (3)I nelevate d and/or eroding areas, they are the shallow LEPTOSOLS over hard rock orhighl y calcareous material,an d (4)th e deeperREGOSOL Swhic h occur inunconsolidate d materials andwhic h have only surficialprofil e development,e.g .becaus e of low soil temperatures,prolonge d dryness orerosion .

SET 5 accomodates (other) soils that are only moderately developed on account of their limited pedogenetic age or ofrejuvenation . Moderately developed soils occur inal l environments, from the sea level to the highlands, from the equator to the boreal regions, and under all kindso fvegetation .The yhav eno tmor e incommo n than 'signso fbeginnin g soilformation 's otha tther e isconsiderabl e diversity amongth esoil si n thisSet .Nonetheless ,the yal lbelon gt oonl yon eMajo rSoi lGrouping :th e CAMBISOLS.

The remaining 18 Major Soil Groupings of the FAO-Unesco system all representminera lsoil swhos eformatio nwa slargel yconditione db yth epas t orpresen tclimat e andb y theclima xvegetatio nwhic hdevelope dunde r this climate.

SET 6accomodate s there dan dyello w soilso fwe t tropical andsubtropi ­ calregions . High soil temperatures and (intimes )ampl ewate r promote rockweatherin g andmineralizatio no fsoi lorgani cmatter .Th eMajo rSoi lGrouping s inthi s Sethav e incommo n thata longhistor y of rapid dissolution and transport ofweatherin g productsha s resulted indee p and genetically maturesoils : (1)th ePLINTHOSOL S ofol dweatherin g surfaces,wit h amixtur e ofcla y andquart z thatharden s irreversibly uponexposur e to the openair , (2)th e deeplyweathere d FERRALSOLS thathav e aver y low cation exchange capacity and arevirtuall y devoid ofweatherabl eminerals , 6 Introduction

(3)th e deep, S inriche r parentmateria l thathav e shiny,nutt y structure elements anda stretche d clayaccumulatio nhorizon , (4)th e strongly leached, red andyello wACRISOL S onaci dparen t rock, with acla yaccumulatio nhorizon ,lo wbas e saturationan d lowactivit y clays, (5)th e less commonALISOL Swit h lowbas e saturation andhig h activity clays, and (6)th eLIXISOL S with ahig hbas e saturation and a lowcatio n retention capacity.

SET 7 accomodates Major Soil Groupings that are typical of arid and semi-aridregions . An importantmechanis m ofhorizo ndifferentiatio n insoil s inth edr yzon e is by redistribution of calcium carbonate and gypsum. Soluble salts may accumulatea tsom edept hor ,alternatively ,nea r thesoi lsurfac e inarea s with shallow groundwater. Thevariou sMajo r SoilGrouping s inSe t 7are : (1)th e SOLONCHAKS ofdeser t and semi-desert regions; these soilshav e a high content ofaccumulate d soluble salts, (2)th e SOLONETZwit h ahig hpercentag e of adsorbed sodium ions, (3)th eGYPSISOL S with ahorizo n of secondary gypsum enrichment,an d (4)th eCALCISOL Swit h secondary carbonate enrichment.

SET8 hold ssoil stha toccu ri nth esteppi czon ebetwee nth edr yclimate s and thehumi d temperate regions athighe r latitudes. This transition zoneha s aclima xvegetatio n ofephemeri c grasses anddr y forest; its location corresponds roughlywit h the transition from adomi ­ nance of accumulation processes in soil formation to a dominance of leachingprocesses .Se t 8include s fourMajo r SoilGroupings : (1)th eKASTANOZEM Swit h carbonate and/or gypsum accumulation at some depth; these soils occur inth e driest parts of the steppezone , (2)th e deep,ver y dark CHERNOZEMSwit h carbonate enrichment inth e , (3)th edusk y PHAEOZEMS ofprairi e regionswit h ahig hbas e saturation butn ovisibl e signs of secondary carbonate accumulation, and (4)th edar kGREYZEM S withhig h contents in their surface soils and signs ofbleachin g alongpe d faces.Thes e soils occur in thewettes t parts of thezone .

SET 9accomodate s thebrownis h and greyish soils of thehumi d temperate regions. Thesoil si nthi sSe tsho wevidenc eo fcla yand/o rorgani cmatte rredistri ­ bution. The cool climate and the short genetic history of most soils in thiszon eexplai nwh ysom esoil sar estil lrelativel yric hi nbase sdespit e a dominance of eluviation over enrichment processes.Eluviatio n and illu- viationo fmetal-humu scomplexe sproduc eth e(greyish )bleachin gan d(brow n toblack )coating s thatar e common inthi szone .Se t9 contain s fourMajo r SoilGroupings : (1)th ebase-ric h LUVISOLS with adistinc t accumulationhorizon , (2)th ebase-poo r PODZOLUVISOLS with ableache d eluviation horizon tonguing intoa cla y enrichedhorizon , (3)th ePLANOSOL Swit ha bleache d topsoilove ra dense ,slowl y permeable subsoil,an d (4)th eaci d PODZOLSwit h ableache d eluviationhorizo n over anaccumula ­ tionhorizo n oforgani c matterwit h aluminium or irono rwit hboth . Introduction 7

NOTE THAT Table 1 indicates merely the most common relationships between Major SoilGrouping s andsoi l forming factors.Th eMajo r SoilGrouping s in Sets 6 through 9 represent soils, which occur predominantly in specific climate zones. Such soils are traditionally referred toa s 'zonalsoils' . However,no t all soils inSet s 6throug h 9ar e zonal soils,no r are soils inothe r Sets always non-zonal. Podzols,fo r instance,ar emos t common in (sub)humidtemperat eclimate sbu tthe yar eals ofoun di nth ehumi dtropics ; Planosols may equally occur in subtropical and steppe climates; Regosols are not necessarily confined to eroding uplands; Ferralsols may occur as remnants outside thehumi dtropics.. . Soilswhos e characteristics resultfro ma stron gloca ldominanc e ofa soi l forming factor other than 'climate' are not zonal soils. They are 'in­ trazonal soils'. In other words, there are zonal and intrazonal Podzols, zonal and intrazonal Gleysols, zonal and intrazonal Histosols, and many more. Therear eals o 'azonalsoils' ,i.e .soil stha tar eto oyoun gt oreflec tth e influence of site-specific conditions in their profile characteristics. Youngalluvia l soils (Fluvisols)an dsoil si nrecen thillwas h (e.g.Cambi - sols)ar e examples ofazona lsoils .

The zonallty concepthelp s toexplai n theworldwid e distribution of soils but cannotb euse d asa basi s for soil classification. The Sets presented in the foregoing may therefore not be taken ashig h level classification unitsbu tmerel y asa reflectio no fcommo nrelationship sbetwee nsoil san d the factorswhic h govern their formation.

Itwa s already mentioned that the FAO-Unesco classification system has ahierarchica l structure.Th e 28Majo r SoilGrouping s atth ehighes t level of generalization are subdivided into a total of 153 Soil Units at the second level.I nth e 'RevisedLegend 'o f1988 ,a thir dhierarchica llevel , that of the Soil Subunits was introduced. Soil Subunits are either intergrades between Soil Units, or mark characteristics in addition to thoseuse d inth edefinitio no fSoi lUnits .Thi sfurthe rsubdivisio nprove d necessary when the system became used in connection with soil mapping at increasingly largerscales . Thesyste maccomodate sals osurfac eo rsubsurfac efeature so fth elan dtha t stem from or influence agricultural use; such 'phases' are designations thathav epractica l significance but arenon-taxonomi e and cancu t across theboundarie s of SoilUnit s and/or SoilSubunits .

DIAGNOSTIC HORIZONSAN D DIAGNOSTIC PROPERTIES

Thetaxonomi eunit so fth eFAO-Unesc oclassificatio nwer eselecte do nth e basis of (present knowledge of) the distribution, formation and charac­ teristics of the soils of theworld , and inlin ewit h their importance as naturalresource san dthei renvironmenta l significance.T ofacilitat e soil identification and correlation in areas far apart, thevariou s taxonomie units inth eFAO-Unesc osyste mhav ebee ndefine di nterm so fmeasurabl ean d observable key properties. The differentiating criteria are essentially properties of the soil itself and have been selected on the basis of generally accepted principles of soil formation. Selected key properties areclustere dt o 'diagnostichorizons' ,th ebasi cidentifier s insoi lclas ­ sification. The system uses individual 'diagnostic properties' as further differentiating characteristics. 8 Introduction

The definitions and nomenclature of diagnostic horizons and diagnostic properties are very similar to the concepts developed by the USDA Soil Taxonomy classificationsyste m (SoilSurve yStaff , 1975).However ,som eo f the definitions have been simplified or modified in accordance with the requirements of the SoilMa p of theWorl d legend. Thevariou s diagnostic horizons and diagnostic soilpropertie s aresumma ­ rized inTable s 2an d3 .

TABLE2 . Summary ofdiagnosti c horizons (seeAnne x 1fo r full definitions).

DIAGNOSTIC HORIZON MOST PROMINENT FEATURES histic H-horizon peaty surface soil of2 0 to4 0c m depth; insom e cases till 60cm . mollic A-horizon surfacehorizo nwit h dark colour due to organic matter;bas e saturation exceeds 50percent . umbric A-horizon similar toa molli c A-horizonbu tbas e saturation lower than 50percent . Fimic A-horizon man-made surface layer,5 0c m ormor e thick,produce d by long-continuedmanuring . ochric A-horizon surfacehorizo nwithou t stratificationan d lacking thecharacteristic s ofa histi c H-horizon, ora mollic,umbri c or fimicA-horizon . albic E-horizon bleached eluviationhorizo nwit h the colour of uncoated primary soilmaterial ,usuall y overlying an illuviation horizon. argic B-horizon clay accumulationhorizo n lackingpropertie s ofa natric B-horizon and/or aferrali c B-horizon. natric B-horizon clay accumulationhorizo nwit hmor e than 15percen t exchangeable sodium,usuall y with acolumna r or prismatic structure. spodic B-horizon horizonwit h illuviation of organic matterwit h iron or aluminium orwit hboth . ferralic B-horizon highlyweathere dhorizon , at least 30c m thick,wit h a cationexchang e capacity of 16 cmol(+)/kg clay or less. cambic B-horizon genetically young B-horizon (therefore notmeetin g the criteria for argic,natric , spodic or ferralic horizons)showin g evidence ofalteration : modified colour, removal ofcarbonate s orpresenc e of . calcic horizon horizonwit h distinct calcium carbonate enrichment. gypsic horizon horizonwit h distinct calcium sulphate enrichment. petrocalcic horizon a continuous cemented or indurated calcichorizon . petrogypsic horizon a gypsichorizo nhardene d to the extent thatdr y fragments dono t slake inwate r and roots cannot enter. sulfuric horizon horizon ofa t least 15c m thick,havin g jarosite mottles andpH(H 20, 1:1)< 3.5. Introduction 9

NOTETHA Ta distinctio nmus tb emad ebetwee n thesoi lhorizo n designations that are given in a soil profile description, and diagnostic horizons as used insoi lclassification . The formerbelon g to anomenclatur e inwhic h masterhorizo ncode s (H,0 ,A ,E ,B ,C an dR ; seeAnne x4 )ar eassigne d to thevariou s soillayer sdescribe d and interpreted inth efield .Th e choice ofhorizo ncod e isb ypersona ljudgemen t ofth esoi l surveyor andbase d on presumedprocesse s ofsoi lformation .I ncontrast ,diagnosti chorizon sar e quantitatively definedan dthei rpresenc e orabsenc e canb eascertaine do n thebasi s ofunambiguou s field and/or laboratorymeasurements . Someo fth ediagnosti chorizon s inth eFAO-Unesc o systemar especia l forms ofA - orB-horizons ,e.g . a 'mollic'A-horizon ,o ra 'ferralic'B-horizon . Other diagnostic horizons are not necessarily A- or B-horizons, e.g. a 'calcic' or a 'gypsic' horizon. In these cases the horizon code is not added to thenam e of the diagnostichorizon .

TABLE 3. Summary ofdiagnosti c properties (seeAnne x 2fo r full definitions)

DIAGNOSTIC MOST PROMINENT FEATURES PROPERTIES

andic properties refer to largely pyroclastic ()material ; normallyhig h inextractabl e aluminium. ferralic properties mark acatio nexchang e capacity (byI MNH40A c atp H 7.0) of less than 24cmol(+)/k g clay inCambisol s and Arenosols. ferric properties mark thepresenc e ofFe-enriche dmottle s ornodule s inAlisols , s andAcrisols . fluvic properties mark ongoing sedimentation or stratification or an irregular organic carbonprofil e inrecen t alluvial . geric properties mark soilmaterial s having 1.5 cmol(+)/kg soilo r less ofextractabl e basesplu s aluminium and apH(l M KCl)o f 5.0 ormore ,o rhavin g a delta-pH (pHKC l minus pHH,0 )o f -0.1o rmore . gleyic and stagnic presentvisibl e evidence ofprolonge d waterlogging properties eitherb y shallow groundwater (gleyic properties)o r by aperche dwate r table (stagnic properties). nitic properties mark amoderat e tostron g angularblock y soil structure,easil y falling apart into smaller blocky elements, showing shinype d faces. salic properties mark anelectrica l conductivity of the saturation extract ofmor e than 15dS/m , or ofmor e than4 dS/m if thepH(H 20,l:l)exceed s 8.5. sodic properties markhig h saturation ofth eexchang e complexb y sodium (15percen t ormore )o rb y sodium plus magnesium (50percen t or more). vertic properties mark cracks,slickensides ,wedge-shape d orparal - lelipiped structural aggregates thatar eno t ina combination,o r areno t sufficiently expressed for the soil toqualif y asa Vertisol . abrupt textural marks adoublin g of thecla y content,o r an increase change of thecla y contentb y 20percent ,ove r avertica l distance of less than 5cm . 10 Introduction

Table 3cont' d calcareous refers tosoi lmateria lwhic h shows strongefferves ­ cence incontac twit hHC l and/orhavin gmor e than2 percent ofCaC0,-equivalent . continuous hard materialwhic h issufficien t coherent andhar d when rock moist tomak e diggingwit h aspad e impracticable, gypsiferous refers to soilmateria lwhic h contains 5percen t or more gypsum. Interfingering narrowpenetration s of analbi c E-horizon into an underlying argic ornatri c B-horizonalon g mainly verticalpe dfaces . organic soil saturated soilmaterial s (unless drained), having materials >=18percen t organic ifhavin g>—6 0percen t clay, orhavin g>=1 2percen t carbon ifwithou t clay, orhavin g aproportiona l carbon content ifth e clay content isbetwee n zero and 60percent ; or soil materials thatar eneve r saturated formor e thana few dayshavin g>=2 0percen t organic carbon, the condition of soil temperatures being perennially ato rbelo w 0°C . an iron-rich,humus-poo rmixtur e of clay and quartz thatharden s irreversibly ondrying , slickensides polished and grooved surfaces that areproduce d by one soilmas s slidingpas t another, smeary consistence a consistence which changes underpressur e and returns to theorigina l state after thepressur e is released ('thixotropic'material s inAndosols) . soft powdery lime calcium carbonateprecipitate d insit uan d soft enough tob e cutwit h a fingernail . strongly humic refers to soilmaterial s havingmor e than1. 4 percent organic carbon as aweighte d average over adept h of 100c m from the surface. sulfidic materials waterlogged mineral or organic soilmateria l contain­ ing 0.75 percent ormor e sulfur and less than three times asmuc h carbonates as sulfur, tonguing relatively wide penetrations of analbi c E-horizon into anunderlyin g argic ornatri c B-horizon mainly alongvertica l pedfaces . weatherable minerals that release plantnutrient s and irono r minerals aluminiumb yweathering .

NOTE THAT the generalized descriptions of diagnostic horizons and diag­ nostic soilpropertie s giveni nTable s 2an d 3ar esolel ymean t asa firs t introductiont oFAO-Unesc oterminology .Actua lsoi lclassificatio nrequire s that exact concepts and full definitions be used. These are presented in Annexes 1an d2 . Final taxon identification isdon eb y identifying theMajo r Soil Grouping with the 'KEYT OMAJO R SOILGROUPINGS' ,presente dhereafter .Th eidentifi ­ cation of the SoilUni t (withinth e selectedMajo r Soil Grouping) isdon e using the 'KEYT O SOILUNITS' .A full Key to SoilUnit s is contained in each chapter which describes (theRegiona l Distribution,Geneti c History, Characteristics,Managemen t andUs e of)a particula r Major SoilGrouping . Introduction 11

KEYT OMAJO R SOILGROUPING S

This 'Keyt oMajo r SoilGroupings 'refer s toth echapte r inthi s texti n which a particular Major Soil Grouping is described. Chapters on related Major Soil Groupings areprecede db y adescriptio n of themajo r landforms withwhic h the groupings are commonly associated.

Soilshavin g anH-horizon ,o ra nO-horizo no f4 0c mo rmor e (60c mo r more if theorgani cmateria l consistsmainl y of sphagnum ormos s orha s a bulkdensit yo fles stha n0. 1Mg/ m) eithe rextendin gdow nfro mth esurfac e ortake ncumulativel ywithi nth euppe r 80c mo f the soil; the thickness of the H- or O-horizon may be less if it rests on rocks or on fragmented material ofwhic h the interstices are filledwit h organicmatter : HISTOSOLS (HS) (page 19)

Other soilsi nwhic hhuma nactivitie sresulte d inprofoun dmodificatio no f the original soil characteristics: ANTHROSOLS (AT) (page 35)

Other soils which are limited in depth by continuous hard rock or highly calcareous materials or a continuous cemented layer within 30 cm of the surface, orhavin g less than 20percen t fine earth over adept h of 75 cm from the surface; having no diagnostic horizons other than a mollic, umbric, or ochric A-horizonwit h orwithou t acambi c B-horizon: LEPTOSOLS (LP) (page 115)

Other soils having, after the upper 20 cmhav e been mixed, 30percen t or more clay inal lhorizon s to adept h ofa t least 50cm ; developing cracks from the soil surface downwardwhic h at someperio d inmos tyear s (unless the soil is irrigated)ar e at least 1c mwid e toa dept h of 50cm ;havin g oneo rmor e ofth efollowing : intersecting slickensideso rwedge-shape d or parallelipiped structural aggregates at some depth between 25 and 100 cm from the surface: VERTISOLS (VR) (page 67)

Other soils showing fluvic properties and having no diagnostic horizons othertha na nochric ,a mollic ,o ra numbri cA-horizon ,o ra histi cH-hori ­ zon, or a sulfuric horizon, or sulfidic material within 125 cm of the surface: FLUVISOLS (FL) (page 93)

Other soils showing salic properties and having no diagnostic horizons other thana nA-horizon , ahisti c H-horizon, acambi c B-horizon, acalci c ora gypsichorizon : SOLONCHAKS (SC) (page 181) 12 Introduction

Other soils showing gleyic propertieswithi n 50c m of the surface;havin g no diagnostic horizons other than an A-horizon, a histic H-horizon, a cambic B-horizon, a calcic or a gypsic horizon; lacking plinthite within 125 cm of thesurface : GLEYSOLS (GL) (page 105)

Other soils showing andic properties toa dept h of 35c m ormor e from the surface and having a mollic or an umbric A-horizon possibly overlying a cambicB-horizon ,o ra nochri cA-horizo n anda cambi cB-horizon ;havin gn o other diagnostichorizons : ANDOSOLS (AN) (page 47)

Other soilswhic h are coarser than sandy to a depth of at least 100 cm from the surface, having no diagnostic horizons other than an ochric A-horizon or analbi c E-horizon: ARENOSOLS (AR) (page 59)

Other soils having no diagnostic horizons other than an ochric or umbric A-horizon: REGOSOLS (RG) (page 119)

Other soilshavin g aspodi c B-horizon: PODZOLS (PZ) (page 259)

Other soils having 25 percent or more plinthite by volume in a horizon which isa tleas t1 5c mthic kwithi n5 0c mo fth esurfac eo rwithi na dept h of12 5c mi funderlyin ga nalbi cE-horizo no ra horizo nwhic hshow sstagni c properties within 50c m of the surface or gleyic propertieswithi n 100c m of the surface: PLINTHOSOLS (PT) (page 139)

Other soilshavin g a ferralic B-horizon: FERRALSOLS (FR) (page 147)

Othersoil shavin ga nE-horizo nshowin gstagni cpropertie sa tleas ti npar t ofth ehorizo nan dabruptl yoverlyin ga slowl ypermeabl ehorizo nwithi n12 5 cm of the surface,exclusiv e ofa natri c ora spodic B-horizon: PLANOSOLS (PL) (page 253)

Other soilshavin g anatri c B-horizon: SOLONETZ (SN) (page 191) Introduction 13

Other soilshavin g amolli c A-horizonwit h amois t chroma of 2o r lesst o a depth of at least 15 cm, showing uncoated and quartz grains on structural ped surfaces;havin g anargi c B-horizon: GREYZEMS (GR) (page 231)

Other soilshavin g amolli cA-horizo nwit h amois t chroma of 2o r less to a depth of at least 15 cm;havin g a calcichorizon , or concentrations of softpowder y limewithi n 125 cm of the surface,o rboth : CHERNOZEMS (CH) (page 219)

Other soilshavin g amolli c A-horizon with amois t chroma ofmor e than 2 toa dept ho fa tleas t1 5cm ;havin gon eo rmor eo fth efollowing :a calci c orgypsi chorizon ,o rconcentration s ofsof tpowder y limewithi n12 5c mo f the surface: KASTANOZEMS (KS) (page 215)

Other soils having a mollic A-horizon; having a base saturation (by IM NH.OAca tp H 7.0) of 50percen t ormor e throughout theuppe r 125c mo fth e soil: PHAEOZEMS (PH) (page 227)

Othersoil shavin ga nargi cB-horizo n showinga nirregula ro rbroke nuppe r boundary resulting from deep tonguing of theE-horizo n into the B-horizon or from the formation ofdiscret e nodules larger than 2cm , the exteriors ofwhic har eenriche dan dweakl ycemente do rindurate dwit hiro nan dhavin g redderhue s and stronger chromas than the interiors: PODZOLUVISOLS (PD) (page 247)

Other soilshavin g agypsi c or apetrogypsi c horizonwithi n 125 cmo f the surface;havin g no diagnostic horizons other than an ochric A-horizon, a cambic B-horizon or an argic B-horizon invaded by gypsum or calcium car­ bonate, acalci c ora petrocalci chorizon : GYPSISOLS (GY) (page 197)

Other soils having a calcic or apetrocalci c horizon, or a concentration of soft powdery lime,withi n 125 cm of the surface;havin g no diagnostic horizons other than an ochric A-horizon, a cambic B-horizon or an argic B-horizon invadedb y calcium carbonate: CALCISOLS (CL) (page 203)

Other soilshavin g anargi c B-horizonwit h acla y distributionwhic h does not show a relative decrease from its maximum of more than 20 percent within15 0c mo fth esurface ;showin ggradua lt odiffus ehorizo nboundarie s between A- and B-horizons; having nitic properties in some subhorizon within 125 cm of thesurface : NITISOLS (NT) (page 157) 14 Introduction

Other soilshavin g anargi cB-horizo nwhic hha s acatio nexchang ecapacity - equalt oo rmor etha n2 4cmol (+)/k g clayan da bas esaturatio n (byI MNH.OA c atp H 7.0) of less than 50percen t at least insom epar t of the B-horizon within 125 cm of thesurface : ALISOLS (AL) (page 167)

Other soilshavin g anargi cB-horizo nwhic hha s acatio nexchang e capacity of less than 24cmol(+)/k g clay and abas e saturation (by IMNH^OA ca tp H 7.0) ofles s than5 0percen t ina tleas tsom epar to f theB-horizo nwithi n 125 cm of thesurface : ACRISOLS (AC) (page 161)

Other soilshavin g anargi c B-horizonwhic hha sa catio nexchang e capacity equalt oo rmor etha n2 4cmol (+)/k g clayan da bas esaturatio n (byI MNH 40Ac atp H7.0 )o f5 0percen to rmor ethroughou t theB-horizo nt oa dept ho f12 5 cm: LUVISOLS (LV) (page 241)

Other soilshavin g anargi cB-horizo nwhic hha s acatio nexchang e capacity of less than 24cmol(+)/k g clay and abas e saturation (by IMNH,0A c atp H 7.0) of 50percen t ormor e throughout theB-horizo n toa dept h of 125 cm: LIXISOLS (LX) (page 171)

Other soilshavin g acambi c B-horizon: CAMBISOLS (CM) (page 125)

CORRELATION

Since the beginnings of soil science, at the end of the 19th century, many soil classification systems have been developed. The first systems published reflected clearly theirplac eo forigi nnamel yregion s dominated by quaternary materials of periglacial origin in the USSR, Central and WesternEurop e and theUSA .A smor eexperienc ewa s gained inothe r regions as well, including the tropics and subtropics, novel approaches to soil classification were developed which embrace the soil cover at a global scale.

Truelyinternationa lclassificatio nsystem swer edevelope di nFrance ,th e USA and theUSSR . The FAO-Unesco classification of soils was set up as a common denominator to facilitate correlation and comparison between systems. An in-depth discussion of the subject of correlation is clearly beyond the scope of this syllabus; it may suffice to comment on the analogies and differences which existbetwee n the FAO-Unesco concepts and definitions and those of theUSD A Soil Taxonomy.

The FAO-Unesco classification is less than a full-fledged taxonomie system inth e sense that itha s only two categories (Major Soil Groupings andSoi lUnits )wit ha thir don e (SoilSubunits )t ob e developed. Itmain ­ tained traditional nomenclature asmuc h aspossible . Introduction 15

TheUSD A Soil Taxonomy comprises six categories:Orders, Suborders ,Grea t Groups,Subgroups ,Soi lFamilie san dSoi lSeries .A tth ehighes thierarchi ­ cal level Soil Taxonomy defines 11 Orders. The broad criteria by which Orders are differentiated are asfollows :

(1) theproductio no forgani cmaterial sove rdeterioratio nan ddestructio n (Order of Histosols); (2) theproductio nan d illuvialaccumulatio n ofmetal-humu s complexesan d associated compounds (Order of Spodosols); (3) thepresenc e of low-charge,mainl y pH-dependent constituents (Order of ); (4) alimite d capability to formhorizon s due toa significant amount of shrinking and swelling clays. (Order ofVertisols) ; (5) aver y low rate of transformation due to the lacko f soilmoistur e (Order ofAridisols) ; (6) thepresenc e of illuviated crystalline clays and arelativel y high base reserve inth e substratum (Order ofAlfisols) ; (7) theproductio n andmaintenanc e of abase-rich , organic enriched surficial horizonan d ahig hbas e reserve (Order ofMollisols) ; (8) thepresenc eo filluviate dcrystallin eclay san da relativel y lowbas e reserve in the substratum (Order ofUltisols) ; (9) thealteratio n ofparen tmaterial s and initiationo fhorizo n differentiation (Order of ); (10)th epresenc e ofmaterial s thatma yb e altered to formhorizon s with the advent ofmor epédologi e time (Order of ); (11)th etransformatio no f(mainl yvolcaniclastic )paren tmaterial san dth e presence ofextractabl e aluminium (Order ofAndisols ; proposed, at present inth e Inceptisols).

Bothsystem sus ediagnosti chorizon san ddiagnosti cpropertie sa staxonomi e identifyers but there exist considerable differences between the two systems inth edefinition s ofth ecriteri ause dan d inth elevel s atwhic h they are applied. One important difference is that,unlik e SoilTaxonomy , the FAO-Unesco system doesno tus e the soil temperature and soilmoistur e regime as distinctive criteria in the classification but projects these data on the soilma p asa noverlay .

There are also differences and similarities betweenbot h systems inth e naming of taxonomie units. Both systems use formative elements. In part these were derived from traditional soil names that acquired general acceptance and are firmly established in the literature on soils. For a limitednumbe r of soils itwa s needed tocoi nne wname s inorde r toavoi d confusion. In both systems names are meant to sum up, in an easily remembered term, a set of characteristics which have been found to be representative ofa particula r soil indifferen t parts of theworld . The FAO-Unesco classification uses one name to denote the Major Soil Groupingwit h adjectives for SoilUnit s and SoilSubunits . The formative elements in thename s of the Soil Orders are used as name- endings at lowerhierarchica l levels,a s inth e followingexample :

SOILORDE R SOIL SUBORDER GREAT SOILGROU P SOIL SUBGROUP

Spodosol Orthod Fragiorthod Typic Fragiorthod 16 Introduction

The Major Soil Groupings of the FAO-Unesco classification compare with either Orders or Suborders of Soil Taxonomy; the Soil Units correspond roughly with GreatGroups . ORGAJSriG SOILS : HISXOSOLS

Histosols 19

HISTOSOLS

Soils having 40 cm ormor e organic soilmaterial s (60 cm ormor e if the organic materials consist mainly of sphagnum or moss or have a of less than0. 1 Mg/m) eithe r extending down from the surface or taken cumulatively within the upper 80 cm of the soil; the thickness of the organic surface horizon may be less if it rests on rock or on frag­ mentai material inwhic h the interstices are filledwit h organicmatter .

Key toHistoso l CHS')Soi lUnit s

Histosols havingpermafros t within 200 cm of thesurface . Gelic Histosols (HSi)

Other Histosols having a sulfuric horizon or sulfidic materials3 at less than 125 cm from thesurface . Thionic Histosols (HSt)

Other Histosols that are well drained and are never saturated with water formor e than afe wdays . Folic Histosols (HSI)

Other Histosols having raw or weakly decomposed organic materials3, the fiber content of which is dominant to a depth of 35 cm or more from the surface;havin gver y poor orbein g undrained. Fibric Histosols (HSf)

Other Histosolshavin g highly decomposed organic materials with strongly reduced amounts of visible fibers and a very dark grey to black colour to a depth of 35 cm ormor e from the surface,havin g an imperfect topoo r drainage. Terric Histosols (HSs)

Diagnostic horizon; seeAnne x 1fo r full definition. 3 Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OFHISTOSOL S

Connotation: andmuc k soils;fro mGr .histos . tissue.

Parent material: incompletely decomposed plant remains,wit h or without admixtures of , silto rclay .

Environment: themajorit y of allHistosol shav e formed inborea lregions . Elsewhere,Histosol sar econfine dt opoorl ydraine dbasin san ddepressions , swamp andmars h landswit h shallow groundwater, andhighlan d areaswit h a highprécipitation/évapotranspiratio nratio . 20Histosol s

Profile development: mostly H or HCr profiles. Transformation of plant remainsthroug hbiochemica ldésintégratio nan dformatio no fhumi csubstanc ­ es create a surface layer of mould. Translocated organic material may accumulate indeepe r tiersbu t ismor e often leached from the soil.

Use: pea t lands are used for various forms of extensive forestry and/or grazing or lie idle.I fcarefull y reclaimed andmanaged ,Histosol s canb e veryproductiv eunde rcapital-intensiv e formso farabl ecropping/horticul ­ ture. Deep Histosols arebes t leftuntouched .

REGIONAL DISTRIBUTION OFHISTOSOL S

Peats cover an estimated 240 millionhectare s worldwide (Figure1) . Some 200 million hectares of peat land are situated in the boreal and temperate regions of North America, Europe and Asia. The rest is in the tropics andsubtropic swit hth emos tprominen t formations occurring inth e coastal lowlandso fsoutheas tAsia ,wher e some2 0millio nhectare s ofaci d forestpea tborde r the SundaFlat .

Fig. 1.Histosol sworldwide .

GENESIS OF HISTOSOLS

Peat accumulation isalway s associatedwit h conditionswher e organic material isproduce d by an adapted (climax)vegetation , and bio-chemical ofplan t debris isretarde d dueto : (1)lo w temperatures,and/o r (2)persisten twaterlogging , and/or Histosols 21

(3)extrem e acidity oroligotrophy , and/or (4)th epresenc e ofhig h levels ofelectrolyte s ororgani c toxins. Figure 2 indicates that a net surplus of organic material can build up under swamp conditions even inth etropics .

10° 15° 20° 25° 30° 35° 40° 45° 50° 55° C

Fig. 2.Comparativ e rates ofproductio n (A)an d decomposition (B)o f organic matter as a function of mean annual temperature and aeration of the soil. Source:Va n Dam,1971 .

In view of the limited agricultural significance of the (extensive) boreal moss , the following discussion of Histosol development will focuso npeat si ntemperat ean dtropica lclimates .Th eexample san dillus ­ trationsuse dwil lmainl y refert opea tarea s inth e (humid)tropic swher e soilformin gprocesse s areaccelerate dan dsoi lformatio nmanifest s itself more clearly than in cooler climates. However, the mechanisms discussed apply topea tbog s andHistosol s inal lclimates .

Themajorit y of allpea tbog s occur inlowland s areas,e.g . in stagnant coastal plains and in lagoons that are separated from the sea by beach ridgesparalle l toth eshor e (Andriesse,1974) . Highgroundwater ,frequen t floodingan dsealin gwit hminera lsedimen tand ,i ncoasta lpeats ,hig hsal t contents retard the decomposition of plant debris. As a consequence, near-stagnant pools in depression areas are gradually filled inwit h the remains of aquatic in their deeper parts and with reeds,sedges , grasses and ferns in their fringe areas. The shallow fringe areas of a depression are the first tobecom e filled in and 'dry'. This prompts the vegetation,differentiate d inflora lbelt si nrespons et odifferen tdegree s ofwetness ,t o shift toward the centre of thedepression . Eventually, the entire depression is filled with 'topogenouspeat ' (groundwater peat). Therema yb e anabrup t change from theminera l substrata to the overlying peat body or the transitionma y be gradual; a thin transitional layer of black,smear yan dtotall y decomposedorgani c ('gyttja')ca noccu r aswel l (seeFigur e3) . 22Hlstosol s

Topogenous peats are shallow by nature. Only where their accumulation coincideswit hgradua ltectoni c loweringo fth elan dsurfac eca nthe yreac h agrea tdepth .Th etopogenou spea tdeposit s inth eDram aPlain ,Greece, fo r instance,ar e inplace s deeper than 300meters .

reeds,sedge s

aquatic plants

1 grasses,shrub s

permanentlywe t fringeo fdom e

Fig. 3.A depression isgraduall y filled inwit h topogenous peatwhic h is then overgrown by a laterally expanding ombrogenous peat mass. Note the changing composition of thevegetation . Histosols 23

After a depression is completely filled with topogenous peat, further accumulation ofpea tma y takeplac e ifrainfal l ishig h and evenly spread over the year, and microbial activity is suppressed by severe acidity, oligotrophy and/or organic toxins.Th e then formed rain-dependent 'ombro- genouspeat 'mas srise sove rth emea nwate rleve lan d isn olonge renriche d with mineral material (clay, nutrients) from outside. The continual precipitation surplus (a precondition for the formation of ombrogenous peat) drains away to the fringe of the bog where it creates a wet zone. Topogenous peat can grow here and become covered with ombrogenous peat lateron .Pea tbodie sfro mvariou snucle i (depressedareas )i na plai nwil l eventually merge into one coherentpea t formation.

In temperate regions, topogenous 'low moor' peats include woody forest peats aswel l as peats derived from a grassy marsh vegetation. The 'high moor' peats of ombrogenous raisedbog s aremostl y moss (Sphagnum)peats . Inth etropics .almos tal llowlan dpeat sar eombrogenous ;th epea tconsist s of woody rain forest debris. At high elevations and in the subtropics, peatsar epredominantl ytopogenou san dles swood y (Papyrusswamps ,sawgras s peats, etc.)

As long as a peat body is still shallow, the vegetation can draw nu­ trients from the underlying mineral base. Once the peat has grown to a depth that puts the subsoil out of reach of the living roots, losses of nutrients through ,fixatio no rotherwise ,forc eth evegetatio nt o survive on a gradually decreasing quantity of cycling nutrients. The (climax)vegetatio n adaptsb ybecomin g poorer inqualit y and species com­ position. An initially heavy mixed swamp forest degrades slowly into a light monotonous forest, and ultimately into a stunted forest which produces insufficient organicmateria l for furthervertica l growth of the bog.Thi s limitt overtica l growth,i ncombinatio nwit hcontinuin g lateral expansion, explains the characteristic dome shape of ombrogenous 'raised bogs'.

Therat eo fvertica lpea taccumulatio ndepend so nth evegetation ,o nth e geneticag eo fth ebog ,an do na numbe ro foutsid efactors .Th egrowt hrat e seems to decrease with time according to a roughly exponential pattern. Carbon datings ofbog s in Sarawak and Indonesia suggest that the initial accumulation rate of 2.5 to 4.5 mm/yr decreases in the course of 3 to 5 millennia to0. 5 mm/yr and less indee p (8t o 12m )dom earea s (Anderson, 1964). Figure 4present s avie w from the flat top ofa (drained)ombroge ­ nous raisedbog .

Topogenouspeat sar eofte nwel ldecompose d throughout.Botanica lstrati ­ fication is partly still recognizable but most of the identifiable plant fibers desintegrateupo ngentl erubbing .Suc hsapri cmateria lmake su pth e body ofman y groundwaterpeat sbu tther ear enumerou s exceptions.Mangrov e peat of raw andbrittl e fibricmateria l isa nexample . Intermediate hemic material isals o quite common intopogenou speats . 24Histosol s

Fig. 4. The steep edge of apea t dome near Pontianak, Indonesia (site at km 11 in Figure 5).Not e the steep sides and flat top of the dome which extends 7meter s over theminera l coastalplai n inth ebackground .

The ombrogenouspeat s of the temperate zone aremostl y moss peats;the y are lessdecompose dnea r thesurfac e thana tsom edept hwherea s theombro ­ genous (forest) peats of the wet tropics show the highest degree of decomposition inth e top 10-30c mlayer . Thesituatio ni nSphagnu mdome si sclear :th emosse sgro wa tthei rto pend s and the deadbas e partsbecom emor e andmor ehumified . Thegreate rdecompositio nnea r thesurfac eo ftropica lraise dbog s thana t some depth is a consequence of the better aeration and higher nutrient levels of the surface tiers which promote microbial activity. Once the relatively well decomposed surface material becomes covered with younger peat inth e course of furthervertica l growth of thebog , thewate r table rises (bogsar esponges! )an dbring s theforme rsurfac ehorizo nwithi nth e permanently saturated zone. This retards further decomposition of the material (see Figure 1) while soluble and fine-grained insoluble decom­ positionproduct s continue tob e removedwit h effluentwater . Theremain ­ inglignin/wood-ric hskeleto nform sth eloos ean dcoars e fibric subsurface peat typical ofwel l developed dome formations inth etropics .

Inhorizonta l direction there isa similar differentiation. Permanently saturated central dome areas consist of less decomposed peat thancompa ­ rable layers nearer to the fringe of the domewher e the peat isyounger , (relatively)ric hi nnutrient san doccasionall yaerate dan dsubjec tt omor e intensivemicrobia lattack .Figur e5 show sth edifferen tpea ttype san dth e different degrees ofpea t decomposition ina tropical raisedbog . Histosols 25

^hemic/ :=8^Ü Peat dome near Rasau Jaya, West Kalimantan

Fig. 5.Cros s section ofa smal l coastalpea t formationnea r Pontianak, Indonesia. Note the fibric core of the ombrogenous dome, the better decomposed (hemic and sapric) shell, and the occurrence of sapric and clayey topogenous peat at the footo f the dome. Source:Driessen ,1978 .

CHARACTERISTICS OFHISTOSOL S

It is evident that the characteristics of a Histosol are determined by the type of the peat bog, its depth, the floristic composition of the peat, the stratification/homogenization, packing density, wood content, mineral content and degree of decomposition of the peat material, and a score of other factors.Hence , only a general description of 'peat soil characteristics' ispossible . MostHistosol shav ea nH-horizo no fmor etha n4 0c move ra strongl y reduced mineralsubsoi lwhic hi softe na tgrea tdepth .H ,H/C ran dHC rprofile sar e the commonest configurations. Soils with an organic surface horizon shallower than 40 cm qualify as Histosols only if the organic materials rest directly on bedrock; more commonly they are Fluvisols, Gleysols or (Gleyic)Podzols ,Majo r SoilGrouping swhic hcommonl y occur inassociatio n withHistosol s (Schmidt-Lorenz, 1986). 26Histosol s

Hydrological characteristics

Mostvirgi npeat sar epermanentl ywet ,wit h thewate r tableclos e toth e surfaceo fth esoil .Th ecentra larea so fvirgi ntopogenou spea tbodie san d of extensive ombrogenous formations are always saturated with (near) stagnantwater .Th efring earea so fextensiv eraise dbog shav ea les smono ­ tonous water regime, with drier areas near natural depressions due to gravitydrainag eo fth eimmediat esurroundings .Th eopposit eseem stru efo r smaller domeswher e thewate r regime is lessbuffered ; radial drainage of such domes results inoccasiona l floodsnea r themargin swhil e the centre may,a t times,b e quite dry at thesurface .

Physical characteristics

Ombrogenous Fibric Histosols are loosely packed in the natural state, witha bul kdensit y (BD)tha t istypicall ybetwee n0.0 5 and0. 1 Mg/m ,th e surface tiers contain (still) more solid material than the subsurface layers andhav ebul k densities of0. 1 to0. 2 Mg/m. SeeTabl e 1. Reclaimed (drainedan dcropped )peat s acquirea highe rbul k densityo f (up to 0.4 Mg/m) afte r a fewyear s of consolidation and decomposition of the peat (Terric Histosols). The specific gravity (SG)o fpea tmateria lwit h low contents ofminera l constituents (lesstha n3 percen tb yweight ) isalway s close to1. 4 Mg/m. It follows that the total pore space fraction (= 1-BD/SG) of fibric ombrogenous peat exceeds 0.9 m/m; skeletal subsurface tiers consist for only 5 to 7volum e percent of solid matter!Value s measured on the peat dome of Figure 5 are given in Table 1. Note the vertical and horizontal differentiation inpackin gdensit yan dit scorrelatio nwit hth evegetatio n type.

TABLE1 . Total Pore Fractions (TPF= 1 -BD/SD )calculate d fora pea t dome near Pontianak, Indonesia.

VEG. TYPE Mixed Swamp Forest Transition Monotonous Forest (dome fringe) (dome center ) SITE (km) 13 11.4 10.9 9.4 8.0

10-20c m BD: 0.20 0.15 0.13 0.14 0.13 SG: 1.39 1.28 1.42 1.52 1.44 TPF: 0.86 0.88 0.91 0.91 0.91

70-80c m BD: 0.23* 0.11 0.10 0.10 0.09 SG: 1.67* 1.24 1.35 1.48 1.29 TPF: 0.86 0.91 0.93 0.93 0.93

* clayey peat from shallow fringe area ** SeeFigur e 5.

Virgin peats can retain considerable quantities of water. However, if dried to the extent that adsorptive water is lost, irreversible changes occur in the colloidal components of the peat resulting in a marked and Histosols 27

permanent reduction of the water retention capacity. Overheated peat becomes hydrophobic and shrinks to a granular powder with unattractive physical and agricultural properties and ahig h sensitivity toerosion . In peat areas under monotonous forest, where the peat consists for a large part of recognizable wood in various stages of decay, the peat material shrinks less than in the better decomposed fringe areas under mixed swamp forest. Fibric peat in particular has many wide pores. Its saturated hydraulic conductivity exceeds 1.6 m/d and may evenb e greater than3 0m/d .Wel ldecompose d sapricpea tha s finerpore san d lowerhydrau ­ lic conductivityvalues .

NOTE THAT the above statements are generalizations to which there are numerous exceptions. Woody peats, for instance, are nearly always very permeable to water. Compacted (reclaimed/drained) peats have much lower Total Pore Fractions thanvirgi npeat s and stratified peatsma yb evirtu ­ ally impermeable, irrespective of their fibercontent .

The loose structure and flexible peat fibers are also accountable for the low bearing capacity and poor trafficability of most peats. The low penetration resistancemake s itdifficul t tous enorma l farmmachiner yan d even light equipmentma y get stuckbecaus e ofhig h rolling resistance and slip. The bearing capacity is a function of the potential, internalfrictio nan d 'effectivenorma lstress '(stres stransmitte dthroug h the peat skeleton). Effective normal stress is influenced by the bulk density of the peat; the bearing capacity therefore increases upon reclamation (consolidation) of thepeat .

Chemical characteristics

The wide variation in the physical characteristics of Histosols is matched by an equally wide variation in chemical soil conditions. Most peats are acid; mature ombrogenous peats arever y acid (pH 3 to 4.5) and contain less than 5percen t inorganic constituents. The organic fraction consists of lignin,cellulose ,hemicellulos e and small quantities ofpro ­ teins, waxes, tannins, resin, suberin, etc. Ombrogenous moss peats in temperateclimate sconsis tlargel yo fcellulos ewherea sdee plowlan dfores t peats from Indonesia appeared to consist for two-thirds of lignin with cellulose and hemicellulose accounting for only 1-10 percent of the dry sampleweight .Th ecellulos econtent so ftropica ltopogenou speat sma ywel l behighe r thanth e lignincontent s (as inpeat s intemperat e regions)bu t there islittl e quantitative information on tropical topogenouspeats .

Oneimportan torgani cfractio ni norgani csoi lmaterial s isno tcontaine d infres hplan tdebri sbu t issynthesize d inth ecours e ofmicrobia ltrans ­ formationo fth eorgani cmaterials : 'humicsubstances' ,a mixtur eo fhumin s and humic and fulvic acids. Humic substances form stable complexes with metal ions. These are easily leached out of the peat mass with effluent water. (Geographic names such asRi o Negro,Blakkawatra , Cola creek,Ai r hitam, Zwarte water, and many more testify of the constant leaching of organiccompound s frompea tbogs. )Tabl e2 present s ranges intota lmicro ­ element contents as observed in a number of deep and extremely poor In­ donesianpea t soils.Th ehighe r element contents of surface tiers reflect 28Hlstosol s

the cycling of elements by thevegetation . Note that a considerable part of all micro-elements in the system are stored in the vegetation; the values presented in Table 2 are by no means indicative of the total quantities present immediately after felling and burning of the (forest) vegetation.

TABLE2 . Micronutrientcontent s (kg/ha)measure d insurfac ean d subsurface tierso f deep ombrogenous forestpeat s from Indonesia.

-0 25c m 80 -10 0cm -

Cobalt 0.1-0.2 0.05-0.1 Copper 0.8-8.0 0.2-0.8 Iron 143-175 67-220 Manganese 4.1-25 1.1-7.1 Molybdenum 0.6-1.0 0.3-0.6 Zinc 2.8-4.4 1.8-4.8

With the respective differences considered, the same applies to the levels of macro- and secondary nutrients in (virgin)pea t soils: element contents arehighes t inth e top 25c mo f the soil. Felling of thenatura l forest (part of the 'reclamation' process of peat lands) upsets this patternbecaus eo finterrupte dnutrien t cycling,releas eo fnutrient s from decaying organic materials and biomass, increased leaching ofnutrients , volatilizationupo nheatin g (burning)o f thepeat ,etc .

TABLE3 . Nitrogen and total ash contents and C/N-ratios in a moderately deep (A) and a deep (B)Fibri c Histosol from theNetherlands . Source: Dine et al, 1976.

DEPTH BD pH(H20) TOTALAS H CARBON NITROGEN C/N (cm) (Mg/m3) (%) (%) (%) A 0-7 0.20 3.1 5.8 54.3 1.01 53.8 7-18 0.14 3.0 3.1 58.8 1.03 57.1 18-92 0.12 3.3 2.7 62.9 1.13 55.7 92-117 0.10 4.3 3.9 61.7 1.55 39.8 117-127 0.14 4.9 84.2 6.8 0.22 30.9

B 0-7 0.24 3.1 8.8 56.1 1.28 45.8 7-24 0.15 3.1 1.2 57.1 0.69 82.7 24-35 0.14 3.1 0.8 62.7 0.82 76.5 35-55 0.13 3.0 1.2 57.9 0.80 72.4 55-86 0.10 3.2 5.8 53.7 0.64 83.9 86-120 0.13 3.0 1.4 61.3 0.96 63.9 120-142 0.10 3.1 1.0 64.1 0.93 68.9 >142 n.d. 3.4 3.1 56.9 0.65 87.5 Hlstosols 29

The contents of 'total ash', K-O, P205 and Si02 of the surface soil decrease after clearing of the forest vegetation whereas CaO and MgO contents tendt oincrease .Th equantitie s ofNa ,C Ian d S04depen dstrongl y on local conditions such as the distance from the sea and thepresenc e of pyrite inth epea t (seeals o the chapter on Fluvisols).

The quantities of nitrogen contained in the peat are of the order of 2,000t o4,00 0k gN/h ape r 0-20 cm, ofwhic h only aver y smallportio n is available toplants .Tabl e 3illustrate s the foregoingwit h data from two Fibric Histosols from The Netherlands (note the high C/N-ratios of the subsurface peat).

Figure 6 presents the distribution of mineral constituents in a young and amatur e Fibric Histosol fromKalimantan . As in the DutchHistosols , nutrient elements are concentrated (and available) in theuppe r 10 cm of the soil where a dense root mat occurs,wherea s living roots are absent (andas hcontent s low)i ndeepe rlayers . Virgin shallow or moderately deep peats are less depleted of plant nutrients thandee ppeats .Deficiencie sar emos t severe ingeneticall yol d peats,wit hth eexceptio no ftopogenou speat swhic hreceiv enutrient s from outside and cannotb e included inan y generalized account of the chemical properties ofpea tsoils .

MANAGEMENTAN DUS EO FHISTOSOL S

Naturalpeat snee dt ob edraine d topermi tth ecultivatio no fdrylan d crops.Centrall y guidedreclamatio nproject sar e (almost)exclusiv e toth e temperate zonewher emillion so fhectare shav ebee ndrained .Tropica lpea t landsar emor e commonlyopene db y individual farmersalthoug h somemedium - sized governmentsponsore dproject shav ebee ncarrie dou ta swell .Despit e small capital inputs, farmers are sometimes more successful than govern­ ment-inspired reclamation projects because they are more flexible in the selection of their locations and use less advanced but time-tested techniques. Thereclamatio no fpea tland sstart snearl y alwayswit h theconstructio n of shallow drainage ditches. As a rule, the vegetation is left standing because it helps drying of the peat. One-meter-deep drains at 20-40 m intervals are satisfactory in most cases but well decomposed or clayey peatsma y require narrower spacingswherea sundecompose dwood y domepeat s could insom ecase sb edraine dwit hditche s10 0m apart .A comple xdrainag e system is not advisable because land subsidence is likely to disturb the connections between sucker drains and collecting drains. In practice, draining peat lands is a matter of experience and the standard formulas applicable tominera l soils are oflittl evalue . 30Histosol s

• deep "padang" - peat

• deep peat under mixed swamp forest

Estimated quantities of mineral constituents in the upper 80 cm.

Total Structurally Involved in Vegetation quantity part of the peat cycling (hatched)

"Padang" 5,630 kg/ha 2,380 kg/ha 3,250 kg/ha

Mixed swamp 13,250 kg/ha 2,400 kg/ha 10,850 kg/ha forest

1000 1500 mineral constituents (kg/ha.cm)

Fig.6 .Distributio no fminera lconstituent si ntw odee pvirgi nombrogenou s forestpeat s inKalimantan .

Aftersom etim eo foperation ,th edrainag esyste mwil lhav et ob eadjuste d because peat properties change. The soil's hydraulic conductivity might decreasei nth ecours eo fdrainag ebecaus e ofth ecollaps eo flarg epores , the formation of an illuvial horizon, or the effects of tillage. On the other hand, the soil may actually become more permeable after some time because of decaying wood providing passage for the escape of water or becauseo fincreasin gbiologica lactivit y (roots, )o rth eformatio n ofcracks .Farmer s inth etropic sprefe r shallowhand-du g drainageditche s thatca nb e deepened asneeded . Histosols 31

Fig. 7.Controlle dburnin g ofpea t torais e thep Ho fth esurfac e soilan d 'liberate'nutrien t elementsbefor e planting.

It is difficult to say exactly when peat reclamation is completed.Re ­ clamation and cultivation overlap and in most instances cropping is even part ofreclamation . Inth efirs tfe wyear s after theremova l ofth enatu ­ ralvegetation , suitable annual crops may produce fair yields, thanks to the nutrients that are still contained in the surface soil (and in the asheso fburn tplan tmaterial) . Theuneve ndistributio no fthes enutrient s over a fieldexplain s the irregular growth typical ofyoun g reclamations. After a few years,whe n subsidence of the land surface has slowed, trees and shrubs canb e planted. They may grow satisfactorily for some timebu t yields will eventually decrease ifth e land isno t fertilized. Where fertilizers are not sufficiently available, farmers often turn to some sort of controlled burning of the peat to 'liberate'nutrient s and to raise thep H of the surface soil. See Figure 7.

Burning has undoubtedly a stimulating effect onplan t performance, but the desirability of burning and its precise effects are still open to discussion.Thos ei nfavou ro fcontrolle dburnin gclai mtha ti ti sno tmor e destructive than oxidation in the long run but concentrates certain nutrients (N, P, K, Ca, Mg, S) and renders them more available to the 32Histosol s plant. Others are of the opinion thatburnin g shouldb e discouragedalto ­ getherbecaus emos t of the liberated nitrogen and sulphur are lost toth e atmospherean dothe rnutrient sar elargel yleache dou to fth esurfac esoil . Theoveral ldeterioratio no fth esoi lstructur e (theburn tlaye ri susuall y by farth ebes tpar to fth eprofile )an d theresultin guneve nsoi l surface are additional arguments againstburning . Liming and full fertilization areneede d for goodyield s on ombrogenous peat. Inadditio nt omassiv eapplication s ofgroun d (magnesium)limestone , N,P an dK fertilizer smus tb eapplie dtogethe rwit hsmal ldose so fsulfur , copper,zinc ,manganese ,molybdenum , andiron .

The reclamation of peats is further complicated by the fact that the loosean dperishabl enatur eo fmos tpeat s leadst oconsiderabl e subsidence of the land surface once the land is drained and the suspending force of thegroundwate r removed.Th etota l lossi nsurfac eelevatio n isdetermine d by thecombine deffect so fconsolidatio n ofth epea tmas s (throughsettle ­ ment, shrinkage and compaction of the peat), and ofmineralizatio n of the peat through biochemical oxidation andburning . Removal of peat, e.g. by erosion, isa proble m inman yHistoso l areas inth e temperatezone .

Initial subsidence starts directly after the water table is lowered and canreac hvalue s ofon emete r andmor e inth efirs tyea r indeepl y drained ombrogenous peats. This high rate levels off to a much lower continuous subsidence after a number of years. The initial subsidence is primarily caused by consolidation of the peat mass after the loss of groundwater buoyancy and -i nth e case ofwood y forestpeat s -th e decay of the roots and timber skeleton of thepeat .Continuou s subsidence ismainl y amatte r ofmineralizatio n of thepea t andrepresent s atru e loss of soilmateria l rather thana loss of soilvolume . Inn o case should drainage of the land be deeper than strictly necessary. MINERAL SOILS CONDITIONED BY HUMAN INFLUENCES: ANTHROSOLS Anthrosols 35

ANTHROSOLS

Soilsi nwhic hhuma nactivitie shav eresulte di nprofoun dmodificatio n orburia l of the original soilhorizon s through removal or disturbance of surfacehorizons ,cut s and fills,secula r additions of organicmaterials , or long-continued .

Key toAnthroso l (AT)Soi lUnit s

Anthrosolsshowin gremnant so fdiagnosti chorizon sdu et odee pcultivation . AricAnthrosol s (ATa)

OtherAnthrosol shavin g afimi c A-horizon. FimicAnthrosol s (ATf)

Other Anthrosols having an accumulation layer of fine sediments, thicker than 50 cm and resulting from long-continued irrigation or artificial raising of the soilsurface . Cumulic Anthrosols (ATc)

Other Anthrosols having, to a depth of more than 50 cm, admixtures of wastes frommines ,tow nrefuses ,fill s fromurba n developments,etc . Urbic Anthrosols (ATu)

Diagnostichorizon ; seeAnne x 1fo r fulldefinition .

SUMMARYDESCRIPTIO N OFANTHROSOL S

Connotation: soils whose most prominent characteristics result from the activities ofman ; fromGr .anthropos .man .

Parent material: various parent materials, modified by man through deep cultivation or through addition ofmateria l fromelsewhere .

Environment: Anthrosols occur in all environments; Fimic Anthrosols are most common innorth-wes t Europe where they occur in areas of quartzitic sandswit h alon ghistor y ofmixe d farming.

Profile development: the influence of man is mostly restricted to the surface horizon(s); a buried soil can still be intact at some depth and testify of soil conditions as existed before Man modified the land. In recent reclamations, heavy equipment often destroyed all previous soil development to a great depth or it buried existing soil profiles under thick layers of allochtonousmaterial .

Use: EuropeanAnthrosol swer e traditionally grownt owinte rrye ,oats ,an d barley; FimicAnthrosol swer evaluabl e tobaccosoils .A tpresent ,the yar e 36Anthrosol s used for forage maize, potatoes, horticultural crops (strawberries) and tree nurseries; in places also for pasture, or for vegetable and flower productioni ngreenhouses .Cumuli cAnthrosol si nirrigatio narea sar egrow n tocas h crops or foodcrops .

REGIONALDISTRIBUTIO N OFANTHROSOL S

Small areas ofAnthrosol s are foundwhereve r peoplehav e lived fora longtime .Fimi cAnthrosol s areth ebes tknow nman-mad e soils;the yoccup y a total of some 0.5 millionhectare s inwester n Europe (Holland, Belgium, FederalRepubli co fGermany ;se eFigur e 1.); smalleroccurrence s arefoun d along theenglis h coast and inScotland ,Wale s and Ireland.

40 80k m

Fig. 1. FimicAnthrosol s innorthwester n Europe. Source: Pape,1970 . Anthrosols 37

GENESIS OFANTHROSOL S

Aric, Cumulic andUrbi c Anthrosols were mostly formed by short term human interference such as deep cultivation, irrigation or 'landscaping' in the context of land reclamation, and regional development and/or urbanization schemes.

Aric Anthrosols formed as a result of deep cultivation. In Europe, for instance, sandy Podzols with compacted or cemented B-horizons at shallow depth were widely mixed and are now Aric Anthrosols. Deep cultivation of landwit ha shallo whisti c topsoilove rsan dproduce dAri cAnthrosol swit h alternating oblique layers of peat and sand, that are excellently suited for arable cropping (the peat layers act as a sink in times of wetness whereas the sand layers keep capillary rise going intime s of drought).

Most Cumulic Anthrosols formed through prolonged sedimentation of silt from irrigation water. In depressed areas, trees are commonly planted on man-made ridges that alternate with drainage furrows. Eventually, the original soilprofil e of theridg e areas isburie dunde r a thick cover of soil and . The ridge-and-furrow system is known from such different environments as thewe t forests ofwester n Europe and the coastal swamps of southeast Asia where the ridges are planted to dryland crops and rice is grown in the shallow ditch areas (the 'sorjan' system; see also the chapter on Fluvisols).

TheUrbi cAnthrosol s area ver yheterogeneou s taxonomieunit .Wel lknow n examples are soils inmin e spoils,o r in filled-in openpi t mining areas and soils in wetlands that were covered with layers of sand as part of urbanization schemes.

Fimic Anthrosols have a characteristic diagnostic horizon: the fimic A-horizon, produced by long continued addition of a mixture of organic manure andearth .Th eman-mad echaracte ro fth efimi cA-horizo n iseviden t from bits of brick and pottery and/or from high levels of extractable phosphorus (normallymor etha n25 0m gP 205pe rk gb y1 percen tcitri cacid ). The formation ofmos t fimic A-horizons started in theMiddl e Ages when inorganic fertilizers werevirtuall y nonexistent.Wher enatura l soilfer ­ tility was insufficient for sustained production of arable crops, the farmersuse da syste mo fmixe dfarmin gi nwhic harabl efield swer ecombine d with sheep grazing on communal pasture land. During the nights and in wintertime,shee p andcattl ewer ekep t instable swit hbeddin gmateria lo f thinsod so fheat hand/o r forest littert oabsor b thevaluabl e dung.Fres h beddingmateria lwa s regularlyprovide dunti lth ebeddin gbecam e toothic k andha d tob e removed. Itwa s thensprea dou to nth efield s asa n 'organic earthmanure' .

Evidently,thi smanur ecoul ddecompos eonl yi npart ;th eminera lfractio n remained behind, with some of the organic matter, and raised the land surfaceb y some 1millimete r per annum. Inplaces, thi s systemwa s inus e for more than one thousand years and produced overthickened fimic A- horizons ofmor e than 100cm . 38Anthrosol s

Dependingo nth ecompositio no fth ebeddin gmaterial ,th efimi cA-horizo n is black (bedding material from heathlands with Podzols) or brown (from forest litter) in colour. In places, sods from low-lying grasslands were incorporated inth eeart hmanure .Thi sgav eth eA-horizo na somewha thighe r clay content than the deeper solum. Some 10 hectares of heathland were needed to maintain the nutrient level of one hectare of arable land. Removal ofth esod smad e theheathlan d susceptible towin derosio nan dno t seldom large tractso fheathlan d turned tobarre n shifting sands thatwen t completely out ofcontro l (seeals ounde r Arenosols). Most arable fields were situated on favourable sites near villages and were well drained even before acquiring a fimic A-horizon. They were selected, among other reasons,o n account of a lower frost hazard to the winter rye crop; only in areas with ahig h population density were less well drained soils also used. This arrangement of arable fields on well drained (higher)terrai nposition s andpasture s innearb y depressions can stillb e seen inAnthroso l areas inwester n Europe (seeFigur e2) .

Fig. 2.Mixe d farming near Winterswijk, The Netherlands. Arable cropping on well drained Fimic Anthrosols (in foreground) is combined with dairy farming indepresse d areas.Source :Va nd eWesteringh ,1979 .

Themixe dfarmin g systemdescribe dproduce d theworld' s largestcontinu ­ ous area of Fimic Anthrosols (see Figure 1). Differen t types of fimic Anthrosols3 9

A-horizons occur elsewhere in western Europe, formed for instance by gradually covering peat soils with layers of sand, bagger from drainage ditches,an dorgani cmanure . Inpart s ofIrelan d andEngland , calcareous beach sandswer e cartedt o areaswit haci dArenosols ,Podzols ,Podzoluvisol san dHistosol swhic h thus becameFimi cand/o rCumuli cAnthrosols .Othe rexample so fFimi cAnthrosol s are found along river terraces insouther n Maryland, U.S.A., where deep, blackA-horizon s formedi nlayer so fkitche nrefus e (mainlyoyste r shells) from early Indianhabitation . Similar soils occur along theAmazo n river inBrasi l('Terr aPrêta') .Man ycountrie sposses ssmal larea so fsoil stha t weremodifie db yearl y inhabitants.

CHARACTERISTICS OFANTHROSOL S

Anthrosols cannotb echaracterise d byan yparticula r arrangemento f soilhorizons ;th eonl yhorizo n that isdiagnosti c isth efimi c A-horizon ofFimi cAnthrosols .Thi ssurfac ehorizo nha sa thicknes so f5 0c mo rmore , a black orbrow n colour,an di si nalmos t allcase s free ofmottles .Th e surface horizon ofa nunderlyin g buried soilma yb e incorporated inth e (lowerpar to fthe )A-horizon ; spademark sma ystil lb edetectabl e there.

Hvdrological characteristics

Fimic Anthrosols are well drained because of their overthickened A-horizon andbecaus e thesyste m ofeart h manuringwa sonl y practicedo n selectedwel l drained sites.Th einterna l drainageo fth efimi c A-horizon is excellent because ofit ssand y texture andhig h porosity (highbiolo ­ gical activity); aerationi sneve ra problem . Soilunit s other thanFimi c Anthrosols range from well drained topoorl y drained depending on their texture, theirporosit yan dthei rpositio ni nth elandscape .

Physical characteristics

The physical characteristics ofth efimi c A-horizon areexcellent :th e total pore fraction is nearly always close to 0.5 cm/cm , pores are interconnected ando fdifferen t sizes,th epenetratio n resistance islo w andallow sunhindere drootin gand ,las tbu tno tleast ,th estorag ecapacit y of 'available'soi lmoistur e ishig hi fcompare dt otha to fth eunderlyin g soil material.Th etyp e oforgani c matter ('moder')further s stable soil structures andlimit s thesusceptibilit y toslaking .Th euppe r part ofa fimic A-horizon maybecom e somewhat dense iftillag e isdon e with heavy (vibrating)machinery .

Chemical characteristics

Mostfimi cA-horizon shav epH(KCl )value sbetwee n4 an d4.5 ,excep twher e foreigncalcareou smateria lwa sbrough t intoth esoi l(e.g .se asand) .Th e organiccarbo ncontent so ffimi cA-horizon sar etypicall ybetwee n1 percen t and 5 percent. The organic carbon content ofblac k fimic A-horizons is higher thano fbrow n ones; theC/ Nrati o isgenerall y between 10an d20 , with thehighe r values inblac k soils. TheCE C depends largely onth e 40Anthrosol s

quantityo forgani cmatte ri nth esoi lan do nit scomposition ;reporte dCE C values arebetwee n 5 and 15 cmol(+)/kg soil. Characteristic of all fimic A-horizons isth ehig h totalphosphoru sconten twhich ,i nplaces ,buil tu p to200 0mg/k gan dmore .Othe relement sar eofte ni nshor tsuppl y (N,K ,Mn , B,Cu) .

MANAGEMENTAN DUS EO FANTHROSOL S

Aric, Cumulic and Urbic Anthrosols vary widely in properties and requirements and cannot be included in any generalized account of the management and use of Anthrosols. Fimic Anthrosols are characterised by favourable physicalpropertie s (porosity, rootability,moistur e availabi­ lity)an d ratherunsatisfactor y chemical properties (acidity,nutrients) . Ryeyield sobtaine dwithou t theus eo f fertilizerswer e 700t o110 0k gpe r hectare, or4 to 5time s thequantit y of seedused . The good drainage and the dark colour of the surface soil make it possible to till and sow or plant early in the season. Rye, oats, barley, potato and also the more demandingsuga rbee tan dsumme rwhea tar ecommo ncrop so nFimi cAnthrosols . Inplaces, th esoil sar euse d fortre enurserie s andhorticulture ,notabl y forvegetable , small fruit and flowerproduction .Unti l the 1950's, soils with deep fimicA-horizon s inTh eNetherland s were indeman d for the cul­ tivation of tobacco.Wit hmoder n fertilization,yield s of*5000k g ryepe r hectare,450 0k gbarle y andsom e 5500k gsumme rwhea tca nb e obtained.Th e production figures for sugarbee t andpotat o are of theorde r of4 0 to5 0 tonspe rhectare .Fimi cAnthrosol s are increasinglyuse dfo rproductio no f silage maize and grass;yield s of 12 to 13 tons of dry maize silage per hectare and of 10t o 13 tons ofdr y grass are considerednormal . MINERAL SOILS CONDITIONED BY THE PARENT MATERIAL : ANDOSOLS ARENOSOLS VERTISOLS

Landforms involcani c regions 43

MAJOR LANDFORMS IN VOLCANIC REGIONS

Volcanic regions differ from allothe r regions intw oways : (1)b y their structure and origin,an d (2)b y theparticula r composition of thevolcani cmaterials .

The world distribution of active volcanoes is shown in Figure 1. The chemical andmineralogica l composition ofvolcani c materials ismarke db y anabundanc eo feasil yweatherabl ecomponents ,accountabl e forth eremark ­ ablepropertie s thatsoil s involcani c regionshav e incommon .I nspit eo f theoverridin g importanceo fth eparen tmateria lfo rsoi lformatio ni nvol ­ canic regions, different soils (may)occu r indifferen t landforms.

Fig. 1.Volcani c regions of theworld .Not e thatmos tvolcanoe s are found nearplat e margins, inzone s ofyoun g orogeny. Based on:Robinson , 1975

As magma composition has a strong bearing on thenatur e of the erupted productsbu tals oo nth eexplosivity ,character ,an dmorpholog yo fvolcani c phenomena,w eshal ldiscus sth emajo rlandform so fvolcani cregion sb ycom ­ positional group,viz .fo r regionswit hbasaltic , andesitic and rhyolitic volcanism.

LANDFORMS INREGION SWIT HBASALTI C VOLCANISM

Basaltic volcanism is most commonly found in situations of divergent platemargins , incontinenta l riftvalleys ,an d inhot-spo tareas . The classic example ofbasalti c hot-spotvolcanis m isHawaii , the largest shieldvolcan o inth eworld ,wit ha diamete r of25 0k m atth ebas e (onth e ocean floor)an d a total height of 9km . Basaltic magma is comparatively fluid and gases escape easily. Therefore, eruptions are relatively quiet 44Landform s involcani c regions andproduc e lava flows, lava lakes and lava fountains,bu t little ash. Basaltic volcanism in continental settings (e.g. the East-African rift valley, theBaika l graben,o r theRhin e -Rhon e and related riftvalleys ) didproduc e 'strombolian'cone san dmaar ebut ,her e too,as hproductio n is limitedan dlocalized .Th eas hdeposit sexten d seldomoutsid e thevolcani c areas themselves,an dwher e extensive ashblanket s occur, as insom e rift valleys, they areusuall y more acid incomposition . Basaltic lava flows, because of their fluidity, may follow river valleys over considerable distances from the eruption centres.Late r differential erosion may result in relief inversion with the former basaltic valley fills extending asplateau s above thesurroundings .

LANDFORMS INREGION SWIT HANDESITI C VOLCANISM

Andesitic volcanism is a characteristic element in 'Cordillera'-type mountain belts such as theAnde s and the Barisan range in Indonesia, and in island arcs. The classicvolcan o type is the stratovolcano. Literally, theter mmean s 'stratified'volcano ,whic h ismisleadin g inth esens e that all volcanoes are built up of layers,b e it of basalt flows, as in the Hawaiian shieldvolcanoes ,o r ofpyroclastics , as the scoria cones ofth e Eifel.Wha t the term indicates,actually , is that this type ofvolcan o is composed of alternating layers of lavas and pyroclastic rock, mostly of andesitic composition.

Andesitic magmas hold an intermediate position between basaltic and rhyolitic magmas with regard to their SiO, content, viscosity and gas content.Wherea sbasaltic ,lo wviscosit ymagma shardl yproduc epyroclastic s ('tephra'), and high-viscosity rhyolites hardly produce lavas, andesitic magmaswil lnormall yproduc eboth .Becaus eo f thegreate rviscosit y ofth e magma, a higher pressure must build up before an eruption can occur; eruptions are less frequent andmor eviolen t than inbasalti c volcanism.

Lava flows emitted by stratovolcanoes are more viscous than those of basaltic shield volcanoes, and do not extend as far from their point of emission,usuall yno tmor etha na fe wkilometres .Thi sexplain swh ystrato ­ volcanoes have the 'classical' cone shape and steeper slopes than shield volcanoes. Lahars arevolcani c mudflows. They can form insevera lway s: (1)whe n acrate r lake is thrownou tdurin g aneruption ,o r (2)whe nnumerou s condensationnucle i inth e air (volcanic ash)generat e heavy rain showers,o r (3)whe n thevolcan o was coveredb y snow or glaciersbefor e the eruption. The term lahar stems from Indonesia,wher evolcani c mudflows of the first typewer e particularly disastrous. Pyroclastic flows are frothy masses of ash and pumice, deposited by glowing avalanches.Th eresultin grock sar eknow na signimbrites . Theyca n have a variety of structures depending on the flow conditions during emplacement and on thedegre e ofpostdepositiona l welding.

Volcanicair-fal lashe sar ewidesprea di nvolcani cregions ,wherea slava s andpyroclasti c flowsar econfine d toth eimmediat evicinit y ofvolcanoes , ashes areblow nove rvas tdistance s andtrave lhundred s ofkilometres .Th e thicknesso fas hdeposit sdecrease swit hincreasin gdistanc efro mth epoin t oforigin .I tma yb e difficult torecogniz e thepresenc e ofvolcani c ashes Landforms Involcani c regions 45 in soils as it is incorporated in the solum by burrowing animals and weathersrapidly .Nonetheless , 'rejuvenation'o fa soi lmateria lwit hfres h volcanic ash isofte n ofgrea t importance as itrestore s or improves andpromote s physical soil stability.

LANDFORMS INREGION S WITH RHYOLITIC VOLCANISM

Partialmeltin g of the continental crust inCordillera nmountai n ranges mayproduc erhyoliti cmagma .Rhyolite srepresen t theextrusiv e equivalents of granites; they are acid volcanic rocks with a high content of SiO,. Rhyolitic magmas are therefore very viscous and withstand very high gas pressures. As a result, rhyolitic eruptions are rare,bu t also extremely violent. Ifa rhyoliti cmagm achambe r ispresen tbelo wa stratovolcano ,i t builds up such tremendous pressures that, once a vent for eruption is opened, the magma chamber empties itself completely, leaving a cavity in theearth' scrus ti nwhic hth eentir estratovolcan ocollapses .I nthi sway , craters of severalkilometre s indiamete r are formed: thecalderas . Themai nemissio nproduct sar eashes , inastonishin gquantitie s andsprea d over vast areas,an d ignimbrites. The latter stem from pyroclastic flows of a fluid, glass-rich mass; the flows (can)exten d over several tens of kilometres and fill depressions and valleys to depths of tens or even hundreds ofmetres . Incontras twit h the irregular surfaces of lava flows andlahars ,ignimbrit esurface sar ecompletel yfla tan dfeatureless .White , porous and fibrous pumice inclusions arecommon .

Bothashe san dignimbrite sconsis tfo rth egreate rpar to fvolcani cglas s and weather rapidly. Phenocrysts (mainly biotite andhornblende ) make up less than 20 percent of the ash. The only historic ignimbrite-forming eruption was that of the Katmai in Alaska in 1912. The eruption of the Krakatoa, in 1883, did produce a caldera, but as the main eruption took place under water, ignimbrite formation was not conspicuous on the land surface.Th elarges teruptio ni ncomparativel yrecen ttime stoo kplac esom e 40,000year s ago and led to the formation ofLak e Toba inSumatra .

Rapid weathering of porous volcanic material in environments with sufficient rainfall, translocation of (part of) the weathering products and accumulation of stable organo-mineral complexes and of short-range- ordermineral s sucha sallophane ,imogolit ean dferrihydrit ear eessentia l processes inth eformatio no fth echaracteristi c soilso fvolcani cregions : theANDOSOLS . 46 Notes

NOTES Andosols 47

ANDOSOLS

Soils showing andic properties to adept h of 35 cm ormor e from the surface andhavin g amolli c or anumbri c A-horizon possibly overlying a cambic B-horizon, or anochri c A-horizon anda cambi c B-horizon; having noothe rdiagnosti chorizons ;lackin ggleyi c propertieswithi n5 0c mo fth e surface; lacking the characteristics which are diagnostic forVertisols ; lacking salic properties.

Key toAndoso l (AN)Soi lUnit s

Andosols havingpermafros t within 200c m of thesurface . GelicAndosol s (ANi)

OtherAndosol s showing gleyic properties within 100 cm of thesurface . Gleyic Andosols (ANg)

Other Andosols lacking a smeary consistence ,o r a texture which is silt loam or finer on the weighted average for all horizons within 100 cm of the surface,o rboth . Vitric Andosols (ANz)

OtherAndosol shavin g amolli c A-horizon. Mollic Andosols (ANm)

OtherAndosol s having anumbri c A-horizon. Umbric Andosols (ANu)

OtherAndosols . Haplic Andosols (ANh)

Diagnostichorizon ; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARY DESCRIPTION OFANDOSOL S

Connotation:soil si nvolcani cmaterials ;fro mJap .an ,dark ,an ddo ,soil .

Parentmaterial :pyroclasti cmaterial ,notabl yvolcani cash ,bu tals otuff , pumice, cinders and other volcanic éjecta of various composition. Exceptionally,weatherin go fnon-volcani cmateria lma yresul ti nandi csoi l properties andhenc e inAndosols .

Environment:undulatin gt omountainous ,humid ,arcti ct otropica l climates with awid e range ofvegetatio ntypes .

Profile development: AC- or ABC-profile. Rapid weathering of the porous materialresult s inaccumulatio no fstabl eorgano-minera lcomplexe s ando f short-range-ordermineral s such asallophane , imogolite and ferrihydrite. 48Andosol s

Use: many Andosols are intensively cultivated with a variety of crops, their major limitation being the high phosphate fixation capacity. In places, steep topography is the chief limitation.

REGIONAL DISTRIBUTION OFANDOSOL S

Andosols occur in volcanic regions all over the earth. Important concentrations arefoun dnea rth ePacifi cBasin :o nth ewes tcoas to fSout h America,Centra lAmerica , theRock yMountains ,Alaska ,Japan , thePhilip ­ pine Archipelago, Indonesia, Papua New Guinea and New Zealand. They are alsoprominen to nman y islandsi nth ePacific :Fiji ,Vanuata ,Ne wHebrides , New Caledonia, Samoa,Hawaii . InAfrica ,Andosol soccu ri nEthiopia ,Kenya ,Rwanda ,Cameroo nan dMadagas ­ car. Other regions with Andosols are the West Indies, Canary Islands, Italy, France, Germany, Iceland. The total Andosol area is estimated at some 124millio nhectare s or0.8 4percen to fth egloba l land surface. Some 80 percent of the Andosol area is potential crop land which corresponds withabou t 2percen to f thetota lpotentia l crop landarea .Mor e thanhal f ofthi s issituate d inth etropics .Figur e 1show s theworldwid e distribu­ tion ofAndosols .

Fig. 1.Andosol sworldwide .

GENESIS OFANDOSOL S

Andosol formationdepend s essentially on the rapid chemicalweather ­ ingo fporous ,permeable ,fine-graine dparen tmateria lcontainin g 'volcanic glass' in the presence of organic matter. The hydrolysis of the primary Andosols 49

minerals microcline and augite may serve to illustrate this type of weathering ('glass' being an amorphous mixture with similar chemical composition and reacting inth e sameway) :

+ 3+ KAlSijOg+ 2H 20 = K + Al + 3Si0 2+ 4 OH " microcline

2+ 2+ CaFeSi206+ 2H 20 = Ca + Fe + 2Si0 2+ 4 OH " augite

Under relatively acid conditions (pH <5), thishydrolysi s is fostered by protons from organic acids; th eproton s stem from carbonic acid ifth ep H is higher. Depending on the intensity of leaching, the liberated basic cations are toa great extentwashe d outwherea s the Al- and Fe-ions, and most prominently the former, are tiedu p in stable complexes withhumus . The ferrous iron isfirs toxidize d toth eferri c statean d isthe nfo rth e greater partprecipitate d as ferrihydrite:

Fe2+ = Fe3+ +e

3+ + Fe + 3 H20 - Fe(OH)3 + 3 H ferrihydrite

2+ + ( or: 2 Fe + \ 02 + 5 H20 = 2 Fe(OH)3 + 4 H )

TheA l inth ecomplexe s protects theorgani cpar tagains t bio-degradation (toxic to micro-organisms). The mobility of these complexes is rather limited because rapid weathering yields sufficient Al and Fe to produce complexeswit ha hig hmetal/organi c ratio thatar eonl y sparinglysoluble . This combination of low mobility and high resistance against biological attackpromote s theaccumulatio no forgani cmatte r inth etopsoil .B ycon ­ trast, a similar combination in Podzols leads generally to metal-un- saturated complexes which aremuc hmor emobile .

The fate of the liberated silica depends largely on theexten t towhic h Al is complexedb y thehumus . Ifmos t or allA l iscomplexed , the silica concentration of the soil solution will increase and while part of the silica iswashe d out,anothe rpar tprecipitate s asopalin e silica. Ifno t all Al is complexed, the remainder may co-precipitate with Si to form

Ferrihydrite is the approved new name for hydrous iron oxides of short- range-order previous-ly termed 'amorphous ferric oxide' or 'iron oxide gel'. Neitherit sstructur eno rit scompositio nha sye tbee nestablishe dbeyon ddoubt ; a good approximation isprobably : Fe20j.2Fe00H . 2.6H20. Ferrihydrite isth edominan t ironoxid eminera l ofmos tvolcani c soils and some of the properties ascribed to allophane may in part be due to ferrihydrite. Recent evidence suggests thatmuch , ifno tall ,o fth eorganicall yboun d Fe (as extractedb ypyrophosphate ) isferrihydrite-Fe . 50Andosol s

allophane ofvaryin g composition and often imogolite ,a swel l as,unde r certain conditions, halloysite. The formation of Al-humus complexes and the formation of allophane associations are mutually competitive. Both groups appeart ooccu r ininvers eproportions ;thi sconstitute s thebinar y composition ofAndosols . Itseem s thatallophan e and imogolite are stable undermildl y acidt oneutra l conditions (pH>5 )wherea sAl-humu s complexes are dominantunde r more acid conditions (pH <5). If there isan yA l still available under the latter conditions, thisma y combinewit h excess Sit o form2: 1an d2:1: 1typ ephyllosilicat ecla ymineral s (e.g.chlorite )a sar e often found in association with Al-humus complexes. Because of the acid conditions, thesesoil sma yhav eexchangeabl eA lwhic h isno tfoun do nal ­ lophane.Th e stability conferred onth eorgani cmatte rb yA l isn o lessi n thepresenc e of allophane.Apparently , the activity ofA l inallophan e is highenoug h tointerac twit horgani cmolecule s andpreven tbio-degradatio n and leaching.

The described competitionbetwee nhumu s and silica forA l is influenced by anumbe r offactor s: (1)Th eAl-humu s complex + opaline silica+ phyllosilicat e clay associa­ tion ismos tpronounce d inacidi c types ofvolcani c ash that are subject to strong leaching. Primary quartz,a typica l indicator of acidparen tmaterials , isofte npresen t inth eAl-humu s complex association. Inpractice ,ther e isa continuou s range inth ebinar y composition ofAndosol s from apur eAl-humu s complexes association ('non-allophanic')t oa pur e allophane/imogolite association('allo - phanic') inwhic h theextreme s are rare.Thi svariatio n occursbot h within oneprofil e andbetwee nprofiles . (2)I nth ever y early stage ofAndoso l formationunde rhumi d temperate conditions, (near-)completeAl-complexatio nb y organic matter may constrain the formation of allophane.Onl ywhe nhumu s accumulation levels off isaluminiu mbecomin g available forminera l formation. This explainswh y B-horizons inAndosol s areusuall ymuc h richer in allophane and imogolite thanA-horizons : theweatherin g ofprimar y minerals proceedsbu t the supply oforgani cmatte r is limited so that Al ishardl y used forcomplexation .

During theintensiv eweatherin gprocesses ,th epor e spaceo fth ealread y quiteporou s material isgreatl y increased, typically from some 50volum e percent tomor e than7 5percent ,becaus e of lossesb y leaching and strong stabilization of the remaining material by organic matter and weathering

Allophanes are non-crystalline hydrous aluminosilicates with Al/Si molar ratiostypicall y between1 an d2 (Al/S irati oo fkaolinit e is1 ). The yconsis t of hollow spherules with a diameter of 3.5 - 5 nm and have a very large (reactive)specifi c surface area.

Imogolite is a paracrystalline aluminosilicate consisting of smooth and curved threadswit h diametersvaryin g from 10t o3 0n man d several thousands of nm in length. The threads consists of finer tube units of 2n m outerdiameter ; their outer wall consists of a gibbsite (Al) sheet and their inner wall of a silica sheet.Th eAl/S i molar ratio is2 . Andosols 51 products (silica,allophane , imogolite, ferrihydrite), some ofwhic h have quite open structures themselves.Biologi c upgrading of thepor e space is of less importance inAndosols .

The genesis ofAndosol s is often complicated by repeated deposition of fresh ash. This may involve surficial rejuvenation of the soil material withthi nas hlayer sbu tals ocomplet eburia lo fth eprofile .A ne wprofil e will then develop inth e fresh depositwhil e soil formation inth eburie d A-horizon isaffecte db y the suddendecreas e inorgani cmatte r supply and a different composition of the soilmoisture . WhenAndosol s get older,th ecla ymineralog y changes,particularl y that of the subsoil,a sallophan e and imogolite are transformed tohalloysite , or gibbsite,dependin g on the silica concentration of the soil solution. In addition,A l from the humus complexes will gradually become availablean dferrihydrit e turnsint ogoethite .Evidently ,thi sproces si s strongly influencedb ysuc h factors asth erat eo frejuvenation ,th edept h and composition of the overburden, the composition of the remaining material and themoistur e regime. Eventually, anAndoso lma y grade intoa 'normal' soil, e.g. apodzoli c soil, ora soilwit h ferric properties,o r with clay illuviation.

CHARACTERISTICS OFANDOSOL S

Thetypica lAndoso lha sa nA Co rAB Cprofil ewit ha dar kAh-horizo nfro m 20t o5 0c mthic k (thinnero rthicke roccurs )ove ra brow nB -o rC-horizon . There isa clea r difference incolou rbetwee n the and the subsoil butcolour sar egenerall ydarke ri ncoo lregion stha ni ntropica l climates where net accumulation of organic matter is less. The organic matter content averages about 8percen t but may reach 30percen t in the darkest profiles. TheA-horizo n isver y porous,ver y friable, fluffy,non-plasti c and non-sticky, andha s a crumb or granular structure. In the field, the soilmateria lma yb e smearyan dfee lgreas yo runctuous ; itbecome s almost liquidwhe n rubbed ('thixotropy').As h layers are often clearlyvisible .

Hvdrological characteristics

Because of their high porosity and their occurrence in high terrain positions, most Andosols have excellent internal drainage. Gleyic soil properties occurwher e thegroundwate r ispresen t at shallowdepth .

Mineralopical characteristics

The quantities of volcanic glass, ferromagnesian minerals (olivine, pyroxenes,amphiboles) ,feldspar san dquart zi nth esil tan dsan dfraction s ofAndoso l material varies with the locality. Some of theminera l grains mayhav e a 'coating' ofvolcani cglass . The mineral composition of the clay of Andosols varies with the genetic age of the soil, thecompositio n of theparen tmaterial ,th epH , thebas e status,th emoistur e regime,th ethicknes so foverburde nas hdeposits ,an d theorgani c matter. The 'X-rayamorphou s materials' inth ecla y fraction ofAndosol s include allophane and, lesscommonly , imogolite,and/o rhumu s 52Andosol s complexes ofA l and Fe together with opaline silica. Allophane/imogolite and Al-humus complexes may occur together although the two groups have conflicting conditions of formation. Besides primary minerals, other components may occur such as ferrihydrite, (disordered) halloysite and kaolinite, gibbsite andvariou s 2:1 and 2:1:1 layer silicates and inter- grades.

Physical characteristics

Thehig h aggregate stability ofAndosol s and theirhig hpermeabilit y to water make that these soils are not very susceptible to water erosion. Exceptions to this rule are highly hydrated types of Andosols when they dry out strongly (e.g. upon deforestation). The difficulty to disperse Andosol material gives problems in texture analysis; caution should be takenwhe n interpreting suchdata . The bulk density of Andosol material is low, not just in the surface horizon; iti stypicall y lesstha n0. 9 Mg/m butvalue sa slo wa s0. 3Mg/ m havebee n recorded inhighl y hydrated Andosols.Th ebul k density doesno t change much over a suction range of 1500kP a (limited shrink and swell). Therefore, the bulk density in the field-moist condition can in practice be substituted for the bulk density at 33 kPa suction, which is a diagnostic criterion. Unless thesoi l isver yyoung ,th emoistur e contenta t 1500kP a suctioni s high. Yet, the quantity of 'available water' inAndosol s (difference in water content retained at 33an d 1500kP a suction) isgenerall y not lower and often higher than in other mineral soils. Air-drying of Andosol material may irreversibly lower the water holding capacity, the ion exchange capacity,volume , liquid limit andplasti c limit.Thi s effect is stronger with the more hydrated types.Air-dryin g may also cause loss in cohesion of theparticles ; the resulting dust isver y susceptible towin d erosion.

Chemical characteristics

Andosols are notorious for their highly variable exchange properties: thecharg ei sdependen to np Han delectrolyt econcentratio nand ,therefore , very similar to thatdescribe d for the Ferralsols (seechapte r onFerrai - sols).A differenc e istha tth enegativ e charge ofAndosol s canreac hmuc h highervalue s thantha to fFerralsol sbecaus e ofth ehig hcontent s ofsoi l organic matter and allophane. The CEC can reachvalue s of 100 cmol(+)/kg soil orhigher . The variation incharg e as a function of thep H is given for some Andosol components in Figure 2; montmorillonite, having a dominantlypermanen t charge, is included forcomparison .

Because of the variable charge properties, the base saturation values are alsovariable .Wit h the exceptiono fver y youngAndosol s and Andosols innon-humi d areas, thebas e saturation values are generally low because of strong leaching. If the clay assemblage is dominated by allophane (and imogolite), the pH of the soil isnormall y higher (pH>5 ) than if the clay consists largely of Al- and Fe-humus complexes together with phyllosilicates (pH <5).I n the latter case exchangeable Al is often present in quantities that are Andosols 53

toxic toplants .Th e strong affinity forphosphat e ions ('chemisorption') isa diagnosti c 'andic'property .

The strong chemical reactivity ofAndosol s has long been attributed to X-ray amorphous compounds of Al, Si and humus. It is more appropriate, however,t oascrib e thisAndoso l characteristic toth epresenc e of 'active aluminium' whichma y occur invariou s forms: 1. inshort-range-orde r orparacrystallin e aluminosilicates sucha s allophane and imogolite. 2. as interlayerAl-ion s in2: 1an d 2:1:1 layersilicates . 3. inAl-humu s complexes,an d 4. asexchangeabl e Al-ions on layersilicates .

The roleo factiv e Fema yofte nno tb e ignoredbu t isgenerall y considered of less importance than that ofactiv e Al.

-100 NH,

-80 -15 -60 -10 -40 -5 -20 5 6 7 CI pH 0 -1 1 r- 5 6 7 CI" pH

bu

+ Fig. 2.NH 4 and CI"retentio n curvesusin g 0.01 M NH4C1 (0.1M NH^C lfo r montmorillonite). (a)Montmorillonite ; (b)Halloysite ; (c)Allophan e 905 (Al:Si=2:l,containin g some imogolite); (d)Allophan e PA (Al:Si-l:l).Afte r Wada &Okamura ,1977 . 54Andosol s

MANAGEMENT ANDUS EO FANDOSOL S

Andosols have ahig h potential for agricultural production but many of themar eno tuse d tothei rcapacity .Thei rnatura l fertility ishigh ,par ­ ticularly ofsoil sdevelope d inintermediat e orbasi cvolcani c ashan dno t exposed to excessive leaching. A major problem is the strong phosphate fixationo fAndosols .Amelioratio n involvesmeasure s toreduc e thiseffec t (causedb yactiv eAl )throug happlicatio no flime ,silica ,organi cmateria l and 'phosphate' fertilizer. Because of the strongbuffering , raising the pH isdifficul t andexpensive . Andosols are easy to cultivate and have a good rootability and water availability. The most hydrated types,however , have only limited traf- ficability and soilma y stick to theploughshar e during tillage. Andosols areuse d for awid evariet y ofcrop s sucha s ,tobacco , sweet potato (little sensitive to phosphate fixation), tea,vegetables , wheat, orchard crops and forest (onstee p slopes). Paddy rice cultivation requires that drainage is impeded by high groundwater or a dense subsoil layer. Landforms insand s 55

MAJOR LANDFORMS IN REGIONS WITH SANDS

Sandy regions canb e divided intw obroa d categories: (1)residua l sands, formedupo nweatherin g ofold ,usuall y quartz-rich soilmateria l orrock , commonly under tropicalconditions . (2)shiftin g or only recently deposited sands, e.g. indesert s andbeac h lands. (Regionswit h on-going deposition of fluvial sandswil lb e discussed in thechapte r on lowland formation.)

LANDFORMS INRESIDUA L SANDS

Largeexpanse so fhorizonta lsandston eplateau s intropica l shieldarea s have adee pweatherin g mantle ofwhit e sands.Wel lknow n examples are the Precambrian Roraima sandstones on the Guiana Shield and the Voltaian sandstone inWester nAfrica .The yhav e incommo nwit h formations ofuncon ­ solidated fluvial sands, such as the Tertiary White Sands in Guyana and Suriname and their equivalents in eastern Peru, northeastern Brazil, or Liberia (westernAfrica) , thatthe yar eal lcompletel ywhite ,ver y richi n quartz,poo r inclay ,an d excessively drained. Most rivers on old sandstone plateaus and inwhit e sand areas have black watero fp H4. 0 orlower .Th edar kcolou rindicate stha tFe-humu scomplexe s areleache d from thesands ;thes e lose the ironcoating s aroundth egrain s andbecom e increasinglywhite .I nfact ,a nextremel y thickalbi c E-horizon is formed, with a spodic B-horizon either at several metres depth, or lacking altogether. However,becaus e of the geologic dimensions of these processes, the albic horizons are parent material for the formation of 'normal' soils:ARENOSOLS .

LANDFORMS INSHIFTIN G ORRECENTL Y DEPOSITED SANDS

Sandyparen tmaterial s areals oabundan t inarea swher e sandaccumulate s by the selective action ofwin d orwater . carry silt-sized particles overver y large distances (Sahara 'dust' settlesregularl y incentra l Europe); sand-sizedparticle s travelles sfa r (bysaltation )an dfin egrave l isonl y transportedb ycreep .Thi s explains why the purest sands,wit h a uniform particle size, are found in eolian deposits. Many such deposits show a characteristic large-scale cross bedding, indicative of sand deposition on the slip faces ofdunes . Fixed dunes are formed when transported sand settles in the lee of an obstacle such as abrus h or apiec e of rock. The obstacle thus grows in size andmor e sand settles:th edun e grows.Whe n thewin d drives the sand grains to the top of thedune ,it s carrying capacity decreases so thata n increasing part of the transported sand settlesbefor e reaching the crest of the dune. This steepens the angle of the slope,particularl y near the crest.Whe nth eslop eangl eexceed sth eangl eo fres to fth edeposite dsan d (typically 34°fo r dry particles), shearing takes place along a slightly 56 Landforms insand s

less steep plane.Thus , asli p face is formed on the lee-ward side of the duneb y aproces s ofoversteepenin g and shearing. Along coasts, the dunes are often 'parabolic dunes', shaped in part by landward migration of thedun estrips .

Free duneshav en o fixedpositio nbu tmigrat e downwindb y erosiono nth e gently inclined windward side and deposition on the leeward side (slip face) in the same way as described above. The smallest free dunes are ordinarywin d ripples andmeasur e only afe wcentimetre s inheight .Large r structures are indigenous todeser t areas and large coastal sandflats.

If thewind s arepredominantl y uni-directional , free dunes canassum e a crescent shape ('barchan dunes'). As the rate of advancement of the sand is roughly inversely proportional to theheigh t of the crest, the flanks of a shifting dune advance more rapidly than the central part until they become shelteredb y themai nmas s of thedune .Th e speed atwhic h barchan dunesmov evaries ;value smeasure d inth eMiddl e Eastrange dbetwee n5 an d 20 metres per year. Figure 1 summarizes the process of barchan dune formation.

wind

wind

Direction of wind flow \ \ • • -j*2j \---\...V..W,j

Fig. 1.Th e formation ofbarcha ndunes .Source :Bagnold ,1965 . Landforms insand s5 7

Coalescingbarchan sproduc etransvers e dunes,wherea slongitudina l dunes parallel toth ewin d direction ('seifs')ar eforme db yspirallin g 'screw­ driver'winds .Mor eo rles sstabl esta rdune s ('ghourds')for mwit hvaryin g wind directions.

The area ofdun e formation isdelimite d byth e15 0mm/y r isohyet. This boundary appears tohav e moved up and down in the recent past. Between 20,000 and13,00 0 BP,th esouther n limit ofactiv e dune formation inth e Sahara was 800k m south of itspresen t position andmos t of the present Sahelianzon ewa sa nactiv edun eare aa ttha ttime .Thes edunes ,mostl yo f the longitudinal type, are now fixed by vegetation, but their eolian parentage isstil l obvious.A similar story canb e told forth eKalahar i sands.Overgrazin gi nrecen ttime sha sreactivate d theeolia ntranspor to f these sands. The cover sands andparaboli c dunes ofth etemperat e climate zone formed under periglacial conditions during thesam e arid intervalbetwee n 20,000 and 13,000BP .Forest ,notabl y pine forest,re-establishe d itselfo nmos t of these sandsbu tovergrazin gb y sheep inmedieva l times ledt orenewe d (wind)erosion ,s otha tyoun ganthropogeni c duneswit hARENOSOL Sar efoun d inman y parts ofnorthwester n Europe.Se eFigur e2 .

Fig. 2.Cove r sands inEurope . Source:Koster ,1978 . 58 Notes

NOTES Arenosols 59

ARENOSOLS

Soilswhic har ecoarse r thansand y loam toa dept ho fa tleas t 100c m ofth esurface ,exclusiv eo fmaterial swhic hsho wfluvi c orandi c proper­ ties; having no diagnostic horizons other thana n ochric A-horizon or an albic E-horizon.

Key toArenoso l (AR)Soi lUnit s

Arenosols showing gleyic propertieswithi n 100c m of thesurface . GleyicArenosol s (ARg)

OtherArenosol s having analbi c E-horizon with aminimu m thickness of 50 cmwithi n 125 cm from thesurface . AlbicArenosol s (ARa)

Other Arenosols which are calcareous at least between 20 and 50 cm from the surface. Calcaric Arenosols (ARc)

OtherArenosol sshowin gsom ecla yincreas eo rlamella eo fcla yaccumulatio n within 125 cm of thesurface . LuvicArenosol s (AR1)

OtherArenosol s showing ferralic propertieswithi n12 5c mo fth esurface . Ferralic Arenosols (ARo)

OtherArenosol sshowin gcolourin go ralteratio ncharacteristi co fa cambi c B-horizon. Cambic Arenosols (ARb)

OtherArenosols . Haplic Arenosols (ARh)

Diagnostic horizon; seeAnne x 1fo r full definition. Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OFARENOSOL S

Connotation:weakl ydevelope dsoil so fcoars etexture ;fro mL .arena .sand .

Parentmaterial :mostl yunconsolidated , inplace scalcareous ,translocate d material of sand texture; in the wet tropics, relatively small areas of Arenosols occur onresidual , intensively leached sandstoneweathering .

Environment: fro m arid to (per)humidan d from extremely cold to extremely hot; landformsvar y fromrecen tdunes ,beac hridge san dsand yplain sunde r a scattered (mostlygrassy )vegetation , tover yol dplateau sunde ra ligh t forest. 60Arenosol s

Profile development:A(E) Cprofiles .I nth edr y zone,a nochri c A-horizon is the only diagnostic horizon. Arenosols in the perhumid tropics often havea thic kalbi cE-horizo nwherea smos tArenosol so fth ehumi d temperate zonesho wsign so falteratio no r transporto fhumus ,iro no rclay ,bu tto o weak tob e diagnostic at theMajo r SoilGroupin glevel .

Use:mos tArenosol s inth edr yzon ear euse dfo rlittl emor etha nextensiv e grazingbu t they couldb e used forarabl e cropping if irrigated. They are permeablesoil swit hgoo dworkabilit ybu tthei rlo worgani cmatte rcontent , low CEC, low water holding capacity and low coherence require adapted management. Arenosols in the temperate zone are moderately suitable for mixed arable cropping andgrazing ; supplemental (sprinkler)irrigatio n is needed duringdr yspells .Arenosol s inth eperhumi d tropicsar e chemically exhausted andhighl y sensitive to erosion.The y arebes t leftunde r their naturalvegetation .

REGIONAL DISTRIBUTION OFARENOSOL S

Arenosols cover some 400 million hectares, mainly in the dry zone, viz. in the southern Sahara, southwest Africa and western Australia. Severalmillio nhectare s ofhighl y leachedArenosol s arefoun d inth eper ­ humid tropics,notabl y inSout hAmeric a (Brazil,th eGuianas )an d inpart s of southeast Asia (e.g. Sumatra, Kalimantan, Sarawak, The Pilippines). Smallarea s of (young)Arenosol s occur inal lpart s ofth eworld ; theyar e not represented inFigur e 1.

Fig. 1.Arenosol sworldwide . Arenosols 61

GENESIS OF ARENOSOLS

Thehistor y of theArenosol s of the dry zone isdistinctl y different from thato fArenosol s inth ewe t tropics.Th eforme r showminima l profile developmentbecaus e soilformin gprocesse s area ta standstil ldurin glon g periodso fdrough tand/o rbecaus e theparen tmateria l iso fyoun g age.Th e latterforme di nyoun gsand ydeposit sor ,th eothe rextreme ,constitut eth e thickalbi cE-horizo n ofa Gian tPodzo lan drepresen t theultimat e insoi l formation.

Arenosols of the dry zone

In the dry zone, Arenosols are frequently associated with areas with areaso f(shifting )san ddunes .Evidently ,soi lformatio ni nsuc hdun esan d isminima l until thedun e iscolonize db y avegetatio n andhel d inplace . Then, somehumu s can accumulate inth e surface soil and ashallow , ochric A-horizon can develop. The individual sand grains of aridArenosol s have notseldo ma coatin go f (brownish)cla yand/o rcarbonate s orgypsu mwhile , in places, the desert sand is deep red by coatings of goethite (fer- rugination, areli c feature according to some). Where theparen t material is gravelly, sand isblow n out of the surface layer and the coarser con­ stituents remain behind at the soil surface as a 'desert pavement' of polished pebbles and stones.

Arenosols of the temperate zone

Arenosols in the temperate zone show signs ofmor e advanced soilforma ­ tion than those in arid regions. They occur predominantly in fluvio­ glacial, alluvial, lacustrine,marin e or eolian quartzitic sands of very young to Tertiary age. Calcareous materials are often deeply decalcified and beginning podzolization (see under Podzols)wit h accumulation ofFe - and Al-humus complexes in thin lamellae is not uncommon. A true spodic B-horizon isno t (yet)formed ,no teve ni nth epoores tArenosols .I nloam y sands that are relatively rich, biological homogenization counteracts cheluviation (see under Podzols) and deep,homogeneous ,brow n or reddish profilesdevelop .Thei rochri cA-horizo ncontain shumu so fth e 'moder'typ e thatconsist s for the greaterpar to fexcrements .Man yhav e an orange-red colour below the ochric A-horizon, attributable to thin (<10 m) iron coatings on the sand grains.

Arenosols inth ehumi d tropics

Arenosols in the humid tropics either are young soils in coarsely textured alluvial, lacustrine or eolian deposits, or they are very old soils in residual acid rock weathering which lost all primary minerals other than (coarse grained) quartz in the course of an impressive pedogenetic history. Theyoun gArenosol so fbeac hridge san dcoasta lplains, ar eazona lsoils ; theymerel yhav ea thi nbrow nochri cA-horizo nove ra dee psubsoi ltha tma y show gleyic properties below a depth of 50 cm and/or signs of beginning horizondifferentiatio n thatar e taxonomically insignificant. 62Arenosol s

.The old (Alble)Arenosol s constitute thedeep ,bleache d surface soilso f Giant Podzolswhic h extend downward toa dept h ofmor e than 125 cm. These zonal soils are the result of intense and prolonged dissociation of weatherable minerals and translocation of theweatherin g products through cheluviation. (The processes of podzolization, cheluviation and chil- luviationar ediscusse d inth echapte ro nPodzols. )Figur e 2show sa nare a withAlbl cArenosol s developed insandston eweatherin g inSarawak .

**<4 ß

Fig. 2.Albi c Arenosols in the deep eluvialhorizo n of a Giant in Sarawak. Photob y courtesy of ISRIC,Wageningen .

CHARACTERISTICS OFARENOSOL S

TheArenosol s of the arid zone have abeginnin g A-horizon with weak single grain or crumb structure over a massive C-horizon. Those of the temperate zone have better developed but still ochric A-horizons over a substratum which may have thin iron coatings throughout, or contain lamellae of illuviated humus, clay or iron compounds that are too thin, too fewo rcontai n too littlehumu s toqualif y asa diagnosti c B-horizon. Young tropicalArenosol sar emorphologicall yno tver ydifferen t from those of the temperate zone. Old tropical Arenosols under forest on residual quartzitic rock weathering have a dark brown O-horizon over a shallow greyish brown A-horizon which tops a deep, grey to white coarse sandy E-horizon. A shallow mini-Podzol may form in the A-horizon; it remains intactbecaus e of thevirtua l absence ofbiologica l activity. Arenosols 63

Mineralogical characteristics

With partiele size being the classification criterion and not particle composition,ther ei sconsiderabl emineralogica ldiversit yamon gArenosols . Those in temperate and humid tropical climates contain ahig h proportion of weathering resistant minerals, mainly quartz but also zircon, tour­ maline,rutile ,staurolite ,etc .Th eArenosol so fth edr yzon eca nhav ean y mineralogical composition; many are calcareous and/or gypsiferous. The parent material may contain some material finer than sand which, upon weathering, releases ironcompound s thatcolou r the soilmatri x orange or red.

Hvdrological characteristics

Arenosols are very permeable soils; their saturated hydraulic conduc­ tivity varies with the packing density of the sand and have any value between 300 and 30,000 cm/day. Arenosols store only between 5 and 12 percent 'available'water ; in arid regions they need irrigation for good productionbu twit h infiltrationrate srangin g from4 t o40 0cm/hour ,man y of them areunfi t for surface irrigation due tohig hpercolatio nlosses .

Physical characteristics

Due tothei r lowcoherence ,Arenosol sar esensitiv e tocompaction ;tota l pore fractions vary between some 0.35 and 0.55 cm/cm . A cemented or indurated layer may occur at some depth but tillage and/or rooting are normally not severelyhindere db yit .

Chemical characteristics

Arenosols inhumi d temperate or tropical regions are leached and often deeply decalcified soils with a low capacity to store bases. Their A-horizons areshallo wand/o rcontai nonl ya fe wpercen torgani cmatte ro r less. On poor quartzitic sands, the natural (forest)vegetatio n survives oncyclin gnutrient s and roots almost exclusively inth e0-horizo n and in a shallowA-horizon . Inth ericher ,loam yArenosol so fth etemperat ezone , rooting is deeper and nutrient cycling lessvita l to the vegetation. The Arenosols of the dry zone are mostly rich inbases . Because of the high permeability, the low water storage capacity and the low biological activity, there can be shallow decalcification despite an extremely low annualprecipitatio n sum.

MANAGEMENTAN DUS EO FARENOSOL S

Arenosols occuri nwidel ydifferen tenvironment s andvar y accordingly in genesis,compositio n andproperties .Th e only characteristic that all Arenosols have in common is their coarse texture, accountable for their generallyhig hpermeabilit y and lowwate r storage capacity. 64Arenosol s

Arenosols inari d lands,wher e theannua lrainfal l sum islowe r than30 0 mm, are predominantly used for extensive (nomadic) grazing. From 300 mm/year onwards, dry farming is possible. High yields of small grains, melons,pulse s and fodder cropshav ebee n realized on irrigated Arenosols but high percolation losses oftenmak e surface irrigation impracticable. The low coherence, low nutrient storage capacity and high sensitivity to erosion are further limitations ofArenosol s inth e dryzone . Arenosols in the (sub)humid temperate zone have similar limitations as thoseo f thedr yzon ealbei t thatdrough t isa les s seriousconstraint .I n some instances, e.g. in capital-intensive horticulture, the low water storage ofArenosol s isconsidere d advantageousbecaus e the soilswar m up early in the season. In (much more common) mixed farming systems with cereals, fodder crops and grassland, supplemental sprinkler irrigation is applied topreven t drought stress during dry spells.A large part of the Arenosols ofth etemperat ezon ear eunde r forest,eithe rproductio nfores t or 'natural'stand s incarefull ymanage d naturereserves . Arenosols inth ehumi d tropicsar ebes t leftunde r theirnatura lvegeta ­ tion, particularly so the deeply weathered Albic Arenosols. As nutrient elements are all concentrated in thebiomas s and in the top 20 cm of the soil, clearing of the land will inevitably produce infertile badlands withoutecologica lo reconomi cvalue .Unde rforest ,th elan dca nstil lpro ­ duce some timber (e.g.Agathis )an dwoo d for thepape r industry. Landforms inregion swit h smectites 65

MAJOR LANDFORMS IN SMECTITE REGIONS

Soil materials whose properties are dominated by an abundance of expanding 2:1 lattice clays can occur in a many landscape elements. Extensive areas are foundin : (1) (Former)sedimentar y lowlands,an d (2)Denudationa l plains

LANDFORMS IN (FORMER)SEDIMENTAR Y LOWLANDS

Sedimentary lowlands of 'heavy' smectitic clays covervas t tracts along the southernborde r of the Sahara,i nth e Sahelianzone .Thes e landswer e lakes and floodplains between 12,000 and 8,000 BP,whe n the climate was more humid than at present. The Saharian lakes, notably Lake Chad, the inland delta of the Niger and the alluvial plains of the Nile (in the presentSudan )expande di ntha tperiod .Th eleve lo fLak eCha dwa si ntime s 40 m higher than today and the lake had the size of the present Caspian Sea.Mos t rivers in the Sahelian zone,eve n those which are intermittent in our times, flowed continuously in meandering channels and deposited finely textured sediments. Much ofwha t isknow n about the former expansion of the Saharian lakes was revealed by palynological studies of diatoms and pollen in the lake sediments.Apparently , a savannahvegetatio n colonized largepart s ofth e Sahara. Rock drawings in the area suggest that ostriches, rhinoceroses, crocodiles andgiraffe s lived therea tth etime .Th emor ehumi dclimat ei s attributed to southward penetration of polar air masses when subtropical high-pressure cells were weaker than at present. Later in the Holocene, notablyafte r500 0BP ,th eclimat ebecam edrie ragain ;lak elevel ssubside d and rivers became intermittent. Under this regime of alternating dry and wet spells,VERTISOL S could form inth ealluvia ldeposits .

LANDFORMS INDENUDATIONA L PLAINS

Denudational plains of smectitic clays occur inth e same, semi-desertic climate zone,bu t are restricted to areaswher e theparen t rock was rich enough inCa ,Mg ,an dN afo rsmectite s toform .Thes ear eessentiall ybasi c volcanic rocks,suc ha sth eDecca nTrap sbasalt s inIndia ,an dbasi cbase ­ ment rocks,suc h asamphibolite s and greenschists. VERTISOL formation is especially plausible where shallow groundwater carried these ions insolutio n andneoformatio n of smectites couldoccur . 66 Notes

NOTES Vertisols 67

VERTISOLS

Soils having, after the upper 20 cm have been mixed, 30 percent or more clay inal lhorizon s to adept h of at least 50cm ;developin g cracks from the soil surface downwardwhic h at someperio d inmos tyear s (unless the soil is irrigated)ar e at least 1c mwid e to adept h of 50cm ;havin g intersecting slickensides or wedge-shaped or parallellepiped structural aggregates at some depthbetwee n 25 and 100 cm from the surface,wit h or withoutgilgai* .

Key toVertiso l (VR)Soi lUnit s

Vertisolshavin g agypsi c horizonwithi n 125 cm of thesurface . Gypsic Vertisols (VRj)

OtherVertisol shavin g acalci c horizono rconcentration s ofsof tpowder y lime within 125 cm of the surface. Calcic Vertisols (VRc)

OtherVertisol s having abas e saturation (byI MNH.OA c atp H 7.0) of less than 50percen t at leastbetwee n 20an d 50c m from thesurface . Dystric Vertisols (VRd)

OtherVertisols . Eutric Vertisols (VRe)

Diagnostic horizon; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r full definition Phase; seeAnne x 3fo r fulldefinition .

SUMMARYDESCRIPTIO N OFVERTISOL S

Connotation: churning heavy clay soils;fro mL .vertere . toturn .

Parent material: sediments that are finely textured and contain a high proportiono fsmectit eclay ,o rproduct so froc kweatherin gtha thav ethes e characteristics.

Environment:Depression san dleve lt oundulatin gareas ,mainl yi ntropical , semi-arid to (sub)humidan dmediterranea n climateswit h analternatio no f distinct wet and dry seasons.Th e climax vegetation is savannah, natural grassland and/orwoodland .

Profile development: A(B)C-profiles. Alternate swelling and shrinking of theexpandin gcla yproduce sdee pcrack sdurin gth edr yseason ,an dslicken ­ sidesan dwedge-shape d structuralelement s inth esubsurfac e soil. Inman y areas a 'gilgai'microrelie foccurs . 68Vertisol s

Use: Fine texture and poor internal drainage account for the often poor workability of Vertisols, both in wet and dry conditions. Vertisols are productive soils ifproperl y managed.

REGIONALDISTRIBUTIO N OFVERTISOL S

Vertisols covera tota lo f31 1millio nhectare s or2. 4percen t ofth e globallan darea .A nestimate d 150millio nhectare si spotentia lcro pland . Intropica l areas,Vertisol s cover some 200millio nhectare s or4 percent of the land surface. A quarter of this is considered to be useful land. Most Vertisols occur in the semi-arid tropics, with an average annual rainfallbetwee n 500an d100 0mm .Larg eVertiso larea s occur inth eSudan , inIndi a and inAustrali a (Figure 1). Smalle r areas ofVertisol s exist in wetter,drie ran dcoole rclimates .Th eannua lrainfal lma yb ea slo wa s15 0 mm (e.g.Sudan )o r ashig h as 3000m m (e.g.Trinidad) .

Fig. 1.Vertisol sworldwide .

GENESIS OFVERTISOL S

The formation ofa verti c structure

The genesis ofVertisol s centers around the formationo f characteristic structural aggregates ('vertic structure') that occur throughout most of the solum; the aggregates change only gradually with depth in grade of development and size ofpeds . The formation ofa verti c structurewil lb e explained assuming a level plain with smectitic clayey sediments and a semi-arid tropical climatewit h adistinc t rainy season: Vertisols 69

At the end of the rainy season, the clay plain is flooded but most of theexces swate rwil leventuall yevaporate .Whe nth esaturate dsurfac esoi l starts to dry out, there is initially one-dimensional shrinkage, and the soilsurfac esubside swithou tcracking .Upo nfurthe rdrying ,th esoi llose s itsplasticity ; tension in the soil material increases until its tensile strength is locally exceeded and the soil cracks. Cracks are formed ina pattern that becomes finer as desiccation proceeds. In most Vertisols, complete drying out of the surface soil turns it into a 'surface mulch' with a granular or crumb structure.Vertisol s are 'self-mulching' ifthe y develop asurfac emulch .Granule so rcrumb s ofth emulc hhorizo nfal lint o the cracks partly filling themu p (Figure2) .

0- i mulch subangulart o angular blocky

vertic structure

80 J depth(cm )

Fig. 2.Cracks , surfacemulc h and soil structure ina Vertiso l during thedr yseason .

Upon rewetting, part of the space that the soil requires for its in­ creasedvolum ei soccupie db ymulc hmaterial .Continue dwate ruptak ecause s pressures that result in shearing: the sliding of soilmasse s along each other.Shearin goccur sa ssoo na sth e 'shearstress 'tha tact supo na give n volume ofsoi lexceed s its 'shearstrength' .Th eswellin gpressure ,actin g inal ldirections ,i sresolve db ymas smovemen talon gobliqu eplane sa ta n angle of 20 to3 0degree swit h thehorizonta lplan e (Figure 3).Th e shear planesar eknow na sslickensides ,polishe dsurface stha tar egroove di nth e direction of shear. Intersecting shearplane s definewedge-shape d angular blockypeds .Th epe dsurface s areslickenside s orpart s thereof,sometime s knowna spressur e faces.Althoug h thestructur econform s toth edefinitio n of angularblocky , the specific shape of theped sha s prompted authors to coinspecia lname ssuc ha s'lentils' ,'wedge-shape dpeds' , 'tiltedwedges ', 'parallelepipeds'an d 'bicuneatepeds' ;th etyp eo fstructur e issometime s 70Vertisol s called 'lenticular' or 'bicuneate' but shall be referred to as 'averti c structure' inth e following.

Thesiz eo fth eped s increaseswit hdepth .I nunifor m soilmateria lthi s isattributabl e to: (1)th emoistur e gradient during drying andwetting . This gradient is steepestnea r the surfacewher e small aggregates are formed ina loosepackin g ('mulch'). Themoistur e gradient decreaseswit h depth except around crackswher ewettin g and drying aremuc hmor e rapid than inth e interior ofcrack-bounde d soilprisms . (2)th e increasing overburden, i.e. the increasing load of the overlying soil.A t greater depths,highe r swellingpressure s areneede d to exceed the soil's shear strength; only a largevolum e of swelling soil cangenerat e suchpressure s and,consequently , structural aggregateswil lb e larger.

ERTICAL CRACK

~ INFILLED 8Y MULCH

SOIL MASS

Fig. 3.Idealize d stress diagram. Soil at three-dimensional expansion stage (source:D eVo s &Virgo , 1969).

The typical vertic structure (see Figure 4) extends from some 15 or 20 cmbelo w the surfacemulc h down to the transition ofsolu m to substratum, just below the depth of cracking. There are no seasonal moisture changes in the substratum; if itha s averti c structure, then that structure is .Vertisol s with aver y deep,fossi lverti c structure are common in situations where sedimentation has alternated with periods of geogenetic stand-still.

The sliding of crumb surface soil into the cracks, and the resultant shearinghav e some important consequences: (1)Subsurfac e soil ispushe dupward swhil e surface soil falls into cracks. Inthi swa y surface soil and subsurface soil aremixed , a process knowna s churning or (mechanical)pedoturbation . Churningha s longbee n considered anessentia l item inVertiso l formation; this is also suggestedb y thenam e (theLati n 'vertere'mean s 'to turn'). However,morphologica l studies and radio-carbon dating inrecen t Vertlsols 71

years showed thatman yVertisol s dono t exhibit stronghomogeniza - tion. Insuc hVertisol s shearing isno tnecessaril y absentbu t itma y be limited toup-and-dow n sliding of soilbodie s along shearplanes . (2)I nchurnin gVertisols ,coars e fragments like quartz gravel andhar d rounded carbonatic nodules are concentrated at thesurface , leaving the solum gravel-free.Th e coarse fragments arepushe d upwards with the swelling soil,bu tmos t of thedesiccatio n fissures that develop inth e dry season are toonarro w to let them fallback . (3)Aggregate s of softpowder y lime indicate absence ofchurning ,unles s such aggregates arever y small and form rapidly. Softpowder y lime is a substratum feature inVertisols .

Fig. 4.Th e typicalverti c structure ofVertisols .

Not all Vertisols develop a surface mulch. Some develop ahar d surface crust. Cracks in such soils are sharp-edged, remain open throughout the dry season,an d little surface soil falls into them. Because ofdifferen ­ tialwettin gbetwee nadjoinin gpart so fsoil , swellingpressure s canstil l develop,an dsuc hsoil sd ohav e averti c structurebu t thegrad eo fstruc ­ ture isweake r than inself-mulchin gVertisols .

CrustyVertisol sar ebu ton eexampl eo fth evariatio ni nstructur eforma ­ tion among Vertisols. Factors thathav e abearin g on structure formation usuallyac tthroug hthei reffect so nth etensil ean dshea rstrength so fth e 72Vertisol s

soil material: fine peds or,alternately , cracks at close intervals, are generally formed in soil materials that have low tensile and shear strengths under moist conditions,wherea s large peds and cracks at wider intervals are formed in soil materials with high tensile and shear strengths.Vertisol s thathav e ahig h ESPhav e greater tensile and shear strengths than soils with lower sodium saturation and commonly have a surface crust rather than a mulch. If the ESP is low and there is much finely divided lime, surface mulching is at amaximu m and peds arefine. . Vertic structure forming processes become stronger with increasing clay content and with a higher proportion of swelling clay minerals. Sandy Vertisols have limited swell and shrink; they develop narrow cracks anda surfacecrust .

The formation ofa gilgai surface topography

A typical self-mulching Vertisol has an uneven surface topography: the edges of crack-bounded soil prisms crumble down,wherea s the centres are pushed upward. The scale of this surface irregularity is that of the crackingpattern ,usuall ya fe wdecimeters .Microrelie fo na large rscale , superimposedo nthi sunevenness ,i sknow na s 'gilgai'an dconsist so nleve l terrain of small mounds surrounded by a continuous network of small depressions, or depressions surrounded by a continuous network of narrow ridges.

Erosion Erosion r * A r"""^ *i i /\ i\ / \ / \ S V_ —vC" II ^-^l

desiccationcrac k partlyfille dwit h shearstres s » surfacesoi l

.- -" ' directiono f massmovemen t

Fig. 5. Sketch showing thekinematic s ofmas smovemen t inVertisol s that result ingilga imicrorelie f (after Beinroth, 1965).

Several hypotheses have been put forward to explain the gilgai micro- relief. These have incommo n that they relate gilgai tomas s movement in swell/shrink soils. The soil must have sufficient cohesion in order to transfer pressures all the way to the soil surface. Gilgai is sometimes explained by sloughing of surface mulch into cracks and an upward thrust of soil between cracks as a result of subsurface soil swelling. However, Vertisols 73

gilgai is a feature that is superimposed over the cracking pattern; it originates inth e subsurface soilan d substratum.

There are two observations to support a subsurface origin of the gilgai microrelief: (1)A trench profile through acomplet e 'wave'o fmoun d and depression shows the slickensides inth e lower solum anduppe r substratum tob e continuous frombelo w the centre of the depression toward ahighe r positionbelo w thecentr e of themound . The oblique shearplane s show a preferential direction. Substratum material ispushe d upwards alongside such setso fparalle l slickensides. See Figure 5. (2)A gilgaied land surface that islevelle dwil lhav e gilgai reappearing ina fewyears .

The commonest form ofgilga i is thenorma l orroun d gilgai.O n slightly sloping terrain (0.5 to 2 percent slope) wavy or linear gilgai occurs; lattice gilgai is a transitional form onver y slight slopes.Wav y gilgai consists ofparalle lmicroridge san dmicrovalley s thatru nwit h theslope , i.e. at right angles to thecontours .

In most gilgais, the wave length (from centre of mound to centre of depression) isbetwee n 2an d 8m ; thevertica l interval or 'amplitudo'i s oftenbetwee n1 5an d5 0cm .Figur e 6present s somecommo nform so fgilgai .

n

ft

normal or round gilgai lattice gilgai linear or wavy gilgai

//// mound or micro-ridge 1 direction of slope

Fig. 6. The commonest forms ofgilgai . 74Vertisol s

Most gilgaied areas have Vertisols, but not all Vertisols develop a gilgaimicrorelief . Somearea swit hgilgaie dVertisol shav ebee nmappe da s a 'gilgai phase' on the 1:5,000,000 Soil Map of theWorld . In the Sudan, where Vertisols occur in a more or less continuous clay plain over a distance ofsom e 700k m fromnort h tosouth ,an dwher e theannua l rainfall increases intha tdirectio n from15 0t o100 0mm , thegilga imicrorelie f is restricted to the 500-1000 mm rainfall zone.Th e gilgaied Vertisols have a thinneran dles sclearl yexpresse d surfacemulc han dar eles s calcareous than thenon-gilgaie dVertisol s inth enorth .

The profile morphology of gilgaiedVertisol s isdifferen tbetwee nmoun d anddepression .O nth emounds, th eA-horizo ni sthi nwherea sth edepressio n profile has a deeper (thickened) and usually darker A-horizon. Coarse componentso fsubstratu mmateria lwhic hreac hth esoi lsurfac ea tth emoun d site such as quartz gravel and carbonatic concretions, remain at the surfacewherea s finer soilmateria l iswashe d down to thedepressions .I n Australia, 'highgilgais 'occu rwit hwavelength su pt o12 0m an damplitude s of up to 240 cm. These high gilgais may well have formed in an entirely differentway .

The formation of the smectite clayparen tmateria l

The environmental conditions that are instrumental in the formation of averti c soil structure alsopromot e the formationo f suitable parentma ­ terials.Rainfal lmus tb e sufficient toenabl eweatherin gbu tno t sohig h that leaching ofbase s occurs.Dr y periods are required for the crystal­ lization of clay minerals that form upon rock or sediment weathering. Impeded drainage hinders leaching and curbs the loss of weathering products. High temperatures,finally ,promot eweatherin g processes.Unde r suchcondition ssmectite sca nb eforme d inth epresenc eo fsilic aan dbasi c cations -especiall y Caan dM g -an d ata p H aboveneutral . Thelarges tVertiso larea sar efoun do nsediment s thathav ea hig h content of smectite clays orproduc e such claysupo npost-depositiona lweatherin g (e.g.Sudan ,describe db yBuursink ,1971) ,an do nextensiv ebasal tplateau s (e.g. India).

The formation ofVertiso l parentmaterial s andVertiso l profiles canb e illustratedb y examining 'red-black' soil catenas,a s abundant inAfrica , the Indian subcontinent andAustralia .Usuall y there arere d soils (Luvi- sols)o n crest and upper slope positions, shallow or moderately deep red soils (Leptosolsan dCambisols )o nsteepe rsection so fth eslope ,an dblac k soils (Vertisols) on lower-lying sites. Depending on parent rock and environmental conditions,Vertisol scove ronl yvalle ybotto mposition sor , in addition to the valley bottoms, also contiguous lower footslopes, or even, asresidua l soils, (gently)slopin ghillsides . In the semi-arid to subhumid tropics, smectite is the first secondary mineral that forms upon rockweatherin g (Kantor & Schwertmann, 1974). It retains most of the ions thathav ebee n liberated from the primary sili­ cates, like Ca and Mg. Iron, contained as Fe2+ in primary minerals, is preserved inth esmectit e crystal lattice asF e .A sweatherin gproceeds , smectites become unstable.Th e clay minerals decompose, and the released basiccation san dpar to fth esilic aar eremove db yleaching .F e -compounds Vertisols 75

remain inth e soil, lending ita reddis h colour; aluminium is retained in kaolinite and Al-oxides. The leached soil components accumulate at the lower terrain positions where they precipitate and form smectites. Smectites in these poorly drained positions are stable as long as the pH isabov eneutral . There are some additional reasons why there is a relative dominance of smectite inth e lowermember s of thecatena : -latera l (surficial and sub-surficial)physica l transport ofclay ,par ­ ticularly of fine clay inwhic h theproportio n of smectites isgreate r than inth e coarse clay,an d -decreasin g drainage and leaching of soluble compounds fromhig h tolo w terrainpositions . Internal drainage is impededb y the formation of smectites. (It is increasedwhe nkaolinit e forms: ferric iron, released from the smectite lattice,cement s soilparticle s to stable structural peds andmaintain s apermanen t system ofpore s inth e soil.)

Theprocesse s of rockweathering ,breakdow n ofprimar y and formation of secondaryminerals ,an dtranspor to fsoi lcomponent sresul ti nth ecatenar y differentiation mentioned above: reddish well-drained soils on higher positions, andblack ,poorl y drained soils indepression s (seeTabl e1) . Minor colour differences between Vertisols are often an indication of differences indrainag estatus .Th emor e reddishhu eo rstronge rchrom ao f relatively better-drained Vertisols is due to a higher content of free iron-oxides.Poorl ydraine dVertisol sar elo wi nkaolinite ;th ehu ei sles s reddish and the chroma weaker. Formerly, colour chroma was used as a differentiatingcharacteristi cbetwee nVertisol s (SoilTaxonom ystil ldoe s so)bu tthi spractic ewa slef ti nth eFAO/Unesc oclassificatio nsyste mwhe n it became clear that pale-coloured, apparently poorly drained Vertisols whichwer e thoughtt ob e typicalo fleve land/o rdepresse dareas ,occurre d alsoo ngentl ean deve nmoderat eslopes .Th esimpl elandscape-soi lrelatio n could thenn o longerb ehel dupright .

TABLE1 Analytical data of thehighes t (Luvisol)an d the lowestmembe r (Vertisol) ina 'red-black' soil catena inth eSudan .

PROFILE ABC DEPTH CLAY pH CEC CECclay BS Si02/Al203 orgC lime (cm) (%) (cmol(+ )Ag) (%) (clay fr.) (%) (%)

LUVISOL A 0-10 4 6.6 nd nd nd 3.8 0.5 0 AB 10-30 15 6.1 9.0 60 71 3.3 0.6 0 Btl 30-60 23 4.7 15.5 68 49 3.5 0.3 0 Bt2 60-85 33 4.5 21.4 66 45 3.3 0.3 0 Bt3 85-105 43 4.5 22.6 50 51 3.1 0.2 0 BC 105-135 39 4.4 27.4 69 47 3.0 0.1 0 C 135-160 39 4.7 27.4 71 57 3.2 tr 0

VERTISOL A 0-30 78 6.6 66.6 86 100 4.3 0.9 0.7 Bwl 30-90 78 7.2 78.4 100 100 4.6 0.9 1.2 Bw2 90-150 81 7.3 78.2 96 100 4.6 0.7 1.2 BCwk 150-180 79 7.3 80.2 103 100 4.5 0.4 1.5 76Vertisol s

CHARACTERISTICS OFVERTISOL S

Vertisols are commonly seena sAC-profiles , theA-horizo n consisting ofbot h the surfacemulc h (orcrust )an d theunderlyin g structure profile that changes only gradually with depth. Surely, the subsurface soil with its distinct vertic structure conforms to the definition of a cambic B-horizon,bu twher e does theA-horizo n end and theB-horizo n begin? Soil colour, texture, element composition, cation exchange capacity, etc are allunifor m throughout the solum. There ishardl y anymovemen t of soluble orcolloida lsoi lcomponents .(I fsuc htranspor toccurs ,i tI scounteracte d by thepedoturbatio n process.) There is,nevertheless , a growing tendency to describe Vertisols asABC - profiles, and todefin e asa (cambic)B-horizo n thatprofil e section that has a vertic structure. The A-horizon is then considered limited to the surfacemulc h (orcrust )an da nunderlyin g subangularblock yzon eo fa fe w decimeters thickness.A calcichorizo n or aconcentratio n of softpowder y lime isofte npresen t ino rbelo w thehorizon(s )wit h averti c structure, whereasgypsu mma yhav eaccumulate d inplaces, eithe runiforml ydistribute d over thematri x or innest s of gypsumcrystals .

Hydrological characteristics

Dry (cracked)Vertisol swit ha surfac emulc ho ra fin e tilthhav ea hig h initial infiltration rate. However, once the surface soil is thoroughly wettedan dth esoi lha sswolle nan dth ecrack sclosed ,furthe r infiltration isalmos tnil .Th ever yproces so fswell/shrin k impliesa discontinuou san d non-permanent pore system. If, at this stage, the rains continue (or irrigation is prolonged), flooding occurs easily because infiltration of water ina mois tVertiso l isextremel y slow.Vertisol swit ha considerabl e shrink/swell capacity, but maintaining a relatively fine class of struc­ ture, have the highest infiltration rates. Not only the cracks transmit waterfro mth e (first)rain sbu tals oth epor espac etha tdevelope dbetwee n slickensided ped surfaces as theped s shrunk.

Data on thewaterholdin g capacity ofVertisol s differ widely which may be due to the complex pore space dynamics.Wate r is retained both at the clay surfaces andbetwee n the crystal lattice layers.A large proportion ofal lwater , andnotabl y thewate rhel dbetwee n thebasi c crystalunits , is generally unavailable to plants. By and large,however ,Vertisol s are considered soilswit h arelativel y goodwate rholdin g capacity. Investigations inth eSuda nGezir a (Farbrother,1972 )showe dtha twhe nth e clay plain was flooded for several days or even several weeks, the soil moisture content midway between large cracks had hardly changed; it decreased gradually from more than 50 percent in the upper 20 cm to 30 percent at 50c mdepth . Deeper than 100cm , the soilmoistur e contentwa s almost invariate (at about 20percent , corresponding to amatri c suction of some 1500kPa )throughou t theyear . Vertisols 77

Physical characteristics

Vertisols that are subject to strong pedoturbation are uniform in particle size throughout the solum. An abrupt change may occur when the substratum is reached. When dry,Vertisol s have aver y hard consistence; they arever y plastic and sticky whenwe t and friable only over arathe r narrowmoistur erange .Thei rphysica l conditions aregreatl y influencedb y high levels of soluble salts and/or adsorbed sodium.

Chemical characteristics

Most Vertisols have a high cation exchange capacity and a high base saturation.Th e soil reactionvarie s fromweakl y acid toweakl y alkaline; pH-values are in the range 6.0 to 8.0. Higher pH's (8.0-9.5) occur in Vertisols with ahig h ESP. The cation exchange capacity isver y high, in the range of 30 to 80 cmol(+)/kg of dry soil. The CEC of the clay is of theorde r of 50 to 100cmol(+)/k g clay. Thebas e saturationpercentag e is high, usually above 50 and often close to 100 percent with Ca and Mg occupyingmor etha n9 0percen to fth eexchang e sites;th eCa/Mg-rati o lies between 3an d 1.

Saline, sodic and saline/sodic phases occur in the more arid parts of the Vertisol coverage. Sodicity occurs incidentally in higher-rainfall areas, e.g. indepression s withoutoutlet . The effect of sodicity on thephysica l properties ofVertisol s is stilla subject of debate. As stated earlier, Na-clays have greater tensile and shear strengths thanCa-clays ,an da hig h ESPproduce s asoi lstructur e of arelativel ycoars eclass .I nVertisol stha tar ebot hsalin ean dsodic ,th e effect thata hig h ESPha s on thediffus e double layer iscommonl y offset by the high ionic strength of the soil solution. Clay dispersion accom­ paniedb y claymovement , thenorma l consequence ofhig h sodium saturation incla ysoils ,canno ttak eplac e insoil swit hsuc ha lo whydrauli cconduc ­ tivity and such a low volume of soil that ever becomes saturated with water. In India, ESP's above 7 were found to have an effect on the hydraulic conductivity of Vertisols; an ESP value of 16 is considered optimal for irrigated cotton cultivation on saline/sodic Vertisols inth e SudanGezir a (possibly because a relatively high ESP increases the water holding capacity ofVertisols) .

Salinity inVertisol s may be inherited from the parent material orma y be caused by irrigation. Leaching of excess salt is hardly possible; it is,however ,possibl e toflus hsalt stha thav eaccumulate do nth ewall so f cracks. There are strong indications that the fallowyea r observed inth e rotations ofth eGezira/Manaqui lirrigatio nschem e inth eSudan ,i sindis - pensible formaintainin g a low salinity level inth e surface soil.

MANAGEMENTAN D USEO FVERTISOL S

Largearea so fVertisol s inth esemi-ari d tropicsar estil lunuse do r usedonl y forroug hgrazing ,woo dchopping ,charcoa lburnin g andth elike . Areas that are inus e for rainfed and irrigated agriculture are evidence 78Vertisol s thatthes esoil sfor ma grea tagricultura lpotentia lbu tspecia lmanagemen t practices arerequire d tosecur e sustainedproduction .Problem sar e inth e areas ofphysica l soil characteristics andwate r management.Asset s area ratherhig hchemica l fertility,an d thelocatio no fVertisol s inextensiv e levelplains, wher e reclamation andmechanica l cultivation are relatively easy. Vertisol plains lend themselves to large-scale mechanised forms of agriculture and are less suited to low-technology farming on account of theirpoo rworkability . takesplac e even inslightl y sloping plains and land slides occur where awe t and plastic surface soil slides over acoheren t subsurface soil. Cultivation of annual crops on landwit h a slope ofmor e than 5degree smus t therefore be discouraged.

Examples from the Sudan and Indiama y illustrate the potential and the problems encountered infarmin gpursuit s onVertisols :

The 740,000 hectares Gezira/Manaqil scheme in the Sudan, gravity-irri­ gatedwit hBlu eNil ewate ro fexcellen tquality , isth ebest-know n example oflarge-scal e irrigatedfarmin go nVertisols .Cotto ni sth eprincipa lcas h crop; other crops grown arewheat ,groundnut s and sorghum. Inadditio n to irrigated farming,extensiv e forms ofrainfe d cropping are practiced in large Vertisol plains; sorghum is the main crop. This low- inputagricultur e isincreasingl y confrontedwit hsoi ldeterioration .Wit h rainfed cropping spreading out onto slightly sloping clay plains, soil erosionha sbecom e amajo r problem.

InIndia ,ver yslightl yslopin gVertiso lplain scove rth ebasalti cDecca n plateau. The Vertisols are partly in residual materials and partly in colluvium, and they are also found in alluvial plains. In places, the rainfall sum is low and the pattern erratic. Management practices have thereforebee ndevelope d thatserv ethre epurpose s:increase d infiltration ofwater ,reduce d erosionhazard , and optimum use of theavailabl ewater . Some of themeasure s applied at the Soil and Water Conservation Research Centrea tBellary ,Karnatak aState ,ar ea goo dillustratio n (RamaMoha nRa o & Seshachalam, 1976): Contour cultivation and contour bunding improve infiltration and make it possible to make better use of the available rain water. These measures also reduce the slope length but there is a danger of water stagnating against bunds and breaking through. This danger is less when bunds are graded. Graded bunds follow 'graded contours' which have a very slight slope. Channels are dug on the upslope side of the bund and these drain towardsgrasse dwaterways .Th ewaterway sen di nfar mpond swher eth eexces s water is stored for supplemental irrigation of rainfed crops. Broad-base terraces, consisting ofa shallow channel anda broad-bas e low ridge,ar e also laid out on graded contours. The terraces drain towards grassed waterways. Verticalmulchin g isa metho dwhereb ysorghu m stubble isplace dverticall y intrenches ,3 0c m deep and 15c mwide ,wit h the stubbleprotrudin g 10c m above the soil surface.Trenche s are4 to 5m apart; they are laid out on the contours. Sorghum yields reportedly increased up to 50 percent by verticalmulching . Vertisols 79

The broad bed and furrow system is another example how soil and water conservation can be achieved through improved infiltration and re-use of excessrai nwater .I ti ssimila r toth ebroad-bas e system outlined above but the difference is that two crops are grown, one in the rainy season and one immediately after the rainy season (or a combination of a short-season and a long-season crop). Traditionally, only one crop is grown, viz. directly after the rainy season. Double-cropping became possible after adetaile d analysis of therainfal l regime overman yyear s permitted optimum timingo fmanagemen t operations.Th esyste m isdescribe d byKanwa re t al. (1982). 80 Notes

MOTES MINERAL SOILS CONDITIONED BY THE TOPOGRAPHY/PHYSIOGRAPHY: FLUVT SOLS GLEYSOLS LEPTOSOLS REGOSOLS

Landforms in lowlands 83

MAJOR LANDFORMS IN ALLUVIAL LOWLANDS

Inth epresen tcontext ,th eter m 'lowlands'connote s flatan dleve lwet ­ lands,normall ysituate da to rnea rse aleve lan dconsistin go fPleistocen e and/or Holocene sediments that are too young to be strongly weathered. Sedimentary lowlands are prominent in fluvial, lacustrine, marine and glacial landscapes, and particularly in areas of crustal subsidence. Climate played an important role inth e shaping ofmos t lowlands,nex t to tectonic processes. Climate determines the sediment load and bedform patterns in fluvial environments; the climate determines whether a sediment-trapping mangrove vegetation develops along a coast, and the climate influences theweatherin gan dripenin go fth edeposite dsediments .

Climate changes in the Quaternary have greatly influenced today's low­ lands.Mos t coastal lowlandswer e shaped to theirpresen t form and extent after the melting of Pleistocene ice caps stopped some 6,000year s ago, and some 15percen t of the present land surface was influenced by Pleis­ tocene glaciers thathav e now disappeared. It is impossible to understand thecomplexit y ofth elandform si nlowland swithou treferrin gt obot hthei r present climate and their climatichitetory .

Themajo r landforms inalluvia l lowlandswil lb ediscusse d fortw obroa d categories oflowlands : (1) (inland)fluvia l lowlands,an d (2) (coastal)marin e lowlands.

LANDFORMS IN (INLAND)FLUVIA L LOWLANDS

River systems are complex systems and a watershed area will normally harbour a variety of landforms. These correspond with the nature of the river and with the position in the river system. In most systems, three zones canb e distinguished: (1)th euppe r parts.wher e erosion outweighs sediment deposition. These stretches arenormall ymountainou s orhill ybu tbecom e gradually levelledb y erosion.Thes epart swil lno tb e discussedhere ,a s they areusuall yno tlowlands . (2)th emiddl e stretch, inwhic h erosion anddepositio n roughlycompen ­ sate each other.Thi s zone is largely azon e of transport; rivers flow inthei r ownalluviu m andhav emeanderin g orbraide dchannels , inplace swit h terraces oralluvia l fans. (3)th e lowerpart swit hne t sediment accumulation. This zone usually grades into adelta ,a nestuar y or some other coastal form, or (in arid regions) itfade s away ina dr y drainlessbasin .

In the following, attention will be given to some common landforms in the systems of three types ofrivers : (1)braide drivers, (2)meanderin g rivers,an d (3)anastomosin grivers . 84 Landforms inlowland s

Braided rivers

Braided rivers have numerous shallow channels of low sinuosity. At maximum discharge, the sediment islands between the channels are usually submerged but they reappear when the water level falls, not seldom at a completely different place. Gravel or coarse sand is deposited at the downstreamend so fth eisland swherea sth eside sar eerode da sth echannel s shift. This river type with its continually changing pattern of shallow channelsan dgrave lbars ,occur s inwatershe darea swit ha highl y irregular discharge and anabundanc e ofcoars eweatherin g products. Such conditions prevaili narea swit ha spars evegetatio ncove ran dtorrentia lrai nshower s sucha sari dan dperiglacia lregion san dalon gglacie r fronts,wher e sudden peaks occur in the discharge ofwate r and sediment due tomeltin g of the ice inspring . The actual riverbe d isnormall y unsuitable for arable farmingbecaus e of the risk of flooding but agriculture may be practised on river terraces which extend above the flood plain. Terraces are not realwetlands ; they areusuall y remnantso f ice-agefloodplain swit ha highe rbas e levelwhic h became dissected later, in (morehumid ) interglacialtimes . Braidedrive rdeposit scontai nalternatin garea s (lenses)o fcoars egrave l and sand and onlymino r inclusions of finer sediment.Th e coarse deposits may have become covered with fine-grained sediment later, e.g. when the braided stream turned intoa meanderin grive ro rdurin gexceptiona l floods ('Hochflutlehm').No tseldom ,on eca ndetec tforme rgrave lbar san dgullie s inth emicrorelie f (less thana metr ehigh )o farea s underlainb y braided river deposits. Figure 1 shows the basic elements of a typical braided river system.

Vegetated Sandwaves

Bar top deposits + (Fining- upward sequence)

Fig. 1. Basic elements of a typicalbraide d river. BarA isbein g driven laterally toward the far bank and is forming a protected part of the channel (slough) in which mud is deposited. B is a complex sand flat, originally formed fromgrowt ho fa nemergen tnucleu s sucha stha tshow ni n the left central part of the diagram. Source: Blatt,Middleto n &Murray , 1980:base d on thewor k ofD.J . Cant inth e South SaskatchewanRiver . Landforms inlowland s 85

Alluvialfan sar eformation swhic hhav eth ecoars esediment san dshiftin g channels of braided rivers. They develop where the gradient of a stream decreases sharply, e.g. where a tributary stream leaves themountain s and enters thefloodplain .There ,th esedimen t loado fth erive rca nn o longer be carried and most of it is dropped right at the entrance to theplain . Thisrapidl yblock sth echanne lwhic hthe nsweep slef tan drigh tt oobviat e the obstacle. The result is a fan-shaped, low-angle sediment cone. There is often a grain size gradient, from coarse to fine, from the 'proximal' part of the fan (close to the apex) to the 'distal' part (far into the plain).Th efan sar eexcellen taquifers ,an didea lsite sfo rtown san dvil ­ lages.

Meandering rivers

Meandering rivers consist normally of one single channel of high sinuosity. The channel is bordered by 'natural levees' behind which the 'basins'o r 'backswamps'occur .Laborator y experimentshav econfirme d that meandering channels are typical of streams with a regular, rather steady dischargepatter nan da bedloa do frelativel yfine-graine dmaterial s (sand, silt, clay). See Figure2 . Meandering rivers occur invegetate d areas in ahumi d climate. The dense vegetationcove ri sassociate dwit ha preponderanc eo fchemica lweathering , with formation of clay insoi l and saprolite,an dwit h stable slopeswit h littlesurfac erunof fan dmuc hundergroun dfeedin go friver s (throughflow).

EARLIER POINT- BAR CREVASSE-SPLAY CHANNEL-FILL DEPOSIT DEPOSIT DEPOSIT DEPOSIT

LEVEE BACKSWAMP CHANNEL LAG DEPOSIT DEPOSIT DEPOSIT

Fig. 2. The classical point bar model for a meandering stream. Source: Allen,1964 . 86 Landforms inlowland s

Meandering rivers may show considerable fluctuations indischarge ,du e to seasonality of rainfall and snow melting, but they never rundr yan d sudden floodsar erare .I fth echanne l isfille dt oth eto po fth enatura l levees, the river is in 'bank-full stage'. Occasionally, more water is supplied than can be held between the natural levees. Then, the river overflows itslevee s andth ebasin s areflooded .

Sedimentation takes place in the channel, on the levees and inth e basins.Th ecoarses tsediment sar efoun da tth ebotto mo fth echanne l ('lag deposits') and consist normally of coarse sand or gravel. Finer sand settles along theinne rbend s ofth erivers ,o nth e 'pointbars' . During floodings,fin esan do rsil ti sdeposite do nto po fth elevees ,an d clay inth ebasins . (Peatma yaccumulat e therea swell. ) The whole system ofla ggravels ,pointba r sands, levee andback - swamp claysmigrate s laterally asth emeande r loops erode theoute rbank s and deposit sediments on the inner point bars. In this way a 'fining upwards'sedimentar y sequencema ydevelop . 'Crevasse splays'ar edeposite d in the basins where the levees break; they are 'coarsening upwards' sediment sequences. Channel cut-offs, ('oxbow lakes') become eventually filled inwit h clay and/orpeat . Althoughmeanderin griver sar emos tabundan ti nhumi dregions ,the yoccu r alsoi nari denvironment sbu tthei rsource sar einvariabl youtsid eth eari d region.Th eEuphrate san dTigris ,fo rinstance ,originat ei nth esnow-cla d Taurus range inTurke ybefor e they reach theMesopotamia ndesert .

Anastomosing rivers

Anastomosing rivers resemble the braided rivers in having several branching channels,bu thav e their fine-grained sediments andthei rwell - developed levees andbackswamp s (often covered with peat) incommo n with the meandering rivers. They differ from the braided and the meandering riversi ntha tth epositio no fth echannel s changes littleunles sa sudde n breacho fth eleve e ('avulsion')occurs .Anastomosin g rivershav ever ylo w gradients. There islittl e erosion ofth eoute r banks and little lateral displacement of the meander belt. Anastomosing rivers occur mainly in rapidly subsiding interiorbasins ;example s areth eLowe rMagdalen a Basin inColombia ,o rarea sclos et oth ecoas tsuc ha sth eAlblasserwaar d inth e Netherlands.

LANDFORMS IN (COASTAL)MARIN E LOWLANDS

The coastal lowlands contain a great variety of landforms. For all practicalpurposes ,th elowland s canb edivide d inthre ebroa d groupings: (1)deltas , (2)wave-influence d coastal lowlands,an d (3)tide-dominate d coastal lowlands. Landforms inlowland s 87

Deltas

Deltas are formed where silt-loaded rivers debouch into the sea or a lake.A s the oceans reached theirpresen t level only some 6000year sago , most surface features of deltas are quite recent. However, drillings in large delta bodies have revealed long and complex histories. Such large delta bodies can be more than 10 kilometres thick and their sediments contain evidence of numerous sea level fluctuations and changes of the drainage base. The different sedimentary facies of delta bodies reflect changes inon eo rmor e ofth e following externalfactors : (1)th e density of therive rwate r relative to the density of the seao r lakewater , (2)th enatur e andvolum e of the sediment carriedb y theriver , (3)th e influence ofwave s andtides , (4)th e climate,an d (5)th e rate of (tectonic)subsidenc e of thebasin .

The oldest description of deltas dates back to 1885 and is given in a publication on the outflow of glacially fed rivers into the former (Pleistocene) Lake Bonneville. In this case, in which no differences in the apparent densities of river water and lake water existed and the influence ofwave s and tideswa s negligible,a fan-shaped classical delta or 'Gilbert-type delta' is formed. In this type of delta, the coarsest (sand-sized)materia l isdeposite d inth e delta plain close to themouth ; finer (silty) sediments accumulate in the submerged delta slope and the finest clay particles travel farthest (to the 'prodelta'). Thus, a grain size gradation evolves across the delta. When the delta progrades under continuing sediment supply, progressively coarser sediments cover (or 'onlap' as sedimentologists say) finer sediments: a coarsening upwards sedimentary sequence results.Suc h sequencesma yb e tenso r evenhundred s of metres thick. The fine prodelta sediments are the 'bottom-set beds', the sloping delta-front sediments the 'fore-set beds', and the topmost deltaplai n sediments the 'top-setbeds' .

If (lowdensity )fres hwate r flows out inth e sea, itact s as abuoyan t jet stream driving its suspended sediment far into the sea. With the river's natural levees extending into the sea, the delta system expands rapidly, particularly if littlewav e or tidal action disturbs sedimenta­ tion. In this way, a 'birdfoot delta' is formed, inwhic h the tributary channels of thedelt aplai n form the 'toes' of the foot.Swam p lands form between the toes; they are the brackish equivalents of the backswamps in purely fluvial environments. Occasionally, a channel breaks through its levees and deposits a crevasse splay; the channel may then be abandoned and a new one formed on top of the crevasse sediments. So-formed fining upwards sequences canb e severalmetre s thick. Where tidal influence isstrong , thechannel swide n tofunnel s similar to theone sfoun di nestuarie s (seeth eparagrap ho ntide-dominate dlowlands) . The areas inbetwee n thechannel s develop into 'tidal flats'. In tropical areas, these are normally colonized by mangroves which add much organic debris to the freshsediment . This combination ofa shallow ,brackish , tidal swampwit h reduced sedi­ mentsan dmuc horgani cmatte rpromote s themicrobia lreductio no fsulfate s 88 Landforms Inlowland s

(fromth ese awater )t o'pyrite ' (ironsulfide) . Pyritic sedimentsar eth e parentmateria lo f'Aci d Sulfate soils'.

A. INITIAL PROGRADATION NATURAL LEVEE

DEL SILT Y SAND AND SILTY CLAY

B. ENLARGEMENT BY FURTHER PROGRAOATION

DELTA-PLAIN

DELTA-PLAIN INORGANIC SIITY CLAY

DELTA-PLAIN NATURAL-LEVEE CLAYEY SILT AND SILTY CLAY

C. DISTRIBUTARY ABANDONMENT AND TRANSGRESSION

MORIBUND DISTRIBUTARY;

TRANSGRESSIVE OELTA-MARGIN- ISLANO SAND

TRANSGRESSIVE DAY DEPOSITS

D. REPETITION OF CYCLE REOCCUPATION OF OLD DISTRIBUTARY COURSE

Fig.3 .Developmen t ofdelt a sequences inth eMississipp i delta. Source: Frazier, 1967. Landforms in lowlands 89

Wave-influenced delta plainshav e commonly cuspate or straight outlines formedb ysan dridges ,i nplace s toppedb yeolia ndunes .Lagoons , commonly filled inwit hpeat ,li ebehin d the (parallel)ridges .An y sedimentwhic h enters the sea isquickl y dispersed bywav eaction .

The Mississippi delta is an example of abirdfoo t delta without strong tidal influence (see Figure 3); th e deltas of theMekon g and Brahmaputra rivers are tide-dominated delta plains. Wave-dominated deltas are, for instance,th edelta so fth eRhone ,Ebr oan dNil e inth eMediterranea nSea , and theDanub e delta inth eBlac kSea . i^^: 3=^

MAXIMUM TIDAL CURRENTS

5 CM/SEC

Fig. 4. Wave and tidal currents acting offshore in modern Niger delta. Source:Allen ,1965 .

Wave-influencedcoasta l lands

The importance of the tides isusuall ymeasure db y the tidalamplitude : coastswit ha tida lamplitud e of2 metre so rles sar ecalle d 'microtidal', thosewit h 2t o4 metre samplitud e are 'mesotidal'an dcoast swit ha tida l amplitude ofmor e than4 metre s are 'macrotidal'coasts . 90 Landforms in lowlands

Microtidal coasts are shapedb ywav e action rather than tidalaction . Wavesrollin gu p toth ecoas tproduc e asur f (washan dbackwash )resultin g inne ttranspor to fsedimen ttoward sth ecoast .Compensatin gcurrent soccu r parallel to the shore: 'longshore currents'. If the waves approach the coast under an angle, longshore currents and 'beach drift' displace the sediment parallel to thecoast . SeeFigur e4 . Mostwave-dominate d coastshav e sandybeache swhic h are attached toth e hinterland; ifthe yar e separated from thehinterlan db ya narro w stripo f sea, they are called 'barriers'. See Figure 5.Alon gmeso - andmacrotida l coasts, inletsar epresen tbetwee n individualbarrier s andtida lflat sar e formedbehin d thebarriers .Thes ear einundate da thig htide san dar ebar e orcarr y ahalophyti cvegetation .

Aeolian Beach flat Dune / Shoreface

Fig. 5. Main morphological components of the barrier model. Source: Blatt, Middleton &Murray , 1972. Landforms inlowland s 91

Somewave-dominate d coasts consistmainl y ofmud . Small sand ridges may occurwithi nthes ecla yplains ;the yar ecalle d 'cheniers',an d theplain s themselvesar e 'chenierplains' .Example so fcoast swit hchenie rplain sar e the coasts of theGuiana s and the coast ofLouisiana . The Surinam chenier plainha sgrow nseawar dove rten so fkilometre sdurin gth elas t600 0years .

Delta growth isdirecte d away from thehinterland ; inth ecas e of sandy beaches andbarrie r coasts thedirectio no flandfor m development isno ts o clear, whether accretionwil l takeplac e or transgression depends onsuc h factors asth ealong-shor e sedimentsupply ,th eslop e ofth eforeshor e and thesubsidenc e rateo fth ebasin ,al lfactor swhic h influence thesedimen ­ tary sequences found in wave-built sandy coastal lowlands. Seaward progradationproduce scoarsening-upward s sequenceswher eth ese abotto mi s formedb yfine rmateria ltha nth esediment sadded ,an dwher eth esand snea r the shoreline are finer-grained than those higher up. On the other hand, landward shifting ofbarrier s may cause the deposition of sand on top of older lagoon material, so that coarsening upwards sequences may develop. The interpretation ofobserve d sedimentary patterns iscomplicate db y the fact thatregressio nan dprogradatio nhav e inplace salternate d duringth e Holocene.

An overview of thehistor y of theDutc h central coast,betwee nHoe kva n Holland andDe nHelder , illustrates how the landforming processesdiscus ­ sed inth e foregoinghav e shaped thiswave-dominate d coastal lowland (the present tidal amplitude isonl y 1.5 metres). When thepost-glacia l sea level risewa s still rapid,betwee n >7,000 and 5,000year s BP,th ethe nexistin gcoasta lbarrier smove d inland.The ywer e separated by inlets behind which clayey tidal flats were formed, the 'Calais deposits'. On the landward side, the Calais deposits interfinger withfluvia lclay san dpeat .Whe nth ese aleve lris eslowe ddow n (5000-2000 BP), the barrier system closed and grew seawards, forming the 'Old Barriers' ('Oude Strandwallen') that are particularly well developed in the stretch between The Hague and Haarlem. The slow rise of the drainage base during this period created ideal conditions for the accumulation of thick layers ofpea t (the 'Holland peat') on top of the Calais deposits. Much of thepea twa s latermine d for fuel,unti l theCalai s deposits were reached. The resulting lakes have largely been drained and form some of the lowestpolder s of theNetherlands .

Tide-dominated coastal lowlands

Especiallyalon gmacrotida lcoasts ,th erive rmouth sar estrongl ywidene d byincomin gtides .'Estuaries 'ar eforme dwhic hhav efunne lshape dchannel s with extensive sand flats in between. Sand waves may develop there with wave lengths of several metres ('megaripples'); the interior parts are commonlybrackis h orsaline .

Where the tides arehig h enough to flood parts of deltaic or estuarine systems, they build tidal flats. The tides enter and leave through deep gullies (geulen) between barrier islands. The gullies are permanently submerged andconduc thug emasse so fwate ran d (coarse)sedimen ta t incom­ ingan doutgoin gtides .Th esedimen treache sth eflat sthroug hsmal lcreek s 92 Landforms inlowland s

(prielen).Thes ehav ea meanderin gpatter nan dwide ntoward s theirmouths . 'Intertldal flats' (platen), between mean high water level and mean low water level, are flooded twice a day. Fine sandy and clayey sediment settles onto p ofthes e flats.Th epart swhic har e floodedonl ya textrem e tides are called 'supratidalflats ' (saltmarshes ,kwelders ,schorren) .

In temperate regions, intratidal flats are normally barren whereas supratidal flats carry ahalophyti c vegetation ofherb s and shrubs. This vegetationtrap ssedimen twhe nflooded ; theresultin gdeposit sar estrati ­ fiedwit ha typica lalternatio no ffin ean dcoars elayer s (kweldergelaagd- heid).Tida lflat si nth ehumi dtropic scarr ya mangrov evegetatio nalread y inth e intertidal zone and siltu pmuc hmor e rapidly thanmos t supratidal flats intemperat e regions.Shiftin go fcreek san dgullie s leads tofinin g upwards sequences,wit h finesupratida l and intertidal sedimentsö nto po f coarser creek and gully sediments.

The southwestern part of the Dutch coastal plain is an example of a tide-dominated coastal lowland. Its 'Duinkerken deposits'wer e formed by continuously.shiftingestuar y channels,mainl y inconnectio nwit h extreme floods.Th eoldes tpart so fth eprovinc eo fZeelan d (Oudland)ar eestuarin e tidal flats datingbac k to pre-Roman times. Inplaces , sandy creek fills in (former)tida l flats stand out asa resul t ofrelie f inversionb ydif ­ ferential compaction of clays and peat around the creeks. The youngest parts in this area (Nieuwland) are estuarine channel fills that were deposited only inth e 16th century.

The soils of alluvial lowlands are marked by prolonged wetness (unless empoldered)an dyoun g age.Wher e sedimentation isstil l going on,strati ­ fied FLUVISOLS occur. GLEYSOLS are found in depressions which do not receiveregula raddition s ofsediment ;thei rprofile s testifyo fa shallo w water table during all or most of the year. Both Fluvisols and Gleysols include Thionic SoilUnit swhic h formed inpyriti c sediments. Fluvisols 93

FLUVISOLS (with special reference toThioni c Soils)

Soils showing fluvic3 properties and having no diagnostic horizons other than an ochric ,a mollic or an umbric A-horizon, or a histic H-horizon, ora sulfuri c horizon,o rsulfidi cmaterial s within 125c mo f the surface.

Key toFluviso l (FL)Soi lUnit s

Fluvisols having a sulfuric horizon or sulfidic material, or both, at less than12 5c m from thesurface . Thionic Fluvisols (FLt)

Other Fluvisolshavin g amolli c A-horizon ora eutri c histic H-horizon. Mollic Fluvisols (FLm)

Other Fluvisols which are calcareous at leastbetwee n 20 and 50 cm from the surface. Calcaric Fluvisols (FLc)

Other Fluvisols having anumbri c A-horizon or a dystric histic H-hori­ zon. umbric Fluvisols (FLu)

Other Fluvisols having abas e saturation (by IMNH^OA ca tp H 7.0) of less than 50percent ,a t leastbetwee n 20an d 50c m from the surface. Dystric Fluvisols (FLd)

Other Fluvisols showing salic3properties . Salic Fluvisols (FLs)

Other Fluvisols. Eutric Fluvisols (FLe)

Diagnostic horizon; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OF FLUVISOLS

Connotation: soilsdevelope d inalluvia ldeposits ;fro mL .fluvius .river .

Parent material: mostly recent, medium- and fine-textured fluviatile, lacustrine ormarin edeposits .

Environment: periodicall y flooded areas (unless empoldered) in alluvial plains, valleys and (tidal) marshes, in climates ranging from arctic to equatorial, and from semi-arid toperhumid . 94Fluvisol s

Profile development: AC-profiles; a distinct Ah-horizon may be present. The lower horizons show evidence of stratification andhav e no or only a weak soil structure.Gleyin g iscommo n in the lowerpar t of theprofile .

Use: Fluvisol s are used for awid e range of crops,o r for grazing. Flood control,drainag e and/or irrigation arenormall y required.Thioni cFluvi ­ sols suffer from severe soil acidity andhig h levels ofnoxiou s Al-ions.

REGIONAL DISTRIBUTION OF FLUVISOLS

Fluvisols occur on all continents and inal l kinds of climates.Ou t of the 316 million hectares of Fluvisols worldwide, some 200 million hectares are inth e tropics.Majo r areas canb e found: a) along rivers and lakes, e.g. the Amazon basin, the Ganges plain of India, theplain snea r Lake Chad inCentral "Africa ,o r themars h lands of Bolivia andnorther nArgentina ; b) in deltaic areas, e.g. the deltas of the Ganges/Brahmaputra, Indus, Mekong,Nile ,Zambesi ,Orinoco ,Ri oPlata ; c) in areas with recent marine deposits, e.g. the coastal lowlands of Sumatra,Kalimantan , and Irian inIndonesia ,an d the coastal zones ofth e abovementione d deltas.

Thionic Fluvisols ('AcidSulfat e Soils') cover an estimated 13 million hectares; the largest concentration (6.7 million hectares) is in the coastallowland so fsoutheas tAsi a (Indonesia,Vietnam ,Thailand )bu tthe y occur also in West Africa (Senegal, the Gambia, Guinea Bissao, Sierra Leone, Liberia) and along the north-eastern coast of South America (Venezuela, the Guyanas). Figure 1 shows the worldwide distribution of Fluvisols.

Fig. 1.Fluvisol sworldwide . Fluvlsols 95

GENESIS OFFLUVISOL S

Fluvisolsar ear eyoun galluvia lsoil swhic hhav e 'fluvicsoi lproper ­ ties'.Fo ral lpractica lpurpose s thismean s thatthe y receive freshsedi ­ ment during regular floods (unless empoldered)an dhav e still stratifica­ tionand/o r an irregular organicmatte rprofile .

Inth eupstrea mpar to frive rsystems ,soil swit h fluvicsoi lpropertie s arenormall y confined tonarro w strips alongside the actual riverbed . In themiddl e and lower stretches, the floodplai n isnormall ywide r andha s theclassica larrangemen to flevee san dbackswamps ,wit hcoarsel y textured Fluvisols on the levees and more finely textured soils in the backswamps further away from theriver . In areas with marine sediments, relatively coarse-textured Fluvisols are present on barriers, cheniers, sand flats and crevasse splays; finely textured Fluvisols arepresen to nclaye y tidalflat san d inchenie rplain s inbetwee nth eridges .Wher eriver scarr yonl yfin egraine dmateria lt oth e sea, thecoasta l plains (and their Fluvisols)ar e entirely clayey.

Genesis ofAci d Sulfate Soils

Theonl ydifferenc ebetwee nth eparen tmateria lo fThioni c Fluvisolsan d thato fothe r Fluvisols isth epresenc e ofpyrit e (FeS,)i nThioni cFluvi ­ sols.

Formationo fpyrit e cantak eplac e inmarin eenvironment s ifth e following conditions areme t during sedimentation: (1)Iro nmus tb e present.Easil y reducible ironoxide s orhydroxide s are normally present incoasta lsediments . (2)Sulfu rmus tb epresent .Thi s originates from sulfates in the sea water. (3)Anaerobi c conditions mustprevai l toallo wreductio n of sulfate and ironoxides .Thi s condition isme t infres h coastalsediments . (4)Iron - and sulfate-reducing microbesmus tb epresent ; they occur in all coastalsediments . (5)Organi c matter isneede d asa source ofenerg y for themicrobes ;i t is suppliedb y a lushpallustri c vegetation (mangrove forests,reeds , sedges). (6)Tida l flushingmus t remove thealkalinit y which forms inth e process ofpyrit e formation. (7)A slow sedimentation rate isnecessary . Otherwise, the timewil lb e too short to form sufficient pyrite fora potentiall y acid sediment.

Themechanis m ofpyrit e accumulationboil s down to the following:unde r oxygen-poor conditions (under water), microbes reduce ferric (Fe ) iron to ferrous (Fe2+) ions, and sulfate (SO^2"' to sulfide (S2"}. During this process, organic matter is decomposed, ultimately to bicarbonates. Thus, a potentially acid part (the pyrite) and an alkaline part (bicarbonates) are formed in an initially neutral system. Tidal flushing removes the alkalinity (HCOj"5, andpotentiall y acidpyrit e remainsbehind . 96 Fluvisols

Pyritei sforme di na numbe ro fsteps ,bu tth e'overall 'reactio nequatio n is:

Fe,0=2v,,3+, 4, S ^*~O +8 CH 20+ 1/ 20 2= 2 FeS 2+ 8 HC0 3"+ 4 H 20

Importantintermediat eproduct si nthi sproces sar esolubl eferrou ssulfat e (FeS04)an d(meta-stable )jarosit e (KFe(S04)2(OH)6). Jarosit e isforme da t a pHbelo w 4 if sufficient K+-ions arepresent . Jarosite hasa typical straw-yellow colourwhic hi seasil yrecognize di nth efiel dan di sindica ­ tive ofActua l Acid Sulfate Soils (asdistinguishe d from Potential Acid Sulfate Soils;th edifferenc e willb ediscusse d later). Thecompositio no fth evariou siro nmineral swhic hfor mi nth eproces s of pyrite oxidation islargel y determined byth eredo x potential ofth e soil material (ameasur e ofth equantit y ofoxyge n present) andth epH . The stability of the iron components, and their dependence on redox potential andpH ,i sshow ni nFigur e2 .

\ \ \ ""-, \ +18- ~"~^ oxygen

jarosite KFe3 (so4)2 .

+6-

^^^^ \ water

0- \ \ pyrite ^^^ \ "°\ • FeS2 ^\ \

-6-

hydrogen "-~

1 1 I 1 ! 1 I 1 pH

Fig. 2: pe:pH diagram of pyrite, limonitic Fe20j, ja iarosite and dissolved + 2 2 3+ 2 ] + 3+ 2+J~„3+1 K , SO,„4 "' Fe,» * an„d^ Fe,„ a.t. log[S0,^6L^4 " -- -2.3-..., lo_g& [Kt.. ], =- -3.3, log [Fe^+Fe ] -5 and 1 atm. total pressure. Source: Miedema, Jongmans & Slager, 1973. Fluvisols 97

When oxygen penetrates a reduced pyritic sediment, the pyrite is oxidized:

2+ 2 + FeS2+ 7/2 02+ H 20= Fe + 2 S04 "+ 2H (pyrite)

The ferrous iron ions formed are oxidized to ferrichydroxide :

2+ + 6 Fe + 15H 20+ 20 2= 6 Fe(OH)3 + 12H

In the presence of carbonates. no lowering of the pH takes place even thoughmuc h acidity isreleased . Gypsum isformed :

+ 2+ 2 CaC03+ 2H - Ca + H 0+ 6 C02 and: (carbonates)

2+ 2 Ca + S04 "+ 2H 20= CaS04.2H20 (gypsum)

Inth eabsenc e ofcarbonates .th ehydroge nproduce d isno tneutralize d and thep H of the sediment falls sharply. The ferric hydroxyde is transformed tojarosite :

+ 2 + 3 Fe(OH)3+ K + 2 S04 "+ 3H = KFe3(S04)2(OH)6+ 3H 20 (jarosite)

Some of the soluble ferrous sulphate that is formed upon oxidation of pyrite,migh tdiffus et olowe rlayer swher ean dreac tdirectl yt ojarosite :

2+ 2 + + Fe + 2/3 S04 "+ 1/3 K + 1/4 02+ 3/2H 20= 1/3 Jarosite+ H

With time, the hydrogen ions will be removed from the system by per­ colation or lateral flushing, and jarosite will be hydrolyzed to ferric hydroxide:

+ 2 + 1/3 Jar +H 20= 1/3 K + Fe(OH)3 + 2/3 S04 " +H

Thepoorl ycrystallize d ferrichydroxid ewil leventuall yb e transformedt o goethite:

Fe(OH)3 = FeOOH+ 3H20 (goethite)

The foregoing shows that aeration of fresh, non-calcareous pyritic sediments, e.g.b y forceddrainage ,result s inlarg equantitie s ofH +-ions beingrelease d toth esoi lsolution .Thes eH +-ionslowe rth ep Ho fth esoi l and exchange with bases at the cation exchange complex. Once the pH has fallen toa levelbetwee np H 3an dp H4 , the claymineral s themselves are 98 Fluvisols attacked.M gan dFe ,bu tmostl yA lar erelease dfro mth ecla ylattices ,an d noxiousA l -ionsbecom e dominant inth esoi l solutionan d at the exchange complex.

Fig. 3.Geolog y andphysiograph y of theMekon g Delta,Vietnam . Legend: (1)Granit ehills , (2)Pleistocen ealluvia l terraces, (3)Holocen e complex ofrive r levees andbackswamps , (4)Holocen ebrackis hwate r sedi­ mentsi nwid edepressions ,(5 )Holocen emarin esan dridge san dcla yplains, and (6)Holocen e peat dome.

Figure 3 shows the geology and physiography of the Mekong delta in Vietnam,t oillustrat eth egenesi san dphysiographi c positiono fFluvisol s (mapunit s 3,4 an d 5)i ngenera l and ofThioni c Fluvisols inparticular . Pyrite occurs in the sediments of broad depressions (map unit 4) where extensive areaso fAci dSulfat e Soilsar e found.Ther e isn opyrit e inth e soilso fma punit s3 (fres hwate rdeposits ;n osulfat epresen tdurin gsedi ­ mentation),an d 5 (sedimentation rate too high). The lower tiers oftopo - genousdeltai cpea t (mapuni t 6)ma ywel lcontai npyrite .Eutri c Fluvisols Fluvisols 99

areconfine d tonarro w strips along theriver s inma puni t 3.A largepar t of the river sediments and most of the marine sediments have lost their fluvicproperties , i.e. they aren o longer stratified orhav e developed a cambicB-horizon .A sgle yphenomen aar enormall ypresen twithi n5 0cm ,the y classifya sGleysols .Larg earea swit hThioni cFluvisol soccu ri nth ebroa d depressions onbot h sides of therivers .

CHARACTERISTICS OF FLUVISOLS

Fluvisolsar ever yyoun gsoil swit hwea khorizo ndifferentiation ;the y aremostl yAC-profile san dpredominantl ybrow n (aeratedsoils )and/o rgre y (waterlogged soils).Thei rtextur eca nvar yfro mcoars esan di nleve esoil s toheav y clays inbackswamps . Most Fluvisols show mottling due toalter ­ nating reducing and oxidizing conditions. Even if such 'gleyic soil pro­ perties' occur inth euppe r 50c m of theprofile ,th e soils areno t clas­ sified as Gleysols because their fluvic properties have priority in the key toMajo r SoilGroupings . Fluvisols may show some structure development, but only in part of the profile. Heavy clay soils have commonly a coarse blocky or prismatic structure,wherea s lighter textured soils canhav e avariet y of structure types (granular,crumb , subangular blocky). Strongly stratified sediments tend to develop platy structures upon drying. It is evident that the characteristics of Fluvisols are dominated by their recent sedimentation andwetnes s:stratification ,beginnin gripening ,chemica lpropertie sinflu ­ encedb yalternat e reducingan doxidizin gconditions ,an di nsom eenviron ­ mentsals osoi lsalinity .Rathe rspecia lar eth echaracteristic so fThioni c Fluvisols; theywil lb e addressed insom edetail .

Hvdrological characteristics

Most Fluvisols are wet in all or part of theprofil e due to stagnating groundwater and/or floodwate r from rivers or tides. Terraces are a part of many river systems but are much better drained than the active flood plain; terrace soils are normally well homogenized and lack fluvic properties.

Physical characteristics

The 'ripening stage' of sedimentary material is expressed by its 'n-value' which indicates the approximate quantity of water that is ab­ sorbedb y one gram of clay. In the field, the ripening stage isjudge db y sqeezing a lump of soil material through one's fingers and interpreting theresistanc e felt.We tclay san dsilt swhic hhav elos tlittl ewate rsinc e deposition,ar esof tan dunripe .Suc hsoil spos eproblem s foragricultura l use;the yhav ea lo wbearin gcapacit yan dmachine scanno tb euse do nthem . Many clayey Fluvisols have few pores and a low hydraulic conductivity. However, many of coastal landforms were once colonized by a pallustric vegetation (e.g.mangrove so rreeds )whic hlef tlarg e tubularpore s inth e sedimenttha tar eofte nstil lintact .Fluvisol so nrive rlevee san dcoasta l sand ridges areporou s aswell . 100 Fluvisols

Chemical characteristics

Most Fluvisols are fertile soils; they have neutral ornear-neutra l pH valueswhic hd ono timpai rth eavailabilit yo fnutrients .Coasta lsediment s contain usually some calcium carbonate (sea shells !), and the exchange complex issaturate d withbase s from the seawater .O n the otherhand , an unfavourably high sodium saturation isno t uncommon and high salt levels inth e soilmoistur e canb e aproblem .

Thehydrologica lan dphysica lpropertie so fThioni cFluvisol sar elargel y similar tothos eo fothe rFluvisol sbu t theirchemica lcharacteristic sar e decidedly different. Within the Thionic Fluvisols, a distinction must be madebetwee nPotentia lAci d Sulphate Soilswhic h areno tye toxidize dbu t contain pyrite in the soilmaterial , and Actual Acid Sulfate Soils which are oxidized and acidified.

Unfavourable properties ofPotentia lAci d Sulfate Soilsare :

(1)Salinity : PotentialAci d Sulfate Soils aremostl y situated incoasta l areaswit h tidal influence. (2)Stron g acidificationupo ndrainage . (3)Lo w accessibility/trafficability: PotentialAci d Sulfate Soils occur inunripe, sof tmuds . (4)Hig hpermeability : theroo t channels of the (former)pallustri c vegetationmad e the sediments excessively permeable towater . (5)Floodin g of the land: flooding at spring tidema y cause damage to crops. (6)Engineerin g problems arisewhe n dikes,et c are constructed onth e softmuds .Acidit y from theoxidize d dikes attacks steel and concrete structures.

Unfavourable properties ofActua lAci d Sulfate Soilsare :

(1)Lo wpH :mos tplant s cantolerat e pHvalue s as lowa sp H4 ,bu t only if the supply ofnutrient s iswel lbalanced . (2)Aluminiu m toxocitv canoccu r insoil swit h ap Hbelo w 4.5; generally valid toxicity limits cannotb e givena s the toxicity depends onth e availability ofnutrients ,th egrowt h stage of theplant ,an d the crop. (3)Salinit y inActua lAci d Sulfate Soils isno t always causedb y salts from seawater ; sulfate canbuil du p inth e soil solution to the extent that the soil ist ob e regarded saline. (4)Phosphoru s deficiency:hig h aluminum levels inth e soil solution causeprecipitatio n of insolubleAl-phosphates . (5)Ferrou s iron (Fe )toxicit y isa commo nproble m when rice isculti ­ vated onActua lAci d Sulfate Soils.Reductio n of insoluble ferric iron to soluble ferrous ironcomponent s takesplac e infloode d rice fields. (6)Acidificatio n of thesurfac ewater :whe nActua lAci d Sulfate Soils are flooded forric e cultivation, soluble ferrous ironca ndiffus e to the surfacewater .Thi s surfacewate r contains oxygen,an d the ferrous ironca nb e oxidized to ferric ironagain : FIuvlsols 101

2+ + 2 Fe + 1/2 02+ 5H 20= 2Fe(0H) 3 +4 H

This process acidifies the surfacewate r and cancaus e irreparable damage to engineering structures and fish ina ver y shorttime . (7)N-deficiency :mineralizatio n oforgani cmatte r bymicrobia l activity is slow inth ewe tActua lAci d SulfateSoils . (8)Engineerin g problems:acidit y from surface water attacks steelan d concrete structures. (9)H.. Stoxicit y becomes aproble m whenActua lAci d Sulfate Soils are flooded for longperiod s (ayea r or longer). Sulfate can thenb e reduced toH 2S,whic h is toxic atver y lowconcentrations .

MANAGEMENTAN DUS EO F FLUVISOLS

Thehig hnatura l fertility ofmos t Fluvisols allows cultivation ofa wide range ofdrylan d crops onrive r levees and onhighe r parts inmarin e landscapes. Intropica l lowlandswit h ayear-roun d supply of freshwater , three crops per year are possible. Such places are among the most densily populated parts of theworld , andhav e been under intensive use since pre-historic times.Lan d tenure isofte ncharacterize db y smallplots ,an dmos tproduc ­ tion is forhom e consumption or for localmarkets . Paddy rice cultivation iswidesprea d on tropical Fluvisolswit h satisfac­ tory irrigation anddrainage .Padd y land shouldb e dry for atleas ta few weeks inever yyea r topreven tth esoil' sredo xpotentia l frombecomin g so low that nutritional problems (iron,H 2S)develop . Besides, a dry period stimulates microbial activity and promotes mineralization of organic matter. Other suitable crops besides rice may be jute,kena f and various tubercrop s (Colocasia,Xanthosoma ). Coconu tsurvive speriodi cfloodin gan d somedegre eo fsalinity .Tida lland stha tar estrongl ysalin ear enormall y undermangrove so rsom eothe rsal ttoleran tvegetation .Suc harea sar euse d forfishing ,hunting , saltpans ,o rwoo d cutting forcharcoa l orfirewood . Acid Sulfate Soils are widely left idle. Reclaimed and/or carefully managed Acid Sulfate Soils inth e tropics arepredominantl y used forric e growing; those inth e temperate zone are inus e aspastur e land.

There are two (opposite) strategies thinkable for reclaiming and using Potential Acid SulfateSoils : Thefirs ton e ist odrai nan dcompletel y oxidize the soil,an dleac hth e acidity formed. Leaching canb e done with saline orbrackis h water; this willno tonl yremov esolubl eacidity ,bu tals oexpel lundesirabl ealuminiu m ions from the exchange complex. This approach solves theproble m once and foral lbu tha s thedisadvantage s that it isexpensive ,pose s a threat to the environment (acid drain water !) and depletes the soil of useful elements together with the undesirable ones. The method has been applied with some success in coastal rice growing areas in Sierra Leone and in areas with fish ponds in the Philippines,bu twa s desastrous inSenegal , whereinsufficien twate rwa savailabl efo rleaching ,an di nth eNetherland s where the first generation of settlersbarel y survived the reclamation of theHaarlemmermee r polder. 102 Fluvlsols

The second strategy is to try to limitpyrit e oxidationb y maintaining ahig hgroundwate r table.A preconditio n isth eavailabilit y ofsufficien t water.Thi smetho dals orequire s ratherheav y investments inwate rmanage ­ ment,an dth epotentia ldange ro facidificatio nremain spresent .Th emetho d was widely applied, both in temperate regions and in the tropics, often with ingenious adaptations to suit local conditions andpractices .

When discussing management and use of Actual Acid Sulfate Soils, a distinctionshoul db emad ebetwee narea swit hshallo winundatio n (lesstha n 60cm )an darea swit hdee p inundation.I narea swit hshallo winundatio nan d at least some tidal influence increek s or canals,fla p gates canb e used for water control. In the rainy season, water can be discharged at low tide, or irrigation water can be applied as needed. In some areas, the springtid eca nb euse dfo rirrigatio ni nth edr yseason ,provide d thatth e tide isfreshwate r tide.I fth e tide isno thig h enough to flood the land forrice ,drylan d crops canb e cultivated byusin g flap gates tomaintai n a shallow groundwater table. In many coastal lowlands, the 'intensive shallow drainage system' is practiced: shallow ditches are duga tnarro w spacings.Thi s system relies on sufficient leaching of the surface soil at the start of the rainy season. Dryland crops can only be grown on raised beds whereby care must be takenno t to turn theprofil e upside downan dbrin g themos t acidpar t (thesubsurfac e soil)t oth etop .Ric e isgrow ni nth eshallo w depressions between the raisedbeds .

It isvirtuall y impossible to eliminate the acidity problem completely by applying lime to the soil. Table 1 shows"th e lime requirement for complete neutralization of soils with various contents of oxidizable sulfur. The neutralizing capacity of a 10 cm layer (without lime; only neutralizationb yexchang e ions)i sals ogiven .Th etabl edemonstrate s the practical impossibility of the liming option:ver y few farmers can afford to apply to an average Acid Sulfate Soil (say 1.5 percent sulfur and an apparent density of 1.0 Mg/m) a total of (47minu s 19),o r 28, tons of lime.An dtha tcover sonl yth eneed so fth eto p1 0cm ,assumin g thatn one w acid forms during a subsequent dryspell .

TABLE1 . Lime requirement for complete neutralization ofa 10c m soil layer.Afte r Dent& Raiswell ,1982 .

Lime requirement Neutralizing capacity ofa 10c m layer, of the 10c m layer, inton s of lime/ha (no limepresent ) in a clayey soil Apparent density percent oxidizable sulfur of the soil (Mg/m3) 0.5 1 1.5 2 3 4

0.6 9 19 28 37 56 74 11 0.8 12 25 37 50 7411 2 14 1.0 16 31 47 62 19 1.2 19 37 56 74 22 Fluvisols 103

Many lands with Actual Acid Sulfate Soils are not used for agriculture at all. Such hostile 'wet desert' lands have a pallustric vegetation, a limited fauna,an d acid surface water,especiall y early inth ewe t season when acid substances dissolve. The situation seems to be a little less bleak in the equatorial climatic zone than inmonsoona l climates.There , Acid Sulfate Soilswil lno tdr you ta seasil y as inth emonsoo narea ,an d morewate r isavailabl e formanagemen t measures throughout theyear . 104 Notes

NOTES Glevsols 105

GLEYSOLS

SoilsI nunconsolidate dmaterials ,exclusiv eo fcoarse-texture dmaterial s and alluvial deposits which show fluvic properties; showing gleyic propertieswithi n5 0c mo fth esurface ;havin gn odiagnosti chorizon sothe r than anA-horizon , ahisti c H-horizon, a cambic B-horizon, a calcic or a gypsic horizon; lacking the characteristics which are diagnostic for Vertisolso rArenosols ;lackin gsalic apropertie s;lackin gplinthite 3withi n 125 cm of the surface.

Key toGlevso l (GL)Soi lUnit s

Gleysolshavin g permafrost3withi n 200c m from thesurface . Gelic Gleysols (GLi)

Other Gleysols having a sulfuric horizon at less than 125 cm from the surface. Thionic Gleysols (GLt)

Other Gleysolshavin g andic properties. Andic Gleysols (GLa)

Other Gleysolshavin g amolli c A-horizon ora eutric 3histi c H-horizon. Mollic Gleysols (GLm)

OtherGleysol shavin ga numbri c A-horizono ra dystri c histic H-horizon. Umbric Gleysols (GLu)

Other Gleysols having a calcic horizon or a gypsic horizon, or both, within 125 cm of thesurface . Calcic Gleysols (GLk)

Other Gleysols having abas e saturation (by IMNH^OA c atp H 7.0) of less than 50percent ,a t leastbetwee n 20an d 50c m from thesurface . Dystric Gleysols (GLd)

OtherGleysols . Eutric Gleysols (GLe)

Diagnostic horizon; seeAnne x 1fo r full definition. Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARY DESCRIPTION OFGLEYSOL S

Connotation:soil swit hclea rsign so fexces swetness ;fro mR . pley.muck y soilmass .

Parentmaterial :a wid erang eo funconsolidate dmaterials ,mainl ysediment s ofPleistocen e orHolocen e age,wit hbasi c toacidi c mineralogy. 106 Glevsols

Environment: depresse d areaswit h shallow groundwater.

Profiledevelopment :mostl yA(Bg)C ro rH(Bg)C rprofiles .Evidenc eo freduc ­ tionprocesse s with orwithou t segregation of ironcompound swithi n 50c m of thesurface .

Use: waterloggin g is the main limitation. Gleysols are mostly used for grazing or are covered with swamp forest. Gleysols in the tropics and subtropics arewidel y planted torice .

REGIONALDISTRIBUTIO N OF GLEYSOLS

Thegreates texten to fGleysol s isi nth eborea lan dcoo lhumi dpart so f theworld . Gleysols occupy anestimate d 623millio nhectare sworldwid e of which some 200 million hectares are in the tropics and subtropics. See Figure 1.

Fig. 1. sworldwide .

GENESIS OFGLEYSOL S

The formationo fGleysol s isconditione db yexcessiv ewetnes s atshallo w depth (less than 50 cm from the soil surface) in some period of the year or throughout the year. Low-redox conditions,brough t aboutb y prolonged saturation of the soilmateria l inth epresenc e oforgani cmatter , result in the reduction of ferric iron compounds to (mobile) ferrous compounds. Thisexplain swh yth epermanentl ysaturate dsubsoi llayer so fGleysol shav e Gleysols10 7 grey, olive orblu e matrix colours:wit h theiro ncompound s mobilizedan d removed,th esoi lmateria lshow sit sow ncolour .Not etha tth echaracteris ­ ticgle ycolour swil lonl y formi fth esoi li speriodicall y saturatedwit h water which contains thedissolve d products oforgani c matter decomposi­ tion.

Subsequent oxidation oftransporte d ferrous compounds (back)t oferri c iron oxides cantak e place near fissures orcrack s inth esoi l andalon g formerroo tchannel swher e therei sa suppl yo foxygen .Hysteresi sbetwee n (comparatively rapid)oxidatio nan d(slow )reductio nprocesse s leads toa netaccumulatio no fferri ccompound snea r suchaerate d spotsi nth eother ­ wisereduce dsoi lmatrix :th esubsoi ldevelop sa patter no fmottle s (around airpockets )an d'roo tprints ' (former roothole s linedwit h iron oxide).

NOTETHA Tsuc h 'gleyic soilproperties 'ar estrictl yassociate dwit hmove ­ mento fth egroundwate r table:mottled , oxidizedhorizon s occuro nto po f a fully reduced subsoil. Adifferen t typeo fmottlin gi sfoun dwher eperche dwate r occurso nto po f a slowly permeable subsurface horizon,whil e therea l groundwater occurs at greater depth. Such soilshav e 'stagnic properties':a reduce d horizon occurso nto po fa noxidize dsubsurfac ehorizon .I fped soccur ,th ereduce d horizon tongues into the oxidized subsoil along the faces of the peds (Figure 2).

depth (cm) A-horizo n predominantly oxidized, 0 oo $ © rich in Fe III predominantly reduced, poor in Fe III

100H ground surface water water gley gley

Fig. 2. The different morphologies of soils with gleyic properties ('groundwater gley') and soils with stagnic properties ('surface water gley').

Stagnic properties are marked by the following morphometric charac­ teristics :

(1) Evidence of reduction as in materials with gleyic properties, and 108 Gleysols

(2)I fmottlin g ispresent , adominan tmois t chroma of 2o r less onth e surface ofped s andmottle s ofhighe r chroma occurringwithi n the peds, or adominan tmois t chroma of 2o r less inth e soilmatri x andmottle s ofhighe r chroma or iron-manganeseconcretion s orbot h occurringwithi n the soilmaterial . Ifmottlin g isabsent , adominan tmois t chroma of 1o r less onth e surfaces ofped s or inth e soilmatrix ,an d (3)Th e dominantmois t chroma of the soilmatri x and of the surfaces of peds increases invalu ewit h todepth .

Soilswit h stagnic soilpropertie s areno tGleysols ;the yke you teithe r at the Major Soil Grouping level as Planosols or Plinthosols, or at the second level as 'stagnic'Soi lUnit s inothe rMajo r Soil Groupings. Soils which are subject to flooding or showreductio na sa resul to f irrigation aremarke db y 'inundic'an d 'anthraquic'phases .

A special category of hydromorphic soils are the soils of the rice paddies.The yma yb e trueGleysols ,bu tar emor e oftena nanthraqui c phase ofsom eothe rMajo r SoilGroupin gwhic h developed stagnic soilpropertie s due to long continued irrigation or inundation.Thi smean s that there are two different types of 'PaddySoils' : (1)th eGleysol s and Fluvisols ofwetland s (witha completel y reduced subsoil), and (2)anthraqui c Paddy Soils formed inoriginall ywel l drained land.

Athin ,compacte dan dslowl ypermeabl eploug hsol ei sforme da tth edept h ofcultivatio n inal l Paddy Soils.Anthraqui c Paddy Soils oftenposses s a reddishbrow nt oblac kaccumulatio nhorizo no firo nand/o rmanganes eoxide s at some depthbelo w theploug h sole.Thi s accumulation layer formed as a result of intensive reduction processes in the reduced surface layer and translocation and precipitation of iron and manganese compounds in the (oxidized)subsoil .Th eaccumulatio nlaye rma y induratet oa nimpenetrabl e .

CHARACTERISTICS OF GLEYSOLS

Gleysolshav enormall y aspong yo rmatte dlitte rlaye rrestin go na dar k greyAh-horizon .Thi shorizo nchange ssharpl y intoa mottle dgre yo roliv e cambicBg-horizon .Wit hdepth ,th eBg-horizo n grades intoa grey ,oliv eo r blue, completely anaerobic Cr-horizon. InGleysol so fth esavannahs ,th etopsoi lconsist snormall y ofa ver ydar k grey toblac kheav y clay. Insom eprofile s this continues to 2m ormore , whereas in others it gives place to a pale grey horizon with prominent mottling (Fitzpatrick, 1986). Close to the surface, the soil structure is medium blocky or even crumb, but it becomes coarsely prismatic at some depth.Th e soilmateria l ishar dwhe n dry and stickywhe nwet . Where Gleysols remainwaterlogge d throughout theyear ,excep tperhap s for shortperiods ,th etopsoi l istypicall y amixe dorgani c andminera l (muck) H-horizon; itoverlie sa mottle dcla yo rsand ycla ybelo wwhic hi smateria l subject topermanen t anaerobic weathering. Gleysols 109

The dominant attribute of Gleysols is their prolonged saturation with water, associated with lack of aeration, anunfavourabl e root environment and poor conditions for the soil fauna. Repeated wetting and drying may also cause soil densification due to weakening of interparticle bonds during saturation and contraction of soilparticle s upondesaturation . Gleysols indepression s or at the foot of slopes compare favourably with similar soils at higher positions with respect to nearly all chemical properties. They have higher organic matter levels, a higher cation exchange capacity andbas e saturation, and usually also higher levels of phosphorus and potassium. This ispartl y the result of a generally finer texture and slower organic matter decomposition, but is also caused by influx of ions from adjacent (higher)lands .

"»••fe**;.

Fig. 3.Wetlan d rice fields in a river valley near Rokupr, SierraLeone . The surrounding uplands areplante d tooi lpalm .

MANAGEMENT ANDUS E OF GLEYSOLS

The main obstacle to the utilisation of Gleysols is the necessity to installa drainag e system,eithe rdesigne d tolowe rth egroundwate rtable , or to intercept seepage or surface runoff water. Adequately drained 110Glevsol s

Gleysols are widely used for arable cropping, dairy farming orhorticul ­ ture. Where the surface soil ishig h inorgani cmatte r andp Hvalue s arelow , limingcreate sa bette rhabita t formicro -an dmeso-organism san denhance s the decomposition of soil organicmatter . If (too)we t soils arecultiva ­ ted,th estructur e ofth esoi lwil lb edestroye d fora lon gtime . Gleysols inlow-lyin garea swit hunsatisfactor ydrainag epossibilitie sar etherefor e bestkep tunde r apermanen t grass cover orunde r (swamp)forest .

The use ofGleysol s forwetlan d rice cultivationha s already beenmen ­ tioned. The difficulties discussed for Thionic Fluvisols apply also to Thionic Gleysols (see under Fluvisols). Where Gleysols occur in narrow valley floors, these aremodifie d intobroa d and level steps,eac hwit ha water-retaining bund. Figure 3present s a picture ofwetlan d rice fields onGleysol s ina smallvalle y inSierr aLeone .

Gleysols canb epu tunde r treecrop sonl yafte r thewate r tableha sbee n lowered with deep drainage ditches.Alternatively , the trees are planted onridge s thatalternat ewit h shallowdepression s inwhic h rice isgrown . This 'sorjan' system iswidel y applied in tidal swamp areas with pyritic sediments insouth-eas tAsia . Landforms inerodin g uplands 111

MAJOR LANDFORMS IN ERODING UPLANDS

Eroding uplands are marked by the occurrence of unstable rocky slopes and outcrops of bedrock. They are particularly common in the following environments: (1)Hig hmountai n areaswhic hwer e exposed to glaciation in the recent past; any old soil coverwa s scraped off during the latest glacial advances). These regions include almost allmountai n chains inth e temperate zone (Alps,Canadia nRockies )an dmuc h of the tropical highmountai nbelt s above the limito f Pleistocene glaciationwhic h lies above 3000m at theequator . (2)Middl emountain s (Caledonian andHercynia n orogenic areas), mainly inth e temperate zone.Thes e regionswer e not or only slightly glaciated inth e recent geological pastbu t suffered somuc hperi - glacial slope action that theyposses s extremely stonysoils . (3)Arcti c shield areas such as Scandinavia andnorther n Canada,whic h experienced glaciation and consistpredominantl y ofsoli d rock (scraped clean)o ro f stonymorain edeposits .

LANDFORMS INHIG HMOUNTAI N AREAS

Thehighes tmountain s inth eworl dwer e allforme dcomparativel yrecent ­ ly,viz .i nth eTertiar yan dQuaternary .The ybelon gt omountai nbelt stha t aresituate da tplat eboundarie san dthei rpropertie sdepen dt osom eexten t on thenatur e of theseplat eboundaries .Ther e are threebasi ctypes : (1)Islan d arcs (acurve d array ofvolcani c islands)for mwher e one oceanic plate isbein g subductedbelo w another. Ina nearl y stage of the subductionprocess ,th e islands are almost entirelyvolcanic ; examples of such islands are theLesse rAntilles ,o r theAleutia n Islands offAlaska .Later , the islands acquire amor e complex geological structure such asJapa nan d the Philippines.Volcanis m is initiallybasalti c toandesiti c andbecome s increasingly andesitic lateron . (2)Cordillera nmountain s formwher e anoceani c crust isbein g subducted below acontinenta lplate . Examples of this type are theAnde s and theAmerica n Cordilleras.Andesiti c and rhyolitic volcanism are prominent incordillera nmountains . Theuplif t to thepresen t elevation israthe r recent (Plio-Pleisto- cene) inal l cordilleranmountain sbu t orogeny (mountain formation) begannormall y already inth e Paleozoic oreve n in thePrecambrian . Folded sedimentary rocks,igneou s intrusivebodie s (granite,grano - diorite), and oldermetamorphi c rocks from all geologic periods can be found inthes emountai nbelts . (3)Continent-continen t collisionmountain s differ from the previous types intha tvolcanis m plays amuc hmor e restricted role.Moreover , compression ismuc h stronger than incordillera nbelts ,s o that 'nappes', low-angle overthrust sheetswhic hma yhav ebee n displaced hundreds ofkilometre s from their original position,ar ever y common. 112Landform s inerodin g uplands

Otherwise,a similar array of igneous,sedimentar y andmetamorphi c rocks of greatly differing ages canb e found as incordillera nbelts . Examples ofcontinent-continen t collisionmountain s are theAlp s and their continuation into Eastern Europe,Ira nan d theHimalayas ; these mountains arebein g formed as aresul t of the collision ofth e African Shield, theArabia n Peninsula and the Indian Shieldwit h the Eurasian continent.Tha t orogeny continues inthes e areas isdemon ­ stratedb y the fact that theKaukasu s isstil lbein g uplifted ata rate of2 c mpe ryear .Th eunderthrustin g ofth e IndianPlat ebelo w the Eurasian continentha smad e theHimalaya s thehighes t mountains onearth .

Rockoutcrop san dshallow ,ston yweatherin gmantle sar eprominen t inal l threemountai ntypes ,especiall y inth ezon ebetwee nth eactua lsno wlimi t and the lowermost extension of the glaciers in the last glaciation. (The present glaciers and snow fields areno t considered here.) Typical erosional forms inthi s zone are the 'cirques',amphitheatre-lik e formerglacie rbasins ,an dtroug hvalleys ,forme rfluvia lvalley s modified into aU-shap e by glacial abrasion. Bothhav e steepuppe r slopes,usuall y consisting of rough rock outcrops, and extensive lower slopes of rock debris.Th esiz eo fth eroc kfragment svarie swit hthei rlithology ;highl y fissile rocks such as shales, schists and dolomite desintegrate to fine debriswherea s granites givehug e angularboulders .

Besides rock outcrops and in-situ rock weathering, this zone contains soilparen tmaterial s of glacial,fluvioglacia l and lacustrine origin. The glacialdeposit sconsis to funsorted ,clay-ric h 'glacialtills 'o nth e bottoms of trough valleys and cirques, and 'moraine ridges' of coarse, unsorted, andusuall y clay-poormaterial . The fluvioglacial deposits aremeltwate rdeposit so fwell-sorte d sandsan d gravels similar to the deposits of (other)braide d streams. The lacustrine deposits, formed in temporary lakes at retreating glacier margins,ar e stratified asa resul to fseasona lvariation s insedimen tin ­ flux( 'varves').

Soil formation in these materials is slow (high altitude, low tempera­ ture) and went on for only 10,000 years or less. As a result, (poorly developed)LEPTOSOL San dREG0S0L Sar estrongl yrepresente di nhig hmountai n areas. The zone below the Pleistocene glacial limit contains older weathering profiles,wit hmor edevelope d soilstha noccu r inth erecentl y deglaciated regions.

LANDFORMS INMIDDL EMOUNTAIN S

The 'middle mountains' of Central Europe and eastern North America are gigantic continental collisional structures.The y formed onbot h sides of theAtlanti c and are known as the Caledonian and Hercynian Massifs. They extend from northern Norway toMauretani a in Eur-Africa and from eastern Greenland to the southern USA. Their folded sedimentary and metamorphic rocks indicate acomple x orogenichistory . Landforms Inerodin pupland s 113

Bothmassif s have an internal zone of crystalline rocks,an d an external zoneo flow-grad emetamorphi c andsedimentar y rocks.Typica l internal-zone crystalline provinces are thePiedmon tprovinc e of theUSA , Brittany, and the Central Massif, Vosges, Black Forest and Bohemian Massif in central Europe. The Valley and Ridge province in the USA, and the Ardennes, the RheinischeSchiefergebirg ean dDevonshir ei nEurop ear emainl ysedimentary . Asorogenesi s endedlon gago ,essentiall ybefor eth estar to fth eMesozoic , mostCaledonia nan dHercynia nMassif shav edegrade d tolo wpeneplain s such as Brittany (France). Where differences in elevation still exist in such areas, they are causedby : (1)differen t rock typeswit h different resistance:har d limestones, dolomites, sandstones or quartzites form 'cuestas'o r 'hogbacks'; soft shales andmarl s form gently slopingvalleys .Granite s form 'tors' (hugeheap s ofboulders )embedde d insand y saprolite, sucha s those atDartmoor .Muc h of this structural reliefwa s smoothed by slopeprocesse s under (peri)glacialcondition s during the ice ages. (2)late ruplif t generated byyounge r orogeny outside themassif s them­ selves; theAlpin e orogeny caused stronguplif t and incision ofen ­ trenchedmeander s inth eArdenne s andRheinisch e Schiefergebirge. TheNorwegia n Caledonides and theAppalachian s wereuplifte d asa corollary of the separation ofNort hAmeric a andEurop e inth e Mesozoic andTertiary .

Peneplain formation has continued until the Tertiary in many of these areas; relic (sub)tropical soils in some shielded areas on peneplain remnants testify to theoccurrenc e ofwarme r climates inth epast . The Caledonian ranges of western Norway, Scotland and Ireland were strongly modifiedb y glacial action.Th e fjords ofNorwa y are exceptional inthei renormou sdepth ,dow nt o130 0m ,mor etha n1 k mbelo wth econtinen ­ talshelf . Incontrast ,Hercynia nmassif s inCentra lEurop ewer ehardl yaffecte db y glacial action because they remained below the snowline. But they have experienced harsh climates: there is ample evidence of frost shattering, frost heaving, periglacial slope processes and typical structures related to thawing and freezing on top of permafrost. All these processes were instrumental inth emixin go fexistin g soilmateria lwit h freshroc kfrag ­ ments fromdeepe rdown ,s otha tman ysoils ,als oo nth euplifte dplateaus , arever y stony.

Rock outcrops are rare in plateau areas of sedimentary and low-grade metamorphic rocks but very common on the steep slopes of rejuvenated valleys. LEPTOSOLS andREGOSOL S are also commonthere .

LANDFORMS ONARCTI C SHIELDS

Precambrian shields such as the Scandinavian and Canadian shields are highly complex structures; their geologic history spans more than three billionyears .Arcti c shieldarea shav e incommo nwit hhig hmountai narea s that they were glaciated in the Pleistocene and consist of fresh rock outcrops alternating with glacial and fluvioglacial deposits. The latter 114Landform s inerodin g uplands stem almost exclusively from the last déglaciation when the vast conti­ nental ice sheets of theNorther nHemispher e melted down. Thearcti c shield areasar egenerall y subdued lowlandswit h littlerelief ; frostweathering , steeproc koutcrop s andslope s aremuc h lesscommo ntha n in thehig h mountains. Otherwise, much ofwha twa s said in the preceding paragraphs about landforms in glacial deposits applies also to arctic shields. LEPTOSOLS andREGOSOL S are common soilsthere . Leptosols 115

LEPTOSOLS

Soils which are limited in depth by continuous hard rock or highly calcareous3 material or a continuous cemented layer within 30 cm of the surface, orhavin g less than 20percen t of fine earth over a depth of 75 cm of the surface; having no diagnostic horizons other than a mollic, umbric ,o rochri c A-horizon,wit h orwithou t acambi c B-horizon.

Key toLeptoso l (LP)Soi lUnit s

Leptosolswhic har e limited indept hb y continuoushar droc k within 10c m of thesurface . Lithic Leptosols (LPs)

OtherLeptosol shavin g permafrost3withi n 200c mo f thesurface . Gelic Leptosols (LPi)

Other Leptosols having a mollic A-horizon which contains or immediately overlies calcareous3materia lwit h acalciu m carbonate equivalent ofmor e than4 0percent . Rendzic Leptosols (LPk)

Other Leptosols having amolli c A-horizon. Mollic Leptosols (LPm)

Other Leptosols having anumbri c A-horizon. Umbric Leptosols (LPu)

Other Leptosols having an ochric A-horizon and abas e saturation (by IM NH40Ac atp H 7.0) of less than5 0percent . Dystric Leptosols (LPd)

OtherLeptosols . Eutric Leptosols (LPe)

Diagnostichorizon ; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r full definition.

SUMMARY DESCRIPTION OF LEPTOSOLS

Connotation:weakl y developed, shallow soils;fro mGr . leptos.thin .

Parent material:variou s kinds of rock, or unconsolidated materials with less than 20percen t fineearth .

Environment: mostly lands at high or medium altitude, and with strongly dissected topography. Leptosols are found inal lclimat e zones,i nparti ­ cular inarea swit h ahig h rate oferosion . 116 Leptosols

Profile development:mostl yA(B) Ro rA(B) Cprofile swit ha thi nA-horizon . Leptosols in calcareous weathering material have normally a mollic A-horizonwit h ahig h degree ofbiologica l activity.

Use: unattractiv e soils for arable cropping; limited potential for tree cropproductio no rextensiv egrazing .Leptosol sar ebes tkep tunde rforest .

REGIONALDISTRIBUTIO N OF LEPTOSOLS

Leptosolsconstitut e themos textensiv eMajo rSoi lGroupin go nearth , witha nestimate dtota lare ao f226 0millio nhectare s (900millio nhectare s in the tropics and subtropics). Lithic Leptosols inmontan e regions form themajorit y of allLeptosols . See Figure 1.

Fig. 1.Leptosol sworldwide .

GENESIS OF LEPTOSOLS

Leptosols are genetically young soils and evidence of soil formation isnormall ylimite dt oa thi nA-horizo nove ra beginnin g (cambic)B-horizo n or directly over theunaltere d parentmaterial . Theprincipa l soil formingproces s inRendzi c andMolli c Leptosols isth e dissolutionan dsubsequen t removalo fcarbonates .A relatively smallresi ­ due remains behind and is thoroughly mixed with stable humifying organic matter and, in Rendzic Leptosols, fragments of undissolved limestone. Swelling and shrinking smectite clays inth eminera l residue areaccount ­ ablefo rth edominanc eo fblock y structures.Umbri cLeptosol s occurmostl y Leptosols 117

on siliceous parent rock in (montane) regions with a cool climate and a high precipitation sum. Leptosols arehighl yvariabl e insoi ldepth ,textur ean dcomposition ,par ­ ticularly inarea swit h active erosionwher e they oftenoccu r inassocia ­ tionwit h Cambisols.

CHARACTERISTICS OF LEPTOSOLS

Most Leptosols have an A(B)R configuration of only weakly expressed horizons. Rendzic and Mollic Leptosols have pronounced morphological features.Thei rdar kbrow no rblac kcalcareou sorgano-minera lsurfac esoil , inRendzi c Leptosols speckledwit hwhit e fragments of limestone rock,ha s a well developed crumb or granular structure, or a vermicular structure with abundant earth worm casts. Below, there is an abrupt change to the underlying rock or therema yb e anarro w transitionhorizon . The physical, chemical and biological properties of non-calcareous Leptosols vary considerably because they are largely determined by the characteristics of theparen tmateria l and the climate.Calcareou sLepto ­ solshav e generally better physical andchemica l properties thannon-cal ­ careous ones andar e also lessdiverse .

Fig. 2.A Dystric Leptosol underpin efores ti nNorway . 118Leptosol s

AllLeptosol s arefreel ydrained ; theylac kgleyi co rstagni cpropertie s atshallo wdept han dthe y areals o free fromhig h levels ofsolubl esalts . Their shallowness and/or stoniness, associated with a low water holding capacity, are serious limitations.Th enatura lvegetatio nvarie swit h the climatebu t isgenerall y richero ncalcareou s Leptosols thano naci dones . Earthworms ,enchytraei dworms ,arthropod s andbacteri aar eth echie fsoi l organisms. The soil faunama yb e temporarily inactive due todrought .

MANAGEMENT AND USEO F LEPTOSOLS

Mostnon-calcareou sLeptosol sar eno tcultivated .The yhav ea resourc e potential forwet-seaso n grazing and as forest land.Rendzi c Leptosols in southeast Asia are planted to teak and mahogony; those in the temperate zone are under deciduous forest whereas acid Lithic, Umbric and Dystric Leptosols aremor e commonly underpin e forest (Figure2) .

The chemical soil fertility ofLeptosol s isofte nhighe r onhil l slopes than onmor e level land. One or a few good crops canperhap s be grown on such slopes but at thepric e of severe soilerosion . Erosion is a serious problem, particularly inmontan e areas where ahig h populationpressur epromote soverexploitatio no fth eland .Summe r tourism, theconstructio no fskiin gground s andth erecen tdeterioratio no fforest s byaci datmospheri cdepositio nthreate nlarg earea so fvulnerabl eLeptosol s intemperat e regions. Steepslope swit h shallowan dston y soilsca nb e transformed intocultiva ­ ble land through terracing,th eremova lo fstone sb yhan d andthei rus ea s terrace fronts. Agro-forestry (a combination or rotation of arable crops and forestunde r strict control)hold s promisebu t is largely still ina n experimental stage.Th eofte nexcessiv e internaldrainag eo fLeptosol sca n cause drought even ina humi d environment. Regosols 119

REGOSOLS

Soils from unconsolidated materials, exclusive of materials that are coarse-textured or show fluvic3properties ,havin g no diagnostic horizons other than an ochric or umbric A-horizon; lacking gleyic properties within 50 cm of the surface; lacking the characteristics which are diagnostic forVertisol s orAndosols ; lacking salic properties.

Key toRegoso l (RG)Soi lUnit s

Regosols having permafrost3withi n20hi 0n c20 m o0f c thm oefsurface thesurf .. Gelic Regosols (RGi)

OtherRegosol shavin g anumbri c A-horizon. UmbricRegosol s (RGu)

Other Regosols which are gypsiferous3 at least between 20 and 50 cm from thesurface . Gypsic Regosols (RGj)

Other Regosols which are calcareous at least from 20 to 50 cm from the surface. Calcaric Regosols (RGc)

Other Regosols having a base saturation (by IMNH.OA c atp H 7.0) of less than5 0percen t at least from 20t o5 0c m from thesurface . Dystric Regosols (RGd)

OtherRegosols . Eutric Regosols (RGe)

Diagnostichorizon ; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OFREGOSOL S

Connotation: soils in theweathere d shell of the earth; from Gr. rhegos. blanket.

Parentmaterial :finel y grained unconsolidated weatheringmaterial .

Environment: al lclimat e zones,bot h at lowan dhig haltitudes ;mostl y in lando f level torollin g topography.

Profiledevelopment :AC-profiles ,wit hn oothe rdiagnosti chorizo n thana n ochric or an umbric A-horizon. Profile development is minimal as a con­ sequence ofyoun g ageand/o r slowsoi l formation e.g. because of lowsoi l temperatures orprolonge ddryness . 120Regosol s

Use:lan dus ean dmanagemen tvar ywidel yamon gth edifferen t soils inthi s grouping, from capital-intensive irrigated farming to lowvolum e grazing. ManyRegosol s areno tuse d at all.

REGIONALDISTRIBUTIO N OFREGOSOL S

Regosols cover an estimated 900 million hectares worldwide, mainly in the arctic region. Some 100millio nhectare s ofCalcari c andGypsi cRego ­ sols lie in the semi-arid (sub)tropics. Regosols occur in montane areas above thevegetatio n zonebu t also atse alevel .Se eFigur e1 .

Fig. 1. sworldwide .

GENESIS OFREGOSOL S

Soilformin gprocesse shav eha da ver ylimite deffec ti nRegosols ,eithe r because pedogenesis went on for a relatively short period of time or because of conditions which retard soil formation such as a dry and hot desert climate orpermafrost . Profile development is typically limited to a thin ochric A-horizon over the unaltered parent material. The paucity of pedogenetic transformation products accounts for the low coherence of thematri x material and makes that soil colours are normally (still) determined by the composition of theminera l soil fraction (Schmidt-Lorenz, 1986). Regosols in acid parent material may develop anumbri c A-horizon if the soilorgani cmatte rdecompose s slowly,e.g . inregion swit ha shor tsumme r anda lon g andcol dwinte r season. Inregion swit ha considerabl eevaporatio nsurplu sove rprecipitation ,som e limeand/o rgypsu mma yhav eaccumulate da tshallo wdept hi nth eprofil ebu t not to the extent that acalci c or gypsichorizo n ispresent . Regosols 121

CHARACTERISTICS OFREGOSOL S

The morphology of Regosols isprimaril y determined by the character of the parent material and the climate. In cold climates, the A-horizon containspoorl y decomposed organicmatte rwherea s (ochric)A-horizon s tend tob ethin ,lo wi norgani cmatte ran dgenerall yweakl ydevelope d inho tan d dry climates. The content of weatherable minerals varies from low to extremely high (little transformation).Th e low coherence of the matrix material makes most Regosols sensitive to erosion; the lowwate r holding capacity andhig h permeability towate r make them sensitive todrought .

MANAGEMENTAN DUS E OFREGOSOL S

Landus e andmanagemen t procedures applied onRegosol svar ywidely . The extensivearcti cRegosol shav eminima lagricultura l significance.Regosol s inth estepp eregio nwit h500-100 0m mo frainfal lpe rannu mnee d irrigation forgoo dproduction .Th elo wmoistur eholdin gcapacit yo fthes esoil smake s frequentapplication snecessary ;sprinkle ro rtrickl eirrigatio nsolve sth e problem,bu t israrel yeconomic . Inarea swher e thetota lrainfal l sum exceeds 750m mpe rannum ,th eentir e profile israise dt oit s (low)fiel dcapacit yquit eearl yi nth ewe tseaso n and, there, improvement inmethod s ofdr y farmingma yb e abette r form of investment (Fitzpatrick, 1980). Regosols in tropical montane areas are mainly used forextensiv e grazing;man y areno tuse d atall . 122 Notes

NOTES MINERAL SOILS CONDITIONED BY THEIR LIMITED AGE: CAMBISOLS

Cambisols 125

CAMBISOLS

Soils having a cambic B-horizon and no diagnostic horizons other than an ochric or an umbric A-horizon or a mollic A-horizon overlying a cambic B-horizonwit h abas e saturation (by IMNH.OA c atp H 7.0) of less than 50 percent; lacking salic properties; lacking the characteristics diagnostic forVertisol s orAndosols ;lackin ggleyi c propertieswithi n5 0 cm of the surface.

Key toCambiso l (CM)Soi lUnit s

Cambisolshavin gpermafros t within hi200nc 20m o0fc thm oefsurface thes u . Gelle Cambisols (CMi)

Other Cambisols showing gleyic propertieswithi n 100c mo f th0e c surfacemo f the surface. . Gleyic Cambisols (CMg)

Other Cambisols showingverti c properties. Vertic Cambisols (CMv)

OtherCambisol shavin ga numbri c A-horizono ra molli c A-horizonoverlyin g a cambic B-horizonwit ha bas e saturation (byI MNH.OA ca tp H7.0 ) ofles s than 50percent . Humic Cambisols (CMu)

Other Cambisols which are calcareous at least between 20 and 50 cm from the surface. Calcaric Cambisols (CMc)

Other Cambisols having acambi c B-horizonwit h ferrallc3properties . Ferralic Cambisols (CMo)

Other Cambisols having abas e saturation of less than 50 percent (by IM NH40Ac atp H 7.0) at least insom epar t of theB-horizon . Dystric Cambisols (CMd)

Other Cambisols which have a strong brown to red B-horizon (rubbed soil has a hue of 7.5Y R and a chroma of more than 4, or a hue redder than 7.5YR ). Chromic Cambisols (CMx)

OtherCambisols . Eutric Cambisols (CMe)

Diagnostichorizon ; seeAnne x 1fo r full definition. Diagnostic property; seeAnne x 2fo r full definition. 126 Gambisols

SUMMARYDESCRIPTIO N OF CAMBISOLS

Connotation: soilswit hbeginnin ghorizo n differentiation through changes incolour , structure,and/o r texture; from L. cambiare. tochange .

Parent material: medium and fine-textured materials derived from a wide range of rocks,mostl y incolluvial ,alluvia l oreolia ndeposits .

Profile development: ABwC profiles. Cambisols are moderately developed soilscharacterize db ysligh to rmoderat eweatherin go fth eparen tmateria l and by absence of appreciable quantities of illuviated clay, organic matter, aluminium and/or ironcompounds .

Environment: level to mountainous terrain; arctic to tropical climates; wide range ofvegetatio n types.

Use:a wid evariet yo fagricultura luses ;climate ,topography ,shallowness , stoniness, or lowbas e status may pose restrictions on theuse . In steep landsmainl y grazing and/or forestry.

REGIONALDISTRIBUTIO N OF CAMBISOLS

Cambisolscove r92 5millio nhectares ,o r7 percen to fth eearth' slan d surface; 500millio nhectare s are actual orpotentia l crop land. Cambisolsar eparticularl ycommo ni ntemperat ean dborea lregion stha twer e under the influence of ice during recent glacial periods,partl y because thesoil' sparen tmateria l isstil lyoung ,bu tals obecaus e soil formation is slow under the low temperatures (or even permafrost) of northern regions. See Figure 1.

Fig. 1. sworldwide . Cambisols 127

In the tropics, the largest continuous surface of Cambisols is in the (young) alluvial plains and terraces of the Ganges-Brahmaputra system. Cambisols arewidesprea d inarea swit hactiv e geologic erosion,wher ethe y may occur inassociatio nwit hhighl y developed soils such asAcrisol s and Ferralsols; themountainou s regions ofPapu aNe wGuine a area nexample .A s Cambisols occurofte ni nassociatio nwit hothe rsoils ,man yCambiso larea s are of limited extent andno t represented inFigur e 1.

GENESIS OF CAMBISOLS

Thecambi cB-horizo ni sth eonl ydiagnosti cfeatur etha tal lCambisol s have in common. A cambic B-horizon can also occur in other Major Soil Groupings but it is not a differentiating characteristic there because other properties are given ahighe r priority (salicpropertie s in Solon- chaks, gleyic properties inGleysols ,andi c properties inAndosols ,etc. ) The fact that Cambisols key out last in the taxonomiehierarch y ofMajo r SoilGrouping s implies thatCambisol s includeman y soils thatjus tmisse d out onon e ormor e requirements for other soil groupings. Inothe rwords , manyCambisol sar ei na transitiona lstag eo fdevelopmen tfro ma youn gsoi l to a mature soil with an argic, natric, spodlc, or ferralic B-horizon. Nonetheless, a cambic B-horizon can be quite stable, viz. where the en­ vironment counteractspedogeneti c change,e.g .b ylo wtemperature so reve n permafrost, or by low precipitation, or impeded drainage, or highly calcareous or weathering-resistant parent materials, or by a continuous supply of ions to replenish ions lostb y leaching, orb y a slow butcon ­ tinuous rateo ferosio n that is inequilibriu m withweatherin gprocesses .

A cambic B-horizon mustb e seena sa 'minimum B-horizon'wit h beginning soil formation. It ismarke dby : (1)recognizeabl e soil structure or,alternatively , absence ofroc k structure,and/o r (2)greyis h reductioncolours ,and/o r (3)a stronger chroma, redderhu e orhighe r clay content than theunder ­ lyinghorizon .

Inpractice ,a cambi c B-horizon isan y section ofa soilprofil e situated between anA-horizo n and a relatively unaltered C-horizon, thatha s soil structure rather than rock structure and that differs from the C-horizon in colour and/or clay content. The fact that Cambisols are in an early stageo fsoi lformatio ni sfrequentl yevidence db yth epresenc eo fappreci ­ able quantities of weatherable minerals and the absence of any signs of advancedpedogenesis .

Most cambic B-horizons formedupo nweathering/transformatio n ofprimar y minerals ina situatio no ffre einterna lan dexterna ldrainage .I na weakl y acidenvironment ,hydrolysi so firon-containin gmineral s(biotite ,olivine , pyroxenes, amphiboles, etc) produces ferrous iron that is oxidized to ferric oxides andhydroxide s (e.g.goethite ,haematite) . This 'freeiron ' coatssan dan dsil tparticles ,an daggregate s clay,sil tan dsan d topeds . 128Camblsol s

The soil becomes structured and yellowish brown to reddish in colour. Besides ferricoxides ,aluminiu m oxidesan dhydroxides ,an dsilicat eclay s are formed. There issom e leaching ofbase sbu tn o clearmigratio n ofFe , Al, organic matter or clay. This oxidative weathering process is not limited to the cambic B-horizon; it occurs just aswel l in the A-horizon andma y evenb e stronger there than inth eB-horizon ,bu t its effects are outweighedb y thedar kcolour s ofaccumulatin g soilorgani cmatter .

Weathering under conditions of impeded drainage leads to new formation of clays, and segregation or removal of free iron oxides at some depth. Soil formingprocesse s are impededb y the lacko fdrainage ,s o that there isonl ylimite dleachin go fsolubl ecompounds ,an dlittl ecla yilluviation .

The processes leading to the formation of a cambic B-horizon are fun­ damentally the same in all climate zonesbu t the intensities of chemical and biological transformations are considerably higher in the (humid) tropics thanelsewhere . Inth ehumi d tropics,Cambisol s canfor m ina few years time. In dry tropical regions, soil formation is interrupted for shorter or longerperiods .

The wide variation among Cambisols is perhaps best illustrated by mentioning anumbe r of typical situationswher e these soilsoccur : In the humid tropics. Cambisols are widespread in highland regions (Umbric Cambisols) and inhill y to mountainous terrain, mainly at medium altitudes.Th e steepest slopeshav en o soila tall ,o ronl yLithi cLepto - sols;Dystri co rFerrali cCambisol soccu ro nth emoderatel ystee phillside s and (residual) s or Ferralsols inmor e stablesites . In the drier subtropics. Calcaric and Chromic Cambisols may form upon erosion ofLuvisol s orKastanozems .Verti c Cambisols occur in association withVertisol s onth eDecca nPlatea ui nIndia ,wher e long-continuedculti ­ vationan dsoi lerosio nhav eproduce d shallow soilstha td ono tqualif ya s Vertisols. In the temperate zone. Cambisols are particularly common in alluvial, colluvial and eolian deposits; they are predominantly Eutric or Dystric Cambisols,dependin go nth enatur e ofth eparen tmaterial .Th emajorit yo f allCambisol s inth enorther nhemispher e areGeli cCambisols . Inwetlands .Gleyi c Cambisols canb e found inassociatio nwit h Gleysols andFluvisols ,and ,i nsomewha tbette rdraine dposition s sucha sterraces , togetherwit h Luvisols,Acrisols ,Plinthosols .

CHARACTERISTICS OF CAMBISOLS

The typical Cambisol profile has an ABwC horizon sequence with an ochric,molli co rumbri cA-horizo nove ra cambi cB-horizo ntha tha snormal ­ lya yellowish-brow n colourbu ttha tma yals ob e an intense red.I npoorl y drainedterrai npositions ,oxidation/reductio nmottle soccu ri nth ecambi c B-horizon. Textures are loamy to clayey. Some clay coatings may be detectablei nth ecambi cB-horizo nbu tth ecla yconten ti scommonl y (still) highest inth eA-horizon . Camblsols 129

It is not well possible to sum up all mineralogical, physical and chemical characteristics of Cambisols in one generalized account because Cambisols occur insuc hwidel y differing environments.However : (1)mos t Cambisols contain at least someweatherabl e minerals inth e silt and sandfractions . (2)mos t Cambisols occur inregion swit h aprecipitatio n surplusbu t in terrainposition s thatpermi t surficialdischarg e ofexces swater . (3)mos t Cambisols aremedium-texture d andhav e agoo d structural stability, ahig hporosity , agoo dwate rholdin g capacity and good internal drainage. (4)mos t Cambisolshav e aneutra l toweakl y acid soil reaction,a satis ­ factory chemical fertility anda nactiv e soilfauna .

NOTETHA T there arenumerou s exceptions to theabov e generalizations !

MANAGEMENTAN DUS EO F CAMBISOLS

Onth ewhole ,Cambisol smak egoo dagricultura llan dan dar eintensive ­ ly used. The Eutric Cambisols of the temperate zone are among the most productivesoil so nearth .Distri cCambisols ,thoug hles sfertile ,ar euse d for(mixed )arabl efarmin gan da sgrazin gan dfores tland .Stonines s (rudic phase; seeAnne x 3)an d shallowness are the commonest limitations in the temperate zone.Cambisol s onstee p slopes arebes tkep tunde r forest;thi s isparticularl y true for theUmbri c Cambisols ofhighlands .

TheVerti c and Calcaric Cambisols in (irrigated)alluvia l plains inth e dry zone are intensively used for the production of food and oil crops. Eutric, Calcaric and Chromic Cambisols in undulating or hilly (mainly colluvial)terrai n areplante d toa variet y ofannua l andperennia l crops and tree crops or areuse d forgrazing .

The Dystric and Ferralic Cambisols of the humid tropics are poor in nutrients but still richer than neighbouring Acrisols or Ferralsols and they have a higher cation exchange capacity. The Gleyic Cambisols of alluvial plains under paddy rice arehighl y productive soils. 130 Notes

NOTES MINERAL SOILS CONDITIONED BY A WET (SUB)TROPICAL CLIMATE : FLINTHOSOLS FERRALSOLS NITISOLS ACRISOLS ALISOLS LIXISOLS Landforms inth ewe t tropics 133

MAJOR LANDFORMS IN TROPICAL REGIONS

Large parts of the tropicsbelon g toon e of the following threemorpho ­ structural units: (1)Precambria nshiel d areas,whic h constitute majorpart s of eastern SouthAmerica , equatorial Africa, and central and southernIndia ; (2)Youn g alpine foldbelts , such as the equatorialAnde s and Central America,an dmajo r parts of SoutheastAsia ; (3)Tropica l alluvial plains, including fluvial sedimentary basins such as theAmazo nbasin , the Zaïrebasi n and the Indus-Gangesbasin , aswel l as coastalplains ,e.g . thecoasta lplain s of theGuianas , theNige r delta and theMekon gdelta .

Each of these units have their characteristic landforms, developed in characteristic parent materials through characteristic geomorphic and tectonic processes. Landforms inhig h mountain areas (above 3000m )hav e been discussed ina previou s chapter,jus t as the (tropical)lowlands . The present chapter is concerned with themajo r landforms in Precambrian shieldarea san di nth elowe rrange so fyoun galpin efol dbelt s (below300 0 m) inth ehumi dtropics .

LANDFORMS INPRECAMBRIA N SHIELDAREA S

Precambrian shieldarea sconstitut e theoldes tcore so fcontinents .The y areremnant so fmountain swhic hforme dmor etha n60 0millio nyear sag oan d have sinceerode ddow nt oundulatin gplain s thatexten donl ya fe whundre d metres above the present sea level. Shield areas form part of the litho- spheric plates which move along the earth's surface at a rate of several centimetrespe ryear .Horizonta lmovemen tbring sabou tvertica ladjustment , and thereforezone s ofactiv e subsidencear e foundnex tt ozone so factiv e uplifting (thoughmuc h lessvigorou s than inyoun gmountai n belts).

The Precambrian spans 80percen t of thegeologica lhistor y of the earth and includesman yperiod so fmountai nbuilding ,erosio nan dsedimentation . Igneous, sedimentary and metamorphic rocks of Precambrian age exist in greatvariet ybu tcrystallin e (plutonican dmetamorphic )rock spredominate . The Precambrian formations aremad eu p of the followingbroa dunits : (1)High-grad e metamorphic belts areusuall ynarro w (tens ofkilometre s across)an d consist largely ofstrongl ymetamorphose d rocks,man y of which originate from sedimentary rocks.Th e lithology of themetamor ­ phicbelt s isaccordingl y varied:metamorphose d limestones (marbles), metamorphosed sandstones (quartzites),metamorphose dbasal t flows or dykes (amphibolites),an d aconsiderabl e proportion ofrock s ofmor e or less granitic composition.Th evariatio n inchemica l andminera - logical composition of these rocks explains thewid evariatio n in landforms andsoils . (2)Greenston e belts arenarro w (afe w tens orhundred s ofkilometre s across)bu t can stretch over thousands ofkilometres .The y consist mainly ofvolcani c rocks (basalts,andesites , rhyolites)wit hvaryin g 134Landform s in thewe t tropics

amounts of intercalated sedimentary rocks thathav e normally been converted to schists andphyllite s by low-grade metamorphism. A characteristic feature ofgreenston e belts is theoccurrenc e of tonalité intrusions,normall ywit h oval outlines on the geological map. Tonalité isa granite-lik e rock ofplutoni c origin and usually has plagioclase as the sole feldspar.A splagioclas e weathers easily, tonalité areas aremor e deeplyweathere d thannearb y graniteareas . Examples are theKoid uBasi n inSierr aLeon e and theBrokopond o lake inSurinam . (3)Granit e areas are oftenassociate dwit h eithermigmatite s (banded rocks formed through partialmeltin g of sediments deep inth e crust), orrhyoliti c volcanic rocks (the extrusive equivalent of granites). (4)Platfor m areas consist ofhorizonta l sedimentary rocks,commonl y sandstones, on top of the Precambrian shield; the surrounding,un ­ covered shield areas are referred toa s 'basement'areas .

Tropical shield areaswer e not affectedb y glacial,periglacia l or eolian processes in the recent past; they are mainly modified by the action of rain and by fluvial processes.Th e amount and intensity of precipitation onth eon ehand ,an d theprotectio nprovide d toth elan db y thevegetatio n covero nth eother ,determine dho wwate rcoul d shapeth esurface .W e shall restrict thediscussio n toarea sunde r either of twovegetatio ntypes : (1)a tropical rain forest,o r (2)a savanna hvegetation .

In areas under rain forest, most precipitation is intercepted by the canopyfro mwher ei ttrickle sdow nt oth efores tfloo ran dinfiltrate s into the soil. There, it reaches the groundwater and is eventually discharged by rivers. On its path, it contributes to strong chemical weathering of rockbecaus e dissolutionprocesse s are favouredb y the low ionic strength and comparatively high temperature of the water. The saprolite (rotten rock)unde ra rai nfores tvegetatio nextend softe nt oten so fmetre sdepth , oreve nt ohundred so fmetres .Th esaprolit e isles sthic ko ngranit e (say, 10-20metres )tha no nmetamorphi c rock (40-70metres ;dat a from Surinam). The strong chemicalweatherin g and comparatively low surface runoff cause gradual deepening of the weathering front, aproces s known as 'etching'. The saprolite isnormall y clayeybecaus e all feldspars and ferromagnesian minerals areweathere d tocla ymineral san d (sesqui-)oxides; th esan dcon ­ tent of the saprolite reflects theconten t of coarse quartz inth eorigi ­ nalparen trock .Thoroughl yweathere d saprolites arechemicall yver ypoor , despite the luxuriant rain forestvegetation .

In very arid areas. the vegetation is widely spaced because each in­ dividual plantneed s a largevolum e of soil for itswate r supply. Insuc h open land, a single downpour can cause torrential 'sheet floods'.Highe r precipitation sums allow a denser vegetation coverbu t gowit h increased erosivityo fth erain .I nth eintermediat esituation ,i.e .i nth esemi-ari d savannahs andprairies ,surfac e runoffan ddenudatio n aremos t severe.

Under aspars evegetation , therat e oferosio nma ywel l exceed the rate ofweatherin g and soil formationwhic h leads to 'stripping' of the land. Under more protective vegetation types, e.g. under rain forest, etching Landforms inth ewe t tropics 135 predominates.Th epresen thumi d tropicshav e experienced profound climate changes in the recentpast .Man y of today's rain forest areas were savan­ nahs duringglacia lperiod s and thepresen t savannahswer e evendrie r than they are today. Figure 1present s aschemati c model of thedevelopmen t of summit levels by differential etching and stripping. The balance between etching and stripping was almost certainly different from the present situation for longperiods .

Fig. 1. The development of summit levels by differential etching and stripping. Source:Kroonenber g& Melitz ,1983 .

Many shield areas consist ofvas t dissected plainswit h solitary eleva­ ted areas that either arebare , dome-shaped granite hills ('inselbergs') sucha sth esuga rloa fo fRi od eJaneiro ,o rheap so fhug egranit eboulder s known as 'tors' (see Figure 2).Th e plains consist of a deep, flat or undulating, residual weathering crust, dissected by a network of small V-shaped valleys that are only a few metres deep. Where the drainage pattern iswidel y spaced, remnants of the original flat surfacema y still be detectable but in areas with densely spaced drains there remain only rounded, convexhills . (Thedrainag epatter n isusuall y determined by the structureo fth eunderlyin gbedrock ;lo wridge san ddepression sresul tfro m differential etchingan dstrippin go fresistan tan dles sresistan trocks. ) 136 Landforms Inth ewe t tropics

Fig.2 .Granit etor si nBotswan aextend sove rth esurrounding safte ra lon g history ofweathering . Photographb y courtesy of PaulienBrombacher .

The pattern of isolated inselbergs and tors amidst vast expanses of undulating lowland is more clear in savannah regions than in areas with rain forest.A difference is that thevalley s (called 'dambo' or 'vlei' in Africa) are broad and shallow in savannah regions as a result of colluviation and slope wash. Surface runoff ishighe r there than in rain forestarea swher einfiltratio nrate sar ehighe ran d (linear)erosio nalon g channelspredominates .Fla tsurface si nsavanna harea sar eessentiall ywas h pediments thatwer e shapedb y sheet floods and surficialwas hwherea s any (fossil) wash pediments in rain forest areas are merely dissected by channeled vertical erosion.

Theorigi no f inselbergsan dtor s isconnecte dwit hclimat e fluctuations in the past. Inwe t eras, some rock masses were less deeply etched than othersbecaus eo fdifference si nchemica lcompositio nand/o rjoin tspacing . In a subsequent drier period, these more resistant parts carried a less densevegetatio n cover andwer e subject to less intense chemicalweather ­ ing. Eventually, they came to stand out above theirsurroundings . Landforms in thewe t tropics 137

Climate fluctuations have also played a role in the formation of the characteristic 'lateriteplateaus' .Highl yweathere dsaprolit ewit hquart z- richclay s ('plinthite')forme ddurin ga humi dperio dan dbecam e truncated and hardened to 'ironstone' during a subsequent drier era. The plateaus either constitute weathering residues under ironstone, or formed through relief inversion of iron-cemented valleyfills .

LANDFORMS INALPIN E FOLDBELT S (LOWERTHA N 3000METRES )

High mountain areas in the Tropical Zone were partly glaciated in the Pleistocenebu ttract sbelo w3,00 0metre swer eneve rreache db y descending glaciers. Rain forest is the preponderant vegetation type below this altitude in humid tropical mountain. The relation between rainfall and land surface transformationi ssimila rt otha ti nth eshiel dareas .Infiltratio nreache s deep levels in the rain forest zone and weathering is advanced, despite steep slopes. Fresh rock is difficult to find, even in deeply dissected terrain, and tors and inselbergs areabsent .

The dominant geomorphic controls inhumi d tropicalmountai n areasare : (1)th epresenc e of deepweatherin gmantles : (2)stron g tectonic uplifting: (3)rapi d downcuttingb y rivers,an d (4)underminin g of slopes and subsequentmas smovements .

Slope forms are mainly determined by landslides which continue further downwards as mudflows. The landslides are often triggered by earthquakes and are especially common in lands with primary forest because the trees add to the weight of the unstable soil masses. In the forested mountain areas ofNe wGuinea ,Hawaii , theAndes ,an d elsewhere, shallow landslides arever y common and aprovisiona l chronology can oftenb e established by considering the degree of forest regeneration. Counter to commonbelief , thefores tvegetatio ncanno tpreven tlandslide s fromhappenin gbecaus eth e sliding land mass detaches itself at the 'weathering front', i.e. the contact plane between saprolite and fresh rockwhic h isbeyon d the reach of the roots.

Regions with crystalline rocks have often symmetrical hills with sharp crests and rectilinear slopes, separated by steep V-shaped valleys. The topography resembles thato f other erodedbadland s inhomogeneou smateri ­ als, only much enlarged (hence the name 'megabadlands'). Interfluv e and slopemorphologie s are dominated by landslidescars .

Siliceoussedimentar yrock shav eles sdee pweatherin gmantle stha ncrystal ­ linerocks .Th ealternatio no fresistan tan dles sresistan t strata infol d patterns isth emai ncontrollin gfactor .Often ,landslide soccu ralon gbed s of impermeable shales, especially in times of heavy rainfall. The alternation of strata accounts also for theoccurrenc e ofmesas , cuestas, and especially hogbackmorpholog y ina nAppalachia nstyle . /

138Landform s inth ewe t tropics

Calcareous rocks are associated with typical tropical karst phenomena, startingwit hth eformatio no fsin khole san dcaves ,bu tultimatel y leading tofa rmor eadvance ddissolutio nphenomen atha nfoun doutsid e thetropics . 'Tower karst' is an example, just as 'cockpit' or 'mogote' hills, or razor-sharp ridges of limestonebetwee n deephole s and ravines,a s inth e forest-clad 'brokenbottl e country' ofNe wGuinea .

Volcanic rocks and the characteristic landforms of regions with volcanic materialshav ebee n discussed ina previou s chapter of thistext .

In summary, thewe t tropics and subtropics aremarke d by large, deeply weathered areas on old, geologically stable shields and by (smaller) occurrences of younger materials, e.g. residual material which became exposed by truncation of the soil, transported and mixed colluvial and alluvial deposits and materials which became rejuvenated by (volcanic) foreignadmixtures .Th echaracteristi c soilso fth ewe ttropic sar ere do r yellow incolour ,ol dan dstrongl yleached .The yar edeep ,finel ytextured , contain no more than traces of weatherable minerals, have low-activity clays, less than 5 percent recognizable rock structure and gradual soil boundaries. Typical Major Soil Groupings are the plinthite-containing PLINTHOSOLS, the deeply weathered and chemically poor FERRALSOLS, the richerNITISOLS ,strongl yleache dACRISOL Swit ha cla yilluviatio nhorizon , ALISOLSwit hlo wbas esaturatio nan dhig hactivit yclays ,an dLIXISOL Swit h high base saturation and low CEC.Th e differences among the soils in the wet tropics can largely be attributed to differences in lithology and (past)moistur eregime . Plinthosols 139

PLINTHOSOLS

Soilshavin g2 5percen to rmor eplinthit e byvolum ei na horizo nwhic h is at least 15 cm thickwithi n 50c m of the surface orwithi n a depth of 125c mwhe nunderlyin g analbi c E-horizon ora horizo nwhic hshow sgleyi c or stagnic properties within 100c m of thesurface .

Key to Plinthosol (PT)Soi lUnit s

Plinthosolshavin g analbi c E-horizon. Albic Plinthosols (PTa)

Other Plinthosols having an umbric A-horizon or a dystric histic H-horizon andwhic h are stronglyhumi c. Humic Plinthosols (PTu)

OtherPlinthosol shavin g anochri c A-horizonan da bas e saturation (byI M NH.OAca tp H7.0 ) ofles stha n5 0percen tthroughou t theuppe r 50c mo fth e plinthite horizon. Dystric Plinthosols (PTd)

OtherPlinthosols . Eutric Plinthosols (PTe)

Diagnostic horizon; seeAnne x 1fo r full definition. Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OF PLINTHOSOLS

Connotation: soils with mottled clayey materials which become ashar d as abric kwhe nexpose d to theope nair ; fromGr .plinthos .brick .

Parentmaterial :plinthit ei smor ecommo ni nweatherin gmateria lfro mbasi c rocktha nfro maci drock . Inan ycas e it isessentia l thatsufficien t iron ispresent ,originatin g either from theparen tmateria l itself orbrough t inb y seepagewate r from anadjacen tuplan darea .

Environment: plinthit e is formed in level to gently sloping areas with a fluctuating water table.A petroferric phase with continuous 'ironstone' develops where plinthite becomes exposed to the surface, e.g. on old erosion surfaces thatar e above thepresen t drainagebase . Skeletic soils with aconcretionar y layer ofhardene d plinthite that isno t continuously cemented, occur mostly in colluvial and alluvial materials. Plinthite is associatedwit hrai nfores tareas ;petroferri c andskeleti c soils aremor e common inth e savannahzone .

Profile development:mostl yAB C orAEB C profiles.Accumulatio n ofsesqui - oxidesb yferralitizatio nan denrichmen t fromoutsid e sources;segregatio n of ironmottle s ina zonewit h a fluctuatingwate r table. 140 Plinthosols

Use: lo wagricultura l valuebecaus e of frequentwaterlogging , dense plin- thite layeran d lowchemica l fertility.Th e iron-richmateria l isuse dfo r building purposes and road construction.

REGIONAL DISTRIBUTION OF PLINTHOSOLS

Areas where the formation of 'plinthite' (an iron-rich, humus-poor mixtureo fcla yan dquartz )i sreportedl yver yactiv e includewester nIndi a (whereBuchana nfirs tuse d theter m ''fo rthi smateria l in 1807), West Africa and parts of South America. All these areas have a hot and humidclimat ewit ha hig hannua lrainfal l sum (inplace smor etha n200 0mm ) and ashor t dry season.

Groundwaie' Latente Soils predominating or forming an imporlant associate

ISNLCS unii s FcJ.Fe;Fi;Qg1 ;Qg2; Z1; Edel. S3 Pe13 and L

Phnthic Podzolic Soils predominating or lorming an important associate, locally with Groundwater Laterile Soils and/or Concretionary Soils included

(SNLCS units Ppd;Pp e ;Pd1 2; Pol5 ;Q1 0; Ws4 and La6)

stone Ipetroplinthite)

Undifferentiated Concretionary Soils predominating (SNLCS units Ld and Lde>

Undifferentiated Concretionary Soils occurring as important associate (SNLCS units La3.Lld4.9.1l;Lrd2 and Q3I

Fig. 1. Occurrence of plinthite and ironstone soils in Brasil. Source: Sombroek& Camargo ,1983 . Plinthosols 141

Areas with 'ironstone' (which develops upon repeatedwettin g and drying ofplinthite )ar efa rmor eextensiv etha nthos ewit hplinthit eo rPlintho ­ sols. The reason is that, once formed, ironstone is highly resistant to weathering. Ironstone materials are oftenrelic s fromperiod swit h adif ­ ferentclimat e thana tpresent .Figur e 1show sth eoccurrenc e ofplinthit e and ironstone inBrasil ;bot hmaterial s areparticularl ywidesprea d inth e Amazonregion .

GENESIS OF PLINTHOSOLS

river

WAV.-.»--'.) . plinthite Z&&ZÏÏ indurated ironstone lowerrive rleve l — •— upper river level (afterSivarajasingha me tal. ,1962 )

Fig. 2.Fou rphysiographicall ydistinc tlandscap eposition swher eplinthit e and ironstoneoccur . A: indurated ironstone (massive ironpa n or gravel)cappin g anol d erosionsurface . B: plinthite and ironstone (gravel andboulders ) ina colluvia l footslope (subject to iron-richwate r seepage). C:plinthit e insoil s ofa lowleve lplai n (river terrace)wit h periods ofhig h groundwater. D: along thebank s ofriver swher eplinthit ebecome s exposed andharden s toironstone .

The formation ofplinthit e

Plinthite isa niron-rich ,humus-poo rmixtur eo fcla yan dquartz .It s formation involves the followingprocesses : 142Plinthosol s

(1)accumulatio n of sesquioxldes, through: (a)Relativ e accumulation as aconsequenc e of the removal of silica andbase sb y ferralitization (seeunde r Ferralsols), and/or (b)Absolut e accumulation through enrichmentwit h sesquioxides from outside (vertical or lateral,se eFigur e2) . (2)segregatio n of ironmottles , causedb y alternating reduction and oxidation. Intime s ofwate r saturation,muc h of the iron is inth e ferrousform ,ha sa hig hmobilit yan di seasil yredistributed .Whe nth e water table falls, this ironprecipitate s as ferric oxide thatwil l not, oronl ypartially , redissolve inth enex twe t season.

Thehardenin g ofplinthit e to ironstone

Plinthite forms in low andwe t areas; if the land becomes drier, e.g. because of a change inbas e level or a change in climate, the plinthite hardens to ironstone.Thi shardenin g involves the followingprocesses : (1)crystallizatio n of amorphous ironcompound s into continuousaggre ­ gates ornetwork s of ironoxid eminerals ,especiall y goethite. (2)dehydratio n ofgoethit e (FeOOH)t ohematit e (Fe203)an d of gibbsite (A1203.3H20), i fpresent , toboehmit e (A1203.H20).

change of climate change of base level

|ground-and river water I I 11l I I I I I edaphic savannah [levels 9p swamp forest ironstone

Fig. 3.Inversio no frelie f ina nerodin g landscape.Hardenin g of exposed plinthite produces a protective (petroferric) shield against further erosion. Source:Buring h& Sombroek ,1971 . Plinthosols 143

Hardening of plinthite is often initiated by removal of the vegetation, especially forest, as this induces erosion and exposure of plinthite to the air. Ironstone materials occur in many tropical soils, either in a 'skeletic' (concretionary) or a 'petroferric' (continuous pan)form . They occuri nth erai nfores tzon ean di nth esavannah sbu tar eespeciall yabun ­ dant in the transition zone: thewe t climate of the rain forest promotes the formation of plinthite, whereas the drier savannah environment is inducive toit shardening . Inplaces ,ironston ecap s forma shiel d against erosion. The result is an inversion of the original relief; parts that initially were the lowest of the landscapebecom e thehighes t (see Figure 3).

Themai ndiagnosti cfeatur eo fPlinthosol s isth epresenc eo fa tleas t 25percen t (byvolume )plinthite .No t every redmottle d clayhorizo ncon ­ tainsplinthite . It isno t always easy to distinguish between red rust mottles,plinthit e and ironstone gravelbecaus e they grade into each other. Important field criteria for identificationo f redmottle s that qualify asplinthit e are: (1)th e redmottle s are firm tover y firmwhe nmois t andhar d tover y hardwhe ndry ; (2)the y canb e cutwit h aknif ebu t onlywit h considerable difficulty; (3)the yhav e sharpboundaries ; (4)the yhardl y stain the fingerswhe n rubbed; (5)the y dono t slake inwater .

Themai n criterion is,o fcourse ,th e irreversible hardening to ironstone uponrepeate dwettin gan ddrying ,bu t thiscanno talway sb eestablishe d in the field.

Ironstone (alsoreferre d toa s 'laterite', 'murram', 'ferricrete',o r 'petroplinthite') can be divided, on basis of its morphology, in the following types: (1)Massiv e ironpan s (thepetroferri c phase;se eAnne x 3), either: (a)residual : formedb yhardenin g ofplinthit ewher e the redmottle s are interconnected; the lighter colouredpart s that dono tharde n form irregular cavities ('vesicular' laterite); insom e instances the original rock structurema y stillb evisibl e ('ferruginized rock'), or (b)recemented : colluvial ironstone gravel,stone s andboulder s that are cemented together. (2)Discontinuou s ironstone (the skeletic phase; seeAnne x 3), either: (a)residua l gravels formedb yhardenin g ofplinthit e where there d mottlesmak e up 40percen t of the soilvolum e ormore ,an d are not connected. The gravels are round concretions ('pisolithes'o r 'pea iron')o r irregularnodules ,o r (b)colluvia l gravel,stone s andboulder s formedb y disintegration of amassiv e ironpan . 144 Plinthosols

Thepetroferri c layercontain s littleo rn oorgani cmatte ran doccur sa t shallow depth. Figure 4 shows an ironstone 'duricrust' (Fr. 'cuirasse') developedo na decapitate d (former)Plinthosol .Skeleti clayer sma yconsis t for aconsiderabl e part of ironstone (up to 80percent) .

Mineralopical characteristics

Plinthitean dironston ehav ea hig hconten to fsesquioxides .Fre eiro n is present as the oxide minerals lepidocrocite (FeOOH), goethite (FeOOH) orhematit e (Fe,03); fre e aluminium occurs as gibbsite (A120,.3H20)and/o r boehmite (A120,.H20). Old ironstone crusts contain more hematite and boehmite and less hydrated Fe- and Al-oxides than plinthite. Free silica ispresen t as quartz,inherite d from theparen tmaterial .Easil yweather - able primary minerals have disappeared. The dominant clay mineral is kaolinite.

Fig. 4. Petroferric ironstone 'duricrust' in Ivory Coast. Photograph by courtesy of ISRIC,Wageningen .

Hvdrological characteristics

Plinthosols are indigenous to regionswit h adistinc t annualprecipita ­ tionsurplu sove revaporation . Percolating rainma ycaus eeluviatio nsymp ­ tomssuc ha sa nalbi cE-horizon ,particularl ywher e thesurfac ehorizo ni s strongly humic. Plinthosols with gleyic or stagnic soil properties are commonphenomen a inbotto m lands. Plinthosols 145

Physical characteristics

Theplinthit e layer isdens e and obstructs the flow ofwate r aswel l as deeppenetratio n ofroots .Th ecapacit y toharde n isa potentia l dangero f all Plinthosols.Th e specific density of ironstone ranges from 2.5 to 3.6 Mg/m and increaseswit h increasing ironcontent .

Chemical characteristics

All Plinthosols have ahig h content of ironand/o r aluminium,wit hpro ­ portions varying from more than 80 percent iron oxides with little alu­ minium, to about 40percen t of each.Mos t Plinthosols have a low CEC and a lowbas e saturationbu t there areexceptions .

MANAGEMENTAN DUS E OF PLINTHOSOLS

Plinthosols come with serious management problems. Waterlogging and lownatura l fertility are their main limitations. If the plinthite layer hardens, e.g. because of deep drainage or erosion, ironstone will limit thepossibilitie s for rootgrowt h and lower thewate r storage capacity of the soil. Most petroferric soils are unsuitable for arable farming; they areuse dfo rextensiv egrazin gan dfirewoo dproduction .Skeleti csoil swit h highbu tvariabl e contents ofpe a ironar eplante d to food crops and tree crops (e.g.coco a inWes tAfrica )bu t thecrop s sufferfro mdrough t inth e dryseason .

Civil engineers have a different appreciation of ironstone and plinthite than agronomists have. To them, plinthite is avaluabl e material tomak e bricks (massive ironstone canb e cut intobuildin gblock s too).Ironston e gravel canb e used for foundations and as a surfacing material for roads andai rfields .I nsom e instances iti sa valuabl e oreo firon ,aluminium , manganese or titanium. 146 Notes

NOTES Ferralsols 147

FERRALSOLS

Soilshavin g aferrali c B-horizon.

Key to Ferralsol (FR)Soi lUnit s

Ferralsols having plinthite3withi n 125 cm of thesurface . Plinthic Ferralsols (FRp)

Other Ferralsols having geric properties in at least some part of the ferralic B-horizon within 125 cm of thesurface . Geric Ferralsols (FRg)

Other Ferralsols which are strongly humic ,havin g an umbric A-horizon, or amolli c A-horizon and abas e saturation (by IMNH,OA c at pH 7.0) of less than 50percen t ina tleas t apar t of theB-horizo n within 100c m of the surface. Humic Ferralsols (FRu)

OtherFerralsol shavin g are dt odusk yre dB-horizo n (rubbed soilha shue s redder than5Y Rwit h amois tvalu e ofles s than4 an da dr yvalu eno tmor e thanon euni thighe r than themois t value). Rhodic Ferralsols (FRr)

OtherFerralsol shavin ga yello w topal eyello wB-horizo n (rubbedsoi lha s hues of 7.5Y R or yellower with a moist value of 4 or more and a moist chroma of 5o r more). Xanthic Ferralsols (FRx)

OtherFerralsols . Haplic Ferralsols (FRh)

Diagnostic horizon; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r full definition.

SUMMARYDESCRIPTIO N OF FERRALSOLS

Connotation: red andyello w tropical soilswit h ahig h content ofsesqui - oxides; from L. ferrum. ironan d alumen. alum.

Parent material:pre-weathered ,mostl y transported materials of oldage , derived from a wide variety of rocks; mor e often in weathering material frombasi c rock than insiliceou smaterial .

Environment: level toundulatin g stable land surfaces of Pleistocene age orolder ; less commonly inyounge r land oneasil yweatherabl e rocks.Per - humid or humid tropical climates; small occurrences elsewhere are relics from past eras with a wetter climate than today. Tropical rain forest, semi-deciduous forest and savannah. 148 Ferralsols

Profile development: ABC profiles. Deep and intensive weathering has resulted in a residual concentration of resistant primary minerals, and the formation of kaolinitic clays and iron and aluminium oxides and hydroxides. This mineralogy and the low pH account for the stablemicro - structure (pseudo-sand) and yellowish (goethite) or reddish (hematite) colours ofFerralsols .

Use: Ferralsols have good physical properties but are chemically poor. Their lownatura l fertility,virtua l absence ofweatherabl e minerals,an d verylo wcatio nretentio ncapacit yar eseriou slimitations .Man yFerralsol s are (still)use d for shifting cultivation. Liming and full fertilization are required for sustainable sedentary agriculture.

REGIONAL DISTRIBUTION OF FERRALSOLS

There are close to one billion hectares of Ferralsols worldwide. Nearly all Ferralsols are situated in the tropics where they occupy some 20percen t of the land surface with major occurrences on the continental shields of South America and central Africa. These regions have not been affected by intensive folding or glacial action during recent geological periods. Outside the continental shields, Ferralsols are restricted to regions with easily weatherable basic rock and aho t and humid climate, e.g. in southeast Asia and on some Pacific islands. The main Ferralsol areas are shown inFigur e 1.

Fig. 1. Ferralsolsworldwide . Ferralsols 149

GENESIS OF FERRALSOLS

Wateraffect sprimar ymineral sthoug hth eprocesse so f 'hydration'an d 'hydrolysis'.Hydratio n iswate r absorptionb y solidparticles . Inhydro ­ lysis,H +-ionspenetrat e theprimar y silicate structures ofmineral s such as feldspars which then release cations (K,Na , Ca,Mg) .A s the hydrogen ion ismuc h smaller thanan y of the cations itreplaces ,th e structure of themineral s isweakened . This facilitates dissolution of Si and Al from the claylattices . 'Ferralitization'i shydrolysi s ina nadvance dstage .A combinatio no fslo w release and leaching of Si,K ,Na ,M g and Ca keeps the concentrations of these ions in the soil solution low. If the soil temperature ishig h and percolation intense, all weatherable primary minerals will ultimately be removedfro mth esoi lmass .Les ssolubl ecompound s sucha siro nan dalumin ­ ium oxides and hydroxides and coarse quartz grains, remain behind. The process of ferralitization (or 'desilication' as it is also called) is accelerated by the following conditions: (1)A relatively lowp H and lowconcentration s of dissolved weathering products inth e soil solutionpromot e desilication and enrichment of the soilwit h (residual)F ean dAl . Highproductio n ofCO ,(fro m respirationb y roots and soil organisms feeding onorgani c matter) andhig hpercolatio n ratesdepres s thep Han dlowe r theconcentration s ofweatherin gproducts . (2)Geomorphi c stability overprolonge d periods of time isa precondi ­ tion. Ferralitization isa ver y slowprocess ,eve n inth e tropics wherehig h temperatures increase reaction rates and solubility limits. In temperate regions,ol derosio n surfaces are rarebecaus e of the influence ofrecen t glacialprocesse s on landscape formation. (3)Ferralitizatio n proceeds most strongly inbasi c parent materials which contain relatively much ironan daluminiu m ineasil yweather - ableminerals ,an d little silica.O nweathering , ironremain s behind incrystallin e compounds,mainl y goethite (FeOOH)an dhematit e (FeJD,), an d aluminium ingibbsit e (AL(OH),). I n soils from acid rock, ferralitizationproceed s much slower.Ther e are less easily weatherable minerals andmor e quartz.Althoug h most silica disappears through leaching (hence 'desilication'), the silica content ofth e soil solution remainshighe r than insoil s frombasi c rock. This silica combineswit h aluminium to the 1:1 clayminera l kaolinite (kaolinitization). Gibbsite isnormall y absent. Impeded internal drainagema y enhance theproces sbecaus e silica insolutio n isno t removed quickly enough (seeTabl e1) .

TABLE 1. Theoccurrenc e ofgibbsit ean dkaolinit e instrongl yweathere d soilsunde r various drainage conditions.

PARENT INTERNAL DRAINAGE MATERIAL very good good moderate poor

Mafic rock gibbsite gibbsite kaolinite 2:1 clays Felsic rock gibbsite kaolinite kaolinite kaolinite 150 Ferralsols

Uponweathering ,ferrihydrit e (Fe(OH),;se eals oth echapte ro nAndosols ) forms ifth eiro nconcentratio n ishigh .Hematite ,th eminera lwhic hgive s many tropical soils theirbrigh tre dcolour ,form sou to fferrihydrit e if: (1)th e iron concentration ishigh ,an d (2)th eorgani c matter content islo w (Fe-humus complexes inactivateFe) , and (3)th e temperature ishig h (accelerates dehydration of ferrihydrite and decomposition of organic matter), and + (4)th e soil-pH ishighe r than4. 0 (else Fe(OH)2 -monomers are formed). Goethite. more orange in colour than the bright red hematite, is formed when one ormor e of the above conditions areno t (fully)met .

CHARACTERISTICS OF FERRALSOLS

The essential diagnostic feature of Ferralsols is the presence of a ferralic B-horizon. A typicalhorizo n sequence ina Ferralso lwoul dbe :

0- 30 cm A ochric A-horizon 30- 60 cm AB transitionalhorizon ,A-characteristic s dominant 60- 90 cm BA transitional horizon,B-characteristic s dominant 90-210 cm B ferralic B-horizon 210- 240 cm BC transitional horizon to C-horizon

Characteristic morphological featuresare : (1)a dee p solum,usuall y severalmeter s thick,ove rweatherin g rock; (2)diffus e or gradualhorizo nboundaries ,unles s astonelin eoccurs . (3)a hig h ironconten t inth e ferralic B-horizon, togetherwit h the good internal drainage responsible fordistinc t red (hematite)o ryello w (goethite)matri x colours,usuall ywithou tmottles ; (4)a wel l developed microstructure; kaolinite with anegativ e surface charge combineswit h sesquioxideswit h apositiv e surface charge to form strongmicro-aggregate s of silt (pseudo-silt) or sand (pseudo- sand)size .Soil swit h acla y content of 60percen t ormor e 'feel loamy' inth e field andhav e the samemechanica lpropertie s asmediu m or even light texturedsoils ; (5)a wea kmacro-structure ; absence ofwel l developedblock y or prismatic structures;ver y fine granules that aremor e or less coherent ina porous, friable soilmass .

Mineralogical characteristics

Ferralsols arestrongl yweathere dan dtherefor echaracterize db yprimar y and secondary minerals of great stability. Easily weatherable primary minerals such as glasses and ferro-magnesian minerals and even the more resistant feldspars and micas have disappeared completely. Quartz is the mainprimar yminera l (iforiginall y present inth eparen t rock). The clay fractionconsist smainl y ofkaolinite ,goethite ,hematit e and gibbsite in varying amounts and reflects the kind of parent rock and the drainage conditions (seeTabl e1) . Ferralsols 151

Hvdrolopical characteristics

A consequence of the advanced weathering of Ferralsols is that most of these soils have a high clay content and strong water retention atper ­ manent wilting point. The formation of micro-aggregates reduces their moisture storage at field capacity which explains the rather limited 'available' water holding capacity: some 10m m of available water per 10 cmsoi ldept h isa rul eo fthumb .Mos tFerralsol sar esensitiv e todrought , particularly those inelevate dpositions .

Physical characteristics

Stable micro-aggregates and many biopores account for the excellent porosity, good permeability and high infiltration rates of Ferralsols. Clayey soils with ahig h content of (positively charged) iron oxides and (negatively charged)kaolinite ,hav e aparticularl y stable soil structure due tobondin g of opposite elements. Ferralsols with a low ironcontent , low pH, high Al-saturation and low organic matter content, as occur in Surinam and Brasil (Xanthic Ferralsols), have less stable structures, especially thesand yones .A slon ga ssuc hsoil sremai nunde rforest ,ther e isn oproblem ,bu t surface sealing and compactionbecom e serious problems once such soils are taken into cultivation. Thestron gcohesio no fth e (micro-)aggregates andth erapi d reflocculation of suspended particles complicate the determination of theparticl e size distribution of Ferralsol material. The percentage clay which is found after removal of the ironan d addition ofa peptisin g agent iscalle d the 'total clay' content.Th e amount ofcla y that isdetermine db y shaking an aliquoto fsoi lwit hdistille dwate r (withoutremovin g theiro nan daddin g dispersion agents) is called the 'natural clay'. In ferralic B-horizons, thenatura lcla yconten ti sver ylow ,whic htallie swit hth ehig hstabilit y of the aggregates and the absence of claymovement . The ratio of natural clayove rtota lcla yma yb euse da sa ninde xo fstability ; iti son eo fth e criteria by which Ferralsols are distinguished from the (less stable) Acrisols.

Chemical characteristics

Ferralsols are chemically poor soilswit h a low ion exchange capacity. The exchange ofcation s and anionsbetwee n the solid and liquidphase s in soils isconditione d by the type and quantity of theclay , the oxides and the organic matter. The exchange capacity is composed of a permanent and avariabl e component:

The 'permanent charge' is caused by isomorphic substitution of e.g. Si byA l ,o r Al3+ byMg 2+, in the crystal lattice of the clayminerals .Th e resulting negative charge is independent of soilp H or ion concentrations ofth esoi l solution.Kaolinit e isth emai ncla yminera l inFerralsol s and has only aver y smallpermanen t charge.

The 'variable charge' isdu eto : (1)dissociatio n ofH +-ionsfro mmolecule s located at theperifer y ofth e exchange complex (creatingnegativ e sites), or 152 Ferralsols

(2)protonatio nwhic h results ina positiv e charge. Protons (H+)may , for example,b e releasedb y acid groups at the edges of clayparticles , orb y carboxylic orphenoli c groups inth eorgani c matter,o rb y aluminium and ironhydroxides .

Dissociationo fH +-ionsi sstronges ta thig hconcentration s ofOH "i nth e soilsolutio nan di stherefor ecalle dth e 'pHdependent 'par to fth ecatio n exchange capacity (CEC).

Thecatio nexchang ecapacit y ofa ferrali cB-horizo nmay ,b ydefinition , not be in excess of 16 cmol(+)/kg of clay; the CEC is determined ina I M NH^OAcsolution ,buffere d top H 7.A Ferralso lwit h5 0percen t clayi nit s B-horizon contains less than 8cmol(+ )cation spe rk g atp H 7.0.However , the field-pH of Ferralsols isnormall y around 5.Th e net negative charge ofth eexchang ecomple xi sneutralize db yexchangeabl ebase s (Na+,K +,C a , Mg2+)plu s exchangeable acidity (Al3+ +H +). Therefore ,th e 'effective CEC, or ECEC, is the sum of thebase s plus the exchangeable acidity. The ECEC represents the cation exchange capacity at field conditions and thus of greatpractica l importance.

NOTETHA T theECE Co fFerralsol s ismuc h lower thanth eCE Can dth eactua l cation adsorption is often not higher than a mere 3 or 4 cmol(+) per kg soil.

Protonation of hydroxylic groups at low pH-values may boost the anion exchange capacity (AEC)o f the soil to the extent that theAE C equals or exceeds the CEC. This canb e detected by comparing the pH of two samples ofth esam esoil ,on ei nsuspensio ni nH- 0an dth eothe ri nI MKCl .Normall y (with ane tnegativ e charge), thepH(KCl ) is lower thanpH(H-O ), bu twit h ane tpositiv e charge therevers e istrue .

Inpublication so nth eexchang epropertie so fstrongl yweathere dtropica l soils, the following terminology isused : -Th ep Hvalu e atwhic h theAE C fully compensates the CEC (permanent plus variable charges)i scalle d the 'point of zerone t charge' (PZNC). -Th e differencebetwee npH(KCl )an dpH(H20 ) iscalle d 'deltapH' .

Figure 2 presents in a schematic way the exchange characteristics of strongly weathered tropical soils atdifferen tp Hlevels .

Biological characteristics

The gradualhorizo nboundarie s inFerralso lprofile s must,accordin g to some, at least partly be attributed to termites. These animals increase the depth of the solum by destroying remnants of stratification or rock structure.Thei rnests ,tunnel san dventilatio nshaft sincreas eth eporosi ­ ty andpermeabilit y of the soil.A s termites preferentially move finean d medium sized particles, they leave the coarse sand, gravel and stones behind. This explains the frequent occurrence of a stoneline in or below the ferralic B-horizon; the depth of the stoneline reflects the depth of termite activity. Ferralsols 153

pH

Fig.2 .Schemati c relationbetwee nexchangeabl e aluminium level,AEC ,CEC , net surface charge and thepH(H 20)o f the soil.

MANAGEMENTAN DUS E OF FERRALSOLS

Ferralsolshav e goodphysica l properties andpoo r chemicalproperti ­ es. Their great depth, high permeability and stable microstructure make them less susceptible toerosio n thanman y other soils,wit h exception of theshallo wan dth esand ytypes .Al lFerralsol shav ea friabl e consistence undermos t conditions and are easy towork .The y arewel l drainedbu tma y in times be droughty because of their low 'available' water storage capacity. The chemical fertility of Ferralsols is poor;weatherabl e minerals are absentan dcatio nretentio nb yth eminera lsoi lfractio ni slow .I nFerral ­ sols under a natural vegetation, the bulk of all 'available' plant nutrients (ando f all livingplan t roots)i sconcentrate d inth euppe r 10 to 50 cm of the soil because elements that are takenu p by the roots are eventuallyreturne dt oth esurfac esoi lwit hfallin gleave san dothe rplan t debris. If this process of 'nutrient cycling' is interrupted, e.g. by introductiono flo winpu tsedentar ysubsistanc efarming ,th eroo tzon ewil l rapidly become depleted of plant nutrients. Maintenance of the organic matter content by manuring, mulching or adequate fallow periods and prevention of surface soil erosion are importantmanagemen t requirements.

A special problem with Ferralsols (and other soilswit h ahig h content of sesquioxides) is the strong inactivation of phosporus. Nitrogen and potassiumar eofte ndeficien ttoo ,jus ta sth esecondar ynutrient scalcium , magnesium andsulphu r anda scor eo fmicronutrients .Eve nsilic adeficien ­ cyi spossibl e ifsilic ademandin gcrop s (e.g.grasses )ar egrown .Element s such asmanganes e and zinc,whic h arever y soluble at lowpH ,migh t reach 154 Ferralsols toxic levels but theyma y alsobecom e deficient on account of their high susceptibility toleaching .

The easily upset ion balance, high losses of cations through leaching (low CEC),an d high phosphorus fixation complicate liming and fertilizer application.

Liming isa mean s torais e thep Ho fth eroote d surface soil. Thiscombat s aluminium toxicityan draise sth eCEC .O nth eothe rhand ,limin glower sth e AECwhic hmigh t lead tocollaps e of structure elements and slaking at the soilsurface .Frequen tapplicatio no fsmal ldose so flim eo rbasi c slagi s therefore preferable over onemassiv edose .

Fertilizer selection and themode/timin g of fertilizer applicationdeter ­ mine toa grea t extent the success of fertilizerus e onFerralsols . slow-release (rock)phosphate ,fo rinstance ,i sa popula rphosphoru ssourc e inFerralso lareas ;i ti sapplie da ta rat eo fsevera lton spe rhectar ean d eliminates phosphorus deficiency for anumbe r ofyears .Alternatively , it couldb eprofitabl e tous e themuc hmor esolubl e (Triple)Supe rPhosphate , applied inmuc hsmalle rquantitie san dplace di nth edirec tvicinit yo fth e roots.

Fig. 3. Xanthic Ferralsols under equatorial forest in the Amazon basin. Photograph by courtesy of theConselh oNaciona l deGeografia ,Brasil . Ferralsols 155

Ferralsols are grown to a variety of annual and perennial crops,by - sedentary subsistence farmers and shifting cultivators. Grazing is also widely practiced and considerable areas are not used for agriculture at all. The good physical conditions and the often level topography would encouragemor e intensive formso flan dus e ifth eproblem swit h respectt o the poor chemical soil properties could be overcome. Between Ferralsols, the Rhodic Ferralsols (basic parent material,hig h clay content) and the HumicFerralsol smak eth ebes tarabl eland .Th ePlinthi c (plinthite),Geri c (very low CEC) and Xanthic Ferralsols (especially the sandy ones; see Figure 3) have severe limitations. Haplic Ferralsols take up an inter­ mediate position. 156 Notes

MOTES Nitisols 157

NITISOLS

Soilshavin g an argic B-horizon showing acla y distribution which does not show a relative decrease from its maximum of more than 20 percent within15 0c mo fth esurface ;showin ggradua lt odiffus ehorizo nboundarie s between A- and B-horizons; having nitica properties in some subhorizon within12 5c mo fth esurface ;lackin gth etonguin g whichi sdiagnosti c for Podzoluvisols;lackin gferri c and/orvertic 3properties ;lackin gplinthit e within 125 cm of thesurface .

Key toNitiso l (NT)Soi lUnit s

Nitisols which are strongly humic, having an umbric A-horizon, or a mollic A-horizon and abas e saturation (by IMNH^OA c atp H 7.0) of less than 50percen t ina t least apar t of theB-horizo n within 125 cm of the surface. Humic Nitisols (NTu)

Other Nitisols having a red to dusky red argic B-horizon (rubbed soil having hues redder than 5YR with amois t value of less than 4 and a dry value notmor e thanon euni thighe r thanth emois t value). Rhodic Nitisols (NTr)

OtherNitisols . Haplic Nitisols (NTh)

Diagnostic horizon; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OFNITISOL S

Connotation:deep ,re dtropica lsoil swit hcla yilluviatio nan dshin yped s; fromL .nitidus . shiny.

Parentmaterial : finely texturedweatherin g products from intermediate to basic parent rock; inplace s rejuvenatedb y admixtures ofvolcani c ash.

Environment: level tohill y landunde r tropical rainfores t or a savannah vegetation.

Profile development: ABtC profiles. Reddish brown clayey soils with a deeply developed clay illuviationhorizo n ofhig h structural stability.

Use:moderatel y tohighl y productive under awid e range ofcrops . 158Nitisol s

REGIONALDISTRIBUTIO N OFNITISOL S

There are approximately 250 million hectares of Nitisols worldwide. More thanhal fo fal lNitisol s occur inAfrica ,especiall y inKenya ,Zair e and Cameroonbu t they are also represented intropica lAsi a (22percent) , SouthAmeric a (12percent) , Central America (5percent ) and Australia (5 percent). See Figure 1. Nitisols are most extensive in regions with clearly defined wet and dry seasonsbu tthe yoccu rals ounde ra (per)udi cmoistur eregime .Thei roccur ­ rencei ndr yregion se.g . inpart so fAustralia ,i sconsidere da paleo-fea ­ ture.Nitisol sar emos tcommo no nslightl yundulatin gt odissecte dsurface s of early tomiddl e Pleistocene age. In EastAfrica , they occur mainly at altitudes of 1000m ormor e but they arewel l represented at lower alti­ tudes inSout hAmeric a and southeastAsia .

Fig. 1.Nitisol sworldwide .

GENESIS OFNITISOL S

Nitisols are formed in intermediate to basic parent materials under a vegetation type ranging from wooded grassland to (montane) rain forest. The formation ofNitisol s involves the followingprocesses :

(1)ferralitization : intensivehydrolysi s ofweatherabl e minerals combined with leaching of silica andbases ,an d relative accumulation ofkaolinit e and sesquioxides.Th eproces s isth e same as inth e formation ofFerralsol s but iti sstil l ina nearl ystage . Nitisols 159

(2)translocatio n of clay from the surface to the subsoil.Cla y cutans arepresen t onpe d faces and invoid s inth eBt-horizon . Because of thewel l drained andpermeabl e nature of theprofile ,cla y canb e transported down toconsiderabl e depth.Thi sproces s is responsible for the stretched bulge incla y content that is typical ofNitisols . (3)nltidlzatlon : formation of strong angularblock s that fall apart into smaller angular blockswit h shinype d faces ('niticproperties') . The exactproces s is stillunknown . It isprobabl y acombinatio n ofcla y translocation andmicro-swellin g and shrinkingwhic h caused the formation ofpressur efaces . (4)homogenization : (biological pedoturbation)b y termites,ants ,worm s and other soil fauna.Thi sproces s isparticularl y prominent inth e top 100c m soil layerwher e itcause s destruction ofcla y cutans and formationo f crumb and subangularblock y structures and of gradual ordiffus e soilhorizo nboundaries .

CHARACTERISTICS OFNITISOL S

Nitisols are well drained soils with nitic properties and a deeply developed argic B-horizon.A typicalhorizo n sequence is shownbelow :

0-30 cm Ap ochric,molli c orumbri c A-horizon 30-90 cm AB transitionalhorizo nbetwee nA - and B-horizon 90-150 cm Bt argic B-horizon withniti c properties 50-210 cm BC transitionalhorizo n to C-horizon

Thesolu mextend snormall ydow nt oa dept ho f15 0c mo rmor ean d includes an argic B-horizon ofmor e than 100 cm thick. Theboundarie s between the A- and theB-horizon , andbetwee n theB - and theC-horizo n are gradual to diffuse, and there arenormall y transitionalhorizons . Most Nitisols are red soils,wit h Munsell notations in the 2.5YR and 5YR hues; yellower (10YR,7.5YR )o rredde r (10R)soil s are less common. The soil structure is moderate to strong. Crumb and subangular blocky structures are present in the surface soil; there are mainly subangular blocky elements between 50 and 100 cm, and very fine to medium angular blocky elements deeper down. The angular blocks break down upon pressure into ever smaller units thathav e shiny surfaces, especially under moist conditions. The aggregate stability isver y high. During dry periods the soilmateria l shrinks and cracks develop inth e surface soil. Most Nitisols contain more than 35 percent clay throughout, with a gradualincreas eo fth ecla yconten tfro mth etopsoi ldow nt oth eB-horizo n anda ver ygradua ldecrease ,i fany ,o fth ecla ypercentag e inth esubsoil .

Mineralopical characteristics

Kaolinite isth ecommones tcla yminera l inNitisols ,followe db y (meta)- halloysite. Minor quantities of illite, chloritized vermiculite and randomly interstratified clay minerals may also be present, just as hematite, goethite and gibbsite.A n amorphous iron content (Fe203b y acid oxalate extraction) that is at least 5 percent of the free iron content (Fe20,b y dithionite-citrate extraction) istypical . 160Nitisol s

Themineralogica lcompositio no fth esan dfractio ndepend s strongly onth e nature of the parent material. Although weathering-resistant minerals (notably quartz)predominate ,mino r quantities ofmor e easily weatherable minerals (feldspars,volcani c glass,apatite , amphiboles)ma y be present and show thatNitisol s are less stronglyweathere d thanFerralsols .

Physical characteristics

Nitisolsar ever yporou san dhav ea hig hmoistur estorag ecapacity .Thei r permeability is rapid tomoderate , even in the deeper subsoil.A Nitisol cannormall y be tilled within 24hour s after itwa s moistened, without a detrimental effect on the soilstructure .

Chemical characteristics

The cation exchange capacity ofNitisol s ishig h ifcompare d to thato f othertropica lsoil ssuc ha sFerralsols ,Lixisol sandAcrisols .Th ereason s are: (1)Althoug h theCE C of thecla y (by IMNH 40Ac atp H 7.0) isofte n less than 16 cmol(+)/kg, thecla y content ishig h (more than 35percen t andno t seldommor e than6 0percent) ; and (2)th econtributio n of theorgani c matter to theCE C isconsiderable , especially ifa molli c orumbri cA-horizo n ispresent . Thebas e saturationvarie s from less than 10 tomor e than 90percent .Th e pHusuall y liesbetwee n 5.0 and 6.5; P-fixationma yb e aproblem .

Biological characteristics

Faunal activity is to some extent accountable for the typical gradual horizonboundaries .Termite shomogeniz e thesoil ;volcani cglas sdeposite d onth e (present)surfac ewa s found ata dept h of7 m inNitisol s inKeny a (Wielemaker, 1984).

MANAGEMENTAN DUS EO FNITISOL S

Nitisols are among the most productive soils of the humid tropics. Their high porosity and deep solum allow deep rooting. This, and their stablesoi lstructure ,make sNitisol sles ssusceptibl et oerosio ntha nman y othersoils .Thei rinterna ldrainage ,wate rholdin gcapacit yan dworkabili ­ tyar egood .Thei rchemica lfertilit ycompare sfavourabl yt oothe rtropica l soils because of their moderate CEC, the relatively high organic matter content and thepresenc e of (some)weatherabl eminerals . Nitisols are intensively used forplantatio n crops such as cocoa, coffee and pineapple, and for food crop production. They respond well toferti ­ lizer (N,P) applications. Erosion control measures help to conserve the organic matter content of the A-horizon which contributes much to the productive capacity ofNitisols . Acrisols 161

ACRISOLS

Soilshavin g anargi c B-horizonwhic hha sa catio nexchang e capacity of less than 24 cmol(+)/kg clay and a base saturation (by IM NH^OAc at pH 7.0) of less than 50 percent in at least some part of the B-horizon within 125 cm of the surface; lacking the E-horizon abruptly overlying a slowly permeable horizon, the distribution pattern of the clay and the tonguing which are diagnostic for Planosols, Nitisols and Podzoluvisols respectively.

Key toAcriso l (AC)Soi lUnit s

Acrisolshavin gplinthit e within 125c m of thesurface . Plinthic Acrisols (ACp)

OtherAcrisol s showing gleyicapropertie s within 100c m of thesurface . GleyicAcrisol s (ACg)

Other Acrisols which are strongly humic ,havin g an umbric or amolli c A-horizon. Humic Acrisols (ACu)

OtherAcrisol s showing ferric3properties . Ferric Acrisols (ACf)

OtherAcrisols . HaplicAcrisol s (ACh)

Diagnostic horizon; seeAnne x 1fo r fulldefinition . Diagnosticproperty ; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OFACRISOL S

Connotation:strongl yweathere daci dsoil swit ha lo wbas esaturation ;fro m L. acer.ver y acid.

Parentmaterial :mainl y silica-richweatherin gmaterials .

Environment:mos t common in old land surfaces with ahill y or undulating topography, inwe t tropical andmonsoona lclimates .Ligh t tropical (rain) forest isth enatura lvegetatio ntype .

Profile development: mostly A(E)BtC profiles. Variations among Acrisols are mainly connected with variations in terrain conditions (drainage, seepage). The shallow A-horizon with dark, raw and acid organic matter grades into a yellowish E-horizon. The Bt-horizon has stronger or more reddish colours than the overlying E-horizon. 162Acrlsol s

Use: prolongedweatherin g and advanced soil formationhav e led to adomi ­ nance of low activity clays and a general paucity of plant nutrients. Aluminium toxicity, strongphosphoru s sorption, slaking/crusting andhig h sensitivityt oerosio npos esever elimitation st opermanen tcropping .Larg e areas ofAcrisol s areuse d for subsistence farming,partl y under shifting cultivation. These soils are not very productive but canb e grown toun ­ demanding, acidity-tolerantcrop s such ascashe wnu t andpineapple .

REGIONALDISTRIBUTIO N OFACRISOL S

Acrisols and Alisols together cover some 800 million hectares, situated almost exclusively in the equatorial tropics. See Figure 1. As Alisolswer e only recently introduced asa Majo r SoilGrouping , the exact division of theAcrisol-Aliso l area isno t yet clear; it is assumed that moretha nhal fo fth etota lare aconsist so fAcrisols ,wit hmajo roccurren ­ ces onold ,deepl yweathere d land surfaces insoutheas tAsia ,wes tAfric a and thecentra l part of SouthAmerica .

Fig. 1.Area swit hAcrisol s andAlisols .

GENESIS OFACRISOL S

Acrisols are characterized by thepresenc e of an argic B-horizon, a dominance of stable lowactivit y clays and agenera lpaucit y ofbases . For a description of the processes of clay dispersion, clay transport andcla yaccumulatio n ina Bt-horizon ,referenc e ismad e toth echapte r on Luvisols.I tmus tb ementione dhere ,however ,tha tsom eauthor sdismis sal l clay illuviationhorizon s inhighl yweathere d soils inth ewe t tropics as relics from adistan tpast . Acrisols 163

The process of ferralitization by which sesquioxides accumulate in the profile, isdescribe d insom e detail inth echapte r onFerralsols .Subse ­ quent redistribution of ironcompound sb y cheluviation/chilluviation (see under Podzols)ha sno tseldo mresulte d ina colou rdifferentiatio ndirect ­ lybelo w anA(h)-horizo nwher e aneluviatio n layerwit hyellowis h colours lies on top of a more reddish coloured Bst-horizon (the 'Red-Yellow Podzolics' of southeast Asia). In situations with periodic waterlogging, theeffluen t percolationwate r canb eblac k as inPodzols . Acrisols aremor e stronglyweathere d thanAlisols ,hav eles sprimar ymine ­ rals and astron g dominance ofwel l crystallized 1:1 clays.

CHARACTERISTICS OFACRISOL S

Most Acrisols have a thin, brown, ochric A-horizon; darker colours occurwher e mineralization ofsoi lorgani cmatte r ishindere db y (period­ ic)waterlogging , augmented perhaps by oligotrophy. (Amolli c A-horizon, though separately mentioned in the definition of Humic Soil Units, is atypical.) Most Acrisols have bright red and yellow subsurface colours. However,gleyi c soilpropertie s and/orplinthit ear ecommo n inAcrisol s in low terrain positions. The structure of the surface soil is weak; the individual elements collapse under the impact of heavy tropical rain showers,particularl ywher e theorgani cmatte rconten to fth eA-horizo ni s lowand/o r theE-horizo nha sbecom eexpose db yerosio n (seeFigur e 2). The structure of theBt-horizo n ismor estable .

Fig. 2. Severe erosion of (denudated)Acrisol s under the impact ofrain . 164Acrisol s

Mineralogical characteristics

Acrisolshav e littleweatherabl emateria l left.Th econtent so fFe- ,Al ­ andTi-oxide sar ecomparabl et othos eo fFerralsol so rslightl ylower ;th e Si02/Al20jrati o is 2 or less. The clay fraction consists mainly of well crystallized kaolinite and somegibbsite .

Hvdrological characteristics

Under protective natural forest, Acrisols have a porous surface soil which permits adequate infiltration of (rain) water. If the forest is cleared, the surface soil will lose its organic matter and slake, with crusting, surface runoff during rain showers and (in sloping positions) devastatingerosio na sa result .Man yAcrisol ssho wsign so fperiodi cwate r saturation, particularly those in depressed areas. There, the A-horizons arever y darkbrow n toblac kwherea smatri x coloursma yb e close towhit e directlybelo w theBt-horizon .

Physical characteristics

Most Acrisols have awea k microstructure and amassiv e macrostructure, particularly where theorgani cmatte r content of the surface soil islow , becauseo fth euneve ndistributio no fsesquioxide s overth esoi lmas swit h low amounts residing in the surface horizon(s). The bonding between sesquioxides (AEC)an dnegativel y charged lowactivit yclay s (CEC)i sles s stable than in Ferralsols. The ratio ofwate r dispersible 'natural clay' over 'totalclay ' (seeunde r Ferralsols) ishighe r than inFerralsols .

Chemical characteristics

Acrisols have poor chemical characteristics; their nutritional limita­ tions include widespread aluminium toxicity and strong P-sorption as in Ferralsols. The pH(H20) is close to 5.5. As Acrisols are very poor soils withlittl ebiologica lactivit yan dn oexpandin gclay ,natura lregeneratio n ofth esurfac e soil,e.g .degrade db ymechanize dagriculture ,i sver yslow .

MANAGEMENTAN DUS EO FACRISOL S

As with other highly weathered tropical soils, preservation of the surface soil with its all-important organic matter is imperative. Mechanical clearing of thenatura l forestb y extraction ofroo tball s and filling of thehole s with surrounding surface soil produces land that is largelysteril ebecaus etoxi clevel so faluminiu m (theforme rsubsoil )kil l offan y seedlings planted outside the filled-inspots . Adapted cropping systems with careful management, including liming and full fertilization,ar erequire d ifsedentar y farming ist ob e takenu po n Acrisols. The commonly used 'slash-and-burn' agriculture ('shifting cultivation')ma y seemprimitiv e at first sightbu t isreall y asophisti - Acrlsols 165 cated type of land use, developed over centuries of try-and-error. If occupation periods are short (one or a fewyear s only)an d followed by a sufficiently longregeneratio nperio d (upt osevera l decades), thissyste m probablymake s thebes tus e ofth e limitedpossibilitie s ofAcrisols . On thewhole , low input farming isno t possible onAcrisols . Undemanding acidity-tolerant crops such as cashew nut and/or pineapple can be grown withsuccess . 166 Notes

NOTES Alisols 167

ALISOLS

Soilshavin g anargi c B-horizonwhic hha s acatio nexchang e capacity equal to or more than 24 cmol(+)/kg clay and a base saturation (by IM NH,OAc at pH 7.0) of less than 50 percent in at least some part of the B-horizon within 125 cm of the surface; lacking the E-horizon abruptly overlying aslowl ypermeabl ehorizon ,th edistributio npatter no fth ecla y and the tonguing3whic h are diagnostic for Planosols,Nitisol s andPodzo - luvisols respectively.

Key toAliso l (AD SoilUnit s

Alisolshavin gplinthit e within 125 cm of thesurface . Plinthic Alisols (ALp)

OtherAlisol s showing gleyic propertieswithi n 100c m of thesurface . Gleyic Alisols (ALg)

OtherAlisol s showing stagnic3propertie s within 50c m of thesurface . Stagnic Alisols (ALs)

Other Alisols which are strongly humic ,havin g an umbric or a mollic A-horizon. Humic Alisols (ALu)

OtherAlisol s showing ferric properties. Ferric Alisols (ALf)

OtherAlisols . Haplic Alisols (ALh)

Diagnostic horizon; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r full definition.

SUMMARYDESCRIPTIO N OFALISOL S

Connotation: acid soils containing high levels of 'free'aluminium ; from L. alumen. alum.

Parent material: given the right conditions, Alisols can form in nearly anyweatherin gmaterial .

Environment: mos t common in old land surfaces with ahill y or undulating topography, in wet tropical and monsoonal climates. Light forest is the naturalvegetatio ntype .

Profile development: mostly ABtC profiles. Variations among Alisols are mainly connected withvariation s inenvironmenta l conditions (irrigation, drainage, seepage). 168Allsol s

Use: thes e weathered soils have a general paucity of macro- and micro- nutrients; free aluminium ispresen t intoxi c quantities.Limin g (tocor ­ rect thep Ho fth esoi lan d todepres s theleve lo ffre eAl )an d fullfer ­ tilization areneede d forpermanen t croppingbu t areno t always economic. Alisols are traditionally used in shifting cultivation or for low volume production of aluminium tolerant crops. They have some potential for the productiono festat e crops (oilpalm) . s instee p landar ebes t left under their naturalvegetatio ncover .

REGIONALDISTRIBUTIO N OFALISOL S

Alisols are exclusive to the tropics where an estimated 100 million hectares are being used for agriculture. The biggest concentrations of Alisolsar efoun di nsoutheas tAsia ,wes tAfrica ,th ecentra lpar to fSout h America and in the southeastern USA. As Alisols were only recently introduced as a separate Major Soil Grouping, their exact regional dis­ tributioni sno tye tclear .A nindicatio no fthei rmai narea so foccurrenc e is given inth e chapter onAcrisols .

GENESIS OFALISOL S

Alisols are characterized by the presence of an argic B-horizon, a mixed clay assemblage that is in a state of transition, high aluminium levels inth e subsoil,an d agenera lpaucit y ofbases . For a description of the processes of clay dispersion, clay transport and clay accumulation ina n illuviationhorizon ,referenc e ismad e toth e chapter on Luvisols. Note that some authors regard all clay illuviation horizons in highly weathered soils in the wet tropics as relics from a distantpast .Th eproces so fferralitizatio nb ywhic hsesquioxide saccumu ­ late in the profile, is described in some detail in the chapter onFer - ralsols. Alisols are comparable with Acrisols but are less strongly weathered, and contain (still) some weatherable primary minerals. The formation of secondarymineral s is less advanced than inAcrisols .

CHARACTERISTICS OFALISOL S

MostAlisol shav ea brow nochri cA-horizon ;darke rcolour soccu runde r a virgin forest vegetation and/or where mineralization of is hindered by (periodic) waterlogging. Stagnic soil properties develop where downwardpercolatio n ishindere db y adens e argic B-horizon and/or the soil isuse d forwetlan d rice cultivation, as inAliso l areas in southeast Asia. Gleyic soil properties and/or plinthite are common in lowposition s inth elandscape .Th esurfac e soilstructur e israthe rweak , particularlywher e theorgani cmatte rconten to fth eA-horizo n islow .Th e structureo fth eBt-horizo ni sclearl ymor establ etha ntha to fth esurfac e horizon(s). Alisols 169

Mineralogical characteristics

Dissociation and transformation of 2:1 clay minerals is in process, resulting inth ereleas e ofconsiderabl e quantities of free aluminium and in a gradually decreasing cation exchange capacity of the clay which becomes more and more kaolinitic. Gibbsite is present in negligible quantities.A sweatherin gi sstil lgoin gon ,th esil tconten ti srelativel y high and the content of sesquioxides has not yet reached its maximum. Therefore,Alisol shav e alowe rAE C thanAcrisols .

Hvdrological characteristics

Alisols with a dense argic B-horizon that are used for wetland rice cultivation, develop pronounced stagnic soil properties ('Anthraquic phase'; see Annex 3).Gleyi c properties and/or plinthite are common in Alisols indepresse d areas and inlowe r footslopepositions .

Physical properties

The presence of swelling and shrinking clays explains the relatively dense,prismati cBt-horizon so fman yAlisols .Thei rhig hsilt-to-cla yrati o andrelativel y lowsesquioxide scontent ,o nth eothe rhand , limit (micro)- structure stability. Another reason for the unstable surface horizons may lie in the lowbio ­ logicalactivit ywhic h isa consequenc e ofth epoo rnutrien t statuso fth e surface soil and thehig h aluminium content of the subsoil. The level of sesquioxidesi nth esoi li s(still )comparativel ylo wand/o rth esesquioxi ­ desar eunevenl ydistribute dove rth eprofile ,a switnesse db ypal ecolour s incombinatio nwit hmottle so rnodules .Therefore ,th epositiv e effectso f sesquioxides on the stability of the soil structure are less than in Acrisols andmuc h lesstha n inFerralsols .Slakin go fth esurfac e soilan d reduced permeability restrict the infiltration of rainwater , reduce the internal drainage of the soil and increase the erosionhazar d in sloping land (see Figure1) .

Chemical characteristics

Where easilyweatherabl e minerals arepresen t ina naggressiv e environ­ ment (low inbases ,lo w inpH) , aluminium isrelease d from decayingmine ­ rals. Th e low permeability and poor internal drainage prevent adequate dischargeo fthi saluminium .A sa result ,a considerabl epar to fth ecatio n exchange capacity of the subsoil is occupied by aluminium ions and many crops onAlisol s suffer fromaluminiu m toxicity.

Alisols have low levels of 'available' nutrients but have still some nutrient reserves. This makes these soils suitable for crops which grow andproduc e over a longerperio d of time (treecrops , forestry). 170Allsol s

MANAGEMENTAN DUS EO FALISOL S

Liming, needed to increase the soil-pH and to decrease the level of freealuminium ,raise sth eCE Can ddepresse s theAEC .O nth eon ehand ,thi s may lower soil structure stability but, on the other hand, liming boosts (theactivit yof )th esoi lfaun awhic hmigh tactuall y improveth estabilit y of the structure. On average,Alisol s are unstable soils and susceptible toerosion .Tre ecrop san dperennia lcrop swhic hminimiz e soildisturbanc e by tillage,ar et ob epreferre d overannua lcrops ,particularl y onslopin g land.Oi lpal m issuccessfull y growni nplantation s onAlisol s inMalaysi a and onSumatra .

ifrC-

^ÄN'*- , i

v. 'iL. i {•M-*mr>' .

Fig. 1. Slaking and aluminium toxicityharme d upland rice on this eroding hillsidewit hAlisol s inCentra lSumatra .Not etha tpreservin g thesurfac e soil isessentia l for (food)cro pproductio n on thesesoils .

Alisolshav ea limite dcapacit yt orecove rfro mchemica lexhaustio n(e.g . inshiftin gcultivation )o rphysica ldegradation .The yar eonl ymarginall y suited for sedentary agriculture.Aluminiu m toxicity at shallow depth can restrict rooting and causewate r stress inth e dry season.Dee p placement of soil amendments (lime, phosphorus fertilizer) proved beneficial in (experiments in)region swit h apronounce d dryseason . Lixisols 171

LIXISOLS

Soilshavin g anargi c B-horizonwhic hha s acatio nexchang e capacity of less than 24cmol(+)/k gcla y at least insom epar t of theB-horizo n anda base saturation (by IMNH.OA c atp H 7.0) of 50percen t ormor e throughout theB-horizon ; lackinga molli c A-horizon; lacking theE-horizo n abruptly overlying aslowl ypermeabl ehorizon ,th edistributio npatter no fth ecla y and the tonguing which are diagnostic for Planosols,Nitisol s andPodzo - luvisols respectively.

Key toLixiso l (LX)Soi lUnit s

Lixisolshavin g plinthite3withi n 125 cm of thesurface . Plinthic Lixisols (LXp)

Other Lixisols showing gleyic propertieswithi n 100c mo f thesurface . Gleyic Lixisols (LXg)

Other Lixisols showing stagnic properties within 50c m of thesurface .

OtherLixisol shavin g analbi c E-horizon. Stagnic Lixisols (LXs)

Other Lixisols showing ferric properties. Albic Lixisols (LXa)

OtherLixisols . Ferric Lixisols (LXf) Haplic Lixisols (LXh)

Diagnostichorizon ; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OFLIXISOL S

Connotation: strongly weathered soils inwhic h clay is washed down from thesurfac esoi lt oa naccumulatio nhorizo na tsom edepth ;fro mL .lixivia , washed outsubstances .

Parentmaterial : thoroughlyweathere d and strongly leached unconsolidated materials;mainl y alluvial and colluvialdeposits .

Environment:fla tt oslopin gland ,predominantl yi nmonsoona lan dsemi-ari d regions.Lixisol sar ewidel ysee na spolygeneti csoil swit hcharacteristic s formedunde r amor ehumi d climate inth epast .

Profile development:matur eABt Cprofiles ; inplaces, th eargi c B-horizon isa t the surface or at shallow depthbecaus e oferosion . 172Llxisol s

Use: Lixisol s have a low nutrient reserve. The unstable (surface) soil structure of Lixisols makes them prone to slaking and to serious erosion inslopin g land; Conserving management isimportant .

REGIONALDISTRIBUTIO N OF LIXISOLS

Lixisols cover vast areas in the tropics and subtropics,notabl y in east-central Brasil, the Indian subcontinent and in west and southeast Africa (Figure 1).Du e to the recent introduction of Lixisols as aMajo r SoilGroupin g inth eFAO-Unesc o Legend, it isno tye tpossibl e toprovid e a reliable figure for their worldwide extent; an estimate of 200 million hectares isprobabl y on theconservativ eside .

Fig. 1.Lixiso l areas worldwide

GENESIS OF LIXISOLS

It is widely accepted that (many) Lixisols started their development under awette r climate thanprevail s today. Strong weathering during the early stage of soil formationcoul db e followedb y chemical enrichment in more recent timeswhe n theclimat eha d changed towards anannua l evapora­ tionsurplus .Lixisol scoul dals ob eenriche db ybase-ric heolia ndeposits , by biological activity (import ofbase s from the subsoil), orb y lateral seepage. The occurrence of fossil plinthite, and/or coarse reddish iron mottleso rindurate d ironnodule si nth esubsurfac e layerso fsom eLixisol s are also indications ofwetnes s inth epast . Many Lixisols have a reddish oryellowis h argic B-horizon as a result of Lixisols 173

'rubéfaction', a process of dehydration of iron compounds in long dry seasons. Themineral s which cause these colours arenormall y goethite (in the subtropics) andhematit e but other iron compounds may be involved as well.

CHARACTERISTICS OF LIXISOLS

Lixisols have normally abrow n ochric A-horizon (not seldom shallow asa resul to ferosion )ove ra brow no rreddis hbrow nargi cB-horizon .Th e subsurface soil may show signs of iron redistribution. The horizon of eluviation of clay and free iron oxides may be sufficiently developed to qualify asa nalbi c E-horizon.

Mineralogical characteristics

Advanced weathering accounts for a low silt-to-clay ratio, a dominance of1: 1 clays (leachingo fSiO, ), an dfo rFe- ,Al-an dTi-oxid econtent stha t arehighe r than those ofth e lessweathere d Luvisols. The Si02/Al20,rati o of Lixisols is lower than 2.0; gibbsite contents are only slightly lower than inmos tFerralsols .

Hydrological characteristics

Lixisols with evidence of periodic water saturation in theuppe r metre of theprofil e occur indepresse d areaswit h shallow groundwater butmor e often because a perched water table forms above the argic B-horizon in periods ofwetness .

Physical characteristics

Lixisolshav e ahighe rp Han dlowe rAE C thanmos tothe rweathere dtropi ­ cal soils. Consequently, the structure stability is lower than, for instance, inAcrisol s and Ferralsols (for an explanation see the chapter on Ferralsols) and slaking and caking of the surface soil is a serious problem. Stable pseudosand and pseudosilt are virtually absent from Lixisols; themoistur e content at lowp Fvalue s ishighe r than inFerral ­ sols orAcrisol swit h the same contents oforgani cmatte r andclay .

Chemical characteristics

Asal lhighl yweathere dsoils ,Lixisol shav eonl ylo wlevel so favailabl e nutrients andlo wnutrien treserves .Yet ,th echemica lpropertie s ofLixi ­ solsar e generallybette r thanthos eo fFerralsol s orAcrisol sbecaus e of theirhighe r soil-pH and theabsenc e of seriousAl-toxicity .

MANAGEMENTAN DUS EO F LIXISOLS

The lowaggregat e stability inth e surfacehorizon(s ) ofLixisol s is conducivet oslakin gand/o rerosio ni fth etopsoi li sexpose dt oth edirec t impacto frai ndrops .Tillag eo fwe tsoi lo rth eus eo fheav ymachiner yca n 174Lixlsol s cause serious structure deterioration and compaction of the surface soil and interfere with the rooting of crops. Minimum tillage and erosion controlmeasure ssuc ha sterracing ,contou rploughing ,mulchin gan dth eus e ofcove rcrop shel pt oconserv eth esoil .Spli tapplication so ffertilizer s (lowCE C !)ar eneede d for goodyields .

SS 3& •fV* .59 -",«5

Fig. 2. Where Lixisols are limited in depth by concretionary layers, farmers inZambi a increase the rootingvolum e by scraping surface soil on to little mounds.Photograp h by courtesy ofFAO .

Chemicallyexhauste dan dphysicall ydeteriorate dLixisol sregenerat ever y slowlyunles sactivel y reclaimed. Perennial cropsar e tob epreferre d over annual crops, particularly on sloping land. Cultivation of tuber crops (cassava, sweet potato, etc) or groundnut increases the danger of soil deterioration and erosion. It is often better to devote Lixisols to extensive grazing or forestry. MINERAL SOILS CONDITIONED BY A (SEMI-)ARI D CLIMATE : SOLONCHAK1S SOLONETZ GYPSISOLS CALCISOLS

Landforms in (semi-)aridrepion s 177

MAJOR LANDFORMS IM ARID REGIONS

Arid and semi-arid regions are,a s such,no t amorphotectoni c category. Dry lands occur on Precambrian shields and inplatfor m areas in the sub- tropics, e.g. in the Sahara, the Kalahari and in Western Australia, but also inmontan e rain shadow areas such as in central Asia, north of the Alpine-Himalayanchai nfro mTurke y toChina ,an di nsom earcti carea s (see Figure1) .

Fig. 1.Deser t areas of theworld .

Nevertheless, it is evident that important geomorphic processes, and therewith landforms,diffe r from those inhumi dregions : (1)stream s are intermittent orephemeral ; (2)braide d rivers andunconfine d sheet floods aremor eprominent ; (3)man y rivers dono t debouch into the sea (theirbas e level is setb y inland depressions without outlet); (4)sal t lakes are acommo n landscape feature,an d (5)eolia nprocesse s play an important role,particularl y inarea s below the 150mm/y r isohyet.

Many regions that are arid todayhav e knowna mor ehumi d climate inth e past. Conversely, many of the present humid regions were much drier in glacial periods, especially between 20,000 and 13,000 BP when eolian processes influenced land formationmor e thana tpresent . 178 Landforms in(semi-)ari drepion s

Thischapte rwil lonl yconcer nitsel fwit hfluvia lan dlacustrin e landforms in arid environments; sandy eolian deposits were treated in an earlier chapter and deposits will be dealt with later, when the major landforms ofsteppe s andsteppi c regions arediscussed .

FLUVIALLANDFORM S INARI DAN DSEMI-ARI DREGION S

Contraryt opopula rbelief ,deser tarea sar eb yn omean s endless seaso f sand;onl y some1 5percen to ftoday' sdeser tarea si scovere db ysand san d afa rgreate rportio nconsist so frock sand/o rgravel .Ai rphoto so fdeser t lands show deep V-shaped river valleys that are now completely dryan d filled with sediment. Such valleys ('wadis' or 'oueds') formed during a more humid climatic episode, e.g. between 13,000 and 8,000 BP, atth e transition from theLas t Glacial toth eEarl yHolocene .

Inrea ldeserts ,wadi scarr ywate ronl yafte rtorrentia lrai nstorm stha t normally occur once ina fe wyears .A tth eonse t ofth erains ,th ewate r can still infiltrate into the soil but if the downpours continue, the supplyo fwate r soonexceed s theinfiltratio n capacityo fth esoi lan dth e excess water runs off over the surface. Slaking and caking of the soil surface are common insuc h barren lands and enhance run-off towards the wadis which become torrential braided streams with ahig h sediment load. Whenth estrea mreache sa n(inland )basin ,th esedimen tsettle squickl yan d formsa nalluvia l fan,no tseldo m deep inth ebasin . Atmountai nfronts ,gentl epedimen t slopesar eforme db yth evehemen tshee t floods. French and Spanish scientists refer to gently sloping fans as 'glacis d'accumulation' and to erosive pediment forms as 'glacis d'érosion'. Pediments are commonly composed of angular, poorly sorted gravel embedded inmud .

LACUSTRINE LANDFORMS INARI DAN DSEMI-ARI DREGION S

When thefloo d water evaporates, itssolute s precipitate inth e lowest partso fth ebasin . First,CaCO ,precipitate s ascalcit e oraragonite .A s thebrin ei sfurthe rconcentrated ,gypsu m(CaSO,.2H,0 )segregates ,an dstil l later,whe nth elak ei salmos tdry ,halit e (NaCl)an dothe rsolubl esalts . Such saltlake sar ecalle d 'playas'o r 'shott'. Whenplay alake sdr yout , themu do nth elak efloo rshrink san dcracks ;sal tcrust s formo nth eplay a flooran di nth ecracks .Muc ho fth eaccumulate dsal tstem sfro mevaporiti c marine sedimentsoutsid eth ebasin ;man yMesozoi c (Triassic,Jurassic )an d Tertiary sediments arever y rich inevaporites .

Itdepend so nloca lhydrographi econdition swhethe ra play ai swe taroun d the year or dries out.Som e playas are fedb y groundwater and staywe t (almost)permanently ,eve ni nclose dbasins .Basins ,suc ha sth eDea dSea , are fedb yperennia l rivers andwil l never dryou tbu tthei r water iss o salty that salts precipitate. Laminated evaporites of considerable thicknessca nfor mi nthi sway ;th elaminatio nreflect s theperiodicit yo f the seasons.Th elarges t evaporitebasi n inth erecen t geological history Landforms in (semi-)arid regions 179 isth eMediterranea nbasin :whe n theStree to fGibralta rbecam eblocke db y mountainbuildin gsom e6 millio nyear sago ,a close do ralmos tclose dbasi n remained. Before the Street opened again,hal f a million years later, a layer of 1kilometr e ofevaporite s accumulated on thebasi nfloor .

Many lakes inpresent-da y arid regionswer e freshwater lakes inth ewe t period between 12,000 and 8,000BP . Terraces and/or shorelines from that period extend well above the present lake or lacustrine plain. The same lakeswer ecompletel ydr yi nth eari dLat ePleniglacia l (20,000-13,000BP ). It is evident that a comparatively minor change in climate could cause a fundamental change inth e sedimentation regimes of arid lands.

Arid and semi-arid regions harbour awid e variety of soils that occur also in other environments (e.g. Leptosols, Regosols, Arenosols, Fluvi- sols).Typica ldr yzon esoil sar esoil swhos e formationwa s conditionedb y aridity; such soils are marked by accumulation and/or redistribution of anorganic compounds. High levels of soluble salts built up in SOLONCHAKS; these soils are particularlycommo ni ndrainles sdepressio narea ssuc ha splaya san dinlan d basins.SOLONET Zar eno tmarke db ya hig helectrolyt econten tbu tb ya hig h proportion of sodium ions in the soil solution; they occur predominantly in temperate and subtropical semi-arid environments. GYPSISOLS have a gypsic or petrogypsic horizonwithi n 125 cm of the surface and CALCISOLS are marked by redistribution of calcium carbonate. The latter two Major Soil Groupings occur in a wide range of landforms, including pediments, lakebottoms ,terrace s and alluvial fans. 180 Notes

MOTES Solonchaks 181

SOLONCHAKS

Soils which dono t show fluvic properties,havin g salic properties and having no diagnostic horizons other than an A-horizon, a histic H-horizon, acambi c B-horizon, acalci c ora gypsi c horizon.

Key to Solonchak (SO SoilUnit s

Solonchaks havingpermafros t within 200 cm of thesurface . Gelic Solonchaks (SCi)

Other Solonchaks showing gleyic3propertie swithi n 100c mo f10 th0ec surface mo f thesurface . . Gleyic Solonchaks (SCg)

Other Solonchakshavin g amolli c A-horizon. Mollic Solonchaks (SCm)

Other Solonchakshavin g agypsi c horizonwithi n 125 cmo f thesurface . Gypsic Solonchaks (SCj)

Other Solonchakshavin g acalci c horizonwithi n 125 cm of thesurface . Calcic Solonchaks (SCk)

Other Solonchaks showing sodic properties at least between 20 and 50 cm of the surface. Sodic Solonchaks (SCn)

Other Solonchaks. Haplic Solonchaks (SCh)

Diagnostichorizon ; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OF SOLONCHAKS

Connotation: saline soils;fro mR . sol, salt,an dR . chak. saltyarea .

Parent material:virtuall y any unconsolidated material, except for young alluvial sediments thatmee t the criteria for fluvicproperties .

Environment:mos tcommo n inseasonall yo rpermanentl ywaterlogge d areasi n (semi-)aridregion swit h avegetatio n of grasses and/orhalophyti cherbs . Also in (previously inundated)coasta l areas inal lclimates .

Profile development:mostl yA C orAB Cprofiles ,ofte nwit h gleyicproper ­ ties at some depth. In low-lying areas with a shallow water table, salt accumulation is strongest in the upper few centimeters of the soil. Solonchaks with a deep groundwater table have their highest salt con­ centration at some depthbelo w the surface. 182 Solonchaks

Use: Solonchak s have a limited potential for cultivation of salt-tolerant crops.Man y areuse d forextensiv e grazing or lieidle .

REGIONAL DISTRIBUTION OF SOLONCHAKS

Solonchaks covera nestimate d 260millio nhectare sworldwide ,mos to f which are inth eari d subtropics.The y aremos t extensive inth e northern hemisphere,notabl y inNort hAmerica ,norther nAfrica ,th eMiddl e Eastan d centralAsia ,bu tcove rals ovas tarea s inSout hAmeric aan d inAustralia . Figure 1show s themajo r occurrences ofSolonchaks .

Fig. 1. Solonchaksworldwide .

Solonchaks occur predominantly in inland river basins, (former) lake bottoms, and depressed areaswhic h collect seepagewate r from surrounding uplands. Coastal lowlandswit h Solonchaks aremuc h lessextensive . The most pronounced Solonchaks are found in regions that were once the bread baskets of prosperous civilisations (e.g. Mesopotamia, the Nile delta,Asi aMinor , the Indus floodplain)bu t succumbed to overpopulation, mismanagement andman' s disregard of the limits to ecosystem resilience. The same processes are going on today and for the same reasons,bot h in developedcountrie san di ndevelopin gcountries .Th eworld' sSoloncha kare a isgrowin g ata nalarmin gpace . Solonchaks18 3

GENESISO FSOLONCHAK S

Solonchaks form where there is a considerable evapo(transpi)ration surplus overprecipitatio n (plus irrigation),a tleas t duringpar t ofth e year. Saltsdissolve d inth esoi lmoistur e remainbehin d after evaporation of thewate r andaccumulat e atth esurfac e ofth esoi l ('external Solon­ chaks')o ra tsom e depth ('internal Solonchaks'). ExternalSolonchak sfor mi narea swit hshallo wgroundwater ;interna lSolon ­ chaks developwher e groundwater isdeepe r andcapillar y rise cannot fully replenish evaporation losses in the dry season. Internal Solonchaks may also form through leaching of salts from the surface to deeper layers duringwe tspells . The accumulation of salts in the soil is enhanced if a cool wet season alternates with aho tdr yseason : thesolubilit y ofmos t salts increases in the warm dry season when there is a net upward water flux from the groundwater table to the surface soil, and decreases in the cooler wet season when salts are leached from thesurfac e soilb y surplus rainfall. See Figure 2.

•AJ&" j>'

___J*»Ü—~~-7^ -/y~^

^-*~^*~*^ Î / sfç> . /<&• 7/ST **/"s** / ^<$0 / if / 1vvv~^ / o '1 / j / / **v / y 0" / / \ -!V°« ' ^ /ƒ / •'' *7l\ •9V / / &/ // / Q /o' Fig. 2.Th esolubilit y ofcommo n saltsi nSolonchaks ,expresse di n moleanhydrou ssal tpe rk gH 20,a s */^^CiS04.ÎH20 afunctio no fth esoi ltemperatur e 7/ ^~-^~~^>? (Braitsch, 1962). ^KjSO^ZHjO

The 'criticaldepth 'o fth egroundwater ,i.e .th edept hbelo wwhic hther e islittl e danger that salicpropertie s will develop inth eroote d surface soil,depend so nsoi lphysica lcharacteristic sbu tals oo nth eevaporativ e demand ofth eatmosphere .Th eUSD A Staffconsider s adept ho f 6 feet critical, "especially ifth esurfac e isbarre n andcapillar y rise ismoderat e tohigh" . 184Solonchak s

Normally, thebul k of the salts that accumulate ina nare a are imported fromelsewhere ,carrie do nb yriver s fromfar-awa y catchment areaso rwit h seepagewate r or surface runof f from adjacent uplands.Accumulate d salts can often be traced to deeper geological strata of marine origin (chlorides)o ro fvolcani c origin (sulphates,nitrates) . Figure3 present sa diagra mo fa commo nsituatio nwit hSolonchak s inbotto m land that receiveswate r (and salts)fro m adjacentuplands .

Spatialan dtempora lvariation si ngroundwate rdept han dsoi ltemperatur e make certain salts accumulate faster than others. This is especially evident inth e case ofexterna l Solonchakswhereb y themineralog y of salt efflorescences iso fparticula r importance.

prec ipitation < I I U PLANDS ! fyf, STRUCTURAL ;% ? 'TE RRACE S < """?? ' BAJADAS ALLUVIAL FAN irMARL PLAIN

springs evaporation /; HIM saltcrust

deep losses

watertable A in spring (May) Bi nsumme r(September )

Fig. 3. Import and redistribution of salts in the Great Konya Basin, Turkey. External Solonchaks develop inth ebotto m lands.Source :Driesse n & v.d. Linden,1970 .

Figure 4 presents the stability diagram ofmineral s in anNaCl-saturate d NaCl-Na2SO,-MgCl2-H20 system. The diagram demonstrates that annual and diurnal temperature fluctuations inducemineralogica l transformations. Solonchaks 185

Fig.4 . Stability diagram of minerals ina nNaCl-saturate d NaCl-Na2S04-MgCl2-H20system . Source: Braitsch,1962 . Mg10 0 o so4

An example of a specific type of Solonchak which forms under the in­ fluence ofdiurna l fluctuations inth emorpholog y of salts is the 'puffed Solonchak',a nexternall ysalin esoi li nwhic hth egreate rpar to fal lsal t consists of sodium sulphate. At night,whe n the temperature at the soil surface is relatively low and air humidity high, crystalline sodium sulphate is present in the surface soil as needle-shaped Mirabilite (Na2S0,.lOHpO) . The particular morphology of this mineral makes that it pushes the soil aggregates apart when it is formed and this creates a fluffy salt-soil mixture, directly at the surface of the soil. When the temperature rises again during the day, the Mirabilite is converted to water-free Thenardite (Na2S04) crystals that have the appearance of fine flour. Figure 5show s the soft and fluffynatur e of the surface soil ofa puffed Solonchak.

Anotherexampl eo fdiurnall ychangin gexterna lSolonchak sconcern ssoil s witha dominanc e ofhygroscopi c saltssuc ha sCaCl 2o rMgCl 2.The y formso - called 'Sabakh'soil s (sabakhi sarabi cfo rmorning )tha tar edar kcoloure d inth emornin ga sa resul to fmoistur e absorptiondurin g thenight .Sabak h soils lose their dark colour again in the course of the day when the temperature rises and airhumidit y drops to alo wvalue .

An example of an annual cycle in which the morphology of salt minerals plays arol e isth e formation of 'slick spots', isolatedpatche s ofmudd y and very saline soil in a field. Slick spots develop early in the dry season in shallow depressions (often hardly recognizable with the naked eye) that are covered with a salt crust. These crusts,e.g . a glass-like Halite (NaCl)crust ,ar es oeffectiv e insealin gth eunderlyin g salinemu d fromth eai r thatth esoi lremain swe tthroughou t thedr yseaso nan dform s nopore s orcrack s thatca nprovid epassag e torai no rleachin gwater .Th e crust may dissolve in a subsequent wet seasonbu t the unripe mud remains salinean drestore sit sprotectiv ecrus ta ssoo na sth ewe tseaso ni sover . 186Solonchak s

The spots thus remain soft, untrafficable and very saline and cannot be reclaimed with conventional (leaching)techniques .

vat-

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.*,-'#*

Fig. 5. A spot of puffed Solonchaks in a former lake bottom in Central Anatolia,Turkey . Note the fluffynatur e of the surface soil causedb y an abundance ofneedle-shape d mirabilite crystals.

CHARACTERISTICS OF SOLONCHAKS

The horizon differentiation of Solonchaks is normally determined by other factors than their high salt content. Many saline soils inwater ­ loggedbackswamp s areGleyi c Solonchaks; theywoul dhav ebee n Gleysols if itwer e not for their salic properties. Likewise,man y Mollic Solonchaks have theappearanc e ofa ,Kastanoze m orPhaeozem , andCalci can d GypslcSolonchak sar eofte nstrongl ysalin eCalcisol san dGypsisols .Salin e Histosols,Vertisol s andFluvisol s occur aswell ; they areno t classified as Solonchaks because Histosols, Vertisols and Fluvisols key out before Solonchaks. Their salinity isacknowledge d ina salicphase . (SeeAnne x 3fo r adefinitio n ofth e salic phase.) Solonchaks 187

The structure of the surface soil of Solonchaks is often influenced by thehig h salt content of the soil. The surface layer of Sabakh soils isa muddy mixture of salt and soil particles during early morning hours but becomes ahar dcrus t later inth eday .O nSolonchak swit ha hig h component ofnitrate s (andpossibl y other salts)th esof tcrus t ispushe dupward sb y gases escaping from the underlying mud; prints of gas bubbles remain visiblewhe n the crust,detache d from theunderlyin gwe t soil, driesout . The fluffy top layer ofpuffe d Solonchaks isa morphologica l feature that isexclusiv et oSolonchak swit ha hig hconten to fsodiu msulphate .Th emos t common type of salt crust,however , isa loos e cover of saltcrystals . Especially in heavy clays, very saline surface layers may exist without any clear efflorescence of salts. Examination with a lens reveals tiny crystals onth eface so fcrum bo rgranula r structureelements .Ver y saline pseudo-sandma yeve nb eforme d thataccumulate s tocla ydune swhe nexpose d to strongwinds .

Themorpholog y ofinterna lSolonchak s differslittl e fromtha to fcompa ­ rable non-saline soils. Solonchaks have, perhaps, a somewhat stronger subsoil soil structure with, inver y saline soils, tiny salt crystals on the faces of the structureelements .

With salic soil properties as the only common characteristic, there is considerable diversity among Solonchaks and a detailed account of their hydrological, physical, chemical and biological properties is not well possible.A fewgenera l trends deserve attention inth epresen tcontext .

Hydrological characteristics

InternalSolonchak soccu ri nlan dtha tlie swel labov eth edrainag ebase . Whenleached ,the yma yactuall yfurnis h (partof )th esalt stha taccumulat e in contiguous bottom land with external Solonchaks. Extreme salinization with thick surface crusts occurs in depressions that collect surface run offwate r from surrounding (higher)lan d inth ewinte rbu t dry out inth e warm season ('flooded'Solonchaks) . Figure 6 presents a cross-section through an inland basin with severe : Where the water table is at shallow depth, strongly externally saline Gleyic Solonchaks occur (site 8).Slightl y above thebas e level (site9) , thesoil sar estil lstrongl y salinebu tth ehighes t concentrationo fsalt s is at some depth in the soil. This results from a combination of upward salt transport from the groundwater through capillary rise and downward leaching of salt from the surface soil to the zone with thehighes t salt concentration. In still higher areas with a very deep water table (site 10), non-saline Calcisols and internally saline Calcic Solonchaks occur. Thesite s6 an d7 ar elocate do na nalluvia l fanwhic hdrain sfreel y toth e lowcentr e ofth ebasin .Soil so nth euppe rpar to f thefa nar e invariably non-saline; there isbeginnin g salinization in the lower tract where the groundwater table is at shallow depth. (Note the different scales of the x-axis.) 188 Solonchaks

600 400 200 0 200 400 600 200 100 0 100 200

Fig.6 .A cross-sectio no fth eGrea tKony aBasin ,Turkey ,demonstrate sho w salinity patterns are influenced by topography and hydrology. All concentrations are expressed inmmol/lite r saturation extract or ground­ water.

Physical characteristics

Solonchaks which dry out during part of the year have normally strong soil structures. When the salt content is lowered by winter rains or irrigationwater ,thes estructure sma ycollapse ,particularl y ifth esalt s have a large component of sodium and/or magnesium compounds. Strong peptisation ofclay sma ymak e the soilvirtuall y impermeable towater . In the extreme case, Sodic Solonchaks or even Solonetz may be formed with a dense subsoil and adverse physicalproperties .

Chemical characteristics

Solonchaks have an 'electric conductivity' value (ECe) in excess of 15 dS/ma t2 5° Cwithi n3 0c mo fth esurfac ea tsom etim eo fth eyear ,o rmor e than4 dS/ m ifth epH(H 20,1:1 ) exceeds 8.5.A nEC eo f1 5dS/ m corresponds with some 0.65 percent salt,o rwit h 150 cmol(+)pe r liter soil moisture ina completel ywater-saturate d soilpast e (seeals oFigur e 6). TheEC ei s thereciproca lvalu eo fth eelectri c resistance (inohm/cm )an d isexpres ­ sed inmho/c m (inolde r literature)o rdS/ m (Sstand s for 'Siemens'). Solonchaks 189

For a quick orientation, the electric conductivity is often determined on 1:1 or 1:5 soil extracts (EC.o r EC5) but extracts of saturated soil pastes are used inbas e laboratory work. Values obtained with different methods cannot always be compared, among other reasons because the 'suspension effect' (different at different dilution ratios) influences theoutcom e of theconductivit ymeasurement .

Table 1suggest sa nindicativ e gradingo fsalt-affecte dsoil so nth ebasi s of their saltcontents .

TABLE 1. Indicative soil salinity classes and the effects of soil salinity oncro p performance.

ECe Salt Concentration FAO-Unesco Effect onCrop s at 25° C extract soil qualification (dS/m) (cmol/1) (percent)

<2.0 <2 mostly negligible 2.0-4.0 2-4 <0.15 some damage to sensitive crops 4.0-8.0 4-8 0.15-0.35 salicphas e serious damage to (Solonchak) most crops 8.0-15 8-15 0.35-0.65 salic phase only tolerant (Solonchak) crops succeed >15 >15 >0.65 Solonchak fewcrop s survive

Asmentioned ,th equantit y ofsalt svarie samon g Solonchaksbu tals oth e saltcomposition .Russia nsoi lscientist s characterize saltaffecte d soils (Solonchaksan dsali cphase s inothe rsoi lgroupings )o nth ebasi so fanio n ratios inth e saturation extract. SeeTabl e 2.Th e Solonchaks inFigur e6 qualify as sulphate-chloride soils according to this classification.

TABLE2 . Classificationo f saline soilsbase d onanio n ratios (Plyusnin,196? )

Pljusnin Rosanov Sadovnikov Sulphate soils C1/S04 <0.5 <0.2 <0.2 Chloride-sulphate soils Cl/SO^ 0.5-1.0 0.2-1.0 0.2-1.0 Sulphate-chloride soils Cl/SO^ 1.0-5.0 1.0-2.0 1.0-5.0 Chloride soils C1/S04 >5.0 >2.0 >5.0

Soda soils C03/S04 <0.05 Sulphate-soda soils C0,/S0, 0.05-0.16 Soda-sulphate soils C03/S04 >0.16 190 Solonchaks

Biological characteristics

Faunal activity is depressed inmos t Solonchaks and ceases entirely in soils with 3 percent salt or more. In severely salt-affected lands, the vegetation is sparse and restricted tohalophyti c shrubs,herb s and gras­ ses that tolerate severephysiologica l drought (andca ncop ewit h periods ofexcessiv ewetnes s inarea swit h seasonally flooded Solonchaks).

MANAGEMENTAN DUS EO F SOLONCHAKS

Saltaccumulatio n affectsplan t growth intw oway s: (1)indirectl y by skewing thecompositio n of the soil solutionwhic h upsets the availability ofplan tnutrients ,an d (2)directl yb y inducingphysiologica l drought asa consequenc e of the high osmoticpressur e of the soilmoisture .

High contents ofa specifi c ion inth e soil solution can interfere with the uptake of other ions. Such antagonistic effects are known to exist, forexample ,betwee n sodium andpotassium ,betwee n sodium and calcium and betweenmagnesiu m andpotassium .Ver yharmfu l are excess levels of sodium salts, chlorides (disturb normalN-metabolism )an dMgS04 . Themai n damage is done, however, because the plant cannot compensate for the combined hydrostatic, adsorptive and osmotic forces that hinder water uptake. In practice, salinity augments the effects ofaridity .

Solonchaks cannotb euse d fornorma l cropping. Only after the saltsar e leached out of the soil (which then ceases to be a Solonchak) can good yields be expected. Normal irrigation is inadequate for desalinization. Often,th erevers ehappens :salt scontaine d inth eirrigatio nwate r remain behind in the soil and the salt level builds up. The use of saline irrigationwate rmus tb e avoidedwheneve r possible,an d excesswate rmus t be applied above the irrigation requirement tomaintai n a downward water flow inth e soil.Jus t as important:drainag e facilities mustb e designed to keep the groundwater table at sufficient depth. Amendments such as gypsum are sometimes used to correct imbalances at the exchange complex and tomaintai na goo dpermeabilit y of the soil. Solonetz 191

SOLONETZ

Soilshavin g anatri c B-horizon.

Key to Solonetz (SN)Soi lUnit s

Solonetz showing gleyic properties within 100c m of thesurface . Gleyic Solonetz (SNg)

Other Solonetz showing stagnic propertieswithi n 50c m of thesurface . Stagnic Solonetz (SNz)

Other Solonetzhavin g amolli c A-horizon. Mollic Solonetz (SNm)

Other Solonetzhavin g agypsi c horizonwithi n 125 cm of thesurface . Gypsic Solonetz (SNj)

Other Solonetzhavin g acalci c horizonwithi n 125 cm of thesurface . Calcic Solonetz (SNk)

OtherSolonetz . Haplic Solonetz (SNh)

Diagnostic horizon; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r full definition.

SUMMARY DESCRIPTION OF SOLONETZ

Connotation: 'sodic' or 'alkali' soils with high sodium saturation; from R. sol. salt,an dR . etz. strongly expressed.

Parentmaterial :unconsolidate d materials,mostl y fine-textured.

Environment:majo rconcentration so fSolonet zar ei nfla to rgentl yslopin g natural grasslands insemi-arid , temperate and subtropicalregions .

Profile development: ABtnC and AEBtnC profiles with a black or brown A-horizon over a natric B-horizon. Well developed Solonetz can have a (beginning)albi cE-horizo n directlyove ra natri cB-horizo nhavin g strong prismatico rcolumna rstructur eelement swit hrounde dtops .A calci cand/o r gypsichorizo nma yb epresen tbelo wth enatri cB-horizon .I nman ySolonetz , the pH of the soil is around 8.5 due to dominance of sodium carbonate in the soil solution.

Use: hig h levels of exchangeable sodium affect plant performance, either directly (toxicity) or indirectly (structure deterioration).Solonet z in temperate regions are used for arable farming or grazing; subtropical Solonetz aremostl y inus e asrang e land or lieidle . 192 Solonetz

REGIONAL DISTRIBUTION OF SOLONETZ

Solonetz occur predominantly in the dry interior parts of North America, Eurasia and Australia, with smaller occurrences in the semi- arid/subtropicalAfric aan dSout hAmerica ,mostl yi nfla to rgentl yslopin g plainsan dno tseldo mi nassociatio nwit hSolonchaks .Figur e1 present sth e main areas ofoccurrence .

Fig. 1. Solonetzworldwide .

GENESIS OF SOLONETZ

Theessentia lcharacteristi co fSolonet zi sth enatri cB-horizo nwhic h has anExchangeabl e Sodium Percentage (ESP)o f 15o rhigher .Th e ESP isa functiono f theexchang e properties of the soilmateria l and the chemical composition of the soil solution inequilibriu m with this soilmaterial . Not only the sodium content of the soil moisture is important but also theconcentration s ofothe r ions,particularl y thedivalen t ions thatar e preferentially adsorbeda tth eexchang e complex.A measur eo fth ecomposi ­ tiono fth esoi l solutiona srelevan t forNa + adsorption atth ecatio nex ­ change complex isth e SodiumAdsorptio n Ratio (SAR), definedas :

SAR- Na +/[(Ca2+ + Mg2+)/2]0-5 whereNa* ,C a andMg 2+ are expressed incmol(+ )pe r liter ofsolution .

(NOTE THAT units are important here; other units, e.g. mmol/1, give different SAR values.) Solonetz 193

The theoretical relationbetwee nSA Ran dES P reads:

ESP/(100-ESP)= 0.031*K*SA R whereK ha svalue s of0.7 5 and1. 5 (cmol(+)/l) forkandit ean dsmectit e clays respectively. The above relations have been found satisfactory in most circumstances,bu t poor correlationha s occurredwit h high-carbonate waters. Various adapted relations have been proposed to cope with such conditions. The SAR gives an indication of the chance that an unfavourably high ESP develops when a soil isbrough t incontac twit h aparticula r groundwater or irrigationwater . The 'sodium hazard' is low if SAR<10,mediu m ifbe ­ tween1 0an d 18,hig h ifbetwee n 18an d 26,an dver yhig h ifSAR>26 .

In areas with a marine history the sodium may originate from NaCl but manySolonet zi nth e (sub)tropicshav esod a (Na2C0,)a sth edominan tsodiu m compound. Soda canfor m intw oways : (1)b y evaporation ofwate r containing anexces s ofbicarbonat e ions over divalent cations (Ca2+ andMg 2+), an d (2)biologically ,b y reductiono fsodiu msulphate .

Ifwate rwit h Ca-an dMg-bicarbonate s evaporates,calciu m and magnesium carbonatesprecipitat ean dth eSA Rvalu e increases.Sod asoils ,wit ha hig h pH, develop ifth ebicarbonat e content ofth eevaporatin gwate r ishighe r than its content of (Ca +Mg ).Exces s bicarbonate is inpractic e always sodiumbicarbonat ewhic h iseventuall y transformed toNa 2C0j.

The biological formation of soda from sodium sulphate follows the sequenceNa 2S04 -->Na 2S -->Na 2C03+ H 2S,whereb y thehydroge n sulfide gas leaves the system. In addition to Na2S0,, this reaction requires the presence of organicmatte r and (periods of)anaerobi c conditions.

Soda formation raises thep H of the soil toa valu e nearp H 8.5; silica andalumin aca nthe ndissolv e from silicate clays.Cla ydestructio n inth e upper horizon and clay formation in the middle horizon, together with illuviation ofclay ,ar e theprocesse s responsible for the formation ofa natric B-horizon. Amatur e Solonetzha s anatri c B-horizon with columnar structure elements thathav erounde dtop sa sa resul to fpeptisatio nan ddestructio no fclay . The process is limited to the top of the B-horizon because percolating water stagnates on this dense illuviation layer (see Figure 2). Stagni c properties may even develop directly above the natric horizon (Stagnic Solonetz).

Asmentione d inth echapte ro nSolonchaks ,Solonet zca nals ofor mthroug h progressive leachingo fsalt-affecte d soils.Eve nsoil stha twer einitial ­ ly rich in calcium may eventually develop a natric B-horizon. Prolonged leaching and exchange of adsorbed Na+ by H+ will ultimately produce an eluvial E-horizon and a lowpH . Such strongly degraded soils areknow na s 'Solods'. 194 Solonetz

. .*tf»r Fig. 2. A mature Solonetz. Note the columnar structure elements with rounded tops in thenatri c B-horizon. Photo by ISRIC,Wageningen .

CHARACTERISTICS OF SOLONETZ

Most (Haplic)Solonet zhav ea thi nloos elitte rlaye rrestin go nblac k humifiedmateria labou t2- 3 cmthick .Thi soverlie sa brow ngranula rochri c A-horizonwhic habruptl ychange sint oa natri cB-horizo nwit hcoars e (often round-topped) prismatic or columnar structure elements and grading with depth into amassiv e subsoil. Mollic Solonetz have a thick, dark mollic A-horizon instead of an ochric A-horizon, and Calcic and Gypsic Solonetzhav e acalci c or gypsic horizon below the natric B-horizon. Gleyic Solonetz have a mottled subsoil and sometimes dispersed organic matter is translocated from theA-horizo n to the top of the B-horizon; Stagnic Solonetz have stagnic properties with reducedsurfac esoi lmateria ltonguin gint oth eunderlying ,oxidize dnatri c B-horizon. Solonetz 195

Physical characteristics

Clayey Solonetz are waterlogged in the wet season and even ponding of waterma y occur.Th e dense subsurface soilhinder s rootpenetration .Whe n ploughed, many Solonetz have a lumpy surface with the top of the natric B-horizon showingu p ina polygona lpatter n inth ebotto m of the furrows. On thewhole ,Solonet zhav e poorphysica lproperties .

Chemical characteristics

The high ESP of Solonetz is directly or indirectly detrimental to the suitabilityo fthes esoil sfo rcropping :directl ybecaus ea hig hproportio n ofsodiu m ionsi nth esoi li stoxi c tosom eplant san ddisturb s theuptak e ofessentia lplan tnutrients ,an dindirectl ybecaus esodicit yi sassociate d with dense thatinterfer ewit h downwardpercolatio n ofwate r and hinderth egrowt ho froots .Th eimpressio nexist stha tsensitiv ecrop ssho w true sodium toxicity symptoms already at low ESP values whereas tolerant crops are stunted at high ESP values largely because of sodium-induced adverse physical soilconditions .

MANAGEMENTAN DUS EO F SOLONETZ

How detrimental high sodium saturation is in a certain situation is partlydetermine db yothe rsoi lparameter s sucha sth edept ho fth enatri c B-horizon and the presence or absence of gypsum, and by the sodicity tolerance of thecrop . Soilswit h apredominantl y smectitlc clay assemblage showalread y serious structure deterioration when the SAR exceeds 9; illitic andvermiculiti c soils degrade at SAR>16 and the most stable soils (kaolinitic soils and soils rich in sesquioxides)deteriorat e only ifth e SAR exceeds 26 inth e absenceo fsalinity .Th eindicativ erange sfo rth esodiu mhazar d ofwater s thatwer ementione dbefore, mus tb einterprete dwit hth erelativ estabilit y of the soil structures in mind. Also, it may not be forgotten that the presence of e.g. gypsum in a soil can mitigate the effects of high-SAR (irrigation)water .Ther ear estron gindication s thata hig hpercentag e of exchangeable magnesium affects the soil structure in a similar manner as ahig hESP . The traditionalwa y ofreclaimin g Solonetz isb y flushingwit h calcium- richwater . In the case ofGypsi c Solonchaks,dee p plowing may eliminate the need for (expensive) application of gypsum. Where a low hydraulic conductivity of the soil precludes effective leaching, one ormor e years undera sodium-tolerant ,deepl yrootin ggras scro p (e.g.Rhode sgrass )ma y make the soil reclaimable. Solonetz inCanad a arewidel y grown towheat ; those insparsel ypopulate darea s inth esemi-ari d subtropics are commonly left idleo r theyar euse d for extensive grazing. 196 Notes

NOTES

/•- (

•••• •+•

r ? *

Vv t' '* '' 1- '.. *."'

f ï *i *- . ,."*> C f ï U ( •.% •?," ." ..s 1 .

3. • '"I: C r :;J Gvpsisols 197

GYPSISOLS

Soilshavin g agypsi c ora petrogypsi c horizon,o rboth ,withi n12 5 cm of the surface; having no diagnostic horizons other than an ochric A-horizon, acambi c B-horizon, anargi c B-horizon permeated with gypsum or calcium carbonate, a calcic or a petrocalcic horizon; lacking the characteristics which are diagnostic for Vertisols or Planosols; lacking salicaproperties ;lackin ggleyi c propertieswithi n10 0c mo fth esurface .

Key toGypsiso l (GY)Soi lUnit s

Gypsisols having a petrogypsic horizon, the upper part of which occurs within 100c m of thesurface . Petric Gypsisols (GYp)

Other Gypsisols having acalci c horizon. Calcic Gypsisols (GYk)

Other Gypsisols having an argic B-horizon. Luvic Gypsisols (GY1)

OtherGypsisols . Haplic Gypsisols (GYh)

Diagnostichorizon ; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OF GYPSISOLS

Connotation: soilswit h substantial accumulation of calcium sulfate; from L. gypsum,gypsum .

Parentmaterial :unconsolidate dalluvial ,colluvia lo reolia ndeposit sric h inbases .

Environment: leve l tohill y landan ddepressio n areas (e.g. former inland lakes) inregion swit h anaridi c moistureregime .

Profile development: AB(t)C profiles with a yellowish brown ochric A-horizon over a cambic or (relic ?) argic B-horizon. Accumulation of calcium sulfate,wit h orwithou t carbonates,i sconcentrate d inan dbelo w the B-horizon.

Use: soil s having less than 25 percent gypsum in the upper 30 cm of soil can be grown to cereal crops, alfalfa, cotton, etc.Yield s may still be depresseddu et onutrien timbalances ,stoniness ,and/o rmechanica lhindran ­ ces.Uneve nsubsidenc e ofth elan dsurfac edu e todissolutio no fgypsu m in percolating (irrigation)wate r isa further limitation. Irrigation canals mustb e lined topreven t thewall s from caving in. 198 Gvpsisols

REGIONALDISTRIBUTIO N OFGYPSISOL S

Gypsisols occur in the same regions as Calcisols but are much less widespread. Theworldwid e extent ofGypsisol s isprobabl y of the order of 100 to 150millio nhectares ;majo r areas are found inth eMiddl e East and the southern USSR (see Figure 1),i n southeast and central Australia and the southwesternUSA .

èckfii 1 . * p&f^Ssi^If^r -

\ 1

„>-.....-... AN \ ƒH Xj )vx7x - 0 1000 2000 Km ' Î5? -\

Fig.1 .Majo roccurrence so fGypsisol s inEurop ean dth eNea rEast .Source : VanAlphe n &Romero ,1971 .

GENESIS OF GYPSISOLS

Most of the gypsum in soils originates from Triassic, Jurassic and Cretaceous evaporites with gypsum (CaS04.2H20) and/or anhydrite (CaSO^), and from (predominantly)Miocen e gypsumdeposit s thatar enormall y inter- bedded in marls or clays. Rarely is gypsum formed directly in the soil. This has, for example, been observed in areas with pyritic sediments in southwest where sulfate ions, formed when sulfides oxidized upon forceddrainag e ofth eland ,precipitate d asgypsu ma tdepth so f2 0t o15 0 Gypsisols19 9 cmbelo wth esurface .I nGeorgia ,USSR ,gypsu mformatio nwa sobserve dwher e saline, Na-SO,-containing seepage water came in contact with Dolomite weathering.

Normally, Gypsisols form through dissolutiono fgypsu m from gypsiferous weathering materials followedb ydownwar d transport withpercolatin g soil moisture andprecipitatio n ofgypsu m deeper down.Th egypsu mma yprecipi ­ tatea sfine ,white ,powder ycrystal si npocket so ri nforme rroo tchannel s ('gypsumpseudomycelium') ,a spendant sbelo wpebble san dstone s(se eFigur e 2), ascoars ecrystallin egypsu msan do reve na srosette s('deser troses') . In arid regions with hot,dr y summers, gypsum (CaSO,.2H20)dehydrate s to loose, powdery hemihydrate (CaSO,.0 .5H 20)whic h reverts to gypsum during themois twinters .I fsufficientl y abundant,so-forme d (highly irregular) gypsum crystals maycluste r together tocompac t layers orsurfac e crusts thatca nbecom e tenso fcentimetre s thick.

" •-•• • *

Fig. 2. Gypsum pendants in a gravel terrace near Deir es Zor, Syria. Photographb ycourtes y ofISRIC ,Wageningen .

CHARACTERISTICS OFGYPSISOL S

The typical Gypsisolha sa 2 0t o4 0c mthick ,yellowis hbrown , loamy orclaye yA-horizo n overa pal ebrow nB-horizo nwit hdistinc twhit e gypsum pockets and/or pseudomycelium. The surface layer consists of strongly de-gypsified weathering residuesan dha sa lo worgani cmatte r contentan d a weak, subangular blocky structure. The gypsic horizon ismos t clearly developed inth elowe r B-horizon or slightly deeper andca nb e anything from asoft ,powder y andhighl y porous mixture ofgypsum , lime andclay , toa har dan dmassiv e layero falmos tpure ,coars e gypsumcrystals . 200Gvpsisol s

Hvdrolo^ical characteristics

Gypsisolshav ehighl yvariabl ehydrauli cproperties .Saturate dhydrauli c conductivityvalue svar yfro m5 t o>50 0cm/d ;infiltratio no fsurfac ewate r is close to zero in some encrusted soils whereas very high percolation lossesoccu r insoil s inwhic hdissolutio no fgypsu mha swidene dfissures , holesan dcrack st ointerconnecte d subterraneancavities .Infillin go fth e cavities with surface soil material makes itnecessar y to level the land surface each year which makes the valuable topsoil ever shallower. See Figure 3.

original gypsiferous soil after irrigation after levelling

non gypsic topsoil material

gypsic subsoil material

after repeated irrigation after re-levelling

Fig. 3.Cavit y formation,uneve n subsidence,an d stripping of the surface soila sa consequenc eo fprolonge dirrigatio no fshallo wGypsisols .Source : VanAlphe n& Romero ,1971 .

Physical characteristics

Most de-gypsified surfacehorizon s contain4 0percen t clay ormore ,an d have an 'available' water holding capacity of 25 to 40 volume percent. Surface soilswit hmor e than1 5percen tgypsu mcontai nseldo mmor e than1 5 percent clay and their retention of 'available' soil moisture does not exceed 25 volume percent. Loamy surface soils slake easily to a finely platy surface crustwhic hhinder s infiltration ofrai nwate r andpromote s sheetwas han dgull yerosio nt oth eexten ttha tdee p (petro)gypsichorizon s becomeexpose d inspit eo fa lo wannua lrainfal lsu mo fonl y20 0t o45 0mm .

Chemical characteristics

Small quantities of gypsum do not harm plants in any way but gypsum contents ofmor e than 25percent , as occur inman y gypsiferous subsoils, upset the nutrient uptake by plants and lower the availability of phos­ phorus,potassiu m andmagnesium . Thetota lelemen tcontent so fGypsiso lsurfac ehorizon samoun tt oles stha n 2500 mg N/kg, less than 1000 mg P205/kg (of which less than 60 mg/kg is Gvpsisols 201 considered 'available'),an d less than200 0m gK 20/kg:heav y fertilization is needed for good yields. The cation exchange capacity decreases with increasing gypsum content of the soilmateria l and is typically around 20 cmol(+)/kg in the surface soil and around 10 cmol(+)/kg deeper down. The base saturation isnearl y 100percent .

MANAGEMENTAN DUS EO FGYPSISOL S

Gypsisols withno tmor e thana fe wpercen t gypsum inth euppe r 30c m layer (60c m ifirrigated )ca nb euse d forth eproductio no f smallgrains , cotton, alfalfa, etc. High irrigation rates in combination with forced drainage made itpossibl e toproduc e excellent yields of alfalfa hay (10 tons per hectare), wheat, apricots, dates,maiz e and grapes on Gypsisols withmor e than2 5percen tpowder y gypsum.

Fig. 4. Lining prevents caving in of irrigation canals in Gypsisols. Photograph by courtesy of ISRIC,Wageningen .

Irrigated agriculture onGypsisol s isplague db yquic kdissolutio no fsoi l gypsum (seeFigur e4 )resultin gi nirregula rsubsidenc eo fth eland ,cavin g incana lwalls ,an dcorrosio no fconcret estructures ,an db yth eoccurrenc e of shallow petrogypsic horizons. The latter obstruct root growth, and interferewit hwate r supply to thecro p andwit h soildrainage . Dry farming on Gypsisols makes use of fallow years and other water harvesting techniques but is not very profitable; dry farming Gypsisols withmor e than2 5percen t gypsum cannotb e recommended. 202 Notes

NOTES Calcisols 203

CALCISOLS

Soilshavin gon eo rmor eo fth efollowing :a calci c horizon,a petro - calcic*horizo n or concentrations of soft powdery lime within 125 cm of thesurface ;havin gn odiagnosti chorizon sothe rtha na nochri c A-horizon, acambi c B-horizono ra nargi c B-horizonpermeate dwit hcalciu mcarbonate ; lacking the characteristics which are diagnostic forVertisol s orPiano - sols; lacking salica properties; lacking gleyicapropertie s within 100c m of thesurface .

Key toCalciso l (CD SoilUnit s

Calcisols having a petrocalcic horizon, the upper part of which occurs within 100 cm of thesurface . Petric Calcisols (CLq)

Other Calcisols having anargi c B-horizon. Luvic Calcisols (CLe)

OtherCalcisols . Haplic Calcisols (CLh)

Diagnostic horizon; seeAnne x 1fo r full definition. Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OF CALCISOLS

Connotation:soil swit hsubstantia laccumulatio no fcalciu mcarbonate ;fro m L. calcis.lime .

Parentmaterial :mostl y alluvial (lacustrine)sediment s and colluvial and eolian deposits ofbase-ric hweatherin gmaterial .

Environment: level to hilly land with an aridic moisture regime and a sparsevegetatio n ofxerophyti c shrubs and ephemericgrasses .

Profile development: AB(t)C-profiles with a pale brown ochric A-horizon overa cambi co rargi cB-horizon .Accumulatio no fcarbonate s atsom edept h below the soilsurface .

Use:dryness ,an d inplace s alsostonines sand/o r thepresenc e ofa petro ­ calcic horizon, limit the suitability of these soils for agriculture. Droughtresistan tarabl ecrop ssuc ha swhea tan dsunflowe rar egrow nunde r rainfed conditions. If irrigated, drained (to prevent salinization) and fertilized, Calcisols can produce good yields of fodder and vegetable crops.Man y areaswit hCalcisol s areuse d forlo wvolum e grazingo fcattl e and sheep. 204Calcisol s

REGIONALDISTRIBUTIO N OF CALCISOLS

Iti sdifficul tt oestimat eth eworldwid eexten to fCalcisol swit han y measure of accuracy,partl ybecaus e thegroupin gwa s only recently intro­ duced,bu tals obecaus eman yCalcisol soccu r togetherwit h Solonchaks that are actually salinized Calcisols, and/or with other soils with carbonate enrichment that do not key out as Calcisols. The total area amountsprobabl yt owel love ron ebillio nhectares ,nearl yal lo fi ti nth e aridan dsemi-ari d subtropicso fbot hhemispheres .Figur e 1give sa nindi ­ cation of theregion swher e Calcisols occur.

Fig. 1.Calcisol sworldwide .

GENESIS OF CALCISOLS

ManyCalcisol sar eol dsoil swhe ncounte di nyears ,bu tthei rdevelop ­ mentwa s slowed downb y recurrent periods of drought inwhic h such impor­ tantsoi lformin gprocesse sa schemica lweathering ,accumulatio no forgani c matter, and translocation of clay, came to a virtual standstill. As a result, only an ochric A-horizon could develop and the modification of subsoil layers did normally not advance beyond the formation of a cambic B-horizon.Man yCalcisol sar e 'polygenetic':thei rformatio ntoo kdifferen t courses during different geologic eras with different climates. For instance, the argic B-horizon of Luvic Calcisols is widely considered a relic from eraswit h amor ehumi d climate thana tpresent . Calcisols 205

Themos t prominent soil formingproces s inCalcisol s -th eproces s from which the soils derived theirnam e -i s the translocation ofcalciu m car­ bonate from the surface horizon to an accumulation layer at some depth. Under certain conditions, e.g. inerodin g land or in land that is inten­ sively homogenized by burrowing animals, lime concretions occur right at the surface of the soil.

Dissolution of calcite (CaCO,)an d subsequent accumulation in a calcic horizon isgoverne d by twofactors : (1)th eCOp-pressur e of the soil air,an d (2)th e concentrations ofdissolve d ions inth e soilmoisture .

The following equilibria are involved (thepH-range s overwhic h the equi­ libria are inoperatio nar e shown inFigur e 2):

C02 +H 20= H 2C03°

+ H2C03°= HC0 3"+ H

2 + HC03"= C03 "+ H

-1.5 -1.0 -0.5

logP co (inkPa )

Fig. 2.Solubilit y ofcalcit e atdifferen tC0 2-pressuresan d corresponding pH-value. Source:Bol t &Bruggenwert ,1979 .

Foral lpractica lpurposes ,th eequilibri awhic har erelevan tt oth edis ­ solution or precipitation of calcite in soils (pH <9) can be viewed as follows: 206 Calcisols

C02+ H 20

2+ CaC03+ H 2C03- Ca + 2 HC0 3"

Anincreas ei nth eC0 2-contento fth esoil-air ,associate dwit ha decreas e in soil-pH, drives the reaction to the right: calcite dissolves andth e concentrations ofC a -an dHC0j"'on s inth esoi l solution rise.Alterna ­ tively, calcite dissolves if (rain) water with a low Ca2+-concentration flushes thesoil . Precipitationo fcalcit eoccur si fth ereactio ni sdrive nt oth eleft ,e.g . bya lowerin go fth eC0 2-pressure (witha consequen t risei npH) , orb ya n increase inio nconcentration s to thepoin t where the solubility product ofC a andCO , isexceeded .

The formationo fa calci c horizon isno weasil y understood: Thepartia l C02-pressure of the soil-air is normally highest in theA-horizo n where root activity andrespiratio nb ymicro-organism s cause CO,content s tob e 10 to 100 times higher than in the atmospheric air.A s a consequence, calcite dissolves and Ca -an dHC0," _1ons move downward with percolating soil moisture,particularl y during and directly after arai n shower.Th e waterma ytak eu pmor e dissolved calcite onit swa ydown . Evaporation ofwate r anda decreas e inpartia l C02-pressure deeper inth e profile (fewer roots and less soil organic matter and micro-organisms) cause saturation of the soil solution and precipitation of calcite.Th e precipitated calcite isno to ronl ypartl ytransporte dbac kwit h ascending water because much of this water moves in thevapou r phase. (The water table inCalcisol s isnormall y deep;wher e there iscapillar y rise toth e solum, calcite accumulates at the depth where the capillary water evaporates.)

Calcite doesnot (always) precipitate evenly distributed over the soil matrix. Root channels andwor m holes that are connected with theoutsid e air act as ventilation shafts inwhic h thepartia l C02-pressure is much lower than in the soil around it.Whe n Ca(HC03)2-containing reaches such a channel, it loses C02 and calcite precipitates along the walls. Continued accumulationproduce s the 'pocketso f soft powdery lime' that are mentioned in the definition of Calcisols. Where narrow root channels become filled with calcite, so-called 'pseudomycelium' forms. Other characteristic forms ofcalciu m carbonate accumulation inCalcisol s arehar d concretions ('calcretes')an d calcite pendants belowpebbles .

The high soil temperature andhig h pHo fCalcisol s enhance dissolution ofsilic a fromfeldspars ,ferromagnesia nminerals ,etc .Wher e therei s(o r was) sufficient moisture insom e period of theyea r fortranslocatio n of dissolved silica,thi sma yhav eaugmente d theinduratio no fth elaye rwit h calcite accumulation. However,cementatio n ofa petrocalci chorizo n isi n first instanceb ycalciu m andmagnesiu m carbonates. Calcisols 207

CHARACTERISTICS OF CALCISOLS

Most Calcisols have a thin (=<10cm) ,brow n orpal e brown A-horizon over a slightly darker Bck-horizon and/or a yellowish brown Cck- or Cmk-horizontha ti sspeckle dwit hwhit ecalcit emottles .Th eorgani cmatte r content of the surface soil is low on account of a sparse vegetation and rapiddecompositio no fvegeta ldebris .Th esurfac esoi li scrum bo rgranul ­ ar, but platy structures can occur as well, possibly enhanced by a high percentage of adsorbed magnesium. The subsurface soil has weak blocky structures or is structureless; the structures are coarser, stronger and often more reddish in colour in an illuvial Bt-horizon than in soils without clay translocation. Thehighes t calcite concentration isnormall y found inth edeepe rB-horl - zonan ddirectl ybelo wth eB-horizon .Burrowin gmammal shomogeniz eth esoi l andbrin ghardene dcarbonat enodule st oth esurface ;thei rbackfille dhole s oftenexten d deep into theC-horizo n ('krotovinas').

Hvdrological characteristics

Calcisols arewel ldraine dan dar ewe tonl yi npar to fth e (short)rain y season when there is just enough downward percolation to flush soluble salts to the deep subsoil. One reason why Calcisols as a taxonomie unit have good drainage properties is that (carbonatic)soil s inwe t positions (depressions, seepage areas) quickly develop salic properties in their aridic environment andke you ta sSolonchaks .

Physical characteristics

Most Calcisols have a medium or fine texture and a good water holding capacity. Where surface soils are silty, slaking and crust formation may hinder theinfiltratio no frai nan dirrigatio nwater .Surfac e run-offove r thebar e soilcause s sheetwas han dgull yerosio nand ,i nplaces ,exposur e ofa petrocalci c horizon.

Chemical characteristics

Calcisolsar epotentiall yfertil esoils .The ycontai nonl y1 o r2 percen t organic matter (C/N-ratio<10 )bu t they are rich in nitrate and mineral nutrients. The pH(H20; 1:1) is 7 to 8 in the surface soil and slightly higher at a depth of 80 to 100 cm where the carbonate content may be 25 percent ormore .Th e cationexchang e capacity ishighes t inth eA-horizo n (10 to 25 cmol(+)/kg) and decreases slightly with depth. The exchange complex iscompletel y saturatedwit hbases ;C a andM g makeu pmor e than 90percen t ofal l adsorbedcations . 208 Calcisols

MANAGEMENTAN DUS EO F CALCISOLS

Vastarea so fnon-irrigate dCalcisol sar euse dfo rlow-volum egrazing . A crop of wheat or sunflower could be grown after one or a few fallow years,bu tCalcisol sreac hthei rful lproductiv ecapacit yonl y ifthe yar e carefully irrigated. Fodder crops such as 'el sabeem' (sorghum bicolor), Rhodes grass and alfalfa,ar e toleranto fhig h calcium levels.Cotto nan d a score of vegetable crops have successfully be grown on Calcisols fertilized withnitrogen ,phosphoru s and trace elements (Fe,Zn) .Furro w irrigation issuperio r tobasi n irrigationo nslakin gCalcisol sbecaus ei t reduces seedlingmortalit y due to surface crusting;puls e crops inparti ­ cular arever yvulnerabl e inth e seedlingstage . In places, arable farming is hindered by stoniness of the surface soil and/or apetrocalci chorizo na tshallo wdepth . MINERAL SOILS CONDITIONED BY A STERPIC CLIMATE : KASTANOZEMS CHERNOZEMS PHAEOZEMS GREYZEMS

Landforms instepp e regions 211

MAJOR LANDFORMS IN STEPPE REGIONS

Steppes and steppic regionshav e an annual rainfall sum of some 400mm ; more than twice the quantity that falls in true desert areas.There , the rainfall is insufficient to support avegetatio n which could protect the land from erosion. In strong winds, sand saltates to aheigh t of several metresabov eth eground ,an dit simpac twhe nfallin gbac ki ssuc htha teve n fine gravel ismove d ('creep')an d rock surfaces are polished. The dunes and sand plains which form in the process have been discussed earlier in this text, ina chapte r on the landforms inresidua l and shifting sands. Finerparticle s thansan dar e transported insuspensio nove r largedistan ­ ces until they settle as 'loess', predominantly in the steppe regions adjacent to thedeser tzone .Th e 'redrain 'whic h falls inwester n Europe everyno w and then, isSahara n dust; it isactuall y a thin loessdeposit .

LANDFORMS INREGION S WITH LOESS

Chinese records make mention of major (historical) periods of loess deposition between 400 and 600 AD, between 1000 and 1200 AD and between 1500an d 1900A D (the 'Little Ice Age'), but themos t extensive occurren­ ceso floes so neart h liei nth esteppi c regions ofeaster nEurop e andth e USA and are of Pleistocene age (see Figure 1).Therefore , some attention mustb egive nt oth erelatio nbetwee nIc eAg earidit yan dloes sdeposition .

SOILO FLAS T 10,000YEAR S

Fig. 1. Climatic history recorded ina Czechoslovakian brickyard. Events of thepas t 130,000year s are recorded as asequenc e of soils and ina quarr y atNov eMesto . (Based on thewor k ofG.J . Kukla.) 212Landform s in steppe regions

During the Late Pleniglacial, between 20,000 and 13,000 BP, some 25 percent of the land surface became covered with continental ice sheets (versus some 10 percent today), the sea level sunk to about 100 metres below thepresen t level,an dlarg epart so fth eworl dwer e extremely arid. The Amazon rain forest dwindled to isolated réfugia, European forests disappeared but for small sheltered areas, and large parts of the globe turned to tundra,steppe , savannah ordesert .

Clearly, eolianprocesse s were muchmor e important at that time thana t present. Large parts of the present temperate zone, from the cover sands of theNetherland s to the sand dunes innortheaster n Siberia,ar e IceAg e (eolian)sands .Sout han deas to fthi scove rsan dbel tlie sa bel t ofloes s deposits,extendin g fromFrance ,acros sBelgium , thesouther nNetherlands , Germany and large parts of Eastern Europe into the vast steppes of the SovietUnion ,an d furthereas tt oSiberi aan dChina .Se eFigur e 2.A simi­ lar east-west loessbel t exists in theUS A and less extensive areas occur on the southernhemisphere ,e.g . inth eArgentinia npampas .

Fig. 2.Distributio n of loess inEurope . Source:Flint ,1971 .

Loessi sa well-sorted ,usuall ycalcareous ,unstratified ,yellowish-grey , eolian clastic sediment. It consists predominantly of silt-sized quartz grains (2-50 Urn), andcontain snormall yles stha n2 0percen tcla yparticle s and less than 15 percent sand. It covers the land surface as ablanket , which isles stha n8 metre s thick inth eNetherland s (exceptionally 17 m), but reaches 40metre s ineaster nEurop e and 330metre s inChina .Loes s is aver yporou smateria lan dvertica lwall sremai nremarkabl y stable,bu t it slakes easily so that exposed surfaces arepron e toerosion . Landforms instepp e repions 213

The loessmateria l isprobabl y producedb y abrasion ofroc k surfacesb y glaciers. It isdifficult ,however , to identify specific source areas for specific loessdeposits ,becaus e thevariou s loessdeposit shav e asurpri ­ singly homogeneous mineralogy. A possible explanation might be that glaciers abrade large surfaces ofdivers e mineralogy, so that theminera - logicalvariatio nbetwee n different source areas isaverage d out. Loess is absent from regions which were covered by glaciers in the last glacialperiod ,no r does itoccu r inth ehumi dtropics .

The eolian origin of loess has long been a subject of dispute; until today, some workers regard loess as an 'alluvial-lacustrine deposit', althoughmos tloes sdeposit slac kth estratificatio ntha ttypifie salluvia l deposits.Thos ewh oadvocat e theeolia norigi no floes shav e the following arguments: (1)loes s occurs asa blanke t overa wid e range of surfaces,mor e or less independent of the topography; (2)loes sblanket s are thickest on the leeward sides of topographic obstacles; (3)ther e isabsolutel y no (cor)relationbetwee n themineralog y ofth e loessblanke t and that of the subsurface strata (which rules out the possibility ofweatherin g in-situ); (4)th e grain size distribution of loesswhic h is typical of eolian materials transported insuspension ; (5)grai n sizes showa downwin d fining gradient,awa y from thesource ; (6)loes s depositsbecom e thicker towards theirpresume d source; (7)fossi l terrestrial snailshav ebee n found inloes s deposits,an d (8)loes s deposition isstil l going onaroun d thepresen t desertareas .

Loess settleswhe n dust-ladenwind s slow down to 7 (ondr y surfaces)t o 14metre spe rsecon d (onmois t surfaces).Th eparticula rpor e distribution ofloes smake s that it isquickl y retainedb y capillary forces ifi tland s ona mois tsurface .Th epresenc eo fa vegetatio ncove rma yals oenhanc eth e rateo floes sdeposition ,an dman yauthor smaintai ntha tth enorther nlimi t ofloes sdepositio ncoincide swit hth enorthernmos texten to fgras ssteppe s during aridperiod s inth ePleistocene .

It has already been said that small-scale stratification is usually absent from loess due to the extremehomogeneit y of its grain size.Lami ­ nated 'loessoid' depositswer e normally redistributed by postdepositional sheetwash .Larger-scal e layering, (deci)metres thick,i softe n indicative ofa certai nperiodicit y inloes s deposition.A t least two separate loess sequences were identified in the Netherlands, a Weichselian one, and an older, Saalian,sequence .Th eboundar ybetwee nth etw osequence s ismarke d by apaleoso l (areli c soilprofile )whic h developed in the Saalianloes s during the intervening interglacial, the Eemian.

Thevas tloes splain sar eno wcolonize db ya clima xvegetatio no fgrasse s and/orforest ,an dar eth ehom eo fsom eo fth ebes tsoil so fth eworld :th e 'black earths'. Deep, black CHERNOZEMS occupy the central part of the Eurasian and North 214Landform s instepp e repions

American steppe zones.Dusk y red PHAEOZEMS are the soils of slightly more humid areas such as the American prairies and pampas. Brown KASTANOZEMS occur in the drierpart s of the steppe zone andborde r arid and semi-arid lands,wherea sGREYZEM Sar eprominen t inth eforest-stepp e transitionzon e tomor ehumi d temperate regions. Kastanozems 215

KASTANOZEMS

Soilshavin g amolli c A-horizonwit ha mois tchrom ao fmor e than2 t o a depth of at least 15 cm;havin g one ormor e of the following: acalci c or gypsic horizon or concentrations of soft powdery lime within 125 cm of the surface, lacking a natric B-horizon; lacking the characteristics which are diagnostic forVertisols , s orAndosols ; lacking salic properties; lackinggleyi c propertieswithi n 50c mo fth e surfacewhe nn o argic B-horizon ispresent .

Key toKastanoze m (KS)Soi lUnit s

Kastanozems having agypsi c horizon.

Other Kastanozems having anargi c B-horizon. Gypsic Kastanozems (KSj)

OtherKastanozem s having acalci c horizon. LuvicKastanozem s (KS1)

OtherKastanozems . Calcic Kastanozems (KSk)

Diagnostic horizon; seeAnne x 1fo r full definition. Haplic Kastanozems (KSh) Diagnostic property; seeAnne x 1fo r fulldefinition .

SUMMARYDESCRIPTIO N OF KASTANOZEMS

Connotation: (dark)brow n soils rich inorgani cmatter ; fromL . castanea. chestnut and fromR . zemlja. earth,land .

Parentmaterial :a wid e range ofunconsolidate d deposits;a largepar t of allKastanozem s have developed inloess .

Environment: dr y and warm; flat to undulating grasslands with ephemeric grasses.

Profiledevelopment :mostl yAhB Cprofile swit ha brow nAh-horizo no fmediu m depth over a brown to cinnamon cambic or argic B-horizon and with lime and/or gypsic accumulation ino rbelo w theB-horizon .

Use:man yKastanoze m areasar e inus e forextensiv e grazing; theprincipa l arable landus e isth eproductio n ofsmal lgrain s and (irrigated)foo dan d vegetable crops. Drought and (wind and water) erosion are serious limitations. 216Kastanozem s

REGIONALDISTRIBUTIO N OF KASTANOZEMS

Kastanozems occupy some 400 million hectares worldwide. Major areas of occurrence are in the southern USSR, in the USA, Mexico, southern Brasil, and in the drier parts of the pampa regions of Uruguay and Argentina. Figure 1show s the regional distribution of theKastanozems .

Fig. 1.Kastanozem sworldwide .

GENESIS OF KASTANOZEMS

The genesis of Kastanozems is largely conditioned by the prevailing (climate-determined) type of vegetation. This is typically a short grass vegetation, scanty, poor in species and dominated by ephemers (early ripening grasses). The above-ground dry biomass amounts to only 0.8-1 tons/hectare, and thedr y rootmas s to 3-4 tons/hectare. More than 50percen t of all roots are concentrated in the upper 25 cm of the soil and there are few roots that extend deeper than 1 metre. The greaterpar to fth evegetatio ndie seac hsummer .I nth eequilibriu msitua ­ tion, the organic matter content of the Ah-horlzon lies between 2 and 4 percent and seldom exceeds 5percent . Downwardpercolatio n insprin gleache ssolute sfro mth esurfac et oth esub ­ surface and subsoil layers. Lime accumulates at a depth of 90-100 cm, gypsumaccumulatio noccur scommonl y ata dept hbetwee n15 0an d20 0cm ,an d inth edries tKastanozem s therema yb ea laye ro fsal taccumulatio n deeper than20 0c mbelo w thesurface . In places, a clay illuviation horizon is found as deep as 250-300 cm below the soil surface.Th e occurrence ofargi c B-horizons inKastanozem s Kastanozems 217 is still ill-understood. They may be fossil, as claimed by some Russian soil scientists,bu t there are also theories of amor e recent formation, through 'normal'translocatio n ofclay ,o rb y destruction ofcla y or fine earthnea r the surface and reformation atgreate r depth.

Climatic gradients in the Kastanozem belt are reflected in pedogenic features. In theUSSR , thedarkes t surfacehorizon s occur inth enort h of theKastanoze mbel t (bordering theChernozems )wherea s soilswit hshallow ­ er and lighter coloured horizons are more abundant in the south. There, effervescencewit hHC lstart salread y inth eA-horizo nan dextend sdownwar d throughout theprofile .Th edifferentiatio nbetwee nhorizon s iscleare r in thenort h than inth e south of theKastanoze mbel twhic h isa consequence ofth edecreasin g lengthan d intensityo fsoi lformatio nwit h increasingly aridconditions .

CHARACTERISTICS OF KASTANOZEMS

Themorpholog y ofdar kKastanozem s isno tver y different from thato f the southern (drier) Chernozems whereas light Kastanozems (may) show a considerable resemblancewit h Calcisols.Th enorther n Eurasian Kastanozem has anAh-horizo n of some 50 cm thick, dark brown andwit h a granular or fine blocky structure, grading into a cinnamon or pale yellow massive to coarse prismatic B-horizon. Inth edrie r south, theAh-horizo n isonl y 25 cm thick and colours are lighter throughout theprofile . Reportedly,argi cB-horizon shav ea "mor eintens ecoloration "i nLuvi cKas ­ tanozems. Calcic and/or gypsichorizons ,presen t inmos tKastanozems ,ar e particularly prominent in those of the southern dry steppes. Krotovinas occur inal lKastanozem sbu t are lessabundan t than inChernozems .

Hydroloeical characteristics

Kastanozems have an intermittent water regime. In the dry period, the soils dry out to great depth. In wet periods, the soils are often incompletely moistened, partly because of the low totalprecipitatio n sum and partly because the non-capillary porosity of Kastanozems is often rather low so that surface run-off losses during and after aheav y shower canb e considerable.A 'dead dry horizon' occurs below the limit ofwet ­ ting;thi shorizo nreceive sneithe rpercolatio nwate rfro mabov eno rcapil ­ lary rise frombelo w and is 'physiologically dead'.

Physical characteristics

Thephysica lpropertie so fKastanozem s areslightl yles sfavourabl e than those of Chernozems but otherwise comparable. The lower humus content of the surface layer,particularl y inth e lighter Kastanozems, is associated with a lower degree of micro-aggregation, which translates into a lower total pore volume (40-55 percent), a lower moisture storage capacity, a denserpackin g of the soil anda lowerpermeabilit y towater . 218Kastanozem s

Chemical characteristics

Kastanozems arechemicall y richsoil swit ha catio nexchang e capacityo f 25-30 cmol(+)/kg dry soil, and a base saturation percentage that is typically 95percen to rmore .Th emajorit yo fal ladsorbe dcation sar eC a andM g ;ESP-value s of4 to 20hav ebee n reported. Thehumou ssurfac ehorizo ncontain ssom e0.1-0. 2percen tnitroge nan d0.06 - 0.15 percent "phosphoric acid".Th eC/N-rati o ofth eorgani c soil fraction is around 10 as inChernozems . The soil-pH is slightly above 7.0 but may increase to avalu e around 8.5 at some depth below the surface. Lime and gypsum accumulation are common; lime contents are some 10 to 20 percent higher in the accumulation horizon than in the deeper solum. More easily soluble saltsma y accumulate aswell , deeper inth e darkKastanozem s than in the lighter soils of the drier steppe. The salt content of the accumulation layer is commonly between 0.05 and 0.1 percent and doesnot seriously inhibit the growth ofcrop sbu t salt levelsma ybuil d up to 0.4 percent andhigher .

MANAGEMENTAN DUS EO FKASTANOZEM S

Kastanozemsar epotentiall yric hsoils ;periodi clac ko fsoi lmoistur e isth emai nobstacl e tohig hyields .Irrigatio ni snearl y alwaysnecessar y wherebycar eshoul db etake nno tt ointroduc esecondar ysalinizatio no fth e surfacesoil .Smal lgrain san d (irrigated)foo dan dvegetabl ecrop sar eth e principal commodities. Extensive grazing is another important land use on Kastanozems but the sparsely vegetated grazing lands are inferior to the tall grass steppes with Chernozems. Chernozems 219

CHERNOZEMS

Soilshavin g amolli c A-horizonwit h amois t chroma of 2o r less to a depth of at least 15 cm; having a calcic horizon or concentrations of softpowder y lime3withi n12 5c mo fth esurface ,o rboth ;lackin ga natri c B-horizon; lackingth echaracteristic swhic har ediagnosti c forVertisols , Planosolso rAndosols ;lackin gsalic 3properties ;lackin ggleyic 3propertie s within 50 cm of the surface if no argic B-horizon is present; lacking uncoated silt and quartz grains onstructura lpe dsurfaces .

Key toChernoze m (CH)Soi lUnit s

Chernozemshavin ga nargic *B-horizo nan dshowin ggleyi c propertieswithi n 100c m of the surface. Gleyic Chernozems (CHg)

Other Chernozems having anargi c B-horizon. Luvic Chernozems (CHI)

OtherChernozem sshowin gtonguin g ofth eA-horizo nint oa cambi c B-horizon or into aC-horizon . Glossic Chernozems (CHw)

Other Chernozems having acalci c horizon. Calcic Chernozems (CHk)

0th er Ch ern ozems . Haplic Chernozems (CHh)

Diagnostic horizon; seeAnne x 1fo r fulldefinition . a Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OF CHERNOZEMS

Connotation:blac k soilsric h inorgani cmatter ;fro mR . ehern,black ,an d zemlja. earth or land.

Parentmaterial ;mostl y eolian sediments (loess).

Environment: fla t toundulatin g plains with avegetatio n of tall grasses (forest in thenorther n transitional zone) in regions with a continental climatewit h coldwinter s andho tsummers .

Profile development: AhBC profiles with a dark brown to black mollic A-horizon over a cambic or argic B-horizon; commonly with redistribution of calcium carbonate to acalci c horizon or pockets of softpowder y lime inth e subsurface soil. 220Chernozem s

Use: thehig h natural fertility of Chernozems and their favourable topo­ graphypermi t awid e range ofagricultura l uses including arable cropping (with supplemental irrigation indr y summers)an d cattle ranging.

REGIONALDISTRIBUTIO N OF CHERNOZEMS

Chernozems cover anestimate d 300millio nhectare s worldwide,mainl y in the middle latitude steppes of Eurasia and North America, north of a zone with Kastanozems. Figure 1present s an overview of their main areas ofoccurrence .

Fig. 1.Chernozem sworldwide .

GENESIS OF CHERNOZEMS

Chernozems form inuniforml y textured siltyparen tmateria l (loess), under a tall grassvegetatio n with avigorou s development over a lengthy vegetative period. The above groundbiomas s amounts to some 1 to 1.5 tons of dry matter per hectare; the corresponding root mass, already incorpo­ rated in the soil, weighs 4 to 6 tons/hectare. The main concentration of roots is in the upper 60 cm of the soil, with 80 percent of all roots concentrated inth e top 30 to4 0cm . SeeFigur e2 .

Deep,humus-ric hChernozem soccu r inth ecentra lpar to fth estepp ezon e where the annualprecipitatio n sum isapproximatel y equal to theevapora ­ tion sum. Such Chernozems contain 10 to 16 percent organic matter, are neutral in reaction (pH 7.0, and about 7.5 in the subsoil), and highly Chernozems 221 saturatedwit hbases .Earthworm sar ever yactiv ethere ,i nwe tperiod spre ­ dominantly in theuppe r 50c m layerbu t theymov e to deeper strata atth e onset of thedr yperiod .A considerable part of the surface soil consists of worm casts, a stable mixture of mineral and organic soil material. Further homogenization of the soil is brought about by burrowing small vertebrates, so that theAh-horizo nma y extend toa dept h of 2meters .

*"'•***

Fig. 2.Tal lgras sstepp e inth eChernoze mbel t (Tselinograd, USSR). Photo by courtesy of ISRIC,Wageningen .

Deep percolation during wet spells has removed virtually all easily soluble salt from theprofile ; the salt content isseldo mhighe r than 0.1 percentwithi n theuppe r4 meter so fsoil .A tabou t thesam edepth ,deepe r inth enort h ofth eChernoze mbel tan dclose r toth esurfac e inth esouth , lies a 'dead dry horizon' that receives neither percolation water from above nor capillary rise frombelow .Th e depth atwhic h itoccur s and its thickness vary; the dead dry horizon is not continuous and may even be absent altogether. At adept h of 2 to 3meter s (1.5 to 2.5 m in southern Chernozems), there mayb e someaccumulatio n ofgypsu m and shallower still,sa y ata dept ho f about 1metr e from the surface, accumulation of lime. Migration of clay has resulted in a faint clay bulge at 50 to 200 cm depth in some Cher­ nozems. Thes e phenomena indicate that the central Chernozems are exposed tomoderatel y strong leaching duringwe tperiods . 222 Chernozems

Towards the northern fringe of the Chernozem belt, the surface horizon with organic matter accumulation becomes shallower and more greyish in colour until signs of podsolization such as an eluvial E-horizon and/or horizontallamella ebegi nt ooccur .Th ehorizo nwit hcarbonat e accumulation isofte n separated from thehumi c surface layerb y acarbonate-fre e layer of appreciable thickness. The absence of a horizon with readily soluble salts,th esligh tacidit y (pH6-6.5 )o fth esurfac e soil,an dth epresenc e ofgreyis hcolour sar efurthe rindication stha tth enorther nChernozem sar e subject to stronger leaching than those of the central steppe zone.

Towards thesouther nfring eo fth estepp ezone ,th ewate r regimebecome s more and more intermittent, with increasingly longer dry periods. As a consequence,plant swit ha lon gvegetativ eperio ddisappea ran dxerophyte s and early ripening grasses move in.Moreover , the soil's humus undergoes increasingly intensemineralizatio nan dther ei sa nincreas ei nth econten t of readily soluble salts in the surface soil. In places, the soil's exchange complexma ybecom e saturated with sodium ions to theexten t that the soils develop 'sodic properties' (See Annex 2) and soil structures collapse, which leads to a denser packing and a lower hydraulic conduc­ tivity thancommo n inth e soils of thecentra l steppezone . The colour of the surface soilha s diagnostic value:whe n thechrom a of the (mollic)A-horizon s has become higher than 2, this is seen as a sign that aridity is so severe that the soils are no longer true Chernozems; they are thenclassifie d asKastanozems .

CHARACTERISTICS OF CHERNOZEMS

Mostvirgi n Chernozems have a thin leafy litter layer on top of the dark grey to black A-horizon which is 'vermicular' (i.e. 'full of worm casts'). The surface horizon is normally between 50 and 80 cm thick but may extend to a depth of 2m and more inwel l developed Chernozems. Worm casts andkrotovina s testify of intense faunal activity.

Insom e instances,th eAh-horizo npenetrate s theB-horizo n with tongues that are deeper thanwide . Calcium carbonate accumulation may show up as pseudomyceliumi nth elowe rpar to fth esurfac esoi land/o ra slim enodule s in abrownis h grey to light cinnamon subsoil.Th e B-horizon has ablock y toprismati c structure. LuvicChernozems ,havin ga nargi cB-horizon ,ar eno tuncommo n inth enort h ofth estepp e zonewher e thegras svegetatio ngrade s intodeciduou s forest andth eChernozem sgrad eint oGreyzem so rLuvisols .Chernozem s inwe tarea s may develop gleyic properties; they are classified as Gleyic Chernozems only if they display clear signs ofhydromorph ywithi n onemete r from the soilsurfac ean dhav ea nargi cB-horizon .Soil swit ha molli cA-horizo nan d with gleyicpropertie s already inth euppe r 50c m layerbu tno thavin g an argic B-horizonke y outa s (Mollic)Gleysols .

Mineralogical characteristics

Theminera lan dmechanica lpropertie s ofChernozem s arerathe rhomogene ­ ous, inlin ewit h theunifor m composition of theparen tmaterial .Sesqui - Chernozems 223 oxides and silica are uniformly distributed over the profile (nomigra ­ tion); th e Si02/R20-ratio ishigh ,abou t2.0 .

Hydrological characteristics

Although it iswidel y accepted that Chernozems formed under conditions of good drainage, there are also (Russian) soil scientists who maintain thatcertai nChernozem spasse d througha bogg yphas e ofsoi lformation .A t present,Chernozem sar erarel ywater-saturated ,apar tfro msoil si n(small ) depressions with occasional shallow groundwater. There is approximate equitybetwee nth eannua lprecipitatio n suman devaporation ,wit ha sligh t precipitation surplus inth enort ho fth estepp ezon ean da sligh t deficit in the south. Table 1present s an overview of the occurrence of Eurasian Chernozems as a function of the annual precipitation sum and the type of vegetation.

TABLE1 . TypicalMajo r SoilGrouping s and SoilUnit s inth e Eurasian steppezone .

TEMPERATURE PRECIPITATION VEGETATION TYPE SOIL GROUPING/UNIT

1 > 550m m deciduous forest Luvisols, Greyzems 1 500m m steppe and forest Luvic Chernozem 1 500m m tallgras s steppe Haplic Chernozem increase 450m m tallgras s steppe Calcic Chernozem 1 200-400m m mediumheigh t grass Kastanozems 1 steppe V < 200m m openvegetatio n Calcisols

Physical characteristics

Chernozems possess favourable physical properties. Their total pore volume lies normally between 55 and 60 percent in the Ah-horizon and between4 5an d5 5percen t inth eC-horizon . The soilshav e ahig hmoistur e holding capacity; reported soil moisture contents of some 33 percent at fieldcapacit yan d1 3percen ta tpermanen twiltin gpoin tsugges ta n'avail ­ ablemoisture 'conten taroun d2 0volum epercent .Th establ emicro-aggregat e structure of the humus-rich Ah-horizons (Table 2) ensures a favourable combination ofcapillar y andnon-capillar y porosity andmake s these soils suitable for irrigated farming.

TABLE2 . Mechanical and micro-aggregate composition of the upper horizon of an ordinary (central)Chernozem . Source:N.A . Kachinsky (Plyusnin, 196?).

FRACTION(mm) : 1-0.25 0.25-0.05 0.05-0.01 0.01-0.001 <0.001 after dispersion: l.î 35.2 27.6 35.4 no dispersion: 35.5 45.3 16.7 2.5 224 Chernozems

Chemical characteristics

Chernozem surface soilscontai n 5t o1 5percen t 'mild'humu swit h ahig h proportiono fhumi c acidsan da C/N-rati o that istypicall y around 10.Th e surface horizon isneutra l in reaction (pH 6.5-7.5)bu t thep H may reach avalu eo f7.5-8. 5i nth esubsoil ,particularl ywher ether ei saccumulatio n oflime .Chernozem shav e agoo dnatura l fertility status;th esurfac e soil contains0.2-0. 5percen tnitroge nan d0. 1 to>0. 2percen tphosphorus .Thi s phosphorus isonl ypartl y 'available';crop so nChernozem s tendt orespon d favourablyt oP-fertilizers .I nsouther nChernozems ,th ehumu scontent sar e lower (4-5percent )an dconsequentl yals oth ecatio nexchang ecapacity :20 - 35cmol(+)/k gdr ysoil ,versu s40-5 5cmol(+ )pe rk gi ncentra lChernozems . Normally,th ebas e saturationpercentag e liesclos et o9 5percen twit hC a andM g asth emai nadsorbe dcations ,bu tsodiu madsorptio nma yb ehig hi n southern Chernozems.

MANAGEMENTAN DUS EO F CHERNOZEMS

Russiansoi lscientist svalu eth e(deep ,central )Chernozem samon gth e bestsoil so f theworld .Wit h lesstha nhal fo fal lChernozem s inth eUSS R being used for arable cropping, these soils constitute a formidable resource for the future. SeeFigur e 3.

Fig. 3. Arable farming on Chernozems in the USSR. Photo by courtesy of ISRIC,Wageningen . Chernozems 225

Wheat,barle yan dmaiz ear eth eprincipa lcrop sgrown ,beside sothe rfoo d crops and vegetables; part of the Chernozem area is used for livestock rearing. Summerdrough tmake smoistur econservatio n imperative.Preservatio no fth e favourablesoi lstructur ethroug htimel ycultivatio nan dcarefu lirrigatio n at low watering rates prevents ablation and erosion. Application of P-fertilizers iscommonl y required. 226 Notes

NOTES Phaeozems 227

PHAEOZENS

Soilshavin ga molli c A-horizon;lackin ga calci c horizon,a gypsi c horizon, concentrations of soft powdery lime ;havin g a base saturation (byI MNH 4OAc atp H 7.0)whic h is5 0percen t ormor e throughoutwithi n12 5 cmo fth esurface ;lackin ga ferrali c B-horizon;lackin ga natri c B-hori- zon;lackin g thecharacteristic swhic har ediagnosti c forVertisols ,Niti - sols, Planosols or Andosols; lacking salic properties; lacking gleyic properties within 50c m of the surface ifn o argic B-horizon ispresent ; lackinguncoate d siltan d quartz grains on structural ped surfaces ifth e mollic A-horizonha sa mois tchrom ao f2 o rles st oa dept ho fa tleas t1 5 cm.

Key to Phaeozem (PH)Soi lUnit s

Phaeozems showing gleyic properties within 100c m of thesurface . Gleyic Phaeozems (PHg)

Other Phaeozems showing stagnic properties within 50c m of thesurface . Stagnic Phaeozems (PHs)

Other Phaeozems having anargi c B-horizon. Luvic Phaeozems (PHI)

Other Phaeozems that are calcareous at least between 20 and 50 cm from thesurface . Calcaric Phaeozems (PHc)

OtherPhaeozems . Haplic Phaeozems (PHh)

Diagnostic horizon; seeAnne x 1fo r full definition. a Diagnostic property; seeAnne x 2fo r full definition.

SUMMARYDESCRIPTIO N OF PHAEOZEMS

Connotation: darksoil sric h inorgani cmatter ;fro mGr .phaios . duskyan d R. zemlja. earth,land .

Parent material: eolian (loess) and other unconsolidated, predominantly basicmaterials .

Environment: flat toundulatin g land inwar m tocoo l (e.g.tropica lhigh ­ land)regions ,humi denoug h thatther eis ,i nmos tyears ,som epercolatio n ofth esoil ,bu tals owit hperiod si nwhic hth esoi ldrie sout .Th enatura l vegetation is tall (prairie)gras s and/orforest . 228Phaeozem s

Profile development: mostly AhBC profiles with amolli c A-horizon over a cambic or argic B-horizon.

Use: Phaeozems are fertile soils;the y are grown tocereal s andpulse s or areuse d asgrazin gland .Periodi c drought,win dan dwate r erosionar eth e main limitations.

REGIONALDISTRIBUTIO N OF PHAEOZEMS

Phaeozems cover an estimated 100millio nhectare s worldwide, mainly in theNort hAmerica n Prairie region, thepampa s ofArgentin a andUrugua y andth esubtropica lstepp eo feaster nAsia .Smalle rarea soccu ri nmediter ­ ranean climates and in montane areas in the tropics, e.g. in eastern Africa. Figure 1present s themajo r Phaeozemareas .

Fig. 1. Phaeozemsworldwide .

GENESIS OF PHAEOZEMS

Phaeozems occur inmor ehumi d environments thanChernozem s orKasta - nozemsan do n fine-texturedbasi cparen tmaterial .Consequently , thebio - mass production is high but also the rates of weathering and leaching. Calcium carbonate is leached out of the soil profile but leaching isno t so intense that the soilshav e become depleted ofbases . Besides,earth ­ worms andburrowin gmammal shomogeniz e the soil andbrin g carbonates from below (back) to the leached surface soil. Inplaces, the faunal activity is so intense that the mollic A-horizon is thickened and worm holes and krotovinas extend into theC-horizon . Phaeozems 229

Argic B-horizons do occur in Phaeozems,bu t they are widely regarded as relics from an earlier development towards Luvisols (in eras with a more humid climate).

CHARACTERISTICS OF PHAEOZEMS

Phaeozems have normally abrow n to greymolli c A-horizon of 30-50c m thickness overa brow ncambi cB-horizo n ora yellowis hbrow nC-horizon ,o r overa brow no rreddis hbrow nargi cB-horizon .Th eA-horizon s ofPhaeozem s arethinne rtha nthos eo fChernozem san dlighte ri ncolour .Wher egroundwa ­ ter isa t shallow depth or temporarily aperche dwate r table occurs,e.g . onto po fa nargi c B-horizon, thesurfac e soilma yb emottle d and/ordark . Luvic soil units,polygeneti c ornot , represent amor e advanced stage of soil formation and have oftenmor e reddish colours than other Phaeozems; they include the former 'Reddish PrairieSoils' .

Hvdrological characteristics

Inspit e of their generally goodwate r storage capacity,man y Phaeozems are short ofwate r inth edr yseason .

Physical characteristics

Phaeozems are porous,wel l aerated soils with moderate to strong, very stable, crumb to blocky structures. Where clay illuviation occurs, the illuviation layercontain s commonly 10-20percen tmor ecla y thanth eover ­ lyinghorizon .

Chemical characteristics

Theorgani cmatte rconten to fth etopsoi l istypicall yaroun d 5percent ; theC/N-rati o of theorgani cmatte r is 10-12. pH-valuesar ebetwee n 5an d 7 and increase towards the C-horizon. The cation exchange capacity of Phaeozems is 25-30 cmol(+)/kg dry soil or somewhat lower; the base saturation percentage lies between 65 and 100 percent, with the highest values inth edeepe r subsoil.

MANAGEMENTAN DUS EO F PHAEOZEMS

Phaeozemsar efertil esoil san dmak eexcellen tfar mland .Luvi cPhaeo ­ zems are said to be somewhat 'better' than Haplic Phaeozems because of their higher water holding capacity. In theUS A and Argentina, Phaeozems are widely grown to wheat (and other small grains) and, in the USA, to soybean. Wind and water erosion are serious hazards. Large areas of Phaeozems are inus e forcattl e rearing/grazing. 230 Notes

NOTES Grevzems 231

GREYZEMS

Soilshavin g amolli c A-horizonwit h amois t chroma of 2o r less to a depth of at least 15 cm and showing uncoated silt and quartz grains on structural ped surfaces;havin g an argic B-horizon; lacking the charac­ teristicswhic h are diagnostic forPlanosols .

Key toGreyze m (GR)Soi lUnit s

Greyzems showing gleyic properties within 100c m of thesurface . Gleyic Greyzems (GRg)

Other Greyzems. Haplic Greyzems (GRh)

Diagnostichorizon ; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OF GREYZEMS

Connotation: dark soilswit h agre y tinge due to thepresenc e ofbleache d (uncoated)quart z sand and silt inlayer s rich inorgani c matter, fromE . grey andR . zemlia. land.

Parent material: decalcified unconsolidated materials including eolian, fluvial and lacustrine deposits,solifluctio nmateria l and glacial till.

Environment: flat to gently undulating plains inmil d temperate to cold, subhumidclimates .Onl yi ncontinenta lsumme rperiod swil lth esurfac esoi l dry out. Inmontan e areas,Greyzem s occur at altitudes corresponding with the change of steppe to forest (vertical zonality).

Profiledevelopment :mostl yAhBt Cprofile swit ha molli cA-horizo nove ra n argic B-horizon overunaltere d parentmaterial .

Use: Greyzems produce fairly good crops of cereals, sugar beets, peas, potatoes and fodder crops in the USSR and of small grains, oil seeds, forage and timber inCanada . Inplaces ,gleyin g andunfavourabl e climatic conditions are serious limitations.

REGIONALDISTRIBUTIO N OF GREYZEMS

Greyzems covera nestimate d2 8millio nhectare s inth enorther nhemi ­ sphere where they occupy a (zonal)positio nbetwee n Luvic Chernozems and Luvisols atth etransitio n fromtal lgras ssteppe s toland swit h deciduous mixed forests.Majo r areas ofoccurrenc e are indicated inFigur e 1. 232 Grevzems

Fig. 1.Greyzem sworldwide .

GENESIS OF GREYZEMS

Greyzems hold an intermediate position between Chernozems and Luvisols: they have themolli c A-horizon of the Chernozems and the argic B-horizono f theLuvisols .

It isunclea rwhethe r the argic B-horizon ofGreyzem s isolde r thanth e mollic A-horizon or, alternatively, degradation of the mollic A-horizon initiated the formationo f the argic B-horizon.

If the first hypothesis holds. Greyzems are to be seen as Luvisols that have acquired amolli cA-horizo n after theclimat ebecam ewette r sometime in the Holocene. The steppevegetatio n changed gradually into forest and the mollic A-horizon degraded under the increasing acidification of the forest floor.

Inth esecon dhypothesis , itals o isacknowledge d thatth eclimati ccondi ­ tions under which the mollic A-horizon developed do no longer exist and that the present forest invaded former grasslands. However, it ispostu ­ latedtha tth eformatio no fa litte rlaye ran dacidificatio no fth esurfac e soil under forest caused the degradation of clay-humus complexes in the mollic A-horizon, associated with bleaching of sand and/or silt and formation of an argic B-horizon upon translocation of destabilized fine materialwit hpercolatin g rainwater . Greyzems 233

Whatever the sequence of eventsma yhav e been, there isclearl y ongoing destruction of humus and degradation of the surface soils today. It is likely that the parent material ofmos t Greyzems was calcareous once and thatGreyzem s result froma long continued process ofdecalcificatio nan d gradual loss ofadsorbe dbases .

CHARACTERISTICS OF GREYZEMS

The most striking characteristic of Greyzems is the presence ofun - coated sand and siltparticle s ina molli cA-horizon . Thebleache d grains canb esee nwit ha len so reve nwit hth enake deye .The yma yappea ri nmor e orles shorizonta lband sassociate dwit ha weakl yplat ystructure ,bu tthe y occur also in spots ('pepper and salt' appearance). In Canada, Greyzems with a distinct, albeit thin, albic E-horizon are seen as intergrades to Albic Luvisolswhic h lack themolli cA-horizon .

Hvdrolopical characteristics

Most Greyzems are well drained, have a good moisture storage capacity and suffer from drought in the summer season only in positions near the drier end of their zone (close to the Chernozems). Greyzem s in northern regionsma ysuffe rfro mwetnes swhe nth esoil stha wi nsprin gan dth estil l frozen subsoil doesnotpermi t adequate discharge ofwater .

Physical characteristics

Properly managed Greyzems retaina reasonabl y stable cloddy to granular aggregation in the cultivated layer.Yet , the tendency to deteriorate in wetcondition s and tofor ma surfac e crustwhe ndr ymake s them susceptible toerosio ni nslopin gterrain .Th eargi cB-horizo nma yhinde rroo tpenetra ­ tion ifi ti sdense ,bu t thetextur echang e isnormall y rather gradual.O n the whole, Greyzems are reasonably productive soils with a somewhat in­ creasedproductio n risk.

Chemical characteristics

Greyzems have chemical characteristics similar to those of Chernozems: the organic carbon content of the surface soil liesbetwee n 3 and 5per ­ cent,wit ha C/N-rati oaroun d 10.The yhav e favourablyhig h CECvalue s (25 to3 5cmol(+)/k gdr ysoil )an da bas esaturatio nclos et o10 0percent ,wit h Ca asth edominan t ion.Th enatura l fertility statuso fGreyzem s isgood ; theirpotassiu m level doesno t need correction,bu t they respond favoura­ bly tomoderat e applications ofnitroge n andphosphoru s fertilizers.

MANAGEMENTAN DUS EO F GREYZEMS

Farmers on Greyzems must minimize structure deterioration in the cultivated layer,an db eawar eo fth etendenc y ofthes esoil s topuddl e in (too) wet condition and to cake and crust in a subsequent dry spell. 234Grevzem s

Greyzemsru na somewha tincrease derosio nris ki nslopin gterrain .Forestr y and the growing of fodder crops minimize soil disturbance and structure deterioration. Ifarabl e crops (cereals,suga rbeets ,potatoes )ar egrown , tillage operations should be performed under favourable moisture condi­ tions. Thepoo r drainage of the (mostly eastern Siberian)Gleyi c Greyzems limit the suitability of thesesoils . MINERAL SOILS CONDITIONED BY A (SUB)HUMID TEMPERATE CLIMATE: LUVISOLS PODZOLUVISOLS PLANOSOLS PODZOLS

Landforms intemperat e regions 237

MAJOR LANDFORMS IN TEMPERATE REGIONS

Most of the (present) temperate regions were covered with continental icesheet swhe nth eIc eAge sha dthei rmaximu mexpanse ,an dmassiv eglacia l and fluvioglacial deposits were laid down in these regions when the ice melted. See Figure 1.

Fig. 1.Sketc hma po fEurop ean dnorther nAsia ,showin gsuppose d extento f maximum glaciated area inPleistocen e IceAges . Source:Flint ,1971 .

Periglacial areas, adjacent to the ice-covered regions, are marked by sedimentswit hcharacteristi cdeformatio nstructures ,incurre d inrepeate d freezing and thawing. Strong winds blew sand and silt out of the frozen river plains; this material settled again as cover sands and dunes,and , at greater distance from the source,a s loessblankets . Virtually alllandform si nth etemperat ezon eshow ,i non ewa yo ranoth ­ er, evidence of the greatvariatio n inclimat e conditions during thepas t 100,000years .Bu t theyhav eals oa numbe ro fcommo ncharacteristic s which werebrough t aboutb y theirpresent ,coo l and (sub)humidclimate : - (most)river shav e aregula rregime ; -meander s are thenorma l channel form inalluvia l floodplains ; -river s tend to incise rather thant oaggradate ; - soil formationan dweatherin g predominate over surfacewas h in sloping terrain as long as thenatura lvegetatio n cover remains intact. 238Landform s Intemperat e repions

Within the temperate zone,thre ebroa dmorphotectoni c categories canb e distinguished: (1)Pleistocen e sedimentary lowlandswit h glacial,fluvioglacial ,fluvio - periglacial and eoliandeposits ; (2)uplifte d and dissected sedimentary basins, inplace swit h (Mesozoic) limestones, orwit h sandstones,mudstones , and/or a loessblanket ; (3)uplifte d and dissected Caledonian andHercynia nMassifs ,partl ycon ­ sisting of folded sedimentary and low-grade metamorphic rocks and partly of crystallinerocks .

For a discussion of the landforms in the third category, the reader is referred to theparagrap h onth emorpholog y ofMiddl eMountains .

LANDFORMS IN (PERI)GLACIALAN D EOLIAN SEDIMENTARY LOWLANDS

'Normal' fluvial andmarin e lowlandshav ebee n discussed ina precedin g chapter; the following paragraphs will focus on areas that are underlain by glacial,fluvioglacial ,periglacia l or eolian deposits.Suc h areas are particularly extensive inth etemperat eregion so fth enorther nhemispher e where the continental ice sheets had the greatest expanse during the Pleistocene glacial periods. The southern hemisphere simply lacked suf­ ficientlan da thig hlatitud efo rth edevelopmen to fextensiv eic esheets .

Reconstructions of the Pleistocene climate changes indicate that there were at least four advances ofcontinenta l ice inEurop e ando n theNort h Americancontinent .Eac ho fthes etoo kplac e ina differen tglacia lperiod . Three major advances could be identified in north-west Europe; they are known as the Elster, Saale andWeichse l glacial maxima; still older ones havebee n recognized elsewhere,e.g . inth e SovietUnion .

Glacial advance and retreat conditioned some typical landforms. The commonestare : (1)til lplains ,e.g . theDrent e Plateau inth eNetherlands ; (2)morain e complexes, e.g. theSalpausselk ä inFinlan d and theR a moraine inNorway ; (3)ice-pushe d ridges, e.g. the 'stuwwallen' inth eNetherlands ; (4)tongu e basins such as the 'GelderseVallei 'nea rWageningen , and (5)outwas h plains around terminalmoraine s or ice-pushed ridges.

Regions adjoining the actual ice-covered areas were perennially frozen ('permafrost'), so thathardl y anyvegetatio n could develop; thepresenc e of ice caps was conducive to the occurrence of stronger winds than at present.Larg earea sbecam ecovere dwit heolia ndeposits ;deser tpavements , sandplain s and dunes occur at short distances from the former icefront , and loess deposits farther away. The mechanism of dune formation was discussed in the chapter on residual and shifting sands; loess deposits were treated inth e chapter onsteppe s and steppic regions. Landforms intemperat e repions 239

LANDFORMS INUPLIFTE D SEDIMENTARY BASINS

Upliftedsedimentar ybasin sforme d inwester nEurop eafte r theHercynia n orogeny. Their sediments, mostly shallow-water limestones, marls and calcareous sandstones, date back to Mesozoic transgressions. They were never folded,bu t differential subsidence and lateruplif thav e inplace s resulted in tilting. Cuesta landscapes are a common feature; remnants of Tertiary soils on top of cuesta dipslopes, e.g. the 'clay-with-flint' on Cretaceous chalk ('limonsà silex' inFrance ; 'kleefaarde' or 'vuursteen- eluvium' in the Netherlands), indicate that these basins formed part of extensivepeneplain s inth eTertiary .Uplif ta sa resul to forogen y inth e nearby Alps caused the formation of river terraces and incisedmeanders ; the 'Iled eParis 'i sa nexample . Many of thesebasin sbecam e partially coveredwit h glacial or periglacial deposits.Moraine san dfluvioglacia ldeposit s arewidesprea d onto po fth e Easteuropean Platform; parts of the Paris basin are covered with loess. Onlywher esuc hcover sar eabsent ,e.g .o ncuest aslope stha twer eto ohig h or too steep to collect a thick loess blanket, did soils form in the original parentmaterial .

PODZOLS are strongly represented in fluvioglacial and eolian sands; parabolic dunes of only 2-3 metres height contain fine podzolic catenas which reflect differences ingroundwate r depth. LUVISOLS are among thecommones t soils inth e loessblanket s in temperate regions; they grade into Chernozems towards the drier end of the zone. Luvisols occur also in (less rigidly sorted) fluvial deposits which il­ lustrates thatth eoccurrenc e ofPodzol san dLuvisol s isno tjus ta matte r of climate, but also of parent material and consequently of Quaternary geological history. PODZOLUVISOLS developed in clayey glacial till and fine-textured materials of fluvioglacial or glaciolacustrine origin, but also inloess ,i nregion swit h coldwinter s and short,coo lsummers . PLANOSOLS occur predominantly in subhumid and semi-arid regions on the southernhemisphere . Insom e instances they formed through degradation of Podzoluvisols. 240 Notes

NOTES Luvisols 241

LUVISOLS

Soilshavin g anargi c B-horizonwhic hha s acatio nexchang ecapacity - equal to or more than 24 cmol(+)/kg clay and a base saturation (by IM NH.OAc atp H 7.0) of 50percen t ormor e throughout the B-horizon; lacking a mollic A-horizon; lacking the E-horizon abruptly overlying a slowly permeablehorizon , the distributionpatter n of thecla y and the tonguing whichar ediagnosti c forPlanosols ,Nitisol s andPodzoluvisol srespective - iy-

Key toLuviso l (LV)Soi lUnit s

Luvisols showing gleyic propertieswithi n 100c m of the surface. Gleyic Luvisols (LVg)

Other Luvisols showing stagnic properties within 50c m of thesurface . Stagnic Luvisols (LVs)

Other Luvisolshavin g analbi c E-horizon. Albic Luvisols (LVa)

Other Luvisols showingverti c properties. Vertic Luvisols (LVv)

Other Luvisols having acalci c horizon orconcentration s of soft powdery lime within 125 cm of the surface. Calcic Luvisols (LVk)

OtherLuvisol s showing ferric properties. Ferric Luvisols (LVf)

Other Luvisols having a strong brown to red B-horizon (rubbed soil has a hue of7.5Y Ran d achrom a ofmor e than4 , or ahu e redder than 7.5YR). Chromic Luvisols (LVx)

Other Luvisols. Haplic Luvisols (LVh)

Diagnostic horizon; seeAnne x 1fo r full definition. a Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OF LUVISOLS

Connotation: soils inwhic h clay iswashe d down from the surface soil to anaccumulatio nhorizo na t some depth; fromL . luere. towash .

Parent material: a wide variety of unconsolidated materials including glacial till, and eolian,alluvia l and colluvialdeposits . 242Luvisol s

Environment:mos t common infla t or gently sloping land incoo l temperate regions and inwar m (e.g.mediterranean )region swit h distinctdr yan dwe t seasons. Deciduous forest,coniferou s forest andgrasslands .

Profile development:ABt Cprofiles ;intergrade s toPodzoluvisol shavin ga n albicE-horizo nbetwee nth eA-horizo nan dth eargi cB-horizo nar eno trare . The considerable variation among Luvisols is due to variations in parent material (e.g. lime content) and environmental conditions (wetness, ero­ sion).

Use: becaus e of their moderate stage ofweatherin g and high base satura­ tion, Luvisols with a good drainage status are suitable for awid e range of agricultural uses.

REGIONALDISTRIBUTIO N OF LUVISOLS

Luvisols cover some 500 to 600 millionhectare s worldwide. They are forth egreate rpar t located intemperat eregion s sucha sth ewest-centra l USSR, theUS A and central Europe,bu t they occur also inwarme r environ­ ments,e.g .th eMediterranea nregio nan dsouther nAustralia .Figur e1 give s an indication of themajo r concentrations ofLuvisols .

Fig. 1.Luvisol sworldwide . Luvisols 243

GENESIS OF LUVISOLS

The dominant characteristic of Luvisols is their argic B-horizon, formed by translocation of clay from the surface soil to the depth of illuviation. Theproces s knows three essentialphases :

(1)mobilizatio n ofcla y inth e surface soil. (2)transpor t of clay to theaccumulatio nhorizon . (3)immobilizatio n of transported clay.

Normally, clay isno t present in a soil as individual particles but is contained inaggregate s thatconsis twholl yo fcla yo ro fa mixtur eo fcla y and other mineral and/or organic soil material. Mass transport of soil material along cracks andpores ,commo n incrackin g soils inregion s with alternating wet and dry periods,doe s not necessarily enrich the subsoil horizons with clay. For anargi c B-horizon to form, the (coagulated)cla y mustdispers e inth ehorizo no feluviatio nbefor e itca nb e transported to the depth of accumulationb ypercolatin gwater .

Mobilization of clay canonl y takeplac e ifth e thickness of the electric 'double layer', i.e. the shell around the individual clay particles that is influenced by thenegativel y charged sides of the clay plates, isin ­ creased. If the double layers arewide , thebond s between the negatively charged sides and thepositiv e charges at theedge s of thecla y particles cann o longer hold individual clay particles together in aggregates. The strength ofaggregatio n isdetermine dby : (1)th e ionic strength of the soilsolution , (2)th e composition of the ions adsorbed at the exchange complex,an d (3)th e specific charge characteristics of the clay inth e soil.

Athig helectrolyt econcentration s inth esoi lsolution ,th edoubl elaye r is compressed regardless of the nature of the adsorbed ions so that the claysremai nflocculated .A decreas ei nio nconcentration ,e.g . asa resul t ofdilutio nb yrainfall ,ca nresul t indispersio no fth ecla yan d collapse ofth eaggregates .I fth eexchang ecomple xi sdominate db ypolyvalen t ions, the double layerma y remainnarro w evena t lowelectrolyt e concentrations and consequently theaggregate s may remain intact.Becaus e concentrations ofth evariou s ions insolutio nan dth echarg e characteristics ofth ecla y arenormall y influencedb y thep H of the soil, dispersion ofclay s is,t o some extent,a pH-dependen tprocess . At pH(H,0, 1:1) values below 5, the aluminium concentration of the soil solution isnormall y sufficiently high tokee p the clay flocculated (Al ispreferentiall y adsorbed over divalent and monovalent ions in the soil solution). Between pH 5.5 and 7, exchangeable aluminium contents are negligible and the concentrations of divalent ions low, so that the clay candisperse .A t stillhighe rp Hvalues ,divalen tbase swil lnormall ykee p thecla y flocculatedunles s there isa stron gdominanc e ofNa +-ions inth e soilsolution .I fth ep Hexceed s9 ,calciu man dmagnesiu m ionsprecipitat e from the solution as carbonates. They are replenished from the exchange complexwhic hbecome s increasingly occupiedb ymonovalen t ions. 244Luvlsol s

Certainorgani ccompounds ,especiall ypolyphenols ,stimulat eth emobiliza ­ tiono fcla yb yneutralizin g thepositiv e chargesa tth eedge so fth ecla y minerals.A siron-saturate dorgani c complexes areinsoluble , this process mightb eo flittl e importance inFe-ric h Luvisols inth esubtropics .

Transporto fpeptize dcla yparticle srequire spercolatio nthroug hwid e(>2 0 um)pore san dvoids .Cla y translocationi sparticularl yprominen t insoil s which crack during thedr yseaso nbu tbecom ewe tdurin g occasional down­ pours. Smectite clays disperse more easily than non-swelling clays; they area common constituent ofLuvisols .

Precipitation ofcla y particles takes place atsom e depth inth esoi la s a resulto fflocculatio n ora sa resul to f (mechanical)filtratio n ofth e clay suspensionb yfin e capillarypores . Flocculationca nb e initiatedb ya nincreas e inth eelectrolyt e concen­ trationo fth esoi l solutiono rb ya nincreas e inth econten t ofdivalen t cations (e.g.i na CaCOj-ric h subsurface horizon). Filtrationoccur swher ea cla ysuspensio npercolate sthroug ha relativel y dry soil mass; it forces the clay plates against the faces of peds or against the walls of (bio)pores where skins of strongly oriented clay ('cutans')ar eforme d (seeFigur e 2).Wit h time,th ecutan sma ywholl yo r partly disappear throughhomogenizatio n ofth esoi lb yth esoi l fauna,o r the cutansma yb e destroyed mechanically insoil s with ahig h contento f swellingclays .Thi sexplain swh yther ei softe nles soriente d clayi nth e argicB-horizo n thanon ewoul dexpec to nth ebasi so fa budge t analysiso f thecla yprofile . There could alsob emor e illuviated clay than expected, viz. If (partof )th eeluviate d surface soil islos t througherosion .

Fig. 2. Cutans ('channel ferri-argillans') in the argic B-horizon ofa Luvisol ina Lat e Weichselian alluvial deposit inTh eNetherland s (cross polarized light;1 c m= 10 0/im) . Source:Miedema ,1987 . Luvlsols 245

CHARACTERISTICS OF LUVISOLS

Luvisolshav e abrow n todar kbrow nA-horizo nove r a (greyish)brow n to strongbrow n or redBt-horizon . Insubtropica l Luvisols inparticular , a calcic horizon may bepresen t orpocket s of soft powdery lime occur in andbelo w the reddishbrow n Bt-horizon. Soil colours are less reddish in Luvisols incoo lregion s thani nwarme rclimates .I nwe tenvironments ,th e surfacesoi lma ybecom edeplete do fcla yan dfre eiro noxide st oth eexten t thata greyis heluviatio nhorizo nform sunde ra dar kbu tshallo wA-horizon .

Mineralogical characteristics

Towards thedr y end of their zone,Luvisol shav e increasing contents of swellingan dshrinkin gclay san dpressur eface san dparallelpipe d structure elementsbecom emor e andmor eprominent .Luvisol s aremoderatel yweathere d soils; they contain less AI-, Fe- and Ti-oxides than their tropical counterparts,th eLixisols ,an dhav e an Si02/Al203 ratio ofmor e than2.0 .

Hvdrological characteristics

MostLuvisol sar ewel ldraine dbu tshallo wgroundwate rma y inducegleyi c soil properties in and below the Bt-horizon in Luvisols in depression areas. Stagnic properties develop where adens e argic B-horizon obstructs downwardpercolatio nan dth esurfac e soilbecome s saturatedwit hwate rfo r extended periods oftime .

Physical characteristics

Luvisolshav efavourabl ephysica lproperties ;the yhav egranula ro rcrum b surface soils that are porous and well aerated. The 'available' moisture storagecapacit y isaroun d1 5t o2 0volum epercent .Th eilluviatio nhorizo n has stable blocky structures. Luvisols with a high silt content may be sensitive toerosion .

Chemical characteristics

Although the chemical properties of Luvisols vary with the parent material and pedogenetic history, these soils are generally fertile and havehig hminera l reserves.Th e (decalcified)surfac e soils containa few percent organic matterwit h aC/ N ratio of 10 to 15an dar e slightly acid inreaction ; the subsurface soilshav e aneutra l reaction andma y contain somelime .

MANAGEMENTAN DUS EO FLUVISOL S

With thepossibl eexceptio no fAlbi csoi lunits ,Luvisol s arefertil e soilssuitabl e fora wid erang eo fagricultura luses . Structuredeteriora ­ tionma yoccu ri nLuvisol swit ha hig hsil tconten ti fth esoil sar etille d in wet condition and/or with heavy machinery. Luvisols on steep slopes 246 Luvisols require erosion controlmeasures . Luvisols in the temperate zone are widely grown to small grains, sugar beet, fodder, etc.or , inslopin g land, they areuse d for orchards and/or grazing. In the mediterranean region, where Chromic, Calcic and Vertic Luvisols are common in colluvial deposits of limestone weathering, the lower slopes are often sown towhea t and sugarbeet swherea s the (eroded) upper slopes are inus e forextensiv e grazing or treecrops . Podzoluvisols 247

PODZOLDVISOLS

Soilshavin g anargi c B-horizon showinga nirregula r orbroke nuppe r boundaryresultin gfro mdee ptonguln g ofth eE-horizo n intoth eB-horizon , or from the formation ofdiscret e nodules larger than 2cm , the exteriors ofwhic har eenriche dan dweakl ycemente do rindurate dwit hiro nan dhavin g redder hues and stronger chromas than the interiors; lacking a mollic A-horizon.

Key toPodzoluviso l (PD)Soi lUnit s

Podzoluvisols havingpermafros t within 200c m of thesurface . Gelic Podzoluvisols (PDi)

OtherPodzoluvisol sshowin ggleyi c propertieswithi n10 0c mo fth esurface . Gleyic Podzoluvisols (PDg)

OtherPodzoluvisol sshowin gstagni c propertieswithi n5 0c mo fth esurface . Stagnic Podzoluvisols (PDs)

Other Podzoluvisols having abas e saturation (by IM NH.OAc at pH 7.0) of less than5 0percen t ina t least apar t of theB-horizo nwithi n 125 cmo f the surface. Dystric Podzoluvisols (PDd)

Other Podzoluvisols. Eutric Podzoluvisols (PDe)

Diagnostichorizon ; seeAnne x 1fo r full definition. Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OF PODZOLUVISOLS

Connotation: soils having thebleache d eluviation horizon of Podzols and the illuviationhorizo n ofLuvisols ;fro m Podzols andLuvisols .

Parent material; mostly unconsolidated glacial till, or materials of glaciolacustrine or fluvial origin,o ro f eolian origin (loess).

Environment: flat to undulating plains under boreal taiga, coniferous forest or mixed forest. The climate is temperate to boreal with cold winters and relatively short and cool summers, and an average annual precipitation sum of 500 to 1000 mm. Precipitation is evenly distributed overth eyea ror ,i nth econtinenta lpar to fth ePodzoluviso lbelt ,i tha s apea k inearl y summer. 248 Podzoluvisols

Profile development: mostly AEEtC profiles with a dark but thin ochric A-horizon over analbi c E-horizon that tongues into abrow n argicB-hori - zon. Stagnic soilpropertie s are common inborea l Podzoluvisols.

Use:acidity ,lo wnutrien tstatus ,tillag ean ddrainag eproblems ,th eshor t growing season and severe frost are serious limitations ofPodzoluvisols . Most of these soils are under forest; livestock farming ranks second and arable cropping plays a minor role. In the USSR, the share of arable cropping increases towards the south and west of the Podzoluvisol belt, especially onEutri c Podzoluvisols.

REGIONAL DISTRIBUTION OF PODZOLUVISOLS

Podzoluvisols cover an estimated 262 million hectares, mainly con­ centrated ina broa dbel textendin gfro mPolan dan dwester nRussi aeastwar d intocentra l Siberia. Small occurrences are found outside thisbelt ,e.g . inGermany ,Belgiu man dth eUS A (Michigan).Thei rabsenc efro mCanada ,wit h similar climatic conditions asi nRussi aan dSiberia , isexplaine db y less developed tonguing inCanadia n soils (Albic Luvisols). Figure 1 indicates theworldwid e occurrence ofPodzoluvisols .

er

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Fig. 1. Podzoluvisolsworldwide .

GENESIS OF PODZOLUVISOLS

Thegenesi so fPodzoluvisol sha selement so fpodzolization , discussed in some detail in the chapter on Podzols,an d of argilluviation (treated in the chapter onLuvisols) . Podzoluvisols 249

Podzoluvisols occur in regions with ahars h climate which explains why there islittl ebiologica l activity inthei r surfacehorizons .Leachin go f metal-humus complexes from the litter layer is not offset by biological homogenization so thata distinc t eluvial E-horizon could develop over an illuvialB-horizon .Th esudde nchang e intextur e fromth eeluviatio nhori ­ zont oth e illuviationhorizo naffect sth einterna ldrainag e ofPodzoluvi ­ sols. Periodic saturation of the surface soil and reduction of iron com­ pounds (enhancedb y dissolvedorgani ccompounds )caus e strongbleachin go f the eluvial horizon. The E-horizon extends into the underlying argic B-horizon along root channels and cracks (the characteristic 'tongulng'). Podzoluvisolsar eclosel yrelate dwit hAlbi cLuvisol swhos egeneti chistor y and morphological appearance are very similar to those of Podzoluvisols except that the E-horizon doesnot extend into theargi c B-horizon.

Periodic saturation with water will, in first instance, cause iron compounds toconcentrat e inmottle s orconcretion s ofiro n (hydr)oxidesi n a matrix that is increasingly depleted of iron. After many cycles, the surface soil losesvirtuall y all its iron,eithe r to theunderlyin g illu­ viationhorizo n or to thedeepe r subsoil.Se eFigur e2 . Alternatively, it ispossibl e that ironaccumulation s inth e surface soil grow insiz edu e tohysteresi sbetwee nth eprecipitatio no firo ncompound s in the oxidative phase and their (re)dissolutionwhe n reduced conditions prevail. This latter mechanism produces the discrete nodules, "theexte ­ riors of which are enriched or weakly cemented or indurated with iron", thatar ementione d inth edefinitio n ofPodzoluvisols .

Fig. 2.Iro ndepletio n zones alongvoid s ina Stagnic Podzoluvisol inth e Netherlands (crosspolarize d light;1 c m= 500 jJ,m) . Source:Miedema ,1987 .

The recurrent saturation and leaching of the E-horizon foster acidifi­ cation and loss of exchangeable bases. Ultimately, the leaching of clay 250 Podzoluvisols and sesquioxides from the E-horizon may become sopronounce d that only a sandysurfac esoi lremain si nwhic ha secondar yPodzo lma yeventuall y form. Low organic matter and iron contents explain the generally low structure stability ofth eleache d surface soil; theE-horizo n isnormall y compacted because of its lowresistanc e tomechanica lstress . Alternating wetting and drying affects also thecompositio n of thecla y assemblage. In the extreme case,aluminiu m interlayering and clay decom­ position lead to the development of acid, seasonally wetPlanosols .

CHARACTERISTICS OF PODZOLUVISOLS

Inth enatura l state,Podzoluvisol s occur almostexclusivel y under a forest vegetation. A raw litter layer tops a dark but thin A(h)-horizon over a clearly bleached E-horizon which extends into abrow n Bt-horizon. Both the E-horizon and theB-horizo n aredens e (see Figure3) .

Fig. 3. A Podzoluvisol from the Netherlands. Note the tonguing into the B-horizon. Source:Miedema ,1987 . Podzoluvisols 251

Hvdrological characteristics

Theconfiguratio no fa nE-horizo nove ra Bt-horizo n isi nitsel fevidenc e ofdownwar dwate r flow through the soil during at leastpar t of theyear . However, as the illuviation horizon grows, it becomes more and more an obstacle to percolation and may eventually cause regular stagnation of surface water. The diagnostic features of a Podzoluvisol (iron depletion and tonguing or,alternatively , ironnodule s inth e E-horizon)ma y orma y notb e strong enough toqualif y as stagnicproperties . Permafrost affects thehydrologica l characteristics profoundly.

Physical characteristics

Most Podzoluvisols are medium-textured; areas of coarse and/or finely textured Podzoluvisols occur in glaciolacustrine parent materials. The texture of theE-horizo n isofte nver y sandy.Th e lowbiologica l activity explainswh ymos tPodzoluvisol sar esomewha tcompacte d andwh y theeluvia l horizonha s oftena plat y structure.Th elo worgani cmatte r contento fth e surface soil and the high susceptibility to structure deterioration make it imperative that tillage be done in the proper moisture range. Rooting and uptake of water may be hindered by the dense Bt-horizon and/or by permafrost, either directly, or indirectly by poor internal drainage and inadequate aeration.

Chemical characteristics

The Ah-horizon of Podzoluvisols can contain between 1 and 10 percent organic carbon; theE-horizo n contains rarely more than 1percen t organic C and a similar amount is present in the Bt-horizon. Podzoluvisols are moderately tostrongl y acidwit hpH(l MKCl )value s from lesstha n4 t o 5.5 orslightl yhigher .Th eCE Camount s tosom e 10t o2 0cmol(+)/kg ,exclusiv e of the contribution by organic matter. The base saturation percentage rangeswidely ;value s as low as 10percen thav ebee n reported for Dystric Podzoluvisols with high contents of exchangeable aluminium and Al-inter- layered clays, whereas Eutric Podzoluvisols with little Al interlayering have abas e saturationbetwee n 60an d 90percent . The distinction between Eutric and Dystric Podzoluvisols isbase d on the base saturationo fth eargi cB-horizon ;th eeluvia lhorizo n isalway sver y low inbases .

MANAGEMENTAN DUS EO F PODZOLUVISOLS

Themajo rlimitation so fPodzoluvisol sar epose db ythei racidity ,lo w nutrient levels, tillage and drainage problems and by the climate, with its short growing season and severe frosts. The Podzoluvisols of the northern taiga are almost exclusively under forest; small areas are used as pasture land or hay fields. In the southern taiga zone, less than 10 percent of the non-forested area is used for agricultural production. Livestock farming is themai n agricultural landus e onDystri c Podzoluvi­ sols (dairyproductio n andcattl e rearing); arable cropping (cereals,po ­ tatoes, sugar beets, forage maize) plays a minor role. In the USSR, the 252Podzoluvisol s shareo farabl efarmin gincrease si nsouther nan dwester ndirections ,espe ­ cially onEutri c Podzoluvisols. With careful tillage, liming and application of fertilizers, Eutric Pod­ zoluvisols produce 25-30 tons of fresh potatoes per hectare, 2-5 tons of winterwhea t and 5-10 tons ofdr yherbage . Planosols 253

PLANOSOLS

Soilshavin ga nE-horizo nshowin gstagni c propertiesa tleas ti npar t of the horizon, and abruptly overlying a slowly permeable horizon within 125 cm of the surface,exclusiv e ofa natri c or spodic B-horizon.

Key to Planosol (PL)Soi lUnit s

Planosolshavin g permafrost within 200c m of thesurface . Gelic Planosols (PLi)

Other Planosolshavin g amolli c A-horizon ora eutri c histic H-horizon. Mollic Planosols (PLm)

Other Planosols having anumbri c A-horizon or a dystric histic H-hori­ zon. Umbric Planosols (PLu)

Other Planosols having abas e saturation (by IMNH.OA ca tp H 7.0) of less than 50percen t ina t leasta par t of the slowlypermeabl e horizonwithi n 125c m of thesurface . Dystric Planosols (PLd)

OtherPlanosols . Eutric Planosols (PLe)

Diagnostic horizon; seeAnne x 1fo r fulldefinition . Diagnostic property; seeAnne x 2fo r full definition.

SUMMARYDESCRIPTIO N OF PLANOSOLS

Connotation: soilswit h aneluvia lhorizo n abruptly over adens e subsoil, typically inseasonall y waterlogged flat lands;fro mL . planus. flat.

Parentmateria l:mostl y clayey alluvial and colluvialdeposits .

Environment: level (depressed)area si nfla tt ogentl yundulatin g terrain, mainly insub-tropica l and temperate,semi-ari d and subhumid regions with a climaxvegetatio n of light forest orgrasses .

Profile development: mostly AEBC profiles. Destruction and/or removal of clay has created a bleached and relatively coarse-textured surface soil thati sabruptl yunderlai nb ya claye ysubsurfac ehorizon .Impede ddownwar d movement of water is responsible for alternating oxidizing and reducing conditions in theuppe r part of theprofile .

Use: th emai n limitingfacto r isth e imperviousclaye y subsurfacehorizon . Flooding iscommo n inth erain y seasonan dsever edrough tstres s occurs in 254 Planosols the dry season. Planosols inregion swit h awar m summer season aremostl y used forwe t rice cultivation; elsewhere, for dry crops or as range land.

REGIONALDISTRIBUTIO N OF PLANOSOLS

Themai nPlanoso larea sar eshow ni nFigur e 1.Thei r totalexpans ei s estimated at approximately 150 million hectares worldwide,wit h the most importantoccurrence s inSout hAmeric a (67millio nhectares )an dAustrali a (49millio nhectares) .

Fig. 1. Planosolsworldwide .

Thelarges tarea so fPlanosol sar e insubtropica lan dtemperat eclimate s with aclea r alternation ofwe t and dry seasons,e.g . insouther nBrasil , Paraguay, Argentina, the Sahelian zone, South Africa, theUnite d States, and in easternAsi a andAustralia . Planosols occur predominantly in flat lands with incised drainage channels but they can also be found in other positions in the landscape, e.g. at the foot of slopes, in a strip just above thevalle y bottom, or somewhat higher along the contours of a long and relatively gentleslope .

GENESIS OFPLANOSOL S

Planosolsar e soilswit hon eo rmor euppe rhorizon swit ha relativel y low clay content, abruptly overlying a deeper horizon with considerably more clay.Thi s abrupt textural change canhav e severalcauses : Planosols 255

(1)I tma yb e causedb y geogenetic processes such as sedimentation of sandy over clayey layers,cree p or sheetwas h of lighter textured over clayey material,colluvia l deposition of sandy over clayey material and selective erosionwhereb y the finest fraction isremove d from the surface layers, and/or (2)I tcoul db e causedb y physical pedogenetic processesviz .th e eluviation-illuviation of clay insoi lmateria l with a low structure stability, and/or (3)Chemica l pedogenetic processes mayb e involved,notabl y aproces s called 'ferrolysis',a noxidation-reductio n sequence drivenb y chemical energy derived frombacteria l decomposition of soil organic matter (Brinkman, 1979).

Ferrolysis isthough t toprocee d asfollows :

In the absence of oxygen (e.g. inwater-saturate d soils with reducing organicmatter) ,ferri coxide san dhydroxide sar ereduce dt oF e -compounds which go into solution:

+ 2+ CH20+ 4 Fe(OH)3+ 7H = 4 Fe + HC03"+ 10H 20

TheF e -ionsreplac e adsorbedbasi ccation s andaluminiu ma tth e exchange complex; the replaced ions are partly leached out (together with some of the Fe ).Whe n air re-enters the soil in a subsequent dry period, the exchangeable Fe isoxidize d againt oF e -iron.Thi sproduce s twoH +-ions foreac h Fe -ionoxidized :

2+ + 4 Fe +0 2+ 10H 20- 4 Fe(0H)3 + 8H

Part of thehydroge n ions replace aluminium orbasi c cations in the clay structure; the cations become exchangeable. During the next wet season, Fe -ions are again formed from iron (hydr)oxides and ane w cyclestarts .

During the reduction phase,H +-ions are consumed so that the pHrises . Once itha srise nt oabou tp H 5t o5.5 ,A l -ionsan dOlT'on spolymeriz e to hydroxy-Al-polymerswit h ring structures.Th e polynuclear Al-polymers are thought toremai n 'trapped' inth e interlayerspace so f2: 1 latticeclays . Other exchangeable cations, including Fe , can then be built into the Al-polymers. The polymers in the interlayer spaces do not easily desin- tegrate andthi scause shysteresi sbetwee nwe tan ddr ycycl eprocesse san d gradual alteration of the clay. Eventually, the clay has a lower cation exchange capacity and swells and shrinks less thanbefore .

The abrupt change incla y content and, insom e Planosols, inth enatur e ofth eclay ,ca nonl ydevelo pan dpersis ti fther ei slittl ehomogenizatio n of the soil. Examples are known of established Planosols thatwer e later transformed to Phaeozems because of intense soil homogenization by termites. 256 Planosols

CHARACTERISTICS OF PLANOSOLS

A typical horizon sequence of Planosols consists of an ochric or umbric A-horizon over an albic E-horizon directly on top of an argic B- horizon.I nver ywe tlocation sther ema yeve nb ea dystri chisti c H-horizon whereas inmor eari dregion s thesurfac e soil iscommonl y almostdevoi do f organic matter and cracked, and the soil qualifies as a 'yermicphase' . Thealbi c E-horizon isgreyis han dha s asand yo rloam y texture anda wea k structure of low stability. The most prominent feature of Planosols is themarke d increase in clay contento npassin gfro mth euppe rt oth emiddl ehorizon .Th emiddl ehorizo n may be a slowly permeable argic B-horizon or a heavy clay layer. It is commonly mottled and has a coarse angular blocky, prismatic or massive structure. Clay coatings may be present, but more often the change in texture appears tob edu e tostron gweatherin g insit ui ncombinatio nwit h clay destruction inth etopsoil .

Mineralogical characteristics

Claydestructio nthroug hferrolysi slead st oa lowe ractivit yo fth ecla y fraction (cationexchang e capacity)i nth etopsoi lan dt oa lowe rmoistur e retention capacity.

Hydrological characteristics

Planosols are subject to waterlogging in the wet season because of stagnation of (rain)wate r on level land with a slowly permeable subsoil layer at shallow depth (stagnic soilproperties) .

Physical characteristics

The surface soil horizons have a low structural stability; especially the silty ones become hard as concrete in the dry season and turn to a heavy mud with a very low bearing capacity when waterlogged in the wet season. The sandy topsoil is hard when dry but not cemented. The poor structure stability ofth etopsoil ,th ecompactnes s ofth esubsoi lan dth e abrupt transition from topsoil to subsoil are all detrimental to (the rootingpossibilitie s of)crops .

Chemical characteristics

Planosolswit hsign so fadvance dferrolysi s arechemicall ydegraded .Th e surface soilha s lost (muchof )it sclay ; theexchang e capacityha sbecom e very low andbase s are largely leachedout .

Biological characteristics

The natural vegetation of areas with Planosols is mostly herbaceous. Where treesgrow ,i tconcern s specieswit hextensive ,shallo wroo tsystem s thatar e capable ofwithstandin g severe drought and strongwinds .I nlin e Planosols 257 withth epoo rphysica lan dchemica lpropertie so fPlanosols ,th esoi lfaun a isno tver y diverse andpopulatio n densities arelow .

MANAGEMENTAN DUS EO F PLANOSOLS

Planosols are used forvariou s kinds of land use,albei t at a lower intensity than most other soils under the same climate. Large areas are used for extensive grazing. See Figure 2. Virgin Planosols are under a sparse grass and sedge vegetation or under grass with scattered bushes, treeso rforest .Th ewoo dproductio no ftree so nPlanosol s isnormall yles s thanhal f of that on other soilswit h the sameparen tmaterial .

*•. -o

Fig. 2. Planosols are widely used for extensive grazing. Photograph by courtesy of ISRIC,Wageningen .

Planosols inth e temperate zone aremainl y ingras so rthe y areuse d for arable crops such aswhea t or sugarbeet.Th eyield s are comparatively low asa consequenc eo fth eunattractiv ephysica lan dchemica lsoi lconditions . Root development ishampere d by oxygen deficiency inwe t periods,b y the high density of the subsurface soil andb y toxic levels of aluminium. The low hydraulic conductivity of the subsurface soil makes narrow drain spacingsnecessary .Experiment s inEurope ,spannin ga numbe ro fyear safte r 258 Planosols drainage and deep loosening of the subsoil, failed to show conclusive results.

Planosols in southeast Asia are normally planted to a single crop of paddy rice, produced on bunded fields that are inundated in the rainy season. Efforts toproduc e dryland crops on the same landwit h irrigation during the dry seasonhav eme twit h little success; the soils seembette r suited to asecon d crop ofric ewit h supplemental irrigation. Fertilizers areneede d forgoo dric eyields .Th esoil s shouldb eallowe dt odr you ta t least once a year to prevent or minimize micro-element deficiencies or toxicitiesassociate dwit hprolonge dsoi lreduction .Som ePlanosol srequir e applicationo fmor e thanjus tmacronutrient san dthei rlo w fertility level may prove difficult to correct.Wher e the temperature permits paddy rice cultivation, this isprobabl y superior toan y otherkin d of landuse .

Inclimate swit h long dryperiod s and short infrequentwe t spells,sup ­ plemental irrigation of grassland inth edr y seasonma yhol dpromise . Stronglydevelope dPlanosol swit ha silt yo rsand ysurfac esoi lar eperhap s best left inthei r naturalstate . Podzols 259

PODZOLS

Soilshavin g a spodic B-horizon.

Key to Podzol (PZ)Soi lUnit s

Podzolshavin g permafrost3withi n 200c m of thesurface . Gelic Podzols (PZi)

Other Podzols showing gleyic3propertie s within 100 cmo f thesurface . Gleyic Podzols (PZg)

Other Podzolshavin g aB-horizo n inwhic h asubhorizo n contains dispersed organicmatte r and lacks sufficient free iron to turnredde r on ignition. Carbic Podzols (PZc)

Other Podzols inwhic h therati o ofpercentag e of free iron topercentag e of carbon is 6o rmor e inal l subhorizons of the B-horizon. Ferric Podzols (PZf)

Other Podzols lacking or having only a thin (2 cm or less) and discon­ tinuous albic E-horizon; lacking asubhorizo nwithi n the B-horizon which isvisibl y more enrichedwit hcarbon . Cambic Podzols (PZb)

Other Podzols. Haplic Podzols (PZh)

Diagnostic horizon; seeAnne x 1fo r fulldefinition . a Diagnostic property; seeAnne x 2fo r fulldefinition .

SUMMARYDESCRIPTIO N OF PODZOLS

Connotation: soils with a subsurface horizon that has the appearance of ash due to strong bleaching by aggressive organic acids; from R. pod, under, and zola.ash .

Parent material: unconsolidated siliceous rock weathering, including glacial till, and alluvial and eolian deposits ofquartziti c sands.

Environment:site swit hpoorl ydecompose d organicmatte ran ddownwar dper ­ colationo ffulvi cacids .I nth enorther nhemispher e inleve lt ohill ylan d under heather and coniferous forest; in the humid tropics under light forest.

Profile development: mostly AhEBhsC profiles. Complexes of AI, Fe and organic acids migrate from the surface soil to the B-horizon with per­ colating rain water. The Fe- and Al-humus complexes precipitate in an 260 Podzols illuvial spodic B-horizon; the leached subsurface soil remains behind as ableache d albic E-horizon.

Use: severe acidity, low chemical fertility and unfavourable physical propertiesmak emos tPodzol sunsuitabl efo rarabl ecropping .The yhav esom e potential for forestry and extensive grazing.

REGIONALDISTRIBUTIO N OF PODZOLS

Podzolscove rsom e47 8millio nhectare sworldwide ,almos texclusivel y inth etemperat ean dborea l regionso fth enorther nhemispher e (seeFigur e 1). Besides these zonal Podzols, there are smaller occurrences of intra­ zonal Podzols,bot h inth e temperate zone and inth etropics . Tropical Podzols total not more than 10 million hectares, mainly in residual sandstone weathering in perhumid regions and in old, alluvial quartz sands. The exact distribution of tropical Podzols is not known; important occurrences are along theRi oNegr o and inth eGuiana s inSout h America, in the Malesian region (Kalimantan, Sumatra, Irian), and in northernAustralia . They seem ratheruncommo n inAfrica .

Fie. 1. Podzolsworldwide .

GENESIS OF PODZOLS

Podzolization (theformatio no fa spodi cB-horizon )i sactuall ya suc ­ cession ofprocesses , including (1) 'cheluviation', themovemen t of solublemetal-humu s complexes (chelates)ou t of thesurfac e layer(s)t o greater depth,an d Podzols 261

(2) 'chilluviation', the subsequent accumulation ofAl - andFe-chelate s ina spodic B-horizon. (Soluble organic compounds canmov e to still deeper horizons.)

Cheluviation

Soluble organic substances producedb ymicrobia l (mainly fungal)attac k onplan t litter, form complexeswit hA l -an d Fe -ions andmov e downward with the soil solution. There is little known of the rates atwhic h such reactions proceed in natural soil systems; they seem to depend on the nature andconcentratio no fth eorgani ccompound s insolution ,th epH ,th e mineral surface area that issusceptibl e toattack ,an d thenatur e of the minerals.

Fulvic acidsar eth edominan tcomplexin gorgani ccompounds ;thei rcarbo - xylic and phenolic groups act as 'claws' (Gr. chela, hence the term 'chelation')whic h preferably grab polyvalent metal ions such asA l and Fe .Chelatio n continues as long as the fulvic acids are not saturated withmeta l ions. Fulvicacid sar ecomparativel y abundantwher elo wtemperatures ,lo wchemi ­ cal soil fertility and/or periodic water saturation retard the microbial decompositiono forgani cmatter .Ther ei slittl efauna lactivit yunde rsuc h unfavourable conditions,s otha tmos t soilorgani cmatte r (andth ehighes t concentration of fulvic acids)remain s inth esurfac e soil.

Fulvic acids are soluble in water. However, as more and more reaction sites become occupied by Al and Fe ,th e solubility product decreases; fully saturated fulvic acids are practically insoluble in water. If the soilsupplie sA l ando rF e atlo wrate scompare dwit hth erat eo ffulvi c acidproduction ,fulvi cacid sca nmigrat eove rconsiderabl edistances ,eve n overhundred s of metres.Normally , however, thebul k of all fulvic acids issaturate dwit hA l and/orF e aftermigratio nove ronl y afe wdecimet ­ ress otha ta nilluvia lB-horizo n (spodicB-horizon ) formsi nth eprofile . Insand ysoil s thatar ever y lowi nF e andA l ,th espodi c B-horizonma y occur at a depth of several metres or, in the extreme case, a spodic B-horizon may not form at all and (the bulk of) the fulvic acid is dis­ charged ineffluen t 'blackwater' .

Chilluviation

Normally,water-solubl eorgani csubstance sreac twit hweatherin gproduct s while percolating downward until the ratio of (Al+Fe) to organic carbon becomes sufficiently high that the complex precipitates. Alternatively, saturationma yb ebrough tabou tb yevaporatio no fth esolvent ,o rlow-rati o complexes mayb e absorbedb yhigh-rati o complexes that are alreadypreci ­ pitated. The result isa naccumulatio nhorizo n inal lcases .

Evaporation ofsoi lmoistur e aftera single shower causes precipitation of Al- and Fe-fulvates at the depth to which water penetrated the soil. Partial decomposition of the precipitated complexes increases the degree ofAl -an dFe-saturatio no fth eremainin gfulvi cacids ,whic hprevent stha t 262 Podzols these dissolve again during the next rain. The humus 'fibres' in well drained Podzols areprobabl y formed inthi sway . The accumulation process is reversible to some extent. If uncomplexed humic substances are added at a faster rate than Al -an d Fe -ions are releasedb yweatherin gminerals ,th eAI,Fe-humu scomplexe swil lacquir ea n ever lower metal-to-organic ratio and become increasingly soluble. Ultimately, an entire spodic B-horizon can be moved to a greater depth. Strongly podzolized soils with aver y thick albic E-horizon occur in the humid tropics. The illuvial B-horizon of such soils isver y deep ('Giant Podzols'), orma y evenb e absent altogether.

Podzolizationversu s ferralitization

From thepoin t ofvie w of soil formation,opposit e processes takeplac e in Podzols, where Fe- and Al-oxides dissolve and iron and aluminium are leached out, and in Ferralsols where Fe- andAl-oxide s remain stable and increase inconten tthroug hrelativ eaccumulation .I tha slon gbee na poin t of academic debate whether the Podzol or the Ferralsol represents the ultimate in (weathering and)soi l formation inth ehumi d tropics. One suggestion was that there is only one fundamental process, that of podzolization, and that red tropical soils are essentially soils in an early stageo fpodzolization ,bu t itha sals obee nsai dtha tpodzolizatio n could followferralitizatio n asth esoi lbecome sprogressivel ymor eacid . Thebleache dhorizon s ofwel ldevelope dPodzol sar eforme dwhe nvirtual ­ ly allweatherabl e constituents (including non-quartz silica and suchAl ­ and Fe-compounds as may be present) are removed by intense leaching, leaving behind only a skeleton of quartz sand grains. Strongly leached Ferralsols,however , althoughver y low incation s andwit h ap H of4. 0 or less,sho wn o tendency todevelo p aneluvia lE-horizon ; theferri c ironi s present inhighl y stable forms.

The situation canperhap s be summed up as follows: In thewe t tropics, soil formation will produce a Ferralsol in well drained clayey parent materials that are rich in iron; a Podzol will result in imperfectly drained, coarse-textured and quartz-rich materials which receive organic matter thatdecompose s slowly under conditions of oligotrophy.

CHARACTERISTICS OF PODZOLS

Not all soils with evidence of podzolization qualify as Podzols. The FAO-Unesco classification system stipulates that a Podzol must have a spodic B-horizon,wit h identifiable 'active amorficmaterials ' of organic matter and aluminium.

NOTETHA Talthoug h therear e twocharacteristi chorizon s ina Podzol ,viz . aneluviatio n and an illuviationhorizon , only the illuviationhorizo n is a classification criterion. By convention, a soil with a thin Ah-horizon over aneluviatio nhorizo n that extends tomor e than 125 cm depth, isno t classified as aPodzo l even ifa marke d illuviationhorizo n occurs deeper than 125 cm. Despite a history of strong podzolization, such deeply eluviated soils aren o Podzols anymore; theyhav ebecom eAlbi cArenosols . Podzols 263

A typical zonal Podzol has a pale grey, strongly leached E-horizon between a dark surface horizon with organic matter, and a brown to very dark brown spodic B-horizon. Most Podzols have a surface litter layer of 1 to 5 cm thick, loose and spongy, and grading into an Ah-horizon with partlyhumifie d organicmatter . Inth elitte rlaye r inparticular ,mos to f the individual plant fragments are still recognizable and live roots may bebese twit hmycorrhizae . TheAh-horizo n consists of adar k greymixtur e of organic matter and mineral material (mainly quartz) with many of the grainsshowin gblac k isotropic coatings.Th eunderlyin gbleache d E-horizon has acharacteristi c singlegrai nstructure ;th estructur e ofth ebrow nt o blackBhs-horizo nvarie s fromloos ethroug hfir msubangula rblock y tover y hard andmassive . At thedrie r end of the climatic range forzona l Podzols,th e illuviation horizon has commonly a high chroma signifying accumulation of iron and aluminium oxides.I nmor ehumi d regions,th eBhs-horizo ni sdarke ran dha s ahighe rconten to ftranslocate d organicmatter .Wher e theparen tmateria l consists of (almost)pur e quartz,a Bh-horizon may form inwhic h the il- luviatedmateria l isentirel y organic.Ther ema yb e an indurated layero r Cx-horizon (fragipan) in the lower solum. Figure 2 shows the typical AhEBhsC configurationo fa matur ePodzol .

Fig. 2.A Podzol inful l splendour.Photograph :th e Marbut collection. 264Podzol s

Theprofil e of a typical intrazonal tropical lowland Podzolha s a surface layero fpoorl y decomposed ('raw'), acidhumu swit h ahig hC/N-ratio . This O-horizon isunderlai n by ahumus-staine d A-horizon over a light grey to white eluvial E-horizon of sand texture thatvarie s in thickness from 20 cm tomor e than 2metre s (Giant Podzols). The Bh-horizon iscommonl y dark brown and irregular in depth. In places, there are mottles or soft con­ cretions of secondary ironan d aluminium oxides,and/o r there is slightly more clay in the illuvial horizon than higher in the profile. Brightly coloured B(h)s-horizons with sesquioxide accumulation as occur in the temperate zone,ar e less common inth etropics .

Mineralogical characteristics

The mineralogy of Podzols is somewhat variable but is nearly always markedb y ahig hquart zcontent .I ncool ,humi dclimate swher e leachingi s intense, the parent material may originally have been of intermediate or even basic composition. A bleached E-horizon is hardly noticeable then; there is only a thick, dark B-horizon of intricately mixed organic and mineralmaterial . The highest concentration of free Fe^O, occurs in the middle of the illuviation horizon; the lowest contents are in the upper portion of the eluviationhorizon .Ther e isnormall ymor e aluminium than ironpresen t in the B-horizon but since ferric oxideshav e strong colours, the ironcom ­ pounds aremor econspicuous .

Hydrological characteristics

Most Podzols formed under conditions of free movement ofwate r through theuppe r and middle portions of the soil; where the illuviation horizon is dense, or an indurated layer (fragipan) has formed, water movement through the subsoilma yb e impaired.Althoug h Podzols are associated with regions having an annual precipitation surplus, their low water holding capacity may induce drought stress indr yperiods .

Physical characteristics

The clay content isnormall y lowo rver y low; it seldom exceeds 10per ­ cent by weight in the bleached eluviation horizon but could be slightly higher in the illuvialhorizon .Th e sandy texture facilitates tillage.

Chemical characteristics

The organicmatte r profile ofPodzol s shows two areas of concentration, viz. amajo r one at the surface and amino r one inth eBh(s)-horizon . The C/N-ratio has a characteristic pattern with values of 20 to 50 in the surface horizon, decreasing to 10 to 15 in thebleache d horizon and then increasing again to 15 to 25 in the illuviation horizon. The contents of 'total' iron and aluminium are highest in the spodic B-horizon; Al-humus complexes tend to accumulate somewhat deeper in the illuviation horizon thanFe-humu s compounds. SeeFigur e 3. Podzols 265

The chemical fertility ofPodzol s islo wa sa consequenc e of theirhig h degree ofleachin g and their low CEC.Th e latter islargel y a function of the properties and distribution of soil organic matter. Plant nutrients are concentrated inth e surfacehorizon(s )wher e cycling elements arere ­ leased upon decomposition of organic debris. The surface horizons are normally acid with pH(H20, 1:1) values between 3.5 and 4.5; the pH-value increaseswit h depth toa maximu m ofabou t 5.5 inth e deep subsoil.

HCl-Extr . Fe& Al (mg/g)

o() 10 20 «) 1C > 20 () 10 20 \J a • i i i i à ' ' • ' • à Fe D Fe •"-^LJ— _ _ O VC Os n, n b OD 20 (D a OTJ' A 1 / A O D o a' " A' ' O D If 40 / SM .-'AI w O O D-» o a' OD *•** 1/ W ly co eu JE 60 \ Q_ O/Di « a o r> a 80 r S^ r CD / CD 100 r 1! Q

• ^ CD ion U

Fig. 3.Thre eiro nan daluminiu mprofile so fPodzol sunde rdeciduou sfores t in New Hampshire,USA . Note that the distributions of free iron and free aluminium diverge intw o of the three B-horizons (Wood, 1980).

Biological characteristics

Larger soil animals such as earthworms are scarce in Podzols; decom­ position of organic matter and surface soil homogenization are slow and aremainl y doneb y fungi,smal l arthropods andinsects . 266 Podzols

MANAGEMENTAN DUS E OF PODZOLS

The low nutrient status, low level of available moisture and lowp H make Podzols unattractive soils for agricultural production. Intrazonal Podzols are more frequently reclaimed for arable farming than zonal Pod­ zols,particularl y those intemperat e climates.Dee p ploughing to improve themoistur e storage capacity of the soil and/or to eliminate a denseB - horizono rfragipan ,an dlimin ear eth emai nameliorativ emeasure sapplied . Podzols need full fertilization with micro-nutrients inadditio n to N, P andK .I nplaces, stonines s and/orhig hsensitivit y toerosio nar efurthe r limitations toarabl e farming. Most zonal Podzols are under forest or are used for low volume grazing. Tropical Podzols are normally under a light forest that recovers only slowly after cutting/burning. By and large,matur e Podzols are best left under theirnatura l (climax)vegetation . REFERENCES

References 269

LITERATURE CITED

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NOTES ANNEXES

Annex 1 275

DEFINITIONS OF DIAGNOSTIC HORIZONS

(Source: theRevise d Legend of the SoilMa p of theWorld , 1988,p p 20-27)

Soilhorizon s that combine ase to f featureswhic h areuse d for soil identificationar ecalle d 'diagnostichorizons' .Sinc eth echaracteristic s of soil horizons are produced by soil forming processes, the use of diagnostic horizons for separating soils relates the classification to general principles of soil genesis. Objectivity is secured, however, in that the processes themselves are not used as criteria but only their effects,expresse d interm so fquantitativel y definedmorphometri cproper ­ ties thathav e identificationvalue . Someo fth echaracteristic s usedt oclassif y soilsd ono t (always)occu r in connection with horizons. They are 'diagnostic properties'. The definitions ofdiagnosti c properties arepresente d inAnne x2 .

The histic H-horizon is anH-horizo n which ismor e than 20 cmbu t less than 40 cm thick. Itca nb e more than4 0 cmbu t less than 60 cm thick if it consists for 75percen t or more,b y volume, of sphagnum fibres orha s abul k densitywhe nmois to f less than0. 1 Mg/m. A surface horizon of organic material less than 25 cm thick qualifies as ahisti cH-horizo n if,afte rhavin gbee nmixe d toa dept ho f2 5cm ,i tha s 16 percent or more organic carbon if the mineral fraction contains more than 60percen t clay, or 8percen t ormor e organic carbon if the mineral fraction contains no clay,o r intermediate contents oforgani c carbonfo r intermediate contents of clay. The same criteria apply to aploug h layer which is 25c m ormor e thick.

Themolli cA-horizo n isa nA-horizo nwhich ,afte r the surface 18c mar e mixed as inploughing ,ha s the followingproperties : 1. the soil structure issufficientl y strong that thehorizo n isno tbot h massive andhar d orver yhar dwhe ndry .Ver y coarseprism s larger than 30c m indiamete r are included inth emeanin g of 'massive' ifther e is no secondary structurewithi n theprisms . 2.bot hbroke n and crushed sampleshav e colourswit h achrom a of less than 3.5 whenmoist ,an d avalu e darker than 3.5 whenmois t and 5.5 when dry; thecolou rvalu e isa t leaston euni t darker than thato f the C-horizon (bothmois t and dry). Ifa C-horizo n isno tpresent , comparison shouldb emad ewit h thehorizo n immediately underlying the A-horizon. If there ismor e than4 0percen t finely divided lime,th e limits of colourvalu e 'dry'ar ewaived ; thecolou rvalu e 'moist' shouldb e 5o r less. 3. thebas e saturation (by IMNH.OA ca tp H 7.0) is 50percen t ormore . 4. the organic carbon content isa tleas t 0.6 percent throughout the thickness ofmixe d soil, as specifiedbelow .Th eorgani c carbon content isa t least 2.5 percent if thecolou r requirements arewaive d because of finely divided lime.Th euppe r limit oforgani c carbon content of themolli c A-horizon isth e lower limit of thehisti c H-horizon. 276 Annex 1

5. the thickness is1 0c m ormor e ifrestin g directly onhar d rock, ona petrocalcic horizon,a petrogypsi c horizon, ora duripan ; the thick­ ness of theA-horizo nmus tb e at least 18c m andmor e thanon e third of the thickness of the solumwher e the solum isles s than 75c m thick, andmus tb emor e than 25c mwher e the solum ismor e than 75c m thick. Themeasuremen t of the thickness of amolli cA-horizo n includes transitionhorizon s inwhic h thecharacteristic s of theA-horizo n are dominant.

6.Th e content ofP 205 soluble in1 percen t citric acid is less than 250mg/k g soil. This restriction ismad e to eliminateploug h layers of very oldarabl e soils orkitche nmiddens .Th e restriction does not apply,however , ifth e amount ofP 20csolubl e incitri c acid increases below theA-horizo n orwhe n the soil contains phosphate nodules,a s mayb e thecas e inhighl y phosphatic parentmaterials .

Thefimi cA-horizo n isa man-mad esurfac elaye r5 0c mo rmor ethic kwhic h has been produced by long continued manuring with earthy admixtures. The fimic A-horizon commonly contains artefacts such as bits of brick and potterythroughou tit sdepth .I fth efimi cA-horizo nmeet sth erequirement s of the mollic or umbric A-horizon, it is distinguished from itb y aPpO , content (by 1 percent citric acid)whic h ishighe r than 250 mg/kg soil. (Both the analytical method and the limiting amount of P205 are likely to be changed inth e future).

The umbric A-horizon is comparable to the mollic A-horizon in its requirements concerning colour, organic carbon and phosphorus content, consistency, structure and thickness.Th e umbric A-horizon, however, has abas e saturation (by IMNH.OA ca tp H 7.0) of less than 50percent .

The ochric A-horizon is an A-horizon that is too light in colour, has toohig h achroma ,to o little organic carbon,o r is too thin tob e mollic or umbric, or isbot h hard and massive when dry. Finely stratifiedmate ­ rials, e.g. surface layers of fresh alluvial deposits, do not qualify as anochri cA-horizon .

The albic E-horizon is a horizon from which clay and free iron oxides have been removed, or in which the oxides have been segregated to the extent that the colour of thehorizo n is determined by the colour of the primarysan dan dsil tparticle srathe rtha nb ycoating so nthes eparticles . An albic E-horizon has a colour value 'moist' of 4 or more, or a value 'dry'o f5 o rmore ,o rboth . Ifth evalu e 'dry'i s7 o rmore ,o rth evalu e 'moist' is6 o rmore ,th echrom a is3 o r less either 'dry'o r 'moist'.I f thevalu e 'dry' is 5o r 6, or if thevalu e 'moist' is 4 or 5, the chroma isclose r to 2tha nt o 3eithe r 'dry'o r 'moist'.I f theparen t materials have a hue of 5YR or redder, a chroma 'moist' of 3 is permitted in the albic E-horizonwher e thechrom a isdu e to thecolou r ofuncoate d silto r sand grains. Annex 1 277

Theargl cB-horizo n isa subsurfac ehorizo nwhic hha sa distinctl yhighe r clay content than the overlyinghorizon .Th e textural differentiation may bedu et oa nilluvia laccumulatio no fcla yo rt odestructio no fcla y inth e surfacehorizon ,o rt oselectiv esurfac eerosio no fclay ,o rt obiologica l activity,o rt oa combinatio no ftw oo rmor eo fthes edifferen tprocesses . Sedimentation of surface materials which are coarser than the subsurface horizonma y enhance apedogeni c textural differentiation. However,a mer e lithological discontinuity, such as may occur in alluvial deposits,doe s notqualif ya sa nargi cB-horizon . Ifa nargi cB-horizo n isforme db ycla y illuviation, clay skins may occur on ped surfaces, in fissures, in pores and inchannels . An argic B-horizon mustmee t the following requirements: 1.hav e a texture that issand y loam or finer andhav e at least 8percen t clay inth e fineeart h fraction. 2. lack the set ofpropertie s which characterize the ferralic B-horizon. 3.contai nmor e total clay thana noverlyin g coarser-texturedhorizo n (exclusive ofdifference s which result froma lithological discontinuity only): (a)i fth eoverlyin ghorizo nha s less than1 5percen t totalcla y in the fine earth fraction, the argic B-horizonmus t contain at least 3percen tmor e clay; (b)i f the overlyinghorizo nha s 15percen t ormor e and less than4 0 percent total clay inth e fine earth fraction, the ratio ofcla y inth e argicB-horizo n to that inth eoverlyin ghorizo nmus tb e 1.2 ormore ; (c)i f theoverlyin ghorizo nha s 40percen t ormor e total clay inth e fine earth fraction, theargi c B-horizonmus t contain at least 8 percent more clay. 4. ifa tleas t somepar to f theargi cB-horizo n shows clay skins ona t least 1percen t ofbot hhorizonta l andvertica lpe d surfaces and in thepores ,o r shows oriented clays ina t least 1percen t of the cross section, the increase incla y contentmus tb e reachedwithi na verti ­ caldistanc e of 30cm . If therequiremen t for thepresenc e ofcla y skins isno tmet ,th e increase incla y contentbetwee n the argic B-horizon and theoverlyin ghorizo nmus tb e reachedwithi n avertica l distance of 15cm . 5. the argicB-horizo nmus tb e atleas ton e tentho f the thickness of the sum of all overlyinghorizons . If the argic B-horizon is entirely composed of lamellae, the lamellaemus thav e acombine d thickness of at least 15cm . 6. thecoarser-texture dhorizo n overlying theargi c B-horizon mustb e at least 18c m thickafte rmixing , or 5c m ifth e textural transitiont o theargi c B-horizon isabrupt .I fa soi l shows a lithological discontinuity or ifonl y aploug h layer overlies theargi c B-horizon, at least somepar t of thehorizo nneed s to show clay skins ona t least 1 percent ofbot hhorizonta l andvertica l surfaces and inth epores , or showoriente d clays ina t least 1percen t ofth ecros s section. 7. lack thestructur e and sodium saturation characteristics of thenatri c B-horizon. 278 Annex 1

The definition of the argic B-horizon recognizes distinct textural differentiation as a diagnostic feature, even if clay skins cannot be identified. On the other hand, an accumulation layer marked by a low CEC ofth eclay ,a lo wconten to fdispersibl e clay,an da lo wsilt-cla yratio , asma y occur inFerralsols ,i sexclude d from the argic B-horizon.

The natric B-horizon has the properties 1 through 6 of the argic B-horizon asdescribe d above. Inadditio n ithas : 1. acolumna r orprismati c structure insom epar t of theB-horizon , ora blocky structurewit h tongues ofa neluvia lhorizo n inwhic h there are uncoated silt or sand grains extending more than2. 5 cm into the horizon. 2. a saturationwit h exchangeable sodium ofmor e than 15percen twithi n theuppe r 40 cm of thehorizon ; ormor e exchangeable magnesium plus sodium thancalciu mplu sexchang e acidity (atp H 8.2)withi n theuppe r 40 cm of thehorizo n if the saturationwit h exchangeable sodium is more than 15percen t insom e subhorizonwithi n 200c m of thesurface .

The cambic B-horizon isa naltere dhorizo n lackingpropertie s thatmee t therequirement s ofa nargic ,natri c orspodi cB-horizon ; lacking thedar k colours, organic matter content and structure of thehisti c H-horizon, or themolli c andumbri cA-horizons ;havin g the followingpropertie s: 1.ha s a texture of sandy loam or finer andha s at least 8percen t clay inth e fine earth fraction. 2. isa t least 15c m thickwit h itsbas e at least 25 cmbelo w the soil surface. 3. soil structure isa t leastmoderatel y developed or rock structure is absent ina t leasthal f thevolum e of thehorizon . 4.ha s acatio n exchange capacity ofmor e than 16cmol(+)/k g clay,o rha s a contento f 10percen t ormor eweatherabl emineral s in the 50-200/i m fraction. 5. shows evidence ofalteratio n inon e of the following forms: (a)stronge r chroma,redde rhue ,o rhighe r clay content thanth e underlyinghorizon ; (b)evidenc e ofremova l ofcarbonates . (c)i fcarbonate s are absent inth eparen tmateria l and inth e dust that fallso n the soil, therequire d evidence ofalteratio n is satisfiedb y thepresenc e ofsoi l structure and the absence of rock structure inmor e than 50percen t of thehorizon . 6. showsn o cementation, induration,o rbrittl e consistence whenmoist .

The spodic B-horizon is an Illuviationhorizo n which meets one ormor e of the following requirements below a depth of 12.5 cm, or, if present, below anA-horizo n ora n E-horizon: 1. asubhorizo nmor e than2. 5 cm thick that iscontinuousl y cementedb ya combination oforgani cmatte rwit h irono r aluminium orwit hboth . 2. asand y orcoarse-loam y texturewit h distinct darkpellet s of coarse siltsiz eo r largero rwit h sand grains coveredwit h cracked coatings which consist oforgani cmatte r and aluminiumwit h orwithou t iron. Annex 1 279

one ormor e subhorizons inwhich : (a)i f there is0. 1 percent ormor e extractable iron, the ratio of ironplu s aluminium extractableb y pyrophosphate (atp H 10)t o percent clay is0. 2 ormore ,o r ifther e is less than0. 1 percent extractable iron,th e ratio ofaluminiu m plus carbon tocla y is 0.2 ormor e;an d (b)th e sum ofpyrophosphate-extractabl e ironplu s aluminium ishal f ormor e of the sumo fdithionite-citrate-extractabl e ironplu s aluminium; and (c)th e thickness issuc h that the index ofaccumulatio n of amorphous material (CECa tp H 8.2 minus onehal f thecla y percentage multiplied by the thickness incentimetres ) inth ehorizon s that meet theprecedin g requirements is6 5o rmore .

The ferralic B-horizon isa horizo nthat : 1.ha s a texture that issand y loamo r fineran dha s atleas t 8percen t clay in the fine earth fraction. 2. isa t least 30c m thick. 3.ha s acatio n exchange capacity of the fine earth fraction equal too r less than 16 cmol(+)/kg clay, orha s aneffectiv e cation exchange capacity of the fineeart h fraction equal to or less than 12 cmol(+) perk g clay (IMNH^OAc-extractabl ebase splu s IMKCl-extractabl e Al). 4.ha s less than1 0percen tweatherabl e minerals inth e 50-200 ßm fraction. 5.ha s less than1 0percen twater-dispersibl eclay . 6.ha s a silt-clay ratiowhic h is0. 2 or less. 7.doe sno thav e andic properties. 8.ha s less than 5percen tb yvolum e showing rockstructure .

The calcic horizon is a horizon of accumulation of calcium carbonate. The accumulationma yb e inth eC-horizon ,bu ti tma y alsooccu r ina B -o r ina nA-horizon .Th ecalci chorizo nfeature ssecondar ycarbonat eenrichmen t over a thickness of 15 cm ormore ,ha s a carbonate equivalent content of 15 percent ormor e and at least 5 percent greater than that of a deeper horizon. The latter requirement is expressed by volume if the secondary carbonates in the calcic horizon occur as pendants on pebbles, or as concretions or as soft powdery lime. If such a calcic horizon rests on materials with 40 percent or more calcium carbonate equivalent, the percentage ofcarbonate s need not decreasewit hdepth .

The petrocalcic horizon is a continuous cemented or indurated calcic horizon, cemented by calcium carbonate and inplace s by calcium and some magnesium carbonate. Accessory silica may be present. The petrocalcic horizon is continuously cemented to the extent that dry fragments do not slake inwate r and roots cannot enter. It ismassiv e orplaty , extremely hardwhe n dry so that itcanno tb epenetrate db y spade orauger ,an dver y firm toextremel y firmwhe nmoist . 280 Annex 1

The gypslehorizo n isa horizo n of secondary calcium sulfate enrichment that is 15 cm ormor e thick, andha s at least 5percen t more gypsum than theunderlyin g C-horizon; theproduc t of the thickness incentimetre s and the percentage of gypsum is 150 or more. The percentage of gypsum (CaSO,.2H 20)i s calculated as theproduc t of gypsum content,expresse d as cmol(+)/kg soil, and the equivalentweigh t of gypsum, 86,divide db y 10. Gypsum may accumulate uniformly throughout the matrix or as nests of crystals; in gravelly material gypsum may accumulate as pendants below coarse fragments.

The petrogypsic horizon is a gypsic horizon that is so cemented with gypsum that dry fragments dono t slake inwate r androot s cannotenter .

Thesulfuri chorizo n isa horizo na tleas t1 5c mthic kan d characterized bya pH(H 20, 1:1) ofles stha n3. 5 andjarosit emottle swit h ahu e of2.5 Y ormor e and achrom a of 6o rmore . Annex 2 281

DEFINITIONS OF DIAGNOSTIC PROPERTIES

(Source: theRevise d Legend of the SoilMa p of theWorld , 1988,p p 28-34)

'Diagnostic properties' are features of horizons or of soil materials which,whe nuse dfo rsoi lclassification ,ar equantitativel y defined.The y are defined as follows:

The term 'abrupt textural change'refer s toa cla y increasebetwee n two layers, which takes place over a distance of less than 5 cm, where the lower layer shows the following: 1.a cla y contentwhic h is twice theamoun t ofcla y inth e overlying layer if the latterha s less than2 0percen t clay. 2. anabsolut e clay increase of 20percen t ormor e over the amount of clay inth e overlying layer ifth elatte rha s 20percen t clay ormore .

The term andic properties applies to soil materials which meet one or more of the following threerequirements : 1. (a)aci d oxalate extractable aluminiumplu shal f of theaci d oxalate extractable iron is 2.0 percent ormor e inth e fine earth fraction;an d (b)bul k density of the fine earth,measure d inth e fieldmois tstate , is0. 9 Mg/m or less; and (c)phosphat e retention ismor e than8 5percent . 2. (a)mor e than 60percen tb yvolum e of thewhol e soil isvolcanoclasti c material coarser than 2mm ; and (b)aci d oxalate extractable aluminium plushal fo f the acid oxalate extractable iron is0.4 0 percent ormor e inth e fine earth fraction. 3. the 0.02 to 2.0 mm fraction isa t least 30percen t of the fine earth fraction andmeet s one of the following: (a)i fth e fine earth fractionha s acid oxalate extractable aluminium plushal f of theaci d oxalate extractable irono f0.4 0 percent or less, there isa t least 30percen tvolcani c glass inth e 0.02 to 2.0 mm fraction;o r (b)i fth e fine earth fractionha s acid oxalate extractable aluminium plushal f ofth e acid oxalate extractable irono f 2.0 percent or more, there isa t least 5percen tvolcani c glass inth e 0.02 to 2.0 mm fraction;o r (c)i fth e fine earth fractionha s acid oxalate extractable aluminium plushal f of theaci d oxalate extractable irono fbetwee n 0.40 percent and 2.0 percent,ther e isa proportiona l content of volcanic glass inth e0.0 2 to 2.0 mm fraction (between 5an d 30 percent). 282 Annex2

The term 'calcareous' applies to soil materials which show strong effervescence with 10 percent HCl in most of the fine earth, or which containmor e than 2percen t calcium carbonate equivalent.

The term 'continuous hard rock' applies tounderlyin g materialwhic h is sufficientlycoheren tan dhar dwhe nmois tt omak ehan ddiggin gwit ha spad e impracticable. The material is continuous except for few cracks produced inplac e and (a)withou t significant displacement of the pieces,an d (b) not horizontally distant an average 10 cm or more. Continuous hard rock does not include hard subsurface horizons such as a petrocalcic or a petrogypsic horizon,o ra duripa n or apetroferri c layer (SeeAnne x3) .

The adjective 'dvstric' refers to a base saturation percentage (by IM NH^OAca tp H 7.0) of less than 50percent .

The adjective 'eutric'refer s to a base saturation percentage (by IM NH^OAca tp H 7.0) of 50percen t ormore .

The term 'ferralicproperties ' isuse d inconnectio nwit h Cambisolsan d Arenosols which have a cation exchange capacity (by IMNH.OA c at pH 7.0) ofles stha n2 4cmol(+)/k gcla yo rles stha n4 cmol(+)/k g soili na tleas t some subhorizon of the cambic B-horizon or the horizon immediately underlying theA-horizon .

The term 'ferric properties' isuse d in connection with Luvisols,Ali - sols,Lixisol s andAcrisol s showing oneo rmor e of the following: 1.man y coarse mottleswit hhue s redder than7.5Y Ro rchroma smor e than 5,o rboth ; 2. discrete nodules up to 2c m indiameter , theexterior s of thenodule s being enriched andweakl y cemented or induratedwit h ironan dhavin g redderhue s or stronger chromas than the interiors.

Theter m 'fluvicproperties 'refer s tofluviatile ,marin ean d lacustrine sediments which, unless empoldered, receive fresh materials at regular intervals, andhav e one orbot h of the following properties: 1.a norgani c carbonconten t that decreases irregularly with depth or thatremain s above 0.02 percent toa dept h of 125 cm.Thi n strata of sandma yhav e less organic carbon if the finer sedimentsbelow , exclusive ofburie dA-horizons ,mee t therequirement . 2. stratificationi na t least 25percen t of the soilvolum ewithi n 125c m of the surface. Annex 2 283

The terra 'gerlcproperties ' refers to soilmaterial swhic hhav e either: 1. 1.5 cmol(+)/kg soil or less ofextractabl e bases (Ca,Mg ,K ,Na )plu s unbuffered IMKC l extractable aluminium and apH(l MKCl )o f 5.0 or more,o r 2. adelt ap H (pHKC lminu s pHH 20)o f+0. 1o rmore .

Theter m 'gleyicproperties 'refer st osoi lmaterial swhic har esaturate d withgroundwate ra tsom eperio do fth eyea ro rthroughou tth eyear ,i nmos t years, and which evidence of reduction processes or of reduction and segregationb y iron: 1.reductio n conditions forpar t of theyea r or throughout theyear , in most years,evidence db y one ormor e of the following: (a)a valu e of rH equal to or less than19 . (rH= Eh(mV)/29 +2pH) . (b)th e appearance of asoli d darkblu e colour on the freshly broken surface ofa field-wet soil sample,afte r spraying itwit ha solution inwate r of 1percen tpotassiu m ferric cyanide. (c)th e appearance ofa stron g red colour ona freshlybroke n surface ofa field-wet soil sample,afte r spraying itwit h a0. 2 percent a,adipyridy l solution in1 0percen t acetic acid. 2. saturationb y groundwater evidenced by the following: (a)reductio n conditions asdefine d under 1. for apar t of theyea r or throughout theyear . (b)groundwate r standing ina dee punline dbor ehol e at such adept h that thecapillar y fringe reaches the soil surface; thewate r in thebor e hole is stagnant and remains colouredwhe n dye isadde d toit . (c)whit e toblac k (N),o rblu e to green (GY,BG ,G orB )colour s in more than 95percen t of the soilmatrix ;hig h chroma oxidized mottles, ifpresent ,occu r onpe d faceso r inroo t channels or animalburrows .

NOTETHA Tfo rsoil s inwhic h theconten to firo noxide s isver y low,o ri n which ironoxide s arepresen t insuc hlarg equantitie s orar e inertan ds o wellcrystallize d thatthe yremai nbrow no rre deve ni nreduce dconditions , theabov e colour requirements dono tapply .

The term 'gypsiferous'applie s tosoi lmateria lwhic hcontain s 5percen t ormor e gypsum.

The term 'interfingering' refers topenetration s of an albic E-horizon into an underlying argic or natric B-horizon along ped faces, primarily vertical faces. The penetrations are not wide enough to constitute 'tonguing',bu t form continuous coatings of cleansil t or sand,mor e than 1m m thick,o n thevertica l ped faces ('skeletans').A total thickness of more than 2m m is required if each ped has a coating of more than 1mm . Because quartz is such a common constituent of soils, the skeletans are usually white when dry and light grey when moist, but their colour is determinedb yth ecolou ro fth esan do rsilt .Th eskeletan sconstitut emor e than1 5percen to fth evolum eo fan ysubhorizo n inwhic h interfingeringi s 284 Annex 2 recognized.The yar eals othic kenoug ht ob eobvious ,b ythei rcolour ,eve n when moist. Thinner skeletans that must be dry to be seen as a whitish powdering ona pe d areno t included inth emeanin g of 'interfingering'.

Theter m 'niticproperties 'applie s tosoi lmateria l thatha s 30percen t or more clay, and has a moderately strong or strong angular blocky structure which falls easily apart into flat edged ('polyhedric' or 'nutty')element swit h shinype d faces thatar e either thincla y coatings orpressur e faces.Soi lmaterial swit hniti cpropertie shav emor e than 0.2 percent acid oxalate extractable ironwhic h is at least 5percen t of the dithionate-citrate-bicarbonate extractable iron.

The term 'organic soilmaterials 'refer s to soilmaterial s whichare : 1. saturatedwit hwate r for longperiods ,o rartificiall y drained, and, excluding live roots,have , (a)1 8percen t ormor e organic carbon ifth eminera l fraction is6 0 percent ormor e clay, (b)1 2percen t ormor e organic carbon ifth eminera l fractionha sn o clay,o r (c)a proportionna i content of organic carbonbetwee n 12an d 18 percent ifth ecla y content of theminera l fraction isbetwee n zero and 60percent ;o r 2.neve r saturatedwit hwate r formor e thana fe w days andhav e 20 percent ormor e organic carbon.

The term 'permafrost'refer s to theconditio n that the temperature ofa soil layer isperenniall y ato rbelo w 0°C .

The term 'plinthite' refers to an iron-rich,humus-poo r mixture ofcla y withquart zan dothe rdiluents .I tcommonl y occurs asre dmottles ,usuall y inplaty ,polygona l or reticulate patterns,an dchange s irreversibly toa hardpan or to irregular aggregates on exposure to repeated wetting and drying. Ina mois t soil,plinthit e isusuall y firmbu t itca nb e cutwit h a spade.Whe n irreversiblyhardened , themateria l isn o longer considered plinthite but develops into apetroferri c or skeleticphase .

The term 'salic properties' refers to an electric conductivity of the saturation extract ofmor e than 15 dS/m at 25° Ca t some time of theyea r within 30 cm of the surface, or of more than 4 dS/m within 30 cm of the surface if thepH(H 20, 1:1) exceeds 8.5.

The term 'slickensides'refer st opolishe dan dgroove dsurface s thatar e produced by one soilmas s slidingpas t another. Some of them occur atth e baseo fa sli p surfacewher e amas so fsoi lmove sdownwar d ona relativel y steep slope.Slickenside sar ever y common inswellin gclay s inwhic h there aremarke d seasonal changes inmoistur e content. Annex 2 285

Theter m 'smearyconsistence 'a suse di nconnectio nwit hAndosols ,refer s to thixotropic soil material, i.e. material that changes under pressure, orb y rubbing, froma plasti c solid intoa liquefie d stagean dbac k toth e solid condition. In the liquefied stage, the material skids or smears between the fingers.

The term 'sodic properties' refers to a saturation of the exchange complexb y exchangeable sodium of 15percen t ormore , orb y exchangeable sodiumplu smagnesiu m of 50percen t ormore .

Theter m 'softpowder ylime 'refer st otranslocate dauthigeni clime ,sof t enough tob ecu treadil ywit ha finge rnail ,precipitate d insit ufro mth e soilsolutio nrathe r thaninherite dfro ma soi lparen tmaterial .I tshoul d bepresen t ina significant accumulation. Pseudomyceliawhic h come and go withchangin gmoistur e conditions,ar eno tconsidere d assof tpowder y lime inth epresen t definition.

The term 'stagnic properties' refers to soil materials which are saturated with surfacewate r at someperio d of theyea r or throughout the year, inmos tyears ,an dwhic h showevidenc e of reductionprocesse s or of reduction and segregation ofiron : 1.reductio ncondition s forpar t of theyea r or throughout theyear , in mostyears ,evidence d by one ormor e of the following: (a)a valu e of rHequa l too r less than 19. (rH- Eh(mV)/29 +2pH) . (b)th e appearance ofa solid darkblu e colour on the freshly broken surface of a field-wet soil sample,afte r spraying itwit h a solution inwate r of 1percen tpotassiu m ferric cyanide. (c)th eappearanc e ofa stron g red colour ona freshl ybroke n surface ofa field-wet soil sample,afte r spraying itwit h a0. 2 percent a,adipyridy l solution in1 0percen t acetic acid. 2. ifmottlin g ispresent ,a dominan tmois tchrom a of2 o r less onth e surface ofped s andmottle s ofhighe r chroma occurringwithi n theped s or adominan tmois t chroma of 2o r less inth e soilmatri x andmottle s ofhighe r chroma or iron-manganese concretions,o rboth , occurring within the soilmaterial . 3. ifn omottlin g ispresent ,a dominan tmois t chroma of 1o r less onth e surfaces ofped s or inth e soilmatrix . 4. thedominan tmois t chroma on the surfaces ofped s and of the soil matrix increasingwit hdepth .

NOTETHA T forsoil s inwhic h theconten to f ironoxide s isver y low,o ri n which ironoxide s arepresen t insuc hlarg equantitie s orar e inertan ds o wellcrystallize dtha tthe yremai nbrow no rre deve ni nreduce dconditions , the above colour requirements dono t apply. Soilswit hstagni cpropertie ske you teithe ra tth efirs tlevel ,a sPiano - solso rPlinthosols ,o ra tth esecon dleve la s 'stagnic'units .Soil swhic h 286 Annex 2 are subject to flooding or show reduction as a result of irrigation are markedb y the 'inundic'an d 'anthraquic'phase s respectively. SeeAnne x3 .

The term 'strongl y humic' refer s to soil material with more than 1.4 percent organic carbon (in gpe r 100 g fine earth) as aweighte d average over adept h of 100c m from the surface.Thi s rule assumes abul k density of 1.5 Mg/m3.

The term 'sulfidle materials' refers towaterlogge d mineral or organic soilmaterial s containing 0.75 percento rmor e sulfur (dryweight) ,mostl y in the form of sulfides,an dhavin g less than three times asmuc h calcium carbonateequivalen ta ssulfur .Sulfidi cmaterial sdiffe rfro mth esulfuri c horizon inthei r reduced condition, theirpH ,an d the absence ofjarosit e mottleswit h ahu e of2.5 Yo rmor e ora chrom a of6 o rmore .

The term 'tonguin g' is connotative of the penetration of an albic E-horizon intoa nargi cB-horizo n alongvertica lpe d surfaces, ifped sar e present. Tongues must have greater depth than width, have horizontal dimensions of 5 mm or more in clay, silty clay and sandy clay argic B-horizons, 10m m or more inmoderatel y fine argic B-horizons, and 15m m or more in coarse (silt loam, loam and sandy loam)argi c B-horizons. The tonguesmus t occupy more than 15percen t of themas s of theuppe r part of the argic B-horizon. With Chernozems, the term 'tonguing' refers to penetrations of theA-horizo n into anunderlyin g cambicB-horizo n or into aC-horizon .Th epenetration smus thav egreate r depth thanwidth ,an dmus t occupy more than 15percen t of themas s of theuppe r part of the horizon inwhic h theyoccur .

The term 'vertic properties' is used in connection with clayey soils which at some period in most years show one or more of the following: cracks, slickensides, wedge-shaped or parallepiped structural aggregates that are not in a combination, or are not sufficiently expressed for the soil toqualif y asa Vertisol .

The term 'weatherable minerals'refer s tomineral s that areunstabl e in ahumi d climate relative toothe rmineral s sucha s quartz and 1:1 lattice clays,an dthat ,whe nweatherin g occurs,liberat eplan tnutrient s andiro n and/or aluminium. They include: 1. clayminerals , including all 2:1 lattice clays except aluminium interlayered chlorite. Sepiolite, talc and glauconite are also included inth emeanin go fthi sgrou po fweatherabl ecla ymineral s although they areno t always ofcla ysize . 2. sand and silt-sizedminerals ,feldspars ,feldspathoids , ferromagnesian minerals, glasses,micas ,an dzeolites . Annex 3 287

DEFINITIONS OF PHASES

(Source: theRevise d Legend of the SoilMa p of theWorld , 1988,p p 60-63)

Phases indicate surfaceo rsubsurfac e featureso fth elan dwhic har eno t necessarily related to soil formation and (may)cu tacros s theboundarie s of soil map units. The features concerned may result from or form acon ­ straint to landuse .

An 'anthraquic phase'mark s soils showing stagnic properties within 50 cm of the surface due to surfacewaterloggin g associated with longconti ­ nued irrigation,particularl y ofrice .

A 'duripa n phase ' marks soils having a subsurface horizon that isce ­ mented by silica so that dry fragments do not slake during prolonged soaking inwate r or inhydrochlori c acid. The upper level of the duripan must occurwithi n 100c m of the soil surface. Duripans vary in the degree of cementation by silica and they commonly contain accessory elements,mainl y iron oxides and calcium carbonate. As a result,duripan svar y inappearanc e but all of themhav e aver y firmo r extremely firmmois t consistency, and they are alwaysbrittle ,eve nafte r prolongedwetting .

A 'fragipan phase' marks soils having a loamy (exceptionally a sandy) subsurface horizonwhic hha s ahig hbul k density relative to thehorizon s above it, and is hard or very hard and seemingly cemented when dry, and weakly tomoderatel y weaklybrittl ewhe nmoist .Whe npressur e isapplied , peds or clods tend toruptur e suddenly rather than tounderg o slowdefor ­ mation. Dry fragments slake or fracture when placed inwater . The upper level of the fragipanmus t occurwithi n 100c mo f the soilsurface . A fragipan is low inorgani c matter, slowly orver y slowly permeable and often shows bleached fracture planes that are faces of coarse or very coarse polyhedrons or prisms. Clay skins may occur as patches or discon­ tinuous streaks both on the faces and in the interiors of the prisms. A fragipan commonly, but not necessarily, underlies aB-horizon . Itma y be from 15 to 200 cm thickwit h commonly an abrupt or clear upper boundary, whereas the lowerboundar y ismostl y gradual ordiffuse .

The 'gelundicphase 'mark s soils showing formation ofpolygon s on their surface due to frostheaving .

The 'gileaiphase ' marks soils showing gilgai, the typical microrelief ofclaye y soils (mainlyVertisols ) thathav e ahig h coefficient ofexpan ­ sionwit hdistinc t seasonalchange s inmoistur e content.Th egilga imicro - 288 Annex3

relief consists ofeithe r asuccessio n ofenclose d microbasins andmicro - knolls innearl y level areas,o r ofmicrovalley s andmicroridge s that run up and down the slope.Th eheigh t of themicroridge s commonly ranges from a fewc m to 100 cm.Rarel y does theheigh t attain 200cm .

The 'inundic phase' marks soilswit h standing or flowing water present on the soil surface formor e than 10day s inth e growing period.

The 'lithicphase 'mark ssoil swit hcontinuou shar droc koccurrin gwithi n 50c m of thesurface .

The 'petroferric phase' refers to the occurrence of a continuous layer of indurated material, inwhic h iron is an important cement and inwhic h organic matter is absent, or present only in traces.Th e indurated layer must either be continuous or, if it is fractured, the average lateral distance between fractures must be 10 cm or more. The petroferric layer differs from a thin iron pan and from an indurated spodic B-horizon in containing little orn o organicmatter .Th euppe r part of thepetroferri c layermus t occurwithi n 100c m of the soilsurface .

The 'phreatic phase' refers to the occurrence of a groundwater table within 5metre s from the surface, the presence of which isno t reflected inth emorpholog y ofth esoil .Therefore ,th ephreati cphas e isno tshown , for instance, with Fluvisols or Gleysols. Its presence is important especially inari d areaswhere ,wit h irrigation, special attention should bepai d toeffectiv ewate rus ean ddrainag e inorde rt oavoi d salinization as aresul t of rising groundwater.

The 'placicphase 'refer s to thepresenc e ofa thi n ironpan ,a blac kt o dark reddish layer cemented by iron, or by iron and manganese, or by an iron-organic matter complex, the thickness ofwhic h ranges generally from 2m m to1 0mm .I nspot s itma yb ea sthi na s1 m mo ra sthic ka s4 0mm ,bu t this israre . Itmay ,bu tno tnecessarily ,b e associatedwit h stratifica­ tion in parent materials. It runs roughly parallel to the soil surface, commonlywithi n theuppe r 50c m of the soil, andha s apronounce d wavyo r convolute form. Itnormall yoccur sa sa singl epan ,no ta smultipl e sheets underlying one another,bu t inplace s itma yb ebifurcated . Itmus t occur within 100c m ofth e soil surface and isa barrie r towate r androots .

The 'rudic phase' marks areas where the presence of gravel, stones, boulders or rock outcrops in the surface layers or at the surface makes theus eo fmechanize d agricultural equipment impracticable.Han dtool sca n normallyb e used andals o simplemechanica l equipment ifothe r conditions areparticularl y favourable. Annex 3 289

The 'salie phase' marks soils which, in some horizon within 100 cm of the surface, show electric conductivity values of the saturation extract higher than4 dS/ m at 25°C .Th e salicphas e isno t showno nsoi lmap s for Solonchaks because their definition implies ahig h salt content. Salinity in a soil may show seasonal variations or may fluctuate as a result of irrigationpractices .

The 'skeletic phase' refers tosoi lmaterial s which consist for4 0per ­ cento rmore ,b yvolume ,o fcoars efragment so foxidi cconcretion so riron ­ stone,o ro fothe rhar dmaterials .Thi sconcretionar ylaye rha sa thicknes s of at least 25 cm and its upper part occurs within 50 cm of the soil surface. The difference from the petroferric phase is that the concre­ tionary layer of the skeletic phase isno t continuously cemented.

The 'sodicphase 'mark s soilswhic hhav e more than 6percen t saturation withexchangeabl e sodiuma tleas t insom esubhorizon swithi n10 0c mo fth e surface. The sodic phase is not shown for soils which have a natric B-horizono rwhic hhav e sodicpropertie s sincea hig hpercentag e ofsodiu m saturation isalread y implied inthei rdefinition .

The 'takvric phase' applies to heavy textured soils which crack into polygonal elementswhe ndr y and form aplat y ormassiv e surfacecrust .

The 'vermic phase' applies to soils which have less than 0.6 percent organic carbon in thesurfac e 18c mwhe nmixed , or less than0.2 5 percent organic carbon if the texture is coarser than sandy loam (the organic carbon conditions are waived if the soil meets the requirements of the salic phase), and which show one or more of the following features connotative of aridconditions : 1.presenc e inth e surfacehorizo n ofgravel s or stones shapedb y the wind or showing desertvarnis h (manganese oxide coating at theuppe r surface), orboth .Whe n the soil isno t ploughed, these gravels or stones usually form asurfac e pavement; theyma y show calciumcarbo ­ nate or gypsum accumulating immediatelybelo w thecoars ematerial . 2.presenc e inth e surfacehorizo no fpitte d and rounded quartz grains showing amatt e surface; they constitute 10percen t ormor e of the sand fraction andhav e adiamete r of0.2 5 mm ormore . 3.presenc e of 2percen t ormor e palygorskite inth ecla y fraction ina t least some subhorizonwithi n 50c m of thesurface . 4. surface cracks filledwit h in-blown sand or silt;whe n the soil is ploughed this characteristic mayb e obliterated. However,crack sma y extendbelo w theploug hlayer . 5. aplat y surfacehorizo nwhic h frequently showsvesicula r pores and which isgenerall y induratedbu tno t cemented. 6. accumulation ofblow n sando na stabl esurface . 290 Notes

NOTES Annex 4 291

SOIL HORIZON DESIGNATIONS

(Source: theRevise d Legend of the SoilMa p of theWorld , 1988,p p 85-90)

Asoi li susuall ycharacterize db ydescribin gan ddefinin gth eproper ­ ties of its horizons. Abbreviated horizon designations, with a genetic connotation, areuse d for showing the relationships amonghorizon s within a profile and for comparing horizons among soils. The symbols used to designate soilhorizon s area sfollows :

Capital letters H,0 ,A , E,B , Can dR indicatemaste rhorizons , ordomi ­ nantkind so fdepartur e from theassume dparen tmaterial .A combinatio no f capital letters isuse d for transitionalhorizons .

Lower case letters areuse d as suffixes toqualif y themaste rhorizon s in termso fth ekin do fdepartur e fromth eassume dparen tmaterial .Th elowe r case letters immediately followth ecapita lletter .Tw olowe rcas e letters mayb e used to indicate two featureswhic h occur concurrently.

Arabic figures areuse d assuffixe s to indicate vertical subdivision ofa . For A- and B-horizons the suffix figure is always preceded by alowe r case lettersuffix . Arabic figures areuse d asprefixe s tomar k lithological discontinuities.

MASTER HORIZONS

H-horizon: organichorizo nforme do rformin gfro maccumulation s oforgani c material deposited on the surface, that is saturated with water for long periods, unless artificially drained, and contains 18 percent or more organiccarbo n ifth eminera l fractioncontain smor e than6 0percen tclay , 12percen to rmor eorgani ccarbo ni fth eminera lfractio ncontain sn oclay , or intermediateproportion s oforgani ccarbo nfo r intermediate contentso f clay.

H-horizons form at the surface ofwe t soils,eithe r as thickcumu ­ lative layers inorgani c soilso ra s thinlayer so fpea to rmuc kove r mineral soils.Eve nwhe nploughed , the surface soilkeep s ahig h content of organic matter following themixin g ofpea twit h mineral material. The formation of theH-horizo n isrelate d toprolonge d waterloggingunles ssoil sar eartificiall y drained.H-horizon s mayb e buriedbelo w the surface.

0-horizon: organichorizo nforme do rformin g fromaccumulation s oforgani c material deposited on the surface, that is not saturated with water for moretha na fe wday sa yea ran dcontain s 20percen to rmor eorgani ccarbon .

0-horizons are theorgani chorizon s thatdevelo p on topo fminera l soils. The organic material in0-horizon s isgenerall y poorly decomposed and occursunde r naturallywel l drainedconditions . 292 Annex 4

O-horizons dono t includehorizon s formedb y adecomposin g rootma t below thesurfac e ofth eminera l soil.O-horizon sma yb eburie dbelo w the surface.

A-horizon:minera lhorizo nforme do rformin ga to radjacen t toth e surface thateithe r (a)show sa naccumulatio no fhumifie dorgani cmatte r intimately associatedwit h theminera l fraction,o r (b)ha s amorpholog y acquiredb y soil formationbu t lacks thepropertie s ofE - andB-horizons .

Theorgani cmatte r inA-horizon s iswel l decomposed. As aresult , A-horizons arenormall ydarke r thanth eadjacen tunderlyin ghorizons . Inwar m arid climates,wher e there isonl y slight orvirtuall y no accumulationo forgani cmatter ,surfac ehorizon sma yb eles sdar ktha n adjacentunderlyin ghorizons .I fth esurfac ehorizo nha s amorpholog y distinct from thato f theassume dparen tmateria l and lacks features characteristic ofE - and/or B-horizons, it isdesignate d as an A-horizon onaccoun t of its surface location.

E-horizon: mineralhorizo n showing aconcentratio n of sand and siltfrac ­ tionshig h inresistan t minerals,resultin g from a loss of silicate clay, irono r aluminium or some combination of them.

E-horizons areusuall y eluvialhorizon s whichunderli e anH- , 0- or A-horizonfro mwhic hthe ydiffe rb ya lowe rconten to forgani cmatte r and a lighter colour.Fro m anunderlyin g B-horizon, anE-horizo n is commonly differentiated by colours ofhighe rvalu e or lower chroma, orb y coarser texture,o rboth .

B-horizon: mineral horizon in which rock structure is obliterated or is only faintly evident, characterized by one or more of the following fea­ tures: (a)a n illuvial concentration of silicate clay, iron,aluminium , or organicmatter ,alon e or incombinations ; (b)a residua l concentration ofsesquioxide s relative to sourcemateri ­ als; (c)a nalteratio n ofmateria l from its original condition to the extent that silicate clays are formed, oxides are liberated, orboth ,o r granular,blocky ,o rprismati c structures are formed.

It isgenerall y necessary toestablis h the relationship between overlying andunderlyin ghorizon s before aB-horizo n canb e identi­ fied. Often,B-horizon s need tob e qualifiedb y asuffi x tohav e sufficient connotation ina profil e description. Examples:a 'humus B-horizon' isdesignate d asBh ,a n 'ironB 'a s Bs, a 'texturalB 'a s Bt, a 'colour B'a sBw . NOTETHA T accumulations of carbonates, or gypsum or othermor e soluble salts dono tb y themselves distinguish aB-horizon .

C-horizon:minera lhorizo n(o rlayer )o funconsolidate dmateria lfro mwhic h the solum is presumed to have formed and which does not show properties diagnostic of any othermaste rhorizons . Annex 4 293

Traditionally,C ha sbee nuse d todesignat eparen tmateria lo rrathe r theunconsolidate d material underlying the solum that does notmee t the requirements of theA , E,o rB designations.Accumulation s of carbonates, gypsum or othermor e soluble saltsma yb e included in C-horizons.

R-layer: laye r of continuous indurated rock that issufficientl y coherent when moist tomak e hand digging with a spade impracticable. The rock may contain cracks but these are too few and too small for significant root development.Gravell y and stonymateria lwhic h allows root development is considered aC-horizon .

TRANSITIONAL HORIZONS

Soilhorizon s inwhic hth epropertie so ftw omaste rhorizon smerg ear e indicatedb y thecombinatio n of two capital letters (for instanceAE ,EB , BE, BC,CB ,AB ,BA ,A C and CA). The first lettermark s themaste r horizon towhic h the transitional horizon ismos t similar.

Mixedhorizon s that consist of intermingled parts each ofwhic h areiden ­ tifiable with different master horizons, are designated by two capital letters separated by adiagona l stroke (for instanceA/B ,B/C) .Th e first letter marks the master horizon that dominates. Note that transitional horizons aren o longermarke db y suffixfigures .

LETTER SUFFIXES

A small letter may be added to the capital letter to qualify the master horizon designation. Suffix letters can be combined to indicate propertieswhic hoccu rconcurrentl yi nth esam emaste rhorizo n(fo rexampl e Ahz, Btg, Cck).N o more than two suffixes should be used in combination. In transitional horizons, no use is made of suffixes which qualify only one of the capital letters but a suffixma y be used if it applies to the transitionalhorizo n asa whol e (forexampl e BCk,ABg) .

The suffix letters used toqualif ymaste rhorizon s are the following: b. Buried orbisequa l soilhorizo n (for exampleBtb) . c. Accumulation inconcretionar y form; this suffix iscommonl y used incombinatio nwit h another suffixwhic h indicates thenatur e of theconcretionar y material (forexampl e Bck, Ces). g. Mottling reflectingvariation s inoxidatio n and reduction (for example Bg,Btg ,Cg) . 294 Annex 4 h. Accumulation oforgani c matter inminera lhorizon s (for example Bh). For theA-horizon , theh-suffi x isapplie d onlywher e there hasbee nn o disturbance ormixin g fromploughing ,pasturin g or other activities ofMa n (h-an dp-suffixe s are thusmutuall y exclusive).

i. Occurrence ofpermafrost .

j. Occurrence ofjarosite . k. Accumulation ofcalciu mcarbonate . m. Strongly cemented, consolidated, indurated; this suffix is commonly used incombinatio nwit hanothe rsuffi xwhic hindicate s thecementin g material (forexampl e Cmk,markin g apetrocalci c horizonwithi na C-horizon, or Bms, marking an ironpa nwithi n aB-horizon) . n. Accumulation of sodium (for exampleBtn) . p. Disturbance by tillagepractice s (forexampl eAp) .

q. Accumulation of silica (for example Cmq,markin g asilcret e layer ina C-horizon). r. Strong reduction as aresul t ofgroundwate r influence (for example Cr).

s. Accumulation ofsesquioxide s (forexampl eBs) . t. Accumulation ofcla y (for exampleBt) . u. Unspecified; this suffix isuse d inconnectio nwit hA - and B-horizons which are not qualifiedb y another suffixbu thav e to be subdivided vertically by figure suffixes (for example Aul,Au2 , Bui, Bu2).Th eu-suffi x isprovide d toavoi d confusionwit h the formernotation s Al, A2, A3, Bl, B2, B3 inwhic h the figuresha d a genetic connotation. Ifn o subdivisionusin g figure suffixes is needed, the symbolsA andB canb euse dwithou t the u-suffix. w. Alteration insit ua sreflecte db y clay content,colou r or structure (forexampl eBw) . x. Occurrence ofa fragipan (forexampl eBtx) . y. Accumulation of gypsum (for exampleCy) .

z. Accumulationo fsalt smor esolubl e thangypsu m (forexampl eAz ,Ahz) .

Letter suffixes canb e used to describe diagnostic horizons and features in a profile. For example, argic B-horizon: Bt; natric B-horizon: Btn; cambicB-horizon :Bw ;spodi cB-horizon : Bhs,B ho rBs ; ferralicB-horizon : Bws; calcic horizon: k; petrocalcic horizon: mk; gypsic horizon: y; Annex 4 295 petrogypsichorizon :my ;petroferri chorizon :ms ;plinthite :sq ;fragipan : x; strongly reduced gleyichorizon : r;mottle d layers:g .

NOTETHA Tth eus eo fa certai nhorizo ndesignatio ni na profil edescriptio n does not necessarily indicate the presence of a diagnostic horizon or property since the letter symbolsmerel y reflect aqualitativ e estimate.

FIGURE SUFFIXES AND PREFIXES

Horizons designatedb y a single combination of letter symbols canb e verticallysubdivide db ynumberin geac hsubdivisio nconsecutively ,startin g from the top of the horizon (for example Btl-Bt2-Bt3-Bt4). The suffix number always follows all of the letter symbols. The number sequence applies to one symbol only, so that the sequence is resumed in case of change of the symbol (for example Btl-Bt2-Btxl-Btx2).A sequence is not interrupted,however ,b ya lithologica ldiscontinuit y (forexampl eBtl-Bt2 - 2Bt3).

Numbered subdivisions canals ob e applied to transitional horizons (for example AB1-AB2), inwhic h case it isunderstoo d that the suffix applies toth eentir ehorizo nan dno tonl yt oth elas tcapita l letter.Number sar e notuse d as suffixes ofundifferentiate d A orB symbols toavoi d conflict with the previous notation system. If an otherwise unspecified A- or B-horizon mustb e subdivided, theu-suffi x mustb eused .

Todistinguis h lithological discontinuities,a nara bnumbe r isprefixe dt o thehorizo n designations concerned (forexampl eA , B, 2C,3C) . 296 Notes

NOTES