WAITE INSTIT'UTE 3t.ro,63 UBRARY
MODIFICATION OF SOIL PHYSICAL PROPERTiES
BY ADDITION OF IRON AND CALCIUM COIÏPOUNDS
by
R. T, Shanmugonothon, B,Agr,Sc, (Sri Lonk0)
Thesis subrnitted in the tot0l fulfilment of the requirements for the Degree of Doclor of Philosophv in the UniversitY of Adeloide
Deportment of soil Science hloite Agriculturol Reseorch Institute The Universitv of Adeloide Aprll 1983 I
CONTENTS Page
LIST OF FIGURES LIST OF TABLES
SUMMARY
STATEMENT
ACKNOl,lLEDGEMENTS
1 CHAPTER 1 LITERATURE REVIEl^J 1.1 Definitions of Soil Structure I 1.1.1 Soil structure anC pores ín relation to aggregation 2 I.7.2 Characterization of soil structure ? 1.2 Coagulation and DjsPersion 10 L.2.1 Diffuse double laYer theorY 10 L.2.I.1 Electric double layer equations 11 1.2.?' Point of zero charge 13 1.2.3 A.dsorption of ions on soil surface 14 | .? .3. 1 Cati on adsorPt'i on 14 l -2.3.2 Anion adsorPtìon 15 l.?.4 Di sPersi on 18 1.2.4.1 Fâctors responsible for clay dispersion 19 1.3 Aggregation in Soils 20 1 .3 . I Factors i nf I uenc'ing aggregati on 2L 1.3.1.1 PhYsica'l agents 21 1.3.1.2 Biologica'l agents 22 1.3.1.3 Chemjcal agents 22 1.3.1.3.1 Iron oxides and hydrous oxides 23 1.3.1.3.2 GYPsum 24 1.3.1 .3.2,1 Gypsum treatment and ESP 25 1.3.1.3.2.2 Sôlub'i'lity and feasibitity ot the use of gypsum 25 -1.3.1.3.3 Calcium carbonate (CaCOa) 27 1.3.1.3.3.1 Solubility and feas'ibllity of the use of calcium carbonate 27 1.3.1.3.4 Mixtures of gypsum and calcium carbonate 28 1.3.1.3.5 Other ìnorganic chemicals 29 1.3.1.3.6 0rganic PolYmers ?9 1.4 Physical Properties after Changes in Aggregatìon and Exchangeable Sod'ium Percentage 30 1.4.1 imProved aggregation 30 31 1.4 .? Reduced ESP 1.4.3 Behav'iour of cal cium carbonate 31
CHAPTER 2 AIMS AND OBJICI'IVES OF THT STUDY 33
CHAPTER 3 EXPERIMENTAL 35 3.1 Preparation of PolyIFe(III)-0H]cat'ions 35 3.2 tl ectrophoreti c l''lc¡bi l i ti es 35 1ì 3.3 Aggregate Stabil itY 35 3.+ Hydraul ic Conduct'ivitY 36 3.5 Soil ConsistencY Parameters 36 3.6 Modulus of Rupture 36
3.7 Fri abi 1 i ty 37 3.8 Root Length Measurement 38 3.9 Water Retention 38
CHAPTER 4 MODII.ICATION OF SOIL PHYSICAI. PROPERTIES BY MANIPULATING THE NET SURFACE CHARGE ON COLLOI DS 39 4.1 Introduction 39 4.2 Materi al s 39 4.2.7 DescriPtion of soil 39 4.3 Experimental 40 4.3.1 DisPersed cìaY 4û 4.3.2 Treatments of soil with po'lycations 41 4.3.3 Examination of polycation treated soils 4t 4.4 Resul ts 44 4.4 .1 Di spersed c1 aY 44 4.4 .2 Electric charges 47 4.4 .3 Surface area and electron microscopy 48 4.4 .4 Aggregate stabiìitY 4B 4.4 .5 Dry bul k dens'ity and porosi tÍ es 48 4.4 .6 Pore size and hydrauìic conductivity 52 4.4 .7 Soil consistencY 53 4.4.8 Modulus of ruPture 53 4.4.9 So'il friabilitY 55 4.4.10 Penetrometer resistance 55 4.5 Discuss'ion 58 4.5.1 Surface charges and flocculation 58 4.5.2 Aggregat'ion and Porosi tY 60 4.5.3 Soil mechanical propert'ies 61 4.6 Conclusions 63
CHAPTER 5 MODIFICATION OF SOIL PHYSICAL PROPERTiiS BY ADDITT0N 0F Fe(IiI) P0LYCATiONS. INFLUENCE 0F PLANT GR0l^lTH 64 5.1 Introduction 64 5.2 Experimental 64 5.2.L Preparation of so'il s in pots 64 5.2.2 Rainfal I simuiation 64 5.2.3 Crack Pattern 65 5.2.4 Pore si ze di stri but'ion 6'5 5.2.5 Penetrometer resi stance 6B 5 .2.6 Germi nati on studi es 68 5.2.7 Plant growth stud'ies 6B 5.3 Results and D'iscussion 7T 5.3 .1 Crack formation 7L 5.3 .2 Seedl 'ittg emergence 74 5.3 .3 Plant growth 76 5.4 Conclusions BO llt
AND PHYSICAL CHAPTER 6 INFLUENCE OF ANIONS ON DISPERSION PROPERTIES0FTHEAI.IORiZOI'¡0FARED-BROI'JNEARTH B1 6.1 Int.roduction 81. B? 6 2 Experimental 6.2 .l Treatments of soil wìth polycations 82 6.2 .2 Preparation of fulvic acid B3 6.2 .3 Adcl'itjon clf anions to poìycat'ioti treated soi l susPc:nsìons B3 6.2.4 Adsorption of anions by so]l Y at pH.7 84 6.2.5 Addit'ions of anions to soils treated with Fe PoìYcations B4 6.2.6 Physìco-chemical propertìes 85 6.2.7 Physical and mechanical properties 85 6.3 Resul ts and Di scuss'ion 85 6.3:1 Dispersed claY B5 6.3.2 Eìeät.rophoretic mobi'ìity and point of zero charge 95 6.3 .3 Electric èharges determined by ion exchange 98 6.3.4 Electron mìcroscoPic studies 98 6.3.5 Aggregate stability and porosities 106 6.3.6 Nãiimum water holding capacity and 106 hydrauì ic conductìvitY 'l 6.3.7 t'tó¿ul us of rupture and f ri ab'i i ty 111 6.4 Concìusions 114
CHAPTER 7 EFFECT OF DISPERSIBLE CLAY ON THE PHYSiCAL PNOÞENTiTS OF THE B HORIZON OF A RED-BROI¡N EARTH 115 7.1 Introduction 115 116 7 .2 Materi al s 7.3 ExPerìmental 116 7.3.1 Treatment of the clay B horizon with 116 PolY[Fe( I I I ) -0H]cat'i ons 7.3.2 betêrmination of dispersible clay Lt7 7.3.3 Physìcaì and mechanical properties 118 I .l .9. 1 Swel ì i ng and shri nki ng 118 7.3.3.2 Water retention 119 7.4 Resul ts and Di scuss'ion 119 7.4 .1 Dispers'ibie claY i19 'itY 7.4 .2 Aggregate stabi I 72? 7.4 .3 Swelling behav'iour r22 7.4 .4 Water retent'ion and ava'i I abl e water T?4 7.4 .5 Dispersion index and hYdrauìic conduct'iv i tY t?5 7 .4.6 Soil mechanical ProPerties t26 7.5 Conclusions t32
CHAPTER 8 MODIFICATION OF SOIL PHYSICAL PROPERTIES BY ADDITION OF CALCIUM COMPOUNDS 133 8.1 Introduction 133 8.2 Material s 134 IV
8.3 Experimental 135 -- 8.3.1 Application of gYPsum, calcium c arbonate and cetnent 135 8.3.2 T reatment of soi'ls in Pots 136 8.3.3 E xamination of treated soils after wetti ng and drY'i ng cYcl es 137 8.3 .4 Germinatìon studies 140 8.3 .5 Plant growth studies 140 8.4 Results 14i 8.4.1 Exchangeable cations and electrolyte conceniration 141 8.4 .2 Dì spers'ibl e cì aY I44 8.4.3 Cembntjng effects - aggregate stab'iìity i46 8.4.4 Total aréal PorositY 151 8.4.5 Available tvater and hydraul ic conducti v'i tY 151 8.4.6 So'il mechanìcal propertìes 155 I .4 .7 Seedl 'i ng emergence- ' 155 8.4.8 Plant giowth 155 8.5 Di scuss'ion 163 8.5.1 Coagulat'ion and dispersion 163 8.5 -2 Cementati on 163 8.5.3 Dispersible clay as a measure of structural Probl ems? 165 S.S.4Pronlot.ionofcoaguìatjonandcementation 166 8.5.5 Root growth and soil structure 168 8.6 Concl usions 169
CHAPTER 9 GENERAL DISCUSSION L7T 9.L Introduction t7l 9.2 Flocculation and Coagulation t7r 9.3 DisPersibìe ClaY 172 9.4 Aggregation 773 9.5 Soil Structure 174 9.6 Root Growth t74 9.7 Possibility of Use of Calcium Carbonate to ImProve Soil Structure 175
T7B APPEND I X
REFERENCES 179 V
LIST OF FIGURES
Fi gure Page d'iagrarn of units of soil structure I Schema.tic ? (hlarkenti n , 1982) 42 2 calibration of % dispersjble clay and opt'ical densìty and 3 Influence of poly[Fe(iII)-0H]cat'ion on flocculation dispersion anä eiãctrophoretic mobi'lity of so'iì suspens'ions 45 j 4 El ectrophoreti c mob I'iti es of soi I s as a funct'ion of pH-U.fói'ô and after treatment with polvIFe(III)-0H] cati ons 46 49 5 Transmission electron m'icrograph of clay fraction (A) Untreated soil (B) PolyIFe( I I I ) -0H]cation treated so'il 0.01%Fe (c) il il ll 0.04%Fe l¡ (D) il il 0.07%te (E) ¡l il ll 0. 16%Fe ¡t Í (F) Í 0.32%Fe on the size 6 Effect of poly[Fe(III)-0H]cations 'in di stri buti bn ói water stabl e part'icl es soi I 50
7 Effect of moulding water content on the modulus of iupture of art'ifiðial briquets by addition of poly IFe(III)-0H]cations 55 I Effect of water content on the friability of soils treated with poly[Fe(III)-OH]cat'ions (curves fitted by eye) 56
9 Effect of time on the penetrometer resistance of soils lreated with po'lyIFe(III)-0H]cations at 20% moulding water content 57 l0 Calibration of rainfall simulator for uniform di stri buti on 66
ll Randomized comp'lete block des'ign for crack formation änã germinatioh experiment by rainfall simulation 67
12 Randomized conrp'lete block design and changes-of the âiiung.*.nts oî the pots with tjme for the pìant growth 70 t3 crack format'ion jn the polyIFe(III)-0H]cation treated soi I s after simul ated ra'infal I and dryÍng 72 t4 Effect of poly[Fe(III)-0H]cations on the crack pattern after simuiatêã rainfall and drying of the soil 73
15 Influence of poly[Fe(III)-0H]cat'ions on the plant height of wheat in the soil 78
l6 Changes in dispers'ible clay of 0.07%Fe treated soil after add'ition of anions B6 V]
17 Changes in.electrophoret'ic mob-'il'ity of C'32%Fe trealed soil after additìon of anions B8
]B Changes in dispersible c'lay of 0.32%Fe treated so'il after addition of anions B9 l9 Changes in electrophoretic mob'il ity of 0.07%Fe treated soi I af ter addi t'ion of an'ions 9l
20 Anion adsorption on 0.32%te treated soil at pH 7 93
21 Electrophoretic mobil jties of 0.07%Fe treated SO ils as a function of pH before and after addition of anions 96
22 Electrophoret'ic mobilities of 0.32%te treated soìls as a function of pH before and after addit'ion of anions 97
23 Transmissjon electron micrograph of clay fraction of 0.07%Fe treated soils before and after additÍon of phosphate t00
24 Transmission electron mìcrograph of clay fraction of 0.07%Fe treated soils before and after addit'ion of ful vate t0l
25 Transmission electron micrograph of clay fraction of 0.07%Fe treated soìls before and after addition of c i trate 102
26 Transmission electron micrograph of clay fraction of 0.32%Fe treated soils before and after addition of phosphate 103
?7 Transmission electron mìcrograph of clay fraction of 0.32%Fe treated soils before and after addition of ful vate 104
28 Transmission electron micrograph of clay fraction of 0.32%Fe treated soils before and after addition of ci trate 105
29 Effect of phosphate on the size distribution of water stable partìcles in sojl samp'les treated with 0.A7%te 107
30 Effect of fulvate on the size distribution of water stable particles in soi'l samples treated with 0.07%Fe 108
3t Effect of citrate on the size d'istribution of water stable particles in soiì samp'les treated w'ith 0.07%Fe 109
32 changes in the modulus of rupture at the plast'ic limit of 0-.07%Fe treated soiìs after addition of anions 112
33 Changes in the friabiì'ity at the plast'ic I imit of 0.07%te treated soi I s after addi tion of an'ions ll3
34 Influence of poly[Fe(III)-0H]cat'ions on the percent dispersible ciay- ãnd eìectrophoretic mobiì'ity of soil 120
35 Effect of poly[Fe(III)-OH]cations on the size distribution of water stable particles in soil (B horizon of a Red-brown Earth) 123 vìì
36 Three-dimensional d'iagram for disPersible clay, .mouldìng water content and nlodultrs of rupture of poly(Fe(III)-0H] cation treated soils 128
37 Three-dìmensional diagram for dispersible clay,_nroulding water content ancj friå¡ility of poly[Fe(III)-0H]cation treated soi I s 129
38 Randomi zed conrp'l ete bl ock des'ign for the determi nati on of physìcal ProPerties. I, II, III' IV, V and VI and R' R,, and R, are the number of sets (6) and repl icates (3 ) 16specti Ve'ly l3B
39 Random'ized compl ete bl ock d esign and changes of the rep'l 'icate pos'i ti ons (Rl , R2 and R3) of each set (II, III IV, V and VI) with time f0r the gérmination and Plant growth ì39
40 Influence of gypsum, ca'lcìum carbonate and cement on dispersjble cläy of soils in different wetting and drying cycles t45 4l Effect of gypsum on the size distribution of water stable parlìcles in soil after ll wetting and drying cycl es 147
42 Effect of gypsum on the size distribution of water stable 'partìcles in soii after 23 wetting and drying cycles 148
43 Effect of cement on the size distribution of water stabl e part'icl es i n soi 1 after I I wetti ng and dryi ng cycles ' 149
44 Effect of cement on the size distribution of water stable particles 'in soil after 23 wetting and drying cycles' 150
45 Effect of gypsum and cement on the modulus of rupture of artificial briquets after ll and 23 wetting and drying cycles 157
46 Influence of gypsum and cement on the friability of soils after li-and 23 wetting and drying cycìes 158
47 Influence of gypsum, calcium carbonate and cement on the totaj dry matter (at d'ifferent stages) and gra'in yietd (maturity) of wheat 16l
4B Dispersible clay in relation to the ratio of exchangeable--- sodium percentage (ESP) and electrical conductivity (Ec) 164
49 Effect of exchangeable sod'ium percentage on hydrau'lic conductivìty of gypsum, calciurn carbonate and cement treated soi I s 167
50 Modeì of dispersible clay and soiì structure on the basis of Fe poìycation and calcium compound treatments 176 vl ì'l
LIST OF TABLES
Tabl e Page
I Levels of organizat'ion of soìl structure and pores 4
2 Functional descriptions for pore s'ize groups 5
3 Some soil characteristics 40
4 pH va'l ures for poly[ Fe ( I I I ) -0H]cati on treated soi l suspensions 47
5 Changes in electric charges of soil by addìtion of 47 poly[Fe( I I I ) -011]cati ons
6 Effect of polylFe(IIi)-0H]cations on the surface area of soils 48
7 Chan ges in the bulk density (g cm-3) and.porosity (%v of moulding water content (% w/w) of the iv) sl poly IFe (I I i ) -0H]cat'ion treated soi I s
B Effect of adding poly[Fe(III)-0H]cations on the soils porosity and poie'sizé ¿istributjon.l964 of air-dried Ïtotal êvaluated area was mm2) 52
9 Influence of poly[Fe(tlt)-0H]cations on the water holding capacìtf ãnd'hydrau'l ic conductivity of soil 53 t0 Influence of poly[Fe(tlt)-0H]cations on consistency parameters 54 ll Effect of percentage of soil particles 50-250 um àiameter oir the phisical properties of soils flocculated by addition of polyIFe(ltt¡-0H]cations 62
12 Effect of poly[Fe(IIi)-0H]cations on the area of cracks of tne iolî samp'les after simuìated raìnfall and drying (total area examined ì350 cmz) 71 l3 Effect of poly[Fe(III)-OH]catíons on the size distributìbn ói wàter'stable particìes (% total soil) in soil after s'imulated rainfal I and drying 75 l4 Influence of poly[Fe(III)-0H]cations on the porosity and pore size d'iilrjbution of clods of the cracked soil sampl es (tota'l area exam'ined 1350 cmz ) 75 ì5 Seedling emergence of wheat (va.Wanigaì) and strength of polyIFe(III)-0H]cation treated soils 76 l6 Influence of polyIFe(lll)-0H]cations in the soil on the yield of wheat 77
17 Moisture, nitrogen and phosphorus content in the plant tissue after harvesting 77
IB Root length of wheat pìants after 6 months of growth in polyIFe(III)-0H]cation treated soils 79 IX l9 Identif ication of polyIFe(III)-0H-cation treated soi]s B3
20 pH va'f ues for soi I sampl es treated wi th 0.07%Fe af ter addition of anions 87
21 pH vaìues for soil samp'les treated with 0.32%Fe after addition of anions 90
22 Chemical formula and stabì1ity constants of Fe-anion compìexes in solutjon 9?
23 Stabiìity, number and size of the rings of the organic ani on-Fe comp'l exes 94
24 Changes in electric charge (C g-1) of soiì samples treated with 0.07%te after addition of anions 99
25 Changes in electric charge (C g-1) of soiì samples treated with 0.32%Fe after addition of anions 99
26 Changes in the bulk density and porosity at the pìastic limit of soìì samples treated with 0.07%Fe after addition of anions il0
27 Effect of anions on the water-hol{ing capacify ft w/w) and hydraulic conductivity (cm ¡-l) of soils treated wi th 0 .07%Fe ilr
28 Effect of anions on the plastic limit in 0.07%Fe and 0.32%Fe treated soils ì14
29 pH values for poly[Fe(III)-0H]cation treated soil su spens i on s 121
30
122
3l Effect of adding poly[Fe(III)-0H]cations on the water retention and available water of clay B horizon 125
32 Influence of poly[Fe(III)-0H]cations on the dispersìon index and hydraulíc conductivity of cìay B horizons 126
33 Influence of poìy[Fe(III)-0H]cations on plastic I im'it 127
34 Correlation of the percentage of dispersib'le clay manipuìated by the addÍtion of polyIFe(III)-0H]cations with various physica'l properties of the clay B horizon l3l
35 Some selected soil characteristics 134
36 Some characterjstics of the calcium compounds 135
37 pH of dry mixed soil samples before wetting and drying 136
38 pH, electrical conductiv'ity and exchangeable cat'ions of soil 142 X
39 pH, electrjcal conductiv'ity and exchangeable cations of soi I 143
40 Effect of aclding gypsum, calc'ium carbonate and cement on ln. u..ul porosii,-ôt ajr dried so'i1s after_wetllng and ãwi ng .yäl .t (total area eval uated was 9 ' 8 cmz ) 152 4l Effect of adding calcìum compounds on the water retention and available wáter of soil at different wetting and dry'ing cycl es 153
42 Influence of calc'ium compounds on the dispers'ion'index and hydraulic conductivity of soìì at different wetting and drying cycles 154
43 Influence of calcium compounds on pìastic limit in the .l56 different wett'ing and drying cycìes
44 Seedling emergence of wheat (var.warìga]) in the soil treated with calcium comPounds 159
45 Number of tillers and nitrogen and phosphorus content in tfl" p'lant tissue of wheat (var. Wariga'l) after harvest'ing 160
46 Root length of wheat plants at different stages of growth iñ gypsum, calcium carbonate and cement treated soils 162
47 Correlation of the areal porosity obtained by the different wetting and dryìng cycles in gypsum and cement treated soils with root length of wheat plant t69 x'l
SUMI4ARY Soil structure is defined and various methods for characterizing structure briefly described followed by a description of factors involved in aggregation and flocculat'ion-d'ispers'ion phenontena. In the literature, ìt is shown that phys'ica1 , bioìog'ica'l and chemical agents are involved jn the aggregat'ion. The importance of the inorgan'ic components involved in stabilizing aggregates, iron oxìdes and hydroxides, gypsum and calcium carbonate have been studied. The solubilìty and feasibiìity of the use of gypsum are detailed in the
I i terature. The addition of 0.07%te in the form of poìycation of molecular weight 10,000-50,000 flocculated soi'l suspensions. Higher concentrations of Fe(III) caused redispersion of the clay. Eìectrophoretic and electron microscopic stud'ies confirmed the flocculation-dispersion phenomena. The soiì suspens'ions with higher concentrations of Fe(III) gave points of zero charge (PZC) between pH values 5.0 and 6.0. The flocculation resulted jn microaggregation and created pores 40-100 pm 'in diameter. This ted to an increased water-holding capacity and hydraulic conductivìty and ìower bulk density and modulus of rupture. The soils treated with Fe(III) polycations were shown to be more friable than untreated soils.
The addition of 0.07%Fe or more decreased the total area of cracks and increased the number of transmissive pores of soìls after simulated rainfaìl and drying. This resulted in increased percent emergence and lower mean day of emergence of wheat p'lants. Percent emergence (y) was negative'ly correlated w'ith penetrometer resistance (x). The jncreased germìnation was fol'lowed by greater pìant growth, including'increased pìant height and yieìd. The Fe po'lycation treatments had no significant effect on root ìength measured at harvest. xll
Three groups of anjons were distinguished in order of effectiveness with respect to the dispersion and flocculation of soiì sampìes treated with Fe(III) poìycations: phosphate and fullvate > citrate, oxaìate, s'ilicate and tartrate > salicyìate, catechol, aspartate, lactate and acetate. Th'is was also the order of the amoutrt of anion adsorbed by the soil. The addition of phosphate and fulvate to soil samples with a net charge of zero lowered the PZC producing particles with a net negatìve charge. This increased the amount of dispers'ible clay present from 0 to 9% by weight of soil. The sorption of phosphate and fuìvate by soiì samp'ì., *ith a net posit'ive charge reduced the PZC and caused floccuìation of dispersed cìay. Eìectrophoretic and electron microscopic stud'ies confìrmed the dispersion-fìocculation phenomena.
Treatments which produced dispersed clay led to increased bulk densities, plastic I im'its and modul'i of rupture but lower porosities, water hoìding capacities and hydraulic conductivities. The sorption of an'ions on soil samples with a net charge of zero reduced friability. Three methods were compared for the determination of dispersÍb1e clay in the absence of chemjcal treatments. Treatment of the B horizon of a red-brown earth with a range of amounts (0-0.24%te) of Fe poly- cations allowed product'ion of a range of dispersible clay contents from 0 to 70%. The sampìes of clay B horizon with decreasing contents of dispersible clay showed decreasing e'lectrophoretic mobiìities with zero mobiiity when the content of dispersible clay was zero. In such samples the clay particles were present in aggregates 50-250 pm diameter according to sedimentation techn'iques. The amounts of dispersible c'lay present appeared to control various physica'l and mechanical properties. It is suggested that the content of dispersible clay may be a useful quantitative characteristic of soils as it contro'ls many other properties. In a sodic soi1 to rvhich calcium compounds such as gypsum, calcium carbonate or cement was added the content of d'ispersible clay was xi'ii related to both exchangeable sodium percentage (ESP) and electrical conductivity (EC). The electrolyte concentration in the sodic soil which could be mainta'ined by addit'ion of calcium carbonate was such that an ESP of = 3 was requ'ired to maintain clay coagulation. Smalì amounts of gypsum (0.2n w/w) coagulated most of the clay by lowering the ESP and rai sing the eì ectroìyte concentrat'ion. However, the c'lay gradually dispersed as the soil was subjected to wetting and drying 'lhe cycles and the electro'lyte concentration waS decreased. most efficient use of gypsum would appear to be in small annual additions. The addition of cement resulted'in the stabilization of particles 250
-2000 Um d'iameter, i.e. cementation as opposed to coagulation. Both processes resulted in changes to various phys'icaì and mechanical properties of the soil. It is suggested that both coagu'lation and cementation in a soiì may be possible by the addition of gypsum and cement or calcium carbonate with s'ignificant improvements of soiI structure. xtv
STATIMENT
This thesis contains no material which has been accepted for the award of any other degree,ordjploma in any university and that, to the best of my knowledge and belief, the thesis contains no material previously published or written by another person, except when due reference is made in the text of the thesis.
.l983 April R. T. Shanmug a athan' XV
AC KNC)I^lL EDGEI'IENTS I wouìd like to eipress my sincere gratitude to Professor J. M.Oades, Chairman of the Departnent of Soil Science at the Waite Agricultural Research Institute for his encouragement, guidance and supervision of the project. I also thank other members of the academic staff for their advice and assistance. In particular I thank, DF. L. R. Jarvis, Department of Histo- patho'logy, Fl inders Medi c.rl Centre, for i nstructì ons on the use of the digitizer cursor to determine the crack pattern; Dr. R. S. Murray for the methodo'logy involved in the determination of swelling;
Mr..J. Thompson, Dìvisjon of Soils CSIRQ, for the preparation of the thin sections of soil; Mr. T. l^l. Sherw'in for the transmission electron micrographs and determination of surface area; Mr. T. l,l. Hancock for the three-dimensional diagrams and statistical analysis; Mr. J. A. Denholm for his assistance in rainfall simulation and collection of soil samples (section 8.2); Mr. C. M. P.ivers, Mrs. R. Henderson and Mrs. A. Waters for their help in various experimental anaìysis; Mr. B. A. Palk for preparing the photographs; Mrs. J. M. Ditchfield for typing the manuscript; and Mrs. J. Shierlaw for the preparation of the diagrams. I acknowledge many others at the Waite Agricultural Research Institute who have been helpfuì in one way or another. I am indeed gratefu'l to the Austral ian Government for the award of a Colombo Plan Scholarship, Research Grant Committee and Coconut Research Institute of Sri Lanka for grant'ing leave to pursue this project.
Final]y, this thesis is dedicated to my wife, Raii, and my children, Anujah and Nijegan for their contribution in various ways to the successfuì complet'ion of this project. I WAITç II{STITUTE
TBRARY CHAPTER 1
LITERATURE RTVIEl,J
1.1 Definit i ons of Soil Structure Hi1lel (I980) defìned "so'il structure" as the arrangement and 'in the soil' organizatjon of the particìes (sand, silt and clay) definit'ion l4arshall and Holmes (1979) included pore space in their pore space ,,The arrangement of the sol'id phase of the soil and of the particles differ located between its constituent particles". S'ince soil associated and in shape, s.ize, and orientat'ion, and can be varìously 'irregular interlinked, the mass of them can form complex and to characterize configurations which are in general exceed'ingly diffjcult of clay' in exact geometric terms. In soils with an apprec'iable content to group the pr.imary particles tend under favourable c'ircumsta.nces, Such aggregates are themselves into structural units known as aggregates' they necessariìy not characterìzed by any un'iversalìy fixed sjze nor are Sands of singìe-grain stable when stressed by rapid wett'ing and drying' as structure and clays of mass'ive structure are sometìmes described (Marshall 1962; Baver "structureless" but this term is inappropriate ' et a7., 1972). Greenland (198.l) indjcated that a high proportjon of aggregates but release it between 0.5 and 2 mm diameter hold water agaìnst gravity, the soil eas'ily to plant roots and allow excess water to drain through 'is (Bakker et a7.,ì973). The structure of the surface so'iì more compìex the forces of and more chang'ing than subsoil structure. At the surface wetti ng and dry'ing , mechani ca'l man'i pu'l at'ion , pl ant root and shoot reform aggregates' influences, W'ind and water, all rearrange, breakdown and chem'ical The compìex 'interrelationship of physical , bio'logical and was reactions involved in the formation and degradation of soil aggregates reviewed bY Harris et a7. (1965)' ?
1.1.1 Soil Structure and Pores in Relation to Aggregation
Some system of namÍng pores of djfferent s'izes is useful in cliscussing soil structure. Size, shape, d'istribution and connections between poreslare the ìmportant parameters. The distinction between cap'iìlary and rron-capillary pores is a useful classificat'ion, introduced many years ago. A functional class'ification is ntore useful than a sìze fores classjficatìon, although capillaryl'is probably not the most useful functjon to choose. The d'istinction between inter- and intra-aggregate pore is .l979); important and useful , Greenland (1977, Warkentin (1977 ), lulaeda et at.(1977) and De Leenheer ('1977) have presented classification systems of sizes of pores (F'ig.l, Tables I and 2).
I.I.2 Characterization of Soil Structure The stabilìty of aggregates and pores decreases on wetting dry soil particularly if this is done rapidly. There are a number of reasons for this. The strength of soil decreases with water content, because of 'l reduced cohesi on and the sof teni ng of cements. If macroscopì c swe'l ì ng occurs during adsorption, th'is will cause uneven stra'ins throughout an aggregate so that its structure will be d'istorted and weakened. This effect of uneven wetting wìì'l be greatest when dry cìay so'il wets rap'idly (Panabokke and Quirk, tSSfr¡ . Rap'id wetting of dry soil can a'lso trap air in pores which may be subject to compressed pressures greater than the tensiìe strength of the soil. The air then escapes explosively, breaking the aggregate into small fragments. This slaking effect can be observed when dry aggregates are immersed suddeniy in water. Many soils slake (Yoder, 1936), particuìarly those of low to medium clay content. SoÍl structure can be studied directìy by microscopìc observation of thin sl ices under po'larized 'light. The structural associat'ions of clay can be examined by means of electron m'icroscopy,us'ing transmission or scanning techniques. The structure of single-grained soi'ls, as we1ì as of 3
:.
Glay o,oo2-o tnt €¡ .'-:i o. os- eonsfo 1 ft te MicroPed 5O-5oo¡m
Gonglomerate 1 -5opm
Fig. I Schematic diagram of units of soil structure (Warkentin,l982) Tabte 1 Levels of Qrganization of Soil Structure and Pores
Greenland,1979 Warkentin, 1977:' Maeda et aL-,1977 Units of structure Pores Units of structure Pores
Name Size Name Size Name Size Name Síze
5000um Fissures Aggregates 1000um Interped and and peds ì nteraggregate
500um Transmission 500um MicroPeds 100um IntermicroPed p0res
Con gl omerates LOur,,r Intercl uster 5mm Aggregates 50um Storage Pores 50um 0r cl usters Interdomain 100Oum Mi cro- 0.05um Residual Pores lum Domains 0.05um aggregates
Sum Domains 0.005um Bonding Pores 0.05um Clay crystal 0.005um Intergrain in domains
Þ 5
Table 2 Functional Descrìptìons for Pore Size Groups
Greenland,1977 De Leenheer, 1977
pd (um) Epd(um)
>500 Fi ssures >300 Aeration capacity pFo-pF,
-50 Transmission pores 300-30 Normally drain'ing RF, -RF, pores -0. 5 Storage pores Sl ow'ly dra'in'ing 30-9 Pt z-Pt z.s+ 5-0.0 Residual pores pores 0.2-0.0 Non-useful water content
Epd = equivaìent pore diameter 6 aggregated . soils, can be consiclered quant'itatively ìn terms of the total poros'ity and of the pore-sìze djstribution. The specific structure of aggregated soil s can, furthermore, be characterized qua'l'itatively by specify'ing the typical shapes of aggregates found in various horizons within the soil profì'le, or quantitative'ly by measuring their sìzes. Additional methods of characterìzing soiì structure are based on measuring mechanjcal properties and permeab'i'lity to various fluids. None of these methods has been accepted universa'11y.
Total porosity, f, of a soil sa mple ìs usualìy computed on the basis of measured bul k dens'ity PU, usi ng the fol I ov'ri ng equation f=1-P5/P' where p^'is.S the average partic'le density. Bu'lk density is genera'lly measured by means of a core sampler desìgned to extract "undisturbed" samples of known volume from various depths in the profile. An alternative is to measure the volumes and masses of ind'ividua'l clods by coat'ing with paraffin wax or a suitable resin prior to immersion in water (Blake,l965). Pore-size distribut'ion measurement can be made in coarse-grained soils by means of the pressure-jntrusion method (Diamond, 1970), in which a non-wetting liquid, generally mercury, is forced into the pores of a predried sampìe. l*lhere the aggregates are fairìy distinct, it'is sometimes possible to divicle pore-s'ize distnibution into two dístinguishable ranges, name'ly macropores and micropores. The macropores are mostly the inter- aggregate cavities wh'ich serve as the principa'l avenues for the infiltration and drainage of water and for aeration. The micropores are the'intra- aggregate capi'llaries responsible for the retention of water and solutes. Aggregate size distribution 'is a determinant of pore-size distrjbution and has a bearing on the erodibility of the surface, part'icuìarly by wind. The determination of aggregate size distribution depends on the mechanjcal means employecl to separate the aggregates. Aggregate screen'ing methods were reviewed by Kemper and Chepil (.l965). Screening through fìat sieves 7 is difficult to standardize and entails frequent clogg'ing of the s'ieve open'ings. Chepil (1962) presented a detai'led pìan for a rotatry sieve mach'ine to eliminate tlte clogging.
Various indices have been proposed for expressing the distribution o'F aggregate sizes.If a single characterist'ic parameter is desired, such as might allow correlation with various factors (e.g.erosion, infiltrat'ion, evaporatìon, or aeration), a method must be adopted for assigning an appropriate weightìng factor to each size range of aggregates. A wideìy
used index is the mean we'ight diameter (Van Baveì, 1949; Youker and McGuinness,1956) based on we'ight'ing the masses of aggregates of the various size classes according to their respective sizes. A least square regressìon line was computed from the measurements and resulted in J=0.876x-0.079 where y is the mean weight diameter, and x is a constant which is obta'ined
by summing the multiplication product of mean sieve size and percent
reta i ned .
Soils vary in the degree to which they are vulnerable to imposed destructÍve forces. Aqqreqate stability 'is a measure of this vuìnerabili ty. The classical and still most prevalent procedure for testing the aggregate
stability is the wet sieving method (Tiuìin, 1933; Yoder, 1936). A representative sample of a'ir-dry aggregates is pìaced on the uppermost of a set of graduated sieves and immersed in water to s'imulate flooding.
The sieves are then osci I lated verticaì ìy and rhythm'icaì'ly, so that water
is made to flow up and down through the screens and the assemblage of aggregates. In this manner, the action of flowing water is simulated. At the end of a specified period of siev'ing (e.g. l0 min) the sieves are removed from the water and the oven-dry vreight of material left on each sieve is determined. As po'inted out by Kemper (.l965), the results should be corrected for the coarse primary particles retained on each sieve to avoid designating them fa'lsely as aggregates. I
An alternative approach 'is to subject soil aggregates to s'imulated .l944), ra'in. In the drop method (McCalla, ìndividual aggregates are bombarded wìth drops of water in a standardjzed manner. The number of drops needed for total dissipation of the aggregate, or the fractional mass of the aggregate rema'in'ing after a given t'ime, can be determined. A better way'is to subject an aggregated soil surface in the field to periods of simulated rainstorms of controlìable raindrop s'izes and veloc1ties (Morj n et ar.,1967; Amernlan et a7., 1970). The corrdition of the so'il surface can then be compared to the initial condition, and the degree of aggregate stabì'l 'ity thereby assessed . To determine the stab'ility of microaggregates, comparative seclimentation analysis can be carried out with dispet sed and undispersed samples. An index indicatjng the fractional amount of clay associated in microaggregates can then be calculated.
Emerson (1967) developed a water coherence test for use in assess'ing the stabiìity of soils for earth dam construction. Thus this method is main'ly applied to sub-so'i1s, and is based on the swelling, slaking and dispersion behaviour of soil aggregates when immersed in water. To do this, soil aggregates are immersed in distilled water, and according to their behaviour, the aggregates are classified ìnto eight classes.
Loveday and Py'le (1973) modified the Emerson dispersion test as part of a study of soil hydraulíc conductivity. The Emerson dispersion test has also been successful]y used by Green'land, Rimmer and Payne (1975) to determine the structural stabi'lity of top soils of Eng'lish and l^lelsh soils. This method has been found to be useful in assessing drainage problems'in these soi I s.
Low (1954) and others (Rovira and Greacen,1957; Richardson, 1976) used dispersion ratio as an index of aggregate water stability. This dispersion ratio is defined as the ratio of percenù silt plus clay obtained after weak dispersion to that obtained after complete dispersion during mechanìcal analysis. Rovira and Greacen (.l957) used the weak dispersion 9 method developed by Marshall (1956). This rnethod consisted of measuring wiih a plummet balance the amounts of material smaller tharr 50 unr after successive perìods of end-over-end shaking. A suspension of about 20 g of soil in 1 I of water was used. Still another Índex of soìl structural stability is obtained by cornparing the permeabi'l'ity of the soil to an inert fluid with the pernreab'il ìty torvard water (Reeve, 1965). The permeabil ity of an appropriately packed so'i'l sampìe is first measured by using air, a fluid that is presumed to have no effect on structure, and then by using water. The latter, being a po'lar liquìd, reacts with the soiì, modifying the structure and usualìy reduc'ing the permeability. A ratio of unity, if such were to occur, would indicate perfect stabi'lity. Values greater than unity indicate a scaìe of increasing instability. The change in permeabìlity due to leaching w'ith dilute salt solution was aiso proposed earlier by tnrerson (1954,.l955) as a measure of structural stability. Butler (.l955) has distinguished four cìasses of clay soils based ma i nìy on the consistenc.y of the soil in the hand. The sub p'lastic soi I s are those which increase in texture on prolonged kneading, ê.g. from a gravel to a clay. Superplastic soils show the reverse effect changìng from heavy to light c'lay, while the consistence of normal labile soils is unchanged. Seìf-mulching soils are similar to the last group, except that after pudd'ling in the field, naturaì wetting and drying return the sojl to a granular state. Greacen (.l960) has suggested that the increased suction whjch deve'lops when a soil at a given water content is continuously remoulded is a measure of the effective strength of the und'isturbed soil at that water content. The strength of dry soil aggregates has been measured by a drop- shatter test (Marshalì and Quirk, 1950). Farrell et at.(1967) investìgated this method over a range of water contents using remoulded spheres of a loam soil. 10
1.2 Coaqulation and DjsPersion Dispersìon is an electrochemìcal property of the soil colloid and is consequently influenced by the pH of the media, the electrolyte concentration and nature of the colloid. Prior to a soil colìoid being dispersed, it has to be in a condition to respond to electro.-chemical forces.
I.2.1 Diffuse Double Layer TheorY Interactions between charged colloidal particles are the result of a
balance between van der Waals attractive forces and several repu'ls'ive forces including e'lectric double-1ayer repulsion and other short range repulsive forces (van 01phen, 1963). Electric double-1ayer repuìsive forces are genera'l1y considered to be the result of the work required to
overcome the increase'in free energy associated with the overlapping of the diffuse layers of counter ions as two charged partìcles approach each other. The ionic distribution and hence the repulsive force in the diffuse layer near a given surface is a function of the counterion valence and eìectrolyte concentration in the solution phase, as described by the electrical double-'layer equations, i.e. the thickness of the diffuse layer decreases with increasing valence and electrolyte concentration. The magnitude of the electric double-ìayer repuìsive forces largeìy deternlines whether particJes are dispersed or coaguìated. As the electric doubìe-ìayer equations 'indicate these repu'ls'ive forces are a function of counter ion valence and the ionic strength of the bulk solution. The counter ions in most soil systems are dominated by caz+, l4g2*, H+ or Al3+ aL or Fe'-, al'l of which tend to reduce the electric double-layer repu'lsive forces to the point that they are less than the van der Waals attractive forces. As a result, soì1 part'icìes generaì'ly interact to form aggregates. The ntajor exception is in the case of soil with high Na+ saturation. Under such conditions the repu'lsìve forces may predom'inate over the attractive 11 forces resulting in dispersìon of the soil particles anci poor soil physica'l properties. Such dispersjon tends to occur at a certain ESP.
t.?.L. 1 El ectri c doubl e-'l ayer equati ons The defjnit'ions for the terms in the electric ciouble layer equat'ions (1-5) are given at the end of this section (page 11). Gouy-Chapman theory of the electric double layer for a symmetricaì eìectrolyte was described by Babcock (1963) and Adamson (1967).
z-L o= (ZnDkI / n) s'i nh ( zerio/2kT )
The fundamental charge eìectroìyte concentration-electrical potentiai
relationship can be related to many chemica'l and phys'ica'l properties of
soils. The Gouy-Chapman equation (Eq.1) has limited quantitative application due to the assumption that ions in solution behave as point charges and can approach the surface without limit (golt, j955). Stern
(1924) introduced a nrodification of the Gouy-Chapman theory such that the first layer of ions (Stern layer) is not immediately at the surface, but a distance, ô, away from it (0verbeek 1952). The Stern theory o"i the
electrical double ìayer assumes that the surface charge is balanced by the charge in solution which is distributed between the Stern ìayer at a distance, ô, from the surface and a diffuse layer. The total surface
charge, o, is balanced by the sum of the charge in the two layers:-
oi= -(or+o2) Q)
As outlined by Stern (1924) and van Raii and Peech (1972), the charge
in the Stern layer is gìven by N.ze o1 = (3)
The charge in the diffuse ìayer is given by the Gouy-Chapman theory (Eq.l) except that the reference is now the Stern potentiaì jnstead of the T2 surface potent'ial ,
-L oZ = (2nDkT/r)'sinh(zerfoô/2kT) (4) since a linear drop in potential across the Stern layer is assumed, the surface charge ìs also given by the Gauss equation for a molecular condenser, o = (D'l+nO) (,to- ,i,O) (5)
The equations 1-5 can be used to expìain the surface charge-surface potentía'l relationships for both constant charge and pll dependent charge surfaces. The definitions for the terms of equations 1-5 are as follows. o = surface charge density o, = Stern layer charge o, = diffuse layer charge n = ion concentratÍon in bulk solution
Úo = surface Potent'ia'l ü¿ = diffuse layer potential gô = Stern potent'iaì e = electronic charge k = Boltzman constant D = diabattivity D' = diabattivity in the Stern ìayer N.' = number of ava'ilab]e spots per cm2 for adsorption of ions
NO = Avogadro's number M = molecular weìght of the solvent ü) = solvent density ø = spec'ific adsorption potentiaì z = ion vaìence T = absol ute temperature 13
I.2.2. Point of Zero Charge (PZC) Sjnce dispersion is an electrochem'ical process, the pH of the soil system must be above or below the PZC. PZC is defined as the pH at which the surface charge of the system reduces to zero (Parks and de Bruyn ,196?). The pH where variable net surface charge resulting from the adsorpt'ion of the potentìa'l determining'ions H+ and 0H- is zero. The PZC has proven to be a useful parameter through which to classify surface chemical phenomena (Parks,1967; Arnold, 1977:' Stoop, l9B0). The pH value at which the net adsorbed charge vanished was called the point of zero net charge (PZNC) by Parker et at.(1979) and was regarded by them to be reìiable parameter with whích to characterize the electro-chemjcal surface properties of soils.
The water d'ispers'ible clay approaches minimum values close to the PZNC.
Uehara and Ken,g (1975) showed the variation in water dispersible clay as a function of the net charge. The surface charge densities that appear in Eq. 2 can be employed to define a variety of PZCs in soils. Conventiona'lly, the PZC is the pH vaìue of the soil solution at which there is no net charge on the sol'id soil particles (Parks,1967). If the term'soil part'icle' is taken to include surface comp'lexes, as is customary, the PZC is the pH va'lue of the soìl solution when o = 0 (Pyman et at-.,.1979). Sposito (l98l) indicated that if oZ.0 (specific anion adsorption) the value of o will be larger than when.no specific adsorpt'ion occurs, and the
PZC must shift downward. If o.r0 (specific cation adsorption), the va1ue of o will be smaller than when no spec'ific adsorption occurs and PZC must shift upward. These kinds of shift of +"he PZC caused by specific adsorption have been confirmed experimenta'lly many times (Hingston et at-.,
1972; Huang and Stumm,1973; Wann and Uehara, 1978) and have been simulated in double-'layer model calculations by Pyman et at. (19i9). 14
1,2.3 Adsorpt'ion of Ions on Soil Surface Adsorb'ing ìons can be div'icled into ttr,o categories, those which can adsorb when there ìs no charge on the surface (charging'ions) and those which can adsorb only when the surface is already charged (non-charging or indifferent Íons). Ryden et at.(1977) assigned two classes of adsorpt'ion (chemjcal and physical adsorption) for anions and cations on the soil surfaces. The adsorption of these ions depends on the nature of the soil surface (e.g. negatively or pos'itive'ly charged).
1.2.3.1 Cation adsorption
The common cations Na+, K*, CuZ*, Mgz+ or Fe3+ and Al3+ are adsorbed by so'il mjnerals when surfaces are negatively charged. For minerals with
pH-dependent charge, the negative charge 'increases with pH. However, a more complete description of catjon adsorption by variable charge surfaces must assume different binding constants of different ions. Thus the mechanistic model of Barrow eÈ al.(1980) suggests that ions, such as Na+
and K+ have a low binding constant which is about ten times greater for
ca?+.
Some authors, who consider specific adsorption to mean chernisorption, point out that specifjc adsorpt'ion of cations on the permanently negatÍveìy charged soil will increase pos'it'ive surface charge (Deshpande et at.l964) thus increasing PZC (Parks, 1967). Uehara and Gillman (1980)
reported that at ZPNC some cations may be adsorbed on the variable charge component, so that the total cat'ion adsorpt'ion should be greater than or equal to the permanent charge measured at pHo. The surface charge is
positive and PZC occurs at high pH in highly weathered Oxisols r¡rhich are rich'in iron and aluminium oxides. The nature and distribution of the reactive iron and aluminium components in soils have been described .l963 .l .l eì sewhere (Oades , ; t'li tchel l et aI . ,1 964; Ri ch, 968; Coul ter, 969 ; Greenland, l97l; Jones atrd Uehara,1973). Apparent PZC values from 2.7 to 15
6.5 have been reported for sor¡e Oxisols (fl-Swaify and Sayegh,'l975;
Tama and El -Swaify,l97B). The PZC values of both iron ar,d alum'inium oxides and their hydroxìdes, generally lie above 7 (e.g.Parks,1965; Atkjnson et al.,1967; El-Swaify, 1976). Unless lowered by the presence of sign'ificant amounts of organic matter, the PZC values of 0xìsols lie between those of kaolin'ite and sesquioxides (e.g. Van Raii and Peech,1972;
Tama and Eì -Swaify, l97S). Th'is co-existence of substantial posit'ive and negative charges at fie'ld pH values, gives rise to electrostatic bonding between djfferent constituents and between cons'Lituents of the same type.
Eì-Swaify (1976) showed that such bonding gives rise to mutual flocculat'ion between clays and sesqu'ioxides and suggested a quantìtat'ive model for ìt.
At the same time natural'ly occurring iron oxides have loler PZC values as compared with their pure synthet'ic counterparts (Greenland and Mott,l978). Since the work of Van Raij and Peech (1972) there has been considerable interest in determining the PZC of trop'ical soils (Keng and Uehara,1974; Gallez et a7.,1976; Gillman and Be]1,1976; Morais et al. .,1976; Hendershot, ì978). Iron oxide coatings can be precipitated onto kaolinite, illite and bentonite clays under suitable laboratory cond'itions, leading to an increased net positive charge on the clay surface (Rengasamy and Oades, 1977a, Greenìänd and l4ott, 1978; Kavanagh and Quirk, 1978).
I.2.3.2 Anjon adsorption Since colloids of the constant potential type can under acid conditions acquire net positìve charge (pH-dependent charge) anion
adsorption becomes a particularly important feature in soiìs containing these colloids (Mekaru and Uehara,1972). It is now general'ly believed that aniond can be adsorbed on oxide surfaces in one of two ways. In the first case, anions are retaÍned as counter ions in the diffuse layer opposite a net positively charged surface and in the second case, an'ions enter into coordination with the oxidenetal ion whjch involves displacement 16 of one anion (lisand) by another, these first and second types have been ternled non-spec'ific adsorptìon and spec'ific adsorption respectiveìy (Hingston et at.,1967,1968). The pH-dependent charge'in highly weathered soils, or on oxide surtaces (Bowden et at.,1977) is rrot limited to effects of H+ and 0H-, but rnay also be caused by adsorpt'ion of ions such as P0O3-, Si04-, 5042- and various organic anions. In recent years there has been an increasing voìume of literature dealing with the anion adsorption by soils and soil materials, including'iron and aluminium sesquìoxide components, part'icu'larly in tropicaì and sub-tropical areas. However, these reactive iron and aluminium surfaces are also common in many temperate soi'ls (M'itchell et at-.,1964.; l{ada and Harward ,1974). It has become apparent that the adsorption behaviour of many anions, in particular the important nutrients phosphate, su'lphate and molybdate, is very similar (Barrow, 1970). tlork with synthetic iron and aluminium oxides has shown that fluoride, seìenite, silicate, arsenate, carbonate, and other anions , including organic anions, are also adsorbed by similar mechanisms. There is suff icient evidence (l'lehlìch, 1960; Lutz et ar.",1966; Hingston et at-.,1967; Mekaru and Uehara,1972; Sawhney, 1974; Parfitt and .l978) Atkinson, 1976; Wann and Uehara, to suggest that the surface charge density and therefore the cation exchange capacity of oxides of certain soiìs may be altered by chemicals commonìy used as fertilizers and soil amendments. The evidence suggests that phosphorus not only serves as a nutrient, but as an amendment to increase cation exchange as well. The most frequentìy citedcauses for phosphate or other anion-induced cation exchange capacity are (i) a shift in PZC to lower pH's (Hingston et at.,
1967,1972,1974; Breeuwsma and Lykìema, 1973: Jepson et at-.t 1976; Parfitt and Atkinson, 1976), (ii) neutralization of positive change
(Hi ngston et a-2. ,1972; Schal scha et aJ. ,1972) and ( i i i ) el ectrolyte imbibition (Thomas, 1960). Phosphate and su'lphate adsorption by ligand bonding (GalinOo and B'ingham, 1977; Parfitt and Smart, 1978; Raian, l97B) t7 increases catÍon exchange capacity and net negative charge hence decreasing PZNC (Morais et a-2.,1976; Galindo and Bingham,1977; Yeoh, 1979). Gillman and Uehara (1980) reported that surface areas calculated from negative ion adsor^ption below and above the PZC indicates that permanent and variable charges occupy the same ared. Anion adsorption at different pH vaìues has been studied for arsenate and selenite (Frost and Griffìn,1977), phosphate and nrolybdate (Tanner, l97B), fìuoride (Omueti and Jones, 1977),and sulphate (Couto et at-.,1979). For anions of fully associated acjds (e.9. su'lphate) which have a low affinity constant (Barrow et a7.,1980), adsorption increases as pH decreases. Anions of incomp'lete'ly dissociated acids (e.9. sil icate) are most readì1y adsorbed near the pK of the acid; pH<4 for selenite, fluoride and phosphate, pH 4 for molybdate, pH 9 for silicate and borate (Parfitt,l978). Anion adsorption also accounts for the flocculation-dispersion properties, and increase in cation exchange capacity of soil (Parfitt, 1978; Wann and Uehara, l97B) and could also be detrimental to the stable physica'l condition of many highly weathered soil s, 'insofar as such stability is a function of the extent of mutual neutralization of positive and negative charges (Fey and LeRoux, 1976). The electrostatic contribution to aggregate stability is .pH-dependent and effects the extent of aggregate breakdown at different pH values (Tama and El-Swaify,1978).
An important implication of this fact is that certain management practices which improve the chemical fertility of soil may be detrimental to soil structure (e.g. liming can reduce intra-aggregate bonds by el imi nati ng most pos i ti ve charges ( Kamprath ,l 971 ) . Another i mpl 'icat'ion is that pH dependent charges in colloid stabi'lity and the para'l'lel change from favourable to unfavourable structural conditions can be very abrupt. Fortunateìy, one origin of intra-aggregate bonding is non-electrostatic
(El-Swaify and Emerson,.l975; Tama and El-Swaify,l978) and this tends to reduce the abruptness of such changes. 1B
1.2.3.2.7 CompLexes with metaJ- ions The polyfunctional, an'¡onic character of humic substances also
accounts for the propensity of these materials for forming complexes with metal ions in solution (Schnitzer and Khan,1972; Jackson et a7.,
1978) . The stabi ì i ty and water sol ubi'l i ty of the resul tant cornpl exes depend on the nature of the metal ion, the size of the humjc polymer, the metal poìymer ratjo , pH, and ionic strength. Thus, for fulvic acid at pH 3 and ionic strength 0..l, Schnitzer and Hansen (1970) found the
following order to decreasìng stabil'ity, Fe3*r Al3+, M2+. Appel t et aJ..
(.1975) concluded that the sorption of organic anions by soils vrith Fe and Al could be described by a sorption-pKa-pH relatior¡shjp, âs outlined .l972). previously by Hingston et at.(197.l and They further suggested that chelation was an important mechanism involved in the specific sorption of some types of organic anions, such as salicy'late.
1.2.4 D'ispersion
lr'lhen aggregates are immersed in water, spontaneous dispersion occurs
if the c'lay swells to such an extent that attractive forces between the particles are no longer strong enough to keep them together (Emerson, 1977\. Clay sized particles are released s'low'ly and appear as a spreading cloud around the aggregates (Arnold,l978). Soils with a high proportìon of exchangeable sodium or magnesium tend to disperse easily (Emerson, 1977). Organic matter tends to oppose dispersion when the organic matter increases the attractive forces between partìcles of c'lay (Emerson,1962;
1967). There is some ev'idence that smal I amounts of organ'ic anions may disperse clay (Aìoomfield, 1963), probably by bìockìng the positive s'ites on the clay and by compìexing the poìyvalent cations which coaguìate the clay (Greenland,l965), and thrrt organic anions may mobi'lize fine clay down the prof i'le (Thorp et aJ-. ,1957) . Differences in opinion can be found in the literature as to whether dispers'ion or swelling is the major cause for deterioration of the 19 structure and poor physical propertìes. McNeal and Colenran (1966),
McNeal (1968) and Rowell eË aL(1969) have pubiished equations whjch relate saturated hydrauìic conductivity and swelìing. McNeal and Coleman (1966) considered d'ispersion and particle translocation the dominant mechanisms for decreases in hydraulic conductivity ìn coarse-textured soils and in soils that contain small amounts of expansive minerals.
There u/a.s a suggest'ion that the blockage of the pores, as a result of clay dispersion and movement, was the main mechanisnr which controlled the structure and perrneability of the soils (Shajnberg et at-.,.l971 ; Felhandler et ar.,1974; Frenke'l et at.,1978). The movement of the c'lay to form cìay ,l965; B horizons being termed m'igration (Thorp et at-.,1959; Brydon,
0ertel, l968). Calculations on the Urrbrae loam show that there has been a volume increase in the 82 horizon of 125% and volume decrease in the A2 of 24%, with an overall 20% increase in the whole profi'le (Chittleborough and Oades, l9B0).
1.2.4.1 Factors responsible for clay dispersion The clay may be dispersed main'ìy by the influence of pedologicaì, physical, chemicaì and bioìogica'l activities in the soils or soiì profiles. Velasco-Molina et ar.(1977) found that in the absence of electroìytes soiì dispersed in relation to their domjnant minerals, montmorillonitic > halloysitic > kaol initic > micaceous. It is known that the finer the clay the greater the dispersion and mobility (Seghal etal.,ì976; Dixit, l97B) Chittleborough and Oades (.1980) have also indicated the increase ìn volume of B horizon of Urrbrae soil by the dispersion and movement of fine c1ay. For good dispersion to occur, water is necessary and critical soil/ water ratios are needed (Gombeer and D'Hoore,1971 ô, b; D'Hoorer 1974). 0n the other hand, for the dispersion of wet sheared aggiegates there ìs a minimum water content required before d'ispers'ion occurs. 20
It has been shown that d'ispersion is related to exchangeable sodium (Emerson, 1967). High exchangeable sodium percentage (ESP) due to th¿ presence of exchangeable sodium (USDA, 1954; Yaron et at-.,1973) caused dispersion in soils (Feìhendler et a1.,.I974). Emerson and Bakker (t973) studied favourable condítions for clay dispersion when a'loamy or sandy soil, rvith low ESP values, was leached with dilute solutions. FethendlêF er ar.(1974) found that cìay dispersÍon in soils with low silt content t{as more pronounced than in soils with similar ESP and cìay mineralogy but with higher silt content. The fact that dispersion can occur at lower exchangeable sodium levels than swelling may be explained by the effect of exchangeabìe cation composÍtion on the structural arrangement of clay particles (Aylmore and Quirk, 1959, 1962; Blackmore and Mi1'ler, 1961 ; Shainberg and Qtoh, 1968; Quirk, 1968; Shaínberg and
Cai serman, l 97l ) .
Two conditions seem to favour cìay dispersion: 1) a sharp reduction in the concentration of the soil solution, below the threshold concentration (Quirf and Schofield, 1955) at which the clay particles f I occul ate, and 2) a hi gh concentratÍon gradient betr^reen the di I ute solutions of the macropores and the more concentrated solution in the micropores inside the aggregates.
1.3 Aggregation in Soils , figgregatÍon can be divided very simply into two categoríes 1) microaggregation which is ìargely contro'lled by coaguìation-dispersion phenomena and 2) macroaggregation, )250gm which is dependent on cements and g'lues" 0n the other hand stable aggregation (micro- and macro- aggregat'ion) can be considered the result of at least two processes. Firstly, coìloidal particles should be in a coagulated or flocculated state, perhaps'in the form of domains. Coagulation of clay particles is promoted by polyva'lent exchangeable cations or high electro'lyte 2L concentratiorrs and is reversibìe by exchanging cations or by d'ilution (Van 0lphen, l977). Flocculatjon'is an irreversible process which results when collo'idal particles possess a balance of positive and negat'ive charges and a PZC is created. This situatÍon arises when the cations neutralising the rregative charges on colloids are not exchangeable, e.g. positive sjtes are produced (Rengasamy and Oades, 1977 a, b;1979; Kavanagh and Quirk,.l978). As discussed earlier raising the PZC to the pH of the soil is one means of controlìing dispers'ion and hence soil structure. Second'ly, the clay particles (domaìns,flocs etç.) can be aggregated into larger aggregates .(macroaggregates) which become stable when points of contact between the first stage aggregates are bonded by cementing agents which may be either organic or inorganic.
1.3.1 Factors Influencing Aggregation
Physicaì, bioìogical and chemical agents are involved in micro- and macroaggregation. Mainìy physica'I, biological and some chemical agents create macroaggregates by consolidation or cementation in the soils.
1.3.1.1 Physicaì agents
The cracks and surfaces of weakness that separate natural aggregates are probably largeìy due to movements of soil during shrinking and swelìing. Croney and Coleman (1954) demonstrated the effect that the degree of drying has on the aggregation of a slurried clay soil. Thorough drying has the effect of consolidating aggregates of clayey material to a higher density than they would reach by any external pressure likely to be experienced by soils under natural conditions. The resulting cracks between them may remain effective for water transmission for some months after re-wetting, as noted in a review by I'larshall (ì962). It was found that wetting and drying cycìes resulted in macroaggregation (McHenry and 22
Russell, 1943; I'Joodburn, 1944; Utonro, l9B0). Freez'ing affects aggregat'ion by the expansion of water on chang'ing to ice wi bhin the soil.
Richardson ('tSZ0) showed that three freezing and thawing cycles on a sandy loam that had puddled severeìy gave better aggregat'ion than that of untreated soil.
1.3.1 .2 Bioì og'icaì agents Stable aggregates are destroyed in many soils when treated wjth .l966; hydrogen peroxide to remove organic matter (Kuznetsova, Edwards and Bremner, 1967) indicating that organic matter is involved in aggregation. Microbial polysaccharides in soil or clay stab'iìize aggregates (Mart'in, 1945; Greenìand, 1956, 0ades, 1967; 0ades and Sw'incer, l968; Hamblin, 1977;Oades, 1978; Turchenek and Oades, 1978; Tisdalì and Oades,1979). Naturally occurring organic materials have also been used to improve the aggregation and physical condition of the soils (Baver,1930; Boekel, ì965; Soane et aJ.,1972; Davies, 1975). There are however many exampìes of soils where organic matter does not influence porosity or stable aggregat'ion (Hambl'in and Davies , 1977). These organic materials are generaììy appìied in solid form or as a s'lurry, and whÍle they have been found to improve various physica'l propert'ies, they suffer from drawbacks, such as insufficient suppìy, inconvenience in handling and applications, difficulty in remova'1, or interference with succeeding crops. In many situations the appìication of these natural materials does not offer an adequate solution to the soil structure prob'lem at hand.
1.3.1.3 Chemical agents
Chemical materiaìs may produce an electrolyte or cementing effect resulting in micro and macroaggregation in sojls. In recent years there has tLn- c-ffe"*s oÇ been an increasing volume of literature deal'ing with(ì'norsanic and organic 23 chernicals like iron and aluminiu¡n oxides and hydroxides, phosphoric ac'id, sulphuric acid, gypsum, poìyvinyl alcohol , poìye'lectroìytes, etc. on aggregation in the sodic and non-sodic so'ils. l-.3.7.3.1- lron oxjdes and hgdrous oxides
Ferric oxides and their hydrates occur widely jn soìls and markedìy influence their chemical and physical properties (0ades, 1963; Taylor and Schwertmann , 1974). 0xi sol s behave I'ike the "sub-pì astic c'lays" of Butler (1955). A systematic means of describing aggregate stabi'lity ìs to classify the aggregates according to the scheme proposed by Emerson
(.l967). Aggregates from oxic horizons genera'l'ly fa1ì in the most resistant classes, namely class 8 (no siaking or sweiling) or class 6 (slaking but no dispersion). Aggregates from surface horizons of certain
Oxisols, however, may dispìay some dispersion fol'lowing immersion, re- moulding, and mixing w'ith water in dilute suspensions.
Nvaka and Voronova (.l980) reported that ferruginous soils formed on the weathering products of basaìt have a very high degree of aggregation, and highly water stable m'icro- and macroaggregates. Among the most impressive consequences of the high aggregate stability of 0xisols is their high toierance aga'inst aggregate disruption under sodic conditions .l980; (El-Swaify and Swindale, t968; E1-Swaìfy, 1970, El-Swaify et ar., 1970; Frenkel' and Shainberg, 1980), even at very ìow electroìyte concentrations, ESPs as high as 100 have only a s'light effect on surface 0xisols and none on subsurface (0x'ic) horizons. The orientatjon of constituent minerals within the aggregates, estimated by anisotropy measurements, was reported to affect the stabiìity of natural (unremoulded aggregates: the better the orientation, the higher the stability (Cagauan and Uehara, 1965). Jones and Uehara (1973), using high resolution electron microscopy werê able to show the presence of ge1-like coatings around prev'iously súspended clay particles of certain 0xisols. 24
They suggested that these coatings may be inst.rumental in aggregate formation and stabiì'ity. 0n the other hand, the role of iron-oxides in aggregate stabilization remains an open question. Deshpande et at-.(1964) found that complete removal of free iron oxides had little effect on the aggregation or structural stabi'lìty of a number of soils. 0n the other hand, chemical removal of iron oxides and sub-plastic material s cause decreas'ing aggregate stab'iì ity and complete dispersion (stace et at-.,.l969; McIntyre, 1gl6). Greenland er ar., (1968) and Deshpande er a-2.(.l968) usìng electron niÍcroscopy in coniunctÍon with selective extraction methods, observed that certajn iron oxÍdes occur as discrete particles or clusters. The decline in aggregate stabiìity after selective removal of inorganic and organic complexes from soil aggregates by aqueous solutions cf inorganic salts was also reported (Greenìand et aJ.,'1962; clapp and Emerson, 1965).
In the study of clay minerals, iron has not been shown to be so effect'ive probabìy because above pH values of about 3 the iron is precipitated as a separate phase from Èhe clay minerals as Follett (1965) and Greenland and Oades (1968) have shown by eìectron microscopy.
However, ferrihydrite, freshìy precipitated jn the presence of clay was shown to be effective in aggregating cìay fractions (Blackmore, lg73).
7.3.7.3.2 Ggpsum
Scotter and Loveday (1967) postuìated that by inhibiting dispersion in particular, gypsum has the ability to maintain pore space with aggregation. Also a reduction of clay swelling is likeìy to be invo'lved, its importance relative to the effects on clay dispersion depends on such factors as soil texture, clay mineralogy, ESp and electrolyte concentration (Rowell et at-.,.l969; Frenkeì et ar.,l97B; pupisky and Shainberg,1979). Blackmore (1976) reported that calcium ions from the 25 gypsum exchange wjth sodium on the clay caus'ing some of the d'iffuse
'layers jn the narrower pores to immediately contract anci open pathways to salt from the interior. The more extensive the contact that the soil has with the gypsum solut'ion, the more complete will be the exchange of caìcium for sodium and the more effective the suppression of negative adsorptìon, permitting still freer passage of salt. This is apparent'ly a way that gypsum can aid in prevention of dispersion of sodic soil quìte apart from its effects on swelì'ing and flocculation
ESP is usualìy taken as a criterion for judging the dispersion or aggregatìon and stabif ity of soil structure.
7.3.7.3.2.7 Ggpsum treatment and ESP
Sharma and Tunny ('1978) showed the reduction of ESP of the soils from 2,11 and 18 to 1, 2, and 12 respectiveìy by the addition of gypsum. Several workers (Bridge and Kleinig, 1968; Sharma, l97l; Loveday, 1974; Grierson, 1978) have also found similar behaviour in different soils. At the same time,applying a given amount of three different sizes (.0.S;
0.5-1.0, L.0-2.0 mm diameter) of gypsurn to the surface soil was more effective'in reducing the ESP in a simiìar way (Gobran et ar.,1982).
The reduction of ESP may give hìgh aggregation in the presence of electroìytes (e.g. cu?* , so42-).
7.3.7.3.2.2 SolubiTitg and feasibiLitg of the use of ggpsum The effectiveness of gypsum as an eìectrolyte agent depends on its particle size and dissolutiön properties (Keren and Shaìnberg, l98l). Gypsum is genera'l'ly the most reasonably priced source of Ca2+ in the solubi'lity range needed in preventing the dispersion of sodic soils. For more economical usage, Haider et at.(1972) fecommenú¿d,fbuse of "pit run" stone of large size rather than powdered gypsum. Kemper et ar.(1975) showed gypsum fragments <4 cm in diameter could provide all the gypsum ?6 desired in water flowjng through beds of these fragments. Inspection of "pit run" gypsum indicated that stones were more comìrtonly in the 5 to 50 kg weight and ranged from about 13 to 30 cn in diameter. Haider eÈ a-2.(1974) found that gypsum stones (4-7 kg) on the beds of water courses lost from 0.10 to 0.I5% of their weight/h of exposure to the running water. 0n the other hand, Ahmad et a7. (1979) reported that gypsum stones 5 to 20 kg in size are a usable source of the Ca2+ needed to bring down the sod'ium aclsorpt'ion ratio (SAR) in sodic tube wel I water. The dissolution rate (g g-1) of the gypsum stones was proportional to the square root of the veloc'ity of water flow'ing through the bed, us'ing the equati on , D =ÆTF where D is the dissolut'ion rate of the gypsum stone, S is size (weight), R is average fìowing rate, and G is a proportional'ity coefficient. Glas et ar.(.l979) attempted to describe gypsum dissolution and ion movement by a first order rate equation in combination with the convective- dispersion equation; Melamed et at.(1977) also adopted a similar mode'lìing approach. Equilibrium models, which assume the rate of d'issolution is rapid enough to maintain a saturated solut'ion have been tested by Dutt et a7. (1972); Tanji et a-2.(1972) and 0ster and Frenkel (1980). An application of 92 to 122 cn of irrigation water was recommended to dissolve 9 to ll.2 tons ha-l of applied agricultural grade gypsum of such fineness that 85% of the material will pass through a 100-mesh sieve (U.S. Salinity Laboratory Staff, 1954). Based on the solubi'lity of gypsum mixed
in salt solutions and Na+-Ca2+ exchange equilibria Dutt (1964) and Dutt et a7.(1972) predicted that 52 to 73 cm of irrigation water was required to dissolve 16.5 to 24.0 tons ha-l of appììed gypsum. Hìra and Singh
(1980) found that about 4 cm of water was sufficient to djssolve all the
gypsum <0.26 mm s'ize. The water required for complete dissolution of gypsum computed from a díssolution equation increased from 2.8 to 15.9 cm 27 as gypsum part'icle size increased from <0.1 mm to 0.5-2.0 nlm. An 'important but less invest'igated factor ìikely to influence the dissolution of the gypsum in the soil is the amount of exchangeable sodium. 0ster and Halvorson ('l978) indicated that the solubjììty of gypsum could be enhanced if the chem'ical is mixed in the soil.
7.3.7.3.3 CaLcium carbonate (CaCor)
Calcium carbonate increased the percentage of aggregates with a diameter of less than 20 um, which could only occur with a decrease'in percentage of larger aggregates (Peele, ì936). In a review, Harris et ar.(1966) pointed out that CaCO, often stìmulates microbial activity and in this way may have a beneficial effect on the aggregatìon of certain soils. Diamond and Kinter (.l965) suggested that slower pozzo"lanjc reactions are responsible for the increasing aggregate stability or" CaC0, treated soils with time. In an attempt to expìain the effect of CaC0r, the U.S. Salinity Laboratory Staff (1954) and Rimmer and Greenland (1976) proposed that CaCO, acts in soils as a cementing agent which stabilizes the soil aggregates and prevents cìay dispersion.
7.3.7.3.3.7 Sol-ubil-itg and feasibiJ-itg of the use of caTcium carbonate ' Cqtcil-e is the most abundant, stabìe, and well crystallized CaC0, found in so'iìs (Doner and Lynn, 1977). If welì crystaìl'ized its solubjlity will not be affected by'its crigin. 0n the other hand, additionaì CaCO, may precipitate during irrigation because the water and/ or the soil solution is super-saturated with respect to calcite (Bower et al.,1965; Rhoades et a-2.,ì97,3; Jury et at-.,1978; Levy, 1980). The solubiìity of the freshly precipitated CaCO, is often reported to be enhanced either because the irrigation water or the soil solution contain crysta'llization inhibitors (Doner and Pratt, 1969), or because the solid phase precipitates as an amorphous coat'ing (Greenland and Mott, 1978). ?B
Stud'ies by Suarez (1977) have indicated that ionic activity products for CaCO, 'in so'il s can vary, depend'ing on the experirnental conditions under which they were obtained. Levy (1981) suggested that Mg may hat,e acted as one of the jnhibitors, but the soils also contained other soluble species, such as organìc matter, phosphorus and silica, which could have increased the solubilìty of CaCO, surfaces even at very ìow concentratjons.
The solubility nray be increased if CaCO, is divided into particles 7.3.L.3.4 Mixtures of ggpsum and calcium carbonate A detailed description of Ca-S0O-HC03-C03 interactions in solution is given by Nakayama (1969) and Robbjns er ar.(1980) provides an invaluable overview of the combined CaC0, and gypsum interactions in soil water systems. Jury and Pratt (1980) compared pred'icted Ca, Mg and Cl and S0O 29 values by a proportional nrodel, a CaCO, and gypsum model, and a chemistry plus cation exchange mode1. Kaushansky and Gat (1977) and Keren and Kauschansky (l9Bl) showed that the rate of gypsum dissolution was reduced by a coating of precipitated CaCOr. As ¡nentioned in section 1.3.1.3.3.1, this coating may also occur when 'irrìgation water containing an appreciable carbonate ion concentration is passed through a soil containing gypsum particles. 7.3.7.3.5 Other inorganic chemicals Suìphuric and phosphoric acids have been used to increase aggregation and water intake rates in the soil (Yahiê. et at.,1975; Thein,1976; Yeoh and 0ades, l98l). Easi'ly soluble calcium chloride and calcium nitrate aìso promoted aggregat'ion and water intake rates (Aìperovitch and Shainberg,1973; Magdoff and Bresler,.l973; De Jong,1982). De Jong (1982) compared the efficìency of some of the inorgan'ic materials with gypsum and obtained the following order: gypsum > magnesium sulphate > calcium ni trate . The mechanisms of clay stabilization and aggregation using inorganic materials including ìime and cement have been reviewed by Ingles (i970) and Ingles and Metcalf (1972). 7. 3.7. 3.6 Organìc poTgmers Synthetic organic po'lymers like po'lyvinyì alcohol , polyvinyl acetate, poìyelectrolytes such as HPAN (hydrolysed polyacrylon'itrile) and VAMA (vinyl acetate-maleic acid copolymer) have all been successfuìly used to stabilize the aggregation (Allison and Moore,1956; Bennett et at.,1964; Carr and Greenland, 1975; Oades, 1976; Koff et at.,1977). 30 1 .4 Phys'ica1 Properti es af te¡ Chq¡gçs i rr Aggregat'ion and E4çlr¡¡geqþl e Sodium Percentaqe The physical properties of soils are important to agricu'lture, because the pore space of so'i1s governs gaseous transfen wìth the atmosphere, as well as water movement and storage, root growth, and the ease of di ff icul ty of ti ì I age. Favourab'l e pore s'izes can be created by aggregation (Russell, l97l ; Greenìand, l98l). I .4 . 1 Improved Aggregat'ion The aggregate stability of iron-oxide rich soils leads to infiltration rates of the order of tens of cm h-l (Lugo-Lopez et aJ.,l968; Yokoyama, 1969; l^lolf ,1975). Rengasamy and Krishnamurt'i (.l978) found that hydraulic conductivity was negat'ive1y related to clay content and microaggregat'ion in some ferruginous soils. The cementing act'ion of iron oxìdes increased the aggregation and hydraul'ic conductivity (McNeal et a7.,1968; Eì-Swaify and Swindale, 1969). El-Swaify (1980) indicated that bulk density increased with depth desp'ite the stronger aggregation within the oxic horizon. This may be partìy attributed to the higher organic matter content.ìn surface layers. Van Wambeke (.l974) suggested that the bulk density in the oxic horizon is linear'ly and pos'itiveìy re'lated to the percentage of sand size particìes. 0n the other hand Nvaka and Voronova (1980) found low bulk densities with high degrees of aggregation in ferrugìnous soils. Eì-Swaify and Emerson (1975) stated that the low liquid limits are characteri st'ic of hi gh'ly aggregated i ron oxi de rich soi I s. Converse'ly, McNabb (1979) suggested that FerO, has little effect on Atterberg limits. Briones (1969) determined the shear strength of compacted samples of iron oxide rich 0xisols and Vertisols. He found well aggregated ferrug'inous soils were less resistant to shearing than Vertisols. Some investigators show that iron'is one of the factors in the formation of degree of 31 hardness of the tiìlage-induced pans. Compounds conta'ining iron were aJso found to be probabìe cement'ing agents in fragipans (Hallmark and .l979). Smeck, 7.4.2 Reduced ESP In the presence of low electrolyte concentration in natural soil, permeability is narkedly affected by ESP (Qujrk and Schofield, 1955; Shainberg and Caiserman, l97l; Russo and Bresler, 1977:' Rahman and Rowell, 1979; Dufey et ar.,l9B2). The variation of hydrau'lic conductivity occurred in a narrow range of ESP in which important changes of size and .l968; shape of aggregatesv¡ereobserved (Shainberg and Otoh, Dufey et at-.t 1976; Dufey and Banin,l979). Hydrauìic conductivity can be maintained, even at high ESP values, provided that the electrical conductìv'ity of the soil solution is above the threshold concentration (Quirk and Schofield, 1955). Lowered ESP or low ESP values in the soils gave high total porosity and hydraulic conductivity (Sedgley , 1962; Scotter and Loveday, 1966; Sharma, '1971,1972; Loveday,1976; Shainberg et ar.,1982) and less .l964), swe'lling (Rowell, 1963; Fink and Thomas, modulus of rupture (Aylmore and Siì1s,1982) and crust strength (Frenkeì and Hadas, 1981). 1.4.3 Behaviour of Calcium Carbonate Rimmer and Greenland (1976) showed variable swelling behaviour of soil after add'ition of calcium carbonatq.' At -63.1 bar water potentia'l , swel ì ing was decreased with removal of carbonate, but swe'l'ling was increased at -10 mbar water potential. They explained that decrease in swelling was due to the mobilization and redeposition of hydroxy- alumina polymers during the treatment. The mechanism by which CaCO, stabilizes soil structure is important in studies of infiltration rate and crust formation in calcareous and 32 non-calcareous soils under rainfall (e.9. Agassi eÈ a-2.,.l981 ). If CaCOU acts as a cementing materiaì, then calcareous soils will not be as sensitive to crust formation as the non-calcareous soils. However, if the dissolution mechanism is the dominant one, then the concentration of electroìytes at the soil surface exposed to rain will be sufficient and both soils will be as sensitive to crust formation. Sha'inberg and Gal (1982) found that powdered, 6aC0u (<44 um diam.) prevented clay dispersion and increased the hydraulic conductivity of sodjc soils. They indicated'that the solubility of the fine CaCO, was such that the eìectroìyte effect t'las important. This investigation illustrates the importance of the size of the CaCO, particles on solubility and hence the physica'l conditìon of the soil. 33 CHAPTER 2 AIMS AND OBJECTIVES OF THE STUDY To date little work has been done using poìycations and organic anions to controì dispersible c'lay and thus the physìcaì condit'ion of the soils. In spite of many studies with gypsum on sodic and non-sodic soiìs, there is a lack of information on the influence of the electrolyte effect on the coagulation of clay. Among the chemicals which jncrease the electrolyte concentrat'ion of soil solution gypsum is preferable in terms of both solubility and cost. 0n the other hand CaCOt and cement may improve aggregatÍon by a cementing action. Whether CaCO, js useful to improve structure by an electro'ìyte or cenrenting effect'is unknown as only limited work has been done. Rainfall or water addition is a very'important factor in considering the solubility of the chemicals which are applied to the soil to create the electrolyte or cementing effect. No work has been reported on the influence of wettjng and drying cyc'les on the effjciency of soil amend- ments. The following objectives were defined for th'is study: i) to prevent the dispersion and improve the physica'l condition of the soils in relation to aggregation by manipu'lating the net surface charge; ii) to investigate dispersion caused by sorption of inorganic and organic anions; iii) to study relationships between dìspersible clay and physical properti es ; iv) to compare the efficiency of electroìyte concentration and cementing in the stabilization of aggregates; v) to determine the influence of the wetting and drying cycles on the solubility of gypsum, calcium carbonate and cement and their effect on the physicaì properties of sodic soils; and finally, 34 vi ) to determine the inf I uence of changes in phys'ica'l propert'ies on gennination capac'ity, plant and root growth. 35 CI]APTER 3 EXPERIMENTAL 3.1 Preparation of Poly[Fe(III)-0H] Cations A fresh solution of 0.1M ferric nitrate was prepared using an anaìytical reagent grade of the salt. Fe(III) po1¡,cat'ion solution was prepared by the add'ition of 1M sodium hydroxide to 0.1M ferric nitrate solution until the pH was 2.2. The pH adiusted solution was aged for 4 days and the molecular we'ighb range 10,000-50,000 was separated by ultrafiltratjon using Amicon filters UM10 and XM50 (Oades, 1983). 3.2 Electrophoretic Mobil ities To the soil suspension (10 cm3) zcmt 0.01M NaN0, solution was added to maintain constant ionic strength. After a short equiìibrium period (about 30 m'in.) particles were wjthdrawn from the mid portion of the suspensíon and the mobility measured. All electrophoretic measurements were performed using a Rank MkII particle lnÍcroelectropho,resis apparatus using the procedure described by Kavanagh et al.(1976). The motion of 10 particles over one division of grid was timed first'in one directl'on and then on reversing the polarity of the appìied field in the opposite direction. All results have been expressed in mobi'lity units. U = 2I7 .9/tv where U is the eìectrophoretic mobjlity (um.*u-1.-1¡, t is the average time in seconds for l partic'le,and v is the app'l'ied voltage across the cell. 3.3 Aggregate Stabi 1 i ty Air-dried soil (20.0 g) was wet sieved according to the method of Kemper (1965) jn 4000 cm3 distilled water in a cyìinder (300 mm by 145 mm diameter) for 5 min. The material which had passed through the sieves 36 Was resuspended in the 4000 cnls of water by shak'ing the cyì'incler gently end-over-end four tirnes, and the size distribution of water-stable particles (the term'particles'is used for the fract'ion retaìned by the sieves whìch may'include some s'ing'le grain partìcles in addition to aggregates) was cletermi ned by sed'inrentati on unden gravi ty (equ'ival ent spherìca1 diameters <50 um, <20 ¡lm, .2 um) . 3.4 Hydraul ic Conductiv'ity Hydrauìic conduct'iv'ity of disturbed samp'les (30 rnm dia x 50 mm) was determined by the constant head method of Klute (1965). The soil sampìes in a glass tube were constraìned by a nyìon cloth. The sampìes were saturated by placing the columns of soil 1 cm below the water level overnight and then connected to a constant head of water and the outflow was collected for a known time when a constant rate of flow had been atta i ned . 3.5 Soil Consistency Parameters The consisterrcy I imits, also termed the Atterberg ìim'it, are ind'ices of the workabiìity of firmness of artificial mixtures of soíl and water as affected by the content of water in the mixture. The limits are defined by the water contents required to produce specified degrees of consistency that are measured in the laboratory. Liquid and p'lastic limit and sticky point were determined following the methods described by Sowers (.l965). 3.6 Modulus of Rupture The modulus of rupture of four artificial briquets for each repl'icate was determined at d'ifferent water contents on the apparatus described by Richards (1953). Rernoulded soils were pressed in brass moulds (70 x 35 x 9 mm) , the top was snroothed over and the briquets left to dry overn'ight 37 taken out from the brass nloulds and driecl at 105"C overn'ight. The force requìred to break a briquet when loaded as a horizontal beam was measured and the modulus of rupture, which was the maximum fibre stress, was calculated by the formula 51 = 3FLlZbdz where S, is the modulus of rupture'in dynes per square centimetre; F is the break'ing force in dynes (the break'ing force 'in gram we'ight x 980); L is the distance between the two lower supports, b is the rvidth of the briquet¡ d is the depth or thickness of the briquet, a'l'l expressed in centimetres. Fìnally, modulus of rupture was given in kPa (1 kPa = lo4dynes cm-2). 3.7 Friabiì ity The friability of the soils was determ'ined by the method of Utomo and Dexter (t981). The remoulded soils at different water contents and air-dried aggregates were used in this determ'ination. In the remoulded soils, the aggregates were made w'ith diameters of about 5, 10, 15 and 20 mm by hand rolling. Tensi'le strength was measured by crushing 1.0 aggregates of each size for each treatment between paraìleì p'lates. The tensiìe strength was calculated for each aggregate using the equation, s2 = Q' 576F/dz where S, is the tensile strength, F is the polar force required to fracture the aggregaterand d is the aggregate diameter. It is interesting to note that positive (compressive) forces in one direction can give rise to tensile stresses and fajlure in another. Friability, k, was obtained from the relationship between Log.S, the logarithrn of tensile strength and Log.V,the logarithm of aggregate volume. The intercept A is an extrapo'lated estimate of the logarithrn of the tensi'le strength of 1 ms samples of the bulk soil LoguS = -klog.V+A 38 3.8 Root l-ength Measuretnent The so'i1 samples were a'ir-dried and ground in a njll using a coarse screen of 4 mm diameter nesh. Each ground sample was thoroughìy mixed before subsampì'ing for root separat'ion. Roots were separated frorn the ground so'iì (10 or l5 g) by a flotation method similar to that of Barley (1955). Three subsamp'les were taken from each replicate. Each sample was stirred rapìdìy'in 500 cm3 water for 30 s before decantìng the water onto a sjeve with 250 1.rm aperture. Further water was added to the sediment, allow'ing the iet of water to stir the suspension. The water was aga'in decanted. Th'is was repeated until no further ¡oots could be seen in the sed'iment. The root segments were transfemed from the sieve to a beaker with a fine jet of water, then onto a Whatman No. 50 fi'lter paper held on a porous p'late under suction. The flow bras confined within a 5 x 5 cm perspex reta'ined placed on the filter paper. The roots and other material were drawn onto the paper as the water was sucked through the paper and p'late. The perspex retainer was coated even'ly with wax to prevent roots being held at the edge by surface tension. t^lh'ile stilì wet the filter papers were sprayed with a so-tutìon of 15% po'lyvinyl acetate (Aquadhere) in water which fixed the roots in position on drying. The length of root in each sample was determined either by oS .1966) ft = {*.*ran, or R = 1å*n (Tennant, 1975). Root ìength (R) was measured by counting the number of intercepts (N) of roots in a regular area (A) with randomly oriented I ines of total ìength (H) or lìnes of a grid with grid unit (G). 3. 9 l,Jater Retenti on lJater holding capacities at dìfferent water potent'ials from -10 to -1500 kPa were determined using s'intered glass funnels and pressure p'lates. 39 CHAPTER 4 MODII.ICATION OF SOIL PHYSICAL PROPERTIES BY MqNIPULATING THE NTT SURFACE CHARGE ON COI.LOIDS 4.1 Introduction In most soils structure'is controlled by the presence of the colloidal fraction and more specìfically by the surface properties of the col I oi dal fracti on. The surface reacti ons of the col I oj ds control swel'l i ng and flocculation-dispersìon phenomena which are of prime importance in determining soìl structure and its stabi'lity. In spite of this the relations between the surface properties of soil cìays and varjous parameters used to assess physical and mechanical propertìes of soils are poor'ly defined. In this chapter an attempt is made to bridge the gap betvreen colloid chemistry and soil mechanics by manipu'lating the net charge on the cìay in a soiì using Fe(III) poìycat'ions, and assessing the changes in some of the physical properties of the soil. Recent work on hydro'lysis of Fe(III), in the absence of conrplex'ing anions, has shown that poìycations wÍth a limited range of sizes can be isolated. They are strongìy sorbed on clay surfaces and cause irreverisble flocculation of the clay (Rengasamy and Oades, 1977 a, b; Kavanagh and Quirk, l978). Polycations separated by ultrafiltratìon (mo'lecular weight 10,000-50,000) were shown to be h'ighly charged, roughly spherical with diameters up to 10 nm, and probab'ly related to ferrihydrite (Oades,1983). Sorption of increasing amounts of such poìycations led to decreased negative electrophoretic mobil'ities of clays until flocculation occurred when the point of zero charge (PZC) was reached, followed by redispersion of some of the c'lay with a net positive charge. 4.? Materi al s 4.2.I Descri pt'ion of Soi I A representative sanrp'le (0-10 cm) of the Urrbrae f ine sandy 'loanr from 40 'long. the Waite Agricultural Research Institute ('lat 34"58'S, j38"38'E), a Red-brown earth (Qades et al.,l9Bl), was air-drÍed and sieved <2 nlm. Table 3 shows some properties of the soil. The nrinerals present in the clay fraction were kaol'inite (30-40%), illite (40-50%), with some amounts of randomìy interstratified cìays (10-20%), and hematite and/or goethìte (5-'10%) and quartz (I-5%). Table 3 Some Soil Characteristics clay (%) 19.4 sirt (%) 3l .3 Fine sand (%) 43.8 Coarse sand (%) 2.0 Plastic limjt (% w/w) 16.7 l^later-holding capacity (% w/w) 42.5 pH (1:5, HrO) 5.4 Organic matter (%) 3.5 1 Cation exchange capacity (C g- ^ ) 6.20 Exchangeable Na (C g-1) 0.02 Exchangeable K (c g-l) 0.90 Exchangeable Mg (C s-l) 0.60 Exchangeable Ca (C g-l) 3.1 Exchangeable Fe (C g-1) 0.03 Exchangeable Al (c g-l ) 0.12 C = Coulomb = 6.2 x 1018 electronic charges 4.3 Experimental 4.3..l Dispersed Cìay A suspension (3%) of the soil in water was used to determine the quantìty of polycation requ'ired to flocculate the cìay (Critjcal Coagulat'ion Corrcentrat'ion - CCC). Different amounts of poìycation were added to 1.0 cm3 4T of suspensjon which was adjusted to 20.t3'by addition of distilled water in a 50 .r3 n',.ururing cyì inder. The suspens'ions were shaken thoroughly by hand for 60 s. The amount of clay dispersed by this treatment was determined by measuring the opticaì density at 615 nm in 1m cells 24 h after addit'ion of poìycation solutjons. The opt'icaì density at 615 nm was calibrated against the percentage clay, isolated from the soil and determined gravimetrìca'l1y (Fig. 2). The opt'ical dens'ity at 615 nm of the polycation solution alone was low (<0.04). 4.3.2 Treatments of Soil with Polycations The soiì samp'les were treated w'ith six dÍfferent anrounts of poìy- cations ranging above and below the CCC. The treated so'il samples were air dried in plast'ic pots for 7 weeks. The treatments added 0.00; 0.01; 0.04; 0.07; 0.16 amd 0.32% Fe, on the basis of weight of soil. Each treatment was repìicated four times. 4.3.3 Exam'ination of Poìycation Treated Soil s Electrophoretic mobilities of soiì suspensions were determined and measurements were also made as a function of pH. Electric charges were deterrnìned at the original pH of the soì1 by weighing 5 g of soil into a 50 cm3 centrifuge tube and wash'ing once with 1.0M BaCl Z, 0.2M BaCl, and 0.01M BaCì, separate'ly by centrifugatìon. The excess BaCl, was removed by washing thrice with 0.002M BaC1r. The retained Ba2+ and Cl- in the soil c'lay partjcles were exchanged with 0.5M KN03 solution of the same pH. The positive charge was calculated on the bas'is of Cl- determined in the KNO, extract, nsing an Orion digital ionaì'yzêr (Model 8014) with chloride electrode (C1- Cat.No. 941700). The negatÍve charge was calculated on the basis of Ba2+ in KNO, extract estimatjon, using atomic adsorption spectrometry. Soil suspensions (0.25%) were prepared by suspending 125 mg ; 42 o. o Y:O.58X+0.06 o ¡2: o.g7 o o.7 E c o.6 |o (o o o.5 ú,c o It o o () Ê o o.3 o. o.1 o o.1 o.2 o.3 o.4 0.5 0.6 O.Z O.B O.9 1.O %Dispersible clay Fig. 2 Calibration of % dispersible clay and optÍcal density 43 3 of the soil ìn 50 cnr of djstilled water. A drop of the finer partic'les of the suspension was pìaced onto the grid of a samp'le holder and dried by adsorpt'ion w'ith a blottìng paper belov¡ the grid. The samp'le vlas p'laced and exanrined'in a JEOL JEM 100 CX electron m'icroscope. In the determination of surface area, approximately 600 mg of air dried soil was made into pellet form and placed into the bulb of the surface area apparatus thermostated at 25"C. The adsorpt'ion of nitrogen with increasing pressure at -195'C was then measured in the usual way and the surface a.rea obtained by appl'ication of the BET equation (Brunaer, Emmett and Tel I er ( I 938) ) . Dry bulk density and porosity were measured separate'ly for soils remoulded at different water contents. The sanrples of mou'lded sojls were prepared in pìastic cylinders. The oven dry weight, air filled volume and volume of the core samples were determined for each water content. Pore size distributions of the air-dry (<2 mm) soils impregnated with epoxy resin under vacuum, were determined directly on 5 crn diameter sections 35-40 um thick. An image ana'lysing computer the Quantimet 720 was used to analyse the total porosity and pore size distribution. The image displayed on the screen was 4 x objective. The pores up to 100 um diameter sizes were selected and 5-10 pictures were analysed for each repìicate. The results were passed into a data output system such as a teìetype printer from which a punched computer tape was obtained for further analysi s. The friability of the soils was determined from moulded soil aggregates. The aggregates were aged at -50 kPa water potent'iaì for two weeks, then equ'i'librated at -10 and -20 kPa using sintered glass funnels and at -100, -200, -1000 and-1500 kPa using pressure p'lates. Penetrometer resistance was tested by remouìdíng the soil at 20% water content with a'laboratory knife until the soil was as homogeneous as poss'ib'le. These remoulded soils were allowed to equilibrate overnight, 44 then again remoulded for a few minutes and pressed in plastic cyìinders (40 mrn dia x 2? mn). The resistance of 3 cores of each treatnlent were measured immediately (0 day sample). The remainder, in their cylinders, were wrapped in thin plast'ic sheeting and alum'inium foi'l , then stored in a constant temperature room (20'C) for test'ing after different time intervals. The resistance was measured with a motor driven laboratory penetrometer. The probe had a diameter of 1.008 mm, a tota'l tip angìe 60", and penetrated downward at a rate of 3 nm min-1. The force required to penetrate the soil was measured with an electron'ic balance (Mettler type PC 4400). Three measurements were done on each core, and the strength was calculated as the resistance to probe penetration, QO, us'ing the equati on , Q, = 4F lndz where F is the force required to penetrate the sampìe at a depth of 5 mm, and d is the probe diameter. After each strength measurement, the water 'l05'C. content of the sample v'ras determined at 4.4. Results 4.4.I Di spersed C'lay The concentration of iron (III) required to flocculate alì the clay (dispersible clay = 0) was 0.07% by weight of soil (Fig. 3). When the amounts of iron added exceeded the CCC, electrophoretic mobilities changed from negative to positive. As anticipated, eìectrophoretic mobilit'ies close to zero, i.e. a net charge of zero, occurred urhere most of the clay was flocculated. The pH of untreated suspensions was lowered from 5.4 to 3.6 as polycations were added (Tabìe 4). However, net pos'itive charges were mainta'ined when the pH values were raised to above 5 (Fig. 4). 8'0 1 th =o i ûl 5'0 E o u E oì o o l 3 o -0 tt= o rl a Ê u A 3.0 .9 -c o -o L ''t^ o o- ó)È o .9 ^ o g() òe l¡J 1'0 0.08 lron tur¡ t%¡ a¿ded to the system on soil basis (:<=) Fig.3 Influence of po]y[Fe(rII)-0H]cation on floccu'lation and dispersion Þ andelectrophoretÍcmobj.lity(.--^--(.ve)¡-- --(+ve))ofsoilsuspensions (Jr 46 +3 +2 I U' I +1 E (J Et- l 0 pH 3 78 10 l¡=Ë o E .9 o -1 o A oCL L oU l¡¡ -2 -3 A Fis. 4 Eìectrophoretic mobilities of soils as a function of pH before and after treatment with poìyfFe(III)-OH]catìons. ( o ) control; ( ^ ) 0.01%Fe; ( tr ) 0.04%Fe; ( 0 ) 0.06%Fe; ( | ) 0.08%Fe; ( r,) 0.16%Fe; ( r ) 0.32%Fe. 47 Tabl e 4 ptl Va'lues for Polyffe(IIi)-0H] catíon Treated Soil Suspensions Iron added pH (% of soil 0.00 s.4 0.01 4.9 0.04 4.4 0.07 4.2 0.16 3.8 0 .32 3.6 4.4.2 Electric Charges Eìectrophoretic mobilitìes of suspended colloids as a function of pH show that charge reversal occurred when nrore than 0.06% Fe was added to the sojl (Fig.4). The po'ints of zero charge were raised to between pH 5 and 6 by the three'largest additions of iron. The CCC gave a PZC between pH va'lues of 4.0 and 5.0, i.e. about one pH unit lower than the natural pH of the soil. There was a 40% reduction in the negative charge or cation exchange capacity by addition of A.07% Fe(III) (faUle 5). However, adsorption of more poìycation did not neutralize or block all the negative charges determined by ion exchange methods. Table 5 Changes in Electric Charges of Soiì by Addìtion of Poly[Fe(III)-0H]cations Iron added Positive charge Negative charge (% of soil) (c g-i) (c g-1) 0.00 0. 18 6 .2 0.01 0.20 5.8 0.04 0 .2r 4.8 0.07 0.14 3.8 0. 16 0.29 4.3 0 "32 0.19 3.9 48 Tabl e 6 Effect of Poly[Fe(III)-OH]cations on the Surface Area of Soìls Iron added Surface area & ot soi'l) (mzs-1 ¡ 0.00 11.4 0.01 11.6 0.04 11.9 0. 07 11.1 0. 16 11.1 0.32 tt.7 4.4.3 Surface Area and Electron Microscopy Specific surface areas of the soil were not changed signifìcant'ly by the addition of polycat'ions (Tab1e 6) but electron microscopy showed cìearly that fine clay (Fig.5A) was el'iminated to some extent by ìower amounts of poìycation (Fig. 58 & C) and completeìy by amounts of poly- cation which flocculated the clays (Fig.5D). Fine particles were again evident in the sampìes to which ìarger quantities of poìycation were added (Fig. 5E & F). 4.4.4 Aggregate Stabiì ity There was no s'ignificant difference in the percentage of water stable particìes 250-2000 um diameter due to addition of po'lycations (Fig.6). The histogram for the <2 Um fraction shows clearly the disappearance of clay sized part'icles with po'lycation treatment and the flocculation of the clay caused by add'ition of 0.07% Fe resulted in a sign'ificant increase in particles 50-250 pm diameter. 4.4.5 Dry Buìk Density and Porosities Addition of 0.07% Fe r more in the form of polycations increased porosities and lowered bulk densities of the soil cores moulded with water 49 Fig. 5 Transmission electron micrograph of clay fraction. Hori zontal bar=1um (A) Untreated sojl (B) PolyIFe(III)-0H]cation rreared soil 0.01%Fe (c) rr ¡'| .' o.o4%Fe rr (D) ', ,. o.ol%te (E) rr '|i ¡¡ o.r6%te rr (f ) 'r '. o.3z%Fe 8¡ I D V o a E l J t â a .t F Õ t a Ì a , t a o - 50 Fig. 6 Effect of poly[Fe(III)-0H]cations on the size distribution of water stable particles in soil. E control; F o .oL%Fei Ø0.04%Fe; [ll[ o .07%Fet ffil 0. t 6%Fe; Ç!o rz%Fe. Verticalbar, P<0.05. 54 4 ='õ 42 th 6 o 36 òa Iat I .9 3 o cL I -o 24 o ah o o 1 I I ì 12 6 I I o 1000-2000 500-looo -250 Size of particles(pm) 51 Tabl e 7 Changes in the Bulk Density (g cm-3) and Porosity (y" v/v) of Moulding l,Jater Content (% w/w) of the PolyIFe(III)-0H]cation Treated Soils Iron added (% of soil) 0.00 Ml^lC 8.6 10.3 l4 I lB.2 19.7 Bulk density 1.55 1 .54 1 6B I .60 1.57 Porosi ty 38.3 38.7 32 I 36 .0 37 .0 0.0.| MhlC 8.4 il.7 14.7 18. B 20.5 Bul k Dens'ity I .54 I .5.| I .69 1.62 I .58 Poros'ity 38.9 39.6 32.6 3s.2 36 .8 0.04 Ml^lC 9.8 1? .8 I4.5 21 I 21 B Buìk Density 1.52 ì .50* l.6l* I 59 I 56 Porosi ty 39.4 40 .0 35.6** 36 4 37 I 0.07 MllC ll .2 13.5 l6 .7 22 I 23.0 Bu'lk Densi ty I .51 I .45*** I .48*** l. 59 l. 54 Poros ì ty 39 o 42.0*** 40 . B*** 36. 5 3B 4 0.16 Mï^JC , 10. 6 13 .l 15.2 23 .4 24.1 Bulk Density 1.5? 1.45*** 1.42*** 1.60 I .56 Porosi ty 39.5 4.| .8** 43.4*** 36.0 37.6 0.32 MWC ì 0.3 14 .4 I 5.4 23 .0 23.4 Bulk Density I.s0 I .44*** 1 .42*** ì .59 I .56 Poros i ty 40.0 42.2 43 .2* 36.2 37 .7 * P<0 .0s ** P<0 .01 *** P<0 .00.| Ml^lC = moulding water content 52 contents ranging from I? to 15% wlw (Table 7). Maximum bulk densities and minimum porosities rvere obtained when sampìes were moulded with water contents near the plastic limit of the untreated soil (17% w/w). 4.4.6 Pore Size and Hydrau'ljc Conduct'ivity The addition o'f poìycations increased the porosity'in pores up to 100 ¡rm djameter of the air-dry soi'l by a factor of 3 to 4 compared with the untreated soil (Table B). The total area of pores of 40 to 100 um diameter was measured. The pore size distribution showed the formation of ìarger pores (40-100 ¡rm) and reduction of smaller pores (<40 um) by addition of 0.07%, 0.16% and 0.32% Fe. Table I Effect of adding Poly[Fe(ttt)-OH]cations on the Poros'ity and Pore Size Distribution of Air-dried Soils (tota'l evaluated area was 1964 mmz) Iron added Total area of pores Percentage of total area of pores *<40 (U of soi ì) (mmt ) um 40-60 um 60-100 um 0.00 7.t 100 0.01 5.5 100 0.04 9.2 100 0.07 27.3 27 .5 26.4 46.t 0. 16 32.4 16.6 44 .6 38.8 0.32 25.4 49 .6 - 50.4 *Percentage of pores <40 ¡rm was obtained by subtracting the percentage of 40-100 pm pores from 100. The lower linlit of detection of pores'is probably about 30 pm which'is the thickness of the th'in sections. Increased porosity and formation of pores (a0-100 um) increased the water holding capacity and hydraulic conductivity significantly (Table 9). The water retained in the air-dried soils increased with amount of 53 poìycat'ion added. The hydraul ic conduct'ivìty was increased by a factor of 3 over the control confirming the increase'in nunlbers of transmissive pores. Table 9 Influence of Poly[Fe(III)-0H]cations on the Tdater Holding Capacity and Hydrau'l i c Conduct'ivi ty of Soi I Iron added Mo'isture content Maximum wa ter Saturated hydraul ic of air-dried soiIs hol ding ca pacity conducti vì ty (% of soil ) (% w/w) (% wlw ) (cm h-l¡ 0.00 1 .3 42.5 0. 13 0.01 1.5 44.1 a.r7 0. 04 2.2 44 .8 0.22 0.07 2.r 50.7 0.35 0.16 2.r 49.2 0.36 0.32 2.0 49.4 0.34 LSD (P < o.os) 2.3 _ 0.04 (P < o.o1) 3.1 0.05 4.4.7 Soil Consistency The liquid and plastic Iimit decreased ('in gravimetric water content) significantìy with amounts of Fe(III) poìycations which caused flocculation (faUle l0), but the sticky point showed no signifìcant differencedue to treatments. 4.4.8 Modulus of Rupture Soils with maximum bulk dens'ity (moulded vlater contents near the plastic lìmit, 17% w/w) yielded maximum values for the modujus of rupture of briquets moulded at similar water contents (Fjg. 7). The force requ'ired to rupture the soils varieá with water content at which the samples were prepared and the amount of poìycation added. Flocculation reduced the modul us of rupture by a factor of 3 over the control . The 54 modulus of rupture (y) was lìnearly related to the bulk dens'ity (x) by the equation y - 250x - 350; rz = 0.77 which is based on all the data in Fig. 7 add Table 7. Tabl e 10 Influence of PolylFe(III)-0H]catjons on Consistency Parameters Iron added LJ q uid limit Plastic l'imit St'i c ky po int (% of soil) ( % w/w) (% w/w) (% wlw ) 0.00 23.1 16.7 18.2 0.01 23.1 16.3 17.8 0.04 23.0 16.2 17.6 0.07 22.4 l5.s 17 .5 0.ì6 21 .2 I 5.0 17 .6 0.32 22.2 16.2 l7.s LSD (P < o.o5) 0.7 0.7 NS (P < o.or) 0.9 0.9 4.4.9 Soil Friability Soil friabilities of the polycation treated soils were obtained at different water contents (Fig. 8). The friabilitV (k) increased with the amount of polycation added at alì the water contents. The highest va'lues were obtaìned rvhen the water content was near the plastic limit. At the same time, sma'll k values were obtained when lower (6-9% w/w) and higher (21-24% w/w) water contents were used. 4.4 . 10 Penetrometer Res'i stance Penetrometer resistance as a measure of soil strength was shown for different tjme intervals (Fig.9). There was no signifjcant difference between the untreated and polycatìon treated soils up to five days after 55 80 70 60 tft o- -50-f o CL *40o .h Eso= o E I O r ^ 20 \ 10 I 18 Moulding water content sf soil (% wzfu ) Fig. 7 Effect of rnouldÍng water content on the modulus of rupture of art'ificjal briquets by add'ition of poly[Fe(III)-0H]cations. (Curvesfittedbyeye.) (o) control; (a) 0.01%Fe; (o ) 0.04?áFe; (. ) 0.07%Fe; (^ ) 0.16%Fe; (r ) 0.32%Fe. 56 .:< = 0'18 l¡ rl l¡- 0 6 91215 18 24 27 Water content of soil (?w/w) Fig. 8 Effect of water content on the friability of soìls treated with po'ly[Fe(III)-0H]cations. (Curves f itted by eye). (o ) control; (^ )0.01%Fe; (o) 0.04%Fe; (o) 0.07%Fe; ( r ) 0.16%Fe; ( r ) 0.32%Fe 57 700 6 o o. ! o (, 500 tr o I o 400 o L 1 o o 300 E o o Ê 20 o À 2O%water too conte o 10 20 30 40 50 60 70 80 90 100 Number of days after remoulding Fig.9 Effect of time on the penetrometer resistance of soils treated wìth poìy[Fe(III)-0H]cations at 20% moulding water content. (o ) control; ( o ) 0.07%te; (r ) 0.32%te. 5B remoulding at 20% water content. The water content was also unclianged at the measurement of the res'istance. However, the strength was greatìy different between the untreated and flocculated so'ils from the 1Oth day, and the djfference was reduced after 70 days. The water content (20%) also started to decrease apprec'iab'ly after 45 days and 70 days in the untreated and f I occul ated soi'l s respect'ive'ly. 4. 5 Di scuss'ion 4.5.1 Surface Charges and Flocculation The resul ts i ndi cate eff i c j ent f I occul ati on of soi'l c'lay by very smal I amounts of polycations presumably through the bridging of the finer partic'les by the chemisorptìon of the polycations. The pr'esence of a net positive charge caused the redispersion of some clay after addition of 0.16% and 0.32% Fe. The treatments also reduced the pH of the samples which in part explains the net positive charges. However the change in electrophoretic mobilities from negative through zero to positive confirm the dispersion-flocculat'ion-redispersion behaviour, and it is clear that the PZC was raised to values between pH 5 and 6. Earlier stud'ies with kaolinite and illitic c'lays showed that Fe polycations raised the PZC to more than 7.0 which is higher than the values obtained in the present study (Rengasamy and 0ades, 1977b; .l983). Kavanagh and Quirk, l97B and 0ades, The CCC of polycations also gave a lower PZC than in the earlier studies. Possibly, carboxyl groups on organic matter are responsible for these differences. Negative charges on clays can be blocked by adsorptìon of iron (III) due to mutual .l963). neutralization or by physical blocking of exchange sites (Sumner, The reduction of negat'ive charges or net charges determined by ion exchange methods did not correspondwìth the electrophoretic measurements. During electrophoretic measurements on'ly the finer partìcles are observed but when the cation exchange capac'ity'is determined coarse and fine particles 59 are involved. Similar d'iscrepancies occurred when jron was added to a clay fraction or st.andard cìays by other workers. In particular, Herrera and Peech (1970) observeC a reduced, but constant negatìve charge in an iron (III) - montmorillonite compìex which had a pos'it'ive electrophoretic mobilìty at pH 5.0. The electron micrographs ìndicate that v¡hile the fine clay is flocculated efficiently by the polycatìons it is quite l'ike'ly that a ìarge proportìon of the surfaces with negatìve charges are probab'ìy not accessible to the polycatjons which may be concentrated on the outer surfaces of clay assembìages. This in part may exp'lain the large d'iscrepancies between net charges obtained by ion exchange and electro- phoret'ic procedures. Alternatively the distribution of the polycations on surfaces in the soil by add'ition of a polycation solution to a 3% soil suspension may not be very uniform leaving significant portions of negat'ively charged surfaces unaltered. This is quite likely as poly- cations are rapidly and strongly adsorbed. In most soils, particularìy those of temperate regions the PZC of the clay fraction is very low and jn fact a net negative charge may exist even when the pH is adjusted to 2 due to permanent negative charge. Thus, some of the clay is susceptible to d'ispersion unless the soil is dominated by calcium ions. Gillman (1974) and Gillman and Bell (1976) showed that water dispersible clay was related to the PZC of strongìy weathered oxidic soils. hlhen the PZC coincided with the pH of the soi'l clay was flocculated and did not disperse jn water. Accumuìation of organic matter near the surface of the soil led to a lower PZC and some water dispersib'le clay. At depth in the profile the PZC exceeded the pH of the soil and again water dispersible clay was evident; the c'lay dispersing as a colloid with net positive charge. Acid washing of the highly weathered soil lowered the PZC implicating aluminium and iron oxìdes as the main source of posì t'ive charges . 60 Our results show c'learly the relation of water dìspersible c'lay to the PZC and that the PZC can be nran'ipulated using Fe(III) polycations. A similar result would be expected us'ing A1 polycations. 4 .5 .2 Aggregat'ion and Poros i ty The adsorption of Fe(III) polycations caused the flocculation of fìner particles into microaggregates 50-250 um, but no larger. There was maximum microaggregation when the amount of polycation added resulted in flocculation, i.e. m'inìmunr or zero lvater dispersible clay. The presence of mjcroaggregates led to decreased bulk densjties of aggregates moulded at water contents from 60 to 90% of the plastic limit. This ìs the water content at which cultivation of so'ils'is likely to y'ield the best results in terms of crumbling the soiì. Obviously porosity was inversely related to bulk density and was increased by the addition of polycations. The increase in porosity due to addition of po'lycation was evident over a range of pore sizes as indicated by the increased water hoìdìng capacities and hydrau'lic conduct'ivities (Table 6). There was an increase of more than 50% in water held after air-drying which exists in pores of diameter <0.1 um or as adsorbed water presumably wìthin the floccuìes or microaggregates. There was an increase of 20% 'in maxinrum water hoìciing capacity, i.e. pores up to about 60 um diameter. There vras alnlost a three-fold increase in saturated hydraulic conductivity whjch is related to pores greater than about 50 um diameter. Studìes using the Quantimet confirmed a three to four-fo'ld increase in the cross sectional area of pores, particularìy those with diameters from 40-100 um, i.e. trans- missive pores which explains the marked increases 'in hydraulic conduct- ivitìes. The hydraulic conductivities quoted,3.5 mnl h-l, are still very ìow compared with those obtajned elsewhere (E'l-Swa'ify, l9B0) e.g. 50-90 mm h-l. However Oxiso'ls genera'l'ly contain more'iron in the form of oxides than the present system even with the highest add'ition of iron. 61 4. 5.3 Soi I l'lechanícal Properties Maximum conrpaction (high bulk densities) occurred vrhen thre water content at mouldÍng was between l5 and 18% w/w which is near the p'lastic limit. Mitchell (1960), Lutz et at.,(.l966) and Yeoh and Oades ('l9Bl) also found that high bulk densities were created by mouldíng the soil at water contents near the plastic limit. Presumabìy at this water content shearing forces cause particle reorganization increasing contact between particles and reducing porosity. At lower water contents during moulding there was insufficient water present to carry clay particles into voids between bigger particles, resu'lting in lower bu'lk densities and strength. At water contents above the plastic l'imit the presence of water in pores prevented compaction of the soi'|, agaín resultíng in lower bulk densities and strength. The high modulus of rupture of a'll the sampìes moulded with water contents near the plastic limit showed that maximun compaction caused high bulk density and strength. The modulus of rupture was a línear function of bulk density. Except for very wet and very dry soils (<10 to >24% w/w) the mou'lding of soils in which the clay was flocculated by polycations led to the least compaction, i.e. the lowest bulk densities and low values for the modulus of rupture. The flpcculation of the fine clay caused a reduction in areas of contact between floccules or microaggregates. Hence a higher porosity and reduced bulk strength of the soí1. If an aggregate is made up of smaller aggregates it will have a high porosity and be mechanically weak. This property is desírable for a tilled layer of soil providing the aggregates are water stable and to some extent is expressed in friabí'lity. The friability (k) increased with flocculation due to a reduction of force required to crush the aggregates. Phosphoric acid treatment of the Urrbrae soil also reduced aggregate strength (Yeoh and Oades, lg8l) and increased the friability (Utomo and Dexter, l98l). Dry soils are strong and friabiìity or 62 crumbl iness j s rnaximal at þ/ater contents near the pì asti c I imi t. Th'is is the water content at which the soil should be tìlled to give maximum formation of smaller aggregates. Linear regressions were obtained relating various physical properties of the polycation treated soils to the percentage of part'icles 50-250 pm diameter (fa¡le I ì ). l^l'ith increasing proport'ions of particles 50-250 ¡rnr diameter the l^lHC, hydrauìic cc¡nductìv'ity and friabil'ity increased. This resulted from the creation of pores 40-.l00 ¡rm diameter and low density aggregates. At the same time the l'iquid and plastic limits and the modulus of rupture were decreased due to the reduction in areas of contact between particles 50-250 um djameter compared with areas of contact between a range of particle sizes including cìay. Table ll Effect of Percentage of Soil Particles 50-250 pm diameter on the Physica'l Properties of Soils Flocculated by Addition of PolyIFe(III)-0H]cat'ions Phys i ca'l properti es Linear regress'ions y=physica'l property x=% 50-250 pm s'ize parti cì es Maximum water holding capacity ! = 22.43 + 0.53x 12 = 0.87** (% w/w) Hydrau I i c conducti vi tv J = 0.01x - 0.43 12 = 0.95*** (cm h- 1) Liquid I imit ! = 25.15 - 0.05x 12 = 0.80* (% w/w) Plastic I imit Y = 19.89 - 0.05x 12 = 0.84** (% w/w) Modulus of rupture J =191.6 - 3.01x 12 = 0.76* (kPa) Friab'iìity y = 0.01x - 0.16 12 = 0.81* (k) * P = 0.05 ** P = 0.01 *** P = 0.001 63 The recluction in liquid and pìastic limits caused by addition of polycations must be due to the change in interpart,icle forces brought about by changed surface properties by sorption of poìycations. Low liquid limits exhibited by Oxisots have been attributed to sesqu'ioxides (El-Swaify and Henderson, 1967). The explanation for the non-significant difference in the penetrometer resistance at the early stages is not fully understood. It was reported that age hardening and thixotropic processes were major causes of the increase in strength in the presence of a negl'igible change'in water content (Utomo and Dexter, lg8l). In the present study' the same trend was observed up to 45 days ìn the untreated soils. The reduction in water content after 45 days would have jncreased the strength. There may be a possibility of loss of water because it was observed that some moisture q remained on the thin poìythene sheet. In the flocculated soi15 f[."e $qs signifÍcant increase in strength over the control after 5 days as the water content did not change up to 70 days. This was a result of water retention in pores 40-100 um diameter. 4.6 Conclusions The phys'ica1 properties of the soil studied were correlated wìth the net electric charge on the clay fraction. l,lhen the ZPC was near to the pH of the soil the c'lay,fract'ion was flocculated resu'lting in more water stable aggregates 50-250 Um, increased porosity, particu'larly in pores 40-l0O pm diameter, lower soil strength and Íncreased friabiljty. The flocculation of the c'lay responsible for the changes'in physical properties was brought al¡out by sorption of 0.07% iron in the form of polycati ons . 64 CHAPTER 5 MODrFrcATr0N 0F s0rL PHYSTcAL PR0PERTIES BY ADDiTI0N 0F Fe(III)P0LYCATIONS INFLUENCE ON PLANT GROWTH 5. 1 I ntroduct'ion In Chapter 4 it was shown that treatment of a soil with polycat'ions of Fe(III) resulted'in flocculation of clay, the development of micro- aggregates and transmissive pores. The treated soil had greater water holding capacity and hydraulic conductivìty and lower bulk density and modulus of rupture and was more friable. It was assumed that these changes in physicaì propertìes were beneficial with respect to pìant growth. This chapter describes the influence of addition of po'lycatìons of iron to soil on crack formation, crusting, seedling emergence and plant growth. 5.2 Experimental 5.2.1 Preparation of Soils in Pots The Red-brown earth, the Urrbrae fine sandy loam (0-10 cm) was treated with poìycations of Fe(III) as described in Chapter 4 (4.2.1; 4.3.1;4.3.2). The soil samples were treated with four different amounts of polycation; 0.00, 0.01%Fe, 0.07% Fe and 0.16% Fe on the basìs of weight of soil. The treated soil samples were air-dried in plastic pots for 7 weeks before crack formationn seedling emergence and plant growth were determined. Each treatment was repl'icated four times. 5.2.2. Rai nfal 1 Simul at'ion The untreated and treated soils were placed in p'lastic trays (45 x 30 x 7 cm) . Simulated rainfall at the rate of 27 mn h-l was applied us'ing a rainfall simulator mounted on a two-r¡lheeled traiier (Grierson and Oades, 1977). A calibration was done to obtain the h'ighest coefficient of 65 unifonnìty by usìng 36 tins in the area I x l. nt2 (Fig.10). Each tray was piaced on tlre ground (highest coefficient of uniformity area) directly under the nozzle. After rainfall sinrulation, the soil samples were ajr dried. The rajnfall sinrulatjon (wetting) and a'ir drying (dry'ing) were replicated three times. The trays were arranged in randomized design (Fis.1l). 5.2.3 Crack Pattern One week after the rainfall simulation and drying the crack formations 'in the trays were photographed. The cracks on the photographs were measured us'ing a digìtizer cursor. The observed different crack widths were marked in the phcltographs and the crack lengths for each width were traced to obtain perimeter and area. The system used consìsts of a digitizer tablet (Houston Hìpan, l-louston Instrument, Aust'in, Texas) with an act'ive area of 28 x 28 cm, a v'ideo d'isp'lay termina'l and Andromeda systems ìllB mjnicomputer with twin floppy disc drives. Coordinate information from the tablet was used to compute the area and perimeter of features traced wjth the digit'izer cursor. The data was dispalyed on the screen and stored on disc for later print-out. A total number of 7 classes of crack widths with 0.4 mm as class intervai was selected for crack pattern measurement. The total area of the cracks was also determined. The equation used was w = ZA/L where w is the width of the crack and A is the perìmeter area for ìength L. This instrument was developed for the measurement of crack area and width by Dr. L. R. Jarvis, Flinders Medical Centre. 5.2.4 Pore Size Distribution The undisturbed so'ils after rainfall simulation were impregnated with epoxy resin under vacuum, and 5 cm diameter sections 35-40 um thick were cut with a diamond salv. An image anaìysíng computer (the Quant'imet 720) 66 Fig. 10 Caljbration of rainfall s'imulator for nniform dìstribution. (hL{ highest coefficient of f9-u = uniformity (Cu) and low standard error (Sf); values given are mean of three calibrations; area of each square - 20 x 20 cm3. ) 1 SE: 17 SE:17 SEz27 SE:54 SE:17 Cu:89 Cu:88 Cu : 81 Cu:43 Cu:35 SE : 13 SE : 16 SE: 31 SE:64 SE:46 Cu : 91 Cu:86 Cu: 6l Cu=47 Cu:92 I 13 14t-- 1- I SE 6 SE 5 SE:29 SE:58 SE :41 Cu:97 Cu:98 Cu:87 Cu:69 Cu:54 2 SE:28 SE:25 SE:42 SE:55 SE:49 Cu:81 Cu:86 Cu : 81 Cu:69 Cu:43 I 9 SE:36 SE:30 SE: 41 SE:.72 SE:48 Cu:78 Cu:84 Cu:78 Cu:58 Cu : 18 31 67 Fig. 11 Randomized comp'lete block design for crack formation and germination experiment by rainfall simulation (length and width of p'lastic trays are indicated in the figure). Duration of the experiment: 11.3.1981- 31.3.81. O.1 6VoFe O.O7 %Fe O.O 1%Fe O.OOo/o Fe o/oF o/oF O.O7o/oFe O.O 1 e O.OOo/oFe O.O7 e O.O7 %Fe O.167oFe O.1 6%Fe O. 1 67oFe Ocm O.OO%Fe O.O 1 VoFe O. OOToFe o.o 10,6Fe 6B was used to ana'lyse the total areal porosìty and the areal pore sìze distribut'ion. The displayed ìmage on TV was 4 x obiective. Pores from 40 Unl to 100 um diameter were selected and 5-10 pictures were anaìysed for each samp'l e. 5.2.5 Penetrometer Resistance The crusts formed by ra'infall sìmulat'ion were used to determine the penetrometer resi stance. Tlrey were wetted to saturation by cap'il'lary action and then dried to -10 kPa water potentìa'|. The resistance was measured with a motor driven laboratory penetrometer as described in Chapter 4 (4 .3 .3 ) . 5.2.6 Germination Studies The soils which had been air dried in pìast'ic pots were crushed with a rubber hammer unt'il all part'icles were less than 5 mm and then transferred to plastic trays (4S x 30 x 7 cm) which were arranged in a randomized design in the g'lasshouse. Wheat seeds (Triticum aestivum L. cv. l,larigal) were sown (54 seeds/replicate) on a 5 cm grìd pattern 2 cm below the surface of the soil and then covered. Rainfall simuiation and drying was repeated three t'imes till emergence of coleoptiìes. The co'leoptiles which emerged were counted daily over the entire period of emergence. This yielded results on two aspects of emergence - the time interval between sowing and emergence and the percentage emergence. The mean day of emergence as defìned by Edwards (.l957) was used to determine the time period between sow'ing and emergence. The seed used was tested for germination capacity. 5.2. t' Pl ant Growth Studi es Plant growth studies were conducted in the glasshouse using plastic pots (26 cm deep and 23 cm diameter). The pots contaìning air-dried 69 treated and untreated soi l s were arranged i n a random'ized des'ign (Fi g . 12). Three weeks after germination, four selected seedl'ittgs were trans- planted in soils with respective treatments fronl the trays. The posit'ions of the pots were changed within anci between the treatments throughout the experiment (Fig.l2). Nutrient solutions were added to each pot in the form of ammonium n'itrate, ammonium djhydrogen phosphate and potassium sulphate to suppìy nitrogen (148 mg), phosphorus (j44 mS) and potassium (112 mg). The nutrients were added to each pot at month'ly intervals for four months. Plant heights were recorded each week through- out the growing season. Tiller counts were also recorded for these pì ants . The pìants that were in the pots were harvested with a sickle 2.5 cm from the surface of the soil. The total number of tillers per pot was recorded. The total r"reights of straw and grain from eaclt pot were determined. The heads of the samp'led wheat pìants were separated from the straw and the grain separated from the chaff with a mini-thresher. Grain yie'ld per pot was noted. The grain (20 g) was weighed ínto pre- weighed silica bas'ins and dried at 105'C in an oven overn'ight. The moisture content was calculated as a percentage of oven dry weight. Tillers were dried at 70oC until brown and brittle. The dry straw passed through a I mm sieve in a grinding mill. Nitrogen was determ'ined in the straw by the Kjeldahl method using the alakaline phenate procedure (þ.lilliams and Twine, 1967), and a Techn i con Auto-ana'lyser. Phosphorus in pìant material was determined after dry ashing with 50% magnes'ium nitrate solution (Chapman and Pratt, l96l). One gram of dried ground p'lant material was mixed with 3 cm3 of 50% magnesium nitrate solution and evaporated as near'ly to dryness as possìble on a steam bath. The dried sample was ignited in a muffle furnace unt'il the ash was white. 'The dry ashed material was d'igested with concentrated nitric acid and 70 Fig. L2 Randomized conrpìete block design and changes of the arrangements of the pots with time for the pìant growth. (Numericals given were concentrations of the Fe(III) by weight of soil.) t, tl ! 00 ¿0'0 9t 10'0 ti co 9 t-'0 00 r0'0 9t- : I æ ! I b tl ll I I t ¿ t-'c t0'0 10'c t 0'0 00 I 9t 0'0 I 9t'0 I I I I I I ! I ¿ t0 00'0 ! 10'0 10'0 00'0 9t 9t'c t'0 I b I @ b I co I I 00 t 0'0 I ¿oIt 00'c t_0'0 L0'0 9 t'0 00'0 I I ! ¿oo 00'0 00'0 00'c 00 9 t-'0 0'0 9 t-'0 ¿¡ I o ¿" b I I I L0'c r.0'0 9r.'0 00'0 ¿0'0 ¿0'( I 9 t'0 t0'0 I I I I I I ! : 0'0 t0'0 l-0'0 00'c ; /0'0 10'0 9t 00'0 ;! b 0'0 00'0 9t'0 9t'0 t 0'0 9t t0 0 N t-0 9t'0 N 00'0 t0 t0'0 9t ; 00'0 t 0'0 Þ o" b I I I 9t-'0 0 00'c t0'0 I 9t'0 10'0 00'0 10'0 I I I I N I N I I 9t-'0 oo 0 9t'c 0 o ¿0'0 )0'0 t 0'0 10'0 b t 0'0 t'0 10'0 ¿0'0 t 0'0 9t'0 ¿0'0 00'0 7L perchlo¡ic acids before deternlination of phosphorus, using the method described by Hanson (1950) whjch'invo'lves an anunon'iunr mo'lybdate- ammon'i um vanadate reagent . The length of root'in each samp'le was determined as described'in Chapter 3 (3.8). 5.3 Results and D'iscussion 5.3.1 Crack Formation Rainfall s'imulation and drying caused crack forrnation in the soil s. The impact of s'imulated ra'infall formed crusts in the soil. The untreated and 0.01%Fe treated so'ils showed a pattern of cracks, but 0.07%Fe and 0.16%Fe treated showed none or very few cracks (Fìg.13). Quantitatìvely cracks were reduced by a factor of 3-4 over the control in the 0.07% and 0..¡6%Fe treated so'ils (Table l2). The crack pattern (Fig.la) shor'red the disappearance of wider cracks in 0.07% Fe treated soils. At the same time 0.16% te treated soiìs did not give aìl the classes of width in the crack pattern compared w'ith untreated and 0.0.l%Fe treated soils. This may be one of the reasons for the reduction of the total area of the cracks in 0.07%Fe and 0.16%Fe treated soils. Table I 2 Effect of Poly[Fe(III)-OH]catjons on the Area of Cracks of the Sojl Sampì es af ter Sìmul ated Rai nf al I and Dryi ng ( tota'l area exam'ined l350cm2 ) Iron added Total area of cracks (%) (cm') 0.00 48.4 x 1.2 0 .01 42.5 r 0.9 0.07 13.5 t 0.4 0.16 16.7 t 0.7 72 Fig. 13 crack formation in the poly[Fe(III)-0H]cation treated soils after simulated rainfall and drying. (A) Control (B) 0.01%Fe (c) 0.07%Fe (D) 0.16%Fe 73 A B vt (, rÉ u o l! o, L 1.2 .ú 0.4 .l o D o, c E 25 o o Ct) a! c o,u L q) fL 1-2 Mean width of cracks (mm) Fig. i4 Effect of poìy[Fe(ttt)-OH]cat'ions on the crack pattern after simulated rainfall and drying of the soil' (A) control; (B) 0.01%Fe; (C) 0.07%Fe; (D) 0'16%Fe' 74 The reduction of total crack area was presuntabìy due to the decrease in dispers'ible clay (Tab'le l3) and increase in particles 50- 250 W dianreter in the soi] with 0.07%te added. This resulted ìn 'increased poros'ity (Table l4). The porosity in pores up to i00 pm dìanleter in clods of the cracked soil increased by a factor of 3 over the untreated soil. The pore size distribution showed the formation of larger pores (40-l00um) and reduction of smaller pores (<40 um) by addition of 0.07% Fe and 0.16%Fe. Dispersed cìay is responsible for swelling and shrinking and in the present study, the reduction in d'ispersible clay and creation of 'larger pores reduced the area of cracks by a factor of 3 over the control. In most env'ironments surface seals dry out rapidly after the vrett'ing process ceases and they tend to shrink and crack to an extent depending upon the clay content (Lutz 1952). 5.3 .2 Seedl i ng Emergence Penetrometer res'istance studies showed a decrease in strength in 0.07%Fe and 0.16%Fe treated soils. The strength was reduced sjgnificantìy at -10 kPa water potent'ial (Table l5). Several authors (Gupta and Yadav, 1978; Chaudri and Das,1978) have reported a negative linear relationship between crustìng strength and seedling emergence. The coleopt'iles which emerged were counted daily over the entire period of emergence and the polycation treatment (0.07%Fe) gave a highìy s'ign'ificant increase in emergence of 20% over the control (Table l5). It did not reach 96% presumably due to other conditions of the soil. The mean day of emergence decreased from l9 to l6 'in 0.07%Fe treated soil. The reduction of mean day of emergence is important in establ'ishing seedlings qu'ickìy. The percentage emergence (Vr) was negativeìy correlated with the strength (x) of the sur'í'ace layers as calculat.ed from the data obta'ined using the penetronreter, and mean day of emergence (V2) was correlated with 2 the strength (x) by the equatìons yr= i65.i8-1.16x; r'= 0.82 and 75 l'abl e l3 Effect of Poly[Fe(III)-0H]cations on the Size Distribution of Water Stable Particles (% total so'iì) ìn Soìl after Sìmulated Rajnfall and DrYi ng Si ze of pa rti cl es Iron added (% of soil) 6 (um ) 0.00 0.01 0.07 0.1 2 14.3 4. 5*** 0.2*** 2 .0*** 2-20 8.9 I 3 .5*** 12.7*** I 4.5** 20-50 28. 0 28.6 20 . B*** 27.4 50-2s0 36.5 40.4 52 .0*** 41 .? 2s0-500 4.7 4.8 6.1 6.2 500- 1000 4.1 4.2 4.2 5.5 1000-2000 3.5 4.0 4.0 3.2 *** P < 0.001 ** P < 0.01 Table l4 Influence of Poìyl-Fe(III)-0Hlcat'ions on the Poros'ity and Pore Size Distribution of Clods of the Cracked Soi'l Samples (total area examined 1350 cmz) Iron added Total area of pores Percentage of total area of Pores (% of soil) (cm2 ) <40 ym* 40-60 pm 60-100 pm 0.00 5.0 r 0.2 t00 0.0.| 5.0 r 0.3 100 0.07 l6.l t 0.3 23 B 21.2 55.0 0.16 t5.7 r 0.4 24 5 22.0 53 .4 *Percentage of pores <40 Um was obtained by subtracting the_ percentage of 40-100 pm pores from 100. The lower l'imit of detection of pores is probably about 30 pm which is the th'ickness of the thi n sect'ions. 76 Tabl e '15 Seedl ing tmergence of Wheat (var. ll.larigal ) arrd St.rength of Po'lyIFe(III )- OHlcation Treated Soils I ron added Penetrometer *Percent Mean day of (% of soil ) res'i stance ( kPa ) enìergence emergence 0.00 94.5 t 2.8 57 19.3 0.01 90.4 t 3.1 60 19.3 0.07 78.1 ¡ 3 .2 80 l6 .4 0.16 79 .5 x 2.9 68 l7.r L. S. D. (P < o.o5) 10.9 8.7 1.5 (P <0.0r ) 1s.6 12.4 2.1 *Germination capacity of l^larigal seed was 96% !Z= 0.17x+3.64; 12= 0.96 which are based on the data in Table 15. These correlations show the ìmportance of the low crust strength for germ'ination in this hard setting Red-brown earth. 5.3.3 Plant Growth The seedlings were transp'lanted'in pots 3 weeks after the seeds were sown. The plants were harvested 6 months later. In soils wìth added polycat'ions (0.07%Fe and 0 .16%Fe) the total yield was increased significantìy (Table 16). The number of tillers and grain yields also showed sign'ificant increases fronr plants growrí in polycation treated soils. The numbers of tillers and therefore ears were less in untreated and 0.01%Fe treated sojls than 0.07%Fe and 0.16%Fe treated soils (Table 16). 77 Table l6 Influence of Poly[Fe(III)-OH]cat'ions in the Soil on the Y'ield of Wheat Iron added No.of tìllers Total dry matter Grain yiel d (% of soil) per seedì 'ing ( s/ pot) g/pot % total mat. 0.00 4.6 7ì .1 21 .3 30.1 0. 01 4.2 69.6 18.6 26.4 0.07 6.4 85.0 28.5 33 .9 0.16 6.6 85.7 32.1 37.4 .l.6 LSD P < 0.05 '13.6 5.7 The water, nitrogen and phosphorus contents of the grain were increased signifìcant'ly in the plants grown in soils to whjch 0.07% and 0 .16%Fe was added (Tabl e I 7) . Table ì7 Moisture, N'itrogen and Phosphorus Content in the Plant Tjssue after Harvesti ng Iron added Moisture content N i trogen Phosphorus (% of so'i'l) of grain (?i) ( % by weight) 0.00 11 .2 0.52 0.026 0.01 11 .2 0. 50 0.026 0.07 ll.6 0.69 0.04.| 0.ì6 il.7 0.62 0.040 LSD P < 0.05 0.2 0.027 0.0122 Up to the l2th week of p'lant growth those plants grown in the soils treated with 0.07%Fe and 0.16%Fe were appreciably taller than plants in the controj soil (Fig.l5). From l2 weeks to harvesting plant height remained relative'ly constant. Root lengths obtained at harvest'ing did tssggSEÊ 50 40 E rJ o 16 o- 20 l0 20 Time after .transplanting (weeks) Fig. 15 Inf'luence of poly[Fe(III)-OH]cationson the plant height of wheat in the soi'1. (o )0.01%Fe;( tr )0.07%Fe;( 0 )0.16%Fe. { )contr"ol;( ^ æ 79 not show sìgn'ificant effects (fa¡le lB) due to treatnrents of the soil in spite of the fact that the soìls treated with polycations appeared to offer an improved physical environmen'l for root growth. Table l8 Root Length of Wheat Plants after 6l'lonths of Grówth in Poly[Fe(IIi)-0H] cation Treated Soils Iron added Root I ength (% of soil ) (cm g-t¡ 0.0 9.2 ! 0.2 0.01 8.9 t 0.2 0.07 9.5 t 0.2 0.16 9.7 x 0.2 LSD P < 0.05 0.6 It may be that the root systems in the poìycation treated soils were better developed up to 12 weeks and that this was the main reason for the increases in height and yie'ld and nutrient uptake of the wheat p'lants. It should be noted that the pots were watered regu'lar'ly throughout the growing season and the soils would not have become strong due to dryìng. However porosity (x) was ìinearly correlated with root length (y) by the equat'ion y = 0.05x + 8.80i Y2 = 0.Bl , wh'ich is based on the data in Table 14 (tota'l areas of pores) and Table 18. A better correlation might have been obtained between porosity and root length after l2 weeks of pìant growth. Porosity has been highìy pos'itive'ly correlated with root I ength i n ri ce pì ants (Kar et aJ. . ,'l 979) . 80 5.4 Conclusion Crack formation, seedl'ing emergence, and pìant growth were correlated with the physìcaì propertìes wh'ich were changed by the addition of Fe(III) polycations. The decreased areas of cracks and strength of crusts, 'improved seedl'ing emergence and plant yieid resulted from the reduction of clay content and increased porosity. Root growth is mostly respons'ible for good pìant growth. In the present study root length measurements at harvest were not sìgnificantly correlated with yield. Therefore the measurement of root lengths at different stages of plant growth are requ'ired to define the influence of structure on root growth and plant yie1d. BI CHAPTER 6 INFLUENCE OF ANIONS ON DISPERSION AND PI1YSICAL PROPERTIËS OF THE A HORIZON OF A RED-BROWN EARTH 6.1 Introduction The dispersion of cìay in soils is important in pedogenesis (il'luvìation) and in agriculture as dispersed c'lay causes undes'irable physical properties. The dispersion of soil colloids is control'led by the nature and distribution of the exchangeable cations which counter the permanent and/or pll dependent negative charges on colloid surfaces. These counter ions, held by electrostatic or coulomb'ic forces are located in either the Stern layer or the Gouy-Chapman diffuse part of the electric double ìayer. A s'implified equation (Keng and Uehara, 1974) shows a relationship between double'layer thickness (lirc) and surface charge density (oo) in the form of, 4noo K tYo where e is the dielectric constant of the medium and rpo is the surface potentìaì. D'ispersions of c'lay are stable when most of the counter ìons exist in the diffuse double layer and the th'ickness of th'is ìayer is of the same order of magnitude as the d'iameter of the individual collojdal partic'le. Repul sive forces created by over'lapping doub'le ìayers which are increased by increasing the surface charge density prevent the close approach and coalescence of individual particles. Flocculation occurs when the th'ickness of the double ìayer is reduced to such an extent that short range attractive forces become dominant. Compression of the double layer is caused by any one or all of the followirrg: (i) increasìng the electrolytg concentratìon; (ii)'increasing the charge on the counten ions so they exist largely in the Stern ìayer, or (i'ii) by raising the pH at which there is point of zero charge (PZC). 82 The,pH at which PZC exists can be raised bJ, the adsorpt'ion of poly- cations of Al and Fe (Rengasamy and 0ades , i97h,l97B) and Shanmuganathan and Oades (l9B2a) showed that flocculat'ion-dispersìon could be controì'led by the sorption of Fe polycations which could be used to raise the PZC to pH values above the pH of the soi'ì. The phenomena described apply to collo'ids with a significant negative charge density. Increasing the repulsive forces will jncrease the double layer thickness and therefore stabil'ize colloid dispers'ions. The surface charge density can be increased by the specific adsorption of an'ions" This can also be regarded as a decrease in PZC which favours stabjl'ization of dispersions. Sh'ifts of PZC caused by specif ic anion adsorpt'ion have been conf irmed experimental ly (H'ingston et at-.,1972; Huang and Stumm, 1973i Gillman,1974; Wann and Uehara, l97B) and have been s'imulated in double layer model calculations by Pyman et at.(.l979). Such adsorptìon has ìncreased catìon exchange capacities (El Swaify and Sayegh, 1975; MoraiS et aJ .,1976; Gaìindo and B'ingham, 1977). Natural organ'ic anions from the decompositìon of organic matter and root exudates and art'ificia'lìy appljed inorganic anions in the form of fertiìizers may change the propert'ies of soils. Little'information'is available, however, on the influence of anjons on dispersion of soil colloids. In this chapter soils with a net charge of zero and soi1s with a net positive charge (manipulated by polyIFe(III)-0H]cations) were used to study the surface and physica'l properties created by the addition of a range of anions. 6.2 Experimental 6.2.1 Treatments of Soil with Poìycations The Red-brown earth, the Urrbrae fine sandy loam (0-10 cnr) was treated with 0.07%te and 0.32%Fe as polycations by weight of soil as described in Chapter 4 (4.2.I; 4.3.1; 4.3.2). These amounts were shown 83 previously to g'ive flocculation of all c'lay at the PZC and a net pos'itive charge, j.e . PZC higher than the so'il pH (Shanrnuganathan and Qades, ì982a). The treated soìì sarnp'les were air-dried in plastic pots for 7 weeks. Identificatjon of polycat'ion treated soils is shown in Tabìe 19. Table l9 Identificatjon of PolyIFe(III)-0H]cation Treated Soils Iron added I derrti f i cati on (% of soil) 0.07 soìl x 0.32 soiì y 6.2.2. Preparatjon of Fulvic Acid The Bh horizon of a sandy podzoì (coffee rock) was used to extract fulvic acid using 0.1I,l HCL at 60oC. Follolving extraction the fulvic acid was passed over a column of cation exchange resin (H+ form). The cation exchange capacìty was determined by potentiometric titration (Schnitzer and Desjardins, 1962) and was 442 C g-l (C g-1= 1.04 meq 100 g-1t C = 6.242 x 1018 electronic charges). 6.2.3 Addition of Anions ,to Poìycation Treated Soiì Suspensions ' Phosphate, silicate and nine organic anions as sodium salts (pH = 7) were added to the soì I samp'l es treated wi th Fe-polycati ons . D'ifferent amounts (0.5 cms of 1 mM-12 mM) of the anions listed in Table 20 were added to 10 cm3 of 3% suspension of poìycation-treated soil which was adjusted to 20 cm3 b,v add'ition of dist'illed water in a 50 cms measuring cylinder. The suspensìons were shaken thoroughly by hand for 60 s. The amount of clay d'ispersed by this treatment was determined by measuring the optical dens'ity at 615 nm in I cm cells 24 h after addition of anions. 84 The optìcal density at 615 nm was calibrated aga'inst the percentage cl ay i sol ated f rom the soi I and determ j ned gravimetri caì 1y. 6.2.4 Adsorption of Anions by Soil y at pH 7 The suspensions of soils treated with anions were adiusted to pH 7 and centrìfuged after 24 h. Suitable volumes of the supernatant solutjon were taken for the determìnation of anions. Adsorption was calculated from the decrease in concentration of anion in solution. Methods used for the determinat'ion of the anions are listed below. Anion Method Absorbance Reference nm Phosphate spectro- 820 Murphy and Riley (1962) as photometri c modified by Watanabe and 0lsen (1965) Ful vate il 508 Si I i cate lt 820 hJ'illiams (1979) Ci trate , oxaì ate , 492 Lee et ar.(ì968) & tartrate Aspartate tl 570 Lee and Takahashi (1966) Sal i cyl ate 508 Catechol 400 Beg et at.(1977) Lactate & ti trimetri c Nema and Verma (1979) acetate 6.2.5 Additions of Anions to Soil s Treated with Fe Po'lycatjons The soils x and y (15 g) were treated with 25 cm3 of six different concentrat'ions of anions (0.00; I; 2; 4; I and 12 mM) for electro- phoretic mr¡bi'lity stud'ies. 0n1y three anions (2.5 I of phosphate, fulvate and citrate) were added to the soils (1.5 kg) on which physical and mechanical properties v{ere subsequently assessed. The concentratjons of anions used were 0.00; 2;4; B and 12 mM. Each treatment was repf icated thrice. The treated soil samp'les were air-dried in plast'ic pots for B weeks. B5 6.2.6 Physico-chemical Properties Eì ectrophoret'ic mob'i I i ti es of so'i ì suspens i ons and el ectri c charges on the soil were measured as described'in section 3.3.3. Soil suspensìons (0.25%) were exam'ined in a JEQL JEM 100 CX electron mi croscope . 6.2.1 Phys'ical and Mechanical Propert'ies Dry bulk density and porosity were determir¡ed as described in section 4.3.3. Dry buìk density, porosity, modu'lus of rupture and friab'i'lìty were ¡neasured at the p'lastic limits of the respective treatments as described earl ier. 6.3 Resul ts and Di scuss'ion 6.3.1 Dispersed Clay The different anions caused different amounts of clay to dìsperse when added to samples prev'iousìy flocculated by addjtion of 0.07%Fe (Fig.l6). Three groups of anions were distinguished: firstìy phosphate and ful Vate, second'ly citrate, oxa'late, tartrate and sil icate and f i nal'ly sal i cy'late, catechol , aspartate, I actate and acetate. The first group of anìons increased the dispersible clay from zero to 9% by weight of soil. This was half of the dispersible clay found in the untreated soi I (without add'ition of polycat'ion) . Wickham (.l978) showed that addition of tripolyphosphate to soil caused immediate d'ispersion and high erodibiìity. The second group of anions resulted in an amount of dispersible clay from 5% to 7% by weight of soil. In the thìrd group' only sa'licyìate and catechol caused some dispersion, whereas lactate and acetate did not cause any cìay dispers'ion. Higher concentratìons of phosphate and fulvate, 6 mM or more, did not cause further clay to disperse but 8 mM concentrations of the thìrd group of anions did result in more djspersed cìay. The pH values for the soil samp'les treated with 0.07%Fe (control) and the anion-treated soiì samples show 86 t0 t 7 ^ 'õ 3n 6 I o 'õÞ) E -o 4 rl o ^ g --E P 3 ---'-- t¡, L 6) o- ____-----a o.9 o\ -----E 1 -tf' --Jt----o----- 4 12 Solution anion concentrat¡on (mM ú-r) added to 0.07 Z Fe treated soil. Fig.16 Changes in dispersible clay of 0.07%Fe treated soil after addition of anions. (ê-4 phosphate; (r-r) fulvate: (o-O citrate; (^-^) oxal ate; (o-o) si I jcate; (o-o) tartrate; (o--o) ,sal icylate; (o--o) catechol; (^--^) aspartate; (^--^) I actate ; (o--o) acetate. 87 that an increase in pH would explain in par^t the increased dispersion of cìay (Table 20). For the fìrst group of anions the pH'increased from 4.2 to 6.0 and for the second group 4.2 to 5.0-5.9 and for the third group a smaller increase again. The increase in pH increased negative charges on the c'lay particles and favoured dispersion. Tabl e 20 pH Vaìues for So'il Samp'les Treated with 0.07%Fe after Addition of Anions Conc.of anion added to soil before anion adsorption Anion (mM l -1¡ 01248 12 Phosphate 4.20 4 .40 4.66 5.00 5.40 6.10 Ful vate 4.20 4.?4 4.5? 4.92 5 .40 6 .00 Sil icate 4.20 4.26 4.46 4.52 5.00 5.10 Ci trate 4.20 4.22 4.33 4.78 5.40 5.96 Oxal ate 4.20 4.25 4.sl 4.78 4.98 5.09 Tartrate 4.20 4.24 4.45 4.70 5.00 5.12 Saì i cyl ate 4.20 4.21 4.29 4.32 4.62 4 .88 Catechoi 4.20 4.20 4.?3 4.92 5.10 5.ì6 Aspartate 4.20 4.22 4.44 4.62 4.82 4.91 Lactate 4.20 4.20 4.4s 4.60 4.82 4.94 Acetate 4.20 4.22 4.37 4.38 4.54 4. 58 In soil sample y the net pos'itive charge was reduced by the additjon of anjons at different concentrations. Again, the first group of anions (phosphate and fuìvate) 4 to 6 mM neutralized the excess posìtive charges (Fig.l7). This resulted in flocculation of the d'ispersed clay particles with net positive charge. At higher concentratjons (>6 mM) of the anions the critical coagu'lation concentration (CCC) was exceeded and the system began to redisperse (Fig.l8) presumably as a system wìth a 88 r3 \À -o--- + T !, {1 I (,E E ____{¡ J >l ll= o E .9 I o oL CL I o -1 ^ (, o A ¡¡¡ 6 Solution anion concentration (mMû-rl added to 032 7" Fe treated soil. Fig. 17 Changes in electrophoretic mobility of O.3Z%Fe treated soÍl after additíon of anions. (o-o) phosphate; (r-r) fuì vate; (o-¡¡ citrate; (^-r) oxalate; (o-e¡ sil ícate; (o_p) tartrate; (o--o) salicylate; (o--o) catechol; ê__4 aspartate; (¿-- ¿) I actate; (o--o) acetate. 89 *----'¡-----o------:o------€ I ------4------^ 'õ (ô 6 o ..c .9 5 (¡, ------¡ ì I I 4 ftt ^ E g .€ g o o. .9 o T \" A ^ I t ^ 2 4 I 10 Solulion anion concentrat¡on (mM û-r) added to 0.t22 Fc treated soil. F'ig. 18 changes in dispersibre clay of 0.32%Fe treated soil after addition of anions. (^-^) phosphate; (r_r) fulvate; (o-o) ci.trate; (^-r) oxaìate; þ-d s.ilicate; (o_ o) tartrate; (o--o) saì icyl ate; (o--o) catechol ; (r--r) aspartate; þ--") I actate; (o-- o) acetate. 90 net negative charge. The th jrd group of an'ions clid not cause flocculation at any concentration. The flocculation took p'lace at the pH vaìues (PZC) rang'ing betwee n 4.2 and 4.4 for the first tvro groups of anions (Tab1 e 21.) . Table 21 pH Va'fues for So'il Sampìes Treated w'ith 0.3?%Fe after Addition of Anions Conc. of anion aclded to soil before anion adsorpt'ion An'i on (ml.4 t-i1 0124812 Phosphate 3 .80 3 .88 3 .95 4.20 4.62 5.36 Fu I vate 3 ;80 3 .84 3.91 4.26 4.62 5..l0 Si I i cate 3. B0 3 .87 3.90 3 .93 4.00 4.62 Ci trate 3 .80 3. 84 3..91 3.94 4.20 4.8? Oxal ate 3 .80 3 .81 3 .85 3 .96 4..l0 4.42 Tartrate 3 .80 3.88 3.92 3"95 4.00 4.66 Saì i cyl ate 3 .80 3 .84 3 .87 3 .93 4.r0 4.?2 Catechol 3 .80 3 .84 3 .87 3.92 3.98 4.26 Aspartate 3.80 3.82 3 .86 3.90 3 .94 4.00 Lactate 3 .80 3.Bl 3.86 3.92 3.94 3 .98 Acetate 3 .80 3. 84 3.90 3.94 3.98 4.00 The electrophoretic behaviour of the treated soil sampìes vrith increasing concentrations of anions confirnrs the flocculation-dispersion phenomena (Fig.l9). The values for negatìve mobility clearìy distinguished the three groups of anions with phosphate and fulvate creati ng the greatest negati ve el ectrophoreti c mobi I i ti es . For the soi l sampìe y a net zero charge which resulted in flocculation was produced by concentrations of phosphate and fulvate between 4 and 6 mM (Fig. 17). The seccnd group of anions gave zero net charge with concentrations close to 8 mM. The third group of anions did not create any particles wjth 91 r1 I ,t, 0 I € ---+- å .(,E -__-_-4, E I f ^ -----tr_ ----¡ I Ë ll o E ^ I .9 o o o 0 L gu ^ I t¡¡ I  -2 ^ 2 4 10 Solution anion concentration (mM ú-l) added to ' 0.077" Fe treated soil. Fig. 19 Changes i n el ectrophoì^eti c mob'i 1 i ty of 0 .07%Fe treated soi I after addition of anjons. (a-o¡ phosphate; (r-r) fuìvate; (o-d citrate; (^-4 oxalatet (o-o) sil Ícate; (o-o) tartrate ; (D--tr) sal i cyl ate ; þ--o) catechol ; ê--^) aspartate ; (o- -o) I actate ; (o--o) acetate. 92 negatjve mobilities showinE that these anions were ineffective in balancing all the posìt'ive charges on particles. The effectir¡eness of the various anions with respect to floccttlation- dispersion phenomena 'is related to the anrount of anion adsorbed. Plots of anion adsorbed against concentration (Fig.20) show that considerably more phosphate and fulvate were adsorbed than any of the other anions at each concentration used. Tabl e 22 Chemical Formula and Stabil'ity Constants of Fe-anion Complexes 'in Solution Anion Chemical Formula Equil ibrium ToC t.og K 3- Phosphate PO Fe3++ Po43=-* FePoo(s) 18-20 21 .9 4 - Ful vate Fe3+* rul vnF^ FeFul u(3-n) **6. I 2- Si I icate si0 Fe3++SiOr2+ FeS'i0r+ *9.6 3 3++ c00- Fe Hrc'it2ç= FeHrc'i t+ 25 6.3 Citrate HO-CHZ-CHZ -c-c00- 3** c00- Fe HCit3-==àFeHCit 25 ll.9 C 00 3+ Oxal ate rl Fe + 0x2ç= Fe0x+ 9.4 C 00 HO-CH-COO Tartrate I Fe3++ Tartzç- FeTart+ Zs 7.5 H0-cH-c00 0d NHz-cH2-cH(3 F.3++ FeAspart+ ll.4 Aspartate 00- Rspartþ 20 3+ 2 + Saf icyìate Fe + Sal FeSal lB 16.5 - ?H 3+ 2+ Lactate cH3-cH-c00 Fe + Lact-Felact 20 6.4 Acetate CH 3c00- Fe3++ Acet-ç+ FeAcetz+ 20 3.? From sirren and Martell' 1e64:.i 3:[l]l:.ltllå)*n.,' (1s72) 93 7 A A I I cD = E A E o ll oL Ev, tú +-- c -9c 2 ---û 0 0 3 6 Solution anion concentralion (mM t-r) Fig. ?0 Ani.on adsorption on 0.32%Fe treated soil at pH 7. ("-4 phosphate; (r- r) ful vate; (o-o) ci trate ; (^-^) oxal ate; (o-o) silicate; (o-o) tartrate; (o--o) saìicyìate; þ__o) catechol ; (^- - r) aspartate; (o- - o) I actate; (o-_o) acetate. 94 Table 23 Stab'il'ity, Number and Size of the Rings of the Organic Anion-Fe Conrplexes Organ'ic anion No. of rings Ring size* Stabi I ity Ful vate Mul ti -ri ngs Very stab'le C i trate 2 Two possibil ities 'i) 6 and 5 Stabl e ii)6 and 7 Probably stable Oxal ate l 5 Stabl e Tartrate 2 Both rings 5 Stabl e Aspartate l 5 Very few complexes Saì i cyì ate 2 6and7 Stabl e Catechol 2 6and5 Stabl e Lactate Acetate *This denotes the number and size of rings per complex anion. The ring size is denoted by the number of atoms which form the ring, e.g. 5 indicates a 5-membered ring The adsorption of the anions is controì1ed by the energy of binding with Fe(III) associated with the surfaces of the soil colloids. To some extent sorption is control'led by the stability constant of the anion for Fe(III) (faUle 22) but other factors are also involved such as the stab'ility, number and size of the rings of organic anion-Fe complexes (Table 23). Hingston et aJ.(.l968) obtained absorptìon spectra for phosphate adsorbed on hydrous ferric oxide ge'l and interpreted them as 2- ?- showing that HZP}4-,- HP04' and P0O" probabìy all form monodentate complexes with the Fe by direct coordinat'ion with oxygen. Silicate Ís also adsorbed on Fe surfaces by l igand exchange (l-l'ingston et at-.,1972), and infrared spectra show that A-type 0H groups are replaced during the reaction (Parf.itt and Russelì, 1977).' Silicic acid behaves as a weak monobasic ac'id (pK = 9.6); thus only one 0H can be replaced by sil'icate. 95 The surface complex nlay be jn the fornr of Fe.0Si (0H), or FeSi0r+. Rajan and Watk'inson (1976) suggested that phosphate can be sorbed by dìsp1acÍng silicate in the soils which indicates stronger adsorption. Citrate and tartrate have hydroxyl groups ìn add'ition to carboxy'l groups. Both groups are considered to take part'in the chelation of Fe. For the citrate ion it has been established that it can lose its hydroxyl hydrogen as well as the carboxyl hydrogens and can coordjnate with Fe3+. This suggests the possibì'lìty of formation of both six and seven menlbered rings with citrate complexes. Low levels of oxalate are strong'ly adsorbed on Fe surfaces in the binuclear form. At higher surface coverage oxalate'is more weak'ly adsorbed and ìs in the monodentate form (Parfitt er a-2.1977). The stabjlity of the oxalate complexes is largely due to the formation of the five membered rìngs. The fulvate anion has a high cation exchange capac'ity due to carboxyl groups. Although Fe- fulvate has a low stability constant (6..l), fuìvic acid is strongìy adsorbed on Fe surfaces by ìigand exchange (Parfitt,l978). The carboxyl groups repìace several A type 0H groups, giving mu'ltiple po'ints of contact. Additional mechanisms of adsorption include hydrogen bonds and entropy effects. This is why fulvate is strong'ly adsorbed and influences the surface properties and clay dispersion more effectively than other organic anions. Acetate can bind two metal atoms together, each oxygen of the carboxyì group ljnking to a d'ifferent metal atom. In most cases, the carboxyl group is attached to only one Fe atom. Thus the tendency for acetate to form chelate rings 'is sìight. 6.3 .2 El ectrophoreti c Mobi I i ty and Po'int of Zero ,Charge The adsorption of compìex'ing anions lowered the PZC as stated in the review by Parks (196i) and the electrophoretic mobilìty of soil sample x after add'ition of anions is shown as a function of pH in Fig. 21. The PZC of the soj'l sample x vras between pH va'lues 4.0 and 5.0. After 96 +1.5 PHOSPHATE TULVATE 0 ^ ^ I o o 1 -3.0 ¡th I +f E o SILICATE o CITRATE E ^ = o o Ë'õ o E -3.0 .9 o oL + SALICYLATE ACETATE o. o L ^ gC, 0 l¡J 1 -3.0 4 1024 10 pH pH Fig. 21 Electrophoretic mobilities of 0.07%Fe treated soíls as a function of pH before and after addition of anìons- ( o ) control - 0.07%Fe treated; ( a ) 1mM; ( D ) zmM; ( o ) 4mM; (r )BnrM;(¡ )12mM. 97 +3 o  PHOSPHATE FULVATE o o L I r 0 o À ^ I Ø I -3 +3 (,E E ¿ SILICATE CITRATE :) 0 .= 5 o o E .9 o o -3 CL +3 o L o g SATICYLATE ACETATE t¡¡ 0 -1.5 tR 10 pH pH Fig. 22 E'lectrophoretic mobjli-ties of 0.32%Fe treated soils as a function of pH before and after addition of an'ions. ( O ) controì 0.07%Fe treated; ( 1 mM; ( D 2 mM; ( a mM; ( a ^ ) ) o ) ) 8mM;(r)12mM. 9B phosphate and fulvate treatments, no PZC was obtained. The second group of anions resulted in a PZC between pH 3.0 and 4.0 at a concentration of 1mM. The third grouþ of anions gave a PZC betwen pH 3.0 and 4.0 w'ith most an'ion concentrations. For the soil sanrpìe y the PZC was between 6 and 7 which was lowered to between 3 and 4 by addìtion of phosphat.e and fulvate (Fig. 22). Wann and Uehara (1978) also reported that sorbed phosphorus in Oxisols lowered the PZC to between pH values 3.0 and 4.0 and ìncreased the charge density at any pll above the PZC. The second group of anions lowered the PZC to pH values between 4.0 and 5.0 at all concentrations as did salicylate and catechol. 6.3.3 Electnic Charges Determined by Ion Exchange The negatìve charges on soil sample x increased by 60% after addit'ion of phosphate and fulvate (fa¡le 24). Citrate also resulted in increased negative charges. These three an'ions were most effect'ive ìn dìspers'ing the flocculated cìay. Parfitt and Atkinson (1976) indicated that this was because the anions blocked posìt'ive'ly charged s'ites as well as adding additional negat'ive charges at higher pH values. Positive charges were reduced by 70% after addition of phosphate, fuìvate and citrate in the soi'l sampìe y (Table 25). 6.3.4 Electron Mìcroscopic Studies Electron m'icroscopìc studies showed clearly that the c'lays were flocculated at 0.07%Fe concentration (A of Figures 23, 24, 25). Phosphate, fulvate and citrate (8, C and D of Figures 23, 24,25 respectively) created some finer part'icles in the soil system. The positiveìy charged so'ils (0.32%te) contained dìspersible clay (A of Fjgures 26, 27,28) which was flocculate.d (B and C of Figures 26, 27,28) and redispersed (0 ot Figures 26, 27,28) by add'ition of phosphate, fulvate and citrate respecti ve'ly . 99 l-abl e 24 Changes ìn Electrìc Charge (C g-1) of Soiì Sampìes Treated with 0.07%Fe af ter Addì t'ion of An i ons Conc.of anion added to soi I before an'io Phosphate Ful vate Ci trate adsorption (mlî l-1¡ PC NC PC NC PC NC 0 0.12 3.70 0.12 3 .70 0.12 3.70 I 0.06 4.82 0.08 4.64 0.09 4.42 2 s.24 0.07 5.22 0.07 4 .84 .l6 4 s.6l 5. 56 5. B 5. Bl 5.70 5.34 12 5.94 5.82 5.40 Iabì e 25 Changes'in Electrjc Charge (C g-1) of Soil Samp'les Treated wjth 0.32%Fe after Addition of Anions Conc.of anion added to soi I before an'ion Phosphate Ful vate C i trate adsorption (mM t-1¡ PC NC PC NC PC NC 0 0.33 4.06 0.33 4.06 0 .33 4.06 I 0.26 4.08 0.26 4.02 0.29 4 .00 2 0.22 4.00 0.20 4.00 0.24 4.00 4 0.1B 4.00 0.1 7 4.00 0.20 4.00 8 0.ì4 4.26 0. l4 4.20 0.17 4.1B 12 0.12 4.47 0.12 4.44 0.15 4.20 pç = posìtive charges NC = negative charges 100 Fig. ?3 Transmission electron micrograph of clay fract'ion of 0.07%Fe treated soi'ls before and after addition of phosphate. Horizontal bar = 1 ym. (A) Control (0.07%Fe) (B) 4 mM l-1 (c) I mM l-1 (D) l2 mM ì-1 A l-{ c D 'þ I , 7o I ì ., t I t o a a WAITE INSTITUTE LIBRARY 10 Fig.24 Transmission electron micrograph of clay fraction of 0.07%Fe treated soils before and after addition of fulvate. Horizontal bar = 1 um. (A) Control (0.07%Fe) (B) 4 mM l-1 (C) B mtt l-1 (D) 12 mM l-1 A B .r¿ rD , f1- ? o ì t D' I,l t a. D C a I C e¡l o I t .,, I ¡ i-{b$' - -q 3 t .l û.i t 4.. ^ I r02 Fig. 25 TransmissÍon electron micrograph of clay fraction of 0.07%Fe treated soils before and after addition of citrate. Horizontal bar = 1 um. (A) Control (0.07%Fe) (B) 4 mM l-1 (c) I mN l-1 (D) l2 mtl l-1 r J o þ' , L J a { q, ,f B I ì.t 103 Fig. ?6 Transmjssion electron micrograph of clay fraction of 0.32%Fe treated soils before and after addition of phosphate. [{orizonta] bar = I um. (A) Controt (0.32%Fe) (B) 4 mM l-1 (c) B ml{ l-1 (D) 12 mM l-1 - o J at o \ ¡ o ¡ "t , f € ¡ a r a çri C ; ì {. o a e I a I t lo a a 'ô. a I o a '¿: rt- lle .-- 104 Fig. 27 Transmission electron micrograph of clay fraction of 0.32%Fe treated so'ils before and after addit'ion of fulvate. Horizontal bar = 1 um. (A) Control (0.32%Fe) (B) 4 mM l-1 (C) 8 mM l-1 (D) l2 mM l-1 h D ta a ßù ia I a a î QoI a. a ¡. a I a C) 105 Fig. 28 Transmission electron micrograph of clay fract.ion of 0.32%Fe treated soils before and after addition of citrate. l-lorizontal bar = 1 um (A) Control (0.32%Fe) (B) 4 mM t-1 (c) B mM l-1 (D) t2 mM ì-1 \ 'l a B A a t .fl a t{ t Ûr! o a' a a o ,* a It ) a ,¡ t ¡a s ! lo r. )c f-l a J t o c D t a ô.þ I ! l-f 106 6.3.5 Aggregate Stabil'ity and Porosities The addition of anions to so:il sampìe x influenced aggregate stabìl'ity and porosity. Phosphate, fu'lvate and citrate significantly reduced the jncreased number of part'icles 50-250 um diameter and significant'ly the (Figures and 3l <2 Um fraction confìrming the dispersion of clay 29,30 respectively). No change in water stable aggregates was detected in soil sample y after additjon of anion. In a previous study with Fe(III) polycations (Shanmuganathan and 0ades, 1982a), there was no s'ignìficant difference'in the physica'l properties of soils x and y, but dispersibìe clay was recluced fron 9% to O%. Even though the aggregate stabiljties of these treated soìl samp'les were not nlarkedly changed, the presence of 'in dispers'ible clay ìowered the porosities and increased bul k densities soÍl sampìe x (Table 26). Sojl samp'le y had lower bulk densities and higher poros'ities, whjch were not changed by the addition of an-ions presumab'ly because there was sufficient Fe3+ pr"sent to cement particìes together. 6.3.6 Maximum Water Holding Capacity and Hydraulic Conductivìty The anions decreased significantly the water hold'ing capacity and hydrauììc conductivity'in soil sample x (Table 27). Water reta'ined in the air-dried sojl was about L2% over the control at 4mM l-1 or higher concentrations of anions. This was promising w'ith the water retajned in the air-dried soil 'increased decreased wjth concentration of an'ions acided. The <2 um fraction, decreased 50-250 pm particle size and decreased the porosity and water holding capacity. At the same time hydraul'ic conduct'ivity was reduced by a factor of 2 over the control by the addition of phosphate and fulvate. This was presumably due to a decrease'in the number of transmissive pores. These propertìes were not changed in soil sample y. L07 Fig. 29 Effect of phosphate on the size distributÍon of water stab'l e parti cl es i n soi I sampl es treated wi th 0 .07%Fe . tfll o mr,r t-1; f Hl 2 mM t-1; (Ø) 4 mM t-1; tffil B mM l-1; t El l2 mM l-1. (concentrations of phosphate added to soil before phosphate adsorption.) Verticalbar,P<0.05. Water stable particles(%total soil) l\'trà o o ooo ooo oo L (,t N o oo oooooooooo oooo oooooooooooooooo o ooo o o t, ¡ a 9ru Or o atl H €'o oooo 0 o oo ooooo o 3 l\)^ 108 Fig. 30 Effect of fulvate on the size distribution of water stable panticles in soil samples treatedwith 0.07%Fe. tEl o mM l-1; tEl 2 mM t-1; tØl 4 mM ì-1; (må) B mM l-1; tÇll t2 mM l-1 (concentrations fulvate added to soil before fulvate adsorpt.ion.) Verticalbar,P<0.05. Water stable particles(%total soil) l\) g) å (,| o o o o U' N t-_{ o o o oo o ooo o o o o ooo o oo oo o o o o o o o o o oo oo o oooooooo o o t o gt o o oo ooooo o ooooooooooo o oooooooooo -3 t--{ 109 Fig. 31 Effect of citrate on the size distribution of water stable partic'les in soil sampìes treated w.ith 0.07%Fe. tll 0 mM ]-1; t$l 2 mM t-1; (Ø 4 mM ì-1;( ffil r mM-I; (Çil l2 mM l-1. (Concentratìons of citrate added to soil before citrate adsorption.) Vertical bar, p < 0.05. Water stable particles(%total soil) uî ¡u (r) Þo o Ø N I }_-| o ]\) oo ooo ooooo oooooooo oo o oooo ('| oooo o o tt ¡ gt o I o (rl o o ooooo oooo o ooooo o oo ooo o 3 ^ o 1i0 Tabl e 26 Changes jn the Bulk Density and Porosity at the Plastic l-imit of Soil Samples Treated with 0.07%Fe after Additiorr of Anions An i ons Conc.of anion added Buìk density Porosity to soil before ani on -? adsor ti on mM 1-1 cm" V V Control 0 1 .47 41 .2 Phosphate 2 1.52 39.5 4 I .55* 38.3** B l.6l*** 35.6*** 12 1.63*** 34 . B*** Ful vate 2 I .50 40.l 4 I .55* 38 .3** I 'l .62*** 35.2** .|.62*** 12 35.2*** C i trate 2 1.52 39.5 4 I .55* 39.3** 8 l.6l*** 35.6*** 12 1.63*** 34 . B*** *P < 0.05 **P < 0.01 ***P < 0.001 s 111 Tabl e 27 Effect of Anions orl the Water/Îolding Capaci ty (% w/w) and llydrau'lic t) Conductivìty (cm h ot Soils Treated with 0 .07%Fe Conc.of anion add ed Phosphate Ful vate Ci trate to soi I before an io adsorption (mM l- 1) MC Ml^lHC HC MC MI,JHC HC MC Ml¡lHC HC 0 2.1 50.8 0.35 2.1 50.8 0.35 2 s0.8 0.35 2 2.0 49.4 0.32 2.1 49.5 0 .33 ? ì 50.5 0.36 4 1.8 45.0 0.26 1.7 46.1 0.25 I 9 48 .6 0.32 I 1.7 45.2 0.21 1.7 45.8 0.21 I 9 46. l 0.30 t2 1.8 44.6 0.18 1 .7 44.6 0.1 9 I B 45.9 0.27 LSD P < 0.05 2.8 0.04 2.5 0.04 2.8 0.04 P < 0.01 3.5 0.05 3.3 0.05 3.6 0.05 MC = moisture content of air dried soils (% w/w) MbJHC = maximum water-hold'ing capacity HC = saturated hydrau'l ì c conduct'ivi ty 6.3.7 Modulus of Rupture and Friability The ptastjc limit was increased (in gravimetric water content) significant'ly after addition of anìons to soil samp'le x (tabìe 28) . For soil samp'le x the addìtion of anions increased the modulus of rupture (Fig. 32) and decreased the friabiìity (Fig.33). The modulus of rupture of soi'l samp'le x was changed and jncreased by a factor of 2 over the control. At the same time a decrease in the number of particles 50-250 um diameter caused a reduction in friab'iìity. There were no significant changes in the modulus of rupture and friability in the soil sampìe y after addit'ion of anions with ìncreasing concentrations. In soi'l sample x with zero d'ispersible clay there vlere few areas of contact between floccules or m'icroaggregates, hence the higher porosity 70 60 o o. J o = ct 3 50 o o 2 tt= o = 40 3 2 I o 12 Goncentratlon of anlon added to solt before anlon adsorptlon(mtt!-1 ¡ Fig. 32 Changes in the modulus of rupture at the p'lastic limít of 0.07%Fe treated soils after additionofanions. ( o )phosphate; ( r )fu]vate; ( r )cìtrate. H H 113 o.2 o.26 o.25 ! à o.24 ¡¡ o ¡r o.23 o.22 o.21 o.20 o 2 4 6 I 10 12 Goncentration of anion added to soll before anlon adsorption(mUl-f ) Fig.33 Changes in the friabiìity at the plastic limit of 0.07%Fe treated so'ils after addition of anions. ( o ) phosphate; ( r ) fulvate; ( r ) citrate. 114 Tabl e 28 Effect of Anions on the Plastic Limit in 0"07%Fe and o.32%Fe Treated Soils Conc.of anion Plastic lìmit (% w/w) added to soi l before anion Pho s pha te Fu lvat.e C i trate adsorpti on 0.07%te 0.32%Fe 0.07%Fe 0.32%Fe 0.07%Fe 0.3?%Fe (nrM l- 11 0 1 5.4 16.0 15.4 16.0 ls.4 16 .0 .l6.0 2 I 5.5 I 5.6 I 5.9 15.4 16 .l 4 l5.B ls.B I 5.8 1 5.9 I s.6 I 5.8 8 16. 1 I 5.7 16.? 1 5.7 ì 5.9 ls.B 12 16.2 I 5.7 16.? ì 5.8 t6.0 15.8 LSD P <0.05 0.62 0.60 0.60 and lower bulk strength of the soil. This resulted in less compaction (lower bulk density) and lower values of the modulus of rupture. The friability decreased with increasing concentrations of anions due to increasing force required to crush the aggregates and the crumbliness of soi I samp'le x was reduced. 6.4 Conclusions Addition of anions to so'il samples in which clay was flocculated by addition of Fe po'lycations resulted in some dispers'ion of clay. The were phosphate fulvate > anions in order of decreasing effectiveness = citrate oxalate silicate tartrate > salicyìate > anion of catechol = = = of > aspartate > lactate =acetate. This is the order of the amount anion adsorbed on the soil sampìe but not necessariìy the order of the stabilìty constants of Fe-anion comp'lexes. Dispersion is undes-irable and results in poor physÍcaì conditions. Dìspersion is aìso'important from the point of view of soil genes'is. Dispersed clay particles are mob'ile and may be illuviated to form a B horizon or lost from the profile in dra'inage waters. 115 CHAPTER 7 EFFTCT OI- DISPERSIBLE CLAY ON THE PHYSiCAL PROPERTIES OF THE B HORIZON OF A RED-BROWN EARTH 7.1 Introduction There is little doubt that the clay content of a soil controls the phys'ica1 condition of the soil. Because of its colloìdal properties the clay fract'ion dom'inates 'texture diagrams' and plays the major ro'le in the ability of the soil to hold and conduct water and to shrink and swell. The clay content is determ'ined for the routine descrjption of a soiì, normally after destructjon and removal of the various forces and materìals wh'ich prevent dìspersion of dìscrete and/or compound clay parti cl es . However, it js clear that soils with similar clay contents exhibit very different phys'ical properties, depending on the surface properties of the c'lay and the presence or absence of flocculating and cementing agents. Thus, the susceptìbility of the clay fraction to swe'll and d'isperse appears to be a major factor in the control of the phys'ical condit'ion of the sojl. This has been recognized as shown by dispersion .l967; tests and indices (Emerson Loveday and Pyle 1973) and by stud'ies of subplasticity in soils which have led to the conclusions that cements prevent clay dispersion. In krasnozems and other Oxisols the putative cements are aluminium and Íron oxides but_rin the subp'lastic red-brown earths the cements have not been prec'isely defìned. The most ìike'ly explanatìon for lack of d'ispers'ion in these soils is intergrowth of cìay crystal s (glackmore.- 1976; Brewer and Blackmore, 1976; Butl er, 1976; Mclntyre, 1976; Norrish and Tiller, 1976; I^Jalker and Hutka, 1976). It is a genera'l observati on that the more subp'l asti c soi I s have desi rabl e physical characteristics in the field, and in some of the-earlier work .l982a, (Shanmuganathan and 0ades, l9B2b, l9B3) it was evident that i16 dispersible clay controlled virtualìy aìl the physicaì paranreters determi ned. In this chapter an attempt is made to show that the content of d'ispersible clay, obtained by limited physical dispersion only, is the major single factor controlìing the physical and mechanÍcal properties of a soil. To obtain a wide range of dispersible clay the B horizon of the Urrbrae fine sandy loam, a red-brown earth (Oades et at.,l9Bl) or Rhodoxeraìf (Soiì Survey Staff"1975) was chosen, because it contains about 70% clay whích is dispersible by the methods used and exhibits substantial swelling (Murray and Quirk,lgB0). The B horÍzon has been described as a soiì of normal plasticity (Norrish arrd Tiller ì976). 7.2 Materials The B horizon of the Urrbrae fine sandy loam was co'llected from 24 to 80 cm depth and air-dried. The soil was passed through a 2 mm sieve. The following characteristics were obtained, pH 6,.7 in soil:distilled water ratio l:5; clay 74.2%, siìt 14.4%; fine sand 9.8%; coarse sand 1.6%; organic matter 1.4% and cation exchange capacity 32.0 C g-1. The minerals present in the clay fraction were illite (^,60%) and kaoliníte (1407á) with some expanding lattice minerals. 7.3 Experímental 7.3.1 Treatment of the Clay B Horizon wíth Poly[fe(ftt)-0H]cations A suspension (l%) of the cìay B horizon ín water was used to determine the quantity of polycation required to coo,3uìate the clay (critical coaguìation concentration - CCC). The concentration of íron(IIl) in the polycation preparation was 320 ug cm-3. Different amounts of polycatÍon were added to 10 cmd of suspension which was adjusted to 25 cm3 by addition of distilled water in a 50 cm3 measuring cylinder. The suspensions were shaken thoroughly by hand for 60 s. The amount of clay dispersion (01) by IL7 this treatment was determ'ined by measuring the optical dens'ity at 6.15 nm in I cm cells 24 h after addition of po'lycat'ion solutions and shaking. The opt'ical densjty at 615 nm was calibrated aga'inst the percentage cìay, 'isolated from the soil and determjned gravimetrica'l'ly. The optica'l density at 615 nm of the po'lycation solution alone was less than 0.07. The samples of clay B horizon were treated with nine different amounts of polycations up to the CCC. The treated soil sampìes were aìr dried in plastic pots for B weeks. The treatments were 0.00; 0.02; 0.04; 0.06; 0.08; 0.10; 0.14; 0.ì8, and 0.24%Fe, on tlte basis of the weight of cl ay. Each treatment was repì i cated four t'imes . 7.3.2 Determjnation of Dispersible Clay Norrish and Tiller (1976) state: "There is no'correct'clay content of a soil; the method that gives the most meaningful result wilì depend on the use to which the results are to be put." This statement is clearly'illustrated in the various stud'ies on subpìast'icity (Aust. J. Soil Res. 14,1976). Thus the methods chosen were simpìe and easy to repnoduce, and did not involve chemical treatments. Two methods were used for determjning dispersible clay in soils treated wi th po'lycati ons and subsequent'ly ai r dri ed. i) The method used was devised by Rengasamy (1982). Ajr-dried soil sampìes (10 g), diaìysed free of electrolytes, were suspended 'in 200 cms of distilled water in a stoppered cylinder. The cyìinder was shaken for a few seconds end-over-end and then allowed to stand for 24 h. The dispersìble clay (D2) was determined by measuring the optical density at 615 nm ìn I cm cells. ii) Air-dried soil sampìes (25 g) were mechan'ical'ly shaken end-over-end for l6 h with 200 cm3 of distilled water, and the contents transferred to a 500 cm3 measuring cylinder. After making up to volunle the anlount of dispersìble clay (Dr) was determined by a pipett,e method. 11E 7.3.3 Physical and Mechanical Propertìes Incli ces f or di spers ion of aggregates were determ j ned us'ing a modification of tmerson's method. Hydrauìic conductivity, modu'lus of rupture and friability were determined as described in Chapter 3. The soil samples were moulded with distilled water at different water contents ranging below and above the plast'ic lim'it for the determinations of modulus of rupture and friabi'lity 'l 7 .3 .3 . 1 Swel i ng and shrì nki ng The soil was ì ight'ly ground by hand, s'ieved (<500 pm) , equil ibrated at 96% relatjve humjdity (RH), pressed (about 500 MPa) into thin (1 nrm) discoid cores of 10 mm diameter, and stored at 96% RH; pressing discs at 96% RH was to give pore-space saturation. A range of so'il water potent'ials was estabj'ished by wetting cores slow'ly to -10 kPa water potentia'l (swelìing) using sintered glass funnels, and draining them at -33, -100, -300, -1000 and -1500 kPa water potentiaìs in a pressure plate apparatus. The behaviour of compressed cores was established and total sample porosity was mon'itored by measuring core diameters (t0.002 crn) wjth a travelling microscope. Porosity (cm3g-1) an¿ moisture content (g HZ}/g solid) were expressed on the basis of oven-dried (110'C) materia'1. -3 The average part'icìe dens'ity for this nraterial was found to be 2.69 g cm The porosity was calculated us'ing the equation, p = ad3+b (1) a = t't'þ- 1) +!l e) çttutt. 9s' where d, is the diameter of the air-drjed core, tl is the mass of core at production, mo is the oven-dry we'ight of the core, P* is the density of water and P, is the density of so'i1 particles. _1. (3) b= ps 119 The coefficient of l'inear extensjbif ity was also calculated using the equati on r/3 C0LE = (V0.33/Vd)--- 1 where VO.¡g is the volume of core at -33 kPa vrater potential and VO is the volume of core when air-dry (Grossman et at-.'i968)' 7 .3 .3 . 2 Water retenti ott The air-driediso1l.samples (<2 mm) were pìaced in glass cylìnders and saturated with distilled water be'l'ore apply'ing different water potential s in the sintered glass funnels and pressure plates. The saturated samples were equilibrated at -10, -33, -500,-1000 and -1500 kPa water potentials. The v¡ater retained was calculated as a percentage by weight of the oven- dri ed ( 110"C ) materì a'l . 7.4 Results and Discuss'ion 7.4.I Dispersìble Clay The concentration of Fe(III) required to flocculate all the clay (dispers'ible clay = 0) was 0 .24% by weight of soil (Fig.34). When the amounts of iron added exceeded the CCC, electrophoret'ic mobil it'ies changed from negative to positive. Similar behaviour was observed for the A horizon soils (Shanmuganathan and 0ades, 1982a). As anticipated, electrophoretic mobìlit'ies close to zero, i.ê. a net charge of zero, occurred where most of the clay was 'coc$Ulated. The pH of untreated suspensions was lowered from 6.7 to 4.2 as po'lycations were added up to the CCC (raUt e 29). The results ìndicate efficient coanulation of soil clay by small amounts of poìycations presumably through the bridgìng of the finer part'icles by the chemisorption of the polycations. The presence of a net pos'itive charge causecl the redispersion of some clay above the CCC. The treatments also reduced the pH of the sanrples vrhich in part expla'ins 0 .0 6.0 'õ I Ul tt, I o E u 50 E 'õol 3 = -c¡ 40 È o l! E o o 0) '7n o 0) o o .9 a L o 20 o gu l¡¡ â.€ a 1.0 0 basis lron (ttl) (7") added to the soil on weight Fig.34 Influence of poly[Fe(IIi)-0H]cations on the percent díspersible clay and electrophoretic mobilityofsoil. ( o )Dl, shakingfor60s; ( I )DZ, end-over-endshakingforafew ( r' -ve seconds after dialysis; ( r ) D3, end-over-end mechanical shaking for l6 h; ) +ve mobiì'itY' f\) mobility; ( a ) O t2r the decreased negative charge to net charge of zero' I'he change in electrophoretic mobilities from negative to zero con.tìrnr the di spersion-fl occul ation behaviour. The dispersible clay obtained by the three djfferent methods showed substant'iaI agreement (FiS. 34). The dispers'ible clay contents obtained by methods D, and D, were identical (y = 0.98x + 0.25; 12 = 1,00). Simultaneously there were good correlations between the amounts of dispersible clay given by methods Dt and D, and D, and D, respectiveìy (y = 1.41xf 7.62; Y2 = 0.96 andy = 1.44x'7.61; 12 = 0.96). These correlat'ions are based on all the data in Fig. 34. Gentle shaking for a few seconds after d'iaìysis gave less dispersible clay than v'igorous shaking for 60 s. 0f the three d'ifferent methods,60 s shaking is a quick simpìe method and gives the same content of dispersible clay as the more standard method involving ì6 h end-over-end shaking. Table 29 pH Values for PolyIFe(III)-0H]cation Treated Soil Suspensions Iron added (% of soil) DH 0.00 6.7 0.02 6.2 0.04 6.0 0.06 5.7 0.08 5.3 0.10 5.0 0.14 4.7 0.18 4.5 0.24 4.2 T?2 7 .4.? Aggregate Stabi I i tY The flocculation of clay caused by treatment wjth polycations led to the djsappearance of particles <2 Um esd (Fig.35). There was a 6 to 7 fold'increase jn the percentage of particles 50-250 Um but not'in the percentage of water stable particles 250-2000 um djameter. Blackmore (.l976) also found an increase of aggregation of clay part'icles after add'it'ion of Fe(III). Tabl e 30 j Porosi t'ies 1.nr3g-1) at Di ff erent Water Potent'ial s of C'ì ay B Hor zon Treated w1th PolyIFe(III)-0H]cations. ('llithin experjmental error water contents (g/HZ}/g solid) gave the same values as poros'ities for all treatments. ) Iron added Water potenti a1 s ( kPa (% of soil) -10 -33 -l 00 -300 -l 000 -1500 A'ir dry C0LE 0.00 0.373 0.349 0.327 0.3'l0 0.261 0.227 0.162 0. l0l 0.02 0.360 0.34] 0.299 0.310 0.261 0.227 0.1 62 0.097 0.04 0.34.l 0.345 0.287 0.274 0.244 0.227 0.161 0.ì0.l 0.06 0.329 0.33.| 0.286 0.271 0.242 0.226 0.161 0.09.| 0.08 0.314 0.31 2 0.279 0.269 0.237 0.225 0.162 0.082 0.t0 0.293 0.284 0.243 0.242 0.227 0.220 0.16t 0.065 0.14 0.261 0.246 0.228 0.224 0.217 0.205 0.161 0.044 0.18 0.221 0. 205 0.21 l 0.201 0. 198 0.200 0.161 0.023 0.24 0. 200 0.202 0.200 0.201 0.1 99 0.200 0. 16.l 0.01 7 COLE = coefficient of linear extensibiìity 7 .4 .3 Swel I ì ng Behavì our Pressing discs of cìay equìlibrated at 96% RH at a pressure of 500 MPa eliminates all air-filled pores and changes in poros'ity determìned by measuring the size of discs at different water potentials from -10 to t23 Fig. 35 Effect of po'ly[Fe(III)-OH]cations on the size distribution of water stable particles in soil (B horizon of a Red-brown earth). ( [) controì; ( m) o.oz%Fe; (- Sþ 0.04%Fe; ( N) 0.06%Fe; ( H) 0.08%Fe; ( El 0.10%Fe; (trþ o.r4%Fe; ( m) o.lB%Fe; ( H o.z4%Fe. 70 6 =o .n 5 6 o òa o o !, 40 I o (J o E¡ o A (I, tr 3 D À o o tr o A o I ô o .c¡ 9 o ^A o o 2 D o À A at g o A D o ô o o  (¡) ô o A E¡ o Þ o G o o À o o 1 a o tr t¡ ì o ô o A o A tr o tr o D o ô o o A o o tr o  o o A Þ tr o tl o o o t¡ A ^ 250 -5() <2 S ize of particles(¡m) 124 -1500 kPa give a measure of normal swelì'ing. The porosìty of the discs for all polycatìon treatments was 0.16 cm3g-1 when they were aìr dried indicating that the treatments did not influence tight]¡r bound water. The porosity of the clay discs to which suffic'ient Fe polycatìon was added to give zero dispersìble clay was 0.20 at all water potentìals studied indìcat'ing elimination of swelìing over this range of potentials (fa¡le 30). The data in the table show that swell'ing was d'irectly related to d'ispersible clay at a1l the water potentials studied. The CSLE values confirm the swelling data and show a six-fold decrease from the untreated clay to the sample treated with sufficient poìycatìon to flocculate all the c'lay. Other workers have shown decreased swelling after add'ition of both Al and Fe to soils (tl Rayah and Rowell, 1973; E1 -Swaify and Emerson, I 975) . 7.4.4 l,Iater Retention and Available Water In spite of decreasing the swellìng substantially it is clear that the polycatìon treatments increased the water retention and pìant available water held by the clay (Table 3l). When the water content of the clay not treated wìth polycation, and not compressed, is compared with the water content of the cìay in pressed discs, we see onìy smaì'l increases in water content'in the non-compressed clay at all water potentia'ls. þJith increased addition of polycation the non-compressed cìays contain substantially more water particularly at low water potentia'ls, e.g. at -10 kPa for samples with zero dispersible clay the cìay discs contained only 20% v¡ater by weight whereas the unconsolidated jncrease- clays conta ined 64% water by weìght.Thus b/e seei an of-porosit'y of 0.44'in pores of diameter approximately 1to about 30 irm. The creatjon of pores of this size is presumably due to the creation and stabilisation of particles 50 to 250 pm diameter (Fig.35) as was also shown to occur in the A ho¡izon of this soil (Shanmuganathan and Qades, l9B2a). r25 Tabl e 31 Effect of Addìng Poly[Fe(III)-0H]cation on the l^later Retention and Available Water of ClaY B Horizon Water retention (%w/w) at djfferent water Iron added water Potenti a ls (kPa) Available (%of soil) -l 0 -33 -500 -l 000 -l 500 (%\ 0.00 42.5 37 .9 30 .3 27 .7 24.0 18.5 0.02 42.8 37 .1 33 .6 28.6 2s.0 l7.B 0.04 43.5 38.2 34.3 29.8 26.6 ì6.9 0.06 45.7 41 .4 34 .8 30.2 27 .5 18.2 20.4 0.08 48 .6 42.4 35. I 31 .0 28.2 0.10 50.4 44.9 36.2 3l .4 28.9 ?1 .5 0..l4 54.4 47 .1 37 .l 32.6 28.7 25.7 29.6 0.l B 58.8 49.5 3B .4 32.4 29.2 0.24 64.4 52.5 40.5 32.8 29.2 35.2 The water avaìlable to p'lants which was calculated as the difference between water contents at -10 and -1500 kPa was found to be poly- 100% more in the soiì to which 0.24%Fe was added ìn the form of porosity cat'ions. The amount of Fe added is small and the increased must be due to the flocculat'ion and orientat'ion of c'lay particles creating ìarger pores and eliminatjng swe'll'ing' v'i 7 .4 .5 Di spers i on I ndex and Hydraul i c Conduct'i ty 'index the The addition of polycatjons decreased the d'ispersion of soil aggregates which were moulded at -10 kPa water potent'ia1 ' A reduction of the dispersion index from 7 to 1 was obtained by the addition of po]ycat'ions (raule 32). There v{as a concurrent increase in decreased part.icles 50-250 um diameter by a factor of 6 to 7. The dispersion 'index and increased numbers of partìcles 50-250 ym diameter 726 increased the hydraulic conductivity by a factor of 7" These three properties are c]osely related to each other in the sojls to which 0.24%Fe was added. The hydraul'ic conductÍv"ity (y) was I inearly reìat.ed to the dìspersion ìndex (x) by the equation y = 9.31 - 0.05x; rz = 0.78 which is based on all the data in Table 3?. Loveday and Pyìe (1973) also obtained a negatìve correlation between dispersion index and hydraul ic conductivi ty. The increased hydrau'lic conduct'ivi ty ind'icates increases in pore d'iameter up to about 100 pm' Tabl e 32 Influence of Poly[Fe(III)-0H]cations on the Dispersìon Index and Hydraulic Conductivity of Clay B Horizons Iron added Di spersion index Saturated hydraul ic of soi I conducti vi cm h- 1 0.00 7 0.05 0.02 5 0.05 0.04 5 0. 07 0.06 4 0.08 0.08 4 0.ll 0.1 0 3 0..l3 0.14 2 0..l7 0.18 I 0.27 0.24 I 0.36 7.4.6 Soil Mechan'ical Properties The plastic lim'it was decreased (in gravimetric water content) significant'ly after addition of amounts of Fe(III) polycations which caused flocculation (Table 33). Maximum values for the modulus of rupture (FiS. 36) and friabiì'ity (F'ig. 37) were obtained at these p'lastic I27 limits. The modulus of rupture was decreased and friabiljty (k) was increased as the amount of d'ispersible clay þ/as decreased by the additìon of dìfferent amounts of iron (III)'in the form of polycatìon. This was true at all the water contents. Flocculation (d'ispersìble cìay = 0) reduced the modulus of rupture and increased friabì'ljty by a factor of 6 to 7. At the sanle time lower values for the modulus of rupture and lower frjabilities were obtajned at mould'ing water contents below (15-25% w/w) and above (50-60% w/w) the plastic limit. Tabì e 33 Influence of PolyIFe(III)-0t{]catìons on, Plastic Limit Iron added Plastic I imit (% of soil ) & w/w\ 0.00 46.4 + 0.3 0.02 45.2 + 0.5 0.04 44.7 + 0.5 0.06 43.'l + 0.5 0.08 42.0 + 0.5 0..l0 42.2 + 0.8 0.ì4 40.6 + 0.4 0.lB 37.2 + 0.5 0.24 35. 0 + 0.8 The influence of water content and dispersible clay on the modulus of rupture and friabil'ity are shown in the three djmensional diagrams (Figs. 36 and 37). Maxìmum compaction occurred when the water content at mould'ing was near the plastic lirnit. Presumably at th'is water content shearing forces caused particle reorgan'ization increasing contact between particles and reducing porosity (Yeoh and 0ades, l98l; Sharrmuganathan and 0ades, 1982a). At lower water contents during mou'ld'ing |lt À .':( o CþI o. 1?0 ôêJ o 360 ! o E ,uro %,r, ./o) Fig. 36 Three-dimens'ional diagram for dispers'ible clay, mould'ing water content and modulus f\) of rupture of poly[Fe(III)-0H]cation treated sojls. æ (l) .30 "a"J rl t! 4?ouldùìg t+ahr co¡?6oÍ Qt Fig. 37 Three-d'imensional dìagram for dispersible clay, mou'lding water content and friability N) of poly[Fe(III)-0ll]cation treated soils. \o 130 there was jnsufficient water present to carry clay partjcles into voids between bìgger particles, resultìng in loler strength. At. water contents above the pìast'ic limit the prese.nce of waterin pores pnevented compactìon of the soil, agaìn resulting in lower strength. The flocculation of fine clay caused a reduction in areas of contact between floccules or micro-aggregates. This resulted in reduced bulk strength of the soil. The friabiljtV (k) increased with flocculation due to a reduction of the force required to crush the aggregates. Friabi'lity or crumbl iness 'is maximal at water contents near the plastÍc I imi t. In the present study, the dispersible clay was reduced by the addition of iron(III). A large content of dispersìbìe cìay =70% causes poor physicaì cond'itions in soils, and both the modulus of rupture and friabil'ity were influenced by the djfferent amounts of dispersible c'lay. 0n the basis of the results obtained in F'ig.36 and 37, the fo.l'lowìng tentative classjfjcation ftay be proposed. %DC Degree of influence of the Modulus of rupture* mouldinq water content 0-20 Sì i ght Low (<60 kPa) 20-50 Moderate Medium (60-160 kPa) >50 Hi gh Hi gh ( t0O-Zoo kPa ) Friabi I i 0-20 Hi gh Friable (0.10-0.25) 20-50 Moderate Sl ight'ly friable (0.05-0.10) >50 Sf ight Not friable (<0.05) *Lorv, medium and high va]ues were classified according to the results obtained in the present study **Classes of friability from Utomo and Dexter (1981) Th'is proposed classificatìon may not be suitable for soils w'ith very different textures. Dispersible clay contents and the soi'l physicaì i31 paramet,ers need to be determ'ined on a range of soils and a more detailed classif ication developed for'loam topsoils with di'tferent structures and stabi I i ti es . Tabl e 34 Correlation of the Percentage of Dispersible Clay manìpulated by the Addjtion of PolyIFe(III)-0H]cations with Various Physical Properties of the Clay B Horizon Physical propertìes Regressions y = Physicaì ProPertY x = % dispersible clay COLE y = 0.0014x + 0.02 12 = 0.94*** Di spersi on 'index Y = 0.08x + 0.63 r" = 0.97*** Hydrau]ic conductivity Y = 0.30 - 0.0045x 12 = 0.89** (cm h-t¡ Available water (% w/w) y -- 31.80 - 0.26x 12 = 0.86** Plastic limit (% w/w) y = 0.16x - 36.08 12 = 0.95** Modulus of rupture(kPa) y = 3 .07x + li .33 rz = 0.96**'( Friability (k) y = O.rU.-0'03x rz = 0.98*** Sìgnificance levels: **P - 0.01; ***P = 0.001 Linear regressions were obtained relating the various pliys'icaì properties of the polycation treated soils to the percentage of dispersible clay (Dt) (Table 34). The correlation between dispersib'le clay and friability was obtained from an exponential curve fit equation. tJith decreas'ing proportions of d'ispersible clay, COLE, dispersion index' plastic limjt and modulus of rupture decreased. At the same time hydrau'l'ic conductiv'ity and friabiì ity were 'increased due to the ìncreased contents of partìcles 50-250 pm diameter. 13? 7.5 Conclusions There were excellent correlatìons between the percentage of dispersible clay and a range of physical parameters. The 'indications are that the content of dispersible clay controls to a large degree the physical condjtjon of the soil and since dispersible clay is easi'ly determ'ined in a reproducible manner ìt may prove to be a useful quantìtative characteristic of soiìs, particuìarìy tilled soils. For many soils it may be a nrore useful gu'ide to physìca'l behaviour than the totaì c'lay content obùained by normal methods for determinatic¡n of particle size distribut'ions. 133 CHAPTTR MODIFICATION OF SOIL PHYSICAL PROPTRTIES tsY ADDiTION OF CALCIUM COI'IPOUNDS 8.1 Introduction Gypsurn has been the most popu'lar calcium compound used for amel ioratìon of so jl structure. in jtial ly 'it rvas considered that gypsum improved physica'l properties by exchanging caic'ium for sodium on colloids, thus favouring flocculation of the colloids. There is convÍnc'ing evìdence that clay dìspers'ion was reduced as ESP (exchangeab'ìe sodium percentage) was reduced by addition of gypsum to sodic, hard- setting soiìs (griAge and Klein'ig?1968; Loveday"1974; Sharma and Tunny, 1978; Gobran et a7.,1982). Various physica'l propertìes such as porosity, hydraulic conductivity and water storage were increased (Davidson and Quirk, l96l I Loveday, 1976; Keren et ar.,1980). The changes in phys'ica1 propert'ies have led ùo better plant establ i shment 'l979). and crop yìelds (Grierson, l97B; Quirk, 1978; Abrol and Bhumbla, In more recent years it has become clear that the "eìectrolyte effect" produced by gypsum is perhaps the dominant factor and that both ESP and electrolyte must be considered. A threshold concentration of eìectro'lyte is required to flocculate clays which is dependent on ESP 'l (Qui rK ,1977) Recent'ly Rengasamy ( I 982 ) showed that Ca-c'l ays wi'l disperse in the absence of electroìyte if the clays are sheared by moulding in a wet state. Thjs situation may weì'l occur at the soil surface under heavy rajnfall. It has also been shown that non-sodic clays wili d'isperse on shaking when the electrolyte concentration 'is low (Shanmuganathan and 0ades, 1982a) and that d'ispers'ible clay'is a determinant of a range of so'iì physical propertìes,(Shanmuganathan and .l982c). Oades, In addition to dispersion-coagulation phenonrenon soil structure also depends on cement'ing agents which prevent slaking of soils (Emerson,1977). 134 Based on f jel d observations that, 'in generaì , cal careous soi I s e.re well aggregated and do not slake readily ìt was decided to compare gypsqm, calcjum carbonate and cement to modify soì1 properties. l.1aìn'ly the studies were carried out 1) to compare the effects of ESP' electroìyte effect and a cement'ing agent on dispersible clay and physìca'l properties and 2) to determine the influence of wetting and dry'ing cycìes on the changes in chemical and physical propert'ies of a soil to wh'ich gypsum, calcium carbonate and cement had been added. 8.2 Materì al s A sandy loarn Natrixeralf with ESP 9 from Charlick Experimental Station, Strathalbyn, South Australìa, was collected. The soil (0-i5 cm) was passed through a 2 mm sieve, and some characteristics are listed in Table 35. Table 35 Some Selected Soil Characteristics pH (1:5,Hr0) 6.4 0rganic carbon (%) 1.3 Exchangeable Na (c g-1) 0.8 Exchangeable K (c g-1) 0.3 Exchangeable Ca (c g-1) 3.6 Exchangeable Mg (c g-1) 2.5 Cation exchange capacity (C g-1) g.Z caco, (%) o.o8 clay (%) 12 sirr (%) e Fine sand (%) 53 Coarse sand (%) 25 clay mineralosy o"liiìt:.'o 135 8.3 Expenimental 8.3.1 Application of Gypsum, Calcium Carbonate and Ce¡nenÙ The sa.nrples of sodic so'il were treated with six different amounts of dry gypsunì, calc'ium carbonate and cement (Table 36) and mixed thoroughly ìn a cement mixer. The treatments were 0.0;0.2;0.4;0.8; 1.4; 2.0; by weight of soil. Data for pH of dry mìxed soi] samples are sholn in Table 37. Tabl e 36 Some Characteristics of the Calcium Compounds Part'icle size distrjbution (% w/w) (dry sìeving of 200 S) Calcium Si ze (um) Gypsum carbonate Cement >2000 0.2 0.7 2000-l 4l 0 1.9 ì.3 .l6.3 I 4l 0- 500 8.6 500-250 40.4 16. 5 'l 250-1 50 ì 8.8 5.4 I 50-l 05 8.0 12.9 t 05-90 2.2 4.3 .l.4 90-75 4.1 75-64 0.9 6.8 64-45 1.2 6.8 <45 8.7 22.2 t00 pH (1:5/sol id:Hr0) 7.3 8.0 12.2 Electrical conductiv'ity (uS cm-l) gooo 2730 5020 % CaC} 2.3 97 .1 64.4 3 (as CaO) 136 Table 37 pH of Dry Mixed Soil Samples before Wetting and Dry"ing Chenri cal Treatment pH appl ì ed (% w/w) ( l:5 H"o) Control 0.0 6.4 Gypsum 0.2 6..| 0.4 6.2 0.8 6.3 1.4 6.4 2.0 6.5 Calcium carbonate 0.2 7.0 0.4 7.2 0.8 7.4 t.4 7.4 2.0 7.4 Cement 0.2 9.t 0.4 t0.l 0.8 10. 5 1.4 I0.6 2.0 t 0.9 8.3.2 Treatment of Soils in Pots The treated soìls were placed in pots for study of physicaì characteristics (1.5 kg) and plant growth (3.0 kg). The pots were arranged in a random'ized design in the glasshouse. All treatmer¡ts were repl icated thri ce. Three wetting and dryi ng (l,lD) cycl es were done per pot per month for eight months. Soils were wetted with djstilled water at the rate of 1000 cm3/1.5 kg soil/month (equivalent to 58 mm rainfall) applied three times each month to sojls to be 137 characterìzed physical'ly, and 2000 cm3/3 kg soil/month, applied eight tjmes each month to so'ils ttsed for plant growth studies. The six different sets of repìicates wjth the same treatments (0.0;0.2;0.4; 0.8; 1.4 and 2.0% w/w) were arranged random'ly for the determination of phys'ica'l properties after B, ll, 14, 17, 20 and 23 |llD cycles (Fig. 38) At the same t'ime five different sets of repficates were arranged for '11, plant growth studies after 14, 17, 20 and 23 llJD cycles (F'ig. 39). 8.3.3. Examination of Treated Soils after Wetting and Drying Cycìes pH and electrical condLtctivity were determined in distiì'led water, 1:5 soil suspension. Exchangeable catjons were extracted wìth 1 M NH4CI in 60% v/v ethanol and water at pH 8.5 (Heanes, lg8l). Exchangeable sodium and potassìum were determined by flame photometry and exchangeable calcium and magnes'ium by atomic absorption spectrometry. Cation exchange capacity (CEC) was determined by dÍlution of the residual exchange catjon extract with M/100 NH4CI followed by extraction of the added + forms of NHO- and Cl- with 1 M KN03, 0.5 M Ca(NOt), and subsequent determination after steam distillation. Air-drÍed samples (25 g) were mechanicaì'ly shaken end-over-end for 16 h with 200 cm3 of distilled water, and the contents transferred to a 500 cm3 measuring cylinder to determine the dispersib'le clay. After making up to volume the amount of dispersible clay (<2 vm) was determined by a pipette method.. Total areal porosities of the air-dry (<2 mm) soils impregnated with epoxy resin under vacuum were determined by preparìng sections 20-25 pm thick. Three photographs were taken for each section. An'image analys'ing cotnputer (Optomax MS3 Image Analyser) was used to analyse the total areal porosity. The image of the photograph was directly displayed on the screen. 138 Fig. 38 Randomized complete block design for the determination of physical properties. I, II, III, IV, V and VI and Rr., R, and R, are the numberofsets (6) and repììcates (3) respectively. (Go: Lo, Co: control; Gl, Ll, Cl: 0.2%; GZ, LZ, CZ, 0.41"; G'L'C': 0.8%; G4, 14, C4t 1.4%; G5, 15, CSt 2.0%.) G = gypsum; L = calcium carbonate; C = cement. I Rz Rg L C L C G G c L qE cA Ln Gn G/l LA cn L (81^JD) G0 G4 L co Ls Gs c Lt C G I 5.1 .82-8.3.82 - G2 c2 L tl C¡ L c4 4 5 1 1 I 3 3 2 0 G L c G2 L G¡ c Lz c L0 G 0 L G c L c G1 3 3 4 0 4 4 5 5 5 I 1 Rt Bz t c- G Go Ln cl c4 G¿ LÁ Gl Lt Gz G: c. c? L. L Lul co 5 ¿ 2 15.1 .82-8.4.82 (l I l,,iD) il cl c, (r. b- G) G" c2 C? L? L2 L5 Gn cn c5 Gq Ln __3. 4 _f L !_ G2 tr^ c2 c? L? L) G- Lo cl c¿ G4 L4 Gl Ll L. G? c ? c- t Rz t b^ I L4 L- cq l- G cr c^ Gr L? b^ Lo G^ cz L? oz 5 4 I I U U l5.r.B2-8.5.82 (14 l,tD) cs L4 L- cq b- o4 ct c^ -3_ G^ Ttf 5 l- _1_ _1 l- 5- _h_ -h_ Lv L G C- L4 L- tr- G4 C^ ,.l L L. u Go L0 c0 c2 2 3 f, 4 f, cl J I J 3 2 I Rz Fo Go Lo G? G4 c. L2 c4 co Gr Lr L- c" L? G- Lr^ I 4 I t- L. C? L? GR G^ c- 15.1.82-8.6.82 (17 l^lD) tv c1 cn Gl Gn Lr -n ¡, h_ 9- _h_ :l- :IL Gq J? C G2 Ga C2 L2 L¿ Ca G2 Ga C2 L2 L¡ rL Lq C1 Lj 5 F" ßr Rz Lz cr o- cq L4 G4 c E L- C^ L0 t co (¡- ,0 Gt oz L 5 5 3 J 2 Gn (20 l^lD) G. G+ C- I c? L^ L? cn G. l5.l .82-8.7.82 G Gz Lr L^ C 5 r I 1 2 "4 I :þ_ L^ cn G? Ln ¡.1 G2 L1 L) cr c2 G- c4 L4 G4 C- L- C? L n R q Rz 3 (r^ G Gr Ln G2 Lz u c2 L L: G+ c4 cr Ll cl 0 0 4 .i 5 5 5 G4 c4 L? G.t G/r c" r5.r.82-r.9.82 (23 t^lD) v¡ Ll c- G. o.ì LA L? G? ? à A 5 5 L 5 G- Gr L- co Lo G2 t GO L? cn G? L2 Gn I c( 2 0 ? 139 Fig. 39 Randomized complete block design and changes of the replicate positions (R' R, and Rr) of each set (II, III, IV, V and VI) with tíme for the germination and plant growth (see Fig.38 for the detaíls of sets and replicates). Date of planting: 8.3.1982. II R.t R2 8.3.82 - 4.4.42 q r Rg 8.3.82 - 4.4.42 ¡[ E E 8.4.82 - 8.5.82 Rl rE 8.3.82 - 8.4.82 a.1.a2 - 8.5.82 -tIlv lBz I E E q R, 8.5_82 - 8.6.82 E E E 8.3.82 - 8.4.42 r E E a.4.82 - 8.5.82 V EJ Rz q 8.5.82 - 8.6.82 E q E 8.6.82 - a.7.82 R1 q r 8.3.82 - 9.4.a2 R2 E E a-4.82 - 8.5.82 EE R3 E 8.5.82 - 8.ô.82 Rg e¿ q 8.6.82 - 8.7.42 1.9.82 Rt % Rz 8.7.A2 - 140 Indices for cl'ispersion of aggregates and water retent.jon v/ere deternrined as described earl ier (section 7.3.3). The modulus of rupture was determined at the plastic linl'it of the soils after the respective treatments. Air^-dried aggregates w'ith diameters of 2.0-4.0' 4.0-6.7, 6.7-9.5 and 9.5-l .7 mm were used for the determínation of friability. The aggregates were slowìy wetted close to saturation by capillary action and then dried to -33 kPa water potential on pressure pl ates . 8.3 .4 Gernli nati on Stucli es . After 8 WD cycles wheat seeds (Triticum aest'ivum [.. cv. Wariga]) were sown (8 seeds/replicate) in a circular pattern 2 cm below the surface of the wetted soil. The coleoptiles wh'ich emerged were counted daily over the entire peniod of emergence. This yielded results on two aspects of emergence - on the time interval between sowing and emergence and the percentage emergence. The mean day of emergence aS defined by Edwards (1957) was used to determine the t'ime period between sovring and emergence. The seed used was tested for germination capac'ity. 8.3.5 Plant Growth Studies After seedlings emerged four selected seedlings were allowed to grow in the pots. The positions of replicates were changed in each set throughout the experiment (Fig.39). Nutrient solutions were added to each pot in the form of ammonium nitrate, ammottium dihydrogen phosphate and potassium sulphate to supply n'itrogen (.l48 mg), phosphorus (144 mg) and potassium (112 mg). The nutrients were added to each pot at monthly intervals for 4 months. The wheat p'lants were harvested at ì, 2, 3, 4, 5 (maturity) months (equivalent to ll, 14, 17,2A, 23 |l|D cyc'les) after sow'ing by cui,ting 2.5 cm from the surface o'F the soil. Total numbers of tilìers per pot were recorded at maturìty. The heads of the sampled 141 wheat plants were separated frotn the straw at maturity and t.he graìn separated from the chaff with a mini-thresher. Grain yjeld per pot was noted. Harvested plant materials were dried at 70oC until brown and brittle. After maturity dry straw Ìvas passed through a ì mm sjeve 'in a grìnd'ing mill. N'iùrogen was determjned in sulphunic acid d'igests of the plant material by the alkal'ine phenate method (Wjll'iams and Twjne, 1967) and phosphorus in nitric-perchloric acid d'igests by the vanadomolybdate method (Hanson, 1950). After each harvest the separation of roots was carried out as descrjbed in Chapter 3 (3.8). The length of root in each sample was determined using an 'image analyser (Optomax MS3 Image Anaìyser) by counting ìntercepts. The ìmage of root sampl es were di r"ect'ly di spl ayed on the TV screen. Known lengths of copper urire with similar thickness to the roots were used for calibration to obtain the number of ìntercepts for the respect'ive l engths. 8.4 Results 8.4.1 Exchangeable Cations and Eìectroìyte Concentration Addition of gypsum reduced the ESP and 'increased the exchangeable calcium in the soils which had ll, l7 and 23 l¡JD cycìes (fa¡le 38 and 39). However, this trend was eventua'lly reversed in the soìl to which 0.2% gypsum was added by the 23rd l.lD cycìes compared wjth the llth WD cycìe. After 23 lrlD cycles the exchangeable calcium increased from 3.2 to _1 6.8 C g-' whiìe ESP was decreased from 9 to 6.5, 3.3, 2.7, ì.8 and 2.1 .l.4 respective]y in the soils to which 0.2, 0.4, 0.8, and 2.0 % gypsum was added. Exchangeabìe magnesium was also reduced from 2 to _1 I C g-'. The pH of the untreated soils was lowered from 6.4 to 6.1 during ll hlD cyc'les and from 6.4 to 6.2 after l7 and 23 hlD cyc'les. There was a five fold increase of electrical conductivity to more than _1 600 uS cm'after 23 blD cycìes in soils treated with gypsum. r42 Table 38 pH, Electrical Conductivity and Exchangeable Cations of Soil 11 wetting and drying cycìes on s Che¡ni cal ïreatment pH EC25., Exch. cati CEC ESP ^2+ + appl i ed (% w/w) ( 1:5Hr0) (pScm-') LA Mg2* Na (cg-1 ) Control 6.4 t50 3 .37 2.17 0.78 7.9 9.8 Gypsum 0.2 6.1 380 4.12 1 .74 0.45 8.4 5.4 0.4 6..l 56? 4.92 1.17 0.25 9:0 2.8 0.8 6.2 59? 5.t4 1.24 4.27 9.3 2.9 1.4 6.3 651 6.lB 1.20 0.20 l0 .2 2.0 2.0 6.2 592 6.82 l.l8 0.33 t0.t 3.3 Cal ci unr 0.2 7.1 278 3 .65 2.26 0. 59 7.8 7.6 carbonate 0.4 7.5 248 3.61 2.06 0.60 7.9 7.6 0.8 7.8 290 3.Bl ì .85 0.63 7.7 8.2 1.4 7.8 245 3.69 2.31 0. 58 7.8 7.4 2.0 7.9 ?69 3.76 2.30 0. 55 7.7 7.1 Cement 0.2 8.0 256 3.72 2.34 0. 50 7.8 6.4 0.4 8.4 267 3 .96 2.14 0.52 7.9 6.6 0.8 8.4 ?82 3. 84 2.00 0.41 7.2 5.7 t.4 8.6 297 3.92 2.11 0.43 7.3 5.7 2.0 9.6 249 4..l0 1 .97 0.43 7.1 6.1 I 7 wett'in and d 1n les 23 wettin and dr in les Chemi cal Treatment pH ECzs Exch. catì ons (Cg- cEc EsP pH EC^- txch . cati ons ( Cg- ) cEc ESP app'l i ed ) ¿5- t ^ -l 1 (% w/w) ( 1:5Hr0) (uScrnl) ca?+ llg2* Na+ cg-1 ( 1:5Hr0) (u5Cm ) cuZ* MgZ* Na+ rg Control 6.4 139 3.3? ?.24 0.72 7.9 9.1 6.4 I q3 3.?4 ?.14 0.72 7.9 9.1 Gypsum 0.2 6.4 245 3.47 1.92 0.51 7 .6 6.7 6.5 243 3.20 1.92 0 .49 7.6 6.5 0.4 6.2 512 4.76 2.07 0.25 8.7 2.9 6.3 492 4.32 1 .17 0.27 8.1 3.3 0.8 6.2 562 5.64 2.16 0.28 9 .8 2.9 6.? 632 5.24 I .84 0.?5 9.4 2.7 1.4 6.4 667 7.02 I .9.l 0.20 I 0.9 1.8 6.2 605 7 .13 I .04 0.20 11 .2 1.8 2.0 6.4 675 7.00 1.82 0.20 I 0.9 1.8 6.3 721 6.82 I .00 0.23 I 0.8 2.1 Calcium 0.2 7.3 204 3.72 2.17 0. 55 7.9 7.0 7.5 246 3 .66 2.37 0.57 8.3 6.9 carbonate 0.4 7.3 253 3.73 2.27 0. 55 8.0 6.9 7.7 ?50 3.82 2.63 0. s3 8.4 6.3 0.8 7.8 238 4.04 2.25 0.52 8.2 6.4 8.2 260 3 .94 2.37 0 .49 B.l 6..l 1.4 8.0 249 3.91 2.51 0. 50 8.0 6.3 8.3 242 4.1I 2.41 0. 50 8.5 5.9 2.0 8.0 270 4.00 2.42 0. 5l 7.6 6.7 8.2 313 4.04 ?.42 0. 53 8.2 6.5 Cement 0.2 7.7 254 3.78 2.14 0.51 7.8 6.6 7.8 256 3 .68 2 04 0.40 7.4 5.4 0.4 8.1 259 3.95 I .82 0.47 7.5 6.3 7.8 256 4 .00 I BO 0.40 7.6 5.3 0.8 8.3 257 4.17 1.87 0.42 7.8 5.3 7.9 253 4.17 I B5 0.40 7.3 5.5 1.4 8.4 256 4.12 2.02 0 .30 7.6 3.9 8.2 292 4. 5l I 90 0.3.l 7.6 4.1 2.0 8.8 287 4.21 2.07 0.30 7.7 3.9 8.7 308 4.47 1.92 0.30 7.4 4.1 Tabl e 39 pH, Electrical Conductiv'ity and Exchangeable Cat'ions of Soil (,Þ 144 Coaguìation of the clay particles fo'llowing increased exchangeable calcium occurred in the range of electnical conductivìty from 240 uS _1 to 400 uS cm-r. The critical coaguìation concentratjon (CCC) was presumably reached by add'itions of gypsum between 0.2% and 0.4% which decreased the ESP to 3-5. Soils treat.ed with calciunr carbonate gave lower ESP values than untreated soils, but the ESP was about 6 after 23 [rlD cyc'les with the highest rates of applicatjon. There was a smali increase in exchange- able calcium. As anticipated the pH was'increased by almost two units by the addition of calcìum carbonate. The addition of calc'ium carbonate raised the electrical conductivity to about 250 ttS ìrrespective of the rate appl'ied. The addition of cement decreased the ESP to 6 or less after ll l¡lD cycìes and to almost 4 after l7 and 23 I^JD cycles. There were correspond- ing increases in exchangeabìe calcjum. The pH was ra'ised to values as high as 9.6 after ll l.lD cyc'les, but subsequentìy fe1ì to 8.5 after cement raised the electrical conductiv'ity to 250 to 300 uS irrespect'ive of the amount added. 8.4 .2 Di spers i bì e Cl ay The dispersible clay of the untreated soil remained between 8 and 9% during the 23'l^lD cycles (F'ig.a0). This represents about 70% of the total cìay obta'ined by standard methods used for particle size dìstribut'ion. The additions of gypsunì gave immediate large decreases in dispersible clay to 4% for an add'ition of 0.2% gypsum and to about 2% clay for al'l other treatments. t^lith time and hlD cycles the dispersible clay increased jn the soil to which 0.2% gypsum was added and after 23 l^lD cyc'les the percentage dispersible clay was similar to that in the untreated soil. After 23 I^lD cycles the ESP was 6.5 and the EC 243 which is about the value given by calcium carbonate. Clearly ù 145 GYPSUM ão o IE CALC¡UM CABBONATE ìo ¡¡ o I g .€ ø 0 o CI ol4 àQ CEMENT 2 1l Number of wetting and dry¡ng cycles Fi g. 40 Influence of gypsum, calc'ium carbonate and cement on d'ispersible clay of soils'in different wettjng and dry'ing cycìes. ( o ) control; ( a 7 0.?%; ( E ¡ 0.4%; ( o ) 0.8%; (r. )t.4%;( I )2.0%. 146 thjs electr"olyte concetrtrat'ion is not hìgh enough to main'Lain coagu'lation with an ESP > 6. For all other gypsum treatments low ESPs and high ECs were maintained. For all additions of calcjum carbonate dispers'ible clay rernained close ta B% although after 23 l^lD cyc'les treated soils began to show slightly less dispersible clay than the control soii tSPs dect"eased from ll to 23 |^JD cycles while the EC remained rela.t'ively constant. It would be interesting to continue the l,lD cycles to determine if the decrease 'in ESP continued with the eventual coagu'lation of a.ll the cìay. The additjon of 0.2 to 0.8% cement reduced dispersible clay by l0 to 20% and there was little change with tinre. The higher rates where pH vaìues of 8.6 and 9.6 u¡ere recorded after ll l^lD cycìes immediateìy reduced dispersible clay from I to 4.5% and it was further reduced to <3% after 23 WD cyc'les 8.4.3 Cementing Effects - Aggregate Stabi'lity Calc'ium carbonate had no significant effects on particle size distributions durìng this experiment and was therefore not implicated in cementatjon. There was no sign'ificant difference in the percentages of water stable particles 250-2000 pm diameter due to addÍtions of gypsum after ll or 23 l¡lD cycles (Figs. 4l and 42) and again it is interpreted as no cementation. However, gypsum resul ted in s'ign'if icant increases ìn part'icles 50-250 ym diameter. The histogranls show clearìy the disappearance of clay sized particles, particu'larìy at the h'igher rates of gypsum appìication, and their transfer to particles 50-250 pm diameter. The cement treatment resulted Ín a cementat'ion effect. The larger additions clecreased the dispersible clay content to some extent (Figs. 43 and 44). There was also a loss of particìes 2-50 ¡rm. The cement led to an increase in ìarger particles from 500-2000 pm d'iameter. Thus silt and clay have been cemented into particles with diameters up to 2 mm. r47 Fig. 4l Ef'fect of gypsurn on the size distribution of water stable particles in soil after 11 wetting and drying cycles. t[l control; (ffi) 0.2%; (m 0.4%; ( E] o.B%; t Nt 1.4%; (É) z.o%. 45 o o o 40 o o o o 35 o o =õ o o o E 30 o o o o o òa o o o 2 o g o o o .9 o o o o G o o CL 2 o o o o g o o l¡ o o o o o o o o 1 o o o o o o o o o o o o o o ì o 10 o o o o o o o o o o o o o 5 o o o o o o o o o o o o o o o o o 1000-2000 500-1000 250-500 50-250 2-50 Size of particlcs(¡m) 1.48 Fig. 42 Effect of gypsum on the size distrìbution of water stable particles in soil after 23 wetting and drying c-vcìes. tnl conùrol' (fffl1 o.z%; tØl 0.4%; (EJ) o.B%; tñl 1.4%; tEil z.o%. o o ô o o 3 o o o =o o o 6 3 o o o a o an 25 o g o o .9 o o o o o o € o o ct 20 o o o o o I o o o .c¡ o o o o o o o o at 1 5 o o o o) ô o o o o o o o o 1 o o o = o o o o o o o o o o ô o o 5 o o cl o o o o o o o o o o o o o o o o o 250 1000-2000 500-1 oo 50-5()0 Size of Particles(¡rm L49 Fig. 43 Effect of cement on the size distributi.on of water stable particles in soil after 11 v¡etting and drying cycìes. f [l conrrol; I ffiÍ) o.?%: fØl 0.4%; t Hl o.B%; ( N) 1.4%; (pil z.o%. 45 40 o o o o o o o o 3T' o o 30 o o o o -o ô o o an o 25 o I o o .9 o o o o o cL o o 2 g o o ¡l o o (It o o .D o o o 15 o o o o o o G o o 0 o ì o o o o 10 o 0 o o o o o o o o I o o o o ô o o 5 o o o o o o o o o o o o o o o o o o o o o o <2 1000-2000 500-1000 250-soo 50-250 2-50 Sizc of particles(ym) 150 Fig.44 Effect of cement on the size distribution of water stable part'icles in soil after 23 wetting and drying cycìes. (fl) control; f ffill 0.2%; ( W 0.4%; rBt 0.8%; tÑr 1.4%; (fiil z.o 4 4 o o o o 3 o =o o .1, o o o 3 o o o I o o o 2 o o o o o o o o o o o o ct 2 o I o o ¡ o o o o o o al, o o o 1 o (E o ì I o o o o o o 10 o i o I o o o o o o 5 o o o o o o o o 50-250 o 1 000-2000 500-1000 0-500 Si ze ol part¡cles(Hm) 151 I .4 .4 Total Areal Poros i t,y The total areai poros'ity increased s'ignif icantly in the soil s af ter l,lD cycles and treatment wìth gypsunr and ce¡nent (Table 40). There was a 45 and 65% increase in areal poros'ity over the control after 23 l,JD cycles by the ìargest additìons of gypsum and cement respective'ly. Treatment with 0.2% gypsum gave a reduction of areal, porosity with increasing l,^lD cycìes, as the d'ispersìble clay increased. Treatments rvith 0.8% cement gave areal porosities which were greater than those given by the correspond'ing rates of gypsum. 8.4.5 Available l^later and Hydrauììc Conductivity The treatments with cement increased water retention and p]ant ava.ilable water after 23 l.lD cycìes (1abìe 4l ). Water available to plants was calculated as the difference between water contents at -10 and -1500 kPa. This agrees with an increase of areal porosity caused by treatment with cement. Hydraulic conductiv'ity of soils treated with cement was also higher than for soils treated wìth gypsum (Table 42)- The greatest additions of cement gave hydraul'ic conductivities 50% higher than treatments with gypsum after 17 and 23 lllD cycles' This must be due to cementation of larger aggregates as co¡lcentrations of Na+ in percoìates from the columns used to determine hydraulic conductivities showed s'imilar amounts after the largest additions of cement and gypsum. A reductíon of the dispersion index from 5 to 1 or less was obtained by the additjon of gypsum and cement. The displaced sodium increased the Na+ concentratíon in the percolate by a faCtor of 4 to 5. No dispersion of a'ir-drjed and moulded aggregates was recorded after the largest add'it'ions of gypsum and cement, and l7 and 23 l^lD cycì es r52 Table 40 Effect of Adding Gypsum, Calcium Carbonate and Cement on the Areal Porosity of Air Dried Soil s after Wett'ing and DryÍng Cycles (tota'l area evaluated was 9.4 cmz¡ Chemi cal Treatment Total area of pores (cmt) appì i ed (% w/w) I lI.lD I 4I^lD ì 7t^lD z0l,lD 23I^lD Control 3.9 3.8 3.9 3.9 3.8 Gypsum 0.2 4.6 4.1 3.8 3.8 3.8 0.4 4.4 4.0 4.5 4.5 4.4 0.8 4.7 4.8 4.9 4.7 4.3 1.4 4.7 5.0 5.1 5.0 4.9 2.0 4.7 4.9 5.6 5.7 5.5 Cal ci um 0.? 4.0 4.1 4.1 4.2 4.2 ca rbonate 0.4 3.9 4.2 4.3 4.2 4.4 0.8 3.8 4.3 4.6 4.5 4.0 1.4 3.9 4.2 4.3 4.3 3.9 2.0 3.4 4.2 4.2 4.2 4.1 Cement 0.2 3.6 3.6 3.9 3.7 3.8 0.4 4.1 4.2 4.2 4.3 4.0 0.8 5.4 5.4 5.3 5.2 5.0 ì.4 5.4 5.7 5.9 6.0 6.0 ?-0 6.0 5-9 6-4 6-4 6.4 LSD P < 0.05 0. 75 0.9.| 0.9'l 0.87 0.83 ll wetting & drying cycles l7 wetting & drying cycles 23 wettìng & drying cyôìes Chemi ca I Ireatment i ed (%w/w) (%w/w) Al^J appl l,JR (%w/w) All l^lR ( %w w ) AI^J I,JR (%w¡w1 l0 500 I 500 (%w/w) I 0 500 1500 (%w/w) l0 500 I 500 .l8.4 lB.9 Control 26.2 ll I 7 .8 26.6 ll.0 7.5 l9.l 26.5 ll.0 7.6 26.4 4 18.0 Gypsum 0.2 27.0 ll.3 7.9 l9 .l 26.4 ll.5 8..| I8.3 ll.0 I 0.4 27.9 ll.3 7.8 20.1 26.7 I 0.9 8.1 18.6 26.9 I 0.8 I 2 18.7 0.8 28.2 12.7 8.9 l9 .3 28.2 1?.6 8.6 19.6 28.0 12.7 8 6 19.4 1.4 29.3 12.6 8.1 20.2 28.9 12.1 8..| 20.8 28.6 12.1 I 0 20.2 2.0 28.9 12.6 8.0 l9 .9 28.0 12.6 8.3 19.7 28.0 12.8 I 2 t9.8 7 7 I 8.6 Calcium 0.2 25 2 ll.l 7.8 17 .4 25 7 10.5 7.2 18. 5 26 3 I 0.9 .l0.5 ca rbonat 26 0 10. 5 7 5 18. 5 0.4 26 3 10.9 7.9 t8.3 26 3 7.9 18.4 .2 7 7 18.8 0.8 26. 4 11 .7 7.8 I8.6 26 5 10.7 7.8 18.7 26 5 11 .l.4 7 3 19.4 25.8 ll.3 7.9 17 .9 26 0 lr.3 7.6 18.4 26 7 1l .2 .l8.6 26 2 11 .2 7 0 19.2 2.0 25. 7 il.1 7.1 26 3 11 .2 7.1 l9.l 3 19.7 Cement 0.2 28.1 ll.l 8.9 19.2 28.4 lr.l 8.9 19. 5 28.0 il.1 I 0.4 30.0 12.3 8.8 21 .2 29.9 12.6 8.7 21 .2 29.7 12.3 9 I 20.6 14. I 8 9 23.2 0.8 30. 5 14.4 8.7 21 .8 3l .l 13.7 8.2 2?.9 3l .l 1.4 29.8 13.9 8.5 21 .3 30. 5 13.7 8.6 21 .9 31 .7 l4.l 8 5 23.2 14.0 8 7 24.0 2.0 30. 5 14.2 8.8 2l .7 3l .4 14.0 8.3 23.1 3l .7 LSDp 1.0 . o. os 1 .2 0.8 f,JR = water retention at dìfferent water potentials (f Gypsum 0.2 2 3 .90 0 40 4 3.02 0 28 4 3.10 0. l8 0.4 I 4.12 0 .54 I 4.24 0 52 2 4.r8 0.44 0.8 I 4.57 0 .55 NO 4. 56 0 55 I 4.71 0. 57 1.4 I 4 .14 0 .55 NO 4. 55 0 54 NO 4.52 0. 59 2.0 I 4.06 0. 54 NO 4.72 0 54 NO 4.67 0. 56 Cal ci um 0.2 4 3.14 0.ll 3 3.37 0.1 I 4 3 .30 0. 13 carbonate 0.4 5 3.47 0.17 4 3.82 0.18 4 3.77 0.20 0.8 4 3 .41 0. l6 3 3.40 0.14 3 3.37 0..ì7 1.4 4 3 .40 0. l7 4 3.62 0.17 3 3 .61 0.17 2.0 4 3.82 0.19 4 3.71 0. l9 4 3.71 0.18 Cement 0.2 4 3 .58 0 32 3 3.47 0.24 3 3.62 0.24 0.4 3 4. l0 0 32 3 3 .96 0.32 2 3.94 0.27 0.8 I 4.32 0 44 I 4.71 0.47 NO 4.70 0.46 1.4 NO 6.12 0 49 NO 6.55 0.52 NO 6.47 0.52 2.0 NO 6.07 0 55 NO 6.77 0.61 NO 7..l0 0.60 LSDP.o.os 1.20 r.3r 1.12 DI = dispersion index HC = hydraulic conductivity * = concentration'of Na+ ìn percolate ('r Þ Tabl e 4 Influence of Calcium Compounds on the Dispersìori Index and Hydrauljc Conductivity of Soì1 at Different Wetting and Drying Cycles i55 8.4.6 So j I l4echani cal Propert'ies The plastjc limit was decreased (in gravintetric water content) significant'ly after addition of gypsum and cenrent and 23 hlD cycìes (Table 43). There was a decrease'in the modulus of rupture of soils treated with cement from >20 to <12 kPa and for soils treated with gypsum from >20 to lSkPa (Fjg.a5). There was a signíficant reduction of modulus of rupture with increasing WD cycles after addit'ion of gypsutx and cement. The force requìred to rupture the soils after the ìargest addjtions of calcìum carbonate was greater than for untreated soils (26 kPa). Fríabilities of the treated soils were obtained at -33 kPa water potent'ial . The f rÍab'il'ity (k) jncreased after gypsum and cement were added to the soil (Fì9.46). The last two ìargest additions of cen¡ent gave the h'ighest frÍabilit'ies after 23 l^JD cycles. The friabil'ity of soils treated with gypsum did not change between ll and 23 l^lD cycles. 8.4.7 Seedl i ng Emergence 0nly gypsum treatment gave a sign'ificant increase in emergence (Tab'le 44). The mean day of emergence decreased from 10.2 to 7.2 after the largest additions of gypsum. The largest additions of cement gave the opposite trer¡d in percent emergence and mean day of emergence, presumably due to the effect of the hjgh pH on plant growth. The calcium carbonate had little effect on germination. 8.4.8 Plant Growth The plants were harvested every 30 days and final harvesting was done at maturity (after 5 months). The number of tillers showed significant increases from plants grown in the pots treated wìth gypsum (l'able 45). In so'ils with gypsum added the total yield was increased s'ignif icant'ìy only after 3 months (Fig. 47). The cement treatment gave 156 Table 43 Influence of Calcium Compounds on Plastic Linl'it in the Different Wett'ing and Dry'ing Cycles Chemi cal Treatment 11 l^l&D cycles 23 W&D cyc'ìes appì i ed Pl ast'ic I imi t Pl ast ic l inri t (% w/w) (%waten content) (%water content) Control l5.B r 0.5 I 5.6 r 0.4 Gypsum 0.2 I 5.8 r 0.6 15.8 I 0.4 0.4 13.0 t 0.5 13.3 t 0.5 0.8 14.2 x 0.5 13.7 r 0.4 1.4 14.2 ! 0.3 13.8 t 0.6 .l4 2.0 14. 2 t 0.3 .0 t 0.4 Calcium 0.2 1 5.6 t 0.3 15.5 I 0.5 carbonate 0.4 t 6.0 t 0.6 16.0 t 0. 5 0.8 l5.B t 0.6 15.2 ! 0.7 t.4 15.5 t 0.4 14.9 t 0.5 2.0 1 5.5 t 0.4 15.2 ! 0.4 Cement 0.2 14.3 t 0. 7 14.0 r 0.4 0.4 ì4.3 t 0.5 14.3 10.3 0.8 ì3.9 r 0.5 l3 .l t 0.3 1.4 13.2 t 0.4 10.6 r 0.6 2.0 l3 .0 r 0.4 'll .4 t 0.4 LSD P < 0.05 1.4 l.t 157 GYPSUM a À. J o 5 4 ct e o ,3o ã It o E CEMENT 0.4 basls GWs¡n r¡d csnert(%) added to the sfdem m sc¡ Fig. 45 Effect of gypsum and cement on the modulus of rupture of artificial briquets after 11 and 23 wettíng and dryíng cycles. ( o ) 11 hID cycles; ( a ) 23 I^lD cycles' 158 0 GY PSUM .y -à .5 0.04 o E 0.24 CEMENf 0.4 0'8 1'2 1.6 2.0 qr G¡parn rd carsr(%il ûdcbd to ûÉ qÊn sof bais Fig. 46 Influence of gypsum and cement on the friabiìity of soils after 11 and 23 wetting and drying cycles. ( o ) 11 t^lD cycles; ( a ) 23t.lDcycles' 159 Table 44 Seedl'ing Emergence of hlheat (var.Warìgaì ) in the Soil Treated with Calcium Compounds. (Germination capacìty of Warìgaì seed was 96%) Chemi cal Treatment Percent Mean day of appf ied (% w/w) emergence emergence Control 76 10.2 Gypsum 0.2 78 7.7 0.4 B'l 8.0 0.8 B4 8.2 1.4 B2 7.9 2.0 87 7.2 Cal ci um 0.2 73 10.0 carbonate 0.4 77 ll.3 0.8 71 t 0.7 1.4 73 10. 5 2.0 72 il.0 Cement, 0.2 79 9.7 0.4 79 9.4 0.8 76 ì1.6 t.4 68 13.4 2.0 67 12.9 LSD P < 0.05 3.9 1.2 160 l'abl e 45 Number of Tillers and N'itrogen and Phosphorus Content in the Plant Tissue of Wheat (var.l.'larìga'l ) after Harvesting Chemi cal Treatment No. of tillers Nitro g en Phos p horus app'l i ed (% w/w) per seedl ing ( % by we'ight ) Control 4.s 0.62 0.028 Gypsum 0.2 4.5 0.64 0.026 0.4 5.3 0.74 0.039 0.8 6.7 0.82 0.044 1.4 6.8 0. 89 0.052 2.0 6.6 0. 88 0.050 Caì ci um 0.2 5.2 0.70 0.025 carbonate 0.4 4.7 0.78 0.025 0.8 4.7 0.77 0.020 1.4 5.0 0.79 0 .020 2.0 4.5 0.77 0.020 Cement 0.2 6..| 0. 75 0.023 0.4 6..| 0.75 0.023 0.8 5.0 0. 75 0.018 t.4 3.9 0.79 0.0.l9 2.0 3.8 0.7 0.01 5 LSDP < 0.05 1.6 0.046 0.0.l 2 161 I GYPSUM a tÐ CALC¡UM CARBONATE o ßt o) o o E ÌtÈ E o o F. 90 CEI,I ENT 60 ..Ò A I 2 (maturlty) Stage(months) ol Plant growth F'tg. 47 Influence of gypsum, calcÍum carbonate and cement on the total dry matter (at different stages) and grain y'ield (maturity) of wheat. ( o ) control; ( a ) 0.2%; ( tr ) 0.4%; ( o ) 0.8%; ( ^ ) r.4%; ( r ) 2.0%; ( ) grain yi el d. r62 Tabl e 46 Root Length of Wheat Plants at Different Stages of Growth ìn Gypsum, Calc'ium Carbonate and Cement Treated Soils 1 Chemi cal Treatment Root length (cm g- ) appl ì ed (% w/w) 1M 2N1 3l'l 4M 5M Control 7.2 ll.4 14.2 14.0 10.5 Gypsum 0.2 9.'l 13.2 13.0 12.7 10.2 0.4 8.9 l3 .3 16.2 14.2 10.7 0.8 8.9 14.4 15.8 12.9 9.2 .l3.5 t.4 8.8 14.7 l6 .9 I0.7 2.0 8.1 14.2 16.7 14. ì 10.2 .l3.0 Cal c'ium 0.2 8.2 t3.l 13 .0 9.6 carbonate 0.4 7.6 l3 .9 13.2 13.2 10.2 0.8 7.8 tl .2 tl .2 10.4 l0.l 1.4 7.1 10. B il.4 10. 5 9.2 2.0 6.0 10.5 l0. r ì 0.0 9.7 Cement 0.2 6.5 ì 4.1 l4.l t 4.0 ll.3 .l4.0 0.4 5.7 14.7 13.2 11 .2 .l0.9 0.8 4.2 12.6 13.4 10.2 'l .4 4.7 9.7 13.7 ì 3.8 9.9 2.0 2.1 6.9 10. 5 10.0 9.2 .l.6 LSD P < 0.05 t.6 1.9 1.6 1.4 163 lower yields than unbreated so'ils, but the d'ifferetrce of the total yìeìd between additions and control r,ras reduced after 3 months. The gra'in yìe'ld was halved by the add'ition of 2% cernent and doubled by treatment w'ith gypsum. The nitrogen and phosphorus contents of the plant t'issue were increased sign'if icantly in the pl ants grown in the gypsum treated soils. There was appreciable reductiott in phosphorus content of p'la.nts grown in the pots treated with cement and calcium carbonate. There was a significant increase in root ìength in the so'ils treated with gypsum up to 3 months but the root lengths were the same as in untreated soils at maturity (Table 46). The ìargest additions of cement and calcjum carbonate resulted in poor root growt,h, particularly in the soil to which 2.0% cement was added where root growth vras half that in the control. After 3 months the root ìength increased, but was stilì less than in the untreated soil. 8. 5 Di scuss'ion 8.5.1 Coaguìation and DjsPersìon The critical ESP for soil structure is a matter for discussion and a value of 6 has been suggested for Australian soils (Northcote and Skene,1972). This is considerably lower than the value of 15 proposed by the USDA (1954). Hovrever, the electrolyte effect must also be taken into account and the dispersìbil'ity of Austrál'ian soiìs may be associated with the abundance of fine clay (<0.2 um) and lack of so'luble weatherable minerals to maintain electroìyte concentration in soil solution. Dispersion can be controlled by mainta'ining an appropriate ratjo between electrical conductivjty (EC) and ESP. The relat'ion between these factors and dispersìble clay is shown in Fig. 48 wh'ich'illustrates a pos'it'ive I i near correl ati on between ESP/EC and di spers i bl e c1 ay . The correl ati on appljes notwithstand'ing the differerrces in pH between the treated soils. t 164 Y-2.35x+O.72, r2..O84 o o ¡ oo Ii a I ll rïrt II ¡ I t I -9 o I ã o¡ I I ! I À'g o E o $ CI -0 c¡ àa 3 o OO 1 4 ESP.,€C{X1O'2} Fig. 48 D'ispersible clay in relatíon to the ratio of exchangeable sodium percentage (ESP) and elecbrical conductívfty (EC). ( o ) control; ( o ) gypsum; ( a ) calcium carbonate; ( r ) cement. i65 For this particular sojl Fig. 48 shows that jf dispersil¡le clay ìs to be maintained at <3.5% [SP/EC = 1.4. With an EC of 250uS crn-l which can be maintained by calcjum carbotlate the ESP must be <3.2. This indicates that even in soils to wh'ich calcium carbonate has been added dispersion will occur when the ESP is 6" The partìcle size of the calcjum carbonate'is obvjously crucjal (as it is for lim'ing to increase pH) and finely powdered calcium carbonate (<45 Um) as used by Shainberg and Gal (1982) may ma'intain an electrolyte concentratjon which allows an ESP of >3 and yet prevents significant clay d'ispersion. It should also be added that although sim'ilar linear relatjonships between dispersible clay and ESP/EC may hold for other clays it is ìikely that the sl opes wi'l ì be qui te d ifferent. 8.5.2 Cementati on The increase in larger aggregates in soils treated with cement is due to the cementing effect of calcium silicate. Calcium silicate ge'l is formed from hydration of anhydrous calcium silicate in the cement and the gel binds clay and s'ilt partjcles into larger units. The mechanism by whìch a smal'l portion of cement can change the propert'ies of a larger mass of soil has been described by Ing'les and Metcalf (1972). The increase'in aggregates of 250-2000 ¡rm diameter was evident in the cement treated soils, compared with formation of aggregates 50-250 um diameter by the coagulation of finer colloidal particles in the soils treated with gypsum. In the case of soils treated w'ith cement, formatiott of larger aggregates (cementation) and coagulation of clay were ev'ident. 8.5.3 Dispersible Clay as a Measure of Structural Problems? Determination of dispers'ible clay could prove to be a useful, sìng1e, quantitative parameter of the structural status of soils as indicated earl ier (7.4.6). l,Jhen compared with dispersion index (DI), dìspersible 166 c'lay is more quantìtative and obiective and overconles the varìab'il it'y between 'individual aggregates. It can be expressed as % of total soil or total clay. In the past, porosity of hydrau'lic conductivity have been used to indicate soil structural problems (Sedgley 1962; Scotter and Loveday, 1966; Loveday, 1976). Hov¡ever texture is the main deternl'inattt of hydraul ic conduct'ivity. For example, Ín a sandy 'in soiì, dispersed cìay may be washed ou'b by leaching result'ing an increase ìn hydraul'ic conductivity ìrrespectjve of electrolyte concentration. Hydrauì ic conductivìty is al so generally increased by coaguìation and cementation as discussed in th'is study. Cementation and coagulation resulted ìn significant negative linear correlat'ions between ESP and hydraulic conductiv'ity (F'ig" 49). These correlatìons also show that the improvement of soil Structure requires both coagulation and cementation. Coagulation is readjly assessed by determining dìspers'ible clay. Hydraulic conductivity ìs a sens'itive means of assessing cementation which creates large pores for rapid transmission of water. 8.5.4 Promotion of Coagu'lation and Cementation Both coagulation by gypsum and cementation by cement improve the physical conditìon of the soil. Therefore application of gypsum (0.2% j w/w) af ter cementat'ion (0 .4% cement) may coagul ate a I the d'i spersi bl e clay and the effects of the two treatments should be additive. Gypsum could also be applied with calcium carbonate to give an jmmed'iate effect on ESP and to maintain the necessary EC for a long period of time. In this st.udy, the carbonate treatment djd not g'ive a s'ign'ificant effect on the physical propert'ies of the soil due to low solubjl'ity. However the use of mixtures may not be successful due to a reduct,ion jn the rate of dissolution of gypsum caused by coatìngs of calcium carbonate (Keren and Kauschansky, lg8l). This coating may a'lso Y ¡9.39 - O.64X , r2.O.93' -T- 1 - -.. t \ (da{a wllfih dotted see) \ \l I F I L I I E 6 l¿ à .È l¿ t Ë Y-¡t98-O.22X, É.O82 o 5 .f¡ o I\ rú o ao å I oa I o I 4 oooo o AA \-- I AI ^ I A - ^a ^ A oo ^ A o 10 2 3 4 5 6 7 I 9 Exctungeable sodr.rn P€rcantagô Fig. 49 Effect Qf exchangeable sodium percentage on hydrau'lic conductivìty of gypsum, ( O ( gypsum; Ctr calcium carbonate and cement treated so'ils. ) control; O ) ! ( r. ) calciumcarbonate; ( t ) cement. 168 occur when irrigation water contain'ing an apprec'iable carbotlat.e ìon concentratjon 'is passed through a soil with gypsum particles. 0n the other hand, appìication of gypsum followed by ca'lcium carbonate may be successful by increas'ing the dissolut'ion rate of gypsum'initia'11y. A j 'in j deta.il ed descri pt'ion of the Ca-S0O-HC03-C03 i nteract ons sol ut on has been given by Nakayama (1969) and Robbjns et al.('1980). Cons'idering gypsum alone as an ameliorant, the efficiency of usage is 'important. H'igh rates of appl jcat'ion rvill lead to large losses by sol ut'ion and I each'ing . The resul ts show the I oss of gypsum (0 .2%) after 23 l,tlD cycles and the equ'ivalent of 44? mm rainfall which is approx'imately the annual rainfall jn areas used for cereaì productìon. Thus an annua'l appl icat'ion of this amount is necessary to maintain electrolyte concentration. Larger additions will lead to loss of gypsum with no beneficial effects. 8.5.5 Root Growth and Soil Structure Root lengths obtained at maturity of the wheat plants did not show sign'ificant effects due to treatnrents of the soil ìn spite of the fact that the soils treated with gypsum appeared to offer an ìmproved phys'icat env'ironment for root growth. A similar result was obtained in sojls to which iron cations were added to flocculate c'lays (5.3.3). There was a significant increase in root'length after three months growth and a s'ignìficant positive correlation between areal porosit'y and root'length in soils treated wjth'gypsum (fa¡le 47). However' no differences'in the lengths of roots were obtained after four and five months of plant growth when root lengths were decreasìng (between the soft dough stage and nraturjty). At thjs stage it seems that the clecompositjon of roots exceeds growth. This may have been assoc'iated wìth the WD cycles. The improved germìnation and root growth resu'lt'ing from the addjtion of gypsum jncreased the yield and nutrient uptake. i69 Table 47 Correlation of the Areal Porosìty Obtained by the Different Wettìng and Drying Cycles in Gypsunr and Cenrent Treatecl Soils with Root Lehgth of Wheat Plant Treatment Age of P'lant growth Regress'ion y = areal porosity(cmz) x = r"oot length (cm) Gypsum after 1 month y = 1 .72x 'r 0.76; 12= 0. 56 " 2 months ! = Z.QBx + 4.32; 12= 0.82 ,r3il y=1.96x+6.4I; 12= 0.79 "4rr y = 0.33x +l 2.07 ; 12= 0. 13 ,t5rl .y = 0.07x + 9.93; Y2= 0.01 Cement after 1 month y = 13.2.l - 1.72x; p2 0. 86 ' 2 months V=?2.36-2.32x; r2 0.67 il 3 Í y=18.41-1.06x; pz 0.64 ll 4 ! = 17.58 - 0.92x; ç2 0.45 il 5 J=13.4?-0.63x; Y2 0 .83 8.6 Conclusion Soil clay may be coagulated by lowering the ESP, ra'ising the electrolyte concentration or both. It is suggested that it is worth estabì'ishing a linear correlation between ESP/EC and dispers'ible clay which appears to be a good sìng'le quantitative parameter of the phys'icaì condition of the soil. Experiments with gypsum suggest that the most efficìent manner of using gypsum would be in small amounts added annual ìy to mai ntai n a cri ti cal coagu'lati on concentrat j on of el ectro'lyte. The application of calcium carbonate did not improve the soil physical condjtion although there were indications that a beneficìal effect would arise over a proìonged period. 170 Cement was shown to stab'il'ize aggregates up to 2 nlm diameter presumably by means of a calcjunì siì'icate gel. Various combinations 'lasti of the cal ci utx compounds of f er poss'ib'i I i ti es f or l ong ng structura'l improvements. t7r CHAPTER 9 GENERAL DISCUSSION 9.1 Introduction Under optimurn condit'ions for agricultural production, surface so'il aggregates should not slump to form a crust after a cycìe of drying and wetting. Qtherwjse physical parameters such aS water infiltration and retention w'ill be adversely affected. Crusting wi'l'l be the most severe if dispers'ion occurs at the surface as this gives rise to greater bulk densjties and strength when the so'il Ís dried compared with a flocculated soil (Sedg'ley, 1962; Coughlârì et a-i..'.l973)- Equally it is important to prevent excessive swel'l'ing of subsoil aggregates, otherwise the conduct'ing cracks become closed off, causing waterl oggi ng . Stud'ies on d'ispersion ancl swell ing are described in the I iterature and in this thesis. In partícular the relationships between dispersion and el ectrochem'ical and phys'icaì propertì es have been establ i shed i n the thesis. The importance of iron(III) and calcium compounds on the f I occul ati on , coaguì at'ion , aggregati on and p'l ant growth has been estabì i shed. 9.2 Flocculation and Coaqulation Flocculation and coagulation processes u/ere respons'ible for aggregation in Fe po]ycation and gypsum treated soils. L-a l'|er (1964) different'iated between the terms coagulation and flocculation, the former be'ing respons i bl e for destab j I i zati on of col I o'i dal d'i spers'ion by the reduct'ion of repuì s'ive potenti al and the I atter f or destabi I i zati on of the suspens'ion by a chemical bridg'ing mechani sm wh jch aggregates the partìcles'in a three-d'imensjonal floc network. Coagu'lation'is regarded as reversible, whjle flocculation is irreversible. D'ispersion was controlled by net charges present in the clay fractions (see sectìons t7? 4.4.I, 6.3.1 and 7.4.I). The results indicate eff icient flocculation of soil clay by very small amounts of Fe polycation, through the bridg'ing of the finer particìes by the chent'isorption of the polycations. The flocculation of dispersed soils was confirmed in electron micro- scopjc studies of the Fe polycat'ion and anion treated soils (see sections 4.4.3 and 6.3.4). Flocculation and d'ispers'ion were also controlled by anions in the po'lycation treated soils t^¡hich had net charges of zero and positive net charges (electrophoretic mobì'lìties). 0n the other hand electrolyte concentrations (electrical conductivity) determined the coagulation in gypsum treated soils (see section 8.4.1). Critical coagulation concentrations were determìned for both Fe poìy- cat'ion and gypsum treated soils. 9.3 Dispersible Cìay Dispers'ible clay was controlled by the net charges in the soil system. Manipuìation of the net charges on the A and B horizons of a Red-brown earth enabled the relations betleen djspersible clay and electrophoretic mobilÍties to be established. Clays were coaguìated at or near the point of zero charge and were dispersed when the net charge was either negative or positive. A net positive charge created by addÍtion of IFe(III)] polycations ra'ised the PZC while anions such as phosphate and fulvate created negative sites and lowered the PZC. Therefore, adjusting the PZC so that it is close to the pH of the soil is one means of coagulating. In spite of the undesirable character of dispersion of soils which are tilled jt'is irnportant from the point of view of so'il profile development. Dispersed c'lay particles are mobile and may be illuvìated to form a B horizon or lost from the profile in drainage waters. In Chapter 6, d'ispersible clay was related to physical properties. Dispersible clay may be dependent on the particle size distribution of 173 the clay fractìon (<2 unr). Chittleborough and 0ades (1980) showed that the'increase in fjne clay in the B horizon of Red-brown earths arose by ìlluvjation. In this thesis the relationship between fine cìay and dispers'ion was not studied. Different soils have different textures, but those soils with hìgh clay contents do not necessa.rily have much dispersib'le clay. Factors other than those elucidated 'in double ìayer theory, e.g. particle s'ize and shape need to be studied with respect to d'ispersible c'lay. The dispersible clay was also controlled by electrolyte concentrations and exchangeable sodium percentage (ESP). Gypsum, calcium carbonate and cement treatments gave a relation between EC, ESP and d'ispers'ible c'lay in a sodjc sojì (see section 8.5.1). For this particular soil, if dispers'ible clay is to be maintained at <3.5%' ESP/EC = 1.4 w'ith an EC of 250 uS which can be maintained by caìcium carbonate the ESP niust be 3.2. It is not possible to obta'in a unjversal relationship between the ESP/EC and djspersible clay because other factors are involved. 9.4 AqqreQation Iron(III) jn the form of polycations and calcium(ìi) in the form of gypsum coagu'lated the clay particles and created particles 50-250 pm d'iameter. 0n the other hand cementat'ion by the addition of cement stab- ilized particles 250-2000 pm diameter. From the work in this thesìs, it js concluded that flocculation and coagulation are involved in micro- aggregation and that cementation results in macroaggregation. In the presence of iron(III), floccu'lation is caused by small amounts of iron but larger quantit'ies of iron result in cementation and formation of hard pans (El-Swaify, l9B0). Th'is macroaggregatjon probab'ly develops by gradual "engu'lfing" of soiì partic'les by iron oxides (Flach et aJ.., 1969). In the presence of ca2+ and/or increased electrolyte concentration, coagu'lation is the on'ly process operative and leads to 174 mjcroaggregation. The precipitation of calcium compounds such as CaC{)r, Ca(0H), and CarSi0U creates cenlentation. Therefore to create both mjcro anC macroaggregat'ion in a soil there'is a poss'ibility of using different calcjum compounds to improve the aggregation. 9.5 Soil Structure In the past the phys'ical propertjes such as dispers'ion index (Loveday and Pyle, 1973), poros'ity and hydrauì jc conductiv'ity (Sedgley, 196?-; Loveday,1976) have been used to jndicate soil structural problems. These parameters have limitations as indicators of soil structural probìems (see section 8.5.3). It ìs suggested that dispersibìe clay could be a useful single measure of soil physical condition or structure as the amounts of d'ispersìble clay present appeared to control various physical and mechan'ical properties (see secti<¡n 7.4.6). The t^rork in thÍs thesis shows that the friab'ilit'ies of soils treated with Fe poly- cations and calcium compounds, except CaC0r' were improved. There is no quantitative infor^matìon on the friability of soils except that of Utomo and Dexter (l9Bl). In Chapter 6, a quantitative relationship was establ i shed between f riabi ì "ity and di spersi b'l e cl ay. The proposed classification of soils gives some idea of the relation of friability to amounts of dispersibìe clay. Thus dispersible clay offers a simple and time saving way to assess friability. However more work ìs needed'in d'ifferent so jl s to establ ish a general class'if ication. 9.6 Root Growth The addition of Fe poìycations d'id not give any s'ign'ificant dìfference in root lengths determined at maturity of wheat, but pìant yield was s'ignificantly relatecl to the treatments (see section 5.3.3). Therefore experìments lvere carried out with gypsum, calcium carbonate and cement to determine the reiationship between root growth and porosity at different Lt5 stages of plant growth (see section 8.5.6). After thr'ee months growth, û sìgn'ificant positive correlation between areal poros'ity and root length was obtained jn soils treated w'ith gypsum. After the soft dough stage (3 months) root lengths decreased probabìy due to a greater decornpos'ition than growth of roots. This may have been assocjated with the wetting and drying procedures but may be a general phenonenon and may be the reason for the lack of a signìficant relation between soil structure and root growth at maturity. So'il structure is not on'ly .importlant for root growth, but influences other factors involving water and nutrient movement to roots. 9.7 Possi bi I i tY of Use of Calc'ium Carbonate to ImP rove Soil Structure Calç1unl carbonate treatments did not give a sign'ificant effect on the phys'icaì propert'ies of the sod'ic soil clue to the low solubil ity of the carbonate (see section 8.4). However, in the literature some success has been reported in the use of calcium carbonate to improve aggregation and hydrauì'ic conduct'ivity. Very fìne (<45 Um) calciunt 'ity carbonate poraider was used to i ncrease the sol ubi I . Theref ore the particle size distrjbution is an important factor when calcium carconate is considered. 0n the other hand the cost'involved in obtainìng the very f ine calc.ium carbonate may e'limjnate economic use. Use of suìphuric ac'id in sodic calcareous soils has been successful j i n so'il amel i orat'ion (Overstreet et al ,1951 ; Chr stensen and Lyerly, t954), but handlìng and application difficulties may discourage the use of acid treatments. Growing plants in soils treated wjth calc'ium carbonate or calcareous so'il s may increase the solub'il ity of calcium carbonate. Secretions from the roots and mÍcrobial products may d'issolve the calc'ium carbonate and release Ca2+ into the soil. Long-term glasshouse or fìeld experiments may be needed to'assess this relatively sìow process. 176 Fig. 50 Model of dispersible clay and soiì structure on the basis of Fe poìycation and calcium compounds 'lreatntents (COLE = coefficient of linear extensibiìity; DI = dispersion index; HC = hydraulic conductivity; Alll = available water; MR = modulus of rupture; k = friability.) Soil Colloid ¡2: o-94 Soil claY ¡lluviatlon D¡s ¡2t o.97 shak DtA%,r2:1oo o.8e &%,¿:oe6 l: claY & Da,P: c.96 12: 0.86 Ê: o.e6 12: o.98 effects Electrolyte effects Electrical charge effects lnterstitial # c o lã Formstlon ol ca3(POa)2 Dostablli¿¿tlon Dèsleblllzatlon ol CaSlO3 ln6olublê of a colloldal ot the auspong¡on dispersion bY by productlon r€duction ot ol a not chargo thê rgpulslve ol zoro duo to potenlial dus to chemical br¡dging gxchangeoblo Ca m9c honlams and algctrolyte concentratlon Coatlng and bindlng M acroaggre ga t ion M ic r o a gg r e g a t io n + + CaCQ3 + Gypsum C€m€nt Gypaum +Comont GYPs CaCqo Organic matter caC% * Plants I ï I Coagulation Coagulat¡on Coagulatlon C.msntat¡on Coogulatlon + (Rapld, + Comantation Camontation (lmmsdiate + {Råold'prå".^ SpcculâllYe pfoYon, (R!Pld' (Slow, Prolongod) Trans¡ont) May bs parmanonl) sPoculatlvo' SpcculatrYô, MoY bo Pormsnont) P.olonged) L77 The acldition of calcium carbonate may be benefic'ial in stimulat'ing organic matter decomposìtìon (by raisìng the pH). This production of ac'ids during decompos'ition may dissolve further Ca2+ which may then be jnvolved in organic lnatter complexes. This is an aspect wh'ich requ'ires further study (see section 1.3.1.3'4)' 0n the basis of work described in thjs thesis, a model has been devjsed to show the relations between dispersible clay and soiì propert'ies and the necessity to etiminate dispersible clay to obtain good soil structure and p'lant growth (F'i9.50). It is clear that further fjeld and laboratory work is required to obtain quantjtative information and to validate the model. t7B APPENDIX Modification of Soiì Physical Properties by Manipulating the Net Surface Charge on Colloids through Addition of Fe(III) Poìycations Modification of Soiì Physical Properties by Addition of Fe(III) Po'lycations: Influence on Plant Growth Effect of Dispersible Clay on the Physical Properties of the B Horizon of a Red-brown Earth by R. T. Shanmuganathan and J. M. Oades Journal of Soil Science,l982,33, 451465 Modification of soil physical properties by manipulating the net surface charge on colloids through addition of Fe(tu) polycations R. T. SHANMUGANATHAN & J. M. OADES Department of Soil Science, Waite Agrícultural Research Institute, University of ,Adelaide, Glen Osmond, South Australia 5064 Summary The addition of 0.07 per cent Fe in the lorm of polycation of molecular weight l0 000-50 000 flocculated soil suspensions. Higher concentrations of Fe(rrr) caused redispersion oflthe clay. Electrophoretic and electron microscopic studies conflrmed the flocculation-dispersion phenomena. The soil suspensions with higher concen- trations ofFe(rrr) gave points ofzero charge (PZC) between pH values 5.0 and 6.0. The flocculation resulted in microaggregation and created pores 40-100 ¡rm in diameter. This led to an increased water-holding capacity and hydraulic conductivity and lower bulk densities and modulus of rupture. The soils treated with Fe(tll) polycations were shown to be more friable than untreated soils. Introduction In most soils structure is controlled by the presence of the colloidal fraction and more speciflcally by the surface properties ofthe colloidal fraction. The surlace reactions of the colloids control swelling and flocculation-dispersion phenomena which are ol prime importance in determining soil structure and its stability. In spite of this the relations between the surface properties of soil clays and various parameters used to assess physical and mechanical properties ofsoils are poorly defined. In this paper we describe an attempt to bridge the gap between colloid chemistry and soil mechanics by manipulating the net charge on the clay in a soil using Fe(rrr) polycations, and assessing the changes in some ofthe physical properties ofthe soil. Recent work on hydrolysis of Fe(rtt), in the absence of complexing anions, has shown that polycations with a limited range of sizes can be isolated. They are strongly sorbed on clay surfaces and cause irreversible flocculation of the clay (Rengasamy and Oades, 1977a, b; Kavanagh and Quirk, 1978). Polycations separated by ultrafiltration (molecular weight 10000-50000) were shown to be highly charged, roughly spherical with diameters ol approximately 3 nm, and probably related to ferrihydrite (Oades, 1982). Sorption of increasing amounts ol such polycations led to decreased negative electrophoretic mobilities of clays until flocculation occurred when the point of zero charge (PZC) was reached, followed by redispersion of some olthe clay with a net positive charge. 00224588/8210900-{45 I $02.00 O 1982 Blackwell Scientific Publications 452 R. T. SHANMUGANATHAN & J. M. OADES Malerials and Methods Soil and preparcttion qfpoly IFe(ttt)-OHJ cations A representative sample (0-l0cm) olthe Urrbrae ñne sandy loam, a Red-brown earth (Oades et al., l98l), was air-dried and sieved <2mm. Table I shows some properties of the soil. The minerals present in the clay lraction were kaolinite and illite, with smaller amounts of randomly interstratified clays. Table I So me soi I c haracter ist ics CIay (o/o) 19.4 Silt (o/o) 31.3 Fine sand (o/o) 43 .8 Coarse sand (o/o) 2.0 Plastic limit (o/o w/w) t6.7 Water-holding capacity (o/o w/w) 425 pH (l : 5, H2O) 5.4 Olganic matter (o/o) 3.5 Cation-exchange capacity (cgl)* 6.20 Exchangeable Na (C g ') 0.02 Exchangeable K (C g-l) 0.90 ExchangeableMg(Cg-l) 0.60 Exchangeable Ca (C g l) 3.1 Exchangeable Fe (C g- l) 0.03 Exchangeable Al (C g l) 0.12 *C : Coulom6 : 6.2x1018 electron charges (l C g l: I meq 100 g-l approx.) Fe(rrr) polycations were prepared by the addition of I nt sodium hydroxide to 0.1 vr ferric nitrate solution until the pH was 2.2. The 10000-50000 molecular weight fraction was separated by ultrafiltration using Amicon filters (Oades, 1982). The concentration ofFe(tu) in the polycation preparation was 350 Fg cm -3. Dispersed clay A suspension (3 per cent) of the soil in water was used to determine the quantity ol polycation required to flocculate the clay (critical coagulation concentratiou, CCC). Different amounts ol polycation were added to l0 cm3 of suspension which was adjusted to 20 cm3 by addition of distilled water in a 50 cm3 measuring cylinder, The suspensions were shaken thoroughly by hand lor 60 s. The amount of clay dispersed by this treatment was detelmined by measuling the optical density at 615 nm in I cm cel|s24 h after addition of pol¡rcation solutions. The optical density at 615 nm was calibrated against the percentage clay, isolated tiom the soil and determined gravimetrically. The optical density at 615 nm of the polycation solution alone was low (<0.04). Tre at me nt s o.[ s o il w it h p o lvc at io ns The soil samples were treated with six different amounts of polycations ranging above and below the CCC. The treated soil samples were air-dried in plastic pots for EFFECT OF Fe ON PHYSICAL PROPERTIES 453 7 weeks. The treatments added 0.00, 0.0 l, 0.04, 0.07, 0. l6 and 0.32 per cent Fe, on the basis of weight of soil. Each treatment was replicated lour times. E xami natio n of p o lycatio n- tre ated s oils Electrophoretic mobilities of soil suspensions were performed in 0.01 tu NaNO3 using a Rank Mk II particle microelectrophoresis apparatus using the procedure described by Kavanagh et al. (1976). Measurements were also made as a lunction of pH. Electric charges were determined at the o¡iginal pH ol the soil by washing with barium chloride. The retained Ba2+ and Cl were exchanged with KNO3 and determined using atomic absorption spectroscopy and an Orion Digital lonalyser (Model 8014) with chloride electrode (CI - Cat. No. 941 700). Soil suspensions (0.25 per cent) were examined in a JEOL JEM l00CX electron mlcroscope. Surlace areas were determined by adsorption of nitrogen at - 195'C using the equation ofBrunaer et al. (1938). Aggregate stability was determined by wet sieving (Kemper, 1965) alter direct immersion of air dried (<2 mm) soils in water. Size distributions of water-stable particles (< 50, < 20 and <2 pm) were determined by sedimentation under gravity. Hydraulic conductivity of core samples (30 mm diam. x 50 mm) was determined by the constant head method of Klute (1965). The soil samples in a glass tube were constrained by a nylon cloth. The samples 'were saturated by placing the cores of soil I cm below the water level overnight, the apparatus \ryas connected to a constant head of water and the outflow was collected for a known time when a constant rate of flow had been attained. Dry bulk density and porosity were measured separately for soils remoulded at different water contents. The samples of moulded soils were prepared in plastic cylinders. The oven-dry weight, air-filled volume and volume of the core samples were determined for each water content. Pore size distributions of the air-dry ( < 2 mm) soils impregnated with epoxy resin under vacuum were determined directly on 5-cm-diameter sections 35-40 pm thick. An image analysing computer (Quantimet 720) was used to analyse the total porosity and pore size distribution. The image displayed on the screen was x4 objective. Pores between 40 and 100 pm diameter were selected and five to ten pictures were analysed lor each replicate. Liquid and plastic limit and sticky points were determined following the methods described by Sowers (1965). The modulus ol rupture of lour artificial briquets lor each replicate was determined at different water contents on the apparatus described by Richards (1953). Remoulded soils were pressed in brass moulds (70x35x9 mm), the top was smoothed over and the briquets lelt to dry overnight, taken out from the brass moulds and dried at 105'C overnight. The lriability of the soils was determined by the method of Utomo and Dexter (1981). Soils were moulded with distilled water at about22 per cent water content and allowed to stand overnight. They were remoulded to make aggregates with diameters of about 5, 10, 15 and20 mm by hand rolling, and were aged at -50 kPa water potential for 2 weeks. The aggregates were then equilibrated at - l0 and -20 kPa using sintered glass funnels and at - 100, -200, - 1000 and - 1500 kPa using pressure plates. Tensile strength was measured by crushing l0 aggregates of 454 R. T. SHANMUGANATHAN & J. M. OADES each size for each treatment between parallel plates. Friability (k) was obtained from the relationship between log.S, the logarithm oftensile strength, and log. Z, the logarithm of aggregate volume. The intercept I is an extrapolated estimate of the logarithm of the tensile strength of I m3 samples of the bulk soil: log"S: -klog"V+A. Results Dispersed clay The concentration of Fe(trr) required to flocculate all the clay (dispersible clay:0) was 0.07 per cent by weight of soil (Fig. l). When the amounts of iron added exceeded the CCC, electrophoretic mobilities changed from negative to positive. As antici- pated, electrophoretic mobilities close to zero, i.e. a net charge of zero, occurred T Ì ô E 'õo E ì a a = ¡ Ë a t E à .9 Ê .ø * o tY t lM turl (%) added to ttE syslem ø æil basis Fig. l. Influence of poly [Fe(rulOH] cation on flocculation and dispersion (a-a) and electrophoreticmobility(n---A,negative; A---A,positive)ofsoilsuspensions. Table 2 pH values for poly [Fe(|tt)-OH] cation- treated soi I suspe ns i o n s Iron added (o/o of soil) pH 0.00 5.4 0.01 4.9 0.04 4.4 0.07 4.2 0.16 3.8 0.32 3.6 EFFECT OF Fe ON PHYSICAL PROPERTTES 455 f i .l ,E E f pH o E .9 o -1 ¡o o ü Fig. 2. Electrophoretic mobilities of soils as a lunction of pH belore and alter treatment with poly [Fe(rrrloH] carions. (o) conrror; (A) 0.01 per cent Fe; (!) 0.04 per cenr Fe; (e) 0.06 per cent Fe; (l) 0.08 per cenr Fe; (A) 0. I 6 per cenr Fe; (r) 0.32 per cenr Fe. Table 3 Changes in electric charges of soil by addition of poty [Fe(rr\OH] calrcns Iron added Positive charge Negative charge .^_t, (o/o ofsoil) (LR') (C e-t) 0.00 0. l8 6.2 0.01 0.20 5.8 0.04 0.2t 4.8 0.07 0.t4 3.8 0.16 0.29 4.3 0.32 0. l9 3.9 where most of the clay was flocculated. The pH of untreated suspensions was lowered from 5.4 to 3.6 as polycations were added (Table 2). However, net positive charges were maintained when the pH values were raised to above 5 (Fig. 2). Electric charges ilities ofsuspended colloids as a function ofpH show that charge en more than 0.06 per cent Fe was added to the soil (Fig. 2). The were raised to between pH 5 and 6 by the three largest additions 456 R, T. SHANMUGANATHAN & J. M. OADES 7 ! , a ) I Ë -i lrrn' Fig. 3. Transmission electron micrograph of clay fraction. (a) Untreated soil, (b) poly (0.32 [Fé(¡r)-OH] cation-treated soil (0.07 per cent Fe), (c) poly [Fe(rlrFOH] cation-treated soit per cent Fe). 45g R. T. SHANMUGANATHAN & J' M. OADES Table 4 Changes in the butk water content (0/o w/w) "^":/fo:i",j/,;;!i,:3';;i,'::,y;l,i::!,i::,:,(''r Iron added (o/o ofsoil) 0.00 MWC 8.6 r0.3 14.8 t8.2 t9.7 Bulk density 1.55 1.54 1.68 r.60 |.57 Porosity 38.3 38.7 32.8 36.0 37.0 0.01 MWC 8.4 lt.7 t4,'l 18.8 20.s Bulk density 1.54 l.5l t.69 t.62 l.58 Porosity 38.9 39.6 32.6 35.2 36.8 0.04 MWC 9.8 12.8 t4.5 2t.l 21.8 Bulk density 1.52 1.50* 1.6 l* r.59 1.56 Porosity 39.4 40.0 3 5.61 J6.4 3 7.8 0.07 MWC 1.2 l3.5 16.7 22.8 23.0 Bulk density l.5l l.45+ r.48+ 1.59 ]l54 Porosity 39.9 42.0+ 40.8t 36.5 3 8.4 0. l6 MWC r0.6 l3.l t 5.2 23.4 24.t Bulk density t.52 t.4s+ t.42+ 1.60 1.56 Porosity 39.5 4l .8t 43.4+ 3 6.0 37.6 0.32 MWC 10.3 t4.4 t5.4 23.0 23.4 Bulk density 1.50 t.441 t 42t. 1.59 1.56 Porosity 40.0 42.21 43.2+ 36.2 3'7.7 * P<0.05. fP<0.01. t P<0.001. MWC: moulding water content. (< .07,0. 16 and 0'32 Per cent Fe. d formation of pores (40-100 ¡rm) increased the water- ho draulic conductivity significantly (Table 6)' The water ret soils increased with amount of polycation added' The hydraulic conductivity was increased by a täctor of 3 over the control confirming the increase in numbers oltransmissive pores. Soil consislencv The liquid and plastic limit decreased (in gravimetric water content) significantly with amounts of Fe(rrr) polycations which caused flocculation (Table 7), but the sticky point showed no significant difference due to treatments' EFFECT OF Fe ON PHYSTCAL PROPERTTES 459 Table 5 Efect of adding poly [Fe(ttt)-oH] cations on the porosity and pore size distribution of air-dried soils (total evaluated area was 1964 mm2) Percenlage oftotal area ofpores Iron added Total area ofpores (o/o of soil) (^*2) <40 pm 4040 ¡tm 60-100 pm 0.00 7.1 t00 0.01 5.5 100 0.04 9.2 100 0.07 27.3 27.5 26.4 46.t 0. l6 32.4 16.6 44.6 38.8 0.32 2s.4 49.6 50.4 Table 6 InJluence of poly [Fe(ttt)-OHJ cations on the water-holding capacily and hydraulic conductivity ofsoil Mo¡sture content Maximumwater- Saturated hydraulic Iron added ofair-dried soils holding capacity conductivity (o/o of soil) /o/o w/w) (o/o w/w) km h-t) 0.00 1.3 42.5 0.l3 0.01 1.5 44.1 0.17 0.04 2.2 44.8 0.22 0.07 2.1 50.7 0.3 5 0. l6 2.1 49.2 0.36 0.32 2.0 49.4 0.34 LSD P<0.05 2.3 0.04 P<0.01 3.1 0.05 Table 7 Influence olpoly IFe(ttt)-OHJ cations on consistency paramelers Iron added Liquid limit Plastic limit Sticky point (o/o of soil) (k w/w) (ok w/w) (/o w/w) 0.00 23.1 16.7 18.2 0.01 23.1 I ó.3 17.8 o.ó¿ 23.0 t6.2 17.6 o.6t 22.4 15.5 t7.5 0. l6 2t.2 15.0 17.6 0.32 , 22.2 16.2 t'7.5 LSD P<0.05 0.7 0.7 n.s. P<0.01 0.9 0.9 n.s.:Not signifrcant. 460 R T. SHANMUGANATHAN & J. M. OADES 80 70 60 î ¡À IE 5 *40o I¡ B30 E 20 10 MouHing water colent of soil (% Vw I Fig. 5. Effect of moulding water content on the modulus of rupture ol artificial briquets by addition of poly [Fe(r rrlOH] cations (curves ñtted by eye). (O) Control; (A) 0.0 I .per cent Fe; (!) 0.04 per cent Fe; (O) 0.07 per cent Fe; (A) 0. I 6 per cent Fe; (r) 0.32 per cent Fe. Modulus ofrupture Soils with maximum bulk density (moulded water contents near the plastic limit, l7 per cent w/w) yielded maximum values for the modulus of rupture of briquets moulded at similar water contents (Fig. 5). The force required to rupture the soils varied with water content at which the samples were prepared and the amount of polycation added. Flocculation reduced the modulus ofrupture by a factor of3 over the control. The modulus of rupture (y) was related linearly to the bulk density (x) by the equation y:250x-350; 12:9.77, which is based in all the data on Fig. 5 and Table 4. Soilfriability Soil friabilities of the polycation-treated soils were obtained at different water contents (Fie. 6). The friability (k) increased with the amount of polycation added at all the water contents. The highest values were obtained when the \ryater content was near the plastic limit. At the same time, small k values were obtained when lower (6-9 per cent w/w) and higher (21-24 per cent w/w) water contents were used. Discussion Surfac e c har ge s and fl occ u I at i o n The results indicate effrcient flocculation ol soil clay by very small amounts of polycations, presumably through the bridging of the finer particles by the chemisorp- tion ofthe polycations. The presence ofa net positive charge caused the redispersion of some clay after addition of 0. l6 and 0.32 per cent Fe. The treatments also reduced the pH of the samples, which in part explains the net positive charges. However, the 462 R. T. SHANMUGANATHAN & J. M. OADES Fe(rrrfmontmorillonite complex which had a positive electrophoretic mobility at discrepancies between net ed bY procedures. Alternatively, t of the by addition ofa polycation Per ce uniform, leaving significant atively flocculated and did not disperse in water. Accumulation of organic matter near the surlace of the soil led to a lower PZC and some water-dispersible clay. At depth in the proflle he pH of the soil and again water-dispersible clay was eviden as a colloid with net positive charge. Acid washing of the highly ød the PZC thus implicating aluminium and iron oxides as the main source of positive charges. Our results show clearly the relation of water-dispersible clay to the PZC and that the PZC can be manipulated using Fe(rrr) polycations. A similar result would be expected , the influence of polycations in soils is not likely to properties are likely to be progressively modified ularly phosphate and organic anions' results in terms of crumbling the soil. Obviously, porosity was inversely related to presumably within the floccules or microaggregates. There was an increase of 20 per cent in maximum water-holding capacity (WHC), i.e. pores up to about 60 pm diameter. There was almost a three-fold increase in saturated hydraulic conductivity which is related to pores greater than about 50 ¡rm diameter' Studies using the euantimet confirmed a three- to four-fold increase in the cross-sectional area of pores, particularly those with diameters lrom 40 to 100 pm, i.e. transmission pores, which explains the marked increases in hydraulic conductivities. The hydrautic EFFECT oF Fe ON pHyStCAL PROPERTTES 463 conductivities quoted, 3.5 mm h-1, are still very low compared with those obtained elsewhere (El Swaify, 1980), e.g. 50-90 mm h - I. However, oxisols generally contain more iron in the lorm of oxides than the present system, even with the highest addition of iron. So i I mec hanical p rop e r t ie s Maximum compaction (high bulk densities) occurred when the water content at moulding was between l5 and l8 per cent w/w, which is near the plastic limit. Mitchell ( I 960), Lutz et al. (1966) and Yeoh and oades ( 198 I ) also found thar high bulk densities were created by moulding the soil at water contents near the plastic limit. Presumably at this water content, shearing forces cause particle reorganization, increasing contact between particles and reducing porosity. At lower water contents during moulding there was insuffrcient water present to carry clay particles into voids between bigger particles, resulting in lower bulk densities and strength. At water contents above the plastic limit the presence of water in pores prevented compaction of the soil, again resulting in lower bulk densities and strength. The high modulus of rupture of all the samples moulded with water contents near the plastic limit showed that maximum compaction caused high bulk density and strength. The modulus ol rupture was a linear function olbulk density. Except for very wet and very dry soils (< lo to >24 per cent w/w), the moulding of soils in which the clay was flocculated by polycations led to the least compaction, i.e. the lowest bulk densities and low values for the modulus of rupture. The flocculation of fine clay caused a reduction in areas of contact between floccules or microaggregates-hence, a higher porosity and reduced bulk strength of the soil. If an aggregate is made up of smaller aggregates it will have a high porosity and be mechanically wea,k. This property is desirable for a tilled layer of soil, providing the aggregates are water-stable, and to some extent is expressed in friability. The Table 8 Efect ofpercentage ofsoil particles 50-250 pm diameter on the physica! properties ofsoils flocculated by addition of poly IFe(rrr)-OH] cations Physical properties Linear regressions Maximum water holding capacity y :22.43 + 0.53x ,2:0.87 (o/o w/w) Hydraulic conductivitv y:0.01x-0.43 ,2 --0.g5 (cm h- t) Liquid limit y:25.15-0.05¡ 12:0.80 (o/o w1w) Plastic limit -v:19.89-0.05x 12:0.84 (o/o w/w) Modulus olrupture y: 191.6 3.0Ix ,2 --0.76 (kPa) - Friability y:0.01x 0. 16 12 :0.8 I (k\ - y: physical property, x: percentage 50-250 pm diameter particles. 464 R. T. SHANMUGANATHAN & J. M. OADES friability (k) increased with flocculation due to a reduction offorce required to crush the aggregates. Phosphoric acid treatment ofthe Urrbrae soil also reduced aggregate strength (Yeoh and Oades, l98l) and increased the lriability (Utomo and Dexter, 198 I ). Dry soils are strong and friability or crumbliness is maximal at water contents near the plastic limit. This is the water content at which the soil should be tilled to give maximum lormation of smaller aggregates. Linear regressions were obtained relating various physical properties of the polycation-treated soils to the percentage ofparticles 50-250 pm diameter (Table 8). With increasing proportions of particles 50-250 pm diameter the WHC, hydraulic conductivity and friability increased. This resulted from the creation of pores 40-100 pm diameter and low-density aggregates. At the same time, the liquid and plastic limits and the modulus of rupture were decreased due to the reduction in areas ol contact between particles 50-250 ¡rm diameter compared with areas of contact between a range of particle sizes includirlg clay. The reduction in liquid and plastic limits caused by addition of polycations must be due to the change in interparticle lorces brought about by changed surface properties by sorption ol polycations. Low liquid limits exhibited by Oxisols have been attributed to sesquioxides (El Swaify and Henderson,1967). The influence of the change in physicat properties caused by addition of Fe(rlt) polycation on the growth ofplants is being investigated. Conclusions The physical properties of the soil studied were correlated with the net electric charge on the clay lraction. When the ZPC was near to the pH olthe soil the clay fraction was flocculated, resulting in more water-stable aggregates 50-250 ¡.1m, increased porosity (particularly in pores 40-100 pm diameter), lower soil strength and increased friability. The flocculation ofthe clay responsible for the changes in physical properties was brought about by sorption of0.07 per cent iron in the lorm ofpolycations. Acknowledgemenls R.T.S. is gratelul lor the award of a Colombo Plan Scholarship. The work was supported by the Australian Research Grants Committee. Specific surlace areas and electron microscopy were done by Mr T. W. Sherwin. We thank Dr L. R. Jarvis, Department of Histopathology, Flinders Medical Centre, for use ol the Quantimet 720. REFERENCES Bnun¡.uen, S., Etøt'.terr, P.H. and TrlLsn, E. 1938. Adsorption olgases in multimolecular layers. Jottrnal ofAmerican L'hemical S'ocie1],60, 309-1 10. Er Swnrnv, S.A. 1980. Physical and mechanical properties oloxisols. ln Soils with Variable Charge (ed. B. K. G. Theng), pp. 303-324. Lo\üer Hutt (New Zealand): Soil Bureau Department of Scientifr c and Industrial Research. El Swnrrv, S.A. and HglrorRsoN, D.W. 1967. Water retention by osmotic swelling of certain colloidal clays with varying ionic composition. Journal ofSoil Science 18,223-232. Grr-r-unN, G.P. 1974. The influence of net charge on water dispersible clay and sorbed sulphate. Attstralian Journal of Soil Research 12, 17 3-lT 6. EFFECT OF Fe ON PHYSTCAL PROPERTIES 465 Gtt-Ltr.te,N, G.P. and Bell, L.C. 1976. Surlace charge characteristics of six weathered soils from tropical north Queensland. Auslralian Journal ofsoil Research 14,3 5 l-360. Hennrnn, R. and Pencs, M. 1970. Reaction of montmorillonite with iron (rrr). Soil Science Society of America Proceedings 34,740-742. K,tvnNrcu, 8.V., PosNen, A.M. and Qurnr, J.P. 1976. Effect of polymer adsorption on the properties of the electrical double layer. In Adsorplion in Condensed Phases, Faraday Discussion of the Chemical Society Vol. 59,pp.242-249. K¡,v¡,rncs, B.V. and Qurnr, J.M. 1978. The adsorption of polycationic Fe(ru) on Na-Illite. Geoderma2l,225-238. K¡tr,Ipen, W.D. 1965. Aggregate stability. In Methods of Soíl Analysis (ed. C. A. Black et al.), pp. 5 I l-5 19. Madison, Wisconsin: American Society of Agronomy. Kruro, A. 1965. Laboratory measurement of hydraulic conductivity ol saturated soils. In Methods of Soil Analysrs (ed. C. A. Black et al.), pp.2lO-221 . Madison, Wisconsin: American Society of Agronomy. Lurz, J.M., PrNro, R.4., Gnncl¡-L¡cos, R. and Hllro¡¡, H.G. 1966. Effect of phosphorus on some physical properties ol soils. II. Water retention. Soil Science Society of America Proceedings 30, 433437. MtrcuuLL, J.K. 1960. Fundamental aspects of thixotropy in soils. Journal of Soil Mechanics Foundation Division ASCE 8ó, SM3 Proceedings Paper2522,pp. 19-52. O¡,¡ss, J.M. 1982. Flocculation and dispersion of aluminosilicate colloids by sorption of poly [Fe(tttlOH] cations. Clays and Clay Minerals (in press). Oe.ors, J.M., L¡wls, D.G. and Nonnlss, K. 1981. (eds) Red-Brown Earths of Australia. Adelaide: Waite Agricultural Research Institute, University of Adelaide and Division of Soils CSIRO. ReNcrsnuv, P. and O¡,o¡s, J.M. 1917a. Interaction of monomeric and polymeric species of metal ions with clay surfaces. I. Adsorption of iron (rrr) species. Australian Journal of Soil Research 15,221¿23 . RsNc¡.s¡tr,ry, P. and Ono¡s, J.M. 1977b. Interaction olmonomeric and polymeric species of metal ions with clay surfaces. IL Changes in surface properties olclay after addition oliron (nr). Australian Journal ofSoil Research 15,235-242. RtcH.nnos, L.A. 1953. Modulus of rupture as an index of crusting of soil. Sol/Science Society of A me r ica Pr oce edi n gs L7, 32 1 -323. Sowens, G.F. 1965. Consistency. ln Methods of Soil Analysrs (ed. C. A. Black et al.), pp. 391-399. Madison, Wisconsin: American Society of Agronomy. Suuler., M.E. l9ó3. Effect of iron oxides on positive and negative charges in clays and soils. C lay M ine rals Bulletin 5, 2 18¿26. Urotrao, W.H. and Dexrrn, A.R. l98l. Soil friability. Journal of Soil Science32,203¿13. YEos, N.S. and OnoEs, J.M. 198 I . Properties of clays and soils after acid treatment. II. Urrbrae ñne sandy loam. Austalian Journal ofsoil Research 19, I 59-l 66. (Received 4 September I 98 I) Journal of Soil Science, 1982, 33, 639-647 Modification of soil physical properties by addition of Fe (III) polycations: influence on plant growth R. T. SHANMUGANATHAN & J. M. OADES Department of Soil Science, Waite Agricultural Research Institute, University of Adelaide, Glen Osmond 5064, South Australia Summary The addition ol 0.07 per cent Fe or more in the lorm of polycations decreased the total area ol cracks and increased the number of transmissive pores of soils after simulated rainlall and drying. This resulted in increased per cent emergence and lower mean day of emergence of wheat plants. Per cent emergence (y) was negatively correlated with penetrometer resistance (x). The increased germination was lollowed by greater pìant growth, including increased plant height and yield. The Fe poìycation treatments had no significant effect on root length measured at harvest. Introduclion In an earlier paper (Shanmuganathan and oades, 1982) it was shown that treatment of a soil with polycations of Fe(lII) resulted in flocculation of clay, the development of microaggregates and transmissive pores. The treated soil had greater water holding capacity and hydraulic conductivity and lower bulk density and modulus ol rupturè and was more friable. It was assumed that these changes in physical properties were beneficial with respect to plant growth. This paper describes the influence oladdition of polycations of iron to soil on crack formation, crusting, seedling emergence and plant growth. Materials and Methods A Red-brown earth, the urrbrae fine sandy loam (0-l0cm) was treated with polycations of Fe(llf as described earlier (Shanmuganathan and oades, l9g2). The soil samples were treated with four different amounts of polycation:0.00,0.01 per cent Fe,0.07 per cent Fe and 0.16 per cent Fe on the basis of weight of soil. The treated soil samples were air dried in plastic pots for 7 weeks before crack formation, seedling emergence and plant growth were determined. Each treatment was replicated four times. Rainfall simulation The untreated and treated soils were placed in plastic trays (45 x 30 x 7 cm). Simulated rainfall at the rate of 27 mm h-l was applied using a rainfall simulator 0022-4588/82/ 1200-0639 $02.00 @ 1982 Blackwell Scientiûc publications 640 R. r. SHANMUGANATHAN &J. M. OADES mounted on a two-wheeled trailer (Grierson and Oades, 1911). Each tray was placed on the ground directly under the ¡ozzle to obtain uniform rainlall distribution. Alter rainfall simulation, the soil samples were air-dried. The rainfall simulation (wetting) and air-drying (drying) were repeated three times. The trays were arranged in randomized design. Crack pattern One week alter the rainlall simulation and drying the crack formations in the trays were photographed. The cracks on the phoiographs were measuied by using a digitizer cursor. The observed different crack widths were marked in the photographs and the crack lengths lor each width were traced to obtain perimeter and area. The tablet (Houston Instrument, Austin, 28 x 28 cm, a vi nal and Andromeda with twin floppy rdinate information ompute the area features traced with the digitizer cursor. The data were displayed on the screen and stored on disc for later print-out. A total number ol seven classes of crack widths with 0.4 mm as class interval was selected for crack pattern measurement. The total area of the cracks was also deter-mined. The method of tracing is described in the instruction manual for this instrument. The equation used was w : 2A/L where w is the width of the crack and I is the perimeter area lor length L. This instrLlment was developed for the measure- ment of crack area and width by Dr L. R. Jarvis, Flinders Medical Centre' A ggre gate s tab ility measure me nl Air-dried soil (20.0 g) after rainfall simulation was wet sieved according to the method olKemper (1965). The material which had passed through the sieves was resuspended in 4 litres of water by shaking the cylinder gently end-over-end lour times, and the size distribution of water-stable particles was determined by sedimentation under gravity. The particle size distribution was measured on three subsamples from each replicate. Pore size distribution The uudisturbed soils after rainfall simulation were impregnated with epoxy resin under vacuum, and 5 crn diameter sections 35-40 ¡rm thick were cut with a diamond saw. An image analysing computer (the Quantimet 720) was ttsecì to analyse the total areal porosity and the areal pore size distribution. The displayed image on TV was 4X objective. Pores lrom 40 to 100 ¡rm diameter we¡e selected and five to ten pictures were analysed for each sample. Pe ne tro me tet' res i s lance The crusts formed by rainlall simulation were used to determine the penetrometer resistance. They were wetted to saturation by capillary action and then dried to l0 kPa water potential. The resistance vr'as measured with a lnotor-driven laboratory penetrometer. The probe had a diameter of 1.008 mm, and penetrated downward at a rate of 3 rnm min l. The force required to penetrate the crust was SOIL PHYSICAL PROPERTIES AND PLANT GRO\MTH 64I measured with an electronic balance (Mettler type PC4400). Three measurements were made on each crust. The strength Ìvas calculated as the resistance to probe penetration, 0p, using the equation QP: 4F/ndz where .F is the force required to penetrate the sample at a depth of 5 mm, and d is the probe diameter. Germination studies The soils which had been air-dried in plastic pots were crushed with a rubber hammer until all particles were less than 5 mm and then transferred to plastic trays (45 x 30 x 7 cm) which were arranged in a randomized design in the glasshouse. Wheat seeds (Variety, Warigal) were so\ryn (54 seeds/replicate) on a 5 cm grid pattern 2 cm below the surface of the soil and then covered. Rainfall simulation and drying was repeated three times till emergence of coleoptiles. The coleoptiles which emerged were counted daily over the entire period of emergence. This yielded results on the time interval between sowing and emergence and the percentage emergence. The mean day of emergence as defined by Edwards (1957) was used to determine the time period between sowing and emergence. The seed used was tested for germination capacrty. Plant growlh studies Plant growth studies were conducted in the glasshouse using plastic pots (20 cm deep and 23 cm diameter). The pots containing air-dried treated and untreated soils were arranged in a randomized design. Three weeks after germination, four selected seedlings were transplanted in soits with respective treatments from the trays. The positions of the pots were changed within and between the treatments tilroughout the experiment. Nutrient solutions were added to each pot in the form of ammonium nitrate, ammonium dihydrogen phosphate and potassium sulphate to supply nitrogen (148 mg), phosphorus (144 mg) and potassium (l l2 mg). The nutrients were added to each pot at monthly intervals for 4 months. Plant heights were recorded each week throughout the growing season. Tiller counts were also recorded for these plants. The plants that were in the pots were harvested with a sickle 2.5 cm from the surface of the soil. The total number of tillers per pot was recorded. The total weights of straw and grain from each pot were determined. The heads of the sampled wheat plants were separated from the straw and the grain separated from the chaff with a mini-thresher. Grain yield per pot was noted. The grain (20 g) was weighed into pre- weighed silica basins and dried at 105"C in an oven overnight. The moisture content was calculated as a percentage of oven dry weight. Tillers were dried at 70"C until brown and brittle. The dry straw passed through a I mm sieve in a grinding milt. Ro o t le n gt h me asure ment s The soil samples were air-dried and ground in a mill using a coarse screen of 4 mm diameter mesh. Each ground sample was thoroughly mixed before subsampling for 642 R. T. SHANMUGANATHAN &J. M. OADES root separation. Roots were separated from the ground soil (l0g) by a flotation method similar to that olBarley (1955). Three subsamples were taken from each replicate. Each sample was stirred rapidly in 500 cm3 water lor 30 s before decanting the water onto a 250 F"m aperture sieve. Further water was added to the sediment, allowing the jet olwater to stir the suspension. The water was again decanted. This was repeated until no further roots could be seen in the sediment. The root segments were transferred from the sieve to a beaker with a fine jet of water, then onto a filter paper held on a porous plate under suction. The flow was confined within a 5 x 5 cm perspex retainer placed on the frlter paper. The perspex container was coated evenly with wax to prevent rccts being held at the edge by surlace tension. While still wet the filter papers were sprayed with a solution of 30 per cent polyvinyl acetate (emulsion) in water which fixed the roots in position on drying. The length of root in each sample was determined (Newman, 1966; Tennant, 1975). Nitrogen was determined in the straw by the Kjeldahl method using the alkaline phenate procedure (Williams and Twine, 1967), and a Technicon AutoAnalyser. Phosphonrs in plant material was determined alter dry ashing with 50 per cent magnesium nitrate solution (Chapman and Pratt, 196 l). lg of dried ground plant material was mixed with 3 cm3 of 50 per cent magnesium nitrate solution and evaporated as nearly to dryness as possible on a steam bath. The dried sample was ignited in a muffle lurnace until the ash was white. The dry ashed material was digested with concentrated nitric acid and perchloric acids belore determination of phosphorus, using the method described by Hanson (1950) which involves an am moni um molybdate-ammoni u m vanadate reagent. Results and Discussion Crackþrmation Rainlall simulation and drying caused crack lormation in the soils. The impact ol simulated rainlall lormed crusts in the soil. The untreated and 0.0 I per cent Fe treated soils showed a pattern ofcracks, but 0.07 per cent Fe and 0.16 per cent Fe treated showed none or very few cracks. Quantitatively cracks were reduced by a factor of3-4 over the control in the 0.07 per cent and 0.16 per cent Fe treated soils (Table l). The crack pattern (Fig. l) showed the disappearance olwider cracks in Table I Eflèct of poly IFe(lll)-OH] c'ations on the areu uf crucks of the :;oil samples after simulated rainfall and drying; total area examined was 1350 cm2 Iron added Tolal erea of c'racks (ok) (rm2 ) 0.00 48.4 + 1.2 0.0 r 42.5 + 0.9 0.07 13.5 + 0.4 0. r6 16.7 + 0.7 SOIL PHYSICAL PROPERTIES AND PLANT GROWTH 643 A B 15 llo IJ ñ u o l! o) l! 3 o o c D o o c') zÉ a)o o o- Mean width of cracks (mm) Fig. l. Effect oipoty [Fe(lll)-OH] cations on the crack pattern after simulated rainlall and drying ofthesoil:(A)control; (B)0.01percentFe; (C)0.07percentFe; (D)0,l6percentFe. 0.07 per cent Fe treated soils. At the same time 0. l6 per cent Fe treated soils did not give all the classes olwidth in the crack pattern compared with untreated 0.0 I per cent Fe treated soils. This may be one of the reasons for the reduction of the total area ofthe cracks in 0.07 per cent Fe and 0. l6 per cent Fe treated soils. The reduction of total crack area was presumably due to the decrease in dispersible clay (Table 2) and increase in particles 50-250 ¡rm diameter in the soil with 0.07 per cent Fe added. This resulted in increased porosity (Table 3). The porosity in pores up to l00pm diameter in clods of the cracked soil increased by a factor ol3 over the untreated soil. The pore size distribution showed the lormation of larger pores (40-100¡rm) and reduction of smaller pores (<40¡rm) by addition of 0.07 per cent Fe and 0.16 per cent Fe. Dispersed clay is responsible for swelling and shrinkage and in the present study, the reduction in dispersible clay and creation of 644 R. T. SHANMUGANATHAN &J. M. OADES Table 2 E"fect of poly [Fe(III)-OH] cations on the size distribution of water stable particles (0/o of total soil) in soil alter simulated rainfall and drying Size ofparticles Iron added (o/o qfsoil) (pm) 000 0.01 007 0.16 <2 t4.3 0.2*** 2.0r.** 2¿0 8.9 | 3.5*** 12.7*** 14.5** 20-s0 28.0 28.6 20.8*** 27.4 50-250 36.5 40.4 52.0*** 4l .2** 2 50-500 4.7 4.8 6.t 6.2 500- I 000 4.t 4.2 4.2 5.5 1000-2000 3.5 4.0 4.0 3.2 ***P 40.001;** P <0.01. Table 3 Influence of poly [Fe(lII)-OH] caLions on the porosity and pore size distribution of clods olthe cracked soil samples; lotal area examinecl was 1350 cm2 Percentage Iron added Tolal area ofpores oftotal area ofpores (o/o cf soiA ¡cm2 ) * q40 pm 40-60 ¡rm 60-100 pm 0.00 5.0 + 0.2 100 0.01 5.0 + 0.3 100 0.07 l6.l + 0.3 23.8 2t.2 5 5.0 0. l6 15.7 + 0.4 24.6 22.0 53.4 *Percentage of pores <40¡rm was obtained by subtracting the percentage of 40-100¡rm pores lrom 100. The lower limit of detection olpores is probably about 30¡rm which is the thickness olthe thin sections. Table 4 Seedling emergence o.[ wheat (ttar Warigal) and strenglh of poly [Fe(III)-OH] cation treated soils Peneîromeler Iron added resislance (ok of soil) (kPa) Per cent emergence* Mean day ol emergence 0.00 945+2.8 57 I 9.3 0.0 t 90.4 + 3.1 60 r 9.3 0.07 78,t + 3.2 80 t6.4 0.16 79.5 + 2.9 68 t7.l L.S.D. (P <0.0s) 10.9 8.7 1.5 (P <0.01) 15.6 12.4 2.1 * Germination capacity of Warigal seed was 96 per cent. SOIL PHYSICAL PROPERTIES AND PLANT GROWTH 645 larger pores reduced the area of cracks by a factor of 3 over the control. In most environments surface seals dry out rapidly after wetting process ceases and they tend to shrink and crack to an extent depending upon the clay content (Lutz,1952). Seedling emergence Penetrometer resistance studies showed a decrease in strength in 0.07 per cent Fe and 0.16 per cent Fe treated soils. The strength was reduced significantly at -l0kPa water potential (Table 4). Several authors (Gupta and Yadav, 1978; Chaudri and Das, 1978) have reported a negative linear relationship between crusting strength and seedling emergence. The coleoptiles which emerged were counted daily over the entire period ol emergence and the polycation treatment (0.07 per cent Fe) gave a highly significant increase in emergence of 20 per cent over the control (Table 4). It did not reach 96 per cent presumably due to other conditions of the soil. The mean day of emergence decreased from 19 to 16 in 0.07 per cent Fe treated soil. The reduction of mean day of emergence is important in establishing seedlings quickly. The percentage emergence þ¡) was negatively correlated with the strength (x) of the surface layers as calculated from the data obtained using the penetrometer, and mean day of emergence (y2) was correlated with the strength (x) by the equations yt:165.18 - l.l6x; 12:0.82 and, y2:0.17x + 3.64; 12 :0.96 which are based on the data in Table 4. These correlations show the importance of the low crust strength for germination in this hard setting Red-brown earth. Plant growth The seedlings were transplanted in pots 3 weeks after the seeds were sown. The plants were harvested 6 months later. In soils with added polycations (0.07 per cent Fe and 0.16 per cent Fe) the total yield was increased signif,cantly (Table 5). The number of tillers and grain yields also showed significant increases from plants grown in polycation treated soils. The numbers of tillers and therefore ears were less in untreated and 0.01 per cent Fe treated soils than 0.07 per cent Çe and 0.16 per cent Fe treated soils (Table 5). Table 5 Influence efpoly [Fe(III)-OH] cations in the soil on rhe yield of wheat Grain yield Iron added No. of tillers Tolal dry matter (ok of soil) per seedling k/pot) g/pot o/o tolal mat. 0.00 46 1t.l 21.3 30. r 0.0 r 42 69.6 r 8.6 26.4 0.07 64 8 5.0 28.5 33.9 0. l6 66 8 5.7 32.t 37.4 L.S.D. (P <0.05) l6 13.6 5.7 646 R. T. SHANMUGANATHAN &J. M, OADES Table 6 Moisture, nilrogen and phosphorus content in the plant tissue after harvesting Mo¡slure content Iron added of grain Nitrogen Phosphorus (o/o of soil) (o/o) (o/o by weight) (o/o by weight) 0.00 lt.2 0.52 0.026 0.01 l.2 0.50 0.026 0.07 r 1.6 0.69 0.04 r 0.16 n.7 0.62 0.040 L.S.D. (P <0.05) 0.2 0.027 0.0t22 Table 7 Root length of wheat planls after 6 months of growth in poly [Fe(lll)-OH] cqtion lrealed soils Iron added Root length (o/o ofsoi| (cm g ') 0.0 9.2 + 0.2 0.01 8.9 + 0.2 0.07 9.5 + 0.2 0. l6 9.7 + 0.2 L.S.D. (P <0.0s) 0.6 The water, nitrogen and phosphorus contents ofthe grain were increased signifr- cantly in the plants grown in soils to which 0.07 per cent and 0.16 per cent Fe was added (Table 6). Up to the twelfth week of plant growth those plants grown in the soils treated with 0.07 per cent Fe and 0. l6 per cent Fe were appreciably taller than plants in the control soil. From l2 weeks to harvesting plant height remained relatively constant. Root lengths obtained at harvesting did not show significant effects (Table 7) due to treatments of the soil in spite of the fact that the soils treated with polycations appeared to offer an improved physical environment for root growth. 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