Soils of Central Sudan

SOILS OF CENTRAL SUDAN

J. BUURSINK Stellingen

1. De aard van de kleifraktie van Vertisols en Alfisols in centraal Sudan, be- staande uit montmorillonitische mineralen, bepaalt de nauwe genetische verwantschap van deze gronden. (Dit proefschrift)

2. De verwering van montmorilloniet- tot illietkleien in Nijldelta afzettingen wordt door Hamdi ten onrechte geacht te zijn aangetoond. (Hamdi H. 1959 Alterations in the clay fraction of Egyptian soils. Z.Pflanzenern. Düng. Bodenk. 84 : 204)

3. Voor universele toepassing van de bodemtemperatuur als criterium om bodems van tropischen subtropisch-aride gebieden te scheiden, dienen de definities als geformuleerd door de U.S.Dept. of Agriculture te worden gewijzigd. (Soil Survey Staff, SCS, USDA 1960 en 1967 Soil Classification. A Comprehensive System. U.S. Gov. Pr. Off., Washington)

4. Het ontbreken van sediment aanvoer uit Ethiopië hoeft niet zonder meer de oorzaak te zijn van de afwezigheid van pyroxenen in de zandfraktie van oudere Nijlterrassen, zoals gesteld door Shukri. Het laatste feit dient veeleer te worden toegeschreven aan verwering van dit mineraal. (Shukri N. M. 1950 The mineralogy of some Nile sediments. Q. J. Geol. Soc. 105 : 511-534)

5. Het merendeel van de 'Zwarte Tropische Kleien' (Vertisols) in Afrika dient niet te worden geklassificeerd als Pellusterts, zoals Sys meent, maar als Chromusterts. (Sys C. 1969 Soils of Central Africa in the American Classification — 7th Approximation. African Soils 14 : 33) 6. De silt akkumulatie achter de Assoeandam zal de bodemvruchtbaarheid van de Nijldelta in ongunstige zin beïnvloeden.

7. Gezien de veelal schaarse gegevens die beschikbaar zijn ten tijde van de planning (eerste besluitvorming) van ontwikkelingsprojekten, dient hierbij ruimte gelaten te worden voor wijzigingen en aanpassingen, die noodzakelijk blijken naarmate de veldstudies vorderen.

8. Door de overheid dient op zo kort mogelijke termijn te worden verlangd dat de in talloze oudere woningen nog aanwezige loden waterleidingbuizen worden verwijderd.

9. De door 'Van Dale' gegeven omschrijving van de betekenis van 'rivierleem' berust op een fictie. (Kruyskamp C. 1970 Van Dale Groot Woordenboek der Nederlandse taal. M. Nijhoff, 's-Gravenhage)

10. In verband met de eis 'baas in eigen buik' kan het gepast zijn ook de hand in eigen boezem te steken.

Utrecht, 21 juni 1971 J. BUURSINK Frontispiece; Photograph of northern and central Sudan showing part of the Nile River basin, taken from the Gemini XI spacecraft on September 14, 1966 at an altitude of about 600 km. Some cumuliform and cirriform clouds are seen drifting over the desert on both sides of the Red Sea (Photo credit: National Aeronautics and Space Administration). Soils of Central Sudan Soils of Central Sudan

Proefschrift

ter verkrijging van de graad van doctor in de Wiskunde en Natuurwetenschappen aan de Rijksuniversiteit te Utrecht, op gezag van de Rector Magnificus Prof. Dr. F. van der Blij, volgens besluit van de Senaat in het openbaar te verdedigen op maandag 21 juni 1971 des namiddags te 4.00 uur (precies) door Johan Buursink geboren op 22 augustus 1939 te Enschede.

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Utrecht 1971 Grafisch Bedrijf Schotanus & Jens Utrecht N.V. PROMOTOR: PROF. DR. IR. F. A. VAN BAREN Abstract

This research is concerned with soil formation in the tropical arid zone of the Sudan, N.E. Africa, and deals only with soils developed in the extensive deposits of the Blue Nile and Nile, between 11 and 17° N. Subsequently are treated: the physical environment, the soils proper including field and laboratory data, and the classification and genesis of the soils. Mineralogical investigations show that rocks of the west Ethiopian Plateau (Trap Series basalts, Basement Complex, Mesozoic sandstones) determine the composition of the sediment of the Blue Nile as well as of the Nile. The influence of the White Nile is negligible. The sand component of the Blue Nile-Nile sediments is characterized, as regards its heavy fraction, by a hornblende-augite- epidote association; its light fraction consists primarily of quartz and potassium feldspars and such typical volcanic components as glass and basalt fragments. The silt fraction of the sediments contains mainly montmorillonitic and vermiculitic minerals. The former dominate the clay fraction. Precipitation in Ethiopia forms the major supply of water to the Nile, especially during the summer flood. The quality of the riverwater is rated at C2-S1, its high carbonate content may lead to soil salinization in flooded depressions. The fluviatile landscape — two terraces along the Blue Nile, and up to four along the Nile — has a semi-desert vegetation in the north, and a woodland savannah south of this. The organic matter content of its soils is generally less than that of the present river sediment. Published data concerning the absolute age of landscape units as well as those on paleoclimatological conditions are scarce and often contradictory. The soils are classified into four orders: Vertisols, Entisols, Aridisols, and Alfisols. The chemical composition of the Entisols and recent Vertisols — relative to the content of Si, Al, Fe, Ti, Ca, Mg, K, Ba, Co, Mn, Ni, and Zn — is positively correlated with four grain size fractions. The concentration of Sr is of a deviating nature. Soil formation of the Alfisols, older Vertisols, and Aridisols has generally induced some loss of Si as well as of the elements Zn, Co, Mn, and Fe (in decreasing degree) mainly as a result of intensive disintegration of silt. From a mineralogical viewpoint increasingly older soils (higher terraces) show successive loss of more resistant minerals from the sand fraction: first basalt fragments, augite, and volcanic glass, then hornblende and potassium feldspar and finally mainly epidote. As for the silt and clay fractions, montmorillonite is mainly subject to weathering, this also appears from the cation-exchange capacity of the soils. Weathering of this mineral from the silt fraction presumably led to its enrichment in the clay fraction (terrace II, Gezira and Kenana). The continuation of this process subsequently entailed loss of montmorillonite from the clay fraction of older soils leading to a relative enrichment of kaolinite, combined with new formation of the latter. Micromorphological observations especially show that the soil formation of Alfisols and associated Vertisols (levee and backswamp) is closely related. Argillipedoturbation and translocation of clay occur in both kinds of soil. The former process dominated in the Vertisols, the latter in the Alfisols. The sulphate-carbonate salinity of the cracking clay soils (mainly Vertisols) of Gezira and Kenana is ascribed to evaporation of stagnant floodwater, now a relic feature owing to the incision of the Blue Nile. Samenvatting

Deze studie betreft de bodemontwikkeling in de droge tropen van Sudan, N.O. Afrika. Uitsluitend bodems gevormd in de uitgestrekte afzettingen van de Blauwe Nijl en Nijl, tussen 11° en 17° NB, zijn hierbij betrokken. Achtereenvolgens zijn behandeld: het fysisch milieu, de bodems met veld- en laboratoriumgegevens, en de klassifikatie en genese van de gronden. Mineralogisch onderzoek toont dat gesteentes van het West-Ethiopisch hoogland (Trap bazalten, Basement Complex, Mesozoische zandsteen) de samenstelling bepalen van het sediment van de Blauwe Nijl en van de Nijl zelf. De invloed van de Witte Nijl is minimaal. De zandcomponent van de Blauwe Nijl-Nijl sedimenten is, wat betreft de zware fraktie, gekenmerkt door een hoornblende- augiet-epidoot associatie, de lichte fraktie bestaat vnl. uit kwarts en kaliumveldspaat en zulke typisch vulkanische componenten als glas en bazalt fragmenten. De siltfraktie is vooral opgebouwd uit montmorilloniet en vermiculiet, waarvan de eerstgenoemde in de kleifraktie domineert. De hoeveelheid Nijlwater is in hoge mate bepaald door de neerslag in Ethiopië. De kwaliteit van het water voor irrigatie is geklassificeerd als C2-S1, vloedwater dat stagneert in depressies kan aanleiding geven tot bodemverzouting. Het fluviatiele landschap — twee terrassen langs de Blauwe Nijl en soms vier langs de Nijl — heeft een vegatatie die van noord naar zuid haar woestijnkarakter verliest en geleidelijk overgaat in een boomsavanne. Het gehalte aan organisch materiaal van de bodems is veelal lager dan van het huidig riviersediment. Gegevens betreffende de absolute ouderdom van de landschapseenheden en over paleoklimatologische omstandigheden zijn schaars en tegenstrijdig. De bodems zijn geklassificeerd in vier 'orders': Vertisols, Entisols, Aridisols en Alfisols. De chemische samenstelling van de Entisols en recente Vertisols is, voor wat betreft de concentratie aan Si, Al, Fe, Ti, Ca, Mg, K, Ba, Co, Mn, Ni en Zn, positief gecorreleerd met vier korrelgrootte frakties. De concentraties van Sr wijken af in dit opzicht. Bodemvorming van de Alfisols, oudere Vertisols, en Aridisols heeft in het algemeen vooral geleid tot een verlies aan Si en aan Zn, Co, Mn en Fe (in afnemende mate) vooral tengevolge van een intensieve siltverwering. Mineralogisch gezien blijkt dat naar mate de gronden ouder zijn steeds resistentere mineralen uit de zandfraktie zijn verweerd: eerst bazaltfragmenten, augiet, vulkanisch glas, vervolgens hoornblende en kaliumveldspaat, en ten slotte vooral epidoot. In de silt- en kleifrakties is montmorilloniet hoofdzakelijk onderhevig aan verwering, hetgeen ook blijkt uit de uitwsselingskapaciteit van de gronden. Verwering van dit mineraal uit de siltfraktie leidde waarschijnlijk tot een verrijking in de kleifraktie van terras II gronden (Gezira, etc), terwijl in oudere terrasgronden verlies van montmorilloniet ook uit de kleifraktie optrad. Micromorfologische waarnemingen vooral wijzen op de nauw verwante bodemvorming van Alfisols en geassocieerde Vertisols. Argillipedoturbatie en translocatie van klei treden in beide soorten grond op. Het eerste proces vooral in de Vertisols, het tweede speciaal in de Alfisols. De verzouting van de kleigronden van terras II, vnl. Vertisols, is verklaard door verdamping van stagnerend rivierwater en opgevat als een relict verschijnsel. Acknowledgements

This study would not have been written as it is - if written at all - without the stimulating influence of many. First of all, I wish to express my sincere thanks to you Prof. Dr F. A. van Baren for being privileged to carry out this research under your supervision, and for allowing me to make full use of the facilities of your Department. This investigation has considerably benefited from your admirable scientific insight as well as from your organizational counsel and unflagging interest. W. L. P. J. Mouthaan B.Sc, I am highly indebted to you for the efficient coaching of this study arising from genuine concern and encouragement and in particular for your expert advice in the field of sand mineralogy. It is a pleasant duty to thank Dr Orner M. A. Mukhtar, present Director, and Abdullahi A. R. Tahir M.Sc, former Director of the Soil Survey Department, Wad Medani, and Dr G. H. Robinson, FAO projectmanager, for their support in the Sudan especially with relation to fieldwork. I also highly appreciated the kind cooperation received from Prof. Petezval Lemos, Project Operations Officer, Land and Water Development Division FAO, Rome. It is a great pleasure to thank Prof. Dr P. Buringh of the Agricultural University Wageningen, and Prof. E. H. Templin, formerly of Texas A. & M. College, who during their respective sojourns in the Sudan actively encouraged the undertaking of this research. I also acknowledge the thorough scientific training received from Prof. Dr J. I. S. Zonneveld and the staff of the Department of Physical Geography, and from Professors Dr D. J. Doeglas and Dr J. F. M. de Raaf and the staff of the Depart- ment of Sedimentology of the University of Utrecht. Particular gratitude is due to D. Creutzberg M.Sc. and N. M. de Rooij M.Sc. for their helpful suggestions and constructive criticism of parts of the manuscript. Also the discussions with L. Bal M.Sc, J. H. V. van Baren M.Sc, W. A. Blokhuis M.Sc, J. A. K. Boerma M.Sc, A. L. T. M. Commissaris M.Sc, Dr A. Jongerius, 10

H. G. A. van Panhuys M.Sc, Dr J. J. Reynders, W. Siderius M.Sc, and S. P. Tjallingii M.Sc. refined many defects or impurities. I am under great obligation to the able, technical assistance of Mr A. L. A. Broek- huisen, Mr H. E. Renaud, Mr D. Schreiber, Mr D. de Vries, and Mr J. van der Wal. Many thanks go to Mr F. Henzen and Mr I. Santoe for their drawing of the illustrations. I have been greatly aided by A. Muller M.Sc. and the members of his staff at the Soils Laboratory of the Royal Tropical Institute, Amsterdam, for having carry out part of the analyses as well as for placing X-ray spectrometer and DTA equipment at my disposal. A heartfelt word of thanks to Mr Pier van der Kruk who never wavered to soil his hands with brushing up and soft-pencilling the English of the manuscript. Much credit is also given to Miss Weia S. Smid for the energetic way in which she carried out the laborious task of typing the longhand text.

This investigation was supported by grants of the Netherlands Ministry of Education and Sciences. Thanks are extended to the Ministry of Agriculture of the Government of the Sudan for permission to undertake this research. Table of Contents

GLOSSARY 14

INTRODUCTION 15

PART I THE PHYSICAL ENVIRONMENT 17

Chapter I Climate 19 1.1. Atmospheric and Soil Climate 19 1.1.1. Temperature 20 1.1.2. Moisture 22 1.2. Paleoclimate 27

Chapter 2 Geology 31 2.1. Precambrian 31 2.2. Paleozoic 36 2.3. Mesozoic 36 2.4. Tertiary 37 2.5. Quaternary 39

Chapter 3 Geomorphology 41 3.1. Blue Nile - Nile River System 43 3.1.1. Solid sediment load 45 3.1.2. Dissolved sediment load 51 3.2. Blue Nile Landscape 56 3.3. Nile Landscape 57 3.4. Age of Landscapes 61

Chapter 4 Vegetation , 65 4.1. Semi-desert 67 4.2. Woodland Savannah 68 4.3. Special Areas 69 12

PART II REPRESENTATIVE SOILS AND SOIL MATERIALS 71 Chapter 5 Representative Soils 72 5.1. Location 72 5.2. Field Description and Laboratory Data 72

Chapter 6 Composition of Soil Materials 165 6.1. Investigational Techniques 165 6.2. Discussion of Results 170 6.2.1. Grain size composition 170 6.2.2. Elemental composition 171 6.2.2.1. Low terrace soils 172 6.2.2.2. Soils of higher terraces 179 6.2.3. Carbonates and gypsum 183 6.2.4. Organic matter 185 6.2.5. Specific constituents 188 6.2.5.1. Sand fraction 188 6.2.5.2. Clay fraction 191

Chapter 7 Organization of Soil Materials 199 7.1. Investigational Techniques 199 7.2. Discussion of Results 199 7.2.1. Field observations 199 7.2.2. Investigation of thin sections 200

Chapter 8 Properties of Soil Materials 205 8.1. Investigational Techniques 205 8.2. Discussion of Results 206 8.2.1. Bulk density 206 8.2.2. Cation exchange : 207 8.2.2.1. Cation-exchange capacity 207 8.2.2.2. Exchangeable cations 210 8.2.3. Salinity 213

PART III SOIL CLASSIFICATION AND GENESIS 217

Chapter 9 Soil Classification 219 9.1. Vertisols 221 9.2. Entisols 222 9.3. Aridisols 223 9.4. Alfisols 224 9.5. Comments on Classification 225 9.6. Soil-Landscape Relations 227

Chapter 10 Soil Genesis 229 10.1. General Concepts 229 10.2. Central Sudan 231 10.2.1. Entisols 231 13

10.2.2. Vertisols 232 10.2.3. Alfisols 235 10.2.4. Aridisols 236

REFERENCES 239

CURRICULUM VITAE 249 GLOSSARY jebel : a hill

m.s.l. : mean sea level (at Alexandria); heights are related to 'old Khar-

toum datum', 360 m above sea level at Alexandria

t (in tables) : trace

YBP : years before present Introduction

Soil constitutes one of the main natural assets of the Sudan. As used here, central Sudan refers to the socio-economic heart of the country - the area along the Nile river system. The soils of this part of the Sudan are consequently of primary concern in the build-up of the country. Their optimum management, both in agricultural and non-agricultural sense, requires insight into their location, properties and use capabilities. In due recognition of the above the Government of the Sudan instigated the execution of extensive soil surveys and the production of soil maps. This task is at present being carried out by the Soil Survey Department, Wad Medani. The U.N. Food and Agriculture Organization assisted in this undertaking, and the author participated. This study is the outcome of field and laboratory investigations of selected soils developed in recent and older deposits of the Blue Nile and Nile. Its objective is to present data concerning these soils that provide a means to characterize and eluci- date their development. This research, which also embodies considerable previous work, may further serve to accelerate investigations by highway and construction engineers, agricultural scientists and earth scientists. We divided the report in three parts. Part I is a comprehensive account of the physical environment of the Blue Nile-Nile region in central Sudan.' It |opens with a synopsis of present and past climatic conditions. In connection with the importance to soil formation of the river system, the geology of its catchment area in west Ethiopia is emphasized as well as our investigations in its present sediment load. A description of the fluviatile landscapes is followed by a review on their vegetation. Part II focuses on the soils in question and on the composition, organization, and properties of their component horizons. It comprises all field and laboratory evidence. The bulk of the chemical data is given directly with the soil descriptions. Relevant detail about investigational techniques is presented at the beginning of 16 each of three consecutive chapters. Various aspects of the composition of soil materials are dealt with, directed at determining their derivation as well as the impact of pedogenesis. Micromorphological observations are included under the heading: organization of soil materials. A concluding chapter on soil properties is mainly concerned with salinity and alkalinity problems. Part III is of an interpretative character, it is devoted to soil classification and formation. The classification of the soils - imperative to effective transfer of agricultural knowledge and experience - follows the system developed by the U.S. Department of Agriculture. Drawbacks of this system, when applied to Sudan soils, are discussed. The final chapter on soil genesis, introduced by some widely held views on this subject, provides a synthesis of our investigations. Unless otherwise specified, standard terminology given in the 'Glossary on Soil Science Terms' (Proc. Soil Sei. Soc. Am., 1965) is used. The term 'alluvial soils' as conceived here comprises soils developed in all river deposits, whether recent or old.

In conclusion, it is our considered opinion that the findings presented in this study - which were related already to conditions in the Nile delta - may equally well apply to the soils associated with rivers that drain the eastern and southern portion of the Ethiopian Plateau and that subsequently also flow through arid lands. A comparative study of the Blue Nile-Nile system, the Awash river basin in Ethiopia (FAO/UNSF, 1965), and the Wabba Shebele and Juba rivers flowing through Somalia may well reveal common problems (related to nutrient supply, soil salinity and alkalinity, etc.) and pave the way to common solutions. 'Apart from aiding man in his search for control over his environment' - to cite Hempel (1966) writing on the philosophy of natural science - such investigations would further answer 'another, disinterested, but no less deep and persistent, urge: namely his desire to gain ever wider knowledge and ever deeper understanding of the world in which he finds himself'. PART I

The physical environment CHAPTER 1

Climate

1.1. Atmospheric and Soil Climate

The climate of central Sudan, according to the climatic classification of Koppen (1931), is a low-latitude dry climate. Parts of two climatic regions are involved - an arid zone (Koppen symbol BWh) toward the north, and a semiarid zone (BSh) lying on its equator-ward margin. The essential feature of this climate is by definition that potential evaporation exceeds annual rainfall. Relatively heavy rains occur during a short period in summer with the advance of the sun towards the summer solstice, when unstable air of tropical maritime origin and the Inter- Tropical Convergence are farthest poleward. The I.T.C. normally reaches as far north as 20° N. The interactions of climate and soil are complex. It is, moreover, hard to separate the effects on soils of external climate from the soil climate. It is therefore thought

Fig. 1. Altitude, geographical latitude, average annual rainfall and air temperature of main meteorological stations

Station Altitude in Latitude Average Average meters in°N annual rain annual air above mean fall in mm temperature sea level in°C

Atbara . . . 345 17° 42' 72 29.7 Shendi . . . 360 16°42' 136 29.5 Khartoum . 380 15°36' 164 29.6 Wad Medani 405 14°23' 373 28.4 Sennar. . . 420 13°23' 481 28.3 Singa . . . 430 13°09' 596 28.6 Roseires . . 465 11°51' 776 28.0 20 advisable to discuss those climatic factors, of which data are available and which are known to have an influence on soil and soil formation. Like elsewhere in the world, in this area studies on soil climate lag far behind those on atmospheric climate. Climatological normals used are based on long term observations obtained at seven selected meteorological stations, located along the Blue Nile and Nile rivers. Observations made available by the Sudan Meteorological Service cover the period 1931-1960, unless otherwise stated. A general picture of climatic conditions is given in figure 1, which lists both average annual rainfall and air temperature, as well as geographical latitude and altitude above sea level of the observation posts (for location see figure 8).

1.1.1. Temperature In central Sudan, with a high altitude sun and predominantly clear skies, there is a relatively great direct control of air and soil temperatures by solar radiation. In fact, for many soil processes the supply rate of incoming radiation sets the upper limit of the rates of change in the soil (Yaalon, 1960). Radiation values, as measured at Khartoum during the period 1958-1962, are presented in figure 2 (Oliver, 1966).

Fig. 2. Average solar radiation in calsjcm2lday at Khartoum per month

J F M A M J J A S O N D

470 518 564 581 570 537 522 524 518 489 473 445

Fig. 3. Average daily air temperature in ° C per month

Month J F M A M J J A S O N D Station

Atbara . . . 22.6 23.5 26.7 30.7 33.7 35.0 33.7 32.9 33.9 32.0 27.5 24.1 Shendi . . . 23.1 23.6 27.1 30.5 33.5 35.0 33.1 32.2 33.2 31.6 27.9 23.8 Khartoum . 23.7 24.7 27.9 31.5 33.7 34.1 31.7 33.4 32.0 32.3 28.2 24.9 Wad Medani 24.1 25.1 28.1 31.1 32.4 32.1 29.3 27.7 28.8 30.1 27.4 24.7 Sennar . . . 23.0 26.0 28.9 31.5 32.2 31.2 28.3 27.1 28.0 29.7 28.3 28.7 Singa . . . 25.9 26.9 29.7 32.1 32.1 30.5 27.9 26.7 27.7 29.4 28.7 26.7 Roseires . . 26.3 27.5 29.8 32.1 30.7 28.5 26.7 26.1 26.7 28.0 27.7 26.6 21

With the sun overhead at Khartoum on May 3 and August 10, increased cloudiness during the July-August rainy season clearly shows its influence. Differences among the various meteorological stations as observed with relation to average air temperatures are relatively slight, as appears from the average annual values of figure 1, and from the average daily temperatures tabulated monthly in figure 3. Two temperature maxima occur: the main one in June, the secondary one after the rains in October. Temperature fluctuations recorded at each station are large. At Khartoum both in January and June the difference between mean daily maximum and minimum air temperature is about 15° C. As with air temperature, the mean annual cycle of soil temperatures reflects the supply rate of solar radiation. Only for Khartoum observatory a record of soil temperatures is available for the period 1958-62. Temperatures have been recorded at fixed hours: 0800, 1400 and 2000 hours local time (2 hours ahead of G.M.T.), in an inferentially loamy soil with a bare surface, and are reported by Oliver (1966). Figure 4 gives the monthly mean values recorded at the three hours stated.

Fig. 4. Soil temperature means in "C at Khartoum, 1958-62

Month J F M A M J J A S O N D Depth in era

1 .... 31.0 32.8 37.8 43.2 44.3 44.9 39.5 38.0 40.6 41.1 37.2 32.7 2.5 ... 30.1 31.2 36.2 40.8 42.7 43.4 38.4 36.6 38.7 39.1 35.5 32.0 5 .... 29.3 30.6 35.4 40.3 42.0 42.9 38.3 36.2 38.1 38.4 35.0 31.4 10 .... 29.0 29.7 34.1 38.6 40.4 41.2 37.3 35.5 37.1 37.6 34.2 30.7 20 .... 27.8 28.1 31.7 37.6 37.6 39.2 35.7 34.4 36.0 36.1 33.1 29.9 50 .... 29.0 29.3 31.8 35.2 37.2 38.4 36.5 34.7 35.8 36.1 33.6 30.6 100 .... 30.2 29.8 31.4 34.0 35.9 37.1 36.3 34.6 35.3 35.8 34.4 31.8

These data of the annual soil temperature régime indicate that the lowest mean monthly temperatures occur in January at all depths, except at 100 cm when they occur in February. The highest soil temperatures are recorded in June. The normal progression of heating and cooling is upset by the rainy season such, that after a sharp decline in temperatures August shows a secondary minimum at all depths. With decreasing cloudiness, drying soils and increasing air temperatures after the rains, a partial recovery takes place and a secondary maximum is produced in October. The wide range of variation of soil temperatures, and especially the high maxima, entailing an extreme environment for both flora and fauna, are shown in figure 5. 22

Fig. 5. Soil temperature variation in °C at Khartoum, 1958-62

Depth Yearly Monthly mean Absolute Difference means Highest Lowest Maximai Minima air 29.6 33.7 23.7 47.7 5.2 42.5 1 38.5 65.3 20.7 73.0 12.0 61.0 2.5 37.0 56.2 20.5 65.5 12.7 52.8 5 36.5 52.9 20.9 61.0 13.5 47.5 10 35.5 46.6 22.7 50.5 17.2 33.3 20 33.8 . 40.9 25.4 44.3 22.0 22.3 50 34.0 38.5 28.8 39.8 25.8 14.0 100 33.9 37.2 29.8 38.5 27.9 10.6

Soil temperature currently is used as one of the criteria of classification in the lower soil categories (Soil Survey Staff, USDA, 1967). According to this system soils characterized by the above temperatures belong to the hyperthermic class, since the difference between mean summer (June, July, and August) and mean winter (December, January, and February) temperatures amounts to 6.9° C, while the mean annual soil temperature is 34.0° C at 50 cm depth (see also Chap- ter 9.5). It is noted that the soil at this depth is 4.4° C warmer than the air on a mean annual basis. This relation constitutes a significant departure from the one valid in most of the United States, where the mean annual soil temperature can be very closely approximated by adding 1° C to the mean annual air temperature (Soil Survey Staff, USDA, 1960). This extreme thermal environment has influence upon the activity of soil organisms in view of the fact that microbial activity — reaching optimum values at about 35° C - decreases markedly at temperatures above 45° C. Practically throughout the year the mean temperatures of the surface soil (1 and 2.5 cm) reach values of over 45° C at 1400 hours, and during the summer period this is the case with the soil down to at least 10 cm depth. Physical soil processes are stimulated by the frequent and pronounced changes in temperature (day and night, before and after rains), while chemical processes develop at increased speed under these high temperature conditions, unless other elements interfere, such as water deficiency.

1.1.2. Moisture In connection with the annual movement of the Inter-Tropical Convergence, a direct relation exists between latitude and both average annual precipitation and length of the rainy season in central Sudan. The situation with respect to the 23

annual rainfall appears from figure 1; the seasonal distribution of the rains is evident from figure 6, in which the average precipitation per month is tabulated.

Fig. 6. Average monthly rainfall in mm

Month J F M A M J J A S O N D Station

Atbara . . . 0 0 0 1 4 1 20 38 7 1 0 0 Shendi . . . 0 0 t 1 3 2 43 64 20 3 0 0 Khartoum . 0 t t 1 5 7 48 72 27 4 t 0 Wad Medani 0 0 t 2 20 34 123 138 60 16 0 0 Sennar . . . 0 t 1 4 29 47 146 170 63 21 t 0 Singa . . . 0 0 t 4 26 73 171 198 95 28 1 t Roseires . . 0 t 1 11 58 126 166 221 152 36 5 0

Computation of the average number of raindays (days with precipitation of 10 mm or more) per month gives an idea of the intensity of the rainy season and shows its diminishing importance with increasing latitude.

Fig. 7. Average number of raindays ( & 10 mm) during the rainy season

Month May June July August September October Station

Atbara 0 0 1 1 0 0 Shendi 0 0 1 2 1 0 Khartoum 0 0 1 3 1 0 Wad Medani 1 1 4 5 2 0 Sennar 1 2 5 6 3 1 Singa 1 2 5 7 3 1 Roseires 2 4 5 7 5 3

Of a much more pronounced character is the rainy season in the West Ethiopian highlands, which is briefly discussed here with regard to its importance to the summerfloods of the Nile (see Chapter 3.1). As shown on the map of average annual rainfall in the Nile basin (figure 8), precipitation in the Blue Nile catchment area may well exceed 2000 mm. In figure 9 the average precipitation per month is listed for two Ethiopian meteorological stations considered representative: Gondar at 2270 m altitude (12° 36'N, 37° 29' E) and Göre at 2130 m altitude (8° 10' N, 35° 38'E) (Fantoli, 1965). 24

20'

31* .36" /^. 5. Average annual rainfall (mm) in the Nile basin ( adapted from Hurst, 1952) 25

Fig. 9. Average monthly and annual rainfall in mm in the Ethiopian highlands

Month J F M A M J J A S O N D Year Location

Gondar 1 5 10 57 77 182 369 356 122 47 18 3 1246 Gore 23 75 102 242 299 453 315 451 363 231 102 12 2668

Rainfall has a maximum at a similar period as in central Sudan-from June to September. In central Sudan potential evapotranspiration, theoretically determined according to procedures outlined by Thornthwaite and Mather (1957), exceeds precipitation almost throughout the year. This circumstance is even more pronounced when the actual total open-water evaporation is considered, as derived from Piche evaporimeter readings (applying reduction factor 0.5). Figure 10 shows a comparison between the three groups of figures, as well as with water deficits computed by Satakopan (1965) from the above potential evapotranspiration.

Fig. 10. Average annual opzn-water evaporation, potential evapotranspiration, rainfall, and water deficit in mm

Station Open- Potential Rainfall Water deficit water evapo- • ' • ' ' evaporation transpiration

Atbara 3120 1806 72 1733 Shendi — 1797 - 136 , 1661 Khartoum 2555 1837 164 1656 WadMedani 2810 1783 373 1402 Sennar 2519 1806 481 1337 Singa 2171 1844 596 • 1254 Roseires 1916 1798 776 996

The question as to how much rain water actually is absorbed by the soil is a very complicated one, and the factors involved are hard to measure. An approximation, however, of soil moisture conditions on a monthly basis can be made by means of Thornthwaite and Mather's (1957) concepts. Water stored in the soil, thus determined, is nil in all months of the year for the stations Atbara, Shendi, Khar- toum and Wad Medani.- During at least the months of May and June the soil is dry at Sennar, Singa, and Roseires (Satakopan, 1965). 26

Figure 11 shows moisture conditions of the soil at the three stations last mentioned. Considering the monthly situation, moisture is not directly stored in the soil at the onset of the rains, but as soon as rainfall starts to exceed potential evaporation. This point is clarified by a comparison between figures 6 and 11. Already before the end of the rains soil moisture is diminishing, the watermovement in the soil changes direction, it is upward during several months yearly, until it ceases.

Fig. 11. Soil moisture storage in mm at three stations, according to Thornthwaite and Mather (1957)

Month J F M A M J J A S O N D Station

Sennar . . . 3 2 1 0 0 0 0 11 9 6 5 4 Singa . . . 7 4 2 1 0 0 4 55 46 29 17 11 Roseires . . 19 12 4 1 0 0 28 115 127 83 51 32

For soil moisture it is necessary to take into account values different from the above, in view of the fact that the applied formulae were originally developed for temperate climate conditions. A number of meterorological factors that influence the amount of rainwater retained in the soils of central Sudan, are: a. The character of the diurnal rainfall régime. According to Oliver (1965) a large proportion of the rain is received during night hours. This is shown in figure 12 for three stations.

Fig. 12. Diurnal rainfall distribution in percentages of mean annual total

Station Duration of Night hours Day hours observations 1800-0600 LT 0600-1800 LT

Atbara 1962-63 74.9 25.1 Khartoum Observatory 1956-60 76.5 23.5 Wad Medani 1932-45 and 64.5 35.5 1958-63

During the night, without direct solar radiation for evaporation, water can move into the subsoil with less loss by evaporation whilst the infiltration is taking place. In addition, Oliver (1969) observed that the lowest wind speeds of the day tend to correspond with the time of the usual rainfall. It should be noted that Pedgley 27

(1969), analyzing the diurnal incidence of the rainfall, arrives at somewhat different conclusions with respect to the above stations. According to this author Atbara receives most of its rain between 1500 and 2100 hours, Khartoum between 2100 and 0600 hours, and Wad Medani between 1800 and 0600 hours LT. b. The character of the rainy season régime. Two extreme distribution patterns of rain may be considered (adapted from Oliver, 1965): (1) an extended distribution of small rainfalls throughout the season, a situation conducive to excessive evaporative losses from the heated soil surface, and (2) a well-distributed but infrequent occurrence of heavy falls, resulting in a larger ratio of run-off to infiltration into the soil. c. The character of rainstorms. Many of the individual rainstorms are small in diameter at ground surface, so that the wetted area is limited. Energy advected from adjacent dry areas will then accentuate losses from evaporation (Oliver, 1969).

It is evident that because of the inadequate soil moisture supply chemical processes will each year stagnate for several months. Apart from being soilforming factors, soil moisture and soil temperature are soil characteristics, and as such they are used for defining taxa in the U.S..soil classification system (Soil Survey Staff, USDA, 1967). Soils with,similar moisture and temperature regimes are largely grouped in the same suborder of this system. If there are no readily discernible soil horizons that serve as marks of the soil moisture and soil temperature regimes, these regimes are used for defining the taxa. The criteria applied are then mean summer temperature, mean annual temperature, and the number of days when the soil is dry. For soil classification purposes two classes of soil climate can be recognized in central Sudan - 1) a very dry soil climate toward the north, and 2) a warm and dry soil climate, with soil moisture during the season when the soil is warm, on its equator-ward margin (see further Chapter 9).

1.2. Paleoclimate

As soil development is time related a consideration of climate not only in space but also in time is desired. The alluvial soils of central Sudan do not seem to be older than Pleistocene (see Chapter 3), the following discussion can therefore be restricted to Quaternary climatic conditions. 28

Archeological, biological, geological, geomorphic, and pedological observations support the theory that climate conditions in the past were subject to substantial changes; for the arid zone of Africa specifically the evidence points to a succession of periods more humid or more arid than at present. Whether these differences in moisture conditions were due to differences in rainfall or temperature (and thus evaporation) remains questionable. Pluvials, in the sense of more humid periods, on the polar side of the Sahara are assumed to correspond with (part of) the glacials of higher latitudes (Budel, 1963; Fairbridge, 1963; Wolstedt, 1965; and Frenzel, 1967). The interpretation and timing of equatorial pluvials, whose fossil relics have been observed on the south side of the Sahara (approximately between latitudes 14° and 20° N), however, is far more controversial. Although both Penck (1938) and Balout (1952) supported the view that the major climatic changes occurred simultaneously from the pole to the equator, Penck concludes to pluvial periods on the south side of the Sahara during glacial times, assuming a narrowing of this desert, and Balout comes to the conclusion of more arid periods at that time on the southern Sahara fringe, due to a southward movement of the desert. Büdel (1963), in reviewing the Sahara pluvials, states that along the southern side of the desert traces of only one Pleistocene pluvial were found, which could not be exactly synchronized with either a high-latitude glacial or interglacial. The fossil traces primarily involved are the West African ferruginous crusts, a subsequent so-called 'younger Rotlehm cover', the sub-Sahara zone of fossil dunes, and its relation to Lake Chad. Büdel's inductive inferences with relation to the Quaternary paleoclimatic sequence of the southern side of the Sahara may be summarized as follows: Subsequent to the Pliocene, which was considerably more humid than at present, the Pleistocene is mostly drier, and only late does a more humid period occur - a Pluvial from late Riss to middle Wurm or from about 200,000-40, 000 YBP. The Holocene, beginning with the Neolithic (5,500 YBP) is humid again up to the present day, and periodically may have been even somewhat more humid than today. The controversial and fragmentary nature of our present knowledge concerning paleoclimates of the southern Sahara, and more specially those of central Sudan, is even more apparent if Büdel's conclusions are compared with those of others, like Grove and Warren (1968), Fölster (1964), Mohr (1963), Herman (1968), Butzer (1967), and De Heinzelin (1967). Grove and Warren (1968), after their géomorphologie research carried out along the south side of the Sahara as far east as the Nile, conclude to the following. A pluvial period that may have begun before 20,000 YBP, and which was well 29 marked about 10,000 YBP, came to an end about 7,000 YBP or somewhat later. A main period or periods more arid than the present probably came earlier. Since 8,000 YBP the climate at times was somewhat drier or somewhat more humid than at present. Fölster (1964), in his study of south Sudan pediplains, concludes that morpho- genesis is characterized and directed by alternation of more humid and more arid periods; no isotopic dating is available. Mohr (1963), in reviewing the Quaternary deposits of Ethiopia, states that the evidence available so far suggests that a two-fold classification of the pluvials is more suitable than the four-fold one given for East Africa. These two pluvials, if they can be distinguished, are considered equivalent to the Kageran-Kamasian and the Kanjeran-Gamblian of Kenya. Herman (1968), on the basis of micropaleontologic and lithologie studies of Red Sea cores, distinguishes four paleoclimatic phases. Sediments of phase I, representing the last 12,000- 13,000 years, have a fauna similar to that of the present day. Phase II, from about 65,000 to 13,000 YBP is a cool period with higher precipitation, interruped by a number of mild fluctuations. Phase III sediments are characterized by a warm climate fauna similar to the present, and about six times longer than phase I. Phase IV represents another cool period of unknown duration. Butzer (1967), in studying the intercalations of local and Nile sediments in southern Egypt, concludes to six periods of greater moisture during the Upper Pleistocene. Four of these are contemporary in Egypt and Sudan or Ethiopia, and they are thought to have occurred during: Early to Middle Wurm, Late Wiirm (17,000 - 13,000 YBP), Upper Dryas to Early Holocene, and Middle Holocene. Possibly Middle Wiirm Nile sediments are ascribed to a sub-Saharan pluvial, local wadi floor conglomerates of Early Wiirm represent a pluvial without apparent nilotic parallel. Finally, De Heinzelin (1967) states, after his research of Pleistocene sediments in northern Sudan (see also Chapter 3.3 and 3.4), that during the last 25,000 years a synchronism holds between stadials - cooler, more humid phases - in Nubia and cooler phases in equatorial regions. More humid phases reached their optimum at about 21,000, 17,000, 12,000, and 9,500 YBP, with the climate subsequently getting less humid. In the light of the above, and in view of the absence of any quantatively defined changes of the precipitation or temperature, the drafting of even a tentative paleoclimatic outline for the central Sudan would be premature at this stage of research. CHAPTER 2

Geology

Apart from the Cenomanian trangression that suddenly invaded the Sahara, the African continent can be said to have been emerged for 250 million years. Hence the great development of continental deposits, of an importance totally unknown on the European continent. Continental deposits, such as presently occur at the surface in central Sudan, are considered to largely consist of weathering products of Ethiopian rocks, transported by the Sobat, Blue Nile and Atbara rivers (see Chapter 3). Since in these deposits the soil formation took place, which is the subject of the present study, the petrography of relevant rock units in western Ethiopia will be emphasized. Consequently, the distribution of the main geological formations of central and east Sudan as well as of western Ethiopia is given on the generalized geological map at scale 1/5,000,000 presented in figure 13.V,

consists •-non-föliatéd'-ignéóüscrocks:' A number-oï-**series-have"%'eén-descrÏDec stratigraphical and structural relations are uncertain (Furon, 1968). 32 Fig. 13. Geological map of eastern Sudan and western Ethiopia (adapted from ASGA-UNESCO Geological Map of Africa, ed. Paris, 1963, 1 : 5,000,000)

Legend of the geological map Nubian sandstone 'Intrusive', post-tectonic granites

Trap lavas Jurassic Basement granites

: il:::!:::: 'Continental Terminal' •.•i>:'.:-'-:'..' Precambrian, undifferentiated Fault, uncertain

'Continental Intercalaire' Basic effusive rocks Location of section figure 14 IVIDC 3600 3600 c 2700 2700 DINDEt 1600 1800 jTiini J mm SUDAN ETHIOPIA DINÙCKK 900 900 Bifiii' ' it P"TÏT 1 34"00' ' -,

SANDSTONE - Trioilie ? Mo*tW*,lin# C BASEMENT COMPLEX- Gronltlet', 0 10 20 30 40 Km. |=——I LAKE BEOS-Voriobly colortd, OLDER VOLCANfCS- Horiienlolly bo to coar«t-grain

Fig. 15. Simplified rock stratigraphie column

Period Rock unit Area* Lithology

Quaternary Kordofan sands S aeolian sands Gezira formation S alluvial sands and clays Aden series lavas E plateau basalts

Tertiary Trap lavas E basalts, some trachytes and rhyolites Umm Ruwaba formation S alluvial clays and sands Hudi formation S cherts

Mesozoic 'Continental Intercalaire' S sandstones Nubian formation s sandstones and mudstones Upper Sandstone E sandstones Antalo Limestone E limestones Adigrat Sandstone E sandstones

Paleozoic Unconformity S,E

Precambrian Basement Complex S,E metamorphic and igneous rocks

* the area designation S and E refers to occurrence in Sudan and/or Ethiopia.

Of the metamorphic rocks in Ethiopia, schists are much more abundant than gneiss, and paraschists greatly predominate over orthoschists. Igneous intrusive rocks are common in the Basement Complex, especially the silicic types (Mohr, 1963). . \' '•'••• •••'.'-•• r! In the Sudan the Sabaloka series is of interest! The rocks óf these series'mainly consist of rhyolites and trachytes, and frequently occur in close association with, younger or contemporaneous sodic granites or syenites, which may form ring structures, as at Sabaloka on the Nile (Delany, 1955;" see Chapter 3). '' . The time stratigraphie position of the Basement Complex is not fully established as yet". Delany (I960) considers the Basement Complex of Sudan as Precambrian. In the eastern Ü.Ä.R. argon ages of basement rock are reported to range from 435 to 540 million years. These dates would fall in the Upper Cambrian or lower Ôrdoviciàn, using the extended Paleozoic time scale (Gheith, 1961). In Ethiopia, the Basement Complex series has been divided into two groups; K/A ages indicate the older group to be 600 million years and the younger group 450 to 500 million years old (Mohr and Rogers, 1965). 36

2.2. Paleozoic

There are no known Paleozoic rocks overlying the basement in central Sudan or Ethiopia (except for two doubtful local occurrences). Throughout the Paleozoic and early Mesozoic the rocks of the Basement Complex # were exposed to sub-aerial denudation, the resulting topography was accidented, present-day mountain forms partially reflect this 'Tassilian' relief (Delany, 1960).

2.3. Mesozoic

The 'Continental Intercalaire' in Sudan is represented by continental Lower Cretaceous deposits with its characteristic flora and fauna (Furon and Lombard, 1964). At present the tendency exists to correlate the Continental Intercalaire with the 'Nubian Sandstones'. The Nubian formation consists of yellow and brown-bedded sandstones with intercalations of mudstones, varying in thickness from 50 to 600 meters. Both the Continental Intercalaire and the subhorizontal Nubian Sandstones lie with marked unconformity on the Basement rocks (Delany, 1960). Marine Jurassic is found in Ethiopia (Mohr, 1963; Mohr and Rogers, 1965) and to a limited extent in N.E. Sudan (Delany, 1960). In Ethiopia these sediments are taken to be deposited by the sea invading from the south-east. A mass of continental and littoral sandstones (the Triassic Adigrat Sandstone), is followed by a shale- gypsum facies, a marine limestone facies (the upper Jurassic - lower Cretaceous Antalo Limestone), and finally, in some areas, by a regressive sandstone facies (the Upper Sandstone). In the Abbai ( = Blue Nile) basin Mesozoic strata show a completely developed succession, they are successively revealed where the upper Abbai has cut southwards more deeply beneath the overlying Trap lavas. Thicknesses of the strata in the Abbai region are as follows: Adigrat Sandstone 550 meters, Antola Limestone 500 to 570 meters, Upper Sandstone 200 meters, and Tertiary basalts less than 500 meters. The Adigrat Sandstone consists of a white, massive quartzose sandstone. Quartz invariably greatly predominates over feldspar in the clastic grains; these grains are usually angular, rarely subrounded. The Antalo Limestone comprises many lithological types of limestone, such as the crystalline, compact and dolomitic ones. The Uppsr Sandstone is characterized by a lithology similar to that of the Adigrat Sandstone. >' Although the Mesozoic rocks thus reach a considerable thickness in the Abbai basin, they are exposed and subject to denudation in a relatively small area only, 37 because their outcrops are confined to the river canyon. The precise relationships of the Nubian Sandstone in eastern Sudan with the Mesozoic succession in northern and northwestern Ethiopia remain to be elucidated (Mohr, 1963).

2.4. Tertiary

The term 'Continental Terminal' is applied to the Tertiary continental deposits (Furon and Lombard, 1964). Apart from a few scattered Neogene deposits in Ethiopia, which are considered of secondary importance here, two formations in the Sudan need to be mentioned: the Hudi formation (not indicated on the geological map) and the Umm Ruwaba formation. The Hudi formation, a fossiliferous chert deposit of the Nile area north of Khar- toum (see figure 27), occurs both in situ, overlying the Nubian sandstones, and as a secondary deposit in ferricrete pebble beds. On the basis of fossil evidence the stratigraphie position is assumed to be Oligocène (Andrew, 1948). The deposits constitute an important element in the interpretation of the development of the Nile system (see Chapter 3). The Umm Ruwaba formation consists of unconsolidated clays and sands, some gravelly, the minerals (feldspar, biotite) are undecayed. Deposits occur in southern Kordofan and along the Rahad and Dinder rivers. The immediate significance of these presumably alluvial deposits is much reduced by the absence of precise evidence of age. -They are considered younger than mid- Tertiary and older than Kordofan sands and Gezira clays by Andrew (1948) and Delany (1960), while Grove and Warren (1968) conclude to a late-Pliocene and early-Pleistocene age. The term Stratiform flow is applied to the thick sequence of basalts and tuffs deposited in Ethiopia mostly during the Tertiary; it is more commonly referred to as Stratoid or Trap Series. Extruding from fissures and centres, these lavas covered the greater part of the Mesozoic rocks in Ethiopia and their flow extended onto Basement Complex rocks in the peripheral regions, as in east Sudan on the Gedaref ridge (the divide between Atbara and Blue Nile). The Trap Series chiefly consists of flood basalts (traps), but with trachytes and rhyolites occurring more especially near the top of the series, and caps the Ethiopian Plateaux except where overlain by rare patches of evidently recent lavas (Mohr, 1963). The Trap Series has been divided by Mohr (1965) into three major units:. (3) the Shield Group, composed of basalts, alkaline silicics and under-saturated lavas, dated as late Miocene-early Pliocene, generally conformable on (2) the Magdala Group, fissure and central-type basalts, with silicic lavas and 38

pyroclasts becoming more abundant near the top, dated as Oligocène and early Miocene, unconformable on (1) the Ashangi Group, fissure basalts with localized silicic lavas in N.E. Ethiopia, dated as Eocene. The Ashangi lavas are considered to predate the initiation of the Arabo-Ethiopian Swell, resulting in a moderate uplift of the whole region in the Upper Eocene. The major uplift of the Swell together with rift faulting on a large scale followed after the extrusion of the Magdala lavas, and occurred during, or shortly before, initiation of the great shield volcanoes, in early and middle Miocene (Mohr, 1965). The petrography of the flood basalts, quantitatively much the most important lava of the Trap Series, is remarkably uniform and summarized by Mohr (1963) as follows. 'The basalts are generally poor or deficient in olivine; the groundmass plagioclase is most commonly labradorite but the phenocrysts, frequently zoned, are usually more calcic; the pyroxene is magnesium rich, clinoenstatite or pigeonite, with an ophitic texture common; magnetite is a common and sometimes very abundant accessory, whilst brown hornblende and magnesium-rich biotite are much more rare. The associated alkaline silicic differentiates and assimilates have sanidine or anorthoclase feldspar, except where a low magmatic SiO2 content gave rise to feldspathoids such as nepheline, sodalite or nosean, together with sodic amphibole or pyroxene; where the magma was potassic such characteristic minerals as leucite, kaliophilite, muscovite and biotite are developed. These differentiates and assimilates are much less abundant than the basic lavas, and generally only come in near the top of the Trap Series.' The mean chemical composition of 18 basaltic lava samples of the Trap Series in Central Ethiopia is as follows:

Fig. 16. Chemical composition of Trap Series basalts after Mohr (1963)

SiO2 Al2o3 Fe2O3 FeO TiO2 CaO MgO K2O Na2O MnO PA H2O

47.43 15.31 4.80 7.42 2.43 9.50 6.40 1.53 2.88 0.22 0.42 1.66

Mohr (1963), in his regional description of the Trap Series, notes that in almost the entire Abbai basin silicic lavas are not developed, and only the basal basalts of the Ashangi Group occur, composed of compact, fine-grained basalts with some pyroclast beds, which are thus subject to erosion by the Abbai. The Trap Series here is perfectly horizontal, thus forming the monotonously 39 flat surface of the plateau away from the river gorges. Peculiar pétrographie types of lava are reported from the Abbai basin, the best-known being the apatite basalts of Mt. Salale(9°43'N, 38°36'E). The Trap lavas thicken away from some 220 m in the Abbai gorge, to an average 1000 m rising to a maximum of over 3000 m toward the west in Gojjam in west Ethiopia. Here also the more recent silicic lavas occur, including trachyte (alkali feldspar and green augite) and flow-banded or brecciated, glassy or cryptocrystal- line rhyolite. Directly south of Lake Tana large areas of the Plateau are covered by recent basalts, discussed below. According to Andrew (1948) the Ethiopian lavas extended farther north (and west ?) into the Sudan than does the present continuous outcrop. In central Sudan there are scattered occurrences of volcanicity: basaltic rocks west of Khartoum and west of Atbara. One effect of this volcanicity has been preservation by burial of the Hudi Chert.

2.5. Quaternary

The term Aden Volcanic Series is used to describe post-Miocene volcanic rocks of Ethiopia. Active volcanoes in the Ethiopian Rift System prove that formation of the series has not yet been completed. Most volcanic rocks are confined to the Rift System, though a number of isolated basaltic eruptions are known from the Ethiopian Plateaux. Such basalts cover an area of at least 3000 km2 south of Lake Tana. This lake owes its existence to the barrier which these recent lavas have formed, their thickness being nearly 100 meters where the Abbai plunges over them at the Tisisat Falls. The lavas almost entirely consist of basalts with some basaltic breccias and tuffs, and lie conformably upon the Trap lavas, a distinction is not always easy to make (Mohr, 1963). Geochemical comparison of the basaltic lavas of the Trap Series and of the Aden Volcanic series, show an increase in total Fe, Mn, Ca, (H2O) and a decrease in Ti, Al, Mg and alkalis with time (Mohr, 1963).

The following most recent continental deposits in the Sudan are not shown on the geological map. The Gezira formation, occupying a large part of the Gezira and Kenana areas between the Blue Nile and the White Nile, rests conformably on the Nubian formation. A subdivision of the Gezira formation into three rock stratigraphie units is reported by Berry and Whiteman (1968): 40

(3) Upper Clay member (2) Lower Sandy member (1) Mungata memter The Mungata member, recognized in the southern Gezira only, may range in texture from clay to gravel, but is predominantly clayey. This subsurface unit is more than 110 m thick. The Lower Sandy member consists mainly of sands with occasional gravel, silt, and clay lenses. The unit varies in thickness from 20 to over 57 m. The Upper Clay member consists mainly of clay and varies in thickness from 6 to 45 m. In several places close to the east bank of the Blue Nile between Wad Medani and Khartoum, and west of Khartoum, outcrops of Nubian sandstone appear through the Gezira formation.

The Kordofan sands form an unconsolidated surface deposit of fixed dunes, covering large areas between 10 and 16° N to the west of the White Nile. These so-called 'Qoz' deposits almost entirely consist of well-rounded quartz grains, with a colour ranging from pale buff to deep red (Edmonds, 1942) and with about 90% of the particles in the size range 0.01 to 1 mm (Jewitt, 1950). The origin is attributed to the weathering of Nubian sandstone by Edmonds (1942), while Grove and Warren(1968) mention a derivation partly from the Umm Ruwaba beds. According to the last mentioned authors two major periods of dune formation can be recognized: one, resulting in the Low Qoz, characterized by N-S running longitudinal dunes (alâb) that show definite signs of weathering; the other more recent, the High Qoz, with a complicated pattern of transversal dunes. Berry and Whiteman (1968) suggest that Qoz deposits were probably formed during late Pleistocene. • CHAPTER 3

Geomorphology

Lands of central Sudan occupy only a part, though from a geomorphological viewpoint a key part, of the entire Nile basin. They are restricted to the section approximately located between latitudes ll°30' and 18°00' N, while the Nile basin itself stretches over some 35 degrees of latitude from 3° S to about 32° N. In this section of the river basin, about at latitude 15°36', the White Nile from Lake Victoria (at 1135 m) and the Blue Nile from Lake Tana (at 1840 m) join to form the Nile at Khartoum (at 376 m). The position of the investigated area within the Nile basin is schematically shown in the generalized section of the Nile river system in figure 17. As indicated the area covers both a part of the Blue Nile basin, from the Ethiopian border to Khartoum, and of the Nile basin, from Khartoum to Atbara.

°70O0 «OOO 9000 4000 »000 2000 Fig. 17. The Nile and its tributaries (adapted from Hurst, 1952)

In Ethiopia the Blue Nile drainage system starts with a number of minor tributaries flowing into the 3000 km2 Lake Tana. About 30 km downstream from the outlet of the lake the water falls some 50 m (Tisisat Falls) and enters a deep V-shaped canyon cut in the Trap basalts. At river kilometer 900 approximately the river emerges again on plains leading to the Sudan (approximate elevation 490 m). 42

32° 36°

Sbendi Wad Ban Naga

KHARTOUM A U.Qeili O

12° - Ingessana/hiiis O\ 32° 34° Fig. 18. Central Sudan 43

The landscape related to the Blue Nile in the Sudan and its tributaries the Dinder and Rahad, essentially consists of a flat and nearly level plain with a maximum width of some 250 km. Its western boundary is formed by the watershed of the Blue and White Nile, passing from south to north over Jebel Mazmum, J. Dali, J. Moya to Managil on the Managil ridge; its eastern boundary is the watershed with the Atbara river, passing from south to north over Gallabat, Gedaref, and J. Qeili (see map fig. 18). This plain ranges in elevation from some 480 m west of Damazin to some 380 m at Khartoum, with an overall gradient of about 22 cm/km. The Blue Nile river, incised in this plain to some 12 m depth, has a slope in this stretch of 12 cm/km between Damazin and Singa, decreasing to 9-10 cm/km between Sennar and Khartoum. The landscape related to the Nile essentially consists of a number of terraces confined to a relatively narrow zone along the river. The Nile river slope in the Khartoum-Atbara section is about 10 cm/km. The main interruption is formed by the 6th Cataract in the Sabaloka Gorge. Before going into any further details on the component parts of these landscapes, and their genesis and development, we will discuss the Nile river system as their main building agent as far as it is thought relevant.

3.1. Blue Nile - Nile River System The present-day Nile system receives an average quantity of 1900 km3 of water per year from rainfall. Some 94% of this is lost to the air primarily through evaporation and through transpiration of the vegetation. Only a relative small amount of water (some 1 to 2%) is used for agricultural purposes in south Sudan on narrow stretches of land along the Nile. This quantity also includes water used for irrigation in the Sudan and the evapotranspiration of wood and grasslands partly being used. The remaining 4% of riverwater, or an average of 83 km3, pass the Nile at Aswan each year. Half of this amount is used for irrigation or lost through evaporation in the U.A.R. below Aswan (Hurst, 1957; Haude, 1961). The regime of the Nile is characterized by considerable variations. Thus the annual discharge at Aswan in the last hundred years has ranged from 151 km3 in 1878/79 to 42 km3 in 1913/14 (Hurst, 1957). As outlined in Chapter 1.1.2. precipitation in the Ethiopian highlands shows a marked maximum in the period between June and September. This is the main factor that causes the very high summer floods in the Ethiopian tributaries of the Nile and thence in the Nile itself. Figure 19 clarifies this point, as it shows the mean flow in the Nile at Aswan throughout the year (effects of reservoirs have been omitted) (Hurst, 1952). 44

May June July Aug. Sept. Oct. Nov. Dec. Jon. Feb. Mor. Apr.

Fig. 19. Mean flow in the Nile at Aswan (Hurst, 1952)

The upper line represents the discharge of the Nile at any time, and the other lines the component discharges. It is noted that, during the summer flood, up to 55% of the total annual flow in the Nile is contributed by the Blue Nile. This river during flood transports some 85% of its total annual flow. Its normal floodstage flow is about 5640 m3/sec, which is contained in a channel some 400 meters wide and 10 meters deep; the low stage base flow is approximately 185 m3/sec which flows in a channel 200 meters wide to a depth of 2-3 meters. The maximum and minimum recorded discharges are 10,800 and 40 m3/sec respectively. To emphasize the effect of the Blue Nile on the Nile flow, data on the average flow in the tributaries of the main Nile are tabulated in figure 20.

Fig. 20. Average water flow in the Nile system in nfijsec, according to Schamp (1959)

River per year in May in September

Blue Nile (Khartoum) 1631 185 5640 White Nile (Khartoum) 800 544 1041 Nile (Khartoum) 2435 729 6681 Atbara 371 — 1355 Nile (Aswan) 2640 556 8060

The sediment load transported by the Nile river system will now be briefly discussed, first the solid load (3.1.1.) and subsequently the dissolved load (3.1.2.). 45

3.1.1. Solid sediment load No data are available on the part of the solid load that moves along the channel bottom in the lower layers of laminar flow: the bed load. This phenomenon is very hard to measure, or even to estimate closely. A considerable number of observations, however, has been made to establish the suspended load carried by the Nile. Simaika (1940) estimates that a total of 100 million tons of suspended matter is carried by the Nile annually. The particle size distribution of this material during flood time given by him and Ball (1939) for three places on the Nile is as follows (in weight percentages):

Fig. 21. Particle size distribution of the suspended load of the Nile

Location > 2 mm 2000-200 (i 200-20 n 20-2 ^ <2ß

Wadi Haifa . 24 42 34 Aswan 15 40 45 Cairo 0.2 18.5 26.3 55.0

The gradual increase in clay content in downstream direction is explained by Ball to be due to the slow fall of the coarser material to form the bed load. A graphic presentation of the average daily amounts of suspended matter passing

750 f i \ i w 500.1 z i \< \ 250 / 'f\ \ / i 1 K July Aug. Sept. Oct. Nov. Dtc.

Fig. 22. Suspended load of the Nile at Wadi Haifa (Hurst, 1952) 46

Wadi Haifa during flood time, and the normal discharge of the Nile, is given in figure 22. The mineral composition of the solid load was studied on samples taken from the riverbed at Damazin (Blue Nile) and Shendi (Nile) by the author (figure 18). In addition, a comparison was made with results of similar studies on Nile alluvia in the U.A.R. In order to facilitate the mineralogical analysis these samples were separated into a number of size fractions: a. sand of 500 to 50 microns, separated into a heavy and a light fraction by means of bromoform (specific gravity 2.89); b. silt of 50 to 2 microns, separated into the fractions 50 to 37, and 50 to 20, 20 to 5, 5 to 2 microns; c. clay of less than 2 microns; For a concise description of analytical procedures followed, one is referred to Chapter 6.1. It should be noted that prior to fractionation the above samples were not subjected to any pretreatment, thus no damage to minerals could occur. a. Minerals of the sand fraction (500 to 50 (i) were examined by means of the polarizing microscope. Results for both the light and heavy fractions are recorded in figure 23. The data on the mineral composition show that the sandy sediments of the Blue Nile and Nile are very similar; this reflects the overwhelming importance of Blue Nile sands in the main Nile system. In the light fraction quartz constitutes about half the amount of minerals present. Further research on this fraction of sands of the Blue Nile at Damazin revealed that quartz accounts for 75% of the minerals in the size fraction 500 to 210 /«, its relative importance gradually decreasing with decreasing grain size to 50 to 35% in the 105 to 50 /.i fraction. The origin of these usually angular and subangular quartzgrains may be attributed to the Mesozoic quartzose sandstones in the Blue Nile basin in Ethiopia. The basalt fragments (often containing laths of labradorite), volcanic glass, labradorite, and biotite, are considered to be derived from the Tertiary flood basalts. The amount of volcanic glass was found to increase with decreasing grain size'to some 7% in the 105 to 50 JU fraction. The potassium feldspars are of non-basaltic origin, and may be derived from Base- ment Complex rocks and/or Mesozoic sandstone. Of more than usual interest is the detection of 2 to 3% minerals of the zeolite group. Zeolites presumably are also present in the basalt fragments, but they could not be specifically identified in them. Among the zeolites observed analcime is the most conspicuous. An indication concerning the source of the zeolite minerals, as well as support for the assumption that zeolitization often proceeds from 47

Fig. 23. Mineral composition of sands (500 to 50 ß) of the Blue Nile and Nile

Light mineral composition: Sample Weight Minerals in mutual percentages location percentage of total 60 glas s fraction c rit e C Quart z Potassiu r Labrad o Zeolit e Biotit e Calcit e product s Basal t Volcani c feldspar s Albit e Weathe r fragmen t

Blue Nile, 87.1 3 45 12 1 6 3 13 6 4 7 Damazin

Nile, 87.9 1 52 17 t 4 2 5 t 8 11 Shendi

Heavy mineral compostition: Sample Opaque Transparent minerals in mutual percentages

0) len e it e nd e S S Rutil e Sillima n Zoisit e Biotit e Saussu r Basal t fragme n Zirco n Garne t Staurol i Epidot e Hornbl e Tit . au g Hypers t Apatit e Titanit e Augit e

Blue Nile, 9 t 2 t 1 t t 19 t 33 11 12 1 15 t 1 15 Damazin Nile, 15 t t 1 1 t 1 11 1 42 12 6 t 1 1 4 19 Shendi hydration of feldspar (Hay, 1966), is provided by the fact that a number of minerals have been noticed to be with characteristics intermediate between those of feldspar and zeolite. Khadr (1960) examined the light fraction of some Egyptian Nile soils in detail. His general results are epitomized in figure 24.

Fig. 24. Ranges in mineral composition of light fractions of Nile soils in the U.A.R., summarized after Khadr (1960)

Quartz Potassium Albite Oligoclase Andesine Muscovite feldspars

70-80 10-17 3-6 2-4 t 1-2 48

Khadr explicitly states that, owing to the pretreatment of these samples with concentrated acids, certain detrital minerals may have been partially or wholly dissolved. Considered to be subject to solution - when compared with his further data - are volcanic glass, intermediate plagioclase, biotite, gypsum, chlorite, and clay-like minerals. This particular aspect presumably accounts for the differences with our determinations, which show less quartz, about equal amounts of potassium feldspars, but more of the other minerals. Khadr also noted a significant decrease of quartz toward the 105 to 50 /.i fraction. The heavy mineral fraction of sands of the Blue Nile and Nile is characterized by a hornblende-augite-epidote association; the three minerals constitute 71 to 75% of the total fraction. The presence of some 20% augite, especially the titani- feröus variety, as well as over 15% basalt fragments, indicate a volcanic provenance. The minerals derived from metamorphic rocks (possibly Basement Complex), such as staurolite and sillimanite, and the stable minerals zircon, garnet, rutile, and titanite, all are very minor constituents. The data on the heavy mineral composition are essentially in accordance with those obtained by Shukri (1950), who reports on the mineralogy of recent sediments of the Nile and its tributaries. According to Shukri the Blue Nile sediments are characterized by an abundance of hornblende, biotite, iron ores, the presence of augite (8 to 27%), and lava fragments in appreciable amounts, the rarity of staurolite, kyanite, garnet, and the presence of olivine and apatite. In his enumeration Shukri fails to mention the occurrence of epidote, which in figures accompanying his article is indicated as some 10%. Shukri also concludes to a great similarity between sands of the Blue Nile and those of the Nile between Khartoum and Atbara. Khadr (1961a), when reporting on heavy residues of recent fluviatile sediments of the Egyptian Nile and Nile delta area, states that the augite-hornblende- epidote association is characteristic of all samples. These three minerals together comprise 79 to 95%, all others are very subordinate. Both the metamorphic minerals staurolite, kyanite, and sillimanite, and the stable minerals such as tourmaline, zircon, rutile, are rare.

b. Minerals of the silt fraction (50 to 2 /<) of the Blue Nile and Nile samples were identified by X-ray diffraction analysis, differential thermal analysis, (both described in Chapter 6.1) and partly by means of the polarizing microscope. The composition of the silt fraction of the sediments of both rivers was also found to be highly similar. Minerals of the 50 to 37 ju fraction were optically determined in order to further trace the occurrence of volcanic glass. Some 10% of glass appeared to be present. The trend of volcanic glass increasing with decreasing grain size in the sand 49 fraction thus continues to at least the coarse part of the silt fraction. Also about 4% minerals of the zeolite group were identified in the 50 to 37 /n fraction. Analysis of the X-ray diffraction patterns of the silt fraction was facilitated by comparison with X-ray diffraction curves of the heavy and light sand fractions. It appeared that about 50% of the silt fraction consists of minerals of the montmorillonite series, the vermiculite series, kaolinite, and mica (in order of decreasing quantities), while in addition about 20% quartz, about 15% feldspars, about 5% calcite, and some amorphous material occur. The presence of minerals of the montmorillonite and vermiculite series was confirmed by the observation that the 18 Â and 14 Â diffraction spacings of these layer silicates in the presence of glycerol, closed to 10 Â on heating at 500° C. The diffraction pattern of kaolinite was destroyed by heating to 500° C. Differential thermograms of the silt fraction show a double endotherm at 110 to 180° C, arising from expulsion of sorbed water, a strong hydroxyl endotherm at 510 to 530° C, a smaller second hydroxyl endotherm at 700 to 740° C, and a small exotherm at 910° C. The main hydroxyl endotherm indicates that the specific species (octahedral composition) of montmorillonite involved contains considerable iron, and may well be nontronite (see also Chapter 6.2.5.2.) The differential thermal curves do not permit further conclusions with regard to the nature of the amorphous material present. Further information on the silts was obtained from X-ray diffraction analysis of the three fractions 50 to 20, 20 to 5, and 5 to 2 /.i. Conclusions are as follows. The amounts of quartz, feldspars, and mica regularly decrease with decreasing grainsize. On the basis of comparison of diffraction intensity of quartz peaks with standard patterns, the amounts of quartz in the three size fractions could be estimated at 35, 20 to 15, and 10% respectively. These percentages are intermediate between those of the light sand fraction (60%) and the clay fraction (3%). The fact that the 14 Â peak height remained the same in the three fractions, but increased in broadness toward the finer silts, suggests a decrease in vermiculite minerals and a concomittant increase in montmorillonite minerals with decreasing grain size. This is confirmed by the fact that minerals of the montmorillonite series dominate in the clay fraction and vermiculite is absent in it. The amounts of kaolinite and of amorphous material (as judged from the diffraction background) gradually increase toward the finer silt fractions. c. Minerals of the clay fraction (< 2 /t) of the Blue Nile and Nile samples were identified by X-ray diffraction analysis and differential thermal analysis. The composition of the clay fraction of the sediments of the Blue Nile and Nile was found to be very similar, which is in conformity with highly similar sand and silt fractions. 50

X-ray diffraction patterns indicated that minerals of the montmorillonite series constitute the dominant component in this fraction, with a limited amount of kaolinite, and no vermiculite, further about 3% calcite, about 3% quartz, and traces of feldspars and (in one case) of mica. The differential thermograms of the clay fraction follow closely those of the silt fraction, showing a double endotherm at 140 to 200° C, a strong hydroxyl endotherm at 535 to 540° C, a smaller hydroxyl endotherm at 740 to 750° C, and a small exotherm at 910° C. They warrant the same conclusions as we put forward with relation to the silt fraction. Further mineralogical research of the Nile clays is limited to deposits of the Egyptian Nile and the delta area. Results obtained are of a controversial nature, as may appear from the following. In a series of investigations, using such techniques as X-ray diffraction, differential thermal analysis, and electron microscope examination, Hamdi (1953, '54, '55, '59, and '61) with co-authors Naga (1949, 1952) and Epprecht (1953, 1955) come to the conclusion that illite is the main component of the clay fraction of Nile alluvia in the U.A.R. One of these investigations - the often quoted 1959 study on five samples of a 12 m deep profile in the middle of the delta area - is of particular interest. In this Hamdi states that alteration of illite clay in submerged Nile deposits is shown. The new products formed are boehmite and a montmorillonite-like clay. This conclusion, however, is considered to be unestablished by the author. The experimental conditions are not sufficiently explained in the publication concerned, this especially pertains to the pretreatment of various samples examined (our determinations showed marked differences in peak height of the 14 Â X-ray diffraction spacing depending on pretreatment with HC1 or not). This inevitably involves that results presented cannot be compared as such. It further remains obscure whether the recently deposited surface sample-used as standard-contains any 14 Â mineral. The character of Nile clays has further been determined by Hashad and Mady (1961) and Khadr (1961b). The former, using the differential thermal method only, also report that minerals of the illite group (with possibly a rather small quantity of montmorillonite) are the main constituent of the less than 1 /t fraction in three alluvial soils located east of the Damietta branch of the Nile. Similar to these observations are the results obtained by Khadr (1961b) on the same size fraction of a series of topsoil samples collected in the Nile valley and the delta. On the basis of X-ray diffraction and differential thermal analysis, as well as on electron microscope examination he concludes to the presence of 60 to 75% illite, 10 to 15% montmorillonite, 10 to 15% kaolinite, and some 3% quartz. 51

Conflicting with this, however, is his table showing X-ray diffraction spacings of the investigated minerals, which only permits the conclusion that montmarillonite is the dominant claymineral, with some kaolinite, and possibly some illite. Then Elgabaly and Khadr (1962) report montmorillonite for the first time to be the dominant mineral of the less than 1 /i fraction of Nile delta soils with an estimated cDntent of 70 to 75%, followed by kaolinite, with a low content of hydrous mica. Hamdi (1967), while stating that the greater part of the clay fraction of soils of the Nile delta consists of particles of less than 1 //, then also showed - through X-ray diffraction analysis of two samples from the southern part of the delta - that the dominant mineral of this fraction belonged to the montmorillonite series. Minor quantities of kaolinitic and illitic clayminerals were found to be present. Our determinations concerning the mineral composition of the clays of the Nile and Blue Nile are thus confirmed by the latest observations in the U.A.R., viz. by Khadr's data - but not by his interpretation - and by the determinations of Elgabaly and Khadr (1962) and Hamdi (1967).

Our investigations also include the determination of organic carbon and total nitrogen in the solid load of the Blue Nile (Damazin) and Nile (Shendi). The concentrations of these constituents were found to be similar in both rivers - complemental to the broadly equal mineral composition of their sediments - and appeared to be texture-dependent, with the highest concentrations in the finest materials. Measurements showed that in the fine-textured load (primarily forming the suspended load) organic carbon values range from 1.65 to 1.50%, and total nitrogen is 0.10%, giving a carbon-nitrogen ratio of 16 or 15. Coarse-textured sediments contained only 0.06 to 0.03% organic carbon and less than 0.01% nitrogen. These results are in accordance with measurements of organic carbon and nitrogen in suspended matter of the Nile in the U.A.R. Zein el Abedine et al. (1964) found 1.12% organic carbon and 0.13% nitrogen during the flood stage of the Nile. They also observed much higher concentrations during the low stage, but this was primarily attributed by the above authors to the water stored in the Aswan reservoir containing relatively high amounts of organic debris and green algae.

3.1.2. Dissolved sediment load Beside the solid load carried by the Nile and its mineralogical and organic matter composition, the dissolved load and its chemical composition should be considered. The average amount of matter in solution in the Nile system of over 160 ppm exceeds both the average dissolved load of river waters of the world (120 ppm) 52 and of Africa (121 ppm) (Livingstone, 1963). The chemical composition of the water of the tributaries of the Nile varies slightly from stream to stream and also throughout the year. Thus Blue Nile water is slightly different from White Nile water (Hurst, 1952). A review of data on the chemical composition of the dissolved load of the Nile system is given in figure 25. The main ions in solution are calcium and bicarbonate. The solubility of CaCO3 in water of 25° C at practically the average CO2 partial pressure of the atmosphere of 0.00031 at. is 52 mg CaCO3/liter, or 52 ppm (Scheffer and Schachtschabel, 1970). This concentration corresponds to 20.8 ppm Ca and 63.3 ppm HCO3, which are respectively about equal to the concentration of calcium and about half the bicarbonate concentration of Blue Nile and Nile water. The water of these rivers is thus shown to be (nearly) saturated with calciumcarbonate. The analyses further show that the water of the Nile system consistently contains less Ca + Mg than HCO3, in amounts ranging from 0.1 to 1.1 meq/1. Upon evaporation of this water - either after flooding under natural conditions or upon irrigation - the residue of carbonate, after precipitation of calcium and magnesium as carbonate salts, is paired with sodium. This 'residual Na2CO3' (Eaton, 1950) may contribute to alkalization of soils. For irrigation purposes the waters of the Blue Nile and Nile are rated in quality- class C2-S1, medium-salinity and low-sodium water, according to the diagram of the U.S. Dept. of Agriculture (U.S. Salinity Laboratory Staff, 1954). The average concentration of dissolved constituents varies from 150 to 185 ppm, equivalent to a conductivity of 240 to 290 micromhos/cm, and the sodium-adsorptionratio varies from 0.3 to 0.9. According to Krauskopf (1967), the kind of salt carried by anyone stream depends largely on the climate. Near the headwaters of a stream the composition may reflect adjacent rocks, but the effects of vegetation, of adsorption processes, and of mixing with tributaries very quickly alter the composition to a type largely responsive to the climate. As outlined in Chapter 1.2, climatic conditions in the past differed considerably from those prevailing at present, thus influencing both water and sediment discharge from Ethiopia. The most recent of the resulting Nile fluctuations are recorded by De Heinzelin (1967) in his Nile highwater curve of the late Upper Pleistocene-Holocene, on the basis of research carried out in the Nile valley in Nubia. This curve (fig. 26) is presented as the most likely interpolation of data obtained from special sedimentary features, usually narrowly limited, which escaped later destruction and at the same time afforded both accurate elevation measurements and C14 dating. Fig. 25. Chemical composition of the dissolved load of the Nile system

River Location Date Parts per million Author sampled Dissolved solids Ca Mg Na K HCO3 SO4 Cl NO3

Blue Nile Wad Medani 1936-37 — 24.4 5.3 9.9 — — 9.4 — — — Greene H. and Snow O. W., 1939

Khartoum 1904-07 T- 20.2 7:1 7.5 2.0 — 6.3 2.4 — 19:5 Beam W. in: Greene H. and Snow O. W., 1939 184 23.8 6 7.1 1.7 108.6 7.3 2.8 0.14 26.7 Livingstone D. A., 1963

White Nile Khartoum 249 17.4 5.2 30.7 11.8 149.2 0.44 8 0.44 25.6 Livingstone D. A., 1963

Nile Giza 1924-26 -— 25.1 7 —- — 102 9 11.6 .003 14.1 Livingstone D. A., 1963 Cairo 1933-36 167 23 9 20 — 127 10 12 — 21 Hurst H. E., 1952 Cairo 1957-58 152-181 22- 8-9 15-22 4 55- 13- 7.5 — Abdel Bar in: 25.3 14.5 15.4 Hassan F. M. H., 1967 54

Nile üeologic BP formotio s time 1 Islom . 1 Christ. • 2 Meroitic . 3 J Aid-New iii - 4 Cingdom . 5 Oodrus CEN I O • 6 _l O • 7 X

8 .r 1KI N • 9 f »

•It Birbet

12 UI -13 UzI •14 ^ÖS'"i.r>if'4'^ m o < O •15 1- CA -16 üï

•IT 0.

•18 'v''VEii% S. Ballono UI -19

-20

21 UI 22 z UoI -23 tr bJ o -3 •24 "^ 1 (O (T UI 25 _l m a. 26 O i (T •27 UI a. 28 a. 3 -29

•30

158 156 154 152 150 146 146 144 142 140 138 136 134 132 130 128 126 124 122 I20ITI abs. elev.

Fig. 26 Interpolated Nils high-zoMer curve (Ds Heinzslin, 1967)

Summarizing, it can be stated that precipitation in the West-Ethiopian highlands largely controls the yearly waterflow and sediment load of the Blue Nile-Nile system; the maximum discharge of water and sediment practically occur simultaneously during summer. The fiuviatile landscape of central Sudan has developed as a result of the action of this Nile system, subject to various fluctuations in the past. 55

Fig, 27. Inselbern on the Er Roseires terrace I Ken ana day plain)

r Fig. 28. Aerial view of relic of m?.inder cut-o < on the Er Roseires terrace, Blue Nile east ba:i!it south of ll'nci Medani 56

3.2. Blue Nile Landscape

In the Blue Nile valley the following landscape elements can be recognized. Along the active river channel recent terraces form discontinuous strips of land, up to some hundred meters wide, from 6 to 10 m above low water level. Deposits of this terrace usually consist of fine silty or fine loamy materials, the original sedimentary stratification has been destroyed in the top 50 to 150 cm, but is clearly preserved below this depth. This terrace is converging in downstream direction with the older and larger Er Roseires terrace (so called by Berry and Whiteman, 1968). The difference in elevation between the two terraces decreases from some 8 m at Er Roseires, to 6 m at Sennar, to 3 or 5 m at Khartoum. The Er Roseires terrace is considered a former floodplain. Natural levees are present in almost continuous stretches along the Blue Nile, they have in places been eroded by recent gullies (so-called 'kerrib land'). Levees also occur along meander cut-offs and other formerly alluviating channels away from the present river. The backslope of these levees is very gentle, generally amounting to a drop of 1 m in the first km. Backswamp areas are located between the levees, and between the levees and the higher grounds toward the divides with the Atbara and White Nile. The floodplain is interrupted by a number of inselbergs (jebels). A sharp contrast exists between natural levees and backswamp surface sediments. Natural levee deposits generally consist of sandy or loamy materials. Because of pedogenetic processes the original sedimentary stratification is obscured at least in the top 2 m, and could not be detected in the field. At deeper levels, in gully cuts, this stratification can be observed .The backswamp deposits are essentially clayey, and without clearly discernible sedimentary strata. While on most floodplains elsewhere in the world gradational sorting of sediment is evident, with coarser alluvium upstream and finer downstream, the reverse is the case on the Er Roseires terrace. There is a steady northward increase in sandi- ness, such that generally the sand and especially the fine sand content, of the clayey soils is about 5% in the Kenana area, increasing to an average 20% toward the north in the Gezira. This phenomenon may very well be the result of soil forming processes (Chapter 6.2.1.). It may also be due to the influence of blown sand, since the Gezira is located at the edge of the Qoz, the fixed dunes and associated sandsheets of west central Sudan that dominantly consist of fine sand and coarse silt. It has also been suggested that the increased sand content of Gezira deposits finds its origin in the late Mesozoic land surface, which is found at shallow depth in the North Gezira, as mentioned in Chapter 2. Throughout the Blue Nile clay plains shallow drainage channels and depressions with or without outlet were observed, differences in height decrease toward the northern part of the plains from about 5 m to 1 or 2 m. 57

Berry and Whiteman (1968) report that in the Gezira area occasional sandy patches and sanddunes occur in association with shallow discontinuous channel systems. Though this will be correct in some places, as confirmed by Williams (1968a), other isolated sand areas (inselberg relics ?) occur without apparent relation to an observable drainage way. Since until now no clear-cut branching of the Blue Nile in a downstream direction, forming a more or less fan-shaped network of distributaries, has been established, Berry and Whiteman's statement that the Gezira possesses many characteristics of an inland delta graded to the White Nile, should be accepted with reserve. Tothill (1946), after his study of the distribution of aquatic, amphibious, and pulmoniferous molluscan remains found in the clays of the Gezira plain (Upper Clay member), concludes that the clays were deposited in a humid or marshy environment in the Blue Nile valley, but that the annual riverflood receded during the deposition, and that there was no permanent lake.

3.3. Nile Landscape

The landscape of the main Nile between Khartoum and Atbara generally consists of three terraces fringing the river at intervals. These occur at estimated heights above low water level of 3 to 5, 11, and 17 m, and are indicated in the following by the Roman numerals I, II, and III respectively. Occasionally a fourth terrace (IV) can be recognized at approximately 22 m above the river, as at Wad Ban Naqa. The bulk of the terrace material is derived from the Blue Nile, with probably a slight influence by the White Nile, as appeared from mineralogical research of soils developed in these terraces. Terrace I, which is mostly under irrigation, sometimes gradually passes into the contemporaneous sandbars of the river. The topography of terrace II is flat, the Khartoum - Atbara railroad along the east bank of the river generally is located on it. The highest terraces usually contain more gravel, and are more eroded (by wind action and by water from intermittent streams coming from the surrounding mountainous country). These terraces often merge into pediment surfaces extensively developed on either side of the river. Although recognition of these terraces in the field generally entails no particular problems, a valuable criterion in terrace correlation was found to be the fact that gravel at the surface of terraces III and IV showed distinct black desert varnish, in contrast to gravels of terrace II and I, which had practically no varnish or none at all. From Khartoum downstream the Nile valley is cut into the Nubian formation, and terraces gradually become narrower until the 6th Cataract is reached. Here the Nile valley, incised into the Basement Complex, consists of a narrow gorge - 58 the Sabaloka Gorge, a feature of considerable interest in the interpretation of the river's evolution. The physiographic pattern of the Nile in the Sabaloka area suggests it having come down from a superimposed position. One may assume that an old Nile system acquired its course upon the Nubian Sandstones, which concealed the underlying Basement Complex. Upon rejuvenation the stream cut through the covering formation and transected the buried Basement Complex. Eventually the covering of Nubian rocks in the area was largely removed. This course of things was already postulated by Andrew (1948). Contrary to the views of Berry and Whiteman (1968), considering the Sabaloka complex an inlier, the term exhumed monadnock is more appropriate, as the Sabaloka complex is not directly surrounded anymore by Nubian sandstone. However, apart from semantics, an old Nile system may be inferred to have existed already soon after deposition of the late Mesozoic Nubian formation. Ever since this deposition the region has been subjected to sub-aerial denudation. Whether or not an early course of the Nile was initiated in late Mesozoic or early Tertiary times north of Khartoum, and took part in this denudation, depends upon the interpretation of origin and occurrence of the Hudi Chert, now found north of Sabaloka (see map of figure 29). Berry and Whiteman suggest (1968) that these fresh water deposits, belonging to the Lower Tertiary were deposited in a depression aligned along the present

30° SEMNA 330

HUDI SERIES XLo BflflChertinSitu 0 200 400Km iHÜ „ Breccio

Fig. 29. Distribution of the Hudi Chert (Tothill, 1948) 59

Fis. -30. Nile terrace III yiear Kabushixa

Fig. 31. Nth terrace II (foreground} and terrace I (irrigated) 60 course of the Nik, and represent a freshwater lake sediment of the Nile. With falling base level through Tertiary times, the river then cut through the Hudi Chert and the not yet completely eroded Nubian Sandstone, or even onto the Basement Complex. From Sabaloka downstream the Nile terraces become wider again, to a total maximum width of about 5 km, and north of Wad Ban Naga the river cuts into the Nubian formation again. Between Shendi and Kabushiya the valley incised in the Nubian sandstone narrows and with it the terrace deposits on either side. In this area Berry and Whiteman (1968) recognize the Shendi surface. It is considered an Early Tertiary erosion surface, a widespread hilltop level; here occuring at some 420 m above m.s.l., and about 61) m above the Nile, mainly developed on the Nubian formation and iron-enriched. Almost to its confluence with the Atbara river the Nile river is flowing over Nubian Sandstone. Geomorphological information on the Nile valley in the northern Sudan, between Atbara and Semna, is scarce. A number of incidental observations are reported, but these mostly have no direct bearing on our subject. Detailed investigations by De Heinzelin (1967), Butzer and Hansen (1967) and Giegengack f 1969), have considerably contributed to the elucidation of the formation of the Nile valley north of Semna in the Sudan and in southern Egypt. They are also of interest with a view to landscape development in central Sudan. De Heinzelin (1967) recognizes three main phases of aggradation, respectively at about 35, 25, and 13 m above the modern floodplain, and not older than the late Upper Pleistocene. The lithological character of these 'Nile Group' deposits indicates a typical Nile formation (i.e. brought by the Blue Nile - White Nile - Atbara system). Older fluviatile deposits, the 'Pre-Nile Group' occur between 38 and 78 m above the floodplain, and have a completely different lithology. They are not a typical Nile formation, but of relative local origin, and Acheulian or even older sites ( ?) rest on the eroded slopes, making the deposits at least Early Pleistocene. Of the period between the formation of the Pre-Nile and Nile Group very little evidence is preserved in Nubia. Only two localities with silts of the 'Proto-Nile Group' are observed. This situation, as well as the observations with relation to the Pleistocene evolution of the Nile valley in southern Egypt (Butzer and Hansen, 1965), are in accordance with the conclusions of Shukri (1950) based on heavy mineral investigations of terrace deposits of the Egyptian Nile valley. Shukri found Plio-Pleistocene (Pre- Chellean) sediments to be devoid of pyroxenes. He recorded these minerals in some abundance in the Early Pleistocene (Lower Paleolithic) of the Nile valley, but in amounts comparable to present-day conditions not until late Upper Pleis- 61 tocene (lower Sebilian) deposits. Shukri concludes to a causal relation between amounts of pyroxenes observed and the influx of material from the Ethiopian tributaries. In our opinion, however, low amounts of augite in the sand fraction of these older sediments may conceivably be due to loss by weathering (see also Chapter 6.2.5). This may pertain especially to Early Pleistocene (Lower Paleo- lithic) conditions. Previous workers have also advocated that the tributaries draining the Ethiopian plateau are younger events in the life history of the Nile. (Arldt, 1918; Sandford, 1929; Huzayyin, 1941). It is tentatively concluded from the above that they are in fact younger than Early Pleistocene. The Egyptian Pliocene and Plio-Pleistocene Pre-Nile still may or may not have been connected with the Sudan White Nile system. On mineralogical grounds Shukri (1950) thinks that a Sudan extension of the Nile did exist - thus explaining the wide valley in northern Sudan and Egypt - but possibly originated in Kordofan (west Sudan). Such a White Nile - Pre-Nile system is supported by Berry and Whiteman's (1968) thesis that during the Tertiary the Hudi Cherts were deposited and subsequently eroded by the Nile.

3.4. Age of Landscapes

Information available on the age-relations of the different landscape elements in the Blue Nile and Nile valleys is limited; a review of isotopic data obtained so far, is given below. In the Blue Nile valley at Singa and Abu Huggar extensive mammalian and human remains and artifacts were obtained from the riverbed, or from the base of the bank covered by about 9 m of younger deposits of the Roseires terrace. Bate (1951) concludes that the animal remains are undoubtedly Upper Pleistocene, but need not belong to the latest phase of the period. Berry and Whiteman (1968) report a radiocarbon date for a crocodile tooth of this deposit of 17,300 ± 200 YBP, but indicate the possibility of false dating due to impregnation with younger radiocarbon. After investigating the artifacts, Lacaille (1951) notes that the closest parallel to the facies is provided by the advanced Levalloisian industry of Lochard, south Rhodesia, and even more advanced stage. Finally, Clark (1967) classifies the industry as undifferentiated 'Middle Stone Age', a main stage covering the period of 38,000 to 20,000 YBP. The Roseires terrace was furthermore observed to be cut by two terraces of the White Nile, with main levels at 386 and 382 m, believed to be lake margin features (Berry, 1962). At two sites on the lower terrace - respectively in latitudes 13°34' and 13°56' N, at elevations of 381.5 and 381 m - buried shells were collected 62 and radiocarbon dated at 11,300 ± 400 and 8,370 ± 350 YBP (Williams, 1966). Berry and Whiteman report the former date as 8,730 ± 350 YBP, and give slightly differing elevations. No information is available with relation to the older White Nile terrace. The above might lead to the tentative conclusion that the Roseires terrace was developed in the period between 38,000-20,000 and about 11,000 YBP. In the Nile valley between Khartoum and Atbara archeological research provides some indications on the time of terrace formation. The dating of artifacts or animal remains at a certain site does not necessarily fix the age of a terrace on which this site is located. The following observations nevertheless are considered relevant. At Esh Shaheinab near Khartoum a famous Neolithic site lies at about 4.5 m above the present flood level of the Nile, probably on terrace I, and a close archeological connection could be established with the Fayum Neolithic of the U.A.R. (Cole, 1954; Arkell, 1965). Two radiocarbon dates from this site are given by Libby (1955) as follows: Shaheinab charcoal 5,060 ± 450 YBP, and Shaheinab shell 5,446 ± 380 YBP. At various sites on the higher terraces in the Nile valley 'Khartoum Mesolithic' has been found, which contains an industry characterized by pottery with wavy lines. Again at Esh Shaheinab and at Khartoum, for instance, sites are at 10 to 11 m above the present Nile level (Arkell, 1965). The Nile terrace II thus predates this industry, which is 10,000 to 12,000 years old at most according to Clark (1967).

The following is known concerning the age of Nile terrace deposits in northern Sudan and southern Egypt. A number of radiocarbon dates obtained on mollusc shells of deposits in the Nile valley are reported by Fairbridge (1962). The only justifiable conclusion is that these shells, collected at various places at levels between 20 and 10 m above Nile level, range in age from 14,950 ± 300 years to 7,300 ± 350 years. De Heinzelin (1967) arrives at the following chronological determinations on his 'Nile Group' deposits. Of the three main stages of aggradation recognized, the oldest one probably reached the 35 m level toward 21,000-20,000 YBP. This so-called 'Dibeira-Jer formation' is correlated with the 'older floodplain silts' of the Kom Ombo plain in southern Egypt, as observed by Butzer and Hansen (1967) at + 22 m and considered to be Middle Wurm. After a period of downcutting, aggradation in two phases of the 25 m level probably culminated toward 13,000-12,000 YBP. Also these deposits, the Sahaba formation, are correlated with the 'younger channel silts I' at + 10-12 m on the Kom Ombo plain, which are considered Late Wurm 17,000-13,000 YBP. During the third aggradation toward 9,500 YBP the 13 m level was formed, the 63

Arkin formation. Large areas of the valley were exposed around 5,300 YBP, because of a sudden drop of the Nile. In view of the fact that the heights of terrace levels above the river already differ so much between northern Sudan and the southern U.A.R., any attempt at correlation on this basis between these terraces and those of the Nile between Khartoum and Atbara, would have too hypothetical a character. For long distance correlation of terraces, inferred pluvial - interpluvial episodes are scarcely reliable guides either. The situation with relation to palaeoclimates in the Sudan is not only far from clear, as discussed in Chapter 1.2, but also a wide variety of interpretation in general exists as to the timing of cycles of stream aggradation and erosion in relation to pluvial - interpluvial cycles (Morrison, 1968). CHAPTER 4

Vegetation

Climate and soil, in liaison with an ensemble of phenomena such as fire and overgrazing, primarily determine the geographical distribution of the vegetation of central Sudan. Two major ecological zones can be recognized according to the classification of the vegetation of the Sudan set up by Harrison and Jackson (1958): (A) the Semi-Desert, and (B) the Low Rainfall Woodland Savannah (see map of figure 32). Together they occupy the area between 18 and 11°N, with average annual rainfall ranging from about 75 to 1000 mm. The two zones approximately correspond to two formation-types distinguished by Eyre (1968): semi-desert scrub and microphyllous forest respectively. Many of the dominants of these two formation-types are quite distinct in their root system. Thus, while the dominant trees of microphyllous forest are characteristically deep-rooted, the grasses of the semi-desert formation have the majority of their roots in the topsoil. In the ecological classification of Harrison and Jackson (1958) the major divisions of the vegetation are determined in the main by rainfall, and the subdivisions by kind of soil. The system benefited from observations made by Smith (1949), who singled out average annual rainfall and soil texture as the dominant elements in determining the vegetation pattern. Smith estimated that sands only need two- thirds of the amount of rainfall required by clays to permit the growth of a given species. Eyre (1968) draws attention to a similar situation in the dry belt of Burma where light soils permit a much greater luxuriance of growth of the various thorn communities than heavy soils. In this context it is of interest to note that Worrall (1960) in studying the distribution of vegetation of the Semi-Desert zone in the Sudan concludes that this is chiefly determined by the variability of soil moisture conditions. In the brief review of the ecological zones and their subdivisions which follows, no attempt has been made to present exhaustive lists of species; mostly type species characteristic of the formations in question are given. 66

32° 34° 36°

18°

•*"'y*it. f.> s- w'

r" M

oO 12°

ET/HIOPIA

32° 34° 36° 7ig. 32. Vegetation of central Sudan (adapted from Harrison and Jackson, 1958) 67

Legend: A. Semi-Desert B. Woodland Savannah Al. Desert scrub Bl. Acacia mellifera thornland A2. Semi-desert grassland B2. Acacia seyal-Balanites savannah C. Hill Catena B3. Anoge issus-Com bretum h. savannah woodland

4.1. Semi-desert The vegetation of the Semi-Desert zone, according to Harrison and Jackson (1958), is a varying mixture of grasses and herbs either without any woody vegetation at all, or, more usually, with a variable scatter of scrub bushes interspersed with bare areas. The rainfall, from about 75 to 400 mm per year, is generally insufficient for grass to grow to the stage where fires are an annual possibility; occasional grass fires do occur toward the higher rainfall limit. The northern, driest part (Al) of this zone is characterized by Acacia tortilis - Maerua crassifolia Desert Scrub. In this part there is an alternation of areas which are more or less bare of vegetation, and depressional areas that have scrub bushes and even some trees. North of Khartoum the flattopped Acacia tortilis is confined to drainage lines mostly. Maerua crassifolia is usually present in considerable amounts. Capparis decidua and Ziziphus spinachristi often occur in clayey depressions. In the northern part of the Gezira, between the Blue and White Nile, Acacia ehrenbergiana is dominant. Grasses usually are Aristida spp. and Schoene- feldia gracilis. The southern, more humid, part (A2) of this zone is the Semi-Desert Grassland, particularly well developed east of the Blue Nile. Most of these lands are entirely without trees or bush, except in areas that receive more water through runoff which have Acacia mellifera. The only other bush occurring, Acacia nubica, is an indicator of overgrazing. Very susceptible to overgrazing is Blepharis edulis which consequently has often been replaced by other herbs and grasses. The grasses dominantly consist of Cymbopogon nervatus, Sorghum sp., and Sehima ischaemoides with locally Aristida sp. and Schoenefeldia around jebels. '['he detrimental influence of man and grazing animals on the natural vegetation, especially on grass cover, and thereby on the soil, was demonstrated by Kassas (1956) and Halwagy (1962a and b) in the desert west of Khartoum. Worrall (1959) noted that the grasses and herbs of the Butana area, east of Khar- toum, occur in the form of repeating bands of up to 20 m width, usually associated with drainage lines, which run more or less at right angles to the bands (bands thus lie on the contour or very nearly so). Grass-patterns, often of a different type, have also been recorded by Ruxton and Berry (1960) on the degradational clay plains between the Blue Nile and the Atbara. 68

4.2. Woodland Savannah

South of the Semi-Desert the Woodland Savannah is met with. The savannah represents a natural vegetation of two different life forms, the grasses and the trees, which have entirely different water requirements {Walter, 1964). The variable proportion of these two components in the Low Rainfall Woodland Savannah, according to Harrison and Jackson (1958), is determined by the fre- quency and intensity of fires. All plants occurring are species with some degree of fire tolerance. The vegetation of this savannah consists of low, thorny trees in the northern dry areas and predominant broad-leaved deciduous trees in the wetter parts; annual grasses prevail over the perennials. On the clayplains of central Sudan this savannah zone is confined to the rainfall belt of 400 to 1000 mm, and three subdivisions have been observed on the basis of the dominant tree species. From north to south these are as follows. The Acacia mellifera thornland (Bl), under annual rainfall ranging from about 400 to 570 mm, is characterized by the dominant tree species Acacia mellifera, often associated with Cadaba rotundifolia, C. glandulosa, and Roscia senegalensis, which often persist after the Acacia mellifera has disappeared through fire. On low-lying soils Acacia seyal occurs. The most abundant species in the grassy areas are Cymbopogon nervatus, Sorghum sp., and Hyparrhenia pseudocymbaria.

Pig. 33, Woodland Savannah, zone B2, with Acacia seyal on foreground 69

The Acacia seyal-Balanites savannah (B2), alternating with grass areas, occurs in the 570-800 mm rainfall belt. In the drier parts of this zone dense belts of Acacia mellifera are found, as well as belts of Acacia Senegal. On areas liable to flooding Acacia fistula and A. drepanolobium occur. The dominant grasses of the grass areas do not differ much from those of the Acacia mellifera thornland. However, Cymbopogon nervatus is confined to the drier parts. The Anogeissus-Combretum hartmannianum savannah woodland (B3), with average annual rainfall in excess of 800 mm, generally occurs in areas where the slope of the land is more pronounced than in the Acacia seyal-Balanites savannah, and is seldom far from hills. There is therefore similarity with areas of the Hill Catenas (see below). Large belts of Acacia seyal alternate with the broad-leaved trees. The vegetation of the drier parts of this belt is composed almost entirely of Combretum hartmannianum, but toward the wetter Ingessana hills many other species appear. Some of these, as quoted from Bunting and Lea (1962), are Lannea fruticosa, L. humilis, Lonchocarpus laxiflorus, and Adansonia digitata. Dominant grasses of the savannah woodland are Hyparrhenia sp., Andropogon gayanus, and Setaria incrassata.

4.3. Special Areas

Separate treatment was given by Harrison and Jackson (1958) of a number of special areas of Low Rainfall Woodland Savannah, among which the Hill Catena is of interest - the vegstation of small rocky hills (jebels) such as Jebel Gargada. Four vegetation zones of the Hill Catena can be described as follows: a. No vegetation, except for a few Ficus species, on the rocky summits of the hills. b. Wide variety of tree species, such as Boswellia papyrifera, Lonchocarpus laxiflorus, Combretum hartm., and of grasses ,such as Hyparrhenia sp., Beckeropsis sp. etc. on the steep slopes of the hills. c. Often no trees and only a scanty grass cover occurs in the zone at the foot of the steep slopes. d. Anogeissus Schimperi and Combretum hartman, characterize a transitional zone toward the surrounding clayplain.

Run-off from inselbergs often collects in 'depressions périphériques', where Acacia fistula is dominant, or which have, as pools, no vegetation at all. 70

The non-clayey, partley eroded soils along the Blue Nile have their own typical vegetation in the Low Rainfall Woodland Savannah. Harrison and Jackson (1958) note that in the Acacia mellifera thornland zone (Bl) in these areas Balanites aegyptiaca, Acacia nubica, A. raddiana and Capparis decidua are characteristic species, with annual grasses such as Aristida sp., Schoenefeldia gracilis, and Sporo- bolus sp. In the Acacia seyal-Balanites savannah (B2) the Acacia savannah is locally replaced by a non-thorny woodland in which Combretum hartm., Sterculia setigera, and Adansonia digitata are dominant. Grasses are fairly similar to those of the thornland zone. PART II

Representative soils and soil materials CHAPTER 5

Representative soils

In order to obtain a better understanding of the variations and differences among the alluvial soils of Central Sudan, and of the development of these soils - the physical environment of which was described in the foregoing chapters — we selected 23 examples of soil units. These soils are divided over seven sample areas, distributed over a stretch of about 750 km along the Blue Nile and Nile. Every attempt was made to locate undisturbed virgin areas in order to describe and sample the very top horizons, so important to an'understanding of soil genesis. The selection of sample areas and soils was carried out during fieldwork subsequent to the completion of soil surveys of over 1,300,000 ha of land in the alluvial plains of Central Sudan. These surveys were made by the Government of the Sudan with the assistance of the United Nations Development Programme and the Food and Agriculture Organization of the United Nations (FAO, 1969); and the author participated. The soils thus selected not only supply information about the range over which soil characteristics and soil formation may vary, they are also considered to be representative for large areas of soils in Central Sudan, unless otherwise indicated.

5.1. Location

The seven sample areas are numbered 1 to 7 from south to north. The first four are located west of the Blue Nile, the last three east of the Nile. Sample areas along the Blue Nile are located as follows: 1. At ll°40'N, 34°17'E approximately, at Jebel Gargada on the edge of the Ingessana hills, 20 km SW of Damazin in Blue Nile Province. 2. At 11°52'N, 34°21'E approximately, 7 km N of Damazin in Blue Nile Pro- vince. 74

3. At 13°35'N, 33°33' to 40'E approximately, about 1 km N of Sennar Junction in Blue Nile Province. 4. At 15°18' to 21'N, 32°40' to 46'E approximately, near Bageir 30 km SE of Khartoum in Blue Nile Province. Sample areas along the Nile are located as follows: 5. At 16°28' to 30'N, 33°O3'E approximately, near Wad Ban Naga, about 150 km NE of Khartoum in Northern Province. 6. At 16°36' to 38'N, 33°15' to 17'E, near El Qoz station, about 10 km SW of Shendi in Northern Province. 7. At 16°51'N, 33°40' to 44'E, near Kabushiya, about 50 km NE of Shendi in Northern Province.

The location of these areas is indicated on the map of figure 34. In addition each area is shown in detail on aerial photographs originally at scale 1 : 25,000, made available by the Sudan Survey Department. The photomaps are presented in Chapter 5.2., together with the descriptions of the soils of each area. The maps are intended to show the position and landscape relations of the sites at which soils were studied in detail. The lines drawn on the photographs separate areas of similar geomorphology, in all instances these lines are also soil boundaries. The soils selected here are considered close to the modal concept of the soil units thus outlined on the photographs. Soil sample sites have been given a two-digit number, the first of which refers to the number of the sample area, while the second digits, whenever the same, imply similar géomorphologie position of the soils. Thus soils 21, 31, and 41 all occur on the lowest terrace of the Blue Nile in sample areas 2, 3, and 4.

5.2. Description

Soil profiles were described in pits to an arbitrary depth of 2 meters in accordance with proposals by Smith (1965) and Van Wambeke (1967), while borings were made and described, where feasible, to a depth of 3 meters. The descriptions are according to the standards recommended by FAO Guidelines for Soil Profile Descriptions (FAO, 1966). Soil samples were taken by horizons with a maximum vertical distance of 30 cm for each sample. For convenience a large part of the data resulting from laboratory analyses of the soil materials sampled - as discussed in Chapters 6 and 8 - is presented here together with the morphological description of the soils in question. 75

32° 34° 36°

O 50 100 Km ^ 18°

16°

14°

12°

32°

Fig. 34. Location of sample areas 76

In the following pages the profile descriptions are grouped per sample area from south to north. In order to facilitate mutual comparison of the soils of a sample area, each group of profile descriptions is preceded by an introductory statement, in which information on the sample sites is presented together with general information on the soils. Values for pH of the soil horizons given with the morphological description of the horizons were determined in the field by means of cresol red or thymol blue indicator, or both (soil/cresol red or thymol blue 1 : 5). In all instances the slopes on which the profiles were sited are flat or almost flat (0-2%). Slickensides are considered to be polished and grooved surfaces with a minimum diameter of 10 cm. Pressure faces have a diameter less than this.

Sample area No. 1 Information on the sites sampled a. Profile numbers: 15, 16 b. Date of examination: March 1968 c. Elevation: both at about 480 m d. Landform : 1. physiographic position: zone at the foot of Jebel Gargada about 3 m above Er Roseires terrace; J. Gargada is an inselberg some 60 m high 2. landform of surrounding country: flat 3. microtopography: 15, very few termite mounds 16, gilgai with amplitude of 10 cm and wavelength of about 2 m e. Vegetation: Low Rainfall Woodland Savannah, Hill Catena; 15, few Dalbergia melanoxylon, few Acacia mellifera 16, dominant Acacia mellifera, very few Acacia seyal, very few Balanites aegyptiaca f. Climate: tropical semi-arid with summer rains, compare data of Roseires meteorological station

General information on the soils a. Parent material: 15, colluvial material derived from granitic rocks, which, in 16, possibly is mixed with Blue Nile alluvia b. Permeability of subsoils: moderately slow c. Moisture conditions in the soils: dry throughout d. Depth of groundwater table: unknown e. Presence of surface stones: few scattered stones f. Evidence of erosion: nil g. Human influence: nil 79

Fig. 3.5. Sample area No. 1 80

Description Profile No. 15

Al 0- 10 cm 7.5YR3/2 dry (moist somewhat darker) sandy clay loam with very few small and large rock fragments; structureless except for the surface 2 cm which is moderate fint; platy; slightly sticky, slightly plastic, friable, hard; very few small hard black iron concretions; non-calcareous; very few termite pores; few fine and very few medium roots; clear wavy boundary; pH 7.2.

A2 10- 25 cm 5YR4/3 dry (5YR3 3, moist) day with few sandgrains and few small and large rock fragments; weak fine angular and subangular blocky; slightly sticky, slightly plastic, friable, hard; non-calcareous; very few small hard black iron concretions; very few termite pores, few fine and very few medium roots; clear wavy boundary; pH 7.2.

Bt 25-150 cm 2.5YR4/2 dry (2.5YR3 2, moist) clay with few sandgrains and frequent angular rock fragments up to 10 cm in diameter which are quartzite, cherts, metamorphic rock; structureless but in some places strong subangular blocky; sticky and plastic, firm, very hard; frequent shiny ped surfaces both horizontal and vertical, may be due to pressure or clay translocation or both; very few small hard black iron concretions; non-calcareous; very few fine and medium roots; clear smooth boundary; pH 7.6.

Cca 150-190 cm 2.5YR3/4 dry (2.5YR3/4, moist) clay with few rock fragments up to some 5 cm in diameter; structureless; nonsticky, non-plastic, friable, hard; few pressure faces; very

few small hard white CaCOa concretions; slightly calcareous; very few small hard black iron concretions slightly more than in horizon above; few termite pores; few fine roots and some decayed roots; pH 7.7.

A boring below 190 cm could not be made due to stoniness. 81

Laboratory Data Profile No. 15

Depth, Horizon °//o Particle size diistribution (fit) % cm >2 mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 10 Al 15 30.5 19.8 12.4 8.6 28.7 10- 25 A2 20 19.3 20.7 7.2 8.7 44.1 25- 65 Bt 25 18.0 12.3 7.5 8.7 53.5 65-105 Bt 25 13.9 13.3 6.7 7.8 58.3 105-150 B2 25 12.0 10.7 7.1 14.7 55.5 150-190 Cca 11 10.2 9.9 8.3 10.0 61.6

Elemental icomposition

SiO2 Al,2O3 Fe2O3 TiO2 CaO MgO K2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

B.D. Organic CaCO3 Gypsum pH Clayminerals (g matter equiv. % 1:5 Mt Mi Vm Chi K o/ per c /o cc) % C/N

1.33 20 6.7 xxxx X XX t 1.18 22 6.8 xxxx X XX t 0.62 14 6.8 xxxx X XX t 6.9 xxxx X XX t 7.0 xxxx X XX t 3.4 7.9 xxxx X XX t 82

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. % Ca Mg Na per cm

28.1 20.5 13.3 6.7 0.5 t 36.3 25.9 16.2 9.4 0.3 t — 37.2 28.1 18.0 9.7 0.4 t — 39.0 30.7 19.7 10.5 0.5 t — 41.2 33.3 21.6 11.2 0.5 t — 46.7 41.2 27.4 13.3 0.5 t

Waterextract 1 :5, soluble (meq/100 g) E.C5 cations anions mmhos per cm Ca Mg K Na Sum Sum co3 HCO3 Cl SO4 NO3

0.2 0.2 t t 0.4 0.4 0.4 t t t 0.1 0.1 0.1 t 0.1 0.3 0.2 — 0.3 t t t 0.1 0.1 0.1 t 0.1 0.3 0.1 — 0.1 t t t 0.1 0.1 0.1 t 0.1 0.3 0.2 — 0.2 t t t 0.1 0.1 0.1 t 0.1 0.3 0.4 — 0.4 t t t 0.1 0.4 0.4 t 0.2 1.0 1.2 1.2 t t t 0.2 83

Description Profile No. 16

Ail 0-15 cm 10YR3/2 dry and moist, clay with very few small and large (up to 10 cm) rock fragments; moderate fine angular blocky with tendency to platiness; very sticky, very plastic, very firm, very hard; few cracks up to 3 cm wide partly filled with surface material; very few pressure faces; very few small hard black iron concretions; non-calcareous; very few termite pores; few fine medium roots; clear wavy boundary; pH 7.6.

A12 15- 50 cm 10YR3/2 dry and moist, clay with very few small and large rock fragments up to 5 cm in diameter; weak fine angular blocky; very sticky, very plastic, very firm, very hard; few cracks up to 2 cm wide most of them ending at the base of the horizon, surface material sloughing down in the cracks; few pressure faces; very few small hard black iron concretions; non-calcareous; very few termite pores; very few fine and medium roots; gradual irregular boundary; pH 7.4. AC? 50-150 cm 10YR3/2 dry and moist, clay with very few small and large rock fragments decreasing with depth in number; structure defined by frequent slickensides (strong fine angular blocky); very sticky, very plastic, very firm, extremely hard; very few cracks up to 1 cm wide ending at 80 cm depth; very few small hard black iron concretions; non- calcareous; very few pores and very few termite nests; very few fine and medium roots; gradual boundary; pH 7.2. 150-175 cm 10YR3/2 with few clear mottles of 10YR4/3, both dry and moist, clay; structure determined by frequent pressure faces; very sticky, very plastic, very firm, very hard; very few small hard black iron concretions; very few small hard white CaCO3 concretions, mostly broken; slightly calcareous; very few termite nests and pores; no roots; pH 7.4.

Boring: 175-220 cm Similar to horizon above, CaCO3 concretions becoming few; pH 7.2. Note: cracks occupy some 7% of the surface area. 84

Laboratory Data Profile No. 16

Depth, Horizon 0/ Particle size distribution (u) % /o cm >2 mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 15 All 18 8.6 6.9 9.8 13.7 61.0 15- 50 A12 16 7.8 7.9 6.2 15.5 62.6 50- 80 AC? 12 9.5 7.4 6.5 14.8 61.8 80-110 AC? 8 9.1 6.6 6.6 13.5 64.2 110-150 AC? 4 6.3 5.3 5.5 14.8 68.1 150-175 C 6.2 5.2 9.5 13.1 66.0

Elemental composition

SiO2 A12O,, Fe2O3 TiO2 CaO MgO K2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

B.D. Organic CaCO3 Gypsum pH Clayminerals (g matter equiv. % 1:5 Mt Mi Vm Chi K Q per c cc) % C/N

0.98 32 — 7.0 xxxx xx t 0.73 27 — 6.8 xxxx XX t 7.2 xxxx XX t 7.4 xxxx XX t 7.8 xxxx XX t 7.9 xxxx XX t 85

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. /o Ca Mg Na per cm

49.0 36.0 16.9 18.6 0.5 t 49.1 36.1 18.2 17.6 0.3 t — 49.5 36.8 19.0 17.3 0.2 0.3 0.6 49.1 39.4 21.3 17.3 0.3 0.5 1.0 52.4 44.1 24.8 18.3 0.4 0.6 1.2 56.2 49.7 28.1 20.5 0.4 0.7 1.3

Waterextract 1 : 5, soluble (meq/100 g) E.C.5 cations anions mmhos per cm Ca Mg K Na Sum Sum co3 HCC-3 Cl so4 NO3

0.1 0.2 t 0.1 0.4 0.3 0.3 t t t 0.1 0.1 0.1 t 0.2 0.4 0.5 — 0.2 0.1 0.2 t 0.1 0.1 0.1 t 0.3 0.5 0.3 — 0.3 t t t 0.1 0.1 0.1 t 0.4 0.6 0.6 — 0.5 t 0.1 t 0.1 0.1 0.2 t 0.6 0.9 1.0 — 1.0 t t t 0.1 0.2 0.4 t 0.7 1.3 1.3 1.3 t t t 0.1 86

Sample area No. 2 Information on the sites sampled a. Profile numbers: 21, 22, and 23 b. Date of examination: March 1968 c. Elevation: 21 at 460, 22 at 474.1, and 23 at 474 m d. Landform: 1. physiographic position: 21, Blue Nile low terrace 22, Blue Nile Er Roseires terrace, levee 23, as 22 but in backswamp, relatively high 2. landform of surrounding country: fiat at all sites 3. microtopography: 21, nil 22, termite mounds 2 to 3 m high 23, gilgai with amplitude of 12 cm and wavelength of 5 m e. Vegetation: Low Rainfall Woodland Savannah; 21, some Hyphaene thebaica, some Calotropis procera, and short grass cover 22, few Balanites aegyptiaca, few Adansonia digitata, few Acacia seyal 23, dominant Acacia seyal, no grasses f. Climate: tropical semi-arid with summer rains, compare data of Roseires meteorological station

General information on the soils a. Parent material: calcareous alluvia of the Blue Nile b. Permeability of the subsoils: 21, moderate 22 and 23, moderately slow c. Moisture conditions in the soils: 21: dry 0 to 130 cm, slightly moist 130 to 250 cm, moist below 22: dry 0 to 175 cm, very slightly moist 175 to 350 cm. 23: dry 0 to 150 cm, very slightly moist 150 to 350 cm. d. Depth of groundwater table: 21, between 1 and 8 m; 22 and 23, unknown. e. Presence of surfacestones: nil f. Evidence of erosion: nil g. Human influence: nil 87

I'"ig. 30. Sample area No. 2 88

Description Profile No. 21

Al 0- 25 cm 10YR 5 2 dry (1OYR3/3, moist) silt loam; weak fine and medium subangular blocky; non-sticky, non-plastic, hard; strongly calcareous; very few termite nests and few termite pores; few tine and common medium grassroots; clear wavy boundary; pH 8.2.

Cl 25- 43 cm 10YR5/2 dry (1OYR3/3, moist) silty clay loam; strong fine angular blocky, presumably due to deposition by Blue Nile; non-sticky, non-plastic, slightly hard; some orange iron flecks on ped surfaces, other ped surfaces have a clear dark brown-darkbluish coating, possibly organic matter; strongly calcareous; frequent termite nests and pores; few fine and few medium roots; gradual wavy boundary; pH 8.2.

C2 43- 70 cm 10YR5/2 dry (10YR3/3, moist) silt loam; weak fine angular blocky; non-sticky, non-plastic, hard; strongly calcareous; very few termite nests and few pores; few fine and very few medium roots; gradual smooth boundary; pH S.4.

C3 70-124 cm The horizon consists of some three 10YR3/1, dry and

moist, silt loam strata occupying some 20° o of the exposed pitsurface, separated by 10YR6/3 (1OYR3/3, moist) silt loam; weak fine angular blocky; non-sticky, non-plastic, slightly hard; strongly calcareous; few termite nests and few pores; few fine and very few medium roots; gradual boundary; pH 8.4.

C4 124-190 cm 7.5YR4/4 and 3/2 dry (both 7.5YR3/2, moist) with some mottles of 10YR6/3 dry (1OYR3/3, moist) silty clay loam; possibly stratification; weak fine angular blocky; nonplastic, nonsticky, slightly hard; strongly calcareous; very- few termite pores; few fine roots; pH 8.2.

Boring: 190-3U0 cm Similar to horizon above, but becoming 1OYR3/3, moist, only below 250 cm; strongly calcareous. 89

Laboratory Data Profile No. 21

Depth, 0/ Horizon /o Particle size distribution (ß) % >2 mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 25 Al 1.1 7.0 41.3 23.7 26.9 3.1 23.8 25- 43 Cl — 0.1 8.1 37.7 33.0 27.4 3.7 23.7 43- 70 C2 — 0.1 0.9 38.2 34.4 26.4 3.6 22.8 70-124 C3 — — — 27.2 50.0 22.8 2.3 20.5 124-190 C4 0.1 0.2 31.1 38.5 30.1 0.8 29.3

Elemental composition

SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

51.0 14.5 10.8 2.1 6.7 2.6 1.4 8.2 270 32.5 1080 37.0 276 105.0 45.5 15.2 11.1 2.1 6.7 2.6 1.2 8.3 570 32.0 1005 37.0 276 104.0 44.6 15.5 11.3 2.2 6.7 2.6 1.3 7.4 575 47.0 1220 47.0 357 97.5 42.5 15.4 11.3 2.2 7.1 2.5 1.4 7.7 445 45.0 1240 53.5 276 111.5 48 9 15.3 11.1 2.2 6.7 7 4 1.4 7.7 410 31.0 1240 68.5 282 112.0

B.D. Organic CaCO3 Gypsum pH Clayminerals o/ (g matter equiv. /o 1 :5 Mt Mi Vm Chi K per 0/ /o cc) C/N

1.34 1.41 13 3.8 8.3 xxxx t XX t 1.32 0.98 13 4.9 8.4 xxxx t XX t 1.47 0.90 3.8 8.5 xxxx t XX t 1.27 0.83 4.6 8.5 xxxx t XX t 1.26 0.62 4.7 8.5 xxxx t XX t 90

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. 0/ per cm /o Ca Mg Na

33.8 35.1 28.5 5.2 1.4 t 36.7 39.6 33.3 5.4 0.9 t — 36.0 37.5 31.2 5.2 1.1 t — 37.7 38.2 31.9 5.6 0.6 0.1 0.3 36.9 40.4 33.5 5.8 1.1 t

Waterextract 1 : 5, soluble (meq/100 g) E.C.5 cations anions mmhos per cm Ca Mg K Na Sum Sum co3 HCO3, ci so4 NO3

0.7 0.2 t 0.2 1.1 1.2 t 0.8 t 0.4 t 0.3 0.7 0.2 t 0.3 1.2 1.1 t 0.7 t 0.4 t 0.3 0.4 0.1 t 0.1 0.6 0.8 t 0.6 0.2 t t 0.2 0.4 0.1 t 0.1 0.6 0.6 t 0.6 t t t 0.1 0.4 0.1 t 0.1 0.6 0.6 t 0.6 t t t 0.2 91

Description Profile No. 22

Al 0- 18 cm 10YR5/2dry(10YR3/2,moist) loamy sand(loosesandgrains covering the surface); structureless; hard, loose moist; non-calcareous; few fine roots; gradual smooth boundary; pH < 7.2.

A2 18- 38 cm 1OYR5/3 dry (10YR4/4, moist) loamy sand; structureless; consistence as above; non-calcareous; few soft iron mottles with unsharp boundaries, very few small rounded black iron nodules; very few big and fine roots; abrupt smooth boundary; pH < 7.2.

Btl 38- 60 cm 10YR5/1 dry (10YR4/1, moist) sandy clay loam; moderate coarse angular blocky; very hard; few clay cutans; non- calcareous; frequent very fine streaky iron mottles 5YR4/6 (5/6, moist), few small rounded black iron nodules; few fine roots and few medium; gradual wavy boundary; pH <7.2.

B2 60-150 cm Mixture of mainly 10YR5/1 dry (10YR4/1, moist) and 5YR3/4 moist, sandy loam; structureless; extremely hard; few clay cutans; non-calcareous; frequent orange and reddish brown iron mottles occupying some 50% of the exposed surface, few black iron nodules; few termite holes some 3 cm in diameter; very few fine roots; gradual wavy boundary; pH 7.8.

C 150-175 cm 10YR5/1 dry (4/1, moist) sandy clay loam; structureless; extremely hard; non-calcareous; very frequent iron mottles 5YR4/4 (moist and dry), occupying some 70% of exposed surface, few black iron nodules; very few fine roots; pH 7.8.

Boring: 175-200 cm Similar to horizon above.

200-250 cm Color determined by iron mottles. 92

Laboratory Data Profile No. 22

Depth, Horizon % Particle size distribution (/t) % cm > 2 mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 18 Al — 64.9 24.1 4.0 2.2 4.3 1.7 3.1 18- 38 A2 — 61.2 24.4 5.1 0.6 8.7 2.9 5.8 38- 60 Btl — 53.2 15.9 4.6 2.3 24.0 1.8 22.2 60- 90 Bt2 — 54.2 14.4 6.6 0.1 24.7 2.2 22.5 90-120 B2 — 53.9 20.6 5.6 2.8 17.1 2.4 14.7 120-150 B2 — 52.4 18.9 5.1 5.7 17.9 1.9 16.0 150-175 C — 41.7 15.3 5.2 3.5 34.3 200-250 — 40.6 16.8 4.2 4.7 33.7

Elemental composition

SiO2 Al 2OS Fe2O3 TiO2 CaO MgO •o Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

82.6 6.6 1.7 0.6 0.7 0.2 1.8 1.0 770 5.5 184 12.0 288 14.5 77.9 6.9 2.2 0.7 0.9 0.3 1.8 1.1 625 14.0 315 12.0 339 17.0 77.7 9.9 3.9 0.7 0.9 0.5 1.4 2.7 425 15.5 220 35.0 359 24.5 74.6 10.6 3.9 0.8 0.8 0.4 1.6 2.7 700 24.0 210 21.5 291 28.5 74.5 9.3 3.7 0.9 0.8 0.4 1.7 2.0 700 24.0 930 26.0 265 24.5 73.1 9.6 4.1 0.9 0.7 0.4 1.7 2.2 480 17.0 610 27.5 240 20.0

B.D. Organic CaCO3 Gypsum pH Clayminerals (g matter equiv. /o 1 : 5 Mt Mi Vm Chi K Q o/ per /o o/ cc) /o C/N

1.59 0.24 13 6.5 xxxx X 1.62 0.14 12 6.2 xxxx X 1.93 0.23 12 6.9 xxxx t X 1.92 7.7 xxxx t X 1.74 8.4 xxxx t X 1.81 8.5 xxxx t X 1.84 8.8 xxxx X 8.8 xxxx X 93

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. % Ca Mg Na per cm

2.7 1.7 1.1 0.5 0.1 t 4.0 2.8 1.6 1.0 0.2 t — 14.6 12.6 8.1 4.0 0.5 t — 15.5 12.9 8.1 4.2 0.6 t — 9.4 8.5 5.6 2.4 0.5 t — 11.5 10.7 7.1 3.0 0.6 t — 19.6 17.8 11.8 5.2 0.7 0.1 0.5 19.6 17.8 12.3 4.7 0.8 t

Waterextract 1 : 5, soluble (meq/100 g) E.C5 cations anions mmhos per cm Ca Mg K Na Sum Sum co3 HCO3 Cl so4 NO3 t t t 0.1 0.1 0.1 t t t 0.1 t 0.1 t t t 0.1 0.1 0.1 t t t 0.1 t 0.1 t t t 0.2 0.2 0.3 t 0.1 t 0.2 t 0.1 t t t 0.3 0.3 0.5 t 0.2 t 0.3 t 0.1 t t t 0.3 0.3 0.4 t 0.2 t 0.2 t 0.1 t t t 0.3 0.3 0.5 t 0.2 t 0.3 t 0.1 t 0.1 t 0.5 0.6 1.0 t 0.4 0.2 0.4 t 0.1 0.1 0.1 t 0.8 1.0 0.9 t 0.8 t 0.1 t 0.2 94

Description Profile No. 23 Ail 0- 1 cm Loose granular surface mulch; clear wavy boundary. A12 1- 18 cm 2.5Y4/2 dry (2.5Y3/2, moist) clay with very few streaks of brown (10YR5/4, dry) sand; moderate fine angular blocky sticky and plastic, hard; very few small (5 mm) hard black rounded iron nodules; non-calcareous; few cracks up to 3 cm wide partially filled with surface mulch; very few fine roots; very few large tree roots; clear wavy boundary; pH 8.2. A13 ? 18- 60 cm 2.5Y4/2 dry (2.5Y3/2, moist) clay with few brown irregular sandstreaks, some of these filling cracks; weak fine angular blocky with a few wedgeshaped aggregates; sticky and plastic, hard when dry; few pressure faces increasing in number and size with depth; few small hard black rounded iron nodules; slightly calcareous; cracks are generally ending at the base of the horizon, few big ones extend down to 90 cm and are some 2 cm wide at 50 cm depth; very few fine roots, very few tree roots; gradual wavy boundary; pH 7.6. AC ? 60-150 cm 2.5Y4/2 dry (2.5Y3/2, moist) clay with very few sandstreaks; angular blocky structure defined by slickensides; very sticky and plastic, extremely hard; frequent slickensides with grooved surface up to 20 cm in diameter, frequent pressure faces; very few small hard black rounded iron nodules; very few small hard white CaCO3 concretions; slightly calcareous; very few fine roots along slickensides; gradual wavy boundary; pH 7.8. C 150-175 cm Subequal mixture of 2.5Y4/2 and 2.5Y4/1 (both dry and moist) clay; structureless; very hard; few pressure faces and slickensides not clearly visible; very few small hard white CaCO3 concretions; slightly calcareous; very few very fine roots; pH 8.O. Boring: 175-200 cm Similar to horizon above. 200-275 cm Accumulation of large (2-3 cm) hard white CaCO3 concretions in 2.5Y3/2 clay. Note: The profile described is underneath the puff of gilgai, cracks occupy from 5-10% of the surface area. 95

Laboratory Data Profile No. 23

Depth, Horizon % Particle size distribution {/j) cm > 2 mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 1 All 16.1 15.8 7.6 10.7 49.8 3.6 46.2 1- 18 A12 16.4 13.5 9.0 7.3 53.8 2.6 51.2 18- 60 A13? 14.6 13.4 8.3 9.6 54.1 3.7 50.4 60- 90 AC? 13.5 13.0 6.6 11.2 55.7 3.0 52.7 90-120 AC? 15.2 12.6 8.0 8.8 55.4 5.2 50.2 120-150 AC? 12.2 11.6 6.7 9.0 60.5 5.4 55.1 150-175 C 12.4 10.7 6.6 12.7 57.6 4.9 52.7 200-250 10.1 9.9 6.5 10.9 62.6 4.9 57.7

Elemental composition

SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2C) Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

61.2 14.6 7.6 1.3 1.7 1.3 1.3 7.0 380 26.0 510 36.5 312 63.0 58.2 14.9 8.2 1.4 1.9 1.4 1.2 6.3 690 17.0 718 35.5 306 70.0 58.8 15.2 7.9 1.4 1.6 1.4 1.2 5.9 420 18.5 860 31.0 291 63.5 63.8 15.2 8.1 1.4 1.7 1.5 1.2 6.0 405 19.0 850 40.5 279 84.5 58.5 15.1 7.6 1.3 1.7 1.5 1.2 5.9 385 20.0 625 40.0 270 58.5 58.5 15.4 8.2 1.4 1.9 1.6 1.2 6.4 385 23.5 580 40.5 234 68.5 59.9 15.5 8.4 1.4 1.2 6.3 54.9 14.5 7.8 1.4 1.1 9.1

B.D. Organic CaCO3 Gypsum pH Clayminerals o/ (g matte_ r equiv. 1 :5 Mt Mi Vm Chi K per c /o cc) % C/N

1.56 1.29 22 6.8 xxxx 1.75 0.64 19 7.5 xxxx 1.78 0.48 22 7.9 xxxx 1.82 0.1 8.6 xxxx 1.82 0.2 8.7 xxxx 1.79 0.6 8.8 xxxx 0.5 8.8 xxxx 7.5 9.0 xxxx 96

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. /o Ca Mg Na per cm

45.2 40.4 22.6 16.5 1.3 t 50.1 46.3 26.4 19.2 0.7 t — 50.0 42.6 23.7 18.1 0.7 0.1 0.2 50.4 46.3 26.4 18.7 0.8 0.4 0.8 45.8 45.1 25.3 18.6 0.8 0.4 0.9 54.1 51.3 29.2 20.7 0.8 0.6 1.1 56.0 54.1 31.4 21.2 0.9 0.6 1.1 48.1 55.5 33.5 20.9 Ó.8 0.3 0.6

Waterextract 1 : 5, soluble (meq/100 g) E.C.5 cations anions mmhos per cm Ca Mg K Na Sum Sum co3 HCO3 Cl SO4 NO3

0.6 0.5 0.1 0.1 1.3 1.0 t 0.8 t 0.2 t 0.2 0.1 0.1 t 0.1 0.3 0.2 t 0.2 t t t 0.1 0.1 0.1 t 0.2 0.4 0.2 t 0.2 t t t 0.1 0.1 0.1 t 0.5 0.7 0.5 t 0.4 t 0.1 t 0.1 0.1 0.1 t 0.7 0.9 0.7 t 0.6 t 0.1 t 0.2 0.1 0.1 t 0.9 1.1 1.3 t 0.9 0.3 0.1 t 0.2 0.2 0.1 t 1.0 1.3 1.4 t 1.2 t 0.2 t 0.3 0.3 0.2 t 1.1 1.6 1.6 t 1.3 0.2 0.1 t 0.3

Sample area No. 3 Information on the sites sampled a. Profile numbers: 31, 32, 33, and 34 b. Date of examination: June 1968 c. Elevation: 31 at 430 m; 32, 33 and 34 at about 440 m d. Landform: 1. physiographic position: 31, Blue Nile low terrace 32, Blue Nile Er Roseires terrace, levee 33, as 32 but in backswamp, relatively high 34, as 32 but in backswamp, relatively low 2. landform of surrounding country: flat at all sites, at 34 somewhat depressional 3. microtopography: 31 and 32, nil 33 and 34, very few potholes e. Vegetation/Landuse: Low Rainfall Woodland Savannah; 31 and 33: under rainfed cultivation 32: very few low Acacia seyal 34: some grasses (Sporobolus helvolus) only f. Climate: tropical semi-arid with summer rains, compare data of Sennar meteorological station

General information on the soils a. Parent material: calcareous alluvia of the Blue Nile b. Permeability of the subsoils: 31, moderate 32 and 33, moderately slow 34, slow c. Moisture conditions in the soils: 31: moist 0 to 14 cm, dry below to 250 cm 32: dry throughout 33: moist throughout 34: moist throughout d. Depth of groundwater table: 31, between 1 and 8 m 32, 33, and 34 unknown e. Presence of surfacestones: nil at 31, 33 and 34; 32, very few scattered quartz gravel f. Evidence of erosion: nil g. Human influence: 31, and 33, cultivation practices 32 and 34, nil l'ig- j~• Sample aren ISo. J 100

Description Profile No. 31

Al 0- 14 cm 10YR6/2 dry (4/3, moist) loam; structureless (plow layer); soft, very friable, nonsticky and nonplastic; no cracks; strongly calcareous; frequent fine and very few medium roots; clear wavy boundary; pH 8.4.

IIAlb 54 cm 10YR6 2 dry (3.5,3, moist) silty clay loam; weak fine angular blocky; slightly hard, very friable, nonsticky and nonplastic; strongly calcareous; below 30 cm depth occasional CaCO3 dendrites; some brown iron flecks on horizontal ped surfaces specially of the heavier textured layers; few fine and very few medium roots; gradual boundary; pH 8.4.

I1C 54-200 cm Sedimentary phenomena prevail. There is a clear sequence of some six 10YR5/3 dry (3/3, moist) silty clay loam layers (10-15 cm thick) within 10YR5.5/2 dry (3/3, moist) somewhat lighter silty clay loam; blocky structure due to deposition; slightly hard, very friable, nonsticky and nonplastic; strongly calcareous; occasional CaCO3 dendrites; some brown iron flecks on ped surfaces which become more reddish brown below 150 cm; no termite activity; very few fine roots down to 2 meter; pH 8.4 from 54—150 cm and 7.8 from 150-200 cm.

Boring: 200-250 cm Continuation of horizon above, at 250 cm frequent large (up to 10-15 cm) hard CaCO3 concretions make deeper boring impossible. 101

Laboratory Data Profile No. 31

Depth, Horizon 0/ Particle size distribution (N) % /o cm >2mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 14 Al 0.3 31.7 26.4 16.6 25.0 14- 54 IIAlb — — 5.9 37.2 27.7 29.2 54- 90 IIC1 — 0.1 4.7 33.2 29.1 32.9 90-150 IIC2 — — 15.2 39.5 20.0 25.3 150-200 IIC3 0.1 13.3 36.3 21.6 28.7

Elemental composition

SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2C) Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

B.D. Organic CaCO3 Gypsum pH Clayminerals equiv. 0/ 1 : 5 matter /o Mt Mi Vm Chi K o/ per /o cc) C/N

0.81 13 3.3 7.4 xxxx X XX t 0.84 11 3.5 7.7 xxxx t XX t 0.73 4.1 7.7 xxxx t XX t 0.61 3.8 7.5 xxxx XX t 4.4 7.4 xxxx XX t 102

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. /o Ca Mg Na per cm

31.6 33.1 27.6 4.2 0.9 0.4 1.3 3.4 38.8 40.5 31.8 7.7 0.4 0.6 1.6 2.0 39.1 38.7 31.1 6.8 0.6 0.2 0.5 35.2 33.3 27.5 4.9 0.8 0.1 0.3 36.7 36.5 27.9 7.7 0.8 0.1 0.3

Waterextract 1 : 5, soluble (meq/100 g) E.C.5 cations anions mmhos

Ca Mg K Na Sum Sum CO3 HCO3 Cl SO4 NC-3 per cm

1.8 0.7 0.1 0.8 3.4 4.7 1.4 t 3.3 t 0.9 0.9 0.6 t 1.0 2.5 3.0 — 1.9 t 1.1 t 0.6 0.6 0.3 t 0.2 1.1 1.5 — 1.4 t 0.1 t 0.3 0.4 0.3 t 0.2 0.9 1.1 — 1.1 t t t 0.3 0.5 0.4 t 0.2 1.1 1.4 1.4 t t t 0.3 103

Description Profile No. 32

Al 0- 12 cm 1OYR3/3 dry (3/2, moist) clay loam, with frequent fine sandgrains and very few quartz pebbles; moderate subangular blocky structure with platy components; hard, friable, very sticky and very plastic; strongly calcareous;

few small hard black CaCO3 nodules; frequent fine roots; clear wavy boundary; pH 8.2.

Bt 12- 44 cm 10YR4/2 dry (3/3, moist) clay with frequent fine sandgrains; coarse prisms breaking into moderate medium angular blocky aggregates; few patchy clay cutans; hard, friable, very sticky and very plastic; strongly calcareous;

very few small black CaCO3 nodules, very few large hard white CaCO3 concretions, few CaCO3 dendrites; very few fine roots; clear wavy boundary; pH 8.2.

B2 44-123 cm Mixed 10YR4/2 (decreasing with depth) and 10YR3/1 dry, clay; weak medium subangular blocky; hard, firm, very sticky and very plastic; very few cracks up to 2 mm wide; strongly calcareous; very few large hard and very few soft

CaCO3 concretions specially in brown tongues; very few CaCO3 dendrites; occasional gypsum crystals; very few roots; clear boundary; pH from 44-85 cm 8.2, from 85-123 cm 8.0.

C 123-175 cm 10YR3/1 - 2/1 dry (3/1, moist) clay; structureless; very hard, very firm, sticky and plastic ; strongly calcareous;

few small and large hard white and yellow CaCO3 concretions; very few small soft CaCO3 aggregates; very few CaCOj, dendrites (zone of accumulation?); very few fine roots; pH 8.O.

Boring: 175-250 cm Continuation of melanic horizon with very few small brown mottles. 104

Laboratory Data Profile No. 32

Depth, Horizon % Particle size distribution (ji) cm >2mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 12 Al 12 25.8 17.4 14.2 10.8 31.8 12- 44 Bt 23.4 9.7 10.3 14.0 42^6 44- 85 B2 22.8 7.7 11.4 13.9 44,2 85-123 B2 22.8 7.8 10.4 15.6 43.4 123-175 C 20.8 7.1 13.1 10.7 48.3

Elemental composition

SiO2 A12O3 Fe2O3 TiO2 CaO MgO I<2o Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

B.D. Organic CaCO3 Gypsum pH Clayminerals (g matter equiv. 0/ 1 : 5 /o Mt Mi Vm Chi K Q Per C/N

1.29 11 4.6 7.7 xxxx 0.73 12 3.9 7.9 xxxx 4.3 7.6 xxxx 4.2 7.2 xxxx 1.9 7.6 xxxx 105

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. % Ca Mg Na per cm

27.5 24.4 18.1 3.7 2.3 0.3 1.1 1.5 38.0 31.9 22.7 6.4 0.7 2.1 5.5 2.9 35.4 39.8 26.2 8.7 0.9 4.0 11.3 3.5 36.8 33.2 20.7 7.5 0.4 4.6 12.5 3.8 42.9 36.0 21.3 9.1 0.4 5.2 12.1 2.3

Waterextract 1 : 5, soluble (meq/100 g) E.C.5 cations anions mmhos

Ca Mg K Na Sum Sum CO3 HCO3 Cl SO4 NO3 per cm

0.7 0.4 0.3 0.4 1.8 2.1 1.9 t 0.3 t 0.4 0.5 0.4 t 3.3 4.2 4.6 — 3.2 0.9 0.5 t 0.7 1.0 0.7 t 6.1 7.8 7.9 — 3.2 0.9 3.1 0.7 1.4 6.5 3.4 0.1 8.1 18.0 21.5 — 0.8 0.8 18.2 1.7 3.5 0 S 0.4 t 5.4 6.3 6.0 0.6 0.6 2.5 1.6 1.4 106

Description Profile No. 33

Ap 0- 10 cm 10YR - 2.5Y3/2 moist, clay with very few sandgrains; structureless (plow layer); friable, very sticky, very plastic; calcareous; very few small hard dark gray CaCO3 nodules also on the surface; very few fine and medium roots; clear wavy boundary; pH <8.2.

All 10- 42 cm 10YR - 2.5Y3/2 both dry and moist, clay with very few 1 sandgrains and occasional quartz pebbles up to 3 cm large; moderate fine angular blocky; very hard, friable, sticky and plastic; calcareous; very few small hard dark gray CaCO3 nodules; very few small (1-2 mm) white CaCO3 concretions; very few shell fragments; very few fine and medium roots; gradual boundary; pH 8.6.

A12+ 42- 89 cm Mixed 10YR3/2.5 (decreasing with depth) and 1OYR3/1.5 Allb? moist, clay; weak fine angular blocky; very hard, friable, sticky and plastic; calcareous; very few small hard dark gray (some with white coatings) CaCO3 nodules; very few small hard white CaCO3 concretions (slightly more than horizon above); some white CaCO3 dendrites on ped surfaces; very few medium roots; diffuse boundary; pH <8.2.

A12b ? 89-146 cm 1OYR3/1.5 moist, clay; wedge shaped structural aggregates as defined by pressure faces; very firm, sticky and plastic; calcareous; few small hard white CaCO3 concretions; occasional CaCO3 dendrites and streaks on ped surfaces; very few slickensides, very frequent pressure faces; very few tiny shell fragments; gradual boundary; pH <8.2.

AC 146-180 cm Mixed 10YR4/3 and 4/2 moist, clay; almost structureless, very few pressure faces; very firm, sticky and plastic; calcareous; few small and large hard white and yellow CaCO3 concretions (zone of accumulation?); pH <8.2.

Boring: 180—250 cm Essentially similar to horizon above.

Note: Cracking not observable, due to recent rains. 107

Laboratory Data Profile No. 33

Depth, Horizon 0/ Particle size distribution (a) % /o cm >2mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 10 Ap 8.8 5.6 5.4 11.9 68.3 10- 42 All — 8.0 4.9 5.0 13.2 68.9 42- 89 A12 + Allb? — 8.0 4.4 7.6 12.9 67.1 89-146 A12b? — 5.0 3.6 5.5 12.7 73.2 146-180 AC 0.8 2.8 3.2 8.6 13.5 71.8

Elemental composition

SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

B.D. Organic CaCO3 Gypsum pH Clayminerals 0/ (g matter equiv. /o 1 :5 Mt Mi Vm Chi K Per c % cc) % C/N

0.56 20 4.7 8.0 xxxx XX t 0.55 20 3.7 8.0 xxxx XX t 0.54 5.3 8.5 xxxx XX t 0.56 4.4 8.2 xxxx XX t 10.0 0.7 7.6 xxxx XX t 108

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. 0/ per cm /o Ca Mg Na 70.9 65.3 51.5 10.3 0.9 2.6 3.7 70.9 64.7 57.8 10.9 0.7 5.3 7.5 71.4 65.0 44.3 11.6 0.5 8.6 12.0 72.0 66.3 41.9 15.1 0.4 8.9 12.4 3.0 66.6 63.0 39.4 14.5 0.5 8.6 12.9 4.7

Waterextract 1 : 5, soluble (meq/100 g) E.C5 cations anions mmhos per cm Ca Mg K Na Sum Sum CO3 HC:o3 ci SO4 NO3

0.2 0.1 t 1.4 1.7 1.6 1.6 t t t 0.2 0.1 0.1 t 1.8 2.0 1.7 — 1.7 t t t 0.2 0.1 0.1 t 1.9 2.1 1.9 0.1 1.8 t t t 0.3 0.2 0.1 t 4.7 5.0 4.9 — 2.3 1.0 1.6 t 0.9 46 2.7 t 9.3 16.6 18.4 11 0.7 16.6 t 2.6 109

Description Profile No. 34

Ail 0- 11 cm 10YR4/2 moist, clay with very few sandgrains; structureless; friable, slightly sticky and plastic; few small hard

black CaCO3 nodules; calcareous; very few fine and big roots; clear smooth boundary; pH 8.4.

A12 11- 65 cm 10YR3/2 dry (almost 3/2.5, moist) clay with very few sandgrains and very few scattered quartz pebbles up to 2 cm; weak angular blocky with platy components, becoming even weaker developed towards the base of horizon; very hard, very firm, very sticky and very plastic;

few small hard black CaCO3 nodules; few small hard white CaCO3 concretions; calcareous; very few fine and medium roots; diffuse boundary; pH 8.6.

A13 65-155 cm 10YR3/2 dry (3/2.5, moist) clay with very few sandgrains and occasional small quartz pebbles; some small faint 10YR4/1 mottles occur at 80-90 cm depth; wedge shaped structural aggregates defined by pressure faces and slickensides; very hard, very firm, very sticky and very plastic; some small slickensides and (increasing with depth)

frequent pressure faces; very few small hard white CaCO3 concretions, some CaCO3 streaks following root channels; calcareous; down to 1 m depth very few fine roots; gradual boundary; pH 8.O.

AC 155-200 cm Mixed 10YR3/2 and 4/4 dry, clay with very few sandgrains; structure elements defined by 20-30 cm long slickensides; consistence as horizon above; few small and large white

(sometimes brownish) CaCO3 concretions; calcareous; pH 8.O. 110

Laboratory Data Profile No. 34

Depth, Horizon % Particle size distribution (fi) % cm > 2 mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 11 All 7.6 5.3 11.1 8.8 67.2 11- 65 A12 5.9 5.6 4.4 10.2 73.9 65-110 A13 7.3 4.6 4.7 11.9 71.5 110-155 A13 6.7 4.7 10.8 1.3 76.5 155-200 AC 4.0 3.8 11.1 12.2 68.9

Elemental composition

SiOj A12O3 Fe2O3 TiO2 CaO MgO K2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

B.D. Organic CaCO3 Gypsum pH Clayminerals (g matter equiv. 1 : 5 Mt Mi Vm Chi K o/ per C /o «0 % C/N

0.56 21 3.2 7.9 xxxx XX t 0.56 19 4.2 8.4 xxxx XX t 4.6 8.4 xxxx XX t 3.9 7.9 xxxx XX t 2.9 7.7 xxxx XX t Ill

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. /o Ca Mg Na per cm

69.5 64.1 50.9 9.6 0.7 2.9 4.2 68.3 58.9 44.1 7.3 0.5 7.0 10.3 68.0 62.1 40.1 11.4 0.4 10.2 15.0 2.1 71.5 66.8 41.6 13.7 0.4 11.1 15.5 4.6 71.6 67.0 39.3 15.2 0.4 12.1 16.9 4.8

Waterextract 1 : 5, soluble (meq/100 g) E.C.j cations anions mmhos

Ca Mg K Na Sum Sum CO3 HCO3 Cl SO4 NO, per cm

0.1 0.1 t 1.2 1.4 1.4 1.4 t t t 0.3 0.5 0.1 t 1.8 2.4 1.7 — 1.7 t t t 0.3 0.1 0.1 t 3.3 3.5 3.2 — 2.1 0.7 0.4 t 0.5 1.3 0.8 t 8.4 10.5 11.1 — 1.2 1.1 8.8 t 1.7 1.1 0.1 t 8.3 10.2 11.1 1.1 1.4 8.6 t 1.7 112

Sample area No. 4 Information on the sites sampled a. Profile numbers: 41, 42, 43, and 44 b. Date of examination: May 1968 c. Elevation: 41 at 378 m, 42 at 383 m, and 43 and 44 at 384 m d. Landform: 1. physiographic position: 41, Blue Nile low terrace 42, Blue Nile Er Roseires terrace, levee 43, as 42 but in backswamp, relatively high 44, as 42 but in backswamp, relatively low 2. landform of surrounding country: flat at all sites, at 44 somewhat depressional 3. microtopography: 41 and 42, nil 43, very few potholes 44, rough surface due to cattle e. Vegetation/Landuse: Desert scrub zone; 41 under rain cultivation, 42, covered by short grass; 43, some short grasses only; 44, no vegetation f. Climate: tropical arid with some summer rains, compare data of Khartoum meteorological station

General information on the soils a. Parent material: calcareous alluvia of the Blue Nile b. Permeability of the subsoils: 41, moderate 42 and 43, moderately slow 44, slow c. Moisture conditions in the soils: all dry throughout d. Depth of groundwater table: 41, between 1 and 8 m 42, 43 and 44 unknown e. Presence of surface stones: nil at 41, 43 and 44; very few quartz gravel at 42 f. Evidence of erosion: some wind deposition at 41, 42 and 43. g. Human influence: 41, cultivation practices nil at 42, 43 and 44 APPR. SCALE

Fig. 3H. Sample area ,Y". 4 114

Description Profile No. 41

Ap 0- 10 cm 10YR5/4 dry (3/3, moist) loam; structureless (plow layer); slightly hard, friable, nonsticky, nonplastic; no cracks; strongly calcareous; few fine medium and large roots; clear wavy boundary; pH below 7.0.

IIAlb 10- 55 cm 1OYR5/3 dry (3,2, moist) silt loam with some lamination of clay {no textural profile development, but recent deposition); weak fine and medium subangular blocky; slightly hard, friable, nonsticky, nonplastic; strongly calcareous; few fine and medium roots; gradual boundary; pH below 7.0.

IICI 55-105 cm 10YR5/4 dry (3/3, moist) silt loam; moderate fine angular blocky (defined by recent deposition), slightly hard, friable, nonsticky, nonplastic; strongly calcareous, very few fine and medium roots; clear boundary; pH below 7.0.

IIC2 105-175 cm Dominant 1OYR5/3 dry (3/2, moist) silty clay loam; strong thin platy structure due to deposition, structure elements have brown iron mottling on horizontal surfaces (in fissures); hard, friable, nonsticky, nonplastic, strongly calcareous; very few fine and medium roots; pH below 7.0. 115

Laboratory Data Profile No. 41

o/ Depth, Horizon /o Particle size distribution (fi) % cm >2 mm ssand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 10 Al 14.9 37.6 22.0 8.2 17.3 2.4 14.9 10- 55 IIAlb — 3.6 10.8 26.7 26.3 32.6 5.6 27.0 55-105 IIC1 — 0.8 10.2 40.3 19.0 29.7 5.2 24.5 105-175 IIC2 0.2 12.4 29.8 21.2 36.4 7.3 29.1

Elemental composition

SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

64.4 9.5 5.9 1.2 4.4 1.6 1.4 4.1 415 26.5 860 32.0 366 83.5 57.6 14.7 10.1 2.0 6.4 2.4 1.3 6.5 515 26.0 1210 49.5 417 112.0 55.7 15.5 10.4 2.0 6.9 2.5 1.3 6.5 610 35.0 1210 61.0 456 130.5 54.4 16.0 10.7 2.1 6.9 2.5 1.4 6.6 415 37.0 1320 74.5 378 139.0

B.D. Organic CaCO3 Gypsum pH Clayminerals 0/ matter equiv. la 1 : 5 Mt Mi Vm Chi K Q o/ per /o o/ cc) /o C/N

1.41 0.58 12 2.4 8.5 xxxx XX t 1.29 0.99 14 2.8 8.4 xxxx XX t 1.17 0.69 2.7 8.5 xxxx XX t 1.13 0.48 2.6 8.5 xxxx XX t 116

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. /o Ca Mg Na per cm

16.8 19.0 15.1 2.9 1.0 t 32.0 33.9 28.5 4.9 0.5 t 31.7 34.7 28.9 5.3 0.4 0.1 0.3 35.8 37.4 31.2 6.0 0.1 0.1 0.3

Waterextract 1 : 5, soluble (meq/100 g) E.C5 cations anions mmhos

Ca Mg K Na Sum Sum CO3 HCO3 Cl SO4 NO3 per cm

0.5 0.1 0.1 0.1 0.8 0.7 t 0.6 t 0.1 t 0.2 0.5 0.1 t 0.1 0.7 0.7 t 0.6 t 0.1 t 0.2 0.4 0.1 t 0.2 0.7 0.7 t 0.6 t 0.1 t 0.2 04 0.1 t 0.2 0.7 0.9 t 0.6 0.2 0.1 t 0.2 117

Description Profile No. 42

Ail 0- 10 cm 10YR5/3 dry (1OYR3/3, moist) sandy loam with very few small quartz pebbles; weak thin platy (possibly wind deposition); soft, loose, nonsticky, nonplastic; frequent small hard dark grey CaCO3 nodules; strongly calcareous; very few shell fragments; few fine grass roots; clear wavy boundary; pH 8.4. A12 10- 28 cm 10YR5/2 dry (10YR4/1, moist) light sandy loam clay with very few small quartz pebbles and frequent sandgrains coming down through fissures; weak medium and fine subangular blocky ; slightly hard, very few small hard dark grey CaCO 3 nodules, very few small hard white CaCO3 concretions; strongly calcareous; few fine roots; clear wavy boundary; pH 8.5. Btl 28- 83 cm 10YR4/3 dry (10YR4/2, moist) sandy clay loam with few patches of brown sand; strong fine and medium prismatic breaking into moderate medium angular blocky aggregates; distinct continuous clay cutans on ped surfaces both horizontal and vertical; hard, friable, slightly sticky, slightly plastic; few small soft white CaCO3 rootlike streaks; strongly calcareous; very few fine roots ending at 60 cm; clear wavy boundary; pH 8.8. Bt2 83- 99 cm 1OYR5/3 dry (10YR5/2, moist) color hard to see because of shiny surfaces, sandy clay loam; moderate medium and fine prismatic breaking into angular blocky aggregates; hard, firm, slightly sticky and plastic; distinct continuous clay cutans on ped surfaces; frequent small and large hard (soft on outside) white CaCO3 concretions; strongly calcareous; clear wavy boundary; pH 8.7. C 99-160 cm 1OYR5/3 and 5/2 dry (1OYR5/3 and 4/2, moist) light sandy clay loam; structureless; very hard, friable ?,nonsticky, nonplastic; frequent small and few large hard (soft powdery on the outside) white (sometimes dark grey inside) CaCO3 concretions; strongly calcareous; very few small hard reddish and black Fe ? concretions; pH 8.8.

Boring: 160-225 cm Similar to horizon above with some CaCO3 concretions. 118

Laboratory Data Profile No. 42

Depth, Horizon 0/ Particle size distribution (fi) 0/ /o /o cm >2 mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 10 All 8 38.6 32.0 5.1 5.1 19.2 3.0 16.2 10- 28 A12 6 34.0 33.4 2.7 4,.8 $4.4 2.4 22.0 28- 55 Btl — 33.0 28.2 2.1 4..7 32.0 2.8 29.2 55- 83 Btl — 32.1 27.7 1.1 6. 1 3Î3.O 2.9 30.1 83- 99 Bt2 — 34.0 30.0 1.7 5..0 28.7 3.2 25.5 99-160 C 38.3 37.7 1.1 4.,4 18.5 1.6 16.9

Elemental composition

SiO2 A12O,, Fe2O3 TiO2 CaO MgO K2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

69.8 6.3 3.4 0.8 5.5 1.5 0.8 6.0 190 14.5 1230 27.0 396 35.5 71.0 8.5 3.8 0.8 2.5 0.5 1.3 3.2 225 12.5 655 20.5 775 49.5 72.4 9.3 5.0 1.0 2.2 0.6 1.5 4.1 205 7.0 485 34.0 675 26.0 74.1 9.0 4.7 0.9 4.5 0.6 1.4 5.4 190 10.0 440 30.0 450 43.0 67.5 8.7 4.6 0.9 4.1 0.6 1.3 5.0 220 12.5 530 36.0 324 39.0 71.8 8.5 4.3 0.9 1.3 4.4

B.D. Organic CaCO3 Gypsum pH Clayminerals 0/ (g matter equiv. /o 1 : 5 Mt Mi Vm Chi K Q per c % o/ cc) /o C/N

1.80 0.18 16 10.2 9.1 xxxx XX t 1.71 0.16 12 1.7 9.1 xxxx t XX t 1.83 0.13 16 1.9 9.3 xxxx t XX t 1.88 5.7 9.5 xxxx t XX t 1.90 6.1 9.5 xxxx XX t 1.74 6.3 9.6 xxxx XX t 119

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. % Ca Mg Na per cm

14.1 16.3 14.5 1.2 0.6 t 14.7 17.0 14.6 1.6 0.8 t — 19.6 21.9 17.4 2.6 1.9 t — 24.3 25.4 20.2 3.3 1.6 0.3 1.2 22.0 23.2 18.0 3.1 2.0 0.1 0.5 15.3 17.4 14.1 2.3 0.9 0.1 0.7

Waterextract 1 : 5, soluble (meq/100 g) E.C5 cations anions mmhos

Ca Mg K Na Sum Sum CO3 HCO3 Cl SO4 NO3 per cm

0.3 t t 0.1 0.4 0.5 0.1 0.4 t t t 0.1 0.4 t t 0.2 0.6 0.6 t 0.4 0.2 t t 0.1 0.2 t 0.1 0.6 0.9 0.8 t 0.8 t t t 0.2 0.2 t 0.1 0.7 1.0 0.9 0.2 0.7 t t t 0.2 0.2 t 0.1 0.7 1.0 0.9 0.2 0.7 t t t 0.2 0.2 t 0.1 0.7 1.0 0.9 0.2 0.7 t t t 0.2 120

Description Profile No. 43 Ail 0- 17 cm 10YR4/2 dry with a few streaks of 10YR6/4 (1OYR3/3, moist) sandy clay loam (few sand streaks along vertical ped faces, possibly wind blown); weak medium and fine angular blocky; hard, friable, sticky and plastic; no cracks; very few questionable clay cutans on ped faces; very few small hard dark grey CaCO3 nodules; strongly calcareous; few fine roots; clear wavy boundary; pH 9.O. A12 17- 40 cm Mixed 10YR4/2 and 4/1 dry (resp. 10YR3/3 and 4/1, moist) sandy clay (few sandstreaks); weak medium and fine angular blocky; very hard, firm, sticky and plastic; very few questionable clay cutans; very few small hard dark grey CaCO3 nodules, very few small soft and hard CaCO3 concretions with etched surfaces; strongly calcareous very few roots; gradual boundary; pH 9.4. B 40- 72 cm 10YR4/1 both dry and moist, with few brown mottles of horizon above, sandy clay; moderate fine angular blocky; very hard, firm, sticky and plastic; few questionable cutans; dark grey nodules are practically absent, few small and large hard white CaCO3 concretions, some of which are dark grey inside; strongly calcareous; no roots; gradual boundary; pH 8.9. B 72-117 cm 10YR5/3 dry (10YR4/3, moist) with few mottles of 10YR4/1 dry and moist, sandy clay loam; structureless; extremely hard, firm, slightly sticky and plastic; few small and large hard white (some dark grey inside) CaCO3 concretions, slightly more than in melanic horizon; pH 8.9. B2 117-162 cm 10YR6/4 dry (10YR5/4, moist) with very few rootlike streaks of 10YR4/1 dry and moist, sandy loam; structureless; extremely hard, firm, slightly sticky and plastic; few small and large hard white CaCO3 concretions; strongly calcareous; clear smooth boundary; pH 8.8. C 162-190 cm 10YR6/3 dry (10YR5/4, moist) with thin (1-2 mm) laminae of dark grey material, loamy sand; structureless; hard, firm, nonsticky and nonplastic; very few small hard whitish CaCO3 concretions; calcareous; pH 8.6. 121

Laboratory Data Profile No. 43

Depth, Horizon % Particle size distribution (f/,) /o cm >2i mm sand silt clay 2000-• 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 17 All 1 27.5 25.4 4.7 8 .8 33.6 0.2 33.4 17- 40 A12 1 20.5 26.8 6.4 8.7 37.6 0.2 37.4 40- 72 B — 23.1 31.1 2.4 7,.1 36.3 4.3 32.0 72-117 B 0.8 20.1 34.4 3.7 3.7 28.1 3.8 24.3 117-162 B2 0.8 34.6 42.7 3.2 2 .7 16.8 2.4 14.4 162-190 C 0.8 6.8 74.8 3.7 3.7 11.0 1.9 9.1

Elemental composition

SiO2 A12O.i Fe2O3 TiO2 CaO MgO K2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

64.5 8.7 4.7 0.9 7.8 1.8 0.9 6.9 270 30..5 1030 34.0 384 55.5 66.7 10.3 5.6 1.0 6.4 2.0 1.1 6.3 400 25..0 860 37.5 567 29.5 61.3 9.6 5.3 1.0 6.4 2.0 1.2 6.2 325 32.0 780 52.0 540 67.5 67.4 9.7 5.3 1.0 4.6 1.8 1.3 4.2 345 38..0 750 44.0 614 61.5 76.4 8.0 3.8 0.9 2.9 1.3 1.1 3.0 400 25,,5 590 41.5 645 44.0' 70.2 10.1 5.4 1.4 6.9 1.7 1.4 3.3 485 26..0 860 45.5 570 67.0

B.D. Organic CaCO3 Gypsum PH Clayminerals o/ (g matter equiv. /o 1 : 5 Mt Mi Vm Chi K Q per Q % cc) o/o C/N

1.82 0.22 18 8.5 9.8 xxxx X t 1.66 0.24 19 6.3 9.9 xxxx X t 1.87 0.09 7.4 9.8 xxxx X t 1.73 0.06 4.1 10.0 XXXX X X t 1.67 2.7 10.2 xxxx X t 1.53 3.7 10.1 xxxx X X t 122

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. % Ca Mg Na per cm

27.8 29.8 23.2 3.1 1.0 2.5 9.0 35.8 34.7 18.1 3.8 0.9 11.9 35.2 0.6 0.3 18.5 2.0 29.2 29.3 15.0 4.6 0.9 8.8 30.1 1.5 0.6 40.0 3.3 23.7 25.1 13.1 4.6 0.3 7.1 30.0 0.8 0.3 26.1 2.8 14.9 17.3 10.2 3.0 0.7 3.4 22.8 0.6 0.4 17.0 1.9 12.8 13.5 8.4 2.4 0.2 2.5 19.5

Waterextract 1 :5, soluble (meq/100 g) E.C.5 cations anions mmhos per cm Ca Mg K Na Sum Sum co3 HCO3 Cl SO4 NO3

0.1 t t 1.9 2.0 1.8 0.6 1.1 t 0.1 t 0.3 0.1 t t 5.0 5.1 4.3 1.1 2.4 0.5 0.3 t 0.8 0.1 t t 4.4 4.5 4.0 0.9 1.0 0.8 1.3 t 0.8 0.1 t t 3.7 3.8 4.1 0.9 2.1 0.5 0.6 t 0.7 t t t 2.9 2.9 2.3 1.0 1.0 0.2 0.1 t 0.5 t t t 2.5 2.5 2.2 1.1 1.0 t 0.1 t 0.4 123

Description Profile No. 44 Ail 0- 25 cm 10YR5/1 dry (10YR5/2, moist) sandy clay loam; moderate fine and medium angular blocky; hard, firm, sticky and plastic; very few cracks not more than 5 mm wide; few small hard dark grey CaCO3 nodules; strongly calcareous; very few fine roots; frequent fine pores; clear wavy boundary; pH 9.6. t A12 25- 50 cm Subequal mixture of 10YR4/1 and 4/2 dry (10YR5/1 and 4/2, moist) sandy clay loam; wtjak fine angular blocky; very hard, very firm, sticky and plastic; very few small hard dark grey CaCO3 nodules mainly as crack fillings, few small hard white CaCO3 concretions; strongly calcareous; no roots; gradual smooth boundary; pH 9.6. \13 50- 95 cm 10YR4/2 dry and moist with frequent mottles of 10YR4/1, moist, sandy clay loam; weak fine angular blocky; very hard, very firm, sticky and plastic; frequent small and large hard (soft powdery on outside) white CaCO3 concretions; strongly calcareous; no roots; gradual smooth boundary; pH 9.4. B 95-135 cm 10YR6/3 dry (10YR5/3, moist) sandy clay loam with few 7.5YR3/1 dry and moist, mottles part of which seem to be translocated along root channels etc. since they occur as distinct skins; structureless; very hard, very firm, slightly sticky and plastic; few small and large hard white CaCO3 concretions decreasing in number and size towards base of horizon; strongly calcareous; gradual smooth boundary; pH 9.4. B2 135-170 cm 10YR6/3 dry (10YR5/3, moist) sandy loam with a few clear mottles of 7.5YR3/1, clay (in some cases the mottles are larger than 5 cm but faint); structureless; extremely hard, very firm, nonsticky, nonplastic; very few small and large hard white CaCO3 concretions (some kankar); strongly calcareous; clear smooth boundary; pH 9.2. C 170-185 cm Several mixed colors like 10YR7/2, 7/3, 6/3, 6/4 dry (resp. 5/1 and 5/2, moist) loamy sand; structureless; slightly hard, loose, moist, nonsticky nonplastic; no CaCO3 concretions; strongly calcareous; pH 9.O.

Boring: 185-250 cm Similar to horizon above, very few large hard white CaCO3 concretions occur. 124

Laboratory Data Profile No. 44

Depth, Horizon 0/ Particle size distribution (u) % /o cm >2 mm sand silt clay '- 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 25 All 33.0 27.3 3.6 5.4 30.7 4.2 26.5 25- 50 A12 — 29.7 25.4 4.7 6.5 33.7 4.0 29.7 50- 95 A13 — 31.9 28.7 2.3 6.4 30.7 3.2 27.5 95-135 •••:.;. B 1 32.4 38.2 4.4 4.4 20.6 2.1 18.5 135-170. B2 0.8 54.4 29.1 2.1 3.1 11.3 1.2 10.1 170-185 C 63.5 23.5 3.7 9.3 0.5 8.8

Elemental composition

SiO2 A21O,, Fe2O3 TiO2 CaO MgO K2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

70.0 ,-;,;..8.6 4.0 0.8 6.0 0.9 6.6 66.6,, , 10.2 5.1 0.9 5.8 1.2 6.4 67.5 9.9 4.7 0.9 5.9 1.2 6.5 71.7' 10.5 5.1 1.1 5.5 1.3 3.8 75:5. 9.3 3.7 0.7 5.6 1.6 1.9 78.8 8.7 3.3 0.6 5.3 1.6 1.7

B.D. Organic CaCO3 Gypsum pH Clayminerals 0/ (g matter equiv. /o 1 :5 Mt Mi Vm Chi K Q per c 0//o cc) % C/N

1.66 0.14 16 9.7 10.0 xxxx X t 1.73 0.20 17 8.5 9.9 xxxx X t 1.80 8.4 10.0 xxxx X t 2.27 1.9 10.1 xxxx X t 1.6 10.2 xxxx X t 3.8 10.2 xxxx X t 125

Cation Extractable cattons, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. % Ca Mg Na per cm

21.9 25.0 13.8 2.2 1.0 8.0 36.5 0.4 0.2 17.0 1.9 27.4 30.8 12.4 2.6 0.1 15.7 57.3 0.5 0.3 40.4 4.2 24.1 25.2 10.8 3.1 0.1 11.2 46.5 0.9 0.3 39.1 3.9 21.3 24.6 11.2 3.2 t 10.2 47.9 0.5 0.3 27.0 2.7 13.7 13.8 6.9 2.2 0.2 4.5 32.9 0.5 0.3 18.4 2.0 10.1 10.6 5.9 1.8 0.2 2.7 26.7 0.6 0.3 15.9 1.7

Waterextract 1 :5, soluble (meq/100 g) E.C.5 cations anions mmhos

Ca Mg K Na Sum Sum, CO3 HCO,, Cl SO4 NO3 per cm

0.1 t t 3.9 4.0 3.5 1.5 1.5 0.3 0.1 0.1 0.7 0.1 t t 5.4 5.5 4.9 1.2 1.2 0.6 1.2 0.7 1.1 0.1 t t 5.0 5.1 4.9 1.2 1.4 0.4 1.5 0.4 1.0 0.1 t t 4.1 4.2 3.9 1.3 1.7 t 0.7 0.2 0.8 t t t 3.2 3.2 3.1 1.2 1.6 t 0.2 0.1 0.6 t t t 2.9 2.9 2.8 1.4 1.0 0.2 0.1 0.1 0.5 126

Sample area No. 5 Information on the sites sampled a. Profile numbers: 51, 57, and 59 b. Date of examination: July 1968 c. Elevation: 51 at about 360 m, 57 at about 366 m and 59 at about 377 m d. Landform: 1. physiographic position: 51, Nile terrace I 57, Nile terrace II 59, Nile terrace IV 2. landform of surrounding country: flat, at all sites 3. micro topography: nil, at all sites e. Vegetation: Desert scrub zone; 51, very few scrub, 57 and 59 without vegetation f. Climate: tropical arid with few summer rains, compare data of Khartoum and Shendi meteorological stations

General information on the soils a. Parent material: calcareous alluvia of the Nile b. Permeability of the subsoils: moderately slow c. Moisture conditions in the soils: 51, moist 0 to 45 cm, dry 45 to 170 cm, moist below 57, moist 0 to 30 cm, dry below 30 cm 59, dry throughout the profile d. Depth of groundwater table: 51, from the surface down to about 10 m; 57 and 59, unknown e. Presence of surface stones: 51, nil 57, very few rounded gravel without varnish 59, 90% of the surface is covered by wellrounded quartz gravel which has a black desert varnish f. Evidence of erosion: 51, winddeposition around scrub 57 and 59, nil g. Human influence: nil 127

i. Km .J'APPR. ^;}

Fig. 39. Sample area No. 5 128

Description Profile No. 51

Al 0-11 cm 10YR4/3 moist, clay loam with very few coarse sandgrains; structureless; friable, nonsticky, nonplastic; calcareous; few fine roots; clear smooth boundary; pH 8.2.

Cl 11- 26 cm 1()YR4'3 moist, clay; structureless; friable, sticky and plastic; calcareous; few fine roots; abrupt smooth boundary; pH 8.2.

IIC2 26- 46 cm 7.5YR6 6 dry (5/6, moist) loamy sand with a very few coarse sandgrains; structureless; loose when dry and moist, nonsticky, nonplastic; calcareous; few fine roots; clear smooth boundary; pH 8.2.

IIC3 46 86 cm 10YR4/3 dry (4/2, moist) clay with sand movement from horizon above through cracks; moderate coarse angular blocky; frequent clear slickensides up to 50 cm long; very hard, very sticky, very plastic; cracks up to 4 mm wide; calcareous; very few fine and medium roots ending at base of horizon; clear wavy boundary; pH 8.5.

IIC4 86-110 cm 10YR6/4 dry (5i3, moist) loamy fine sand; structureless; loose when dry, nonsticky, nonplastic; very few small hard white CaCO;, concretions towards base of horizon mainly; calcareous; no roots; clear wavy boundarv; pH 8.5.

IIIC5 110-15Ucm 10YR4/3 dry (3/2, moist) clay with brown sandstreaks from horizon above, occupying 20-30 percent of the exposed surface; structureless; friable, slightly sticky, slightly plastic; very few pressure faces; strongly calcareous; clear smooth boundary; pH 9.2. 11IC6 150-170 cm 10YR3/2 moist, clay loam with few sandstreaks; structureless; friable, slightly sticky, slightly plastic; very few small hard white CaCO;( concretions, some black inside; strongly calcareous; pH 9.O.

Boring: 170-240 cm same as horizon above

240-270 cm siltv clav without CaCO3 concretions 129

Laboratory Data Profile No. 51

Depth, Horizon % Particle size distribution (ß) % cm >2r •nm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 11 Al 7.7 32.8 16.2 8.1 35.2 11- 26 Cl — 7.3 20.7 14.1 13.1 44.8 26- 46 IIC2 0.6 47.2 42.0 1.6 1.1 8.1 46- 86 IIC3 0.4 11.3 29.2 12.0 6.7 40.8 86-110 IIC4 0.6 37.6 50.9 1.3 1.3 8.9 110-150 IIIC5 0.6 25.2 35.2 4.7 9.5 25.4 150-170 IIIC6 0.6 18.5 42.6 7.7 8.5 22.7

Elemental composition

SiO2 Al,A Fe2O3 TiO2 CaO MgO K2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

395 25.0 700 39.0 405 120.5 335 42.0 820 47.5 324 81.0 355 25.0 350 28.5 312 30.5 385 21.5 880 51.5 342 50.0 310 16.5 380 31.0 324 25.0

B.D. Organic CaCO3 Gypsum pH Clayminerals (g matter equiv. % 1:5 Mt Mi Vm Chi K Q per % cc) C/N

0.75 12 0.7 7.3 xxxx XX t 0.38 14 0.4 7.5 xxxx XX t 0.06 13 0.2 7.4 xxxx XX t 0.28 0.4 8.0 xxxx XX t 0.05 0.1 8.6 xxxx XX t 0.10 130

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. /o Ca Mg Na per cm

29.2 26.6 21.1 3.6 1.5 0.5 1.7 41.1 38.3 32.2 4.6 1.1 0.4 1.0 6.6 6.4 5.3 0.8 0.2 0.1 1.5 35.4 32.4 24.0 6.1 0.4 1.9 5.1 6.6 5.7 4.0 1.2 0.1 0.4 6.1

Waterextract 1 : 5, soluble (meq/100 g) E.C.5 cations anions mmhos per cm Ca Mg K Na Sum Sum CO3 HCOa Cl so4 NO3

0.4 0.1 0.1 0.1 0.6 1.0 — 0.9 t 0.1 t 0.2 0.3 0.1 0.1 0.1 0.6 0.7 — 0.6 t 0.1 t 0.2 0.2 0.1 t 0.1 0.4 0.6 — 0.4 0.1 0.1 t 0.1 0.1 t t 0.8 0.9 1.0 — 0.8 0.1 0.1 t 0.2 t t t 0.8 0.8 0.8 — 0.6 0.1 0.1 t 0.2 131

Description Profile No. 57

Al 0- 11 cm 7.5YR4/4 moist, sandy clay loam with a very few wellrounded pebbles without polish; weak medium subangular blocky; soft, friable, nonsticky, slightly plastic; very few small hard black CaCO3 concretions also on the surface; strongly calcareous; very few roots; gradual smooth boundary; pH 8.2.

B 11- 28 cm 10YR4/3 moist (ill matching), sandy clay loam with coarse sandgrains; moderate medium subangular blocky; slightly hard, friable, nonsticky, nonplastic; very few small hard black CaCO3 concretions, very few large hard white CaCO3 concretions somewhat increasing in number with depth; strongly calcareous; very few black rockfragments (Nubian sandstone?); no roots; gradual smooth boundary; pH 8.2.

IIC1 28- 80 cm 10YR7/7 and 7/8 dry (6/6, moist) sandy clay loam with angular coarse quartz fragments; structureless; very hard; frequent small soft and hard CaCO3 concretions; strongly calcareous; very few fine roots in top of horizon; abrupt wavy boundary; pH 8.3.

IIC2ca 80-130 cm 10YR7/7 dry (6/6, moist) sandy loam; structureless; dominant (90 percent by volume) small and large soft and hard white CaCO3 masses and concretions; gradual boundary; pH 8.3.

IIC2 130-150 cm Similar to horizon above but CaCO3 decreasing to few (some 40 percent) and with few small angular quartz fragments and black angular rockfragments (Nubian sandstone ?). 132

Laboratory Data Profile No. 57

Depth, Horizon 0/ Particle size distributior /o ' (/*) % cm >2 mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 11 Al 28 30.9 37.5 4.7 4.9 22.0 11- 28 B 38 33.0 33.2 4.5 5.1 24.2 28- 80 IIC1 23 50.1 27.7 0.4 1.1 20.7 80-130 IIC2ca 33 43.9 23.9 3.8 6.6 21.8 130-150 IIC2 44 47.8 21.3 2.4 9.6 18.9

Elemental composition

SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm 3.0 445 26.0 700 38.0 552 49.5 24.2 475 22.5 700 32.0 315 46.0 7.5 495 20.5 710 26.5 390 50.0 25.6 485 27.5 620 27.5 204 49.5 31.4 425 28.0 550 23.0 195 32.5

B.ü. Organic CaCO Gypsum pH Clayminerals (g matter equiv. 1 : 5 Mt Mi Vm Chi K per Q /o cc) % C/N

0.13 13 3.6 7.3 xxxx 0.14 17 21.7 7.1 xxxx 0.04 12 7.5 7.5 xxxx 47.4 7.0 xxxx 47.0 7.5 xxxx 133

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. ex eh. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. % Ca Mg Na per cm

18.2 19.3 16.6 1.5 0.8 0.4 2.2 22.3 23.8 20.5 1.0 1.1 0.2 0.9 16.0 16.9 15.9 0.5 0.4 0.1 0.6 12.4 12.9 12.2 0.5 0.1 0.1 0.8 10.7 12.3 11.5 0.5 0.1 0.2 1.9

Waterextract 1 : 5, soluble (meq/100 g) E.C.5 cations anions mmhos per cm Ca Mg K Na Sum Sum CO3 HCO3 Cl SO4 NO3

0.2 t 0.1 0.1 0.4 0.8 0.5 0.2 0.1 t 0.1 0.2 t 0.1 0.1 0.4 0.6 — 0.4 0.1 0.1 t 0.1 0.1 t t 0.2 0.3 0.6 — 0.4 0.1 0.1 t 0.1 0.3 t t 0.4 0.7 0.8 — 0.3 0.2 0.1 0.3 0.3 07 0.1 t 0.5 1.3 1.8 • 0.3 0.4 0.4 0.7 0.5 134

Description Profile No. 59

Al 0- 2 cm 1OYR5/3 dry (4/4, moist) clay loam; structureless; soft, loose, nonsticky, nonplastic; non-calcareous; clear wavy boundary; pH 7.6.

A2 2- 35 cm 5YR4/4 dry (3/4, moist) gravelly sandy loam (80 percent gravel); structureless; slightly hard, looise, nonsticky, nonplastic; gravel has no iron coating; slightly calcareous; very few fine roots; clear wavy boundary; pH 7.6.

Btir 35- 70 cm 2.5YR3/6 dry and moist, gravelly clay, wellrounded gravel, some 80 percent by volume, has average diameter of 2-3 cm, but can be as large as 8 cm; strong fine angular blocky; slightly hard, friable, nonsticky, nonplastic; clear continuous clay cutans; gravel with continuous red iron coating; non-calcareous; no roots; pH 7.4. 135

Laboratory Data Profile No. 59

Depth, Horizon 0/ Particle size distribution /o (jU) % cm >2mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 2 Al 7.2 31.2 15.5 14.6 31.5 8.1 23.4 2-35 A2 16 38.9 25.8 7.3 10.7 17.5 7.5 10.0 35-70 Btir 28 16.2 10.3 4.3 6.5 62.7 25.4 37.3

Elemental composition

SiO2 A12O3 Fe,O, TiO2 CaO MgO K2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

66.8 12.8 7.2 2.0 2.2 1.7 1.4 4.9 555 30.5 680 46.0 234 72.5 77.5 9.9 5.2 1.1 7.6 2.6 1.9 4.0 465 27.5 1120 57.5 222 66.0 SS7 19.5 9.0 1.1 1.4 0.8 1.1 9.5 7 SO 20.0 280 56.5 117 49.5

B.D. Organic CaCO3 Gypsum pH Clayminerals matter equiv. 0/ 1 :5 (g /o Mt Mi Vm Chl K o/ per /o cc) C/N

0.18 12 0.1 8.6 XXX t XX t 0.15 15 1.3 8.2 XXX t XXX t 0.11 16 0.1 7.7 XXX t XXX t 136

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. % Ca Mg Na per cm

24.1 25.7 22.6 2.1 0.9 0.1 0.4 5.7 8.8 7.8 0.7 0.3 t 157.5 23.7 56.5 23.6 13.7 13.3 10.4 2.5 0.3 0.1 0.7 555.0 160.5 139.1 74.0

Waterextract 1 : 5, soluble (meq/100 g) E.C5 cations anions mmhos

Ca Mg K Na Sum Sum CO3 HCC-3 Cl SO4 NC-3 per cm

0.3 0.1 0.1 0.1 0.6 0.6 t 0.3 0.1 0.1 t 0.2 6.4 0.7 0.2 1.5 8.8 8.8 t 0.1 4.2 3.6 1.0 1.8 ?.8,S 7.7 0.1 6.7 43.0 43.3 t 0.1 38.8 0.5 3.9 8.4

138

Sample area No. 6 Information on the sites sampled a. Profile numbers: 61, 67, and 68 b. Date of examination: July 1968 c. Elevation:'61, at about 354 m, 67 at about 360 m and 68 at about 366 m d. Landform 1. physiographic position: 61, Nile terrace I 67, Nile terrace II 68, Nile terrace III 2. landform of surrounding country: flat at all sites 3. microtopography: nil e. Vegetation/Landuse: Desert scrub zone; 61, under irrigation, 67, very few Acacia tortilis, 68, very few desert scrub f. Climate: tropical arid with some summer rains, compare data of Shendi meteorological station

General information on the soils a. Parent material: calcareous alluvia of the Nile b. Permeability of the subsoils: 61, moderate 67 and 68, moderately slow c. Moisture conditions in the soils: 61, dry 0 to 15 cm, moist below 15 cm; 67 and 68, dry throughout the profile d. Depth of groundwater table: 61, between 2 and 10 m 67 and 68, unknown e. Presence of surface stones: 61, nil 67, very few rounded gravel without varnish 68, very few rounded gravel with varnish f. Evidence of erosion: 61, nil; at 67 and 68 some wind erosion and deposition around scrub g. Human influence: 61, irrigation practices, 67 and 68 nil 139

f"0

Fig. 40. Sample area No. 6 140

Description Profile No. 61

Al 0- 15 cm 10YR5/4 dry (3/2, moist) clay loam; structureless; slightly hard, friable, nonsticky and nonplastic; strongly calcareous; very few fine and few medium roots; clear wavy boundary; pH 8.0

Cl 15-17? cm 10YR5/4 dry (3/2, moist) clay loam; structureless; friable, nonsticky and nonplastic; strongly calcareous; few termite pores; very few fine roots down to base of the pit; pH down to 75 cm is 8.0, below 75 cm to 175 cm pH is 7.6; at some 150 cm depth the original sedimentary horizontally laminated structure is weakly preserved with a fine sandy (mica) layer, though discontinuous, still in horizontal position. 141

Laboratory Data Profile No. 61

Depth, Horizon 0/ Particle size distribution (jU) % >/o2 mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 15 Al 5.8 18.6 26.0 15.0 34.6 5.8 28.8 15- 65 Cl 5.4 24.1 28.5 14.1 27.9 4.2 23.7 65-115 Cl 1.0 8.8 30.3 20.0 39.9 7.2 29.7 115-165 Cl 0.2 26.9 32.0 14.4 26.5 1.5 25.0

Elemental composition

SiO2 Al 2O3 Fe2O3 TiO2 CaO MgO K2C) Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

59.4 14.5 8.7 1.6 4.1 1.4 6.8 61.0 14.0 8.6 1.6 4.4 1.3 5.8 53.4 16.2 9.8 1.8 4.4 1.3 7.1 55.6 14.8 9.4 1.7 4.1 1.4 5.8

B.D. Organic CaCO3 Gypsum pH Clayminerals matter equiv. 0/ 1 : 5 /o Mt Mi Vm Chi K Q per cc) 0/ C/N /o

1.52 0.86 12 1.3 8.4 xxxx t XX t 1.42 0.40 15 1.3 8.6 xxxx t XX t 1.20 0.50 2.1 8.6 xxxx t XX t 1.33 0.35 1.8 8.6 xxxx t XX t 142

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. 0/ per cm /o Ca Mg Na 29.6 29.7 23.0 5.1 1.5 0.1 0.3 29.9 29.6 24.3 4.6 0.7 t 34.4 36.4 28.4 7.5 0.4 0.1 0.3 29.5 33.3 28.2 4.7 0.3 0.1 0.3

Waterextract 1 : 5, soluble (meq/100 g) E.C.. cations anions mmhos per cm Ca Mg K Na Sum Sum co3 HCO3 Cl SO4 NO3

0.4 0.1 0.1 0.2 0.8 1.0 t 0.4 0.2 0.1 0.3 0.2 0.3 0.1 t 0.2 0.6 0.7 t 0.5 0.2 t t 0.2 0.3 0.1 t 0.3 0.7 0.6 t 0.5 0.1 t t 0.1 0.3 0.1 t 0.2 0.6 0.7 t 0.5 0.2 t t 0.1 143

Description Profile No. 67 Btl 0- 16 cm 7.5YR4/3 dry (4/2, moist) sandy clay loam (with very few brown sandstreaks in fissures); moderate medium and coarse columnar, although rounded caps are not clear everywhere; clayskins on horizontal and vertical pedsurfaces; hard, friable, noristicky, nonplastic; few and towards the base of horizon frequent small and large soft white CaCO3 streaks, sometimes slightly hard (can be crushed with fingers) concretions, in the top 5-10 cm sometimes (20 percent by volume) no streaks occur; very few small hard black CaCO3 nodules mainly in sandstreaks; strongly calcareous; very few fine roots; clear wavy boundary; pH 9.4 Bt2 16- 46 cm 10YR4/3 dry (4/2, moist) sandy clay loam; moderate coarse angular blocky becoming less developed toward the base of horizon; clayskins somewhat less clear than in horizon above; hard, friable, nonsticky and nonplastic; very frequent small and large soft white CaCO3 streaks and patches and frequent small (1-2 mm) soft white CaCO3 specks; strongly calcareous; very few fine roots mainly on pedsurfaces; clear wavy boundary; pH 8.4 Cl 46-124 cm 10YR5/8 dry (4/4, moist) mixed with some small spots of 1OYR3/3 dry (3/2, moist), sandy loam; weak coarse subangular blocky; hard, very friable, nonsticky and nonplastic; very few small soft white CaCO3 aggregates mainly in top of horizon, very few small and large (up to 2 cm) hard white CaCO3 concretions mainly at bottom of horizon; strongly calcareous; very few small hard white gypsum in fine needles mostly in vertical position in horizontal fissures; almost no roots; clear smooth boundary; pH 8.4. IIC1 124-170 cm 7.5YR3/2 dry (4/2, moist, higher value), clay; strofig fine angular blocky determined by frequent pressure faces and few slickensides; hard, very firm, sticky and plastic; very few small hard white CaCO3 concretions; strongly calcareous; few small hard white gypsum needles up to 1 cm long occurring in nests or patches of some 3 cm in diameter; no roots; pH 8.5. Boring: 170-275 cm essentially the same as horizon above 144

Laboratory Data Profile No. 67

Depth, Horizon % Particle size distribution (fi) % cm >2mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 16 Btl 29.9 33.7 13.5 2.6 20.3 3.4 16.9 16- 46 Bt2 22.4 30.3 17.6 7.1 22.6 3.5 19.1 46- 86 Cl 28.8 45.3 12.3 2.3 11.3 1.7 9.6 86-124 Cl 24.7 44.1 12.0 6.4 12.8 1.9 10.9 124-170 IIC1 17.6 27.3 11.9 20.0 23.2 3.8 19.4

Elemental composition

SiO2 Al 2O3 Fe2O3 TiO2 CaO MgO K2C) Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

73.3 6.7 3.7 0.8 4.9 0.9 5.0 70.8 7.9 3.2 0.9 5.0 1.0 6.4 81.9 5.3 2.9 0.7 4.0 0.8 3.7 75.4 6.0 3.6 0.8 3.7 0.9 3.1 64.1 9.5 6.0 1.1 4.4 1.1 5.1

B.D. Organic CaCO3 Gypsum pH Clayminerals 0/ (g matter equiv. /o 1 : 5 Mt Mi Vm Chi K o/ per c /o cc) o/o C/N

1.73 0.12 7.1 9.5 xxxx X 1.74 0.10 26 8.7 9.0 xxxx XX 1.91 4.0 0.02 9.3 xxxx X 1.87 2.7 0.02 9.6 xxxx X 1.90 4.2 0.02 9.3 xxxx X 145

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. % Ca Mg Na per cm

17.5 18.6 11.0 2.1 0.5 5.0 28.6 3.9 1.0 74.8 9.0 20.6 26.8 10.0 2.8 0.3 9.7 47.1 24.8 12.2 282.6 31.0 12.8 16.3 6.6 1.5 0.2 8.0 62.5 15.1 3.6 208.7 19.6 15.3 17.8 5.9 1.9 0.2 9.8 64.1 8.0 2.5 150.0 13.6 30.4 30.6 7.0 4.5 0.4 18.7 61.5 1.0 0.8 65.2 7.2

Waterextract 1 : 5, soluble (meq/100 g) E.C5 cations anions mmhos

Ca Mg K Na Sum Sum CO, HCO3 Cl SO4 NO3 per cm

0.1 t t 3.8 3.9 4.1 0.6 0.7 2.5 0.2 0.1 0.8 0.3 0.1 t 14.1 14.5 14.0 0.2 0.9 11.9 0.1 0.9 3.0 0.2 t t 10.1 10.3 10.1 0.2 0.8 4.2 4.7 0.4 2.0 0.1 t t 8.0 8.1 7.4 0.3 0.4 3.4 3.2 0.1 1.7 01 t t 8.5 8.6 8.3 0.4 1.4 3.7 2.8 t 1.7 146

Description Profile No. 68

Ail 0- 16 cm 7.5YR5/6 dry (4/4, moist) loamy sand, with very few small rounded quartzpebbles; structureless; soft, friable, nonsticky, nonplastic; strongly calcareous; few fine roots; clear smooth boundary; pH 7.8

A12 16- 30 cm 10YR5/3 dry (4/4, moist) loamy sand with few small broken rounded desert-varnished black pebbles; weak medium subangular blocky; soft, friable, nonsticky and nonplastic; few small hard white CaCO3 concretions; strongly calcareous; few fine roots; gradual smooth boundary; pH 8.5

Bt 30- 65 cm 10YR5/4 dry (4/3, moist) sandy loam, with few rounded quartzpebbles; moderate medium angular blocky; very hard, slightly cemented; very frequent small and large (up to 5 cm) hard white CaCO3 concretions, sometimes soft on the outside, sometimes black inside, some 60 percent by volume; strongly calcareous; few fine roots; gradual smooth boundary; pH 8.2

Cmca 65-150 cm 10YR6/3 and 6/4 dry (5/6, moist) sandy clay loam, with few up to 3 cm large rounded quartzpebbles, with few black rockfragments from pediment up to 20 cm large at base of horizon; indurated; dominant small and large white CaCO3 concretions with some yellowish CaCO3 streaks; strongly calcareous; no roots; pH 8.6 147

Laboratory Data Profile No. 68

0/ Particle: size distribution Depth, Horizon /o (/*) % cm >2mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 16 All 2 51.2 33.6 6.8 1.4 7.0 1.2 5.8 16- 30 A12 9 53.6 29.2 7.2 0.3 9.7 1.7 8.0 30- 65 Bt 44 54.0 23.3 5.5 0.8 16.4 1.9 14.5 65-100 Cmca 28 46.5 23.9 5.7 3.8 20.1 2.7 17.4 100-165 Cca 30 43.4 24.6 6.3 4.5 21.2 3.5 17.7

Elemental composition

SiO2 A12O3 Fe2O3 TiO2 CaO MgO K 2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

86.2 4.1 2.8 0.7 0.8 0.5 1.7 78.9 3.9 2.7 0.6 4.8 0.4 3.3 71.1 3.9 2.2 0.4 6.7 0.4 10.2 56.0 3.8 1.9 0.4 14.9 0.3 16.6 57.5 3.9 2.0 0.4 15.5 0.3 17.8

B.D. Organic CaCO3 Gypsum pH Clayminerals (g matter equiv. % 1:5 Mt Mi Vm Chi K 0/ per c /o cc) % C/N

0.09 0.6 9.3 XXX t XXX t 0.10 17 3.5 8.8 XXX t XXX t 0.10 19 17.0 8.7 XXX X XXX t 33.2 8.7 XXX X XXX t 35.5 9.1 XXX X XXX t 148

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. 0/ per cm /o Ca Mg Na

6.8 7.1 6.3 0.6 0.2 t 9.9 10.5 9.4 0.8 0.3 t — 11.3 11.0 9.9 0.9 0.2 t — 10.4 11.3 9.9 1.2 0.2 t — 16.8 2.7 41.3 6.4 10.0 10.0 8.7 1.2 0.1 t 8.0 1.8 46.7 6.2

Waterextract 1 : 5, soluble (meq/100 g) E.C5 cations anions mmhos

Ca Mg K Na Sum Sum CO3 HCO3 Cl SO4 NO3 per cm

0.3 t t t 0.3 0.5 t 0.2 0.2 0.1 t 0.1 0.3 t t t 0.3 0.5 t 0.3 0.1 0.1 t 0.1 0.4 0.1 t 0.3 0.8 0.8 t 0.2 0.4 0.1 0.1 0.2 0.3 0.1 t 1.9 2.3 2.4 t 0.3 1.6 0.1 0.4 0.5 0.1 t t 2.6 2.7 2.4 0.1 0.2 1.4 0.4 0.3 0.6

150

Sample area No. 7 Information on the sites sampled a. Profile numbers: 71, 71 A, 77, and 78 b. Date of examination: July 1968 c. Elevation: 71 and 71A at about 352 m, 77 at 358 m, and 78 at 364 m d. Landform: 1. physiographic position: 71 and 71 A, Nile terrace I; 77; terrace II; and 78, terrace III 2. landform of surrounding country: flat at all sites 3. microtopography: nil at 77 and 78; 71, depressional; the area in which 71A is located is dissected by a network of some 3 m wide and 1 m deep gullies, covering some 40% of the surface area, 'tabra' channels of White and Law (1969), see figure 41 e. Vegetation: Desert scrub zone; all sites without vegetation, except at 71A where some grass occurs f. Climate: tropical arid with some summer rains, compare data of Shendi and Atbara meteorological stations

General information on the soils a. Parent material: calcareous alluvia of the Nile b. Permeability of the subsoils: 71, moderate; 71A, 77, 78, moderately slow c. Moisture conditions in the soils: 71, dry 0 to 14 cm, slightly moist 14 to 90 cm, moist below 90 cm 71A, dry 0 to 55 cm, slightly moist below 55 cm 77 and 78, dry throughout the profile d. Depth of groundwater table: unknown at all sites e. Presence of surface stones: 71 and 71 A, nil 77, surface covered by rounded gravel (5-6 cm long) without desert varnish 78, surface covered for some 60% by desert varnished gravel (desert pavement) f. Evidence of erosion: possibly some water erosion at 71 A, winderosion at 77 and 78 g. Human influence: nil 151

Fig. -II. Characteristic gully pattern of Xile low terrace

Fig. 42. Sample area No. 7 152

Description Profile No. 71

Al 0- 14 cm 10YR4/3 (3/3, moist) clay; top 2-3 cm fine platy, moderate medium and coarse angular blocky; few clay eutans; hard, friable, nonsticky and slightly plastic; very few fissures; strongly calcareous; very few fine shell fragments; very few fine roots; clear smooth boundary; pH 8.3

Cl 14- 26 cm 1OYR3/3 moist, sandy clay loam; weak fine angular blocky; friable, nonsticky and nonplastic; very few small (up to

2 mm) soft white CaCO:, streaks and patches; strongly calcareous; very few fine roots; gradual smooth boundary; pH 8.5.

C1C2 26- 64 cm 10YR4/4 moist, with common distinct diffuse 1OYR3/3 tongue-shaped mottles from horizon above, sandy loam with very few gravel; structureless; friable, slightly sticky and plastic; few small soft white CaCO;, streaks and patches, and concretions that are sometimes black inside; strongly calcareous; very few fine roots; gradual smooth boundary; pH 9.2

C2 64— 90 cm 10YR4/4 moist, no mottles, sandy loam; structureless;

consistence as above; few small soft white CaCO3 patches (no streaks) and spots; very few small hard white CaCO;( concretions; strongly calcareous; very few fine roots; clear wavy boundary; pH 9.4

C3 90-160 cm 10YR5/1 moist with some brown sandstreaks of horizon above down to 120 cm, sandy loam; structureless; sticky and plastic; very few small hard (soft weathered outside) white CaCO-, concretions down to 120 cm; calcareous; no roots; pH 90-120 cm 8.8, from 120-160 cm 8.

Boring: 160-250 cm same as horizon above, with some small black rock fragments (weathered). 153

Laboratory Data Profile No. 71

Depth, Horizon /o Particle size distribution (//,) % cm >2 mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 14 Al 19.9 24.3 8.6 5.1 42.1 14- 26 Cl — 26.4 28.6 7.9 2.7 34.4 26- 64 C1C2 0.6 41.3 38.6 5.7 0.3 14.1 64- 90 C2 0.6 48.9 38.3 4.0 1.7 7.1 90-120 C3 0.6 39.7 37.4 5.6 2.8 14.5 120-160 C3 38.8 36.3 4.1 2.9 17.9

Elemental composition TiO CaO MgO K O Loss on Ba Co Mn Ni Sr SiO2 A12O;, Fe2O3 2 2 Zn ignition weight percentages ppm

65.0 10.8 6.6 1.1 2.0 1.0 4.8 72.6 9.1 5.8 0.9 2.1 0.8 3.8 82.3 4.4 2.9 0.7 2.0 0.6 2.7 70.8 2.9 2.0 0.7 1.9 0.5 1.8 69.9 4.7 2.7 0.7 1.9 0.7 2.7 69.7 5.9 3.2 0.7 2.0 1.0 2.5

B.D. Organic CaCO3 Gypsum pH Clayminerals matter equiv. 0/ (g /o 1 : 5 Mt Mi Vm Chi K Q per 0/ C /o cc) % C/N

0.28 17 0.6 7.7 xxxx X t 0.28 15 1.3 7.8 xxxx XX t 0.25 16 3.0 9.4 xxxx t XX t 0.13 2.6 10.2 xxxx XX t 0.24 3.0 10.2 xxxx XX t 0.15 2.4 10.3 xxxx t XX t 154

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. % Ca Mg Na per cm

35.7 33.6 23.4 3.2 0.8 6.2 17.4 2.8 27.8 26.9 12.4 1.9 0.6 11.9 42.8 8.4 13.7 14.7 4.8 0.7 0.3 8.9 65.0 10.6 7.0 9.3 3.8 0.3 0.1 5.1 72.9 5.6 13.7 14.7 2.5 0.3 0.2 11.7 85.4 5.0 19.1 19.3 1.8 0.4 0.3 16.8 88.0 4.1

Waterextract 1 :S, soluble (meq/100 g) E.C.6 cations anions mmhos per cm Ca Mg K Na Sum Sum co3 HCO3 Cl SO4 NO3 0.1 t t 2.2 2.3 2.5 1.1 0.2 1.1 0.1 0.5 0.2 t t 8.0 8.2 8.0 — 0.8 0.5 6.4 0.3 1.8 0.1 t t 7.1 7.2 6.9 0.4 0.5 0.5 4.9 0.6 1.4 t t t 3.6 3.6 3.8 1.8 0.4 0.3 1.0 0.3 0.8 t t t 4.8 4.8 4.9 2.2 0.5 0.4 1.4 0.4 1.1 t t t 5.5 5.5 5.3 2.2 0.6 0.6 1.4 0.5 1.2 155

Description Profile No. 71A

\1 ()- 55 cm 10YR4/2 dry and moist, clay; a 5-10 mm slightly hard crust at surface, strong fine and medium angular blocky; cracks 3 -4 cm wide down to base of horizon and 30 40 cm apart; extremely hard, firm ( ?), very sticky and very plastic; strongly calcareous; few fine and medium roots; clear wavy boundary; pH 8.6

Cl 55-125 cm 10YR3/2, moist, clay with occasional rounded quartzpebbles; structure determined by frequent pressure faces and some slickensides; no cracks; very hard, firm, sticky

and plastic; few small soft white CaCO;, patches and streaks; strongly calcareous; very few fine roots; gradual smooth boundary; pH 8.2 C2 125-150 cm 10YR3/2, moist, clay; structure determined by pressure faces; no cracks; very hard, firm, sticky and plastic; no CaCOs masses; strongly calcareous; no roots; pH 8.0 Boring: 150-175 cm continuation of horizon above 175-275 cm texture becomes silt loam to sandy loam and pH at 225 cm is 8.4.

Note: cracks are 5-8 cm wide at the surface, and occupy some 10°o of the surface area.

Fig. 43. Profile No. 71A, Typic Torrert 156

Laboratory Data Profile No. 71A

Depth, Horizon •>„ Partiell; size distribution («) "„ cm >2 mm sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 25 Al _ 0.5 6.1 7,8 14.2 71.4 11.6 59.8 25- 55 Al — 0.4 6.1 7.7 14.4 71.4 13.0 58.4 55- 90 Cl 1 0.6 6.9 8.6 14.2 69.7 11.2 58.5 90-125 CI 1 0.7 8.4 10.8 12.8 67.3 11.2 56.1 125-150 C2 1.0

Elemental composition

SiOa AI,O8 FeaO3 TiO2 CaO MgO K2O Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

52.6 16.8 9.3 1.2 3.2 1.1 1.3 7.8 305 48.0 1180 42.5 300 102.5 52.7 16.9 9.4 1.5 3.2 1.2 1.4 7.7 245 38.5 1120 32.0 330 93.0 53.4 16.4 9.4 1.5 3.5 1.3 1.2 7.0 295 22.5 1270 82.5 375 80.5 55.7 17.1 9.2 1.6 3.3 1.1 1.2 6.8 295 38.5 1120 72.0 318 98.5 55.7 16.7 9.4 1.6 3.2 1.0 1.2 6.9 515 43.5 960 80.5 1170 82.5

B.D. Organic Gypsum pH Clavminerals (g matter equiv. 1 : 5 Mt Mi Vm Chi K Q per C/N

0.36 20 1.2 9.1 xxxx XX t 0.37 19 1.0 9.0 xxxx XX t 0.7 0.1 8.4 xxxx XX t 0.6 1.9 8.2 xxxx t XX t 1.3 0.1 8.3 xxxx t XX t 157

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. /o Ca Mg Na per cm

52.8 53.2 41.3 7.7 1.7 2.5 4.7 53.7 52.4 33.9 7.9 1.3 9.3 17.3 1.3 0.4 23.9 2.8 52.4 54.6 28.7 7.1 1.6 17.2 32.8 19.8 5.5 117.4 12.8 48.9 55.4 28.2 7.7 1.5 18.0 36.8 25.0 8.7 151.1 15.4 50.5 51.0 16.9 7.9 1.7 24.5 49.0 12.8 6.1 133.7 13.4

Waterextract 1 : 5, soluble (meq/100 j E.C.5 mmhos per cm Ca Mg K Na Sum Sum CO3 HCO3 Cl SO4 NO3

0.1 0.1 t 1.1 1.3 1.1 0.1 0.8 0.1 0.1 t 0.2 0.2 0.1 t 3.2 3.5 3.2 0.1 1.0 1.8 0.3 t 0.7 1.7 0.4 0.1 16.1 18.3 18.4 t 0.4 6.4 11.4 0.2 3.6 8.8 1.9 0.2 28.6 39.5 38.4 t 0.2 6.5 31.6 0.1 5.9 1.5 0.6 0.1 22.8 25.0 24.3 0.5 7.0 16.7 0.1 4.4 158

Description Profile No. 77

Al 0- 15 cm 10YR4/2 dry (3/2, moist) sandy clay loam (no gravel); moderate fine subangular blocky; questionable clay cutans; hard, friable, nonsticky and nonplastic; occasional small (1-2 mm) soft white CaCO3 specks; strongly calcareous; very few fine roots; clear wavy boundary; pH 8.4

Bt 15- 55 cm 10YR4/2 dry (3/2, moist) sandy clay; moderate fine and medium angular blocky; clear continuous clay cutans on horizontal and vertical pedsurfaces; hard, friable, nonsticky, slightly plastic; very few black rock fragments; numerous (very few by volume) small (1-2 mm) soft yellowish CaCO3 specks; strongly calcareous; very few fine roots; clear smooth boundary; pH 8.0

IIC1 55- 70 cm 10YR5/6 dry and moist, fine gravelly (up to 5 cm long, 50 percent by volume) sandy clay loam; larger pebbles wellrounded, smaller ones less well rounded; weak subangular blocky; hard, loose, nonsticky, nonplastic; very few small soft white CaCO3 specks, as horizon above; strongly calcareous; no roots; clear smooth subhorizontal boundary; pH 8.O.

IIC2 70-120 cm 10YR5/4 dry (5/3, moist) gravelly (10-20 percent by volume) sandy clay; pebbles are wellrounded except for very few pediment rock fragments, some rounded pebbles have some red iron coating; weak fine subangular blocky; very frequent (some 65 percent by volume) hard white CaCO3 masses; no roots; pH 8.O. 159

Laboratory Data Profile No. 77

Depth, Horizon 0/ Particle size distribution (fi) % /o cm >2 mm !sand silt clay 2000- 200- 50- 20- 2- 200 50 20 2 <2 0.2 <0.2

0- 15 Al . 32.0 33.5 4.6 2.6 27.3 15- 55 Bt 9 32.1 29.5 2.5 5.2 30.7 55- 70 IIC1 50 47.3 24.8 3.4 2.5 22.0 70-120 IIC2 44 34.5 31.9 7.3 3.2 25.1

Elemental composition

SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2C) Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

1.4 0.3 600 17.0 1000 45.0 420 40.0 1.6 0.4 165 14.0 1450 30.0 237 41.5 1.8 0.4 150 21.5 905 33.5 261 31.0 41.7 0.4 300 18.5 670 43.5 171 14.5

B.D. Organic CaCO3 Gypsum pH Clayminerals matter equiv. % 1:5 Mt Mi Vm Chi K per 0/ /o ce) C/N

1.80 0.22 31 0.9 7.2 xxxx t X t 1.90 0.25 31 1.0 7.4 xxxx XX t 1.6 7.0 xxxx t XX t 51.2 7.0 xxxx t XX t 160

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos cap. % Ca Mg Na per cm

27.3 26.0 23.1 1.7 0.6 0.6 2.2 32.5 30.6 27.0 2.1 0.3 1.2 3.7 22.4 21.2 18.5 2.0 0.1 0.6 2.7 • 11.2 11.5 12.8 10.9 1.4 0.1 0.4 3.5 9.0

Waterextract 1 : 5, soluble (meq/100 g) E.C.5 cations anions mmhos

Ca Mg K Na Sum Sum CO3 HCOj Cl SO4 NO3 per cm

0.1 t. t 0.2 0.3 0.7 — 0.5 0.1 0.1 t 0.1 0.1 t t 0.9 1.0 1.2 — 0.5 0.4 0.1 0.2 0.3 1.0 0.2 t 1.6 2.8 3.4 — 0.3 2.0 0.3 0.8 0.9 1-.0 0.3 t 1.4 2.7 3.4 — 0.3 1.8 0.8 0.5 0.8 161

Description Profile No. 78

Bt 0-11 cm Mixed 7.5YR7/4 (60 percent) and 7.5YR5/3 (40 percent) dry (4/4, moist) gravelly sandy clay loam (pebbles are wellrounded, have no desert varnish, and occupy some 50 percent by volume); moderate fine angular blocky; clay cutans; slightly hard, friable; strongly calcareous; very few fine roots; clear smooth boundary; pH <8.0

Btir 11- 41 cm 5YR5/6 dry (4/6, moist) gravelly (and cobbly) sandy clay loam (coarse gravel is wellrounded and fine gravel, absent in top horizon, is subangular, together they occupy some 70 percent by volume); weak fine angular blocky; slightly hard; strongly calcareous; gravel has a reddish iron coating mostly on top and bottom, while the sides are continuous white coated with CaCO3 powder; very few fine roots; gradual boundary; pH 8.2

Bir 41- 80 cm 5YR6/6 dry (4/6, moist) gravelly (and cobbly) sandy clay loam (gravel occupies some 80 percent by volume); structureless; slightly hard; strongly calcareous; reddish iron coating on pebbles is less conspicuous than in horizon above, and is often covered with CaCO3 dendrites; no roots; clear smooth boundary; pH <8.0

Cmca 80-155 cm 10YR7/4 dry (5/4, moist) clay loam; indurated by CaCO3; strongly calcareous; pH <8.0 162

Laboratory Data Profile No. 78

Depth, Horizon % Particle size distribution (/j,) % cm >2mm sand silt clav

0- 11 Bt 24 24.2 39.0 7.5 3.3 26.0 11- 41 Btir 31 32.1 26.8 17.5 3.6 20.0 41- 80 Bir 50 37.1 16.1 20.8 4.5 21.5 80-155 Cmca 37 12.2 13.4 33.1 9.9 31.4

Elemental composition

SiO2 A.21O3 Fe2O3 TiO2 CaO MgO K2C) Loss on Ba Co Mn Ni Sr Zn ignition weight percentages ppm

1.5 0.6 215 9.0 810 31.0 99 16.5 1.6 2.6 90 28.0 1000 44.0 166 28.0 1..9 0.5 185 44.5 500 30.0 654 19.5 37..5 0.5 230 12.0 530 34.0 265 10.0

B.D. Organic CaCO3 Gypsum pH Clayminerals matter equiv. 1 :5 Mt Mi Vm Chi K Q per cc) C/N

0.12 13 0.7 7.4 xxxx xx t 0.11 19 2.3 7.2 xxx xxx t 1.5 0.6 7.0 xxx xxx t 44.9 t 7.1 xxx x XX t 163

Cation Extractable cations, meq/100 g Exch. Saturation extract, E.C. exch. Sum Ca Mg K Na Na soluble (meq/1) mmhos 0/ cap. /o Ca Mg Na per cm 20.0 22.1 17.6 1.7 0.7 2.1 10.5 16.4 18.8 15.3 1.1 0.8 1.6 9.8 7.2 16.0 20.9 17.4 1.9 0.5 1.1 6.9 33.4 13.8 16.5 12.8 2.3 t 1.4 10.1 23.6

Waterextract 1 : 5, soluble (meq/100 g) E.C5 cations anions mmhos

Ca Mg K Na Sum Sum CO3 HCO3 Cl SO4 NO3 per cm

0.2 0.1 0.1 0.1 0.5 0.8 0.6 0. 1 0.1 t 0.1 0.5 0.1 0.2 1.2 2.0 2.1 — 0.4 1.1 0.2 0.4 0.6 9.1 1.4 0.2 4.4 15.1 16.7 — 0.2 7.7 0.3 2.5 3.4 4,S 1.6 0.1 4.1 10.3 11.7 0.2 10.0 0.8 0.7 2.3 CHAPTER 6

Composition of soil materials

The constituents of the soil materials, which form the various horizons in the soils described in the foregoing chapter, were described through the application of size analysis. The constituents were identified and their relative proportion estimated, using such techniques as elemental analysis, CaCO3, CaSO4, and organic matter determination, mineralogical analysis, X-ray diffraction analysis and differential thermal analysis. For convenience most of the data resulting from this study are presented with the morphological descriptions of the soils concerned in Chapter 5.2.

6.1. Investigational Techniques

6.1.1. Size analysis Size analyses of the grains making up the soil material were performed on the fine-earth fraction of all soil samples by sieving and by the pipette method. Material retained by the 2 mm sieve is reported separately as larger than 2 mm. Samples of 10 g were treated with H2O2 to remove soil organic cementing matter, and all of the carbonates were destroyed by means of HC1. The pastes were neutralised by successive washing with distilled water and washed through a 50 micron sieve into 1000 ml cylinders. The dispersion agent used was sodium-pyrophosphate. The cylinders were filled with distilled water, thoroughly shaken, and the suspensions checked for good dispersion. Determinations than continued according to standard procedure. The clay fraction of 63 samples were separated into two fractions, particles less than 0.2 (X and from 0.2 to 2 ß, via tube centrifuge, beginning the separation at 0.2 ju as outlined by Jackson (1968). The main particle-size limits employed — between sand, silt, and clay fractions — are those of the scheme used by the U.S. Dept. of Agriculture; additional limits are at 200, 20 and partly at 0.2 microns; together they are as follows:

coarse sand 2000 to 200 microns fine sand 200 to 50 microns 166

coarse silt 50 to 20 microns fine silt 20 to 2 microns coarse clay 2 to 0.2 microns fine clay less than 0.2 microns

6.1.2. Elemental analysis The concentration of at most 13 elements was determined by means of X-ray spectrometric analysis and atomic absorption spectrophotometer investigation. Oxide percentages of the major elements aluminum, iron, potassium, silicon, and titanium, were determined with the former technique, the oxide percentages of calcium and magnesium as well as the trace elements barium, cobalt, manganese, nickel, strontium, and zinc were analyzed with the latter method. All of these elements were determined in the fine-earth fraction of 40 samples, five major elements in another 23 samples, and the concentration of primarily trace elements in an additional 19 samples. Results of these analyses are given in the laboratory data sheets of Chapter 5.2. In addition the concentration of seven major elements was determined in the clay fraction of samples of the same soils. These data are used below in section 6.2.5.2. Elemental analyses were carried out on ignited samples, the loss on ignition - the difference between oven-dry (110° C) and ignited samples - is therefore given too. The two procedures used in elemental analysis may be described as follows. a. X-ray spectrometry Pretreatment of samples: Samples were fused with lithium borate and cast into a solid button. With this method — a modific- ation of the technique of Claisse (1961) — such effects as interelement interferences, macroscale heterogeinity, and particle size effects can be avoided (Jenkins and De Vries, 1967). The fusion reaction was made at 1300° C in a furnace containing the fusion mixture in a crucible made of platinum + 3% gold. Equipment: A manual X-ray spectrometer was used (Philips type PW 150). The experimental conditions may be summarized as follows:

Element Tube Kv mA Crystal Detector Collimator Medium

Si Cr 50 30 PE F.C. c vac. Al Cr 50 30 PE F.C. c vac. Fe Cr 30 16 LIF F.C. f air Ti Cr 50 30 LIF F.C. f vac. K Cr 50 30 PE F.C. f vac.

The crystals used were PE for Penta-erythritol and LIF for lithium fluoride. Measuring method: The intensities of the various elements were determined by means of measurements of peak and background. The background was computed from the mean of the intensities on both sides of the peak. The percentages of SiO2, A12O3, Fe2O3, TiO2, and K2O were calculated by a method of successive approximations correcting for matrix absorption. 167

Percentages found were corrected for loss on ignition. Results are expressed as percentages on oven-dry basis. b. Atomic absorption spectrometry The investigational procedures applied were verified through analysis of U.S. Geological Survey rocks (second series; Flanagan, 1969), which are used internationally as geochemical standards. Pretreatment of samples: Part of the bulk fine-earth fraction was cone-quartered twice down to some 80 g, which was subsequently crushed in an agate mortar. From this material the 1000 mg required for analysis was taken. The Na2CO3-Na2B4O7 fusion technique was used to prepare the soil sample for flame analysis. The sample subsequently was dissolved in acid solution according to Meyer and Koch (1959); in order to prevent coagulation of SiO2 this was done by mechanical shaking at roomtemperature. Interferences: Spectral interferences — the influence of one element on the excitation of another — in atomic absorption are of no importance as the concentration of unexcited (ground state) atoms is measured (Angino and Billings, 1967). Chemical interferences — resulting from chemical combination of the element concerned with other elements - are known to appear in the determination of Ca, Mg, and Sr, primarily due to the presence of Al, P, or Si. In order to overcome this difficulty Lanthanum was added to 1 % of the solution to be analyzed (Robinson, 1966; Angino and Billings, 1967; Capdevila, 1968). Anion interferences are largely eliminated because of the pretreatment with Na2B4O7 and the measuring method used. Ionization interferences — resulting in a smaller number of atoms remaining in the ground state - occur in the determination of Ba, the problem was overcome by adding 1 % KC1 to the sample solution (Angino and Billings, 1967).

The experimental conditions may be summarized as follows:

Element Cathode current Absorption Flame Fuel Oxidant Mech. slit (+ auxiliary current) wavelength, width, fA mA Â

Ba 10 5,536 highly C2H2 N2O 50 reducing Ca 10 4,227 ,, air 25 Co 10 (+ 400) 2,407 oxidizing ,, 25 Mg 4 2,852 reducing ,, 50 Mn 10 2,795 oxidizing yy 100

Ni 10 (+ 400) 2,320 oxidizing jj 50

Sr 10 4,607 hi.reducing , y y, 100

Zn 6 2,139 oxidizing « 300

Instrumentation : The spectrophotometer used was a Techtron Model AA4, consisting of the following four major units: 1. a set of hollow cathode tubes - one for each element - as light source emitting the sharp-line spectrum of the element to be determined; 168

2. a premix burner, featuring a burner chamber in which sample, fuel and oxidant are mixed before entering the flame where atomic vapor of the sample is produced; 3. an Ebert type monochromator as wavelength selector, which can be set to pass any wavelength of about 1,860 to 10,000 Â; mechanical slits open to a maximum width of 300 f.L, which gives a spectral slit width of 9.9 Â; and 4. a HTV photomultiplier type R 213 whose output is fed into an A.C. amplifier, whose signal is rectified and fed to a galvanometer as readout mechanism, coupled to a 1 mV Hitachi recorder. Measuring method: In order to determine the concentration of the elements in a sample from the absorption measured the addition method as developed by Zeegers (1959) was applied.

6.1.3. Calcium carbonate equivalent determination In a so-called 'Scheibler apparatus' part of the fine-earth fraction was shaken for 1 hour with 25% HC1 and the volume of CO2 evolved was measured. Results, as obtained from all soil samples, are presented as percentage CaCO3 of the oven-dry soil, but MgCO3 may be present too.

6.1.4. Gypsum analysis Gypsum was quantitatively determined by precipitation with acetone (U.S. Sal. Lab. Staff, 1954) in all soil materials where calcium and sulphate ions each constitute 1 meq/100 g soil or more, as determined on water extract 1 : 5, and which occur in a practically equivalent content. The determination was carried out on 16 soil samples.

6.1.5. Organic carbon determination The oxidizable organic matter of 68 soil samples was determined by chromic acid oxydation with spontaneous heating (Walkley-Black method). The soil materials selected are generally from topsoils but include a limited number of subsoils. This method has been found to give approximately 77% recovery of carbon, as compared to the dry-combustion method, for most soils of the temperate zone (Jackson, 1958). The percentage organic carbon can thus be obtained by employing conversion factor 1.3. This procedure was applied here with regard to the requirements for soil classification of the USDA.

6.1.6. Total nitrogen determination Nitrogen was measured on 55 soil samples mostly of topsoils according to the micro-Kjeldahl method.

6.1.7. Heavy and light mineral analysis Samples of the 500 to 50 micron sand fraction were obtained after pretreatment of about 50 g of the bulk fine-earth with H2O2 and HC1, and subsequent sieving. This material was cone-quartered down to the amounts required for bromoform (sp.gr. 2.89) separation and for mounting on slides. The determination and line counting of the mineral species by means of the polarizing microscope was done according to standard methods (Kerr, 1959; Milner, 1962; Heidenreich, 1965). Of the light fractions 100 grains were counted per sample, of the heavy fractions the mutual percentages of opaque and transparent grains were determined first and subsequently 100 transparent grains identified. Data concerning the heavy and light mineral composition of 41 soil samples are reported in section 6.2.5.1. 169

6.1.8. X-ray diffraction analysis X-ray diffraction analyses were carried out with the following Philips equipment: a generator control cabinet(PW 1320/00), a stabilized generator (PW 1310/00) with wide range goniometer (PW 1050/25), a basic recording unit (PW 1352/10) with sealer-timer combination (PW 1353/100) and pulse height analysis combination (PW 1355/10). The experimental conditions applied to obtain penrecorded X-ray diffraction diagrams with the apparatus, may be summarized as follows: radiation CoKa high voltage 35 kV current 30 mA filter Fe divergence slit 1° receiving slit 0.3 mm scatter slit 1° detector proportional detector probe scanning speed 1 ° (2 0) per minute for random powder and parallel orientation specimens full scale 400 counts per second time constant 2 seconds sample holders flat aluminum sample holder and glass slides.

A total of 126 random powder specimens of different clay fractions (not treated with H2O2 or HCl) were analyzed, and 15 random powder specimens of selected fine clay fractions (< 0.2 fi). Whenever required for essential diffraction data parallel orientation specimens were prepared. The saturation with Mg and ethylene-glycol solvation, as well as the saturation with K in preparation for heating treatments (up to 600° C) were done according to procedures outlined by Jackson (1968). In the tables with analytical data of Chapter 5.2. the clay minerals of the fraction < 2 fx are identified by the following symbols: Mt Montmorillonite Chi Chlorite Mi Mica K Kaolinite Vm Vermiculite Q Quartz The relative amounts of the clay minerals were estimated from areas enclosed by the strongest X-ray diffraction peaks, they are indicated as follows: x : small xxx : large xx : moderate xxxx : predominant

6.1.9. Differential thermal analysis The apparatus used for differential thermal analysis includes a pair of cylindrical electric furnaces. The sample heating rate of 10° C per minute was accomplished by an autotransformer on the furnace input which was advanced by means of an electric motor, gear and cam arrangement. Platinum-platinum plus 10% rhodium thermocouples were employed. The differential temperature was measured by recording the thermocouple voltage as a function of furnace temperature with a Philips automatic pen recorder (PR 3210 A/100). This and other procedures are discussed in detail by Arens (1951), Mackenzie ed. (1957), and Jackson (1968). The differential thermal curves obtained were used as supplementary evidence in the determination of clay minerals by X-ray diffraction. 170

6.2. Discussion of Results

6.2.1. Grain size composition Results of grain size analyses generally confirm the field observations with relation to soil texture. In a few instances minor deviations occur. Weight percentages of the component grain size fractions of horizons in the same soil differ relatively little, compared to variations that may exist between various soils. The latter variations are considered to be due to different conditions of deposition, the former, in addition, to soil formation. The content of fragments larger than 2 mm in soil materials deposited by the Blue Nile (soil profiles 16 to 44 inclusive) is small. This fraction, if present at all, then largely consists of CaCO3 nodules up to 5 or 6 mm in size (see section 6.2.3.). Soils of sample area 1 contain additional colluvial material originating from Jebel Gargada. Soils along the Nile, on the other hand, may contain significant proportions of fragments coarser than sand. These fragments mostly consist of wellrounded (often broken) quarzite or quartzose sandstone, and mainly occur in soils of terraces III and IV. The sand content tends to be less than 25% in soils of the low terrace of the Blue Nile and Nile, and usually is over 50% in levee soils of the Er Roseires terrace and in higher Nile terraces. The silt content consistently reaches maximum values of 60 to 70% in soils of the recent terrace of the Blue Nile, in soils of the same terrace along the Nile this percentage may be some 30% lower. All older soils generally have a silt content not exceeding 20%. With the silt fraction varying relatively little in most soils, the bulk clay fraction consequently is generally inversely proportional to the amount of sand present. The proportion of fine clay normally constitutes of from 80 to almost 100% of the total clay fraction. Examination of the silt and clay fractions of the backswamp soils of the former Blue Nile floodplain (Gezira, Kenana, and Dinder clayplains) - which were originally supplied in suspension - shows that the clay-silt ratio of these soils ranges of from 3 to 5. The maximum clay content is about 70% (soil profiles 16, 33, and 34). As may be computed from figure 21 the clay-silt ratio of the presently suspended load of the Nile approximately ranges between 0.5 and 2. The highest clay content is 55%. The high clay content and corresponding low silt percentages of the very extensive central Sudan clayplains may bé theoretically attributed to the following three factors prevailing at the time the Blue Nile formed these plains, a. Amounts of silt transported by the river were less than at present (or clay 171 supply higher) entailing a higher clay-silt ratio. In the light of the very uniform character of rocks in the Ethiopian catchment area, this is considered unlikely. b. Amounts of silt supplied were approximately similar to present conditions, but less of the silt was deposited. This seems even harder to explain than a), when conditions of stream velocity apparently allowed sedimentation of predominantly sand (in levees) and clays (in swamps). It would also remain to explain where silt in fact was deposited. c. Amounts of silt supplied were approximately similar to present conditions, but due to weathering a substantial decrease in silt and a nearly corresponding increase in clay took place. If the latter possibility would indeed hold or constitute the dominant factor, and if it is assumed that the decrease in silt during soil formation resulted in increase in clay content, then simple computation may learn that from 11 to 40% of the currently present clay in the Blue Nile clay plains has been derived from silt through weathering in situ. This process is essentially confirmed by our conclusions of section 6.2.2.2., which, in addition, reveal further details. Pedogenic changes in grain size composition other than the decomposition of mineral grains in the soil system, include the transfer of clay from one horizon to another, and the addition or removal of material by wind. Transfer of clay in soils is clearly indicated in profiles 22, 32, 42, levee soils of the Blue Nile, in profiles 67, 77 on the corresponding terrace II of the Nile, as well as in profiles 68, 78, 59 on older Nile terraces. Migration of clay, resulting in the occurrence of argillic horizons, is evidenced by the organization of soil materials, as observed in the field and discussed more closely in Chapter 7. It is further suggested by higher fine clay-coarse clay ratios. The effects of windaction on the soils is well-marked in the northern, more arid part of central Sudan. Evidence of this is recorded in the description of sample areas 4, 5, 6, and 7. Wind deposition primarily occurs around low scrub, soil profile 67 is considered an example of a truncated profile.

6.2.2, Elemental composition Data concerning the chemical composition show that the bulk of most soil materials consists of the combined oxides of silicon, aluminum, and iron, together constituting of from 70 to 90% of the fine earth. The contents of SiO2 range from 40 to 85%, of A12O3 from 5 to 20%, and of Fe203 from 2 to 12%. A few soil horizons (as in profiles 57, 77, and 78) contain 20 to 50% CaCO3 equivalent. Oxides generally occurring in lesser amounts are titanium, usually from 1 to 2%, 172 calcium usually below 8%, and magnesium and potassium not exceeding 3 and 2% respectively. The total content of six trace elements is completely within the usual ranges given by Mitchell (in Bear, 1965), though mostly on the low side of these ranges. The variation in the total content of trace elements, as measured on 59 samples, is as follows: Mn from 184 to 1450 ppm Sr from 99 to 775 ppm Ba from 90 to 770 ppm Zn from 10.0 to 139.0 ppm Ni from 12.0 to 92.5 ppm Co from 5.5 to 48.0 ppm

The concentration of Mn, Zn, and Co may be below the threshold concentrations of these elements in soils, as established at 400 ( ?), 30, and 5-7 ppm respectively by Kovalsky (1967, in Makeev, 1968). Below these values the functioning of the regulating system of plants is considered to be impaired. Kovalsky gives the following ranges of these elements for normal functioning of biospecies: Mn from 400 to 3000 ppm Zn from 30 to 70 ppm Co from 7 to 30 ppm A small but important fraction of the total of various elements occurs as adsorbed ions; this form of the elements Ca, Mg, K, and Na is discussed in Chapter 8.2.2.2.

6.2.2.1. Low terrace soils We shall first consider the concentration of chemical elements in soils of the recent Blue Nile and Nile terraces. This serves to demonstrate two points, namely that 1) a close relationship exists with rocks in the Ethiopian catchment area of the Blue Nile and with recent Nile soils in the U.A.R., and that 2) the chemical composition of these soil materials is largely texture-controlled. The soils at issue (profiles 21, 31, 41, 51, 61, 71, and 71A) are considered to be comparatively fresh sediments, mostly unchanged by soil formation, they are in fact still being accumulated by the river. The mean chemical composition of 19 samples of these soils appears from figure 44.

Fig. 44. Mean chemical composition of recent Blue Nile and Nile soil materials

SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2O MnO

55.98 13.03 8.30 1.54 4.37 1.77 1.18 0.13 173

These values may be compared with Mohr's analyses of Trap Series basalts of figure 16, which, in figure 45, are adjusted to give ferrous iron oxide percentage only.

Fig. 45. Mean chemical composition of Trap Series basalts after Mohr (1963), as recomputed from figure 16

SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2O MnO

47.04 15.19 12.93 2.41 9.42 6.35 1.52 0.22

In order to assess which part of the various elements of the Sudan soil materials is inherited from the Ethiopian basalts, and which part is derived from some other source, like Mesozoic sandstone and Basement Complex, we will assume that TiO2 is practically completely inherited from the basalts. Since potassium feldspars and quartz may have a non-basaltic derivation, the oxides of potassium, aluminum, and silicon cannot be used as index. The use of TiO2 as index is supported by the mineralogical data of recent Blue Nile and Nile sediments as presented in Chapter 3.1.1. The titaniferous augites of the sand fraction are of volcanic origin. This assumption leads to the conclusion that in the recent Blue Nile and Nile soil materials silicon, aluminum, potassium, and iron are derived, with increasing amounts, from basaltic sources, while manganese, calcium and magnesium, with increasing amounts, are available in excess from the basalts, an excess that may have been transported away by the river. The following is a tabular statement of the percentages for various oxide concentrations of basaltic and non-basaltic origin.

Fig. 46. Origin of major oxides in recent Blue Nile and Nile soil materials

Oxides % Basaltic % Non-basaltic % Loss from in recent origin origin basalts soil materials

SiO2 53.7 46.3

A12O3 74.5 25.5

K2O 82.3 17.7

Fe2O3 99.5 0.5 MnO 100.0 — 8.1 CaO 100.0 — 37.7 MgO 100.0 — 129.3 174

The above does not necessarily imply that MnO, CaO, and MgO are derived only from the basalts, but the supply of these oxides from the basalts is more than sufficient to explain the oxide concentrations in the recent soil materials. The non-basaltic alumina represents 25.5% X 13.03% = 3.32% of the recent soil materials (r.s.m.). We will assume that this alumina occurs in the non-basaltic potassium feldspars, because other feldspars here are of basaltic origin. The chemical composition of KAlSi3O8 is 16.9% K2O, 18.4% A12O3, and 64.7% SiO2. Then, 3.32% A12O3 corresponds to 3.05% K2O and 11.67% SiO2 on r.s.m. basis. This supply of K2O exceeds by far the amount of K2O of non-basaltic origin in the r.s.m., which is 17.7% X 1.18% = 0.21%. The excess 2.84% K2O is lost, presumably during river transport.

The potassium feldspar supplied SiO2 constitutes almost half of the non-basaltic silica in the r.s.m., which is 46.3% X 55.98% = 25.92%. The remaining 25.92% - 11.67% = 14.25% SiO2 may then be inherited from the Mesozoic quartzose sandstones. With the foregoing discussion, showing the predominant basaltic origin of the recent soil materials in the Sudan, the total content of the major elements is also explained, especially the relatively high concentrations of Fe2O3, TiO2, and MnO, characteristic of basic igneous rocks. Also the trace element composition reflects the influence of this basaltic derivation, especially since certain trace elements display very pronounced preferences for specific minerals. Thus, the elements Zn, Ni, and Co are known to occur in relatively high concentrations in basic igneous rocks. Vinogradov (1957) and Kovda et al. (1964) support this view. For Russian basalts Kovda et al. (1964), in their review on micro- elements of the soils in the Soviet Union, give an average content for Zn of 112 ppm (range 75 to 130 ppm), for Ni of 67 ppm (range 8 to 73 ppm), and for Co of 21.6 ppm (range 2.8 to 78 ppm). The maximum concentration of these elements in the soils of the Soviet Union, however, is approximately half of these values. The above data are similar to the proportion of Zn, Ni, and Co found by the author in the recent soil materials of central Sudan, which are for Zn on average 93 ppm (raiijge 30 to 139 ppm), for Ni 56 ppm (range 28.5 to 92.5 ppm), and for Co 33.3 ppm (range 16.5 to 48.0 ppm) (arithmetic means). Mitchel (in Bear, 1965) also reports a preference of Ni and Co (and Cr too) for the more basic rocks in the Skaergaard intrusion of E. Greenland, as well as a Ni/Co ratio above unity (in recent soil materials of the Sudan the Ni/Co ratio averages 1.7). Mitchel further reports that Zn occurs in many basic rocks at 100 to 200 ppm. Since the Trap Series basalts are poor or deficient in olivine, the major portion of the elements Zn, Ni, and Co is presumably located in the pyroxenes and further in the amphiboles. These ferromagnesian minerals are excellent host minerals 175 for trace elements: divalent Zn, Ni, and Co can equally well occupy the place of Mg or Fe cations in their lattice, since they have similar ionic radii of approximately 0.8 Â (the main factor in isomorphous substitution). For similar reasons of crystal structure the trace elements Ba and Sr can be accommodated in K- and certain Ca-minerals (Mitchel, in Bear, 1965). In the recent soil materials of the Sudan the mean concentration of Ba is 408 ppm and of Sr 339 ppm. Thus while Ba is most likely to occur in K-minerals such as the potassium feldspars and biotite, and their weathering products, supplied by the Blue Nile, Sr may, in addition, occur in plagioclase such as labradorite. In the U.A.R. the chemical composition of recent Nile soil materials has been determined by Langenbeck and Luchsinger (1959). They chemically analyzed two samples: 1. from the Aswan area, 2. from Delta Barrages, where the Nile widens to its delta. Hamdi and Fathi (1959, in Hassan, 1967) also analyzed the suspended matter of the Nile. Their data are presented in figures 47 and 48 for comparison with our results on the chemical composition of recent Blue Nile and Nile soil materials (fig. 44).

Fig. 47. Chemical analyses of recent Nile soils in the U.A.R. after Langenbeck and Luchsinger (1959)

Sample Fe2O3 TiO2 MgO MnO Zn Ni Co Cu Mo site in weight percentages in ppm

Aswan 6.29 2.17 2.49 0.17 280 <50 100 180 <100 Delta Barr. 9.58 1.50 0.66 0.16 200 <50 130 130 <200

Fig. 48. Chemical analysis of suspended matter of the Nile in the U.A.R., after Hamdi and Fathi (1959, in Hassan, 1967)

SiO2 A12O3 Fe2O3 CaO MgO K2O Na2O

44.94 14.81 13.99 3.98 1.60 1.77. 1.38

Results demonstrate that a close relationship exists with our findings, this especially with relation to the major elements. The trace element concentrations as given by Langenbeck and Luchsinger, however, deviate considerably from our data as given above, which is not surprising since the former were not determined by atomic absorption spectrometry. The above authors' results on Zn are two to three times as high, and while Ni values are somewhat less, Co is three to four times as high, entailing a Ni/Co ratio far below unity. 176

Study of the total content of chemical elements present in the recently deposited soils of the Blue Nile and Nile is further indispensable for separating the variations in chemical composition due to different parent materials from those due to soil- forming processes in the central Sudan alluvial soils. As in these recent soils the influence of pedogenic processes on the inorganic soil constituents is negligible (for our purposes), the influence of parent material on chemical composition may be separately determined. Applying the principle of actualism - which considers the presence as a key to the past - the original chemical composition of the older alluvial soils of the Blue Nile and Nile, constituting by far the most extensive area, can then be estimated. This presumes that the solid load of these rivers has not differed substantially in the past, and in view of the geology of Ethiopia there seems, as yet, to be no reason to assume otherwise. The control of texture over the chemical composition of the recent soil materials is apparent from the high degree of positive correlation found to exist between the weight percentages of individual elements (except strontium) and the weight percentages of various grain size fractions. The regression of the chemical elements on a number of other soil variables, such as pH, organic matter, CaCO3 percentage, has been checked but no significant relationships could be established. Although the relationship between texture and chemical composition is essentially a one-way and causal relation, the use of correlation coefficients is considered appropriate to determine which grain size fractions show the highest correlation. In figure 49 the correlation coefficients are presented between the concentration of various elements and those grain size fractions that show the maximum correlation. The number of pairs of observations is also included. The computations further serve to demonstrate the interrelationships of the elements. The regression equations suitable for estimating the concentration of major and minor elements (y) from the appropriate grain size fractions (x), as well as the standard error of estimate of y, are listed in figure 50. The correlations in question are visualized in figure 51.

Fig. 49. Correlation coefficients between concentration of elements and selected grain size fractions in recent Blue Nile and Nile soil materials

SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2O Ba Co Mn Ni Zn pairs of observations 21 24 24 24 19 19 24 19 19 19 19 19 Sand 0.95 Silt 0.95 0.95 0.94 0.79 Silt + clay 0.97 0.99 0.85 0.64 0.92 0.76 Fine silt + clay 0.84 177

ppm Ni 75-

* ^^ • 125 -| 50- ppm • • 25- s' 100- Zn n- 75- 0 50 100 FINE SILT + CLAY %

50-

25-

0 50 100 SILT + CLAY % 1500-1 ppm

1200- 50 100 Mn SAND % 900-1 900- Ppm

600- 600 -

300- 300-

0 50 100 0 50 100 SILT+ CLAY % SILT % 4-,

°k TiO2 2- K20 2-

0 0 50 100 50 100 SILT + CLAY % SILT %

16 - „>^> 2 - A i2o3

8 - ^Fe2O3

4 -

0 - 50 100 100 SI LT -I- C L A Y % Fig. 51. Scatter diagrams and regression lines for elemmtal concentration and grain size percentage in recent Blue Nile and Nile soils 178

Fig. 50. Regression equations to calculate elemental composition (y) with from the particular grain size fraction (x) as given in figure 49. Regressions apply to recent Blue Nile and Nile soil materials

Element (y) Regression equation Standard error of y

SiO2 y = 0.406 X + 48.43 2.86%

A12O3 y = 0.152 x + 1.97 1.07%

Fe2O3 y = 0.102 x + 0.87 0.48%

TiO2 y = 0.021 x + 0.76 0.17% CaO y = 0.088 x + 0.94 0.66% MgO y = 0.028 x + 0.67 0.26%

K2O y = 0.008 x + 0.61 0.14% Ba y = 3.584 x + 269.41 66.97 ppm Co y = 0.217 x + 16.55 6.98 ppm Mn y = 9.428 x + 277.90 107.27 ppm Ni y = 0.724 x + 14.67 10.69 ppm Zn v = 0.836 x + 27.83 19.13 ppm

It thus appears that the sand content primarily determines the silicon concentration, that the concentrations of titanium, calcium, magnesium, and barium depend on the amount of silt, that the concentrations of aluminum, iron, potassium, cobalt, manganese, and zinc are closely related to the silt -)- clay content, and, finally, that nickel is determined by the percentage of fine silt + 'clay. A high degree of correlation with the clay fraction alone was not found for any element. A similar relationship could not be established between a grain size component and the concentration of strontium (ranging but little of from 276 to 456 ppm in recent soil materials). The vertical distribution of Sr in the soils (profiles 21, 41, 51, and 71A), however, repeats that of Ba, but while the amount of Sr is less than Ba in soils 21 and 41, it approximately equals Ba in soil 51, and exceeds Ba in soil 71 A.

Research on Nile alluvial soils of the delta area, Fayum Province, and Kom Ombo, in the U.A.R. by Abdel Ghani (1964) showed that there the total iron content is also texture-controlled. Abdel Ghani reports that total iron in these soils does not exceed 7.70% (as compared to 11.30% in central Sudan, profile 21), and that a high degree of correlation exists between total iron and the clay + fine silt content (fraction less than 20 /.i) in soils with a CaCO3 content of less than 7%. Abdel Ghani computed a correlation coefficient of 0.942 and a regression equation y = 0.100 x — 0.084 (wherein y is the total iron concentration and x the weight percentage clay -f- fine silt). Although there is undoubtedly a high degree of correlation, as appears from the 179 results of chemical and grain size analyses also given by Abdel Ghani, his computations are not correct. The negative value of the interception on the y axis in his regression equation is unrealistic, since it would imply that certain soils can have a negative concentration of total iron. When based on 43 pairs of observations (as given in his scatter diagram) - and this includes results on 5 soil samples with over 7% CaCO3 - this regression equation should be y = 0.075 x -f- 0.97, and the correlation coefficent 0.89. In central Sudan the correlation of total iron with fine silt -(- clay is distinctly less significant than with silt + clay (corr. coefficient of 0.80 versus 0.99). This grain size component was not taken into account in Abdel Ghani's work. His findings, however, provide evidence that the direct control of texture on the chemical composition of recent central Sudan soils may also be valid (and to the same extent ?) in Nile alluvial soils of the U.A.R. Relationships between major or trace element content and various grain size components as well as interrelationships of the elements proper, have been reported by other workers on soils of several countries. The following is an enumeration of more recent publications. McKenzie and Taylor (1968) and Taylor 1968) found that in widely different soil types most of the total amount of cobalt is concentrated in mineralized manganese, such as birnessite, lithiophorite, and hollandite. Horowitz and Dantas(1968) report a correlation between total Co and SiO2, A12O3, Fe2O3 content, and also with clay content in certain soils of Brasil. Misra and Kishore (1967) similarly found Co to be correlated with Fe content and with clay content in soils of Uttar Pradesh, India. Bleeker and Austin (1970) showed the association of Cu, Zn, and Ni with clay in Papua-New Guinea soils. More generally, Nalovic and Pinta (1969) mention a very significant correlation between a wide variety of trace elements and the fine fraction of soils of Madagascar. Statistical analysis by Lupinovich and Dubikovskii (1966) established an approximately linear relationship between total content of the elements Ti, Mn, V, Cr, Ni, Cu, and Co and clay percentage in derno-podzolic soils of White Russia. Greinert (1968), finally, reports that the total Co concentration was significantly correlated with the silt -f- clay fraction of soils of western Pommerania (Poland), a similar association for Co as found by the author for recent alluvial soils of central Sudan.

6.2.2.2. Soils of higher terraces Geogenetic factors - as demonstrated - essentially condition the chemical composition of recent soils of the Blue Nile and Nile. Pedogenic influences in addition 180 determine the composition of the older alluvial soils under consideration here. The separate effects of geogenesis and pedogenesis, however, cannot be exactly ascertained. Without the knowledge of either texture or elemental composition of these older soils at the time of their deposition, and more especially without knowing the changes in weight and volume that may have occurred during soil formation, the means are inadequate to precisely explain the chemical composition of the older alluvial soils. But on the basis of a number of assumptions an approximate assessment of the actual pedogenic influence can be made such, that the relative changes in elemental composition become apparent. We will assume that 1) at the time of sedimentation these older Nile soils were characterized by the same correlation between texture and elemental composition as recently deposited soils, and 2) no breakdown of soil particles has occurred, thus leaving the sand, silt, and clay content in tact. The present chemical composition may then be calculated from the various grain size fractions using the regression equations of figure 50. Examination of the residuals (the observed minus calculated values of the element concentrations) as computed for each of the older alluvial soils, reveals the following pattern of deviation. Positive residuals - suggesting relative enrichment of the element concerned - are generally most frequently encountered among the elements, Al, Ba, K, Mn, and Ca Negative residuals - indicating loss of the elements - are most frequently found for Zn, Si, Ni, Ti, Fe, Co, and Mg. The probability of a residual value of up to one standard deviation in recent soil materials is normally 67%. In figure 52 the percentages of residual values of up to one standard deviation in older soil materials are tabulated. The data give a general indication on the influence of pedogenesis, which thus tends to be lowest on the Ni-concentration, and at is maximum relative to Ca-content.

Fig. 52. Percentages of residuals deviating by less than one standard deviation from the computed element concentrations in older alluvial soils

Element Ni Fe2O3 Co TiO2 Zn SiO2 K2O MgO A12O3 Mn Ba CaO

Number of observations 40 45 40 45 40 45 45 40 45 40 40 40 Perc. residuals <1 stand, error 62 56 55 53 50 45 42 30 29 27 22 22

Whether the pedogenic influence commonly entails an absolute loss or gain of a particular element, may appear from figure 53 which lists the number of observed 181 values that differ by more than one standard error from the calculated values. Apart from the numbers of both positive and negative residuals the means of these residuals are given too for comparison.

Fig. 53. Number and mean value of residuals deviating by more than one standard deviation from the calculated element concentrations in older alluvial soils

Element Positive residuals Negative residuals /\number number mean number mean residuals

SiO2 2 4 % 23 6.8% —21 Zn 0 0 ppm 20 36 ppm —20 Co 3 14 ppm 15 14 ppm —12 Ni 4 17 ppm 11 19 ppm — 7 TiO2 9 0.3% 12 0.3% — 3 Mn 13 375 ppm 16 295 ppm — 3 Fe2O3 9 1.1% 11 1.2% — 2 MgO 15 0.7% 13 0.8% + 2 CaO 19 10.3% 13 0.8% + 6 Ba 20 210 ppm 11 140 ppm + 9 K2O 23 0.6% 3 0.5% +20

A12O3 28 2.8% 4 2.1% +24

It appears that because of soil genesis the observed concentrations of Si, Zn, Co, and Ni tend to be less than the computed values as influenced by geogenesis only, while the observed content of Al, K, Ba, and Ca tends to be more. This confirms the general trends noted on all positive and negative residuals mentioned above. Explanation of the changes in chemical composition due to pedogenesis inevitably involves reassessment of our second assumption. Thus, we will now assume that textural changes have taken place, but that no other soil materials have been added during pedogenesis. The changes in the elemental concentrations may then be interpreted as being due to the alterations in grain size composition during soil formation, qualitatively defined as follows: a. a relatively high decrease in silt percentage b. a relatively high increase in clay percentage; c. a relatively low decrease in silt + clay percentage; d. a relatively low increase in sand percentage. This interpretation may be deduced as follows. The observed concentrations of silt -f- clay related Al and K higher than calculated are considered due to a decrease in silt + clay percentage (supply from external sources is excluded), presumably due to loss of elements other than potassium and aluminum. 182

The observed concentrations of Zn, Co, Mn, and Fe, lower, decreasingly, than computed on the basis of clay -j- silt percentages, then, necessarily are due to removal of these elements from the soil. A silt -f- clay increase would contradict our aforesaid statement. The loss of these elements may conceivably be the result of weathering of ferromagnesian silt and clay size minerals. The increase in the sand percentages, concomittant to a decrease of silt -j- clay percentages, may explain the observed Si values lower than calculated. In addition, the observed concentrations of silt related Ba, Ca, Mg higher than calculated, are held to be due to a decrease in silt percentage. Weathering of mostly non-barium, calcium, magnesium minerals of the silt fraction to clay-size particles may account for this. With relation to extremely high Ca-concentrations, however, one needs allow for external contributions of calcium in addition, in the form of CaCO3. Silt-related titanium observed in lower concentrations than computed is considered to be occasioned by loss of this element from the silt fraction, probably from titaniferous augites, thus somewhat lowering the silt percentage. This does not preclude some weathering of sand primarily to clay particles.

Detailed and complete analyses on the chemical composition of alluvial soils of central Sudan were not previously available. A number of data presented by other authors, most of which is confined to the older alluvial soils of the Gezira, confirms our results. Already in 1924 Joseph investigated Gezira soil chemically, his results include data on oxide concentrations per grain size fraction (also in Jewitt, 1955). These data show that A12O3, Fe2O3, and K2O primarily occur in the silt -f- clay fraction, and that TiO2 and MgO have highest concentrations in the silt fraction. Although the CaO content is highest in particles larger than 2 mm, which mainly consist of CaCO3, a secondary CaO maximum is associated with the silt fraction too. These trends clearly parallel our findings on the association of these oxides with grain size components. Data presented by Scharpenseel et al. (1960) and by Blokhuis et al. (1964) approximately complement each other, and are of the same order of magnitude as our results (see profiles 23 and 43), when taking into consideration a somewhat different texture of the soils at issue. Finck (1962) computed the mean total potassium value of Gezira soil to be 1.20%, similar to what was measured in our profiles 23 and 43. Magar and Babiker (1965) reported very high total amounts of iron of 15,000 to 29,600 ppm Fe (or 2.1 to 4.1% Fe2O3) in the Gezira soil, as well as Mn values as high as 1450 ppm (0.19% MnO). This Mn concentration slightly exceeds our maximum Gezira values of 1230 ppm in profile 42, but is equal to one of the Mn 183 concentrations measured in profile 77 on a Nile terrace. The amounts of iron found by the author and others may be two or three times as much as 4.1%.

6.2.3. Carbonates and gypsum Of the nonsilicate material commonly occurring in central Sudan alluvial soils carbonates are by far the most important. From comparison of the CaCO3 equivalent percentages with CaO and MgO concentrations of the same soil materials, especially in horizons with carbonate accumulation as in soils 77 and 78, it may be concluded that the CaCO3 values indeed represent mostly, if not wholly, CaCO3 and not MgCO3. This is substan- tiated by elemental analyses of carbonate concentrations of Sudan soils by Kerpen et al. (1960) and Blokhuis et al. (1968) as well as by the fact that river water of the Blue Nile and Nile is saturated with CaCO3 (see Chapter 3). Pedogenic CaCO3 usually consists of calcite. Most soils were found to be calcareous or strongly calcareous. Noncalcareous soils occur in areas with average annual rainfall exceeding 750 mm (except the recently deposited soil 21) as well as on the oldest Nile terrace IV (soil 59). With relation to the recent soil materials a correlation between total CaO and silt percentage could be established (see foregoing section). A high degree of positive correlation (r = 0.92) also exists between CaCO3 and silt percentage. This serves to demonstrate that most of the CaCO3 - which may constitute up to 5% - is presently supplied in silt-size particles. The regression equation suitable for estimating the calcium carbonate concentration (y) from the silt percentage (x) is: y = 0.066 x - 0.56 with a standard error for y of 0.60%. In the scatter diagram of figure 40 this relation is visualized. The equation is based on 28 pairs of observations of soils 21, 31, 41, 51, 61, and 71A. Soil profile 71 data have been omitted as - because of its depressional setting -

100 SILT % Fig. 54. Scatter diagram and regression line for CaCO3 concentration and silt percentage of recent Blue Nile and Nile soils 184

more CaCO3 has accumulated here from stagnant water than normally occurs (here shell fragments too). In these recent soils carbonates are generally not discernible in the field. Soil profile 31 shows occasional dendrites, and 71A a few small soft white powdery streaks, indicating a slight carbonate redistribution. This fact may account for apparent deviations in the CaCO3 content, which entail a regression line with a negative y-axis intercept of 0.56%, which would theoretically result in negative CaCO3 values. The standard error of y, however, indicates that the y axis intercept may be positive, as it should be. The circumstance that calcite is deposited in silt-size particles along the Blue Nile and Nile throughout central Sudan, may be attributed to the fact that the water of these rivers is saturated with calcium carbonate, thus preventing solution of the carbonate particles in their solid load. In the older alluvial soils CaCO3 percentages have a much wider range and may be as high as 51% (profile 77). Here carbonates are visible and have been observed in the field to occur in at least four major authigenic forms: 1. very few to few shell fragments; observed mainly in the top 50 cm of soils of areas 3 and 4, not in Nile terraces; 2. very few to frequent, small, sometimes large, hard, black or dark gray, discrete nodules; observed to decrease generally in number from the soil surface down to some 1 m depth in soils of areas 3 and 4 (Er Roseires terrace) and Nile terrace II; 3. very few to frequent, small and large, hard, sometimes soft powdery on the outside or with etched surfaces, white, yellow, or brownish, irregular concretions; observed primarily in the subsoil to 1 or 2 m depth of most soils, often showing an increase in number and size with depth; and 4. very few to dominant specks, dendrites, streaks, aggregates, and (often indurated and laminated) masses engulfing soil particles; observed in various horizons. For a systematic and thorough discussion of the morphology and distribution of these carbonates one is referred to a study by Blokhuis et al. (1968), based on thin section analysis as well as stereomicroscopic and field observations. The CaCO3 concentrations in the older Blue Nile and Nile soils are often much higher than in the recent soils, owing to various processes. Redistribution of carbonates, accumulation or complete removal, has played a role in alle soils, furthermore soils of the Blue Nile backswamp areas may have originally received more CaCO3 from evaporating flood water, while a lateral supply of CaCO3 may have occurred especially on the Nile terraces. Gypsum is less common in central Sudan soils than carbonates, if it occurs at all it may range from a trace to 2%. Crystalline gypsum - CaSO4,2H2O - often 185 needle-shaped, occurs in various horizons of widely different soils (such as 31, 71 A, and 67, 78).

6.2.4. Organic matter The organic matter content of central Sudan alluvial soils is low: organic carbon ranges from 0.1 to 1.4%, and total nitrogen varies between 0.01 and 0.12%. Similar low amounts of organic matter are encountered in most arid soils of the world. Climatic conditions entail a sparse plant cover supplying only little organic residue for humification. In addition, the succession of wet and dry seasons may cause a 'partial sterilization effect' - the inadequate moisture supply severely limits microbiological activity in the dry period, an extremely intensive flush of the activity of micro-organisms occurs in the rainy season, when a rapid cycle of humification and mineralization is accomplished (Harmsen, 1959; Kononova, 1961; Finck, 1963). A relatively high organic carbon and nitrogen content is recorded in the recent soils of the low terrace of the Blue Nile and Nile. Here, carbon percentages either decrease irregularly with depth or remain above levels of 0.2% down to 1.25 m from the surface. The average organic matter content of these soils distinctly decreases from south to north (increasing aridity), as may be illustrated on the average content in the top 50 cm of these soils in sample areas 2 to 7 included. The organic carbon content decreases as follows: 1.2, 0.8, 0.7, 0.6, 0.6, 0.4, 0.3%. The nitrogen content respectively is: 0.08, 0.07, 0.06, 0.04, 0.05, 0.02, 0.02%. The carbon-nitrogen ratios of these soils remain relatively constant, they are 13 on average, and usually between 11 and 16. In the older alluvial soils carbon percentages are generally lower, while the nitrogen content does not exceed 0.05%. The carbon-nitrogen ratio of these soils shows wide variations. In the Blue Nile plains (Er Roseires terrace), for example, the carbon-nitrogen ratios of the light-textured levee soils are much like those of the low terrace, averaging 12, with a range of 11 to 16 (soil profiles 22, 32, 42), whereas in the cracking-clay soils of former backswamp areas this ratio averages 19, with a range of 16 to 22 (soil profiles 23, 33, 34, 43, 44). It is noted that Scharpenseel et al. (1960) arrive at somewhat lower carbon-nitrogen ratios for these clay soils. The state of organic matter in the above soils was attained through appreciable loss of carbon and nitrogen during soil formation. This appears when carbon and nitrogen percentages of fresh Nile sediments (Chapter 3.1.1.) are compared. These are approximately similar to or slightly above the maximum values recorded in the alluvial soils, and have carbon-nitrogen ratios of 15 or 16. It is also confirmed by the nitrogen status of Nile valley alluvial soils near Cairo. Jenny (1962) observed here a preponderance of nitrogen depletion from the originally nitrogen-rich Nile sediment. 186

Fig. 55. Mineral composition of sands (500 to 50 ji) of Blue Nile and Nile soils

Light mineral composition Soil Depth, Weight Minerals in mutual percentages profile cm percentage Nr. of total fraction CO orit e 1 :rin g •5 -b6 O BH £ < J N m £ a P 21 0- 25 98.4 17 31 12 1 15 1 t 2 2 25- 43 98.5 same as above 43- 70 98.2 same as above

41 0- 10 95.8 3 61 13 1 10 1 l 5 10- 55 96.7 5 49 15 1 13 1 1 3 i: 55-105 96.5 8 39 14 2 14 1 3 5 105-175 97.2 9 36 18 3 13 2 1 3 1:

61 0- 15 97.6 4 53 15 2 14 2 1 3 15- 65 97.2 2 48 21 2 10 4 1 3 65-115 96.3 same as above 115-165 96.6 same as above

22 0- 18 97.7 t 83 11 3 1 t t 2 — 18- 38 98.2 same as above 38- 60 98.8 same as above 60- 90 98.8 same as above 90-120 97.7 same as above 120-150 97.4 same as above

42 0- 10 96.6 — 87 9 2 2 t t 10- 28 96.3 same as above 28- 55 96.3 same as above 55- 83 96.1 same as above 83- 99 95.5 same as above

67 0- 16 99.3 — 87 11 1 1 t t 16- 46 99.2 same as above 46- 86 99.2 same as above 86-124 99.5 same as above

68 0- 16 98.6 — 95 4 t 1 — t t 16- 30 98.6 same as above 30- 65 98.8 same as above 65-100 99.0 same as above

59 0- 2 94.8 — 93 3 2 1 t 1 2- 35 93.9 "same as above 35- 70 94.1 same as above ON 00 so ^ H ^ to U> — tO tO Zircon

Tourmaline

to W U> Garnet

- Ji 1O W Rutile to to -» i-> Titanite

Staurolite

Kyanite

Sillimanite

U> "^ U) UJ to O O* O ^1 o Epidote u>4*-tsjs/i ^^ \O » Ui si Zoisite

O so 00 O Hornblende

Augite

>-» u» >-* ui O> 4k OOvOO Titraùgite'

Hypersthene

Biotite

» O " U O U Ui M to to to 4k to Saussurite

to LO 4k Ui Basait fragments 188

It is noted that evidence of illuviation of humus was observed in the alkali soil 44. In soils subjected to alkalization, humus, in the form of sodium humâtes, reaches a high degree of solubility and may be transported into deep soil horizons during the rainy season (see also Chapter 8.2.2.2.).

6.2.5. Specific constituents 6.2.5.1. Sand fraction - mineralogy Mineralogical research of the sands (500 to 50 /u) of alluvial soils of central Sudan primarily serves as an aid in characterization of a. origin of the soil materials; b. uniformity of soil materials; c. degree of weathering of the soils; d. classification of the soils. These various aspects will be dealt with on the basis of figure 55, showing the mineral species identified as well as their relative proportions (cf. also Worrall, 1957). Data presented here cover soils of the low terrace of the Blue Nile and Nile (profiles 21, 41, and 61), of terrace II or Er Roseires terrace (profiles 22, 42, and 67), as well as of terraces III and IV of the Nile (profiles 68 and 59 respectively). The occurrence of traces glauconite, chlorite, and phytoliths in low terrace soils only is not tabulated. sub a. The qualitative similarity of the light and heavy mineral suites of profiles 21, 41, and 61, supports our computations of section 6.2.2.1., which indicate a number of Ethiopian rocks (Trap basalts, Basement Complex, Mesozoic sandstone) to be the common source of these soils. The nature of the mineral composition is further similar to the mineral assemblage of sands currently transported by the Blue Nile and Nile (see figure 23). Minor deviations, viz. calcite, may be attributed to different pretreatment of the samples. For a discussion of the derivation of the various minerals observed, one is, therefore, referred to Chapter 3.1.1. The quantitative differences in mineral ratio of the above soils are considered to be due to different conditions of deposition of the soil materials, entailing differences in grain size distribution of the sand fractions. Thus, sands with a dominant fine component have less quartz and less minerals of the epidote group, but more volcanic glass and basalt fragments (both in the light and heavy fractions) than predominantly coarse sands. Soils of the older terraces increasingly contain less identifiable mineral species. Soils of terrace II do not contain basalt fragments, soil profile 68 of terrace III lacks volcanic glass, zeolite, sillimanite, and hypersthene, and finally, soil pofile 59 of terrace IV has no titaniferous augite. Their heavy mineral fraction, however, 189 remains characterized by a hornblende-augite-epidote association, indicating a similar Ethiopian derivation as the low terrace soils. In this light soil profile 59 has an aberrant heavy mineral composition, with zircon making up some 50% of this fraction. A wide range of zircon varieties could be recognized in this soil, ranging from clean, fresh grains to strongly weathered malacon, the metamict alteration product. This high proportion of zircon is not in the first place attributed to a relative increase of this mineral due to weathering of less resistant minerals (quartz content is not extremely high either), but to an absolute increase as a result of zircon supply from disintegrating quartzose sandstone gravel. This well-rounded gravel constitutes a large portion of the soil at issue. Microscopic examination of crushed gravel indeed revealed zircon to be an accessory mineral in it. The gravel is generally present in soils of the Nile terraces III and IV. It may be of Ethiopian origin, supplied by the Blue Nile from the Mesozoic sandstones. It may also be of relatively local origin, transported by the Nile over a short distance only and of Nubian sandstone derivation. In an effort to assess the possible contribution of allochthonous windblown material to central Sudan alluvial soils, mineral analyses were carried out on dune sand of the Qoz deposits of west Sudan (see Chapter 2.5). Dr. A. Warren, University College London, who carried out géomorphologie research in this region (1966), kindly provided a sample of Qoz sand, collected by him in a dune area at 13°42' N, 31°48' E. Our investigations showed that the light fraction constituted 99% by weight of the 500 to 50 fj, fraction, and almost completely consisted of quartz grains with traces of orthoclase. Analysis of the heavy fraction revealed the occurrence of a limited number of minerals only (fig. 56).

Fig. 56. Heavy mineral composition of Qoz sand (500 to 50fi)

Transparent minerals

g- § S s K U 3 -4-t a. .b o 3 O NI H « 78 28 28 19 16

Determination of the influence, if any, of this highly weathered material on the alluvial soils would require further characterization of the quartz, either on the 190 basis of its surface characteristics, kind and amount of inclusions, or trace element content. This is beyond the scope of the present study. sub b. As stated in section 6.2.1. different environments of deposition resulted in soils with different grain size distribution. Compared to these differences, the separate horizons within each one of the soil profiles show minor variations in texture only. This is essentially confirmed by analysis of variations in mineralogy of the sand fraction with depth in the profiles. There is a high degree of similarity of the light mineral composition, suggesting relatively uniform parent material. The differences occurring in the heavy mineral fraction are considered to be chance variations - due to differences in grain size and counting method (Doeglas, 1940). The concentrations of the stable minerals zircon and tourmaline (and the zircon- tourmaline ratio) are too small to be used as criteria for establishing uniformity. The total percentages of heavy minerals of the sand fraction do not differ significantly with depth in each of the profiles. Although generally no sediment will be entirely homogenous, our observations indicate that soils being discussed here have throughout their depth formed from relatively uniform material. sub c. The mineral composition of the sands also reflects the degree of weathering of the soils. This is best revealed when we compare the average mineral make-up of all soils on each of the four landscapes (terraces I through IV), using sand mineralogy of soils 21, 41, and 61 as a basis. Heavy mineral analyses then give the following trends. Soils of terrace II primarily have lost titaniferous augite from the sand fraction, as well as augites and basalt fragments to a less extent. Soils of terrace III in the first place have been subject to loss of hornblende. The amount of pyroxenes has decreased too, but they occur in small amounts only. Basalt fragments are not available for weathering anymore. Soils of terrace IV not only show distinct loss of hornblende, but also of epidote. The pyroxenes, zoisite, and saussurite (alteration product of labradorite) present in small amounts, also have decreased. This sequence, reflecting the relative stability of the various minerals to weathering under Sudan conditions, generally is in accordance with Pettijohn's (1941) table of the relative persistence of mineral species. This table shows the increasing degree of stability from augite to hornblende to epidote. It provides evidence for the increasing degree of weathering (of the sand fraction) of the soils from terrace I to IV. This trend is matched by the average content of opaque minerals. These grains 191 constitute on average about 10% of the heavy fraction in the low terrace soils, their content increases to an average 28% in terrace II, 39% in terrace III, and 57% in terrace IV soils. Under the prevailing conditions this may reflect a relative increase of the opaque minerals due to weathering of less resistant minerals with time. The light mineral analyses parallel the above findings. They too show weathering of increasingly more resistant minerals with terrace soils becoming older. Soils of terrace II primarily show loss of basalt fragments, and further of volcanic glass, potassium feldspars, and labradorite. The relative high susceptibility to weathering of the volcanic glass is in accordance with its basaltic origin. Basic glass contains less SiO2 and more alkaline earths and iron than glasses of rhyolitic origin (Mohr and Van Baren, 1959). Soils of terrace III have been mainly subject to loss of potassium feldspars, the only mineral available - other than quartz - that could be weathered. Soils of terrace IV, finally, show no great changes anymore, with all weatherable minerals depleted. Absolute weathering may be rated according to specific mineral ratios. Ruhe (1956) proposed ratios between the amounts of stable minerals and minerals considered to weather at moderate rates, for both the heavy and light fractions. In the light of data available, however, computation of these ratios here would make their interpretation too speculative. Our data are in accordance with those found by Nicholls (1939) on certain soils associated with basalt in the state of Victoria, Australia (quoted by Milner, 1962, table 14). Also his investigations showed how minerals such as augite, olivine, plagioclase, and rock fragments were removed by weathering. sub d. The taxonomie significance of soil mineral species was recognized early in soil research carried out in Indonesia (Mohr and Van Baren, 1959). In U.S. Dept. of Agriculture soil classifications the mineralogy of the soil has chiefly been applied since the development in 1960 of the Comprehensive Soil Classification System (McCracken, 1968). The approximate mineral composition of selected size fractions presently is used as one of the criteria to distinguish among soil families. Sand of 0.02 to 2 mm is the determinant size fraction in the definition of both siliceous and mixed classes as applied to sandy, silty, and loamy soils. The use of these two classes in our classification of representative soils of central Sudan (Chapter 9) is largely based on mineral data presented in figure 55.

6.2.5.2. Clay fraction - mineralogy and elemental composition Essential differences in mineral and elemental composition of the clay fraction of the alluvial soils of central Sudan could be revealed in our studies, yet these 192 variations are relatively small. Generally the clays are characterized by predominant or large amounts of minerals of the montmorillonite group, with moderate or small amounts of kaolinite, traces of quartz, and traces of mica or none at all. High iron concentrations (up to 15%) are a marked feature of the chemical composition, which otherwise reflects the dominant montmorillonitic character of the clays. The clay mineral composition on the whole is similar to that of the solid sediment load carried by the Blue Nile and Nile (Chapter 3.1.1), thus demonstrat- ing that the minerals are in large part inherited from earlier sediments deposited by these rivers, rather than formed in situ. This especially holds for the Vertisols (Chapter 9.1), cracking clay soils of the Er Roseires terrace, whose montmorillonitic clay fraction might be conceived to have formed locally under prevailing poor drainage conditions, as actually happened in many other parts of the world (Dudal, 1965). The above generalizations require further specification. Differences in clays of various soils are best revealed in a discussion of the soils per landscape unit. This detailed account is essentially an integration of data obtained from elemental analysis, X-ray diffraction and differential thermal analysis. X-ray diffraction patterns of the clay fraction of soils of the recent Blue Nile and Nile terraces consistently show the presence of predominant amounts of montmorillonitic minerals, with moderate amounts of kaolinite, traces of quartz, and often traces of mica. This composition was confirmed by Mg- and glycol-saturated specimens of these clays, as well as by heating to 600° C of K-saturated samples. A set of representative diffraction patterns is shown in figure 57. A characteristic feature of the 14 Â minerals present was revealed by the application of chromium radiation (Cr-Ka, 2.28 Â) which, by giving a greater spread to the diffraction pattern, rendered two distinct basal reflections at 14.2 and 15.8 Â, instead of one diffuse peak obtained with cobalt radiation. This diffraction effect may be explained by a mixture of two montmorillonite 'hydrates' with thicknesses of 14.2 and 15.8 Â, present at the atmospheric watervapor pressure under which the samples were prepared. The same clays analyzed under vacuum conditons showed a 14 Â reflection only, indicating loss of interlamellar water (with subsequent rehydration again two peaks appear). A similar diffraction effect was reported by Mering (1946), who demonstrated that Ca-montmorillonite on hydration gives first a 14 Â reflection, corresponding to octahedral groups of composition Ca(H2O)6. At relative humidities of about 50% both a 14 and 15 Â spacing were obtained, and in the later stages of hydration (relative humidity 90%) a single 15 Â reflection, corresponding to two interlamellar water layers. Whereas with Na-montmorillonite no stationary regions were obtained. This strongly suggests that interlamellar cations are a determining factor in the arrangement of watermolecules in some montmorillonites. 193

Fig. 57. X-ray diffraction patterns representativi of Blue Nile and Nile low terrace soils. A: untreated clay B: K-saturated clay, heated to 600° C C: Mg- and glycol-saturated clay 194

Clays of the low terrace soils presumably also contain Ca-montmorillonite, since calcium dominates the exchange complex of these clays (Chapter 8.2.2.2). The diffraction patterns of random orientation samples further show an 060 reflection at 1.503 Â which, according to Warshaw and Roy (1961), is indicative of the dioctahedral group of montmorillonitic minerals. This spacing further den- otes that the specific species (octahedral composition) involved may well be an intermediate montmorillonite-nontronite mineral (according to Mac Ewan, 1961, the 060 reflections of each mineral are at 1.492 and 1.522 Â respectively). X-ray diffraction analyses of the fine clays of these soils, constituting from 80 to 85% by weight of the bulk clay, reveal the presence of predominant montmorillonitic minerals and small amounts of kaolinite. Both quartz and mica are absent in this fraction, according to Jackson (1968) a characteristic feature of moderately weathered soil materials. In this case weathering obviously took place in Ethiopia. In accordance with the very uniform X-ray diffraction patterns of the bulk c lays, their chemical composition varies only slightly. These minor variations may be largely due to experimental errors. Suffice to present therefore the mean chemical composition of 17 samples of recent soil clays, the range in concentrations observ ed, as well as the molar silica - sesquioxide ratios (figure 58).

Fig. 58. Chemical composition of the clay fraction of Blue Nile and Nile alluvial soils

Landscape Low terrace soils Terrace II soils (incl. Gezira) Concentration Mean Minimum Maximum Mean Minimum Maximum

SiO, 42.4 40.4 45.1 42.3 40.0 45.5

A12O3 20.0 18.6 22.4 17.0 15.0 18.4 Fe2O3 12.1 9.7 14.8 11.5 10.0 13.4

TiO2 1.6 1.4 1.8 1.4 1.2 1.6 CaO 0.3 0.2 0.4 0.3 0.2 0.4 MgO 1.9 1.4 2.3 1.9 1.1 2.6

K2O 0.8 0.6 1.0 0.8 0.5 1.3 Loss on ignition 19.7 18.8 20.7 20.6 19.8 22.4

SiO2/R2O3 2.7 2.5 2.8 3.0 2.7 3.2

SiO2/Al2O3 3.7 3.5 3.9 4.2 4.0 4.7

SiO2/Fe2O3 9.4 7.8 10.8 9.7 8.9 11.1

Al2O3/Fe2O3 2.5 2.3 2.8 2.3 2.2 2.5

Data on the chemical composition of the clay fraction of terrace II soils, based on 28 observations of soils 22, 23, 42, 43, and 67, are also included in figure 58. The concentration of the main constituents is in very good agreement with data given by Blokhuis et al. (1964) on the clay fraction of a soil near Wad Medani 195

(Gezira clay plain). The chemical data further support our observations with X-ray diffraction analysis. The diffraction patterns of the clay fraction of terrace II soils indicate that these clays contain a higher percentage of montmorillonite and less kaolinite than recently deposited clays. Montmorillonite minerals are predominant and kaolinite often occurs in small amounts. Quartz remains present in trace amounts, while traces of mica occur in a few instances only. The montmorillonite minerals appear to be closely similar to those of the recent terrace soils, and are characterized by a high iron content. There exists a discrepancy between the above findings and the observations of Greene-Kelly (1953). The latter devised a method for distinghuishing montmorillonite from beidellite and nontronite by lithium saturation and heating of clay samples, and applied the test to 'black soils' of Sudan, presumably Gezira clays. Greene-Kelly found that the clays contained predominant amounts of montmorillonoids, with the specific species identified to belong to the intermediate montmorillonite-beidellite group, i.e. characterized by a low iron content. The fine clays of terrace II soils, constituting from 80 to over 90% by weight of the total clay fraction, mainly consist of montmorillonite with small amounts of kaolinite. The chemical data of figure 58 show that the clays of terrace I and II soils have about equal SiO2 concentrations, and mainly differ in their A12O3, Fe2O3, and TiO2 content. The latter are lower in terrace II clays, entailing higher SiO2/ R2O3, SiO2/Al2O3, and SiO2/Fe2O3 ratios. These clays also have a somewhat higher loss on ignition. This is in accordance with the higer proportion of montmorillonitic minerals in clays of terrace II soils. Theoretically the formation of terrace II from terrace I clays may have occurred in either one of two ways: a. removal from the soil of clay components with a SiO2/Al2O3 ratio of less than 3.7, involving destruction of clay minerals, or b. addition to the clay fraction of components with a SiO2/Al2O3 ratio of more than 4.2., presumably involving new formation of clay minerals through weathering. Possibility a. is considered unlikely because our determinations on the chemical composition of the total soil (Chapter 6.2.2.2.) indicate aluminum to be the element least subject to removal from the soil. Under prevailing alkaline conditions of pH 7 to 9.5 aluminum is further considered immobile, while silicon is mobile to some extent (Keller, 1957). r Possibility b. cannot be the result of silica supply alone (SiO2/Al2O3 ratio \-) since it would not explain the increase in the proportion of montmorillonite as observed with X-ray diffraction analysis. The addition of some X-amorphous silica remains possible, the quartz content of terrace II soil clays does not show any apparent increase. The new formation of montmorillonitic clay minerals with a high SiO2/Al2O3 ratio from weathering products of presumably plagioclase and 196 ferro-magnesian silt and/or sandsize minerals seems to be a further possibility, it is also in accordance with the concepts on montmorillonite genesis outlined by Jackson (1965). This conclusion is supported by our observations on the weathering of these minerals in the sand fraction (Chapter 6.2.5.1.). The conclusion further supports our ideas, based on the grainsize and chemical composition of the total soil, that soil formation resulted in a substantial increase in clay content in soils of terrace II. Our data on the exchange capacity of the soils involved (Chapter 8.2.2.) indicate that simple breakdown of montmorillonitic silt minerals to claysize grains may well be the most likely way to explain possibility b. Differential thermal investigation of the clay fractions permits the identification of two kinds of mineral assemblages. The differential thermographs for clays of low terrace and terrace II soils are similar, as well as those of terrace III and IV soils. The thermographs of all soils show double endothermic peaks between 120 and 180° C. The endothermic reaction, due to loss of lattice water begins gradually at about 400° C. In terrace I and II soil clays the reaction ends at about 590° C, with a peak loss of water at 530 tot 560° C. In the older clays these temperatures are 640° C and 570 to 590° C. The hydroxyl endotherm temperatures are considered to be fairly diagnostic of the particular montmorillonite species involved. Where montmorillonites proper show a peak temperature at about 700° C, a reduction in the temperature of reaction to 550 to 600° C is caused by a large replacement of aluminum by iron in octahedral position (Grim, 1953; Mackenzie, 1957; Jackson, 1968). The above-mentioned low endotherm temperatures thus are indicative of montmorillonites containing considerable octahedral iron (nontronite). Ross and Hendricks (1945) also noted that nontronites have a lower stability range of temperature than montmorillonites, as they may begin to break down at 400°. The proportion of octahedral iron seems less in soils of terraces III and IV. A minor hydroxyl endotherm at 740 tot 750° C, observed on differential thermal curves of Blue Nile and Nile clays (Chapter 3.1.1.), could not be detected for soil clays. The mineral structure of soil clays, in fact, persists to temperatures of about 900° C. Exothermic reactions, occuring as a result of recrystallization, take place at about 900 to 920° C in clays of terrace I and II soils as well as in river sediments, and at 930 tot 940° C in soils of older terraces. While X-ray diffraction clearly showed a mixture of mainly montmorillonite and kaolinite to be present, the differential thermal data give no definite indication of more than the montmorillonite component. This is considered to be due to the coincidence of the main hydroxyl endotherm of montmorillonite, when at a low temperature, with the kaolinite endotherm. This is also reported by Grim (1953) for a Texas bentonite, and by Martin (1954) for a number of New York State soils. 197

The X-ray diffraction data of terrace III and IV clays reveal a trend other than the possibly lower iron content of the montmorillonites, namely a considerable proportion of kaolinite minerals. Both kaolinite and montmorillonite occur in large amounts, with traces of quartz and mica. The fine clay fraction of these soils has essentially the same composition as the total fraction, except for its lack of quartz. Traces of mica were still found to be present. These observations are confirmed by the results of chemical analysis of the clay fraction of soil 59, characterixed by a low SiO2/Al2O3 molar ratio and low loss on ignition, as given in figure 59.

Fig. 59. Chemical composition of the clay fraction of soil 59 on Nile terrace IV

Depth, SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2O Loss on SiO2/Al2O3 cm ignition

0- 2 43.3 20.4 10.5 1.2 0.3 2.5 0.8 16.5 3.6 2-35 44.1 26.6 10.7 1.4 0.3 2.4 0.9 15.2 2.8 35-70 40.1 29.7 12.0 1.0 0.2 1.1 0.8 14.9 2.3

In terrace II soils, weathering of primary silicates may have resulted in some new formation of montmorillonitic clay minerals, and depletion of less stable minerals in the sand and silt fractions. In the same way, subsequent weathering of the remaining primary minerals, especially K-feldspars and epidote, may well account for formation of (part of) the kaolinite minerals in terrace III and IV soils. An indication of this may be seen in the presence of mica in the fine clay fraction, presumably derived from K-feldspars too. In view of the large proportion of kaolinite, however, weathering of montmorillonite to kaolinite should not be precluded. Such a transformation would entail desilication; the amount of silica which passes into solution from montmorillonites increases with increasing alkalinity (Nutting, 1943). Beside loss of silica, release of iron as oxides is evidenced by the characteristic red colour of these soils. It is also conceivable that the montmorillonitic constituents were entirely removed from the soil, entailing a relative increase of kaolinite and a decrease in clay percentage. Since the clay-silt ratio of these soils is generally between 1 and 2, indicating a relative decrease of clay as compared to terrace II soils (see section 6.2.1.), the latter process may well be dominant. CHAPTER 7

Organization of soil materials

The way in which the constituents of the soil materials are arranged was examined both in the field and through microscopic analysis. A detailed and systematic micropedological examination and description of all horizons of the representative soils is beyond the scope of the present study. Our investigations are limited to specific horizons and focus on specific problems related to soil formation.

7.1. Investigational Techniques

The individual natural soil aggregates, as recognized in the field were described according to the definitions of the USDA Soil Survey Manual (1951). Results are included with the profile descriptions of Chapter 5.2. Microscopic examination was carried out on thin sections, both mammoth-sized and of small size (about 5 cm in diameter). The mammoth-sized specimens were prepared of undisturbed samples of seven selected horizons collected in metal containers (15x8x5 cm). Thin sections were made according to procedures described by Jongerius and Heintzberger(1964). The unsaturated polyester resin Vestopal 130 was used for impregnation. Micromorphological terminology follows Brewer (1964).

7.2. Discussion of Results

7.2.1. Field observations 'Field structure' of soils of the Blue Nile and Nile low terrace is here designated as blocky. This structure is considered to be inherited from originally horizontally stratified sediments. The • grade of structure is usually strong. Sedimentary stratification is mostly destroyed in the topsoil presumably by homogenization processes (Hoeksema, 1953). As a consequence, surface horizons usually have a weak subangular blocky structure, or are considered structureless when ploughed. 200

Of these soils no thin sections were studied. The soils of terrace II (Er Roseires terrace) are generally characterized by two types of structure related.to the grain size composition of the soils (see Chapter 6.2.1.). The loamy levee soils have subangular blocky A-horizons, often prismatic or columnar (soil 67) Bt horizons, and are frequently structureless below. The prism-like structure of the B horizon easily breaks down to angular blocky aggregates. The micromorphology of this horizon in particular will be discussed below. The clayey backswamp soils typically have angular blocky topsoils with a granular surface mulch. Subsoils show structures largely determined by slickensides and other planes due to differential movement upon wetting and drying. The exteriors of some structure aggregates in strongly alkaline clayey soils (as in soil 44) have thin dark-coloured surface films, presumably due to humus translocation. For a detailed discussion on the structure of the Vertisols among these backswamp soils, one is referred to De Vos and Virgo (1969). Some aspects of the micromorphology of the clayey soils are dealt with below.

7.2.2. Investigation of thin sections Our microscopic observations were made primarily with the aim to assess soil forming processes in soils of terrace II. Therefore, the discussion below is essentially concerned with the rearrangement of soil constituents. Inasmuch as the plasma of soil materials forms the constituent which is relatively easily reorganized by pédologie processes, it is of primary importance here. The arrangement of the plasma in relation to the relatively inert skeleton grains in the groundmass of soil materials is characteristically different for the medium textured levee soils and the clay soils of the backswamp areas. Thus soil materials of levee soils dominantly have an intertextic fabric, the plasma forming inter- granular bridges between skeleton grains. Soil materials of the backswamp soils mainly have a porphyroskelic fabric, with the skeleton grains embedded in the plasma. In general, the plasmic fabrics observed predominantly belong tot the sepic class, they have recognizable anisotropic domains that are oriented with regard to each other. The indications are that plasma separations are the result of reorganization of clay domains due to the effects of soil wetting and drying (Brewer and Sleeman, 1969). These separations, characterized by a striated orientation pattern under crossed niçois, were noted to occur in two forms - 1) as patches, largely insepic fabric, and 2) as elongated zones, associated with voids (vosepic fabric) or with skeleton grains (skelsepic fabric), or traversing the groundmass (masepic fabric). These plasma separations are especially common in the Vertisols of backswamp 201 areas (see Chapter 9) owing to the typical churning process of these soils. Their very development is such that Blokhuis et al. (1970), in a quantitative study of plasmic fabrics of Sudan Vertisols, conclude that the width of planar vosepic plasmic fabric and the fraction of masepic fabric of the total plasma may be used as parameters of the stress processes active in these soils.

Whereas plasma with stress-induced fabrics marks the groundmass of the clayey soils, pedological features effected by illuviation of plasmic material, in addition, characterize the groundmass of Bt horizons of the associated levee soils. In the latter horizons plasma concentrations are manifest as cutans. Apart from this kind of reorganization, microscopic observations clearly revealed the presence of more or less deformed cutanic material within the plasma (figure 60). This mixing of soil constituents is considered to be due to the process of argillipedoturbation as described by Jongerius (1970). The clay concentrations ultimately may become so transformed that they can no longer be distinguished from well-developed plasma separations. This type of Bt horizon thus essentially owes its specific make-up to the mineral composition of the clay fraction, which is montmorillonitic (Chapter 6.2.5.2.). The horizon closely resembles what Fedoroff (1968) termed a 'horizon B textural dynamique', and likewise occurs in soils of the Alfisol order (Chapter 9). It is conceivable that similarly illuviation takes place in the backswamp soils - the Vertisols, and that resulting cutans are deformed beyond recognition due to churning. Traces of illuviation may be preserved below the zone of maximum churning. This line of thought is confirmed by observations on horizons of soil 23. At a depth of 120 to 150 cm in this soil minor clay concentrations were detected. Plasma concentrations were found as very thin cutans associated with the walls of voids (figure 61). In addition, small areas of masepic fabric, relatively high in organic matter, were observed and interpreted as deformed cutans. Further evidence of clay illuviation in Vertisols is reported by Jongerius and Bonfils ( 1964) from Argentina.

As a consequence, reconsideration of the conventional AC horizon designation of Vertisols in general seems appropriate. In case prolonged soil formation has taken place the deep A horizon of these soils might be conceived as the extreme development of Fedoroff's dynamic Bt horizon. Similar ideas presumably form the basis for Jackson's (1965) concept of the Vertisol as 'super B' horizon, capable periodically of engulfing the A horizon and part of the C horizon. As compared to the clay illuviation in the Alfisols of the southern levees, rearrangement of soil constituents in the form of cutans is less well observable in the Aridisols in the northern part (more arid) of the Blue Nile floodplain and Nile terrace II. In the Bt horizon of soils 42 and 67, orientation of the internal fabric 202

Fig. 60. Cittern m various singes of deformation. Alfisoî (profile 22, depth 70 cw ). 40 X. Pimn light left, crossed polarizers right.

Fig. 61. Very thin, initial, cutan in Vertisol (profile 23, depth 130 cm). 375 X. 203

Ptg. 62. Crack fill I'd Tilth soil mute na/ shmctni* fia:-: \ti ne/m t . . irnhtdf /profile 42, depth 60 cm :. JftX,

Fig. 6.3. Secondary silica crystallised from the walls of a void inward, Alßsof (proßle 15, depth 80cm). 40 X. 204 of the clay concentrations appears to be less pronounced. Omnipresent carbonates, and dispersed organic matter liberated from the topsoil have an obscuring effect. Apart from this, oriented concentrations of illuviated clay combined with coarser grains, carbonates, and humus particles, occur in these horizons, as visualized in figure 62. This phenomenon presumably is to be ascribed to mechanical destruction of structure elements at the surface through heavy falls of rain (Chapter 1.1.2.) and subsequent translocation of soil material. In our discussion of Chapter 6.2.2.2. in particular, we concluded to the release of silica by weathering. In support of this process we are able to show secondary silica crystallized in situ at a depth of about 90 cm in soil 15 (figure 63). The crystals occur in normal arrangement and show evidence of crystallization from the walls of voids inward by the preservation of a central void. Related thin films of silica were not observed, they are difficult to identify in thin section. CHAPTER 8

Properties of soil materials

A number of properties of the soil materials - due in fact to their composition and organization - were measured. The determinations include bulk density measurement, analysis of exchange properties, and the measurement of reaction and salt content of soil solutions. All results are included in the laboratory data sheets of Chapter 5.2.

S.I. Investigational Techniques

8.1.1. Bulk density measurement Bulk densities were determined for 50 air-dry soil samples, using the clod method with paraffin wax as coating material (Black, 1965). Results are presented in grams per cubic centimeter.

8.1.2. pH Determination Soil was shaken with water (soil-water ratio 1 : 5) for 2 hours, pH readings were determined on all soil samples by means of a pH meter with glass electrode.

8.1.3. Exchangeable metallic cation determination Soil was mixed with purified sand (to facilitate leaching), put into percolation tubes, and leached with 50% alcohol to remove water soluble salts. In calcareous soil materials, appreciable ^mounts of Ca and Mg tend to dissolve from the carbonate or sulphate form in NH4OAc extraction solution, and thus cannot be distinguished from exchangeable cations. In order to better determine the truly exchangeable Ca and Mg, alcohol solution of NH4OAc at pH 8.2 was employed here to extract exchangeable cations. In the leachate Ca, K and Na were analyzed directly by flame photometer, and Mg with the colorimetric thyazol yellow method. However, the sum of exchangeable cations still exceeds the cation exchange capacity in most of our calcareous soil materials — even when low in organic matter — thus showing that the solubility of carbonates is not fully prevented. The analytical results, expressed as meq/100 g soil on an oven-dry basis and determined on all soil samples are therefore presented as extractable cations. The sum of truly exchangeable Ca and Mg can be estimated as the exchange capacity minus exchangeable K and Na. 206

8.1.4. Cation exchange capacity (C.E.C.) determination Following the procedures used for exchangeable cation determination, soil materials were leached with NaOAc of pH 8.2. The excess sodium was removed by one or more washings with 100% alcohol. Finally, the absorbed sodium was replaced by leaching with NH4OAc of pH 8.2, and, in the leachate, was determined by flame photometer. The results, as determined on all soil samples, in meq/100 g of oven-dry soil, give the cation exchange capacity at the equilibrium pH of 8.2.

8.1.5. Soluble salts and ions in soil-water extract 1:5 Soil was shaken with water (soil-water ratio 1 : 5) for 2 hours. In the extract, obtained by filtration, soluble salts and ions were determined as follows. Salt content (EC5) by electrical conductance with a conductivity meter, results expressed in mmhos/ cm at 25° C. Calcium determination by flame photometer. Magnesium determination colorimetrically with thyazol yellow. Potassium determination by flame photometer. Sodium determination by flame photometer. Carbonate and bicarbonate determination by titration with 0.1 NHCl. Chloride determination with a 'Chlor-o-counter'. Sulphate determination turbidimetrically. Nitrate determination colorimetrically with brucine. These determinations were carried out on all soil samples. Results are expressed in meq/100 g soil on an oven-dry basis.

8.1.6. Soluble salts and cations in saturation extract Soil was saturated with water. In the extract, obtained by suction or centrifuging with a high speed centrifuge, soluble salts and cations were determined as follows. Salt content (ECe) by electrical conductance with a conductivity meter, results expressed in mmhos/ cm at 25° C. Calcium determination by flame photometer. Magnesium determination colorimetrically with thyazol yellow. Sodium determination by flame photometer. These determinations were carried out on 23 soil samples, all having an EC5 of 0.5 mmhos/cm or higher. Results are expressed in meq/1.

8.2. Discussion of Results

8.2.1. Bulk density The bulk densities of soils of the low terrace and of terrace II of the Blue Nile and Nile range from 1.1 to 1.9 g/cc, about the variation in bulk density which has been observed on soils in general. 207

The lowest bulk density values, as might be expected, were observed in the recent soils. They are characterized by average values of 1.3 g/cc, and a range of 1.1 to 1.5 largely depending on texture. Soils of terrace II have generally a higher bulk density, ranging from 1.6 to 1.9 g/cc. The highest values were recorded for the sandy soils of the levees of the Blue Nile clay plain (soils 22 and 42) and those of Nile terrace II (soils 67 and 77), they average 1.8 g/cc. The associated dark clay soils have somewhat lower bulk densities (soils 23, 43, and 44) of 1.7 g/cc on average. Similar high, and even higher values, are reported for dark clays of Arkansas, Texas, and Arizona by Dudal (1965). Zein el Abedine et al. (1969) reported lower bulk densities for a Gezira Vertisol of about 1.2 to 1.3 g/cc.

8.2.2. Cation exchange 8.2.2.1. Cation-exchange capacity The ion-exchange properties of soils are generally considered to be almost entirely due to the clay and fine silt fractions and organic matter (Wiklander, in Bear ed., 1965). The sand and silt fractions of soils, however, may have relatively high exchange capacities too, exceeding 20 and 25 meq/100 g respectively, according to Ruellan and Delétangs' (1967) summary. In the recently deposited soils along the Blue Nile and Nile a very high degree of positive correlation was found to exist between CEC and fine silt + clay (r = 0.97). This is in accordance with the substantial amounts of montmorillonitic minerals shown to occur in the silt fraction of the solid load of these rivers (Chapter 3.1.1.). The regression equation to estimate CEC in meq/100 g (y) of these recent terrace soils from the percentage of fine silt -f- clay (x), based on 36 pairs of observations, is: y = 0.56 x + 4.2 (with Sy = 3.3 meq). For the same soils the degree of correlation with total silt + clay as well as with clay alone is lower, the correlation coefficients are 0.87 and 0.88 respectively (see figure 64). From the calculated regression equation for CEC on clay percentage (y = 0.64 x + 10.4) follows that the exchange capacity of the clays is 74 ± 6 meq/ 100 g clay. This is compatible with our investigations on the clay mineralogy of these soils, which revealed the presence of predominant amounts of montmorillonitic minerals with moderate amounts of kaolinite. The situation with relation to the older soils, as might be expected, is different. As may be seen from figure 64, the relation between CEC and a grain size component of terrace II soils cannot be assumed to be linear anymore. The separation of observations at the arbitrary value of 50% clay or fine silt + clay, however, enables one to approximate the relationship by means of two linear correlations for each of these grain size fractions. The intersection points of each pair of regres- 208 sion lines give the (fine silt +) clay percentage, above or below which each one of the regression equations should be applied as the best fit between CEC and grain size component. The regression for CEC (y) on the clay content (x) of these soils is determined by the following equations (with intersection value for x), correlation coefficients, and standard errors of y: (1) y = 0.77 x + 0.9 (x < 38%), r = 0.84, Sy = 4.5 meq (2) y = 1.20 x — 15.4 (x > 38%), r = 0.89, Sy = 4.4 meq. The regression for CEC (y) on the fine silt + clay content (x) may be similarly expressed as: (3) y = 0.63 x + 1.0 (x < 47%), r = 0.84, Sy = 4.0 meq (4) y = 1.10 x — 21.2 (x > 47%), r = 0.90, Sy = 4.5 meq. Mutual comparison of these equations shows that the degree of correlation between CEC and clay, and CEC and fine silt + clay, is about equal. This may indicate a decrease in CEC of the fine silt fraction as compared to terrace I soils. It follows from equation (2) that the clays of these soils have an exchange capacity of 105 ± 4.4 meq/100 g, which is about 30 meq/100 g more than calculated for recently deposited clays. This confirms our results on the mineral composition of the clay fraction of terrace II soils, which, apart from montmorillonite, contains but small amounts of kaolinite. Further, the exchange capacity of these clays is in close agreement with the findings reported by Jewitt (1955), who states that the clays of Gezira soils have a total exchangeable cation capacity of about 1 meq/g of clay (also quoted by Finck and Ochtman, 1961, and Williams, 1968, b). As follows from equation (1), soils low in clay content have a cation-exchange capacity which is about equal to or somewhat less than low terrace soils of the same texture, although the clays of the former soils have a higher exchange capacity (105 versus 74 meq/100 g). The non-clay fraction thus necessarily has a concomittant lower exchange capacity, a conclusion arrived at above on different grounds. This effect is ascribed by the author to weathering of montmorillonite, which, when lost from the silt fraction, enriches the clay fraction. The resulting soil may then have a somewhat lower CEC, whereas its clay fraction has a higher CEC than originally. The cation-exchange capacities of yet older soils (Nile terraces III and IV) are considered collectively in view of the limited number of observations available. The general trends with relation to exchange capacity as outlined above, may be extended to these soils. Thus, the highest degree of correlation between CEC and a grain size component is with clay percentage (r = 0.71), this coefficient for CEC on fine silt -+- clay is 0.64. The regression equations, the lines of which are given in figure 64, are y = 0.51 x (clay %) + 2.6, (Sy = 3.6), and y = 0.32 x (fine silt + clay %) + 5.0, (Sy = 3.9), 209

333 Fig. 64. Scatter diagrams and regression lines f or cation exchange capacity (CEC) and clay percentages or fine silt + clay percentages of various Blue Nile and Nile soils 210 respectively. The cation-exchange capacity of the clay fraction of these soils is relatively low: 53.6 ± 3.6 meq/100 g, which, again, is in accordance with their mineral composition, consisting of large amounts of both montmorillonite and kaolinite (Chapter 6.2.5.2.). Summarizing, it may be stated that with increasing age the cation exchange capacities of soils with comparable clay content decrease, and are correlated with ever finer fractions. The exception is formed by the soils of terrace II which contain more than about 45% clay with a high cation exchange capacity. These soils are generally characterized by the highest CEC observed.

8.2.2.2. Exchangeable cations The exchange complex of most soils investigated is practically saturated with cations. The sum of exchangeable cations even exceeds the CEC of recently deposited soils (terrace I), as well as of the older soils of sample areas 4, 5, 6, and 7, presumably due to the presence of free carbonates (see section 8.1.2.). Calcium and magnesium are the principal cations found on the exchange complex of the greater number of soils, as is normal for soils of arid regions. In soils of specific areas the exchangeable sodium percentage is high. The nonalkali soils are from 70 to 90% Ca-saturated, and from 5 to 15% Mg- saturated, both exchangeable K and Na ions are present in a few percent each. This applies to recent as well as to older soils. Similar percentages are reported by Jewitt (1955) for a series of Gezira soils: exchangeable calcium 77% and magnesium 12%. The soils of sample areas 1 and 2(except the recent soil 21) tend to have lower exchangeable calcium and concomittant higher magnesium percentages, from 50 to 70% and from 20 to 45% respectively. Kelley's (1964, b) suggestion that the kind of cation-exchange material in the soil may largely determine the content of exchangeable magnesium is unfit to explain the above differences, since exchange materials, as far as known, are similar for soils of similar development. The monovalent cations K and Na are generally less strongly adsorbed than divalent Ca and Mg. The content of exchangeable potassium is low in the soils observed, and does not exceed 2 meq/100 g of soil. For a comprehensive account of the K-status of soils of the Gezira area (terrace II), including exchangeable K, one is referred to a study by Finck (1962). In particular areas the soils have a high exchangeable sodium percentage (ESP), sometimes so high as to dominate the exchange complex. Alkali soils occur on two terraces only. On the recent terrace they occupy small depressional areas or are characterized by an unusually high clay content (most soils are silty) which inhibits water movement through the soil, as in soils 71 and 71A respectively. They are saline-alkali soils. The bulk of the alkali soils occurs on terrace II, 211 mainly in backswamp areas of the former Blue Nile floodplain, not on the levees. They are present over extensive areas in the northern, more arid, part of this plain, and are usually saline too. Nonsaline-alkali soils occupy depressions without outlet (soil 44), also under the higher rainfall conditions of the semi-arid Dinder and Kenana plains (see map figure 18). Williams' (1968, b) statement, misquoted from Jewitt (1955), that sodium is the dominant ion in the Gezira clay exchange complex, is fortunately not correct. Alkalization of soils, the process whereby the exchangeable sodium content of a soil is increased, takes place wherever soluble sodium salts accumulate, provided the concentration of soluble calcium is low. The amount of adsorbed sodium as a function of the concentrations of soluble cations in the saturation extract of soils may be expressed by the following empirical equation, based on research of soils of the U.S.A. (U.S. Salinity Laboratory Staff, 1954): 100 (—0.0126 + 0.01475 SAR) ESP = 1+ (—0.0126 + 0.01475 SAR) ' wherein SAR (sodium-adsorption-ratio) represents the concentration of soluble Na divided by the square root of half the concentration of Ca + Mg (concentrations expressed in meq/1). Alkalization, according to Kelley (1964, a), is determined by the concentration and the kind of sodium salts which accumulate in the soil. This is confirmed by Tyurin et al. (1960) who, in their review of the origin of solonets soils in the U.S.S.R., deal with this process in detail. The latter conclude that Gedroits' (1912) theory on the formation of solonets by way of desalinization of neutral saline soils is only valid where the salt composition shows an overwhelming preponderance of sodium salts (up to 90%) over the salts of alkali-earths (Ca and Mg). This statement is based on research carried out by numerous investigators, one of which (Robinson, 1949) worked on soils of the Sudan. The principal mechanism of alkalization, however, is by predominant adsorption of sodium from its alkaline salts (soda), from the very earliest stages of soil salinization under conditions of a shallow water table. The introduction of sodium in the soil solution, in the presence of carbonates and bicarbonates, entails an increase in pH and causes calcium and magnesium to precipitate as carbonates. This enhances Na adsorption even under conditions of low soda salinity. Tyurin et al. (1960) state that solonets soils thus arise in the course of salinization with soda as well as with soda-sulfate present in the soil solutions. This process actually precedes the stage of solonchak formation Whereas flooding by the Blue Nile and Nile presently does not give rise to alkalization of most of the soils of the low terrace, both alkalization and salinization to high levels take place, apparently quite quickly and practically simultaneously, under conditions of impeded drainage in arid climate. This fact, as well as the 212 presence of substantial amounts of sodium carbonate and -sulfate in the soil solutions, leads the author to believe that alkalization proceeded according to the concepts of Tyurin et al. (1960) outlined above (adsorption from soda), and not according to the theory of Gedroits(1912). We will deal with the genetical aspects of soda in the soil solution - a highly controversial matter with relation to Nile delta soils - in section 8.2.3. The same environmental conditions that presently lead to alkalization of limited areas of low terrace soils presumably existed during the development of the landscape of terrace II. The same mode of alkalization (combined with salinization) may thus account for the alkali and saline-alkali soils of terrace II (Gezira, etc.). This is in agreement with the occurrence of the alkali soils in backswamp areas, the clay plains, mainly in the northern part of the former Blue Nile floodplain (terrace II), as well as with the presence of mainly sodium bicarbonate and sulfate in the solution of these soils (soils 23, 33, 34, 43, and 44, and Jewitt, 1955). With gradual incision of the river system from terrace II down to its present level, moisture could still be supplied to these soils for a long time from the groundwater. Information on the depth and fluctuation of present-day groundwater of terrace II is scarce, but points to such levels (of 10 to 20 m below the surface) that at the present day from this source no salts are supplied to most soils, (in view of the presence of highly soluble NaCl soil 67 is probably still influenced by groundwater). After severance of connection with groundwater, supply of soda and other salts ceased and with it the alkalization of terrace II soils. As stated above, the accumulation of exchangeable sodium requires arid conditions. Such circumstances presumably did not prevail in the southern part of the backswamp areas (Dinder, Kenana), where rainfall presently exceeds 500 mm per year. This condition prevented substantial accumulation of soda in the soil solution, except in depressional soils (see section 8.2.3.). We therefore conclude that in the high rainfall part of the clay plains alkalization never occurred, and soils always contained large amounts of exchangeable calcium. In depressions, however, alkalization could take place. The alkalinity of soils of terrace II is considered to be a relic feature, originally largely the result of soda accumulation in these soils. This adds to the importance of the study of soda formation. Under natural conditions of low rainfall the possibility of a reverse displacement of sodium from the exchange complex, when solutions become diluted, is practically precluded, since sodium is not removed from the soil, and the solutions contain very few bivalent cations only. Under irrigation, as is actually the case in the Gezira and proposed for other extensive areas, no further alkalization will take place unless the concentration of Na2CO3 in the soil solution is increased. 213

8.2.3. Salinity

In the more arid part of central Sudan (about north of sample area 3) easily soluble salts have accumulated in soils of various landscapes, often to the extent that both saline and saline-alkali soils developed. Saline soils, in the sense of nonalkali soils containing sufficient salts to impair their productivity (determined here from electrical conductance, ECe), mainly occur on the older terraces III and IV of the Nile, as well as occasionally (soil 77) on terrace II. Saline-alkali soils, defined as soils containing sufficient exchangeable sodium to interfere with the growth of most crop plants and containing appreciable quantities of soluble salts (ECe more than 4 mmhos/cm and ESP more than 15), primarily occur on terrace II, more specifically in backswamp areas of the former Blue Nile floodplain, as well as to a limited extent on terrace I (see also section 8.2.2.2.). We will not deal here with the degree of salt accumulation - soil salinity by nature shows large variations over short distances. Any appraisal of this aspect would require a very large number of measurements. We will restrict the discussion to the kind of salts which accumulated and the explanation of their presence. Each of the two kinds of saline soil, mentioned above, contains salts characterized by specific anions. Whereas the soils of terrace III and IV primarily contain chlorides, younger soils are distinguished by (bi)carbonate salinity. This demonstrates the relation between geomorphology and salt distribution. Every assessment of soil salinity, be it in a pedogenic context or with relation to irrigated agriculture, essentially involves consideration of the balance between supply and removal of watersoluble salts in a given body of soil. Saline-alkali soda soils are presently developing in depressional areas of the Nile low terrace under arid conditions (especially soil 71). This, in a way, is unusual. Research by numerous soil scientists, especially Kovda's (1946) work (in Janitzky, 1957) on salt provinces in the U.S.S.R. testifies a distinct connection between different chemical types of salt accumulation and specific climatic zones. Soda salinity in the U.S.S.R. is mostly a feature of the steppe and forest-steppe zones, with average annual rainfall of between 300 and 500 mm. Local conditions of deserts and semi-desert normally give rise to chloride and sulfate-chloride salinization, and if soda salinity occurs it consequently arises from influx of this salt from outside. Such an influx of soda to the Nile low terrace soils is provided by the river. Most of these soils are non saline, the soluble material is removed again by the passage of water through the soil, their exchange complex is saturated with calcium. In depressions without outlet, however, flood water stays for a long time, its movement restricted to vertical migration, while, in addition, rain water is collected here. Since Nile water is nearly saturated with calciumcarbonate, continued 214

evaporation will, probably rather quickly, lead to its precipitation (as well as of MgCO3), and subsequently increases the concentration of sodium and bicarbonate which is still available (see Chapter 3.1.2.). Because of the exceptionally high accumulation of CaCO3, we have omitted the carbonate values of soil 71 from our computations of Chapter 6.2.3. The increase in 'residual Na2CO3' entails replacement of calcium by sodium on the exchange complex with more or less simultaneous precipitation of the replaced calcium as insoluble carbonate. Left in the soil solution are the easily soluble sodium salts of bicarbonate and sulfate. Very local conditions of evaporation and drainage determine which one of these salts is dominant. The alkalization as well as the sulfate-soda salinization of these soils may thus be attributed to simple evaporation of Nile water in conjunction with Ca-clays. Bazilevich (1965) also concludes that soda lakes of the Nile delta owe their existence to supply of soda by the Nile. His suggestion, however, that it is derived from inland African regions with savannah landscapes (with reference even to the Lake Chad basin) contradicts the observation that most Nile water, and its dissolved load, is supplied by the Blue Nile from Ethiopia (Chapter 3.1.2.). In this connection the observations of Schoonover et al. (1957) are also relevant. They conclude, in a detailed study of salinity and alkali problems of soils of the Nile valley and delta in the U.A.R., that there is some evidence that the use of Nile water for perennial irrigation has an alkalizing influence. The occurrence of soils high in exchangeable sodium as well as of high salinity was found to be usually associated with high water tables or poor permeability. Gracie et al. (1934), on the other hand, proposed a different process of soda formation to explain the 'black alkali' soils which developed in the Nile valley in the U.A.R. after the introduction of perennial irrigation. They conclude that sulfate-reducing bacteria are responsible for the formation of Na2CO3. The anaerobic conditions and the presence of organic matter essential to this process these authors believe to be available. It leaves to explain, however, the original presence of sulfates in the soil solution. The biological way of soda formation has been described in detail bij Kelley (1951), Rode (1955), Tyurin et al. (1960), and Whittig and Janitzky (1963). It is quite conceivable that the saline soils of the backswamp areas of terrace II (Gezira, etc.) also owe their existence to former evaporation of Blue Nile water. The author does not wish to preclude the possibility of soda formation by bacterial reduction of Na2SO4 in these soils. The southern, nonsaline, backswamp areas presumably received enough precipitation at the time of flooding to prevent soda formation by evaporation in these soils. The salts in terrace II soils then essentially constitute relic features, whose present behaviour is entirely determined by rain water, unless irrigated. 215

The chloride salinization which characterizes the older terraces of the Nile may constitute a relic feature too, the easily soluble NaCl and CaCl2 having accumulated from groundwater when the Nile was flowing at the level of terrace II. The presence of these salts may also be ascribed to its influx with rain water from the adjacent pediment surfaces, explaining the chloride concentration of terrace IV higher than the younger terrace III. Summarizing it may be stated that the salinization of the soils of terraces I and II is determined by the influence of river water, whereas the salts of higher terraces correspond to a desert environment. PART III

Soil classification and genesis CHAPTER 9

Soil classification

The variety of soils, to which the sediment masses of the Blue Nile and Nile - as exposed at the surface - gradually have become transformed, reflecting the varying interplay of environmental factors, and perhaps also some relic features of former conditons, and which have been the subject of systematic observation and description in previous chapters, can now be classified. This grouping of the soils on the basis of their common properties serves three objectives, it is intended 1) to organize knowledge gained from field and laboratory observations as specified, 2) to scrutinize the concepts implicit in the classification system selected, and 3) to provide a basis for further discussion on soil genesis. As it is not yet possible to make use of an international system of soil taxonomy, the comprehensive soil classification system as developed by the U.S.D.A. Soil Survey Staff (1960, 1967) has been adopted here. Apart from fundamental objections to this system (for example by Webster, 1968), considerable difficulties have been encountered with its application in tropical areas outside the United States (Allbrook, 1968; Bennema, 1969; Sys, 1967 and 1969). This is not surprising, since the system has been developed to provide "a natural classification of the soils of the United States. If it will accommodate soils of other continents, it will be more useful in the United States" (Soil Survey Staff, 1960). The system here is nevertheless preferred, because it gives a direct link between our findings and the kinds of soil similarly identified in the course of soil surveys and studies carried out by the Government of the Sudan, the FAO/UNDP, and also because the system is increasingly used as an international reference system of soil classification.

From the ten soil orders recognized in the U.S.D.A. Soil Classification System, four can be identified in central Sudan, they are: Vertisols, Entisols, Aridisols, and Alfisols. The representative soils, described in Chapter 5, are classified down 220

Fig. 65. Classification of representative soils according to the U.S.D.A. Soil Classification System (1960 and 1967)

Profile number Order Subgroup Family

15 Alfisol Typic Haplustalf Clayey, montm., isohyp. ( ?) 16 Vertisol Typic Chromustert Very fine, montm., isohyp. (?)

21 Entisol Aquic Ustifluvent Fine silty, mixed, calcareous, isohyp. ( ?) 22 Alfisol Typic Haplustalf Loamy, mixed, isohyp. ( ?) 23 Vertisol Typic Chromustert Fine, montm., isohyp. ( ?)

31 Entisol Aquic Ustifluvent Fine silty, mixed, calcareous, hyp. 32 Alfisol Typic Haplustalf Clayey, montm., hyp. 33 Vertisol Typic Chromustert Very fine, montm., hyp. 34 Vertisol Entic Chromustert Very fine, montm., hyp.

41 Entisol Aquic Ustifluvent Fine silty, mixed, calcareous, hyp. 42 Aridisol Typic Haplargid Loamy, mixed, hyp. 43 Aridisol Typic Camborthid Clayey, montm., hyp. 44 Aridisol Ustertic Camborthid Loamy, mixed, hyp.

51 Entisol Aquic Ustifluvent Sandy over clayey, mixed, calcareous, hyp. 57 Aridisol Typic Calciorthid Loamy, mixed, hyp. 59 Aridisol Typic Paleargid Clayey-skeletal, kaolinitic, hyp.

61 Entisol Aquic Ustifluvent Loamy, mixed, calcareous, hyp. 67 Aridisol Typic Haplargid Loamy, mixed, calcareous, hyp. 68 Aridisol Petrocalcic Paleargid Loamy, siliceous, calcareous, hyp.

71 Entisol Aquic Ustifluvent Loamy, mixed, calcareous, hyp. 71A Vertisol Typic Torrert Very fine, montra., hyp. 77 Aridisol Typic Haplargid Loamy, mixed, montm., hyp. 78 Aridisol Petrocalcic Paleargid Loamy-skeletal, siliceous, calcareous, hyp.

Abbreviations: montm. : montmorillonitic isohyp. : isohyperthermic hyp- : hyperthermic

to the family level in figure 65. The placement into suborder and great group classes is not shown separately, as both are implied in the last word in the name of the subgroup. This grouping of soils is briefly explained below. With relation to orders, suborders, and great groups the sequence of the Key of the classification is used. 221

9.1. Vertisols

Mineral soils without a lithic or paralithic contact within 50 cm of the soil surface that have: 1. After the upper 18 cm are mixed, 30 per cent or more clay in all horizons down to 1 m depth; 2. At some period in most years, cracks at least 1 cm wide at a depth of 50 cm unless irrigated; 3. One or more of the following: a. Gilgai b. At some depth between 25 and 100 cm, slickensides close enough to intersect c. At some depth between 25 and 100 cm, wedge-shaped or parallelepiped natural structural aggregates with their long axis tilted 10 to 60 degrees from horizontal. The soils represented by profiles, 16, 23, 33, 34 and 71A all qualify for Vertisols, on the basis of their clay content, the width of their cracks, and the presence of gilgai. Usually slickensides in these, and similair, soils are not close enough to intersect, while in addition wedge-shaped or parallelepiped aggregates are difficult to observe, if present at all. Vertisols are subdivided primarily on the basis of their cracking properties into: Torrerts Usterts Xererts Uderts The Torrerts are usually dry in all parts and have cracks that remain open throughout the year in most years. Cracks may close for variable periods, if the soil is inundated, this is the case with profile 71A located in an area which is yearly flooded by the Nile. Soils represented by this profile are Typic Torrerts on account of their colour and structure. The Usterts form the class in which the other Vertisols can be placed. All have cracks that remain open for 90 cumulative days or more but not throughout the year, and a mean annual soil temperature of 22° C or more, as can be deduced from data presented in Chapter 1.1. None of the soils has moist chromas of less than 1.5 throughout the upper 30 cm, they all, therefore, are Cromusterts. Except for profile 34, which is named an Entic Chromustert, all Chromusterts presented here are Typic in view of their colour, structure, and cracking properties. The classification at the family level takes into account: the particle-size class, the mineralogy class of the clay fraction, and the soil temperature class. Whether the soil temperature class of the soils of areas 1 and 2 is hyperthermic or 222 isohyperthernric remains questionable, as direct measurements of soil temperature have sofar not been carried out in these areas. It is assumed that soils of the areas 3 and 7 (as well as 4, 5 and 6 treated later) have hyperthermic soil temperatures on the basis of the measurements at Khartoum (see Chapter 1.1).

9.2. Entisols

Mineral soils that have no diagnostic horizon other than an ochric or an anthropic epipedon, an albic or an agric horizon, with or without any of the following: a salic horizon, sodium saturation exceeding 15%, a calcic or gypsic horizon or duripan, plinthite, buried diagnostic horizons, and ironstone - all under specified conditions. The soils represented by profiles 21, 31, 41 and 51, 61, 71 all qualify for Entisols. The order is differentiated into five suborders: Aquents Psamments Fluvents Orthents Arents The 'wet' and 'sandy' suborders, the Aquents and Psamments have not been encountered among the alluvial soils of central Sudan. All Entisols presented here are Fluvents, because they lack fragments of diagnostic horizons that can be identified, and have slopes of less than 25%. They have a carbon content that decreases irregularly with depth (profiles 31,41, 51, 61, and 71) or remains above levels of 0.2% for 1.25 m (profiles 21, 31, 41, and 61). On the great group level these Fluvents do not meet the requirements of Cryo- fluvents, they also are considered to be too moist for the usually dry Torrifluvents, since they receive in addition to rain and surface runoff moisture from river floods. The soil moisture status then necessitates classification as Ustifluvents. Because of the high floods of the Blue Nile and Nile, these soils have horizons at depths within 1.5 m of the surface, which are saturated with water at some period, and thus should be named Aquic Ustifluvents. The classification at the family level takes into account the particle size class of materials between 25 and 100 cm depth, the mineralogy class of the whole soil, the reaction class, and the soil temperature class. 223

9.3. Àridisols

Mineral soils that have no oxic or spodic horizons, but have an ochric epipedon and one or more of the following combinations of properties (summarized): 1. no argillic or natric horizon, but within 1 m of the surface: a calcic, petrocalcic, gypsic, or cambic horizon, or duripan, and either usually dry between 18 and 50 cm depth or ECe is 2 mmhos/cm or greater; or within the C horizon there is an increase in saturation with Na plus K; 2. no argillic or natric horizon, but within 75 cm of the surface: a salic horizon, and saturated with water within 1 m of the surface for one month or more; 3. surface horizon not both hard and massive when dry, an argillic or natric horizon, and are usually dry between 18 and 50 cm depth; 4. have the Vertisol characteristics, except the required clay content of 30%. The soils represented by profiles 42, 43, and 44; 57, and 59; 67, and 68; 77, and 78 are Aridisols. The order is subdivided on the presence or absence of an argillic or natric horizon in: Argids Orthids Both the Calci- and Cambi-great groups of the Orthids are represented here. Profile 57 is considered a Calciorthid, this usually dry soil has a low conductivity of the saturation extract and a calcic horizon with its upper boundary at 80 cm from the surface. The soil is a Typic Calciorthid. Profiles 43 and 44 are considered to be Camborthids. The latter, with a conductivity of the saturation extract exceeding 4 mmho per cm within 50 cm from the surface, is considered to be not usually dry, due to the fact that the soil, occurring in a slight depression, receives additional moisture from surface runoff. This very phenomenon necessitates the placement of this soil in the Ustertic subgroup. Although the coefficient of linear extensibility (COLE) and the potential linear extensibility have not been determined, they are assumed to be 0.09 or more and 6 cm or more respectively on the basis of the mineralogy of the control section, which is dominated by montmorillonite. Profile 43 can be classified as a Typic Camborthid. Both the Pale- and Hapla- great groups of the Argids are represented here. Profiles 68 and 78 are considered to be Paleargids, in addition to an argillic horizon they are characterized by a petrocalcic horizon with an upper boundary within 1 m of the soil surface. The latter characteristic serves to classify these soils in the Petrocalcic subgroup. Profile 59 is also considered to be a Paleargid, as it is characterized by an argillic horizon with more than 35% clay and an increase of more than 15% within 2.5 cm at its upper boundary. This soil is a Typic Paleargid therefore. Profiles 42, 67, and 77 are classified as Haplargids, they all belong to the same Typic subgroup. 224

The classification of the Orthids and Argids at the family level takes into account the particle-size class and mineralogy class of varying control sections as well as the soil temperature class. It is noted that the siliceous class applied to soil profiles 68 and 78 is based on an estimation of minerals present from grain counts (see also Chapter 6.2.5.1.).

9.4. Alfisols

Mineral soils that have no spodic or oxic horizon overlying an argillic horizon; that have no plinthite forming a continuous phase within 30 cm of the soil surface; that have no mollic epipedon; that have no surface horizon and an upper subhorizon of an argillic or natric horizon that are separated by an albic horizon but that together meet all requirements for a mollic epipedon, and that have: no fragipan but have an argillic horizon or natric horizon, are usually moist in some part of the soil between 18 and 50 cm unless the epipedon is both hard and massive when dry, and either have mean annual soil temperature less than 8° C, or have base saturation (by sum of cations) of 35% or more at 1.25 m below the top of the argillic horizon or at 1.8 m below the soil surface, whichever is shallower (the combination of properties allowed with fragipan is omitted here). The soils represented by profiles 15, 22, and 32 are Alfiisols. The order is differentiated into five suborders: Aqualfs Boralfs Ustalfs Xeralfs Udalfs Soils of the 'wet' and 'cold' suborders, the Aqualfs and Boralfs, have not been identified in central Sudan. The Alfisols presented here are Ustalfs, they have a hard and massive epipedon when dry, they are continuously dry in all parts of the soil between 18 and 50 cm for more than 60 consecutive days in more than 7 out of 10 years, but the mean annual soil temperature is estimated at 22° C or higher, and they are dry for more than 90 cumulative days, and have increasing saturation with Na plus K within 75 or 90 cm of the surface. As none of the soils has plinthite, a duripan, a natric horizon, a petrocalcic, a strongly developed argillic horizon, or pronounced red colours, they all can be classified as Haplustalfs. At the subgroup the soils meet all requirements of the Typic Haplustalfs. Particle-size, mineralogy, and soil temperature classes are used at the family level of classification. 225

9.5. Comments on Classifation

The above soil classification is not complete without the following comments: a. Under the prevailing low-latitude arid and semi-arid conditions, the Aridisols and Alfisols can only be distinguished on the basis of the consistence and structure of the surface horizon when dry. Alfisols then have a surface horizon that is both hard and massive when dry, Aridisols do not have such a surface horizon. As presently defined the soil moisture status of these soils cannot be employed as a second criterion for separating between the two orders. For, soils of the Alfisol order with a mean annual soil tempearture of 22° C or more or a difference between mean summer and mean winter soil temperature at 50 cm depth of less than 5° C are dry for more than 60 consecutive days in all subhorizons between 18 and 50 cm in more than 7 out of 10 years, and are dry for more than 90 cumulative days in sjme subhorizon between these depths in most years. And soils of the Aridisol order, which have an argillic horizon or are non-saline, are usually dry between 18 and 50 cm depth. How long a usually dry soil is permitted to be moist, however, is not defined. Usually dry, according to the U.S.D.A. Soil Survey Staff (1960), means that the soil is dry more than half of the time that it is not frozen; if during a 7-months period a soil is dry except for a few brief periods of about two weeks or less following showers, the soil is considered usually dry. The term might thus be conceived as 'dry for more than half the year', as opposed to usually moist for 'moist for more than half the year'. This binomial grouping however, conflicts with the division of the non-cryic Entisols at great group level, and the Vertisols and Alfisols (in part) at suborder level, which is in four soil climatic classes, indicated by the formative elements: Torr, Ust, Xer, and Ud respectively. Usually dry and usually moist then only cover the extremes of the soil moisture regimes, equivalent to the elements 'Torr' and 'Ud' respectively. A usually moist soil is dry for less than 60 consecutive days in all subhorizons between 18 and 50 cm in more than 7 out of 10 years, and is dry for less than 90 cumulatiye days in some subhorizon between these depths in most years (as in Udipsamm'ents or Udalfs). An exact statement of the meaning of usually dry is necessary to distinguish between Torri- and Usti- great groups of Entisols, and to better distinguish between Torrerts and Usterts (cracking is a first criterion), and between Argids and Ustalfs (the surface horizon properties are a first criterion). b. Classification of the Vertisols is hampered by the lack of sufficient quantitative definition of the phenomena gilgai, slickensides, and wedge-shaped natural 226 structural aggregates. This especially pertains to the minimum requirements of these features. The matter is also relevant to the classification of soils of vertic subgroups, that fail Vertisols, primarily because they lack these phenomena. Soils of the vertic subgroups have cracking properties peculiar to Vertisols. In the southern U.S.A. their clay content commonly exceeds even 40%, and they are dominated by montmorillonitic clay (De Ment and Bartelli, 1969). The polyinterpretable definitions result in confusion in the classification of the cracking clay soils of the northern part of the Blue Nile alluvial plain (Er Roseires terrace). b.l. Gilgai, according to the USDA Soil Survey Staff (I960), constitutes a microrelief of clays that have high coefficients of expansion with changes in moisture. This general definition is not contradictory to the one proposed by Hallsworth and Beckmann (1969) in their recent review on gilgai, although the latter recognize five forms of gilgai microrelief compared to two of the USDA Soil Survey Staff (1960). The form of the gilgai relief most commonly present in the Blue Nile clay plains is the normal or round gilgai of Hallsworth and Beckmann. The degree of development generally decreases from south to north - from 20 to 30 cm vertical interval between mound and shelf (often with sinkholes) at the latitude of Roseires to almost no perceptible interval (sometimes sinkholes only) in the North-Gezira. Where both intersecting slickensides and wedgeshaped aggregates are absent, a statement only on the approximate central concept of gilgai is insufficient for classification purposes. b.2. Slickensides, according to the USDA Soil Survey Staff (1960), are polished and grooved surfaces produced by one mass sliding past another. Size limitations for slickensides are not given. The use of unspecified related terms such as pressure faces or slip faces adds to the confusion. As with gilgai, both the size and the amount per unit volume of soil of slickensides decreases from south to north in the cracking clay soils of the Er Roseires terrace. De Vos and Virgo (1969) distinguish between small, polished and grooved, surfaces in the upper profile of Sudan Vertisols and large slip faces of the deeper horizons, for the latter of which they propose the term 'macro-slickensides'. b.3. ' Wedge-shaped natural structural aggregates, which should be due to intersecting slickensides, are often difficult to observe in Sudanese Vertisols. De Vos and Virgo (1969) consider the term not applicable to peds occurring between 20 and 60 cm from the surface, as these are 'double-wedge shaped and resemble lentils without rounded surfaces'. c. Soils very similar to the Chromusterts here have been classified by Finck (1961), Blokhuisi Ochtman and Peters (1964) in the Gezira area, and by De Vos 227 and Virgo (1969) in the clay plains east of the Blue Nile, all using the USDA Soil Classification System (Soil Survey Staf, 1960). Finck(1961) - after reviewing previous attempts to classify Gezira clay soils by Shantz and Marbut (1923), Kenchington (1935), Robinson (1949), Middelburg (1951), and Greene (1958) - concludes that the Gezira soil to a large extent seems to fit into the central concept of the Orthic Grumusterts. Blokhuis et al. (1964) suggest that soil formation took place during sedimentation of the clays, they therefore propose the name Cumulic Grumustert for the subgroup. This implies that the rate of accumulation of new sediments has been slower than the accumulation of organic matter (Soil Survey Staff, 1960). De Vos and Virgo (1969) report the majority of the soils east of the Blue Nile to belong to the Ustert and Aquert suborders of the Vertisols. d. Soil temperature data (at 50 cm depth), as obtained from measurements at Khartoum, show the soil temperature regime to be hyperthermic(see Chapter 1.1). Judging from Smith (1965a), however, it seems to be the intent of the U.S.D.A. Soil Classification System to place soils of dry, tropical climates in the isohypsr- thermic class (with less than 5°C difference between mean summer and winter soil temperature). The soil temperatures recorded at Khartoum, would thus exclud: these soils from the tropical soils. They also show that soil temperature classes, as presently defined (Soil Survey Staff, 1967), do not serve to universally distinguish between tropical and temperate usually dry soils. This is especially relevant to the classification of Aridisols. e. An undesirable inconsistency exists with relation to the classification of the Nile low terrace soils. The Vertisols can be accommodated in the Torr-suborder and great group (which have equivalent definitions) as Torrerts, but the Entisols cannot be placed in the Torr-great group, as Torrifluvents. This arises from the fact that, while Torrerts may be inundated for variable periods, no such provision is made in the definition of Torrifluvents, the reason for which seems obscure.

9.6. Soil-Landscape Relations

The general principles apparent from the above classification of soils of Central Sudan may conveniently be outlined in relation to the major landscape elements. On the low terrace of the Blue Nile and Nile primarily Entisols occur, all of the Fluvent suborder. Especially along the Nile limited areas of Vertisols occur, of the Torrert suborder. 228

On the Er Roseires terrace of the Blue Nile (the Kenana, Dinder and Gezira areas) soils of three orders are present. On the levees Alfisols occur (suborder: Ustalfs), with increasing dryness toward the north passing into Aridisols (suborder: Argids) at some 100 km south of the confluence of the Blue Nile and the White Nile. In the backswamp areas Vertisols are present (Usterts), which also change into Aridisols (Orthids) near Khartoum, owing to a decrease in clay content below 30% in horizons within 1 m depth. In the area around Jebel Gargada, about 3 m above the Er Roseires terrace both Alfisols and Vertisols occur (respectively Ustalfs and Usterts). The continuation of the Er Roseires terrace along the Nile, terrace II, is generally also characterized by the occurrence of Aridisols (suborder: Argids). Soils developed in older Nile alluvia, terraces III and IV, are Argids too. Soils of these three Nile terraces, however, are distinguished at the great group and subgroup level. Thus the Argids of terrace II are Haplargids, those of the terraces III and IV are Paleirgids, while the Paleargids of terrace III are Petrocalcic and those of terrace IV are Typic. It is of interest to note the close correlation existing between a number of soil units classified above according to the U.S.D.A. System, and those traditionally distinguished by farmers in central Sudan, and mostly recorded by Tothill (1948). Thus, Fluvents of the Blue Nile low terrace are known as 'gerf', those of the Nile low terrace as 'qurer' or 'gureir'. Ustalfs of the Blue Nile Er Roseires terrace correspond to the local 'azaza' (even connoting a hard-surfaced soil). Vertisols of this same terrace are equivalent to 'badob', otherwise referred to as 'badobe' (Peters, 1965) or 'bardobe' (Finck, 1963). The Vertisols occurring in depressional sit;s are known as 'lugud' (Peters, 1965), but do not necessarily have the low chroma of soils of the Pell-great group. CHAPTER 10

Soil genesis

10.1. General Concepts

In order to achieve an explanatory insight into the phenomenon: soil, theories have been designed with relation to soil formation. To a large extent these are based upon the observable and measurable soil characteristics. Apart from support by new observations, as derived, for example, from comparative soil studies, experimental investigations may provide a further test for such theories. Hallsworth and Crawford (1965) in particular emphasized the approach of soil genesis by experimental study. The author considers separation between processes active in soil formation and factors that condition these processes of fundamental importance. A theory of soil formation in terms of processes has been outlined by Simonson (1959). The latter, conceiving the soils of the world as a continuum with a number of properties in common, regarded soil genesis as consisting of two steps, 1) the accumulation of parent materials, and 2) the differentiation of soil horizons. Horizon differentiation is ascribed to additions, removals, transfers, and transformations within the soil system, with each of the four kinds of changes covering a wide range of processes. Cline (1961) had an essentially comparable concept. In Russian soil science a somewhat different view prevails. Rode (1955) supports the notion of a single process of soil formation. The rate at which the phenomena, which add up to this soil forming process, occur, however, differ in various soil horizons. Two categories of events are considered to determine soil formation: transformation of substances, and their shifting. If we limit this discussion to soils developed in alluvial deposits, Simonson's two steps may be regarded to be approximately equivalent to what was termed geogenesis and pedogenesis respectively by Pons and Zonneveld (1965). Geogenesis refers to 'any kind of process relating to the origin and transport of sediments, to any form of sedimentation', while pedogenesis designates 'the combined processes by which parent material is transformed to soil'. 230

Some soils do not show clear differentiation of soil horizons. Simonson (1959) conceived of such soils as having been formed by the same four kinds of changes mentioned above, but with processes operating such as to retard the development of horizons. Hole (1961), in a somewhat different approach, distinguished between two kinds of soil forming processes, one which leads to formation of soil horizons, the other which inhibits or disturbs them (called propedanisotropic and propediso- tropic respectively). The interplay of both groups of processes may entail a soil in steady state. Why Hole combines in this context soil forming factors and processes remains obscure. Our understanding of soil genesis is hampered by the lack of further differentiation of these (or analogous) basic kinds of changes. One feels the need of a comprehensive scheme of pedological processes unambiguously defined in physico-chemical terms, first qualitatively, and subsequently, in a quantitative sense. One of the benefits of such a scheme could be that full allowance is made for processes of which no diagnostic marks are left in certain soils. Such processes, for example those that resulted in complete removal of certain consituents from the soil, are otherwise easily overlooked in pedogenetic studies. A wide variety of processes, on different levels of integration, is presently recognized in soil science (for instance in: Duchaufour, 1968; Jongerius, 1970; Schlich- ting and Blume, 1966). Some of these, however, are defined in terms of the marks they leave in soils without specification of the actual process involved, thus adding an unexplained term to still unexplained soil phenomena. Such definitions obviously are tentative. The conclusions reached by Schelling (1970) on a general model of soil genesis are quite elucidative in this context. His model comprises the soil forming processes in a 'black box', which responds to different kinds of input (in the form of state factors) by specific and complex outputs (in the form of soil). With this model the indirect relation between soil forming factors (state factors) and soil is emphasized. It also shows that some explanation of the performance of the black box may result from an assessment of the state factors as input. Jenny (1961) introduced the term state factors, variables that define the state of the soil system, as a better expression of what was traditionally known as factors of soil formation or soil formers: climate, organisms, topography, parent material, age of soil, and unspecified factors (dust storms for instance). The study of the state factors has indeed led to an improved understanding of soil genesis. Pertinent to soils of the tropics this was amply demonstrated by Mohr and Van Baren (1954), who particularly stressed the importance of climate, organisms, parent material, and time. Any consideration of the factor time should not only take into account the age of a soil , but also allow for the other state factors to be different during this time from what is at present observable. This aspect seems 231 to have been somewhat underestimated in tropical soil science, according to De Villiers (1965). Recently more attention has been given to the separation of soil phenomena due to processes operating currently and those which have ceased to operate. This research, in fact, has been formulated as one of the objectives of paleopedology (Ruellan, 1970). The climate, and especially the lack of winter and summer seasons, essentially determines the unique character of tropical soils. Tropical arid and semi-arid soils, however, are closely related to temperate climate soils, since their biologic activity is also interrupted by seasonal lack of moisture (Smith, 1965 a). Buol (1965), in his review of arid and semi-arid soils, concludes that sofar only limited data had been available on these soils, and emphasizes the need for continued re-evaluation and revision of existing ideas about their genesis. It is against this conceptual background that we will briefly discuss soil formation in central Sudan in the following section.

10.2. Central Sudan

The review presented here gives a perspective of soil genesis in central Sudan. It comprises by no means all soil forming processes operating at present or having operated in the past in the area. We summarize in this discussion the soil genetic aspects already touched upon or concluded to in the foregoing chapters. Soil orders classified according to the USDA Soil Classification System (Soil Survey Staff, 1967) represent differences in the kinds of processes that have been dominant in their genesis (Smith, 1965 b). The mode of formation of the soils of central Sudan is therefore discussed within the framework of the four orders recognized: Entisols, Vertisols, Alfisols, and Aridisols. The geogenesis of these soils is con- ditioned by the Blue Nile and Nile.

10.2.1. Entisols Because of their very nature Entisols are usually of little interest in soil genetic research. In the present study, however, their investigation is emphasized and not in the least in view of its substantial implications for soil formation. The Entisols, as confined to the low terrace of the Blue Nile and Nile in the Sudan, form the immediate geochemical link between the Ethiopian basalts and the soils of the Nile delta in the Ü.A.R. Our computations of Chapter 6.2.2.1 show that the chemical composition of the Entisols is primarily determined by the Trap Series basalts of the Ethiopian Plateau, and also that a close chemical relationship exists between these soils 'and the Nile delta deposits. In view of the absence, to 232

our knowledge, of trace element determinations for soils of both these areas the relationship is confined to the major oxides. The connection with the soils along the Nile and its delta is further supported by closely similar mineral assemblages of both the sand and clay fractions (Chapters 6.2.5.1, 6.2.5.2, and 3.1.1). In addition, by examination of the Entisols we eliminated part of the difficulty normally encountered in the study of soils developed in alluvial or colluvial deposits, that is, separation of the variations in soil texture, and inhaerent properties, due to differences in the environment of deposition, from pedogenic influences. In general, the Entisols, whatever their texture, are chemically characterized to a large extent by the high degree of correlation established between various grain size components and the concentration of a dozen elements (see figures 49-51). As explained in Chapter 6.2.3 most calcium carbonate is at present deposited in silt size particles, a likely circumstance also in view of the chemical composition of the water of the Blue Nile and Nile. Research on the cation-exchange capacity of the Entisols forms a key element in our conclusion with relation to this property in soils of older terraces. It helped to reveal the fact that with growing age of the soils their cation-exchange capacity is increasingly determined by ever finer grain size fractions. Three phenomena of the Entisols are interpreted to be due to soil formation: a. homogenization; b. depletion of organic matter; c. salinization and alkalization in depressions. Field observations show the obliteration of the originally horizontal sedimentary strata due to the activity of plants and animals as well as man (Chapter 7.2.1.). The depletion of organic matter is increasingly apparent in downstream direction along the Blue Nile and Nile as demonstrated in Chapter 6.2.4. This is evident when the carbon and nitrogen values of the Entisols are compared with the percentages present in the solid load of the rivers. It substantiates the well-known rapid oxidation of organic matter under arid conditions. The salinization of Entisols is limited to depressional areas, as is found in recent Vertisols. As for our conclusion (Chapter 8.2.3) that the sulfate-soda salinization in question, as well as the associated alkalization, is due to evaporation of Nile water, one might conceive of this process as being of a geogenetic nature. The same process is believed to be responsible for the salinity and alkalinity of the Vertisols of part of terrace II.

10.2.2. Vertisols Under the prevailing environmental conditions of central Sudan - with alternating wet and dry seasons - the formation of Vertisols merely requires deposition of 233 sediments high in clay content (30% or more). As shown in Chapter 3.1.1 the clay fraction of the solid sediment load of the river system dominantly consists of montmorillonitic minerals. Upon deposition these entail the characteristic expansion and contraction of the soil mass - with wide deep cracks as specified - as well as its churning. Vertisols can thus be very recent here, soil profile 71A is an example of such a soil. This observation disproves Buringh's (1970) contention that 'real Vertisols are not recent'. Apart from the physical process of soil mixing, or pedoturbation, these recent Vertisols may also be subject to salinization as well as alkalization. Because of their depressional setting stagnating flood water is believed to induce these processes, especially under arid conditions, as argued by the author in Chapter 8.2. The total extent of these recent clay soils is a minor fraction of that of older Vertisols in the Sudan, which widely occur in areas like the Gezira, Kenana, and Dinder clay plains. These Vertisols of terrace II may have many properties in common with the recent ones, as reflected in the classification (figure 65). We were nevertheless able to establish substantial differences between these Vertisols of different geomorphic surfaces, and to ascribe these to soil formation. Genetic studies of Vertisols are often concerned with the explanation of the presence of high amounts of swelling clays, their usually dark colour, as well as with an understanding of such typical effects as gilgai, churning, and cracking. These aspects were ably summarized by Dudal (1965). Until recently the conclusion has been drawn that Vertisols are in general but little developed soils (Duchaufour, 1970), this apparently in view of the absence of any horizon differentiation. Our investigations show that the terrace II Vertisols are the result of a considerable degree of soil formation. The process primarily produced: a. depletion of less stable minerals from the sand and in particular from the silt fraction, b. increase in the montmorillonite content (by some 15 to 25% absolute), and concomittant less kaolinite, of the clay fraction, c. redistributed calcium carbonates and easier soluble salts, and d. illuviated clay, obscured by soil mixing. The minerals, wholly or partly, lost from the sand fraction include basalt fragments, volcanic glass, potassium feldspars, labradorite, and the pyroxenes (Chapter 6.2.5.1). The indications are that this loss did not result in a decrease in the sand percentage, owing to a higher loss of silt -f- clay components, the latter especially constituting the minerals high in Zn, Co, Mn, Fe, and Ti. We concluded that this happened in older soils in general, not in Vertisols specifically (Chapter 6.2.2.2.). Further, it seems reasonable to ascribe the increase in montmorillonite percentage in the clay fraction to an absolute increase in this mineral, being derived of the silt 234 fraction. This is borne out by our contention in Chapter 6.2.1, that about a quarter of the present clay fraction is derived by weathering of silt particles. This phenomenon might be the result of new formation-of montmorillonite from weathering products of plagioclase and ferromagnesian minerals, as concluded in Chapter 6.2.5.2. In view of cur findings on the cation-exchange capacity of the soils concerned (Chapter 8.2.2.1) and with regard to the very sizable increase in montmorillonite, however, we believe that the increase of this mineral in the clay fraction is dominantly due to disintegration of silt-size montmorillonite. This could be the result of mild chemical weathering. As regards the content of CaCO3 and easier soluble salts considerable variations occur in the Vertisols of terrace II. The main trend is the decrease in total content with increasing average annual rainfall. In the southern part of the Blue Nile plains Vertisols are noncalcareous. In depressions, however, they may still be calcareous. This is considered to be due to the fact that more CaCO3 was originally deposited here by the river from stagnant floodwater. We have given a similar explanation for such depressions when they are saline, and stressed the relic character of salinity as well as related alkalinity (Chapter 8.2.3). One of the forms in which CaCO3 occurs is as black or dark grey coated and im- pregnated nodules, which usually are numerous and of small size (Chapter 6.2.3). Blokhuis et al. (1968) conclude that the accretion of this hard, discrete type of carbonate and the formation of the (mostly) manganese cutans are due to 'processes active in the lower part of the (Vertisol) profile, below the present churning zone' and that their presence in the topsoil is 'inherited by transport within the profile, due to churning'. With this they stress the significance of the churning process of Vertisols (both direct and indirect) to their formation and distribution. However, our observations show that the occurrence of these nodules (or calcitic glaebules) is not restricted to Vertisols. They were also found in substantial amounts in the Aridisols of the same terrace along the Nile (soils 57 and 67, and 43,44). This implies that churning is not essential to formation and notably to distribution of this specific form of carbonate. The nodules were encountered exclusively at the surface of, or in, terrace II soils. The iron-manganese coating probably represents a desert-varnish, in this case the formation of this patina at or near the surface is essential. This was suggested by Worrall (1957), and finds support in our observation that the nodules decrease in number from the soil surface down. In this context it is relevant to note that gravel at the surface of terrace II soils, unlike that of older soils, did not acquire a varnish (cf. the introductory statements of sample areas 5, 6, and 7 in Chapter 5.2 as well as the section below on Aridisols). Regarding the salinity of the Vertisols, one question remains, covering both clear soil genetic and agricultural implications. Apart from salts originally supplied in 235 solution by the Blue Nile, to what extent has weathering of Na-containing minerals contributed to soil salinization ? We intend to deal with this problem in detail in our next publication. Preliminary investigations indicate that sodium supplied from this source to the soil solution is negligible. For example, the content of total sodium, as measured by atomic absorption spectrometry, in nonsaline surface soil samples of soils 71A (recent Vertisol) and 23 (terrace II Vertisol with about 800 mm rainfall) is about equal. After allowing for both some exchangeable and soluble sodium, sodium in the minerals of these samples amounts to 5522 and 5487 ppm respectively. The minerals that contain sodium in these soils are: albite, labradorite, zeolite, and basalt fragments. Micromorphological observations revealed the presence of minor amounts of illuviated clay (figure 61). This connects the formation of Vertisols with that of the associated Alfisols (cf. section below).

10.2.3. Alfisols Soils of the suborder Ustalfs are widely represented in the dry fersiallitic soil belt of tropical Africa (Sys, 1969). In the Sudan their genesis appears to be closely related to that of the higher rainfall Vertisols (sample areas 1, 2, and 3). The clay fraction of the former sous also consists dominantly of montmorillonitic minerals. This, together with a similar mineral composition of the sandfraction, testifies to a common provenance of the soils of both orders. Different environments of deposition of the original sediments (levee versus backswamp) resulted in soils with a varying clay percentage. It is, in particlar, this difference in the content of swelling clays which led to the differentiation of Alfisols and Vertisols. Two physical processes, both intimately connected with the clay fraction, are involved: clay translocation and clay pedoturbation, or in micromorphological terms plasma concentration and its separation. Each of these processes dominates the formation of one of the soil orders. The most plausible concept of clay translocation, according to McKeague and Arnaud (1969), is that of a simple physical process. Wetting of dry soil by rain will result in the suspension of some soil material which will move down cracks and channels. Pedoturbation was defined as the physical process of soil mixing (Hole, 1961). Micromorphological observations support our view of related soil formation. Whereas clay illuviation led to the horizon differentiation of Alfisols, some argillipedoturbation was shown to affect these soils too. This effect is demonstrated by the deformation of cutans (figure 60). On the other hand: while pronounced argillipedoturbation led to the homogenous Vertisols, the translocation of minor amounts of clay was shown too (figure 61). As far as depletion of less stable minerals and increase in montmorillonite content is 236 concerned, no major differences could be established between the Alfisols and associated Vertisols. The indications are that more iron oxide, released by weathering, is stabilized in the Alfisols. In view of the absence of gibbsite, with a molar SiO2/Al2O3 ratio of the clays of 4.2, and a silt/clay ratio in excess of 0.15, the Alfisols are not considered to be ferrallitic. According to the French concepts on soil formation they are Ferruginous Tropical soils (Duchaufour, 1968 and 1970).

10.2.4. Aridisols. Except for the most recent terrace, Argids dominate the arid part of the older terraces, mainly occurring along the Nile. Haplargids are characteristic of terrace II and Paleargids of terraces III and IV (Chapter 9.6). The presence of Calciorthids on terrace II (soil 57) might be associated with slight soil truncation and enhanced carbonate accumulation, as has been shown by Gile et al. (1969) along the border of the Rio Grande Valley, U.S.A. Comparison of the Haplargids and Paleargids reveals the following major soil genetic trends: 1. weathering' of increasingly stable sand minerals; 2. decrease in montmorillonitic silt- and claysize minerals; 3. increased expression of clay illuviation; 4. development of desert varnish; 5. carbonate enrichment and loss; 6. influx of chloride salts. The Paleargids of terrace III have been depleted of potassium feldspar and hornblende with some augite, as compared to the Haplargids. In terrace IV the loss of heavy minerals not only includes hornblende, but epidote too. Little change was observed relative to the light sand fraction, which almost entirely consists of quartz (Chapter 6.2.5.1). The decrease in the content of montmorillonitic minerals is considered to be due not only to some increase in kaolinite, resulting from the weathering of primary minerals (K-feldspars, epidote), but mainly to the removal of montmorillonitic constituents from the soil (Chapter 6.2.5.2). The decrease in the silt component of the montmorillonites is inferred from our investigations in the cation-exchange capacity (Chapter 8.2.2.1). The above reveals that the clay mineralogy of the toposequence of soils of terrace I to IV is primarily determined by one process: montmorillonite disintegration. First, from silt to claysize grains (enriching the clay fraction of terrace II), and subsequently from claysize to elemental components removed from the soil. The above also demonstrates that genetic studies on the clay mineral composition of 237 soils should include the non-clay fraction, a point often overlooked in clay mineralogy. The expression of the argillic horizon of the Argids in the form of cutans on pedsurfaces and as coatings on pebbles is more obvious in older terraces. However, carbonates and dispersed organic matter tend to obscure the internal orientation of cutans (Chapter 7.2.2). Gile and Grossman (1968), in a study on the morphology of the argillic horizon in Aridisols of southern New Mexico, noted that clay skins may easily be disturbed physically, for example by authigenic carbonate accumulation. Buol (1965) concludes that the process by which textural B horizons in arid soils are developed is not clearly defined. He believes that both in situ formation and illuviation of clay contribute to its formation. Desert varnish was observed to cover gravel at the surface of the Paleargids of terrace III. As stated above (paragraph on Vertisols), the Haplargids of terrace II have gravel without, but CaCO3 nodules apparently with, a varnish. The production of desert varnish in arid regions testifies that such a varnish on carbonate nodules does not necessarily have to develop under reducing conditions in the subsoil. Under Eh-pH conditions likely to occur in desert soils some ferric iron will always be in solution, although Mn is much more soluble. This may explain the relatively high Mn content of the varnish, as revealed by its dark colour and the determinations of the chemical composition of carbonate nodules by Kerpen et al. (1960) and Blokhuis et al. (1968). An approximate Eh-pH diagram representing conditions under which varnish occurs and is believed to be forming in the desert was compiled by Hooke et al. (1969). The latter studied desert varnish samples of California by means of the electron microprobe, and found that especially the outer main layer of the varnish is rich in iron and manganese. In view of the often extremely high CaCO3 concentrations in subsoils of both Haplargids and Paleargids it is concluded that vertical translocation of carbonate is not the only process involved in the genesis of the caliche or ca-horizons. Unless much of the carbonates are of a geogenic nature, which is unlikely especially for (originally) well-drained soils or terrace II (see Chapter 6.2.3), considerable quantities of carbonates presumably resulted from lateral migration. The petrocalcic horizon characteristic of the Paleargids of terrace III generally does not have a laminar horizon on top. The horizon appears to be similar to what was termed a K-horizon by Gile et al. (1966). Its content of carbonates is in excess of 35%, Flach et al. (1969) observed that most petrocalcic horizons contain more than 40% CaCO3. The Paleargid of terrace IV (soil 59), which contains only minor quantities of carbonate, was apparently not influenced by addition from outside. With relation to soil salinity the Haplargids as well as the younger Entisols are characterized by soda salinity, the Paleargids typically show chloride salinization. 238

As argued in Chapter 8.2.3 the former type of salinity represents a relic feature, the soda salinity is closely related to the salts of the river system. The calcium- and sodium-chlorides are presumably due to influx from neighbouring pediment surfaces. References

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De auteur werd in 1939 te Enschede geboren. Hij doorliep het Gymnasium B van het Gemeentelijk Lyceum ter plaatse en behaalde in 1957 het einddiploma. Aan de Rijksuniversiteit te Utrecht studeerde hij fysische geografie en speciali- seerde zich in de bodemkunde, hij legde in 1964 het doctoraalexamen af. Gedurende ruim drie jaar (1965-'68) werkte hij als bodemkundige niée aan een trainings- en karteringsproject van de F.A.O. in Wad Medani, Sudan. Onderzoek opgezet in Sudan werd vervolgens aan het Instituut voor Bodemkunde van zijn alma mater voortgezet en resulteerde in deze dissertatie. Deze studieperiode werd enige maanden (1968/69) onderbroken voor een bodem- onderzoek in Noord-Afghanistan (Kunduz-Khanabad Irrigation Feasibility Study). De schrijver is momenteel als wetenschappelijk medewerker verbonden aan het International Soil Museum, Utrecht.

ERRATUM : Stelling 2 lees: De verwering van illiet- tot montmorilloniet-kleien in Nijldelta afzettingen wordt door Hamdi ten onrechte geacht te zijn aangetoond.