SOIL TRANSITIONS IN CENTRAL EAST BOTSWANA (AFRICA) W. SIDERIUS This thesis will also be published as a publication of the International Institute for Aerial Survey and Earth Sciences (ITC) — International Soil Museum. Soil Transitions in Central East Botswana (Africa) Printed by Krips Repro Meppel Scanned from original by ISRIC - World Soil Information, as ICSU World Data Centre for Soils. The purpose is to make a safe depository for endangered documents and to make the accrued information available for consultation, following Fair Use Guidelines. Every effort is taken to respect Copyright of the materials within the archives where the identification of the Copyright holder is clear and, where feasible, to contact the originators. For questions please contact [email protected] indicating the item reference number concerned. Soil Transitions in Central East Botswana (Africa) 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. Sj. Groenman, volgens besluit van het College van Dekanen in het open- baar te verdedigen op woensdag 2 mei 1973 des namiddags te 2.30 uur door Woutherus Siderius geboren op 13 augustus 1940 te Baarn PROMOTOR: PROF. DR. IR. F. A. VAN BAREN ACKNOWLEDGEMENTS This study was conceived thanks to the co-operation of many who directly or indirectly were associated with its development. I am, Prof. Dr. F. A. van Baren, in particular indebted to you for your scientific advice and organizational endowments that enabled me to follow up my work in Botswana at the Soils Institute at Utrecht, and to use the facilities of your Department. Your concurrent effort and personal concern made it possible to bring this study to its conclusion. I owe sincere thanks to you, Mr. W. L. P. J. Mouthaan, B. Sc, not only for your valuable suggestions in the field of the sand mineralogy, but moreover, for your philosophical insight relative to soil genesis and your guiding support. It is a pleasure to thank J. G. Pike, M. Se, FAO project manager, for his interest and help towards the accomplishment of this investigation, and also my colleagues of the FAO integrated survey team, of which I owe special thanks to H. J. van Rensburg, M. Sc. for valuable suggestions concerning the description and recording of the vegetation. I am grateful to the UNDP Office in Gaborone, Botswana, for their assistance in the realization of this study and to the Department of Surveys and Lands for permission to use cartographic material. My gratitude is further extended to Dr. R. Dudal and A. J. Smyth, Esq. of the FAO Land and Water Development Division, for their comprehension comparative to the subject, and to Dr. R. F. Loxton, consultant, for his constructive ideas made during his sojourns in Botswana. Professor Dr. J. I. S. Zonneveld and the Staff of the Department of Physical Geography (Utrecht State University), Prof. Dr. D. J. Douglas and Prof. Dr. J. F. M. de Raaf, I acknowledge the profound scientific teaching received during my University education. I express gratitude for the valuable critisism of Dr. L. Bal, D. Greutzberg, M. Sc, Dr. J. J. Reynders and N. M. de Rooij, M. Sc, on selective chapters of the manuscript. The help of G. A. E. M. Hermans, M. Sc. and Dr. P. Maaskant in the determination of the rock specimen is appreciated, and the latter is thanked for his part in the electron microprobe determina- tion. Discussions with j. i>. Al, B. Se, J. H. V. van Baren, M. Se, J. A. Boertna, M. Se. and S. P. Tjallingii, M. Se, prevented imperfections. I am grateful to the following persons for their expertise help, Mr. J. Drenth, Mr. H. E. Renaud, Mr. J. D. Schreiber, Mr. D. de Vries, Mr. J. van der Wal and Mrs. I. W. J. Wolters. Thanks are also due to Mr. F. Henzen and Mr. I. Santoe for their assistance in drawing most of the diagrams and the reproduction of many illustra- tions. I am grateful to A. Muller, M. Se, and his Staffat the Soils Laboratory of the Royal Tropical Institute, Amsterdam, for the able way in which a number of analyses were carried out, and for the facilities made available to me. The pleasant company of Mr. L. Lebodi on many field trips is memorized with great appreciation. I thank Miss Weia S. Smid for her efficient typing of the manuscript. My grateful thanks go to Madame V. Balikci-Ossent and to my wife, for the correction of the English. This research was for the major part financed by the Netherlands Foun- dation for the Advancement of Tropical Research (WOTRO), The Hague. Its publication was supported by a grant from the Netherlands Ministry of Education and Sciences. Sincere appreciation is bestowed to the Ministry of Agriculture of the Government of Boswana for their authorization to undertake this investi- gation and permission for its publication. CONTENTS INTRODUCTION CHAPTER I ENVIRONMENTAL CHARACTERISTICS 15 1.1. Climate 15 1.1.1. Atmospheric climate 16 1.1.1.1. Precipitation 16 1.1.1.2. Temperature 17 1.1.1.3. Evaporation 18 1.1.2. Soil climate 19 1.1.2.1. Soil temperature ,20 1.1.2.2. Soil moisture 23 1.1.3. Paleoclimate 24 1.2. Geology 28 1.2.1. General 28 1.2.2. Parent material 30 1.3. Geomorphology 39 1.3.1. Topography 39 1.3.2. River systems 40 1.3.3. Landscapes 43 1.4. Vegetation 47 1.4.1. Natural vegetation ,47 1.4.2. Human induced vegetation 51 1.4.3. Vegetation patterns 52 CHAPTER II TYPICAL SOILS 55 11.1. Location of sample sites 55 11.2. Field observations 56 11.3. Soil properties 84 11.3.1. Bulk density 84 11.3.2. Soil reaction • 84 11.3.3. Exchange capacity 85 CHAPTER III SOIL COMPOSITION 89 III. 1. Fragments coarser than 2 mm 89 111.2. Grain size distribution in the fine earth 89 111.2.1. Sand 90 111.2.2. Silt 91 111.2.3. Clay 91 111.3. Elemental composition of the fine earth 95 111.3.1. Major elements 95 111.3.2. Trace elements 100 7 111.4. Mineralogy of special constituents 103 111.4.1. Mineralogy of the sand fraction 103 HI.4.1.1. Heavy minerals 104 III.4.1.2. Light minerals 109 111.4.2. Mineralogy of the clay fraction 112 111.5. Organic matter 116 III6. Salinity 118 CHAPTER IV ARRANGEMENT OF SOIL CONSTITUENTS 121 I V.l. Micromorphological observations 121 IV.2. Evaluation of the field data 140 CHAPTER V ROCK WEATHERING AND SOIL FORMATION 147 V.l. Rock weathering 147 V.2. Soil formation 152 CHAPTER VI SOIL CLASSIFICATION AND LAND USE 167 VI.1. Soil classification 167 VI.2. Land use 171 APPENDIX I METHODS 175 1.1. Survey methods 175 1.2. Laboratory methods 175 APPENDIX II SOIL PROFILE DESCRIPTIONS AND LABORATORY DATA 182 SUMMARY 245 SAMENVATTING 247 REFERENCES 249 CURRICULUM VIT AE 260 UPPER CATCHMENT OF THE MAHALAPSHWE AND BONWAPITSE RIVERS * North iliiilllip ridge i^. J. Location map of the investigated area INTRODUCTION The importance of natural resources for Botswana, as expressed by this country's export data, but even more as the basis for the life of more than 95% of its population, is well understood by the present Government. Inventarization of these resources has proceeded rapidly since the coun- try's independence in October 1966. Within this programme, integrated surveys were initiated in Northern and Eastern Botswana from 1968 to 1972 as part of a FAO/UNDP-SF project. One aspect of these were a number of soil surveys carried out by the author on different levels of intensity. Although much data was gathered, advanced soil research on genetic and related fields of interest could not be conducted within the current project programme. This was felt to be necessary, however, in order to come to a better understanding of the soil's properties and characteristics. For that purpose, an area was selected in Central East Botswana (fig. 1). The area located just north of the Tropic of Capricorn at 26° 18'—26° 50' E and 22°45'—23° 18' S comprises the upper catchment area of two tribu- taries of the Limpopo river, namely the Mahalapshwe and the Bonwapitse. In these catchment areas a number of representative soils were selected after the area was surveyed on semi-detailed level. Differentiating environmental characteristics played an important role in locating the sample sites. A total of 154 soil samples was collected and they form the raw material for this study, together with four samples from streambeds and ten rock specimens. Under the present semi-arid climate, a number of soils or soil character- istics seem allochthone and questions about their present development and potential are better answered by looking into the past. The factors influencing the pedosphere are dealt with in Chapter I. Decisive informa- tion concerns particularly the sections on climate and geology. The bulk of the soil data is presented in Chapter II, field descriptions and laboratory data provide the necessary background for further detailed studies. The results hereof, with special reference to the mineral constit- uents of the soils which account for more than 99% of their assemblage, is the subject of Chapter III. As a logical development, the arrangement of these soil constituents are then discussed and described in Chapter IV. Together with the material from the foregoing chapters, main conclusions about rock weathering and 13 soil formation are presented in Chapter V. The last chapter deals with some aspects of the soil classification, also a short assessment is given on the agricultural use of the investigated soils.