Lëiiiitfak • ' SU 1313.01 the Soils and Geography of the Boliland Region of Sierra Leone
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lëiiiitfAK • ' SU 1313.01 The Soils and Geography of the Boliland Region of Sierra Leone BY A. R. STOBBS Vegetation by Dr. T. S. Bakshi 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. Published by the Government of Sierra Leone 1963 To be purchased through the Government Book Shop, Freetown, Sierra Leone, and the Crown Agents for Oversea Governments and Administra- tions, 4 Millbank, London, SWi Printed by Eyre and Spottiswoode Limited, Her Majesty's Printers, at The Thanet Press, Margate, England CONTENTS FOREWORD 4 PART I : GEOGRAPHICAL BACKGROUND : INTRODUCTION . 5 GEOLOGY . 5 TOPOGRAPHY . 8 GEOMORPHOLOGY . 8 CLIMATE . 9 VEGETATION . 9 HUMAN AND ECONOMIC GEOGRAPHY II THE SOIL MAP . II BOLI SOIL MAPS 13 PART II : SOILS : PEDOLOGICAL CLASSIFICATION 14 SOILS OF THE UPLANDS : PENEPLAIN DRIFTS 15 RED BROWN CONCRETIONARY DRIFTS . 15 YELLOW BROWN CONCRETIONARY DRIFTS 16 COLLUVIAL DRIFTS. .. 17 SEDENTARY SOILS . l8 SOILS OF THE BOTTOMLANDS : SOILS DEVELOPED OVER TERRACE DEPOSITS 18 SOILS OF THE LEVEES . 21 SOILS DEVELOPED OVER SLOUGH DEPOSITS . 22 SOILS OVER OLD RIVER CHANNEL DEPOSITS . 27 SOILS DEVELOPED IN DRAINAGE GROOVES; MIXED BOTTOM SOILS, AND RECENT ALLUVIUM . 27 NUTRIENT STATUS OF BOLILAND SOILS . 27 LAND CLASSIFICATION 28 PART III : VEGETATION : ANADELPHIA/RHYTACHNE ASSOCIATION .. .. .. 31 CHASMOPODIUM/NAUCLEA ASSOCIATION .. .. .. .. 31 LOPHIRA/CHASMOPODIUM ASSOCIATION .. .. .. .. 32 RIVERAIN FOREST AND " SACRED BUSH " 32 PART IV : LAND USE : SIZE OF FARMS AND MANPOWER . • • 35 SYSTEM OF FARMING . 35 CROPS 35 PART V : RECOMMENDATIONS 38 REFERENCES 40 APPENDIX : ANALYTICAL DATA 41 FOREWORD BY DR. H. GREENE Adviser on Tropical Soils, Department of Technical Co-operation THE reconnaissance soil survey of the bolilands of Sierra content just to record the occurrence of different kinds of Leone was begun by J. C. Chisnall and after his lamented soil but are concerned to relate their field observations to death was resumed and completed by A, R. Stobbs. Both potential growth of crops. This involves an interpretation men were seconded from the Colonial Office Pool of Soil of the field observations and adds to the practical value of Surveyors. a soil map such as is presented here. The reader should Stobbs has based his soil map in part on recognised remember, however, that potential growth of crops depends sub-divisions of the land surface (geomorphological units), on changing techniques, and that practical recom- and in part on the system of soil classification that was being mendations depend on a changing economic situation. elaborated in Ghana by the late C. F. Charter, O.B.E. This The soil surveyor cannot do more than offer an assessment system has seemed well suited to the soils of West Africa based on such information as is available to him at the time and its use in this report facilitates comparison with the of writing. His tentative conclusions need to be tested in numerous and valuable soil surveys carried out in Ghana. the field: it is a valuable feature of a soil map that it shows It should be mentioned that soil boundaries recognised by the most useful locations for field experiments. Stobbs in the field are likely to remain unchanged when The accompanying soil maps prepared and printed by Charter's system of soil classification is superseded by the Directorate of Overseas Surveys are part of the others, as is inevitable since all such systems need to be Technical Assistance provided to Sierra Leone by the revised as new information comes in, and as new ideas gain United Kingdom Government through the Department of a hearing. At the present time many soil surveyors are not Technical Co-operation. PART I GEOGRAPHICAL BACKGROUND Introduction the analytical programme to a selected range of Bottomland " Bolilands " is the generic term for the seasonally soils. swampy region adjacent to the Rokel and Mabole rivers Although the region has an annual rainfall of up to occurring over the Rokel River geological series. A " boli " 130 inches most of which arrives in a period of six months, is an individual swamp. it was possible to continue with a modified programme The survey of the bolilands was carried out at the in a rainy season without loss of too many days, by leaving request of the Department of Agriculture, Sierra Leone, to this period work on the upland soils much of which which was already endeavouring to develop a mechanical could be carried out from the Landrover or within easy rice cultivation scheme there. The purpose of the survey reach of shelter. was to locate, delineate, and describe suitable areas for rice At all times bicycles were a great help in keeping up in order to provide a sound basis for further progress. the speed of the survey. The scope of the survey was extended to encompass an Thanks for unfailing help with administrative, supply assessment of the whole region in which these swamp and personnel problems are due to the Agricultural Officer, areas occur, so as to allow a co-ordinated approach to its Makeni, in whose circle most of the survey area lay; also agricultural problems. to the staff of the West African Rice Research Station, Work was begun by Mr. J. C. Chisnall in April, 1957, Rokupr, whose interest and help in providing space and under Colonial Development and Welfare Scheme 3094, equipment for carrying out a number of mechanical and after a lapse consequent upon his unfortunate death analyses and pH tests locally was most useful; and to the in January, 1958, was resumed by Mr. A. R. Stobbs in Agricultural Chemist and other staff at the Departmental December, 1958, under Colonial Development and Headquarters at N'jala; and to all those others, adminis- Welfare Scheme 4019, field work being completed in trators, foresters and geologists, whose interest, advice and June, i960. local knowledge helped the survey. The survey was done at two levels. Firstly a recon- naissance survey of nearly 1,000 square miles was carried out to produce an overall assessment of soil resources. The Geology observations were mapped on R.A.F. 1951 photography The area surveyed overlies the Rokel river series which (1 : 30,000), later transferred to Aero Surveys 1958 photo- extend southwards from the Guinea frontier for about graphy (1 : 40,000) and are now presented on a scale of 140 miles, in a strip about 25 miles wide (Poliert, 1951). 1 : 50,000. Then, additional to this, detailed surveys were Two elements of this geological series have influenced the carried out of separate " bolis " with a view to defining soils with which the survey was concerned:— areas on which further research can be undertaken to 1. Sandstones and sandy shales of the series give rise relate agronomic data to the soil pattern. Some such to low, gently undulating uplands, and provide parent information can already be obtained from comparison of material for the upland soils. In the oldest soils formed the soil survey with the crop records from ploughing sites. on relict peneplain drifts covering these upland summits, In the reconnaissance survey widely spaced traverses biotic activity is slowly introducing this parent material were selected to confirm criteria which could be used to into the profiles. The same parent material is also found interpret the soil pattern from the air photographs, with present as colluvium in many of the topsoils of the subsequent additional traverses where necessary to confirm " inland " boüs. this interpretation. 2. Mudstones and shales of the series underlie most The detailed surveys were carried out along a regular of the " inland " bolis. These are now weathering in situ pattern of traverses laid down 5 chains apart and sampled into compact puddled clay subsoils, thereby exerting an at 5 chain intervals. important influence upon the water regime of these bolis. Soil pits were normally dug to a depth of 6 feet, with The rocks are rarely exposed except in the beds of the a few selected pits deepened as necessary to acquire major river courses, and more rarely on steeper hillsides pedogenetical data. Pits were sited to give a standard approximating locally to low scarp formations. As they description for each series, and demarcate the variation are inherently much poorer in minerals than the crystalline permissible in its characteristics. Other sampling and rocks on either side of the bolilands, the soils derived from identification was done by means of chisel holes (circular rocks of the Rokel river series are low in nutrient status inspection holes about 9 inches across and 36 inches deep) and have attracted less local settlement than the more and augers. fertile soils to the east and west. This inherent poverty Soil samples were analysed by Mr. C. L. Bascomb at must be considered an obstacle to development until means Rothamsted Experimental Station. Circumstances limited of overcoming it have been reliably demonstrated. MAP TO SHOW AREA OF SOIL SURVEY AND LOCATION OF DETAILED SOIL MAPS 1 BATKANU BOLI 4 MADINA-TABAI BOLI 2 ROCHIN BOLI 5 ROMANKNE BOLI 3 BANTORO BOLI 6 KONTOBE BOLI REFERENCE Inter-Territorial Boundary + • +•+• + * + Roads Railways D.O.S. (Mile) 362 Drawn and photographed by Directorate of Overseas Surveys 1963. LANDFORM UNIT Slough Old Slough Terrace remnant Flanking slopes Upland peneplain levee surface Résèquent stream developing as a result of base level adjustment.