Soil improvement in the Sudan Gezira

Abstract

After reviewing the possibility of soil deterioration in the Gezira and its dependence on change in the exchangeable bases in the soil we have given an account of field trials with soil improvers. Increased penetration of water, improved supply of nitrogen and higher yields were obtained, but the effects were not lasting. A part of the initial response is ascribed to the flocculating action of salts formed near the soil surface by interaction between soil and soil improver. With continued watering these salts are washed from the surface soil, but the applications of soil improver were insufficiently heavy and the leaching insufficiently thorough to effect any deep-seated change in the soil. Drainage experiments in which effluents were measured and analysed showed small changes in the right direction, but their main outcome has been strikingly to confirm the view that lateral movement of water through Gezira soil is so slight as to promise small hope of achieving any large-scale improvements by means of subsoil drainage. Analyses of saltbush have been made and showed that although these plants remove much sodium from the soil they can hardly be regarded as a practicable agent in soil improvement, since the amount of sodium in the soil is comparatively large. On the other hand, it appears that by including saltbush in rotation and by removing the crop from the land it is possible to guarantee that no progressive deterioration will occur in consequence of accumulation of sodium introduced in the irrigation water. Further analyses of irrigation water were made in this connexion and substantially confirmed Beam's earlier data. We do not yet know whether, under normal irrigation, the general tendency is for an accumulation of sodium or whether, on the contrary, salts are slowly being washed from the soil column (Greene & Peto, 1934). Until the nature and extent of chemical changes in the soil are accurately assessed it is impossible to say what annual expenditure on soil improvers, drainage, growing and disposal of saltbush is needed for maintenance of soil fertility. Further laboratory work should be directed to the detailed study of base exchange in Gezira soil and to observation of the physical characters on which permeability depends while, in the field, further experiments should be made in the use of soil improvers and drainage; information should also be sought as to the most convenient and least expensive way of including saltbush in the rotations best suited to different parts of the Gezira and as to the economic disposal of saltbush ash.