Alkali investigations in the Sudan

Abstract

(1) The cotton growing area in the Gezira consists of a heavy clay soil, the proportion of clay being about 50–60 per cent. in the upper layers with a maximum at about the 4th foot.(2) The water soluble salts amount to about 0·2 per cent. The proportion is highest at about the 3rd to 5th foot. The alkalinity (pH) is highest at the 2nd foot.(3) In the first 2 feet, the salts consist mainly of sodium carbonate and the third and fourth of sodium sulphate.(4) The irrigation (Blue Nile) water is of excellent quality as judged by its natural chemical composition. The concentrated water, however, contains a very high proportion of alkali salts. It is estimated that a season of normal irrigation would cause an increase of 0·01 per cent. in the alkali content of the first 4 feet of soil.(5) The sodium salts can readily act on the clay and the sodium clay so formed hydrolyses with the formation of sodium carbonate.(6) Samples taken at the same time from good and bad plots in the same area show a strong correlation between salt content and cropyielding power. There is also a correlation between pH and fertility.(7) In the same season and in the same area, virgin (i.e. unirrigated) plots give a higher yield than those which have been previously under the same system of cultivation.