PRESERVING SOIL QUALITY UNDER IRRIGATION IN THE SENEGAL RIVER VALLEY

Soil quality under irrigation in the Senegal River Valley may become a ffected by salinization (Delta) and alkalinization (Middle Valley) pro cesses. The salinity status of 158 irrigated rice fields in the Delta was measured with an electromagnetic conductivity meter (EM38). Double -cropped fields (two rice crops per year on the same field) were least saline (average horizontal EM38 reading: 0.6 dS m(-1)), followed by s ingle-cropped drained fields (1.6 dS m(-1)), single-cropped non-draine d fields (2.5 dS m(-1)), non-cropped sites outside irrigation schemes (4.7 dS m(-1)) and abandoned fields (5.7 dS m(-1)). Results illustrate d that when cultivating rice in the Delta, the ponded water on the soi l surface blocks capillary rise of salt from the water table. In the V alley, the difference in the total amount of carbonates in 1:50 soil e xtracts between cultivated and non-cultivated sites was used as an ind icator for alkalinization risk in 27 irrigation schemes. Highest rates of carbonate accumulation (0.65 meg HCO3- (kg soil)(-1) cropping seas on(-1)) and, therefore, greatest soil degradation risk were observed i n schemes without drainage, compared to 0.10 meg HCO3- (kg soil)(-1) c ropping season(-1) in schemes with good irrigation and drainage facili ties. Plot and scheme level recommendations that may pre serve soil qu ality under irrigation are presented.