Preserving soil quality under irrigation in the Senegal River Valley

Abstract

Soil quality under irrigation in the Senegal River Valley may become affected by salinization (Delta) and alkalinization (Middle Valley) processes. The salinity status of 158 irrigated rice fields in the Delta was measured with an electromagnetic conductivity meter. Double-cropped fields (two rice crops per year on the same field) were least saline (average horizontal reading: 0.6 dS/m), followed by single-cropped drained fields (1.6 dS/m), single-cropped non-drained fields (2.5 dS/m), non-cropped sites outside irrigation schemes (4.7 dS/m) and abandoned fields (5.7 dS/m). Results illustrated that when cultivating rice in the delta, the ponded water on the soil surface blocks capillary rise of salt from the water table. In the valley, the difference in the total amount of carbonates in 1:50 soil extracts between cultivated and non-cultivated sites was used as an indicator for alkalinization risk in 27 irrigation schemes. Highest rates of carbonate accumulation (0.65 meq HCO3-/(kg soil) per cropping season) and, therefore, greatest soil degradation risk were observed in schemes without drainage, compared to 0.10 meq HCO3-/(kg soil) per cropping season in schemes with good irrigation and drainage facilities. Plot and scheme level recommendations that may preserve soil quality under irrigation are presented.