Explaining yield gaps on farmer-identified degraded and non-degraded soils in a Sahelian irrigated rice scheme

Farmers in the Foum Gleita irrigation scheme in southern central Mauritania experienced declining rice yields, and within a decade after its establishment 12% of the scheme's land had been abandoned. Actual rice yields (≤4.0 t ha−1) are low in comparison with potential yield (ca. 8 t ha−1) and with yields elsewhere in the Sahel (4–6 t ha−1). Farmers related the productivity problems to salt efflorescences on the soil surface. Rice yields on the ‘upper and middle slope’ soils were lower (3.4 t ha−1) than the yields on soils further down the slope (> 4.2 t ha−1). Farmers classified the ‘upper and middle slope’ soils as degraded, but following the USDA classification the soils could not be classified saline or sodic. Low yields on the ‘degraded’ soils were related to co-limitation of nitrogen (N) and phosphorus (P), which was due to low soil N supply (ca. 18 kg ha−1), low soil P supply (ca. 8 kg ha−1), occasional low N fertilizer doses (35 kg N ha−1) in combination with low N fertilizer recovery efficiency (0.3 kg kg−1), or non-application of P fertilizer. On the ‘non-degraded’ soils, soil P supply (ca. 16 kg ha−1) was higher and N deficiency prevailed despite a higher soil N supply (ca. 32 kg ha−1) and a higher N fertilizer recovery efficiency (0.4 kg kg−1). Higher contents of carbonate salts in the ‘degraded’ soils increase soil pH (> 7.5) and are, therefore, likely to contribute to low soil P supply and low N fertilizer recovery efficiency.