Sulfur Requirements of Cowpea and Implications for Production in the Tropics

Low S concentrations in rainwater and depleted organic matter reserves of soils are associated with S deficiencies in the seasonally dry West African savanna. The purpose of this study was to provide information on the internal and external S requirements of cowpea, a promising grain legume for tropical Africa. Three cultivars of cowpea [Vigna unguiculata (L.) Walp.] were grown in pots in which seven levels of SO₄-S were maintained in the soil solution. About 7 pprn S was required in the soil solution for 95% of maximum grain production for cultivars ‘TVu 76-2E’ and ‘TVu 201-ID’. Cultivar ‘Sitao Pole’ required about 2 ppm S in solution. The S contents of uppermost, fully expanded leaves at early flowering associated with 95% of maximum grain yield were 0.32% for Sitao Pole and 0.50% and 0.65% for cultivars, TVu 201-ID and TVu 76-2E respectively. The corresponding S content in the petioles was about 0.08%, 0.39%, and 0.42% respectively. Older leaves and leaf petioles accumulated S if it was supplied in excess of requirements. The S percentage of grain was 0.26% when yield was 95% of maximum. Sitao Pole accumulated S to a maximum of 0.36% when SO₄-S was supplied at 45 ppm in solution. About 30% of the plant S was in the grain of plants adequately supplied with S and about 50% was in the grain when the S supply was moderately deficient. The N/S ratio of upper trifoliate leaves associated with 95% of maximum grain yield was about 10 at first bloom. At near maximum yield, cultivar TVu 201-ID contained about 80 mg S per plant in the grain. Assuming (65,000 plants/ha) the S requirement for grain was about five times mean annual S in the rainwater of northern Nigeria. Sulfur fertilization of field grown cowpea in southern Nigeria was accompanied by a small (12%) but statistically nonsignificant increase in yield. Sulfur fertilization (20 kg S/ha) was required to give 0.26% S in the grain.