Response of common legumes in Zimbabwe to application of different fertilizer amendments

Appropriate application of nutrients is important for smallholder farmers for the efficient use of nutrients in legume crops to maximize biological nitrogen fixation and increasing yield in degraded soils in southern Africa. A number of fertilization options were tested for soybean, cowpea and groundnut production under smallholder conditions in Zimbabwe. Compound L (N, P, K, S), Single super phosphate (P, Ca, S), and dolomitic lime (Ca, Mg) were used solely and in combinations to determine the best nutrient requirements for the different legumes. Soybean yield was higher with the P-based fertilizers in poor sandy soil although yields were generally lower than 100 kg ha-1. Soybean grain yield ranged from 25-90 kg ha-1 in un-inoculated plots compared to 160 kg ha-1 to 600ka ha-1 in inoculated plots. Cowpea also responded well to SSP and Compound L application with highest yields attained in the combination of SSP and dolomitic lime (240 kg ha-1) compared to 140 kg ha-1 in the zero control. However in a fertile soil, cowpea residue yield responded positively to nutrient addition (5 000 kg ha-1 in the zero control vs 7 000 kg ha-1 in the SSP + dolomite) at the expense of grain yield which had the highest yield of about 1 000 kg ha-1 in the zero control compared to the lowest of about 500 kg ha-1 in the compound L + dolomitic lime, indicating possible luxury consumption. In soybean, yield was higher in inoculated treatments relative to the un-inoculated control, indicating low indigenous rhizobium numbers in the soil. Despite the strong response to fertilizer and inoculants in soybean, yields remained low in all treatments ( 700 ha-1) in the granitic sandy soil. This suggests the need to combine both organic and inorganic manures for legume production in the highly leached granite derived soils in Zimbabwe.