Effect of organic amendments on nodulation and nitrogen fixation by cowpea

Cornstover, garbage compost, and cowdung were added to an Alfisol (Iwo series) at the rates of 0, 23, 69, and 115 Mg ha(-1) and incubated for 4 weeks in the greenhouse prior to sowing cowpea [Vigna unguiculata (L.) var. Ife Brown]. Two successive plantings were made per pot. Harvesting of the plants was at onset of flowering, 42 days after planting. Nitrogen (N) fixed was assessed using the N-2 difference method. Soil reaction (pH), organic carbon (C), and tissue N contents were increased by all the organic amendments. At both harvests, dry matter yields were significantly enhanced by garbage compost and cowdung. While garbage compost and cowdung increased the amounts of N-2 fixed, the efficiency of N-2 fixation was higher in the cornstover treatments. The cornstover additions also significantly increased nodule size and number. These results suggest that organic amendments would benefit N-2 fixation especially in soils low in indigenous organic matter. The beneficial effects were in the order: cowdung > garbage compost > cornstover. To enhance N-2 fixation in cowpea, a rate of 23 Mg ha(-1) of these organic amendments is suggested.