Mineral N effects on cowpea and soybean crops in a Nigerian soil - I. Development, nodulation, acetylene reduction and grain yield

In a Nigerian soil depleted in available N, fertilizer-N enriched in15N was applied at 25 and 100 kg N/ha to crops of four cowpea and two soybean cultivars. Soil-N availability was estimated with three non-fixing crops, non-nodulating soybean, maize and celosia. With sequential harvests we examined the development of the fixing crops, as well as their nodulation profiles and acetylene reduction activities, and the patterns obtained were correlated with uptake of mineral-N. At low levels of mineral-N, excellent nodulation (up to 580 mg dry weight/plant) and very high acetylene reduction activities (up to 208 μmoles/plant/h) were recorded. Although fertilizer-N utilizations were low, 12% and 28% at 25 and 100 kg N/ha respectively, the lower application had a transient adverse effect on nodule development and the higher application had a long-term adverse effect on nodule formation, nodule development and acetylene reduction activity. Cowpea nodule mass reached maximum levels at early pod-fill except at 100 kg N/ha at which it continued to increase into late podfill. In contrast, soybean nodulation did not peak at any level of N but increased into late pod-fill. © 1982 Martinus Nijhoff/Dr W. Junk Publishers.