Growth, nitrogen fixation and yields of cowpea (Vigna unguiculata L. Walp) and chemical properties of an acid Alfisol in response to applications of organic amendments and inorganic N.

An acid Alfisol (Typic Paleustalfs) was amended in triplicates at the rate of 5 g kg−1 soil (12 t/ha) with organic amendment (cow dung (CD), corn cob (CC) and 1:1 CD:CC (CDCC), and 20 kg N ha−1 as urea, respectively, and planted to cowpea in the screenhouse. At flowering stage, plant shoots, roots and nodules were harvested. N2 fixed was estimated by N-difference method using maize planted and treated the same way as the cowpea as non-fixer. Considering average data for two planting periods, the increases (p < 0.05) obtained with the application of both organic manure with and without N over the control ranged from 125–775% for number of nodules, 96–296% for N2 fixed, 36–99% for dry matter yield and 160–460% for seed yield. The parameters were higher (p < 0.05) in the organic –inorganic N than in sole manure treatments. The organic amendments increased the pH, organic carbon (OC) and available P contents of the soil at both plantings, with CDCCN and CDCC recording the highest values. Average tissue N and P at the end of the two plantings were similar among all treatments, but average N and P uptakes were higher (p < 0.05) with than without amendments by 26–100% and 238–340%, respectively. Therefore, 5 g kg−1 cow dung:corn cob (1:1) or 5 g kg−1 cow dung in sole application or combined with 20 kg N ha−1 can promote cowpea growth, N2 fixation and yields with resultant positive effects on soil chemical properties. [ABSTRACT FROM AUTHOR]