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

Abstract

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.