Utilization of Cowpea Crop Residues to Reduce Fertilizer Nitrogen Inputs with Fall Broccoli

Abstract

Environmental concerns have led to renewed interest in use of sustainable green manure legume cropping systems. Field experiments were undertaken to determine whether cowpea [Vigna unguiculata L. (VValp.)] crop residues could be used to reduce fertilizer N inputs with fall broccoli [Brassica oleracea L. (Italica Group)]. Studies in 1992,1993, and 1994 involved broccoli preceded by cowpeas, with or without factorial combinations of preplant N and sidedress N from urea on the broccoli. Control broccoli plants were grown on fallowed ground and supplied with 168 kg ha−1 of N from urea. Cowpea crop residues alone did not provide sufficient N for optimum production of marketable broccoli. Marketable broccoli yields sometimes showed no significant differences between controls and cowpea residue treatments receiving only 84 kg ha−1 of fertilizer N. However, there was a tendency for increased broccoli transplant mortality in cowpea residue plots. A fourth study in 1994 indicated a negative main effect of cowpeas on broccoli stand establishment, and a complex interaction of cowpeas and trifluralin treatments on broccoli yield. Petiole N concentrations of broccoli plants with reduced fertilizer N inputs which succeeded cowpeas usually were lower than those of control plants. Soil nitrate levels at 15‐ to 30‐cm soil depths at the time of broccoli planting were consistent indicators of apparent nitrate utilization by microbial decomposition of cowpea crop residues. We were unable to demonstrate a cowpea‐broccoli succession cropping system which consistently reduced fertilizer N inputs without some type of adverse effect on the broccoli crop. Therefore, the use of cowpea crop residues to reduce fertilizer N inputs with fall broccoli is not recommended.