Nitrogen credits from cowpea, soybean, groundnut and mucuna to maize in rotation

Abstract

Legumes constitute a major component of sustainable cropping systems due to their biological nitrogen fixing capacity. A field study was conducted in 1999 and 2000 at Ejura in the forest savanna transition woe of Ghana to quantify nitrogen credits to mai:ze from early (65-105 days) and medium/late-maturing (80-120days) grain legumes (cowpea, soybean, and groundnut}, and from Mucuna cover crop. The design was a split plot in randomized complete block with three nitrogen levels (O, 45, 90 kg N ha· 1) applied to maize following legumes as the main plots. There was in addition a basal application of 26 kg P ha·• and 42 kg K ha· 1 • Subplot treatments were 10 cropping systems: 6 grain legumes (an early and later-maturing variety each of soybean, cowpea, and groundnuts), Mucuna pruriens as a planted fallow, a natural fallow, and two maize varieties as reference crops, early (95 days) and medium (105 days). Mucuna had the highest nodule mass (20 kg ha·'), cowpea (7.8 and 10.2 kg ha·') and soybean (10.1 and 17 kg ha·') were intermediate, with the least nodule mass on groundnut (5.2 and 5.4 kg ha· 1). Nitrogen credit to mai:ze from Mucuna and medium-maturing cowpea variety "Soronko" was more than 90 kg N ha· 1 , 22 N ha· 1 from early-maturing cowpea "Asontem" and 16 kg N ha· 1 from natural fallow. No appreciable N credits were measured from soybean and groundnut varieties. Early soybean variety TGX 1478-2E had negative N credit to maize. The species and maturity group oflegumes are important determinants of their N contribution to crops in rotation: