INFLUENCE OF NITROGEN AND PHOSPHORUS ON NODULATION AND YIELD OF SOYBEAN

Abstract

Grain legumes are getting popular all over the world, either because of their high protein or oil content and potential to improve soil fertility. A field study was undertaken with two grain legumes; Soybean and Cowpea, that are economically important in the semi-arid region at the Irrigation Research Farm of Institute for Agricultural Research, Kadawa, Kano in the Sudan Savanna to gain an understanding of fertilizer application rates on nodulation and yields of Soybean (TGx 1904-6F) and Cowpea (Kannanado white). The rates of nitrogen (0 and 20 kg N ha-1), rates of phosphorus (0, 6.6, 13.2 and 26.4 kg P ha-1) were factorially combined in a Randomised Complete Block Design with three replicates on the field. Biological Nitrogen fixation parameters were measured as nodule number, nodule dry weight while agronomic data determined were shoot and root dry matter and yield and yield components were pod weight, seed per plant, 100 seed weight, pod per plant, harvest index and grain yield. Data collected were subjected to analysis of variance, with significant means compared using Duncan Multiple Range Test. Results indicated that increasing levels of nitrogen significantly depressed Soybean nodulation by 50% and 78% at 6 and 8WAP respectively; pod weight by 16% and grain yield by 13.05%. The same trend was observed for Cowpea where increasing levels of nitrogen depressed nodulation by 71.8% and 54.2% at 6 and 8WAP respectively; biomass accumulation decreased too by 8.9%, 6.8%, and 9.7% for shoot dry weight at 6WAP, root dry weight at 8WAP and shoot dry weight at 8WAP respectively; yields and yield components was also depressed by 34.6%, 21.5%, 14.2% for pod weight, seed per plant and grain yield. The native strains of rhizobia were able to produce nodules on plants growing on plots where no nitrogen was applied. On the other hand, no significant difference (P>0.05) were obtained with P levels, although, 6.6 kg P ha-1 consistently gave higher values for the parameters measured in the Cowpea trials, but was at par with the control plot in the Soybean trials. Application of 26.4 kg P ha-1 gave highest grain yield in the Soybean trial. It was apparent that there was minimal P response owing to the inherent P in the soil. The legumes are self-sufficient for nitrogen requirements derived from the symbiotic nitrogen. Amongst the legumes, Cowpea consistently gave higher number of nodules than Soybean.

Keywords: Grain legumes, nitrogen, phosphorus, nodulation and yields