Nitrogen fixation by grain legumes contributes N to tropical soils. But in Sudano – Sahelian region of North-eastern Nigeria, low phosphorus content of the soil may restrict rhizobia population and legumes root development, which in turn, can affect their N2 fixing potential. A two- years field experiment was conducted at the Department of Soil Science Teaching and Research Farm University of Maiduguri North-eastern Nigeria during 2005 and 2006 cropping sessions to evaluate the influence of phosphorus (P) on N2 fixation by groundnut ( Arachis hypogaea L. ), cowpea (Vigna unguiculata L. ) and bambara groundnut (Vigna subterranean L. ). The legume crops and a sorghum variety (Paul Biya) were applied 0, 20 and 40 Kg Pha-1 and grown for 50 days, after which they were harvested and the amount of N fixed was determined. P fertilization significantly increased the amount of N fixed by the crops. Application of 40 KgPha-1 increased N fixation in cowpea, groundnut and bambara groundnut by 378, 169 and 138% respectively, over the control. Cowpea differed significantly from groundnut and bambara groundnut in the amount of N fixed (P<0.05). Cowpea fixed (54.19 KgNha-1), groundnut (39.16 KgNha-1) and bambara groundnut fixed (28.42 KgNha-1). Cultivation of Bornoji red cowpea variety with application of 40 KgPha-1 would improve the soil N status.
Key words: Phosphorus fertilizer; N2-fixation; Grain legumes;Sudano-Sahelian