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BIOLOGICAL NITROGEN FIXATION OF SOME GROUNDNUTS AS AFFECTED BY GENOTYPE AND APPLIED PHOSPHORUS AT SAMARU, NORTHERN GUINEA SAVANNAH OF NIGERIA Gabasawa, A. I. and A. A. Yusuf Department of Soil Science, Faculty of Agriculture/Institute for Agricultural Research, Ahmadu Bello University, Zaria, Nigeria. *Correspondence author: [email protected] ABSTRACT Biological nitrogen fixation (BNF), brought about by both free living soil microorganisms and their symbiotic associations with higher plants, is the major source of N input into agricultural systems. Groundnuts (Arachis hypogaea L.), in symbiosis with rhizobia, in their root nodules, fix atmospheric nitrogen (N2). A field trial was carried out at Samaru, Nigeria, in 2008 with a view to evaluate BNF potentials of five groundnut genotypes (SAMNUT 10, 11, 21, 22 and 23). Each genotype received four rates of P (0, 20, 40 and 60 P2O5 ha ) in an RCBD with three replications. N-Difference method was employed in assessing the BNF of the genotypes. Nodulation performance of the genotypes was observed. Results of the study showed that SAMNUT 11 and 22 significantly recorded the highest nodulation, by number and weight. All the remaining genotypes statistically produced same nodule number. Moreover, the early maturing SAMNUT 21 fixed the highest (82 kg N ha) quantity of N2, while SAMNUT 11 (46 kg N ha ) and 23 (31 kg N ha) were statistically similar and fixed the lowest N2. SAMNUT 11 and 22 were, therefore, found to significantly record the highest nodule number and weigh and SAMNUT 21 fixed the highest N2, and hence the highest BNF, indicating their genotypic desirability in terms of N2 addition to the soil environment, especially for subsequent crops.