[en] Non conventional phosphorus fertilizers have the potential to be used under different soils. Their effect on the biological nitrogen fixation by legumes is not clear. This greenhouse experiment was conducted to evaluate two p -sources, namely super phosphate (SSP), and the partially acidulated phosphate rock (PAPR) on nitrogen fixed by Cowpea, using N-15 isotopic dilution technique. Acid soil was limed with Ca C O₃, treated with P(as SSP or PAPR), at rates of 0, 50, and 150 mg P/Kg soil, and planted with Cowpea. Results showed that, application of P as PAPR or SSP significantly increased the dry matter content of the whole plant, as well as the total nitrogen uptake. Significant biological nitrogen fixation was found as result of P application. Percentage Ndfa ranged from 28.3 to 70.1 depending on the rate and type of P source, also most of Ndfa was translocated to the seeds. 3 tab