Greenhouse and field trials were conducted to evaluate the effects of phosphorus and micronutrients on the response of soybean to rhizobia inoculation on a fallow soil in the northern Guinea savanna of Nigeria. In the greenhouse study, there were significant differences among different nutrient combinations in parameters such as plant height, number of leaves, leaf area, nodulation, dry matter yield, N and P uptakes and N derived from the atmosphere (Ndfa). Application of phosphorus (P) had about 86% higher Ndfa than minus P. However, there were no significant differences among inoculants except in chlorophyll content, nodule dry weight and Ndfa. The effect of the nutrients in the field followed similar trend with greenhouse. The application of P gave higher yield (1.61 t/ha) than minus P (0.47 t/ha). In addition, rhizobia inoculation with Legumefix gave the highest nodule number while yield was not significantly influenced by inoculation. Surprisingly, the study showed that long term fallow, even more than 20 years was not enough to supply the P requirement of soybean in a northern Guinea savanna Alfisol.