In order to assess the effects of P addition and NP addition on nodulation and N fixation this study examines the nodulation of Vigna unguiculata cv. 305 (cowpea) and Stylosanthes hamata cv. Verano in fertilized and unfertilized conditions in NE Thailand. V. unguiculata cv. 305 and S. hamata cv. Verano were studied for their adaptation to low P on the top of the toposequence. The results showed that P addition not only increases above-ground biomass but also nodulation and N2 fixation of V. unguiculata cv. 305 and S. hamata cv. Verano. Nitrogen fixation represented between 50 and 58% of total plant N with P applications. The soil organic carbon increased strongly the weight of the nodules for V. unguiculata cv. 305 when no fertilizer was added. Our results pointed out a positive correlation between N content inside the plant and nodule dry weight for V. unguiculata cv. 305. We noticed that an increase of 1 mg in nodule dry weight produced an increased in total plant N at flowering stage of 1.29 mg plant-1 in the control and of 1.48 mg plant-1 in the PK treatment. Regarding S. hamata cv. Verano, an increase of 1 mg in nodule dry weight at 183 days after sowing (DAS) produced an increase in N content of 3.18 mg plant-1.