Nitrogen is commonly the most limiting plant nutrient in arable farming in the tropics and the most expensive mineral fertilizer element. Biological N fixation, as a biological process of capturing atmospheric N, holds a great promise to small-holder farmers particularly in the Northern Guinea Savanna (NGS) of Nigeria, where the soil is characterized low fertile. There is also a high cost of chemical fertilizers and environmental impacts. Differences among legumes, for ability to grow on low or high P soil exist, the mechanisms involved are not fully understood and few are vaguely described. Only little is known about differences in P uptake, P use and N2-fixation abilities of the genotypes selected for this book, especially in the prevailing low soil-P conditions, and little or no similar studies have been reported on groundnuts in the Nigerian NGS. The genotypes fixed between 31-82 kg N ha-1, derived 63.9-85.8% N from the atmosphere and 31.5-71.7% PUE ranges. This is an important book especially for undergraduate and graduate students; and researchers that have interest in the field of soil microbiology and related disciplines that study nitrogen-phosphorus dynamics of legumes.