Effect of starter nitrogen and phosphorus fertilizer rates on yield and yield components, grain protein content of groundnut (Arachis Hypogaea L.) and residual soil nitrogen content in a semiarid north Ethiopia

Increasing costs of chemical fertilizers, environmental concerns of their application and demand for protein foods, placed an extensive interest in growing of legume crops for human nutrition, and soil fertility replenishment. This study was conducted to investigate the effects of nitrogen (N) and phosphorus (P) fertilizers on parameters of phenology, growth performance, grain yield, yield components, grain protein content of groundnut, and residual soil nitrogen content in the northern Ethiopia during the growing season of 2017. Three levels of N (0, 15 and 30 kg ha(−1)) and four levels of P(2)O(5) (0, 23, 46 and 69 kg ha(−1)) were set in factorial combinations of randomized complete block design with three replications. Results showed that an average total biomass yield increased by 22.5% for separate individual application of 15 kg N ha(−1) and by 16.6% for 46 kg P(2)O(5) ha(−1) compared to control plots. Haulm yield increased by 29.17% for plots treated with N fertilization compared to control plots. Average pod yield increased by 85.4% for a combined application of 15 kg N ha(−1) and 46 kg P(2)O(5) ha(−1) fertilizers compared to the control plots. Plots fertilized with the highest combined rates of N and P have attained lower grain yield compared to the combined application of 15 kg N ha(−1) and 46 kg P(2)O(5) ha(−1). The highest grain protein contents were obtained for a combined application of 30 kg N ha(−1) and zero P, and 15 kg N ha(−1) plus 46 kg P(2)O(5) ha(−1). The highest N harvest index was obtained for control treatments and for plots treated with combined application of 15 kg N ha(−1) and 46 kg P(2)O(5) ha(−1). Residual soil N content increased by 119% on plots with combined application of 15 kg N ha(−1) and 46 P(2)O(5) ha(−1) compared to control plots. Based on our results, combined application of 15 kg N ha(−1) and 46 kg P(2)O(5) ha(−1) was recommended for increasing grain yield, grain protein content and residual soil nitrogen. The results of this study are crucial to improve groundnut productivity, grain protein content and also to provide implication on soil fertility management in a crop rotation system.