Optimizing NPK Fertilizer Use to Improve Yield and Protein Content of Cowpea [Vigna unguiculata (L.) Walp]: A Review

Abstract

Cowpea (Vigna unguiculata L. Walp) is considered an important source of dietary protein for rural and urban populations in sub-Saharan Africa. Its ability to fix atmospheric nitrogen in the soil further emphasizes its important role in improving soil fertility. However, low yield and poor seed quality are the consequences of poor soil fertility. The introduction of improved varieties in recent decades has required different fertilization recommendations adapted to different ecological conditions. This review is based on approximately 103 research papers and critically discusses the effects of nitrogen (N), phosphorus (P) and potassium (K) on growth, yield and seed quality of cowpea, in terms of crude protein content. The data collected show that recommendations for N, P and K fertilization depend on several factors, including total N content and available P and K in the soil, which vary between agro-ecological zones. The use of N-fertilizers can be harmful to the plant, except in cases of extremely low N content in the soil. Effective seed inoculation with Rhizobium bacteria promotes nodule formation. Many sources indicate 15–40 kg N ha-1, 20–45 kg P₂O₅ ha-1 and 30–40-kg K₂O ha-1 as optimal rates for growth and yield, even in conditions of very low soil supply of these nutrients. These rates have a positive effect on plant height, number of branches, flowers and pods, leaf area, root length, dry matter content, biomass (aboveground and underground parts), grain yield, number and weight of nodules and 100-seed weight, and are therefore also recommended in this review. Since N and P are key components in protein, the use of NPK fertilizers is essential to optimize the crude protein content of cowpea seeds, as confirmed by many authors. Increasing the doses of N and P generally increases the crude protein content, but the optimum yield and protein content were achieved at a dose of 30 kg N ha-1 and 37.5 kg P₂O₅ ha-1 in most agro-ecological conditions in Nigeria, combined with seed inoculation with Rhizobium bacteria.