Cowpea (vigna Unguiculata [l] Walp) Response to the Application of Npk Fertilizer in a Ferric Luvisol

This study evaluated the response of cowpea to nitrogen (N), phosphorus (P), and potassium (K) fertilizer applications in a ferric luvisol of the Guinea Savanna zone of Ghana, West Africa. The treatments were arranged in a randomized complete block design with 4 replications and carried out in two seasons. Performance of Omondaw cowpea variety was evaluated at different combinations of Nitrogen (0, 10, 20 and 30 kg/ha); Phosphorus (0, 15, 30 and 45 kg/ha) and Potassium (0, 10, 20, 30 kg/ha) fertilizers making 7 treatments based on existing blanket fertilizer recommendations in the region. The fertilizer rate corresponding to 20 – 30 – 20 kg N-P2O5- K2O/ha gave the highest grain yield but was not significantly different from the yield obtained from the application of 20 – 45 – 20 and 30 – 45 – 30 kg N-P2O5- K2O/ha. Lower application rates of 10 – 15 – 20 and 10 – 15 – 10 kg N-P2O5- K2O/ha were significantly different (P < 0.05) from each other in the 2012 cropping season but not different in 2013. The biomass yield was higher in the second year than in the first year and could be due to better rainfall at the season's onset. Fertilizer rate of 30 – 45 - 30 kg N-P2O5- K2O/ha also gave the highest biomass yield while the control gave the lowest yield for the 2 years. The good fertilizer rates selected for optimal cowpea grain yield in the study area are NPK 20 - 30 - 20.