Growth and yield response of cowpea (Vigna unguiculata [L] Walp) to NPK fertilizer and rhizobia inoculation in the Guinea and Sudan Savanna Zones of Ghana

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The attendant low yield of cowpea among smallholder farmers has increased the need for site specific fertilizer recommendation and integration of biological materials to increase the yield of the crop. The effectiveness of applied fertilizer is constrained by the use of the in appropriate rate and improper timing of sowing. Rhizobia inoculant, on the other hand, needs a balanced nutrient application to increase crop yield. The study was designed to: i) determine the effects of inoculant, P and K mineral fertilizers on N,P,K uptake, growth and grain yield of cowpea, ii) evaluate the effect of NPK fertilizer application on the growth and yield of cowpea in the Guinea and Sudan Savanna zones of Ghana, and iii) simulate the potential yield, yield gap, best sowing date, growth and yield of cowpea using CROPGRO – cowpea DSSAT model. The response of cowpea to rhizobia inoculation and fertilizer application revealed that sole application of inoculant was not sufficient to raise cowpea yield except when combined with 30 kg P2O5 ha-1 and 20 kg K2O ha-1. The yield response to mineral fertilizer also showed that K is very essential in determining cowpea yield and should therefore, not be omitted in any fertilizer recommendation rate. CROPGRO - Cowpea model was used to simulate the response of cowpea to different N rates, potential yield and the best sowing dates for cowpea. The model was calibrated for Omondaw cowpea cultivar using data from the experiment carried out at Lawra (Ferric Lixisol) and Nyoli (Ferric Luvisol) during the 2012 and 2013 cropping seasons. The model performance was evaluated statistically using RMSE (0.13 tons ha-1), CV (RMSE) (9.9%) and Wilmott index of agreement d (0.97). A long term seasonal analysis using the model was able to detect that 15 kg N ha-1 is optimal for Ferric Lixisol while 20 kg N ha-1 was optimal for Ferric Luvisol. To avoid crop failure, sowing window for Lawra ranged from July 11 to July 21st while that of Nyoli was extended to 26th July with consideration for August 5th. The yield gap analysis revealed that a wide gap exists between climatic potential yields and yields obtained from farmers’ fields. The gaps between potential and research station yields (yield gap 1) ranged from 1.05 to 18 % of potential yield, while that of the farmers field was 84.21 % for Lawra soil and 79.55 % for Nyoli soil. It is necessary to reduce the yield gap by using site specific fertilizer recommendation and appropriate timing of sowing dates.
A thesis presented to the Department of Crop and Soil Sciences, Faculty of Agriculture, College of Agriculture and Natural Resources, Kwame Nkrumah University of Science and Technology, Kumasi Ghana, in partial fulfilment of the requirements for the award of degree of Doctor of Philosophy in Soil Science