Combined Application of Inoculant, Phosphorus and Potassium Enhances Cowpea Yield in Savanna Soils

Low soil phosphorus levels in savanna soils of Ghana limit cowpea response to inoculation. A two-year experiment was carried out on 2 soil types of the Guinea and Sudan savanna zones of Ghana based on the hypothesis that Bradyrhizobia inoculant (BR3267) in combination with phosphorus and potassium fertilizer will significantly increase cowpea root nodulation, growth and yield. The study aimed to determine the effect of phosphorus and potassium fertilizer on cowpea response to Bradyrhizobia inoculant. The treatments were laid out in Randomized Complete Block Design, replicated four times. The plot size was 8 × 3 m, with the sowing distance of 60 × 20 cm. The treatment comprises of commercial Bradyrhizobia inoculant, phosphorus (0, 30, 40 kg P₂O₅ ha⁻¹) and potassium (0, 10, 20, 30 K₂O ha⁻¹). Application of Bradyrhizobia inoculant with 30 kg P₂O₅ ha⁻¹ and 20 kg K₂O ha⁻¹ gave the highest grain yield (1.68 and 1.86 tons ha⁻¹) at both soils which did not differ from the yield obtained from BR-40-30 kg ha⁻¹ P₂O₅ and K₂O ha⁻¹ on the Ferric Lixisol and BR-40-20 kg ha⁻¹ P₂O₅ and K₂O on the Ferric Luvisol. The same treatment also gave the highest nodule number and nodule dry weight. The results of this study have shown that the application of Bradyrhizobia inoculant followed by P and K fertilizer was effective for cowpea growth in field conditions.