Compost as an Alternative to Inorganic Fertilizers in Cowpea [Vigna unguiculata (L.) Walp.] Production

Soil fertility management is essential to sustain agricultural production in smallholder farming systems. An experiment was carried out to assess the viability of the combined use of compost and inorganic fertilizers as an alternative to conventional inorganic fertilization under greenhouse conditions. The 10 treatments, arranged in a randomized complete block design (RCBD) with six replications, consisted of a control, conventional mineral fertilization (150 kg NPK ha⁻¹), composts added to the soil alone (2.5, 5, 7.5, and 10 t ha⁻¹), and their combination with 50% of recommended rate of inorganic fertilizers (75 kg NPK ha⁻¹). Application of 7.5 t ha⁻¹ of compost and 50% of the recommended dose of inorganic fertilizer (75 kg NPK ha⁻¹) gave the significantly highest seed yield, corresponding to a 30% increase over NPK-fertilized plants. The combined application of 2.5 or 10 t ha⁻¹ compost with 75 kg NPK ha⁻¹ increased plant height by 38% compared with the NPK treatment. Additionally, stem diameter increased by 53% when 5 t ha⁻¹ of compost and 75 kg NPK ha⁻¹ were mixed. As expected, control plants produced the most nodules (108), 85% more than inorganic fertilization. Plants fertilized with 7.5 or 10 t ha⁻¹ of compost and 75 kg NPK ha⁻¹ produced 17% more pods, seeds per pod, and seeds per plant than NPK treatments. However, fertilization treatments had no significant effects on cowpea fresh and dry biomass or SPAD values. The results reveal that combining compost with inorganic fertilizer reduced synthetic fertilization by 50%, while producing growth and yields comparable to, or even higher than, recommended inorganic fertilization. This experiment demonstrated that integrated soil fertility management can be used as an alternative to the use of inorganic fertilizers in cowpea cultivation.