Site-Dependent Responses of Cowpea (Vigna unguiculata (L.) Walp.) to DAP-Based P2O5 and K2O Fertilization at Three Contrasting Sites in the Colombian Caribbean

Cowpea (Vigna unguiculata (L.) Walp.) is an important food and income crop in the Colombian Caribbean, where fertilization is seldom guided by site-specific soil information. Three randomized complete block field trials with four replicates were conducted at contrasting sites in Cereté, Cotorra and Ariguaní using a 3 × 3 factorial combination of diammonium phosphate (DAP)-based P2O5 and K2O rates (0, 25 and 50 kg ha−1), together with nitrogen, vermicompost and foliar-fertilizer add-on treatments. Site significantly affected grain yield and pods per plant (p < 0.001) and seeds per pod (p = 0.033), whereas 100-seed weight remained comparatively stable across sites (p = 0.313). Grain yield was not significantly affected by increasing DAP application rates, whereas seeds per pod showed a significant positive linear response (p = 0.005); because increasing DAP rates simultaneously increased N supply, this response cannot be attributed to phosphorus alone. Potassium response was site-dependent, with a significant Site × K(quadratic) interaction for grain yield (p = 0.007); at Ariguaní, 25 kg ha−1 K2O produced the highest yield. These findings provide preliminary evidence of site-dependent nutrient responses under the specific year, genotype and environmental conditions evaluated and require validation across additional seasons and genotypes before broader management recommendations are developed.