Heterogeneity effects of plant density and fertilizer application on cowpea grain yield in soil types with different physicochemical characteristics

Context or problem: Although high-density cropping and fertilizer application are plausible techniques to improve crop production, there are difficulties in the growth environment that may undermine the positive effects.Objective or research question: This study focused on soil types as a major environmental factor of the hetero-geneity effects of plant density and fertilizer on grain yield. The objectives were to clarify the responses of grain yield to high plant density and fertilization under soil types with different physicochemical characteristics. Methods: Three cowpea genotypes (KVx61-1, KVx421-2J, and Dja) were grown in three dominant soil types in the Sudan Savanna: Ferric Lixisols (LXfr), Petric Plinthisols (PTpt), and Pisoplinthic Petric Plinthosols (PTpt.px). The experiment was conducted two consecutive years in 2018 and 2019. The effect of plant density (31,250 and 62,500 plants ha-1) and fertilizer application (non-fertilizer and fertilizer application at N:P2O5:K2O = 14:23:14 kg ha-1) on grain yields were evaluated. The fertilizer was applied with two different methods: basal dose and split dose.Results: A larger yield increase by high plant density and fertilization was observed for PTpt and PTpt.px than for LXfr. Split dose of fertilizer drastically increased yield when combined with high plant density. However, fer-tilizer leaching was occurred for split dose applied during the peak of rainy season, which was larger in PTpt and PTpt.px because of higher topsoil permeability. The differences in grain yield among the cowpea varieties were attributed to the shoot growth and flowering date that were differently affected by plant density and fertilization. Conclusions: Excessive moisture stress depressing shoot growth and delaying flowering was a cause of the het-erogeneity effects of high plant density and fertilization on grain yield. The stress was more severe in soil of lower topsoil permeability. In contrast, soil types with higher topsoil permeability caused late-season drought for the plants with delayed flowering. Split dose of fertilizer was a strong option for yield increase but the risk of fer-tilizer leaching should be considered. Genotypic differences in the yield in response to plant density and fertilizer application were presumably influenced by the ability to adapt to excessive soil moisture and drought conditions. Implications or significance: Plant density and fertilizer application should be optimized according to the soil type of the target region. Its optimization would help to achieve food security and better economic income of small households of the region.