Combined lime and biochar application enhances cowpea growth and yield in tropical Alfisol

It is essential to increase the pH of tropical soils in order to reduce acidity and promote soil and crop productivity. Therefore, experiments were carried out in 2020 and 2021 to assess the impacts of biochar and lime on the chemical properties, growth, nodulation, and yield of cowpea (Vigna unguiculata). The study involved various levels of lime (CaCO₃) and wood biochar (ranging from 0 to 10 t ha^-1), organized in a factorial combination. The treatments were arranged in a randomized complete block design and replicated three times. The application of lime and biochar, either separately or in combination, led to improvements in soil chemical properties such as pH, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sodium (Na), magnesium (Mg), and cation exchange capacity (CEC), as well as enhancements in the growth, nodulation, and yield of cowpea when compared to the control. Lime and biochar alone and combined reduced exchangeable acidity (Al + H) relative to the control. Cowpea yield increased with lime rate up to a point, but then decreases. The highest cowpea yield is achieved at a lime rate of 2.5 t ha^-1, whereas cowpea yield increased as the Biochar rate increased from 0 up to 10 t ha^-1. There was a significant correlation between pH and cowpea pod weight in both years (2020 and 2021). The R values were - 0.615 and - 0.444 for years 2020 and year 2021 respectively at P < 0.05. At higher lime levels combined with biochar, there were considerable reductions in cowpea yield, and this decrease can be attributed to unfavorable soil pH conditions. Relative to 2.5 t ha^-1 lime + 5 t ha^-1 biochar, 10 t ha^-1 lime + 10 t ha^-1 biochar, reduced cowpea grain yield by 853% in 2020 and 845% in 2021. Since there were no significant differences between the effects of 2.5 t ha^-1 lime + 5 t ha^-1 biochar, 2.5 t ha^-1 lime + 7.5 t ha^-1 biochar, and 2.5 t ha^-1 lime + 10 t ha^-1 biochar applications on cowpea yield, therefore to prevent waste of Biochar, 2.5 t ha^-1 lime + 5 t ha^-1 biochar is recommended for production of cowpea.