Growth performance of cowpea in spent oil-contaminated soils ameliorated with cocoa shell powder and biochar

Abstract

The study assessed the ameliorative potentials of cocoa shell powder and biochar on spent engine oil (SEO) soils using the growth performance of cowpea. Twenty-four polyethylene bags were set up consisting of seven treatments (T1 to T7) contaminated with 2% v/w SEO and control (T0) each replicated three times. Cocoa shell biochar (CSB) was applied to T2, T3 and T4 at rates 0.25%, 0.5% and 1.0% while uncharred cocoa shell powder (CSP) was incorporated into T5, T6 and T7 at rates 0.25%, 0.5% and 1.0%. Chemical properties of CSB, CSP and soil treatments were determined by standard methods. Cowpea seeds were sown and germination and growth parameters were determined at 3 and 6 weeks after sowing. The result showed CSB was alkaline and rich in exchangeable cations. SEO-contamination negatively impacted soil nutrient composition, weakened germination by 27% and negatively affected growth of cowpea. Plants in T0 had significantly highest growth and biomass. CPB (especially 1%) amendment significantly improved leaf initiation and area compared with plants in T1. Growth declined with increasing CSP amendment. In conclusion, conversion of cocoa shells to biochar is necessary eliminate the acidic effects of the raw cocoa shell and effectively condition the soil.