Impact of pesticides on soil microorganisms, physicochemical properties and cowpea (Vigna unguiculata L.) yield

Abstract

Cowpea (Vigna unguiculata L.) is an herbaceous annual crop mostly grown in the tropics in Latin America, Africa and South Asia with pesticides application to control associated pests. These chemicals are applied indiscriminately by peasant farmers in Nigeria without due consideration of their non-target effects. This study investigates the effects of lambda-cyalothrin, and a mixture of cypermethrin and dimethoate applied at their recommended field application rates and twice the recommended rates on soil microorganisms, physicochemical properties and cowpea yields. Parameters measured include soil pH, organic carbon, nitrogen, phosphorus content and cation exchange capacity. Soil microorganisms were isolated and enumerated using dilution plate method while cowpea yields were determined by seeds weights. Results showed that the pesticides had no significant effects on soil pH, nitrogen, phosphorus and cation exchange capacity. The recommended rate of lambda-cyalothrin significantly increased the soil organic carbon content at (P < 0.05). Both pesticides significantly reduced microbial counts with twice the recommended rates having higher inhibitory effects compared to the control soil (without chemical treatment). Reduction of predominant bacteria and fungi genera as well as the elimination of some genera was observed. The mixture of cypermethrin and dimethoate at twice the recommended rate had the highest cowpea grain yield of 4.5 kg/ha, while the control plot had the lowest grain yield of 1.6kg/ha. It may be concluded that soil microorganisms important to soil fertility were adversely affected by the application of insecticides. Efforts should be made to use alternate non-chemical approaches in ensuring food and environmental sustainability.