11. The Effects of Benomyl and Glyphosate Treated Plant Litter on Nitrogen Mineralization in Mollisols
Abstract
Cocoa production is bedevilled with low inherent soil fertility, pests and diseases, which cause substantial
reduction in the potential yield of the crop. Despite these problems, farmers are often reluctant to use
pesticides, due to lack of knowledge on the effect of the chemicals on non targeted organisms. The experiment
is, therefore, aimed at finding out if Roundup (herbicide) and Benlate (fungicide), with Glyphosate and
Benomyl, as their respective active ingredients r ecommended for use on cocoa plantations in Ghana, have any
effect on nitrogen mineralization by affecting decomposition of plant litter. It is also to find out if the combined
effects of the two pesticides are additive or interactive. Tomato served as a test plant, treated with three levels
of each pesticide. A factorial experiment arranged in a completely randomized design with combinations of five
incubation periods and 10 treatments was established. The 10 treatments were soil alone and nine factorial
combinations of the fungicide and the herbicide at three levels each. The application rates of Roundup were 0,
1.2 and 12 kg Glyphosate ha–1, and those of Benlate were 0, 1.13 and 11.3 kg benomyl ha–1. The treated plant
materials were dried, milled and incorporated into soils of Rowland series and incubated at 25 oC for 0, 7, 14, 28 and 42 days, after which NH4– -N and NO4– -N were determined. The results showed that increasing glyphosate concentration increased available NH4– -N concentration at the day of application. NH4 – -N concentration at the day of application was significantly (P < 0.05) higher than those of days 7, 14, 28 and 42. Increasing glyphosate concentration increased nitrification. There was no net immobilization of soil mineral nitrogen at higher concentration of glyphosate treatment and their combinations. Benomyl treatment at normal concentration (1.13 kg benomyl h-1) depressed nitrification for a longer period (up to day 28) than that at higher concentration (up to day 14). Net mineral nitrogen at day 42 was higher in the glyphosate and the benomyl treatments than the control. It is concluded that pesticide application affected litter decomposition as indicated by changes in mineral nitrogen. There were positive interactive effects between Glyphosate and Benomyl in relation to nitrogen mineralization.
