Comparative Study of the effects of a Synthetic Pessticide (Endocel) and Plant Extracts on litter Decomposition in a Theobroma cacao Linn.Plantation.
This study was undertaken to determine the effects of Endocel (a synthetic pesticide) and water extracts of neem and siam weed leaves on mineral content and litter decomposition in a Theobroma cacao Linn. Plantation. This was with the view of assessing the comparative effects of the synthetic pesticide and the plant extracts on litter decomposition. The experiment was carried out at the Teaching and Research Farm, Obafemi Awolowo University, Ile-ife, Nigeria. The experimental design was made up of four blocks of seven plots each with three trees per plot giving a total of 21 trees per block. Pre-treatment samples of litter and soil were taken a week before treatment. Aqueous extracts of fresh leaves of siam weed and neem leaves collected from other sites within the university were prepared using standard methods: endocel at 0.1 L/ha and 0.05 L/ha; neem and siam at 40,000 and 20,000 mg/kg each and control at 0.00 mg/L. Cocoa plants were sprayed once every week for five consecutive weeks with the six spray mixtures (treatments) using a knapsack sprayer. Soil and litter samples were collected from each plot once every month for five months starting from 4 weeks after treatment. Microbial analysis of soil and litter samples, C:N of litter and exchangeable cations of soil were determined using standard methods. Data on the microbial analysis and exchangeable cations were subjected to analysis of variance and where there were significant differences (p<0.05), treatments means were separated using Duncan’s Multiple Range Test. The mean range of total heterotrophic bacteria (THB) load in the litter samples treated with endocel was 2.45x107±8.17x106 to 1.36x1012±1.36x1012 cfu/g while that for plant extracts was 3.01x107±7.21x106 to 4.44x109±4.22x109 cfu/g and 9.25x106±2.69x106 to 1.15x1011±1.15x1011 cfu/g for neem and siam respectively. Corresponding values for soil were: endocel, 1.56x106±9.92x105 to 5.16x109±3.45x109 cfu/g; neem, 1.70x106±5.60x105 to 2.57x109±2.54x109 cfu/g; and siam, 4.14x106±2.46x106 to 5.20x107±2.51x107 cfu/g. The values for the control was 7.84x106±4.95x106 to 4.58x107±2.14x107 cfu/g, and not significantly different with those of the treated plots (p<0.05). This is an indication that neither the synthetic nor the plant extracts had a significant effect on the decomposer organisms. However, microbial population in soil were in the order neem>siam >control > synthetic and in the litter, synthetic> siam>control>neem. The C:N ratio for the litter samples in all the experimental plots were in the range 12:1 to 15:1. Values of the exchangeable cations for the six experimental plots from November to January were in the range: 0.14±0.01 to 0.21±0.01 cmol/kg for Na+; 0.16±0.01 to 0.39±0.02 cmol/kg for K+; 6.76±0.33 to 10.79±0.61 cmol/kg for Ca2+; 0.40±0.11 to 4.57±1.44 cmol/kg for Mg2+. The corresponding values control were 0.14±0.01 to 0.18± 0.01 cmol/kg for Na+; 0.23±0.23 to 0.41±0.11 cmol/kg for K+; 6.07±0.67 to 8.57±0.84 cmol/kg for Ca2+; 0.76±0.20 to 3.44±0.93 cmol/kg for Mg2+. In conclusion, this study showed that neem, siam and endocel (at manufacturer’s recommended rate) pesticides did not have adverse effect on soil micro-organisms and soil micronutrients. Litter decomposition therefore did not vary with the type of pesticide used.