Effect of pesticide application rate on yield of vegetables and soil microbial communities

Abstract

Lindane is listed among the Prior Informed Consent (PIC) pesticides, and agricultural uses of lindane have been banned in 52 countries due to its hazardous nature. However, lindane is still widely used in vegetable cultivation in Ghana. The effect of increasing rates of application of lindane (156.0, 244.0 and 312.0 g ha -1), unden (propoxur) (125.0, 187.5 and 250.0 g ha -1), dithane and karate (166.6, 209.8 and 333.3 g ha -1) on garden eggs, okro and tomatoes was studied to find out if it has any advantage that makes its use attractive to farmers despite its ban. Yields of garden eggs were suppressed by all the rates of lindane applied. In tomatoes, lower rates of lindane increased yields whereas the higher rates suppressed yields lower than the control. In okro yields were higher than the control at all levels of lindane applied though yield increments were low. Unden application had the highest effect on garden egg yields followed by tomatoes and least on okro. In the garden egg and tomato treatments, increasing concentration of unden resulted in decreasing yields though yields were higher on the control plots. The optimum unden rate for garden egg and tomato was U20 (i.e. 125.0 g ha -1). Increasing rates of unden on okro did not have any significant effect. Pesticide application had a higher effect on fungal population (50-70% reduction) than on bacterial population in the soil (23.0–38.4% reduction). Dithane suppressed bacteria most whereas karate suppressed fungal population most. Lower levels of chloride residues in tomato fruits corresponded to lower rates of lindane application. Lindane did not have any advantage over the other pesticides as it caused the least increase in yield. It is recommend that farmers are educated on the adverse effects of lindane use, and government enforces the restriction on lindane importation and use in agriculture. Introduction Pesticides are used in controlling insect pests, diseases and weeds in agriculture. They are known to increase agricultural production tremendously as these chemicals act on pests that destroy agricultural produce. The behaviour of a pesticide in the environment depends on its stability, physico-chemical properties, the nature of the medium into which it is applied, the organisms present in the soil, and the prevailing climatic conditions (Graham-Bryce, 1981). It has been established that pesticides could become a nuisance if they are misused. Some of the negative effects of pesticide misuse include low crop yield, destruction of soil micro-fauna and flora, and undesirable residue accumulation in food crops (Edwards, 1986). In advanced countries, strict pesticide regulation and enforcement mechanisms are put in place to ensure their safe use and proper handling. The control schemes further ensure that approval for the sale and use of pesticide is based on scientific data that support its effectiveness against target pests and that it is not unduly hazardous to human health and the environment. Pesticide use in most developing countries is, however, based solely on manufacturer's recommen-dations. These recommendations, which include data on toxicological and environ-mental properties of the pesticides, though useful, may not be appropriate under local conditions since they were tested under different agro-climatic and socio-cultural conditions. With the intensification of agriculture and increasing usage of pesticides, the need to study the side effect of pesticides on various soil properties and on the host plant is very crucial. A survey conducted in vegetable growing areas in Ghana identified lindane, unden, karate and dithane as the most used pesticides by vegetable growers. According to Brown (1978), lindane is used primarily as an insecticidal treatment for hardwood logs and lumber, seed grains and livestock. It is also used as an insecticide for several dozen fruits and vegetable crops and for personal hygiene as a scabicide (EPA, 2002). However, lindane is listed among the Prior Informed Consent (PIC) pesticides, and all agricultural uses of lindane have been banned in 52