Soil-applied pesticides

An ecosystem can be thought of as a representative segment or model of the environment in which one is interested. Three such model ecosystems will be discussed (Figures 1 and 2). A terrestrial model, a model pond, and a model ecosystem, which combines the first two models, are described in terms of equilibrium schemes and compartmental parameters. The selection of a particular model will depend on the questions asked regarding the chemical. For example, if one is interested in the partitioning behavior of a soil-applied pesticide the terrestrial model would be employed. The model pond would be selected for aquatic partitioning questions and the model ecosystem would be employed if overall environmental distribution is considered. 

Soil-applied pesticides have been successfully used to control soilbome diseases, weeds, and nematodes in most vegetable and fruit crops over the past decades. Toxicity of these materials to animals and humans and their environmental and economic costs (Pimentel et al. 1992 Ruzo 2006) raised serious environmental and human safety concerns, leading to the phase-out of the most effective and largely used chemical, the methyl bromide (Luken and Grof 2006), and the increasing restrictions on the applications of available pesticides (Perkins and Patterson 1997). The limited availability of chemicals resulted in an increased emphasis on... 

In some situations, foliar treatments can be avoided by using a prophylactic soil-applied pesticide. Treatment of potted plants, especially ornamentals, may be by an admixture of granules to the potting compost, by dipping the roots into a slurry of pesticide at planting, or by a subsequent drench. However, all these treatments have a limited persistence, and are only aimed at protection during the first two to six weeks of plant development. 

The loss of pesticides due to volatilisation should not be underestimated. It has been estimated that much more pesticide is lost from its target by volatilisation than from spray drift (Anon., 1996). The pesticide formulator tends to encounter this problem when working with some soil-applied pesticides, or where a leaf deposit needs to remain on the plant surface. Pesticides taken up by foliage do not tend to suffer from this problem, as plant uptake mechanisms usually ensure that pesticide soon ceases to be available to this loss mechanism. 

In order to properly understand pesticides in the context of the soil microbial ecosystem, it is important to consider the relevant properties of the soil environment, the metabolic capabilities of the soil microbial community, and the uses and dissipation routes of soil-applied pesticides. 

By the mid 1970 s, the replacement of these materials with degradable, soil-applied pesticides In nearly all markets set the scene for the development of current Interest In adapted microbial pesticide degradation. 

Although agronomists periodically observe failures of soil-applied pesticides to control target pests, these have historically been attributed to improper application technique, unusual environmental conditions, or development of pest resistance. 

Adaptation of soil microorganisms for rapid degradation of soil-applied pesticides can occur as a result of the complex interactions between the soil, the pesticide, the microbes, and environmental conditions. The current research addresses the role of breakdown products from the pesticide in the development of the condition. 

Another important variable that determines the microbial metabolism of soil-applied pesticides is the availability of the chemical to the microbial systems degrading it. The hydrolysis product and parent pesticide should be available to microbes so as to exert their toxicity or provide nutrient value. The lack of availability of some chemicals may result in resistance to microbial adaptation. 

The role of soil microorganisms in the enhanced degradation of several soil-applied pesticides has been well documented (1-3). 

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