Application is by spraying a liquid, broadcasting granular formulations, or fumigation. Concerns about environmental and health issues related to pest control make it important to understand the possible hazards associated with their use and to minimize risks. The fate of pesticides in soil is controlled by chemical, biological and physical forces. The downward movement of pesticides through the plant rooting zone of soil reduces their concentrations in the surface soil but may ultimately lead to ground water contamination. Absorbed pesticides are stored in the tissue of organisms, a process called bioaccumulation, or transformed by enzymes of the organisms.
Pesticides remain or persist in soil for a limited time.
Persistence time, which relates to the rate of disappearance, varies from days to years depending on the type of pesticide, soil moisture, organic matter, temperature and pH. Relative persistence times for a variety of compounds are shown in Table 1 . In general, non-persistent pesticides disappear from soil in less than one month, while moderately persistent chemicals take from one to three months. Persistence may differ considerably because of varying environmental conditions and application rates. Most pesticides are chemically stable across a broad range of conditions, but a few degrade spontaneously because of their lack of stability in soil. For example, the herbicides Treflan and atrazine are subject to chemical decomposition.