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23. Determine Your Pesticide's Potential for Leaching

leaching diagram

When a pesticide is put down, it doesn't always stay put. Pesticides follow any number of pathways.

Pesticides can enter plants, insects, and other living organisms. They can volatilize, which means they change into a gas. They can be broken down by sunlight, microorganisms, or chemical reactions with water, and they can chemically attach to soil and organic particles. They can also remain in a dissolved form, then leach with water through the soil, sometimes winding up in groundwater.

pesticide degradation Among the most important factors that affect the risk of pesticide leaching is pesticide degradation. This is the breakdown of a pesticide into compounds that are usually less toxic.
half life The longer that a pesticide persists in the soil, the more chance there is of leaching. Persistence is usually measured in "half-lives. " A half-life refers to the amount of time it takes for one-half of the pesticide to be degraded.
soil absorption Another key factor is "soil adsorption"—the attachment of pesticide to soil particles. When pesticides attach more strongly to soil particles, they are less likely to leach.
volatility and water solubility Two other factors to consider are volatility, the rate at which a pesticide will change into a gaseous form, and water solubility, the likelihood that a pesticide will dissolve in water. When a pesticide dissolves in water, there is more risk that it will be carried into surface water or leached into groundwater.
pesticide leaching rating To find out how these factors affect pesticide leaching, contact the Natural Resources Conservation Service (NRCS). The NRCS takes many chemical characteristics into consideration and ranks pesticides according to their potential for leaching. They group pesticides in four categories: large, medium, small, or extra small potential for leaching.
spraying a field Once you know your pesticide's leaching potential and your soil's leaching potential (see Chapter 22), put the pieces of information together with the soil-pesticide interaction screening procedure (see Chapter 24).

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