Chemicals applied to soils may undergo biological or chemical transformations, be taken up by plants, escape to the atmosphere in vapour form, be lost to surface water by surface water flow and erosion, or remain in the soil and be available for leaching to groundwater. The importance of these different processes depends upon the chemical of interest. Active ingredients in most pesticides break down or degrade over time as a result of microbiological and chemical reactions. Pesticides with low persistence degrade quickly to other products.
Most biological breakdown occurs in the root zone of the crop since more microbes exist there.
Pesticide degradation rates also depend upon soil temperature and water content. Pesticide persistence is usually specified in terms of the degradation half-life. These half-life values are typical values for acifluorfen sodium salt, bentazon sodium salt, and metolachlor, respectively. The curves show the amount of pesticide remaining at different times if 1 kg of each product was applied to the soil. The amount of degradation needed depends upon the initial amount applied, the toxicity of the product, and the manner in which it mixes in the aquifer.