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. The process of photodegradation involves the chemical transformation of substances under the action of sunlight.
Photochemical reactions are limited to the soil surface.
Consequently, once pesticides are incorporated into soil, they are protected from photochemical transformation. The principal reason for the disappearance of most pesticides from soil is microbial decomposition. Common soil bacteria and fungi can degrade the majority of these compounds. On the other hand, 2,4-D disappears quickly because it degrades by a variety of common soil microorganisms. Usually as microorganisms carry out degradation, they obtain carbon and energy that they require for growth.