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. An increase in the size of the pesticidedegrading populations leads to faster rates of degradation.
Most pesticides transform to simple, non-toxic products such as carbon dioxide and water.
Some pesticides are resistant to microbial decomposition because they are toxic to microorganisms or do not support their growth. The most important soil factors that influence biodegradation are temperature, moisture, presence or absence of oxygen, organic matter, and clay content. Some microorganisms function best under aerobic conditions, while others are most active in the absence of oxygen. Microbial abundance and activity are stimulated by the addition of organic materials to soil. This usually results in faster rates of pesticide degradation.