Pesticide Adsorption
The adsorption process binds pesticides to
soil particles and/or plant parts, similar to iron filings or paper clips
sticking to a magnet. Adsorption often occurs because of the attraction
between a chemical and soil or organic particles. For example, positively
charged pesticide molecules are attracted to and can bind to negatively
charged clay particles and plant debris.
Many soil factors influence pesticide
adsorption. Soils high in organic matter or clay are more adsorptive than
coarse sandy soils. This occurs, in part, because a clay or organic soil
has a greater particle surface area or number of sites onto which
pesticides can bind. Soil moisture also influences adsorption. Wet soils
tend to adsorb less pesticide than dry soils because water molecules
compete with the pesticide for the binding sites on soil particles.
Pesticides may adsorb onto plant materials such as litter in no-till or
minimum-till fields, the bark of trees, or thatch in turf. These organic
layers may prohibit pesticide movement to target areas deeper in the soil.
Pesticides vary in their tendency to adsorb to soil particles. Some
pesticides, such as paraquat and glyphosate, bind very tightly, while
others bind only weakly and are readily desorbed or released back into the
soil solution. Adsorption is particularly important because it influences
whether other processes can affect pesticides. |