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.