Soil properties that influence herbicide persistence can be broadly categorized into chemical, physical and microbial. One of the more important soil chemical factors that influences persistence of some herbicides is pH. Soil pH, a measure of the concentration of hydrogen ions present in the soil solution, determines whether a soil is considered acidic, neutral or basic. At higher soil pH values, weakly acidic herbicides, such as many sulfonylurea herbicides, exist in the negatively charged or anionic form. The anionic form is only weakly adsorbed to the clay and organic matter components of the soil, and therefore more of the herbicide remains in the soil solution and available for plant uptake. At lower soil pH values, weakly acidic herbicides exist in the neutral form or, in some instances, the positively charged (or cationic) form. These neutral and cationic forms of the herbicide are more strongly adsorbed to the negative surfaces of clay and organic matter, and therefore are less available for plant uptake.
Soil pH also can impact how much and how quickly certain herbicides are degraded by a chemical process known as hydrolysis. Hydrolysis is a process by which the herbicide molecule reacts with water (“hydro” means water) to cleave (“lysis” means to split or degrade) certain chemical bonds of the herbicide molecule and render it either much less active or herbicidally inactive. The rate of chemical hydrolysis is influenced by the pH of the soil; hydrolysis occurs more rapidly under acidic conditions than under neutral or basic conditions since the weakly acidic herbicide molecule exists in the anionic form under basic conditions and is much less susceptible to hydrolysis. The rate of chemical hydrolysis also can be influenced by soil moisture and temperature, with hydrolysis slowing under dry and cold soil conditions.
Physical properties of soils are related to the relative amounts of sand, silt, clay, and organic matter a soil contains. Soils with higher amounts of clay and organic matter have a greater potential for herbicide carryover than coarse-textured soils or those with less organic matter. Higher amounts of soil organic matter and certain types of clay particles increase the potential for herbicide carryover by adsorbing more herbicide onto these soil colloids. Adsorption is the adhesion of ions or molecules to the surface of soil colloids. Usually these types of attractions are weak such that herbicide molecules move from the colloids into the soil solution in a reversible manner. However, adsorption forces can become stronger over time given certain soil conditions (such as lack of soil moisture). Herbicide bound to soil colloids is not available for plant uptake, movement downward through the soil profile, or microbial degradation. Injury to rotational crops is possible if these bound residues are displaced by water molecules either late in the season they were applied or the following spring.