Dry soil conditions such as in 2012 often slow the rate of herbicide degradation and increase the potential for damage to rotational crops from herbicide carryover, according to a University of Illinois weed science expert.
"Many factors interact to determine how long a herbicide remains active in the soil environment, including factors related to the herbicide, the soil, and climactic conditions," said associate professor of weed science Aaron Hager.
Herbicides vary in their persistence in the soil.
Some herbicides, such as thifensulfuron, have very little soil activity, whereas others, such as picloram, can persist for several months. Chemical characteristics inherent to the herbicide molecule affect persistence, so there is little that can be done to shorten persistence time once it has been applied.
The crop rotation interval specified on the herbicide label is one indication of its persistence. Intervals are longer for herbicides that tend to persist longer and for crops that are more sensitive to residues.
Soil properties also influence persistence. Soil pH can impact how quickly herbicides are degraded by hydrolysis, the process by which the herbicide molecule reacts with water to cleave certain chemical bonds, inactivating it or rendering it less active. Hydrolysis is more rapid in acidic soil. Soil moisture and temperature influence hydrolysis rates, with hydrolysis slowing in dry and cold soil.
Physical properties of the soil are also important. Soils with higher amounts of clay and organic matter have a greater potential for carryover than coarse-textured soils or those with less organic matter. Higher amounts of soil organic matter and certain types of clay particles adsorb more herbicide onto soil colloids, increasing the potential for carryover.
Usually these attractions are weak, and herbicide molecules move from the colloids into the soil solution in a reversible manner. However, lack of soil moisture can cause adsorption forces to become stronger. Herbicides bound to soil colloids cannot be taken up by plants, move downward through the soil profile, or be degraded by microbes. Injury to rotational crops is possible if these bound residues are displaced by water molecules either late in the growing season or the following spring.