Dry soil conditions, such as such as those in Illinois in 2012, often slow the rate of herbicide degradation and increase the potential for damage to rotational crops from herbicide carryover, according to Aaron Hager, 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,” says Hager, associate professor of weed science.



Herbicides vary in their persistence in the soil. Some, 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 the herbicide 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 herbicide residues.

Soil properties also influence herbicide 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 herbicide 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 herbicide 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. Herbicide bound to soil colloids cannot be taken up by the plant, 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.

 

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Many herbicides are degraded by soil microorganisms, such as fungi, bacteria, actinomycetes and algae. Soil type, pH, organic matter content and moisture influence soil microorganism composition. The rate at which soil microorganisms degrade herbicides depends on environmental and soil conditions affecting the microbial species composition and population, such as temperature and moisture. The activity level of most soil microorganisms increases with soil temperature. Minimal herbicide degradation occurs when soil temperatures approach 40° F.



Soil microorganisms need moisture to degrade herbicides rapidly. Under extremely dry conditions, the degradation rate can slow enough to allow the herbicide to persist into the next growing season.