14.14 Fate of Herbicides

What happens to herbicides after they have been applied to the field?  There are several different possibilities.

• Bacteria or other microbes can break down the herbicide molecules into smaller, non-active components. These microbial organisms exist in the soil environment, plant residue, etc.
• Ultraviolet light from the sunlight can also degrade or break down the herbicides.
• Temperature can also have an effect. Freezing or a high heat condition can degrade the herbicide chemistry.
• Normal chemical reactions are taking place in the environment, which can interact with herbicides. For example, a herbicide salt might bind to some of the soil particles.
• The presence of excess soil water/moisture can lead to herbicide runoff or leaching.  Just as we have to be aware of fertilizers that can leach from the soil profile, the same is true with herbicides.  Water can also create a chemical reaction with some herbicides, where hydrolysis breaks down the herbicide chemistry.
• Some herbicides can change directly from liquid to gas form (volatilization).
• Plants can metabolize or break down the herbicide chemistry. In some cases, rather than breaking down the herbicide molecules, the plants can compartmentalize the herbicide molecules into parts of the cell where the herbicide will not have as much of an effect. So rather than actually breaking it down, they put it an area where it is more safe for the plant.