Pesticide Persistence in Soil

Do not apply volatile pesticides in dry, windy weather.

Pesticides that persist or remain active in soil can have either harmful or beneficial effects. Persistent pesticides can injure non-targeted crops. Pesticides that quickly leach from soil are effective for a shorter period of time, and can pollute groundwater. Highly volatile pesticides can drift, injuring not just crops, but also animals and humans. Although persistent pesticides can injure crops in subsequent plantings, they remain effective against weeds and pests for a longer period and are less likely to leach and pollute water.

Soil Composition

Soil composition can affect a pesticide's persistence. Pesticide molecules and soil particles with opposite magnetic charges bind to each other. Rich clay soil particles have greater surface area than coarse soils. According to the Pennsylvania State University Extension, pesticides bind with clay soils and soils that contain at least 3 percent organic matter more easily than with sandy soils. Pesticides quickly leach from sandy, coarse soils, because water easily transports the pesticides through spaces between the soil particles. In wet soils, water molecules bind with soil particles and interfere with pesticide uptake.

Soil Chemistry

According to the University of Illinois Extension, soil chemistry, such as pH level, can affect pesticide persistence. Harmful or beneficial effects depend on factors such as pesticide type, target pests and crops. For example, triazine herbicides bind with alkaline soil particles at a lower rate and are readily available for weed uptake. On the other hand, imidazolinone herbicides adhere to acidic soil particles and may damage non-target crops in later plantings. Through a process called hydrolysis, water can break down organophosphate pesticides within hours in alkaline soil. Strong sunlight can cause pesticides on the soil surface to break down.

Soil Microorganisms

Soil microorganisms play a vital role in pesticide breakdown. Microorganisms such as bacteria and fungi are most active in warm, moist, fertile soil with a pH level near 7.0. These organisms feed on pesticides and can rapidly multiply with repeated applications of the same pesticide. Breakdown can occur so quickly that the pesticide becomes ineffective. To prevent excessive microbial activity and pesticide loss, the University of Missouri Extension recommends applying pesticides at the lowest effective rate according to label instructions, and rotating different types of pesticides. For example, rotate photosynthesis inhibitor atrazine, which breaks down slowly, with growth regulator 2,4-D, which breaks down quickly.

Pesticide Properties

Water solubility, vapor pressure, chemical structure and other pesticide properties can affect a pesticide's persistence in soil. For example, water-soluble pesticides dissolve in water and easily leach from the soil. A pesticide with high vapor pressure quickly turns into a gas and evaporates from the soil. Avoid using volatile pesticides, such as thiocarbamates, during hot, dry, windy weather.

Solutions

To avoid injury from pesticide persistence, always read and follow label directions. Apply pesticides as early in the season as possible for your crops in your area to prevent injury to later plantings. Do not plant rotational crops in soil that contains potentially damaging herbicides. For example, the University of Illinois Extension explains that soybeans are sensitive to triazine herbicides. To encourage microbial activity and pesticide breakdown, till the soil after applying pesticides.

References

• Pennsylvania State University Extension: Persistence of Herbicides in Soil; William S. Curran; 1998
• University of Illinois Extension: Herbicide Persistence and How to Test for Residues in Soil; Aaron G. Hager and Dawn Nordby; 2007
• University of Missouri Extension: Pesticides and the Environment; Fred Fishel; July 1997
• University of Minnesota Extension Service: Herbicide Mode of Action and Injury Symptoms; 1999