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Factors Affecting Soil-Applied Herbicides

To be effective, soil-applied herbicides must be available for uptake by the roots and/or shoots of germinating weed seedlings. This means that they must be dissolved or suspended in the soil solution. Soil properties that affect the availability and activity of soil-applied herbicides include soil texture, organic matter level, and pH. All should be considered when determining herbicide rates.

Soil texture is determined by the relative percentages of sand, silt, and clay in a soil. Names describing texture such as loamy sand and clay loam are assigned to soils depending on these percentages. Clay particles are negatively charged and have a large surface area. As a result, soils high in clay content (heavy soils) have the capacity to adsorb or tie up herbicides and generally require higher herbicide rates than coarse-textured or light soils.

Organic matter content also affects adsorptive capacity of soils. Although un-decomposed plant and animal residues can influence herbicide performance, the well-decayed, fine organic matter particles known as humus are of greatest importance. Like clay particles, humus particles are negatively charged but exhibit an even greater capacity to adsorb or tie up herbicides than clay. Consequently, herbicide rates also have to be adjusted to the soil organic matter level.

Rates for soil-applied herbicides in the chemical weed control tables (Tables 3.7.3, 4.11.1 and 6.7.2) are for medium-textured (loam) soils with organic matter levels of 3 to 4 percent. Fine-tuning the rates for other soils can be done by consulting the herbicide label for different soil textures and for varying organic matter levels.

Soil pH can also affect the availability of some soil-applied herbicides. This is important for the triazine herbicides (*atrazine and *†Princep). These herbicides are most strongly adsorbed (tied up and unavailable for uptake by weeds) on clay and organic matter particles at low pH levels. Although the amount of triazine adsorption increases at all pH levels below 7.0, adsorption is most dramatic at pH levels of 6.0 and below. This is an important consideration for continuous zone/no-tillage fields where the surface inch of the soil profile may have a lower pH than is reflected in the results of a normal soil sample analysis. It may be helpful to check the pH in the top inch of the soil profile with a soil pH kit in fields that have been in zone/no-tillage for extended periods.

Soil pH also affects availability of Python WDG (flumetsulam), of soil applied *†Hornet WDG (clopyralid + flumetsulam) and of *†SureStart/*†TripleFlex (acetochlor + clopyralid + flumetsulam). These herbicides should not be applied to areas where soil pH is greater than 7.8 as this may result in unacceptable crop injury. In addition, soil applications of Python WDG, *†Hornet WDG, or *†SureStart/*†TripleFlex should not be made to soils with more than 5% organic matter if soil pH is less than 5.9 as reduced weed control will result.