Each year we are finding more resistant weeds. As we prepare for upcoming pre-emergent herbicide applications it is important to understand the characteristics of these herbicides and adjuvants.
Leaching is the movement of contaminants, such as herbicides, carried by water downward through permeable soils. Leaching occurs when a herbicide is dissolved in water and moves down through the soil profile. Pre-emergent herbicides that readily leach may be carried away from weed germination zones causing loss of both efficacy and expensive inputs, as well as environmental harm. The fate of these mobile herbicides depends on a race between the various degradation processes and leaching to groundwater.
Herbicide leaching is determined by several factors. These include herbicide-soil binding properties, soil physical characteristics, rainfall and irrigation frequency and intensity, and time of application.
Soil Characteristics that Affect Leaching
Organic matter content is an important soil property when considering leaching and pesticide breakdown by microorganisms. Organic matter in the soil provides more surface area for adsorption (or holding herbicides in the soil profile), and increases the soil’s ability to hold water and degradation by microbial activity. Soils low in organic matter lose some ability to hold herbicides where they are most effective.
The proportions of sand, silt, and clay, also known as soil texture, affect the movement of water through the soil. Coarse-textured soils containing more sand particles have large pores and are highly permeable. This can allow herbicides to move rapidly through the profile. Clay-textured soils have low permeability. A soil containing large proportions of clay holds more water and adsorbs more chemicals. This slows the downward movement of pre-emergent herbicides, keeping them in the weed killing zone and reducing the chance of groundwater contamination.
The amount of water already in the soil has a direct bearing on whether rain or irrigation results in the recharging of groundwater and potential leaching of herbicides. Soluble chemicals are more likely to reach groundwater when soil water content approaches or is at saturation. Saturation is typical in the spring when rain and snowmelt occur. Unfortunately, this is also a key time to apply pre-emergent herbicides. Conversely, when soils are dry due to lack of rainfall, irrigation fills the pores in the soil near the soil surface making leaching less likely.
Mitigating Herbicide Leaching
Applicators can use several approaches to decrease the loss of herbicides due to leaching. Monitor forecasts for a few days after a proposed herbicide application for periods of intense rainfall and avoid, if possible, these application windows. Similarly, schedule irrigation to provide the necessary irrigation required by the label, but not enough to flush herbicides through the root zone. Split applications may also be considered to stretch residual control over a longer period.
In sites with highly leachable soils (coarse, sandy), use of an oil-based surfactant designed to hold mobile pesticides in the root killing zone can be very effective. Grounded® is a specialized blend of surfactants designed to enhance the absorption of herbicide applications. Using this technology, herbicides are held in the soil profile for a longer period of time, allowing optimum performance and reduced leaching.
Contact your local Helena representative for all of your pre-emergence herbicide and adjuvant needs for 2025!