Key Environmental Factors

Acetochlor Registration Partnership

The following information is intended to describe the key environmental factors involved in the movement of a herbicide, such as acetochlor, through soil. In most soil environments, acetochlor is rapidly degraded and therefore poses no threat to ground or surface water quality. However, the ARP has determined that certain soils are particularly vulnerable based on soil texture, organic matter content, and depth to ground water, and have restricted the use of acetochlor on such soils. The following information is meant to help identify where such conditions may exist.

Factors Involved in Transport of Herbicides Through Soil

Several factors can affect the movement of a herbicide through the soil profile. These include soil texture, organic matter, location of water table, and herbicide characteristics such as solubility, persistence (half-life), and the herbicide's adsorption ability.

Soil texture is the percentage of sand, silt and clay. Soil texture directly influences soil's ability to hold herbicides and fertilizer. This process is called adsorption (binding). As silt and clay content increase, adsorption increases because clay and silt bind more than sand.

The following diagram, called the texture triangle, shows the relationship between different kinds of soil textures and their makeup. A soil's total texture needs to add up to 100%. The sum of the percentage of sand, silt and clay at any point in the triangle is 100.

Organic matter content also influences the amount of herbicide and fertilizer adsorption. As organic matter content increases, soil-applied herbicide rates need to increase because the herbicide adsorbs to the organic matter. Soils low in organic matter and high in sand content have the greatest potential for herbicide leaching.

Water tables that are closer to the soil surface generally have the greatest potential for a herbicide, fertilizer or manure to leach into it. The location of the water table may vary depending on the season. Wells are often direct conduits into ground water because they may move through several different levels of aquifers. Therefore, it is important to use anti-backsiphoning devices and properly seal old wells to help prevent contamination.

Definitions Groundwater- the water below the level at which the pore spaces in the soil or rock are fully saturated with water. Aquifer- ground water areas capable of yielding water to springs or wells at a flow rate sufficient to serve as a practical source of water. Unsaturated Zone- the area above the water table. Saturated Zone- the top of the water table. Perched Water- water which saturates a zone above a confining layer, such as clay. When such water is shallow (less than 30 feet) and discharges to surface water without serving as a source of drinking water, it would generally not be considered groundwater.

How are Soil Texture and Organic Matter Determined?

Most herbicide labels use a simplified system for determining soil texture, generally calling it coarse, medium or fine. Acetochlor users who have coarse soil require a more precise determination of soil texture and organic matter.

The determination of the amounts of sand, silt and clay present in soil is called a mechanical analysis or particle size analysis. Analysis is performed by most soil labs. The textural class of soil that contains 15% clay, 65% sand and 20% silt is a sandy loam. A soil containing equal amounts of sand, silt and clay is clay loam.

The following table lists the percentage of sand, silt and clay for each soil texture restriction.

Currently, there are several ways to determine soil texture and organic matter level, including:

• lab analysis - Soil sampling and lab analysis are conducted frequently by most dealers or farmers. However, most labs do not include the soil texture or organic matter level as part of a standard lab fertility analysis; this test usually costs an additional $20-59 per sample.
• use of soil survey - Most counties have had a soil survey conducted by the National Resource Conservation Service (NRCS, formerly the Soil Conservation Services SCS). The soil survey contains information about soil classification and use. The soil classifications indicate the soil texture and organic matter levels. The aerial survey maps give specific field information on soil series. Copies may be obtained at local NRCS offices.
• mapping - In addition, the ARP has completed detailed mapping of certain corn growing counties to determine where the combinations of soil texture and organic matter specified on all acetochlor labels may exist. These maps do not include depth to groundwater data due to extreme seasonal variability.

How to Determine Depth of Ground Water

Most herbicides have the potential for moving into ground water. Point source contamination (such as a spill) is the greatest threat to our ground water resources for all agrichemicals. For acetochlor, the depth to ground water restriction only applies if the soil texture and organic matter level criteria have also been met.

These are several ways to determine the depth of ground water:

• Local water well drillers - These people are probably the most knowledgeable source of information on location of ground water and aquifers. Most well drillers keep logs on local aquifers and drilling depths.
• University or United States Geological Service personnel - Most states land grant institutions have a specialist assigned to soil and water conservation. Some states have offices of the United States Geological Service (USGS) that may have records of ground water depth.
• Irrigation Districts - Irrigation districts often have up-to-date information on ground water depths

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This page was last updated on 13-Jan-03.
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Dave Gustafson
Monsanto Company
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