Testing for and Deactivating Herbicide Residues

Ed Peachey
March 2022

Residues of triazine herbicides (such as atrazine and simazine), substituted urea herbicides (such as linuron and diuron), clopyralid, or fomesafen may persist in the soil for months. Herbicide labels include rotation, or plant-back, intervals for many crops, but it is often prudent to determine whether harmful residues are present in the soil before planting, particularly when planting very sensitive crops, or when renting land, even when label guidelines are followed. Testing for herbicide residues also can be helpful when attempting to determine the cause of unknown crop injury or failure.

Testing for Herbicide Residues: Labs and Bioassays

There are two main options for testing for herbicide residues in soil. The first option is to send a soil sample to a lab for analysis. Chemical screens are performed by many labs for a wide array of herbicides. However, lab analysis may be costly, time consuming, and misleading. Additionally, predicting potential crop damage that may result from herbicide residues detected by laboratory soil analysis is challenging.

Another option is to conduct a bioassay by planting crops of interest in soil collected a few weeks before the scheduled planting date. The following example is specifically designed to test for atrazine residues in soil, and therefore uses oat as the indicator species, because oats are very sensitive to atrazine. A similar tactic can be used to test for residues of other herbicides.

Example: Testing for atrazine residues in soil.

  1. Secure a representative soil sample from the field you suspect contains atrazine residue. Sample from several locations, as when collecting soil samples, to determine fertilizer requirements. Atrazine residue may be found in patches of a field. Sample enough areas to avoid missing areas that might contain a high residue. Take separate samples from areas where residues may be excessive. Always sample to the full depth of the plow slice, whether or not the field is plowed. Remember that the assay is only as reliable or representative as the samples. Each sample to be assayed requires about 10 lb of soil.
  2. Assay samples within a week or two after they are collected from the field. If the samples cannot be assayed immediately, store the soil in a cold place—in a freezer if possible. When samples are stored indoors under warm conditions, the atrazine residue may be lost.
  3. If the soil is wet, spread it out and allow it to dry so it can be worked readily. If the soil is cloddy, crush clods to the size of peas or wheat seed, but do not pulverize the soil.
  4. Adding about 50% by volume of coarse sand will improve the physical condition of silt and clay soils. If sand is added, mix it well with the soil.
  5. Add about 0.5 g of activated carbon to 5 lb of the soil, or of the soil–sand mixture. Mix carbon and soil thoroughly. Carbon deactivates atrazine or other herbicide residue. For purposes of comparison, soil treated this way provides the equivalent of soil without residue.
  6. Partially fill two containers with soil that does not contain carbon, and two containers with the soil–carbon mixture. The four containers should hold about a pint to a quart each. Punch holes in the bottom of the containers to allow drainage. Tin cans, paper milk cartons, or ice cream cartons are satisfactory for this purpose.
  7. Plant five to eight oat seeds (or seeds of vegetable species of interest) in each container; cover seeds with about 0.5 inch of soil. Wet the soil with water but do not saturate. After emergence, thin to three plants to ensure maximum uptake or absorption of possible residue.
  8. Place containers where they will be warm (about 70°F to 75°F), and receive as much sunlight as possible. A strong light source will help development of atrazine injury symptoms.
  9. Injury symptoms on seedlings should appear about 3 weeks after planting. If temperatures are below 70°F, more time is required. Water plants sparingly. Do not allow soil to dry out.
  10. Severe triazine injury is characterized by drooping leaves and by leaf-kill that extends from the leaf tip toward the base. Leaf-kill indicates a significant amount of residue in the soil. Marginal residue content will stunt the oats’ growth without killing the leaves. Stunting can be determined by comparing the growth of oats in soil with carbon. Oats grown in soil with carbon should be normal, and should show no atrazine injury or stunting, unless extremely high residues of atrazine are in the soil sample.
  11. If the oats show any evidence of leaf-kill or stunting, plant an atrazine-tolerant crop in the field from which the samples were obtained.

Using Activated Charcoal to Remediate Contaminated Soil

Activated charcoal (or carbon) can reduce herbicide contamination in specific areas (gardens, greenhouses, lawns, etc.) and can also be used as a root dip to partially protect transplants (tomatoes, peppers, strawberries, ornamentals, etc.) from triazine or substituted urea herbicides. Activated carbon can also be used to mitigate pesticide spills.

Other herbicides that carbon can deactivate include trifluralin (Treflan), bromacil (Hyvar X), benefin (Balan), bensulide (Prefar), DCPA (Dacthal), dichlobenil (Casoron), EPTC (Eptam), 2,4-D, terbacil (Sinbar), sulfentrazone (Zeus or Spartan) and chloroacetamide herbicides such as S-metolachlor (Dual Magnum) or dimethenamid-P (Outlook) and others.

Activated carbon, used in a wide range of applications in diverse industries, is made by heating or chemically treating organic matter to create a porous structure. This gives a large surface area within a relatively small volume. Most activated carbon products are purified by acid and water washes to remove impurities and are available in both granular and powdered form. Charcoal for outdoor grills cannot be ground up to achieve the same pore structure characteristics of activated charcoal on a pound-for-pound basis.

The usefulness of activated carbon is based primarily on its ability to hold molecules within its vast pore structure. The phenomenon of adsorption can take place either in gaseous or liquid phase systems. Adsorption is often selective when applied to systems containing more than one component, for example when using activated carbon in gas masks to remove poisonous vapors, and as an antidote for ingested poisons.

Where to Obtain Activated Charcoal

Some garden supply centers carry packaged activated carbon that is designed for the uses outlined here. Activated carbon is used extensively in dry cleaning and water-purification. Usually, sources of activated charcoal can be quickly found by searching online (e.g., https://buyactivatedcharcoal.com/) or by contacting commercial agriculture retailers in the PNW. Activated carbon is offered in containers of up to 55 lb. Small quantities of purified activated carbon are available at pharmacies and at chemical supply houses.

Charcoal Application

(Modified for PNW conditions)

Use of activated charcoal should be considered as an emergency treatment for herbicide residues from previous crops, spills, or changes in crop rotations. Before using activated charcoal, however, consider that the herbicide label directions for rotational restrictions must take precedence over efforts to deactivate the herbicide. The efficiency of deactivation depends on the soil’s organic matter content and physical condition, the herbicide’s activity, and the crop’s sensitivity. This treatment will work for some herbicides better than others.

If an area is contaminated with undesirable herbicide residues and a susceptible crop is to be planted, apply activated carbon at 200 lb/A (approximately 5 lb/1,000 sq ft) for each 1 lb ai/a of actual residue detected in the soil. A rule of thumb is that in a soil sample from 0 to 6 inches, a 1 ppm test result would be equivalent to 2 lb active ingredient (ai) of the herbicide/acre or 0.046 lb ai/1,000 sq ft.

The carbon can be mixed at the rate of 1 lb carbon to 10 lb sand and applied with fertilizer-spreading equipment, or sprayed using large-capacity nozzles (0.5 gal/minute or larger). Carbon wets and suspends poorly. Local commercial applicators are available in some areas such as the Willamette Valley of Oregon. Otherwise, a grower can add the carbon to a partially filled spray tank and use the remaining water to help mix the floating carbon while the agitator is operating. Incorporate activated charcoal to 6 inches deep in the soil, then irrigate and let set for several days before planting. A bioassay is recommended to confirm effectiveness.

Reference: H. J. Hopen, Horticulture Facts, VC-15-81, Cooperative Extension Service, University of Illinois, Urbana-Champaign, IL 61801.