Testing for and Deactivating Herbicide Residues*
Ed Peachey
Reviewed November 12, 2008
* With permission from H. J. Hopen, Horticulture Facts, VC-15-81, Cooperative Extension Service, University of Illinois, Urbana-Champaign, IL 61801. DTP: put this footnote at the bottom of the first page of this article, then delete this directive text

Residues of triazine herbicides (such as atrazine and simazine) or of substituted urea herbicides (such as linuron and diuron) may persist in the soil for long periods. Attempts have been made to correlate residue persistence with rainfall, temperature, soil characteristics, cultivation practices, method and time of herbicide application, and rate of application. Thus far, however, predicting the extent of residue damage to sensitive crops the year following residual herbicide applications has been only partially successful.
This method is not adequate for, and does not pertain to, testing for sulfonylurea herbicides such as those in Glean, Finesse, and Ally. Consult manufacturer’s label for recommended bioassay method.
Testing for Herbicide Residues
Chemical analyses for herbicide residues are slow and quite complicated. Such tests can be done in only a few specialized laboratories and usually are expensive. Biological assays are more feasible, since they can be done with simple equipment found in most homes or offices. Although the biological assay outlined here does not provide an exact measure of the amount of residue in the soil, the assay will indicate whether residue is enough to harm sensitive crops. The bioassay for atrazine (using oats as a test species) was published in the December 1967 issue of Crops and Soils. That method can also be adapted to test for other residues.
1.  Secure a representative soil sample from the field you suspect has atrazine residue. Sample from several locations, as when collecting soil samples to determine fertilizer requirements. Atrazine residue may be in patches of a field. Sample enough areas to avoid missing ones with a high residue. Headlands and knolls often show the most residue injury. 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 a pea or a 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 with the soil properly.
5.  Add about 0.5 g of activated carbon to half (5 lb) of the soil or of soil–sand mixture. Mix carbon and soil thoroughly. Carbon deactivates atrazine or other 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 others with the soil–carbon mixture. These should be containers holding about a pint to a quart. 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 to 75°F) and get the most sunlight possible. Sunlight usually is essential for the development of atrazine injury  symptoms. Artificial light is much less intense than sunlight and may not be satisfactory for symptom development.
9.  Injury symptoms on seedlings should appear about 3 wk 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. A 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
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 protect transplants (tomatoes, peppers, strawberries, ornamentals, etc.) from  triazine or substituted urea herbicides. Activated carbon can also be used to “clean up” pesticide spills.
Other herbicides that carbon can deactivate include trifluralin (Treflan), bromacil (Hyvar X), benefin (Balan), bensulide (Betasan and Prefar), DCPA (Dacthal), dichlobenil (Casoron or Norosac), EPTC (Eptam), 2,4-D, terbacil (Sinbar), and chloroacetamide herbicides such as metolachlor or dimethenamid. 
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 and the like 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 absorb molecules into its vast pore structure. The phenomenon of adsorption can take place either in gaseous or liquid phase systems. The adsorption is often selective when applied to systems containing more than one component. Two examples of this are using activated carbon in gas masks to remove poisonous vapors and as an antidote for accidentally ingested poisons.
Where to Obtain Activated Charcoal
Some garden supply centers carry packaged activated carbon designed for the uses outlined here. Several brands of a similar type of carbon are available.
Activated carbon is used extensively in dry cleaning and water-purification. Usually, the names of local distributors can be found in the Yellow Pages under “Cleaners and Driers’ Supplies,” by contacting a dry cleaning establishment, or searching the internet (e.g., GROW-SAFE at http://www.buyactivatedcharcoal.com/product/GRO). Some dry-cleaning carbons may contain additives that will make them unsuitable.
Such local businesses are an especially valuable source of carbon in rural areas, where delivery from a distributor may be slow. Activated carbon is offered in containers of 1 to 50 lb. Small quantities of purified activated carbon are available at pharmacies and at chemical supply houses.
Charcoal Application
(Modified for Oregon conditions)
Applying activated charcoal should be considered as an emergency treatment for herbicide residues from previous crops, spills, or changes in crop rotations. The efficiency of deactivation depends on the soil’s organic matter content and physical condition, the herbicide’s activity, and the crop’s sensitivity. For differences between herbicides, see G. F. Warren, 1973, “Use of Activated Carbon to Inactivate Herbicide Residues,” North Central Weed Control Conference, 28:68–69.
If an area is contaminated with undesirable herbicide residues and a susceptible crop is to be planted, apply activated carbon at 200 lb/A (0.5 lb/100 sq ft) for each pound per acre of actual residue. 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 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.
Plan carefully. Using activated carbon is a poor substitute for a well-planned weed control program.
For use in grass seeding, see “Grass Seed Crops” in this handbook.