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Check your soil for herbicide residue

Learn how to conduct plant bioassays to detect potential herbicide residues in your soil

Check your soil for herbicide residue

Plant bioassays are a simple, inexpensive, accurate and direct method of determining if it is safe to grow crops on land previously treated with known herbicides or on cropland or compost with an unknown history of herbicide use.

A bioassay can detect if herbicide or chemical residues are present in the soil or compost at concentrations high enough to adversely affect crop growth, yield and quality.

Six times bioassays are helpful

Bioassays should be conducted:

1. When seeded or established plants exhibit abnormal growth injury.

2. When seeding or planting on areas previously treated with herbicides known to be residual, such as imazethapyr.
3. When using old cropland, which may contain herbicide residues such as picloram or triazines.
4. When using non-cropland, such as railways, roadsides, storage areas, and industrial sites that may contain residues of chlorsulfuron and picloram.
5. When purchasing livestock manure or topsoil of unknown origin.
6. When cropland is treated with clopyralid avoid seeding to legumes.

In all cases of herbicide use, check the manufacture’s guidelines to determine if herbicide residue can be a concern.

The accuracy of a bioassay depends of two factors: your sampling technique, and the depth and area of your soil samples.

For best results, soil samples should be collected and the test conducted about one month before seeding time or during the previous late fall following crop harvest.

Selecting bioassay species

Bioassays are conducted by growing species of plants known to be sensitive to a specific herbicide or class of herbicides in the sample soil, along with the crop to be planted. See the table for a list of plants. If the suspected herbicide contaminant is unknown, a broad range of crop species are grown to help identify the culprit.

If injury occurs to test bioassay plants, then the potential exists for significant field crop injury, particularly if the crops grown are sensitive to that particular herbicide.

Producers, crop specialists, industry representatives and others can all conduct bioassays for herbicide residues.

x photo: File

Seeding and growing bioassay species

To minimize variables in plant growth, make three to four 4″ to 6″ pots for each soil sample.

Seed 10 to 15 seeds of specifically sensitive bioassay species into the submitted “clean” and “contaminated” soil samples. A control, made up of sterilized herbicide-free potting soil, is usually added as a further check.

Make the soil to be bioassayed wet by adding water close to field capacity. Only one plant species is seeded per pot. Label the pot. Place the pots in a greenhouse, in water tight trays, with day/night temperatures of 22 C/18 C and a photoperiod (time the plant is exposed to light) of 16 hours per day, with an eight hour dark night when possible. Otherwise use a bright south facing house window if available.

Let the plants grow for at least three weeks and continue to observe them regularly. Water as required, taking extreme caution to avoid over-watering.

Evaluating plant growth and injury

To accurately diagnose herbicide injury, you need to be able to recognize herbicide-related symptoms and understand herbicide modes of action, fate and behaviour in the soil.

After completing a bioassay for herbicide residue diagnosis, a written report should be prepared. If the bioassay is being done for someone else, provide the client with the written report.

If it is necessary to validate results, send comparable samples to a laboratory for chemical analysis. Chemical analysis confirms the presence and concentration of an herbicide in the soil, but it can be expensive.


When residues of any herbicide or herbicide group are detected (confirmed) in a submitted soil sample plant a crop species that is tolerant of the detected herbicide residue. For example, never plant a crop such as lentils in a field treated the previous year with clopyralid (this includes Lontrel, Tensile and others).

When applied at recommended rates in a crop, many herbicide residues will disappear within a few weeks after application and impose no restriction on cropping options the next year. However, some herbicide residues do not degrade quickly, and can persist in the soil for months or years following application, thereby restricting the crops that can be grown in rotation. Herbicide residues in the soil are deactivated in various ways including:

  • Breakdown by chemical reactions;
  • Breakdown by soil microbe;
  • Escape to the atmosphere as a gas (volatilization);
  • Break down by light (photodegradation);
  • Leaching; and,
  • Binding to soil particles.

Herbicides often disappear form the environment by more than one of these mechanisms. Many herbicides considered to be non-residual are bound temporarily to soil particles while they are broken down gradually by either soil microbes or chemical reactions. The binding action ensures that the herbicide is usually not available to the crop in quantities that will cause damage.

As a general rule, breakdown processes are favoured by warm, moist soil conditions. During the winter, when the ground is frozen, and in the summer when the soil is dry, herbicide degradation is significantly reduced. The residual activity of certain herbicides is also affected by soil organic matter and soil pH, e.g. low pH levels below pH 6 may significantly reduce herbicide breakdown. These soil factors are seldom uniform across a field.

Herbicide carryover is aggravated by low levels of organic matter and is more likely to occur on eroded hilltops than in other parts of a field. The risk of herbicide carryover will also be greater in sprayer overlaps which are most common around headlands and slough margins.

Growers should be aware of the residual properties before applying any herbicide if they are to avoid cropping restrictions in following years. Knowledge of the limitations associated with herbicides that leave a soil residue, along with an accurate record of application (i.e. rates, locations) will serve to minimize rotational problems. Each herbicide used in mixes should be considered separately.

Soil tests using chemical extraction cannot always give a good indication of the potential injury risk from herbicide residue because of the influence of organic matter, clay and pH. Because of this, a field bioassay or laboratory bioassay, where plants are grown directly in the treated soil are best for detecting the potential for injury. These tests are not intended to be used to shortcut restrictions on the label, but provide information on rotational crops where none is available.

Injury symptoms from other causes can resemble herbicide carryover injury (such as cold weather, micronutrient deficiency, flooding, drought, insects, diseases). Consult with your local agronomist or specialist on potential causes before spending money on testing.

Remember, I can explain it for you but I can not understand it for you.

Seven steps to soil sampling

To collect soil reliable examples:

  1. Sample soil from areas suspected of having herbicide residues as well as those areas believed to be free of herbicide. Keep samples separate.
  2. Take separate samples from high spots and low spots, including areas where sprayer overlap could have resulted in an overdose — particularly if the overlap is still visible in standing stubble.
  3. Collect soil samples from the upper surface (0″ to 3″). Most residual herbicides are bound in the upper top of soil (2″), unless the land has been cultivated to a specific depth, in which case, samples should be taken at the depth of cultivation, usually 4″ to 6″.
  4. Using a spade, trowel or soil sampler, take several samples from the suspect area and combine them. Collect enough soil, about four lbs., to fill several 4″ pots.
  5. Along with the soil sample from the herbicide-free area, you may now take one to three additional soil samples, depending on land topography.
  6. Store the samples in cool conditions. If the soil is wet, spread it out to dry, keeping different samples separate to avoid cross contamination. When dry, crush clods to pea-sized particles.
  7. Clearly label the separately bagged samples with detailed information.

About the author


Dr. Ieuan Evans is a forensic plant pathologist based in Edmonton, Alta. He can be reached at [email protected]

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