Contaminated Compost Equals Gardening Problems

Published: August 4, 2014

Organic matter often has been referred to as “a gardener’s best friend”. In addition to improving soil structure, it provides valuable nutrients to garden plants as it decomposes. For every one percent of organic matter in the soil, about 20 pounds per acre of nitrogen are released as it breaks down. Indeed, a “best management practice” for most garden plots is to add and incorporate thoroughly about four inches of well-decomposed organic matter each year.

Compost is an ideal way to add organic matter to garden soil. Properly prepared, its carbon content should be relatively stable and not tie up nitrogen when it is first incorporated into the soil. Additionally, compost is fairly easy to work with, readily available and relatively inexpensive. Recently, however, there has been a widespread outbreak of plant damage which has been traced back to the use of herbicide-contaminated compost. This article is written in an attempt to explain the cause of the contamination and to offer possible preventative measures.

Symptoms of herbicide damage from contaminated compost include poor seed germination, twisted or malformed new growth, elongated fruit and leaves, and death of younger plants. Contaminated compost can result in the total loss of a crop even if plants do not die. Sensitive crops (Table 1) are more at risk than crops which are less sensitive. However, even crops that are somewhat tolerant might respond to contaminated compost by producing yields that are lower than normally would be expected.

Table 1. Horticultural crops sensitive to picloram, clopyralid, or aminopyralid*

* From the Journal of the NACAA; Vol. 6, Issue 1; May, 2013

Initial reports of the afore-mentioned symptoms came from individuals who made their own compost or obtained some from a neighbor who owned livestock. Recently, however, similar symptoms have been reported by gardeners who purchased commercially available, bagged compost and added it to their garden. The problem, apparently, is still escalating in magnitude.

In nearly every case of damage from contaminated compost, the active ingredient of the herbicide responsible was judged to be either aminopyralid, clopyralid, fluroxypyr, picloram or triclopyr (Table 2). These five herbicides are classified as pyridine carboxylic acids. The latter mimic the action of naturally occurring plant hormones which act as plant growth regulators. Since they are much more potent than the naturally occurring compounds, they are toxic to plants. The EPA has approved the application of these herbicides on pastures and hayfields to control a variety of broadleaf weeds. They benefit the agricultural industry by controlling weeds and producing good quality forage and hay. In fact, some of the weeds controlled are toxic to livestock and can sicken (or kill) animals that forage on pastures or consume hay containing them.

Table 2. Active ingredients and trade names of herbicides.*

* From the Journal of the NACAA; Vol. 6, Issue 1; May, 2013

Of the above herbicides, picloram, clopyralid and aminopyralid are of greatest concern because they can remain active in manure, compost and hay for an unusually long time. These herbicides have the ability to go through the digestive system of animals that consume treated forage. They are excreted in the animals’ manure or urine, and still remain relatively active. When the manure and/or bedding of animals which have consumed treated forage is used to make compost, the result is compost which is contaminated with one or more of the herbicides. Manure applied directly to the garden and spoiled hay used for mulch or compost are additional methods of accidental herbicide contamination.

Herbicides are broken down naturally through the action of light, temperature, moisture and soil microbes. For the herbicides being discussed, this can take from one month to several years, depending on environmental conditions. Piles of manure or heaps of compost do not encourage rapid herbicide degradation. Additionally, hay has been reported to contain active amounts of residual herbicide even after three years of storage. Therefore, damage might occur years after pastures or hay fields have been treated.

While the above is discouraging, all is not lost. There is a simple, inexpensive bioassay that can be performed to test for the presence of herbicides in compost. To begin with, a random sample of the compost (or manure) in question must be taken. This involves taking several shovelfuls from the source at random locations and combining them into one sample.

Next, fill several small pots with at 50:50 blend of the compost sample and a commercial, soilless growing medium. Additionally, fill several more pots with the growing medium only. This will serve as the check or control. Plant several green bean seeds into each pot and water thoroughly. Place the pots in an environment conducive to seed germination and plant growth. If the pots are located close to one another, place a saucer beneath each to prevent drainage water from one pot being absorbed by another. Allow the seeds to germinate and plants to grow for 14 to 21 days. By this time there should be at least three sets of true leaves on the plants.

If herbicide-like symptoms appear on the plants growing in the compost blend (i.e. the treatment) but not on the control plants is normal, we can assume the compost is contaminated. If the growth of both treatment and control plants appears to be normal, then the compost most likely is not contaminated. This simple test only is accurate if one does an adequate, thorough job of sampling the compost being tested.

There are additional strategies for preventing plant damage from contaminated compost and, as with most problems, prevention is the best cure. For example, check with the person/company who made the compost to see if they know the history of the manure or plant material used, or if a bean seed test has been performed. Gardeners who make your own compost but use components obtained from others should ask the same questions of the farmer supplying manure or plant material. Gardeners who have animals (and manure) of their own but purchase hay, should make certain the herbicide application history of the hay is known.

Also, avoid the use of grass clipping for making compost unless the herbicide application history of the grass involved is known. The herbicides mentioned above likely would not be used by homeowners, but they are labeled for turf and might be used by golf courses, parks, etc.

Finally, the question arises of what can be done if contaminated composed unknowingly was applied to a garden. It is safe to plant tolerant crops (e.g. sweet corn) in contaminated soil. Since the degradation time of the herbicides in question is uncertain, perform the bean seed test on contaminated garden soil each year before planting sensitive crops in the area. The use of activated charcoal and Zeolite® has been advocated to absorb herbicides from contaminated soil, but probably is not cost-effective.

When all is said-and-done, the worst thing that can happen from this unfortunate circumstance is for gardeners to stop using compost, manure or mulch. Compost and animal manures represent excellent soil amendments that supply plants with needed nutrients while improving soil structure. Mulch is effective in both water conservation and weed control. Gardeners should be encouraged to continue using these resources but to take extra precautions to prevent accidental herbicide contamination. The bean seed bioassay is an easy, inexpensive way to achieve that goal.