Test for hidden poisons in compost, manure, wheat straw

Test peas affected by herbicide in mulch (yellowing leaves)

[Cat’s NOTE: This post, by ESP gardening presenter Don Bates, concerns herbicide damage to garden crops including tomatoes, peas, beans, lettuce and several other crops. The herbicide can be lurking in compost, manure and wheat straw, as well as airborne from sprays. See also my earlier post: Alert: Poisoned Soil, Compost, Manure]

by Don Bates, July 11

(all photos by Don Bates)

Given that many of our ESP community are gardeners, I thought this to be worth posting. It concerns an herbicide which is nearly ubiquitous, and so, probably off people’s radar.

Some of you may remember hearing Jean H’s story from 2009 regarding contamination of her property with the herbicide Milestone [aminopyrlid].  At the time, there were similar stories circulating in the press which prompted me henceforth to test all materials (straw mulch and manure) that I imported into my garden.


A simple test

There is a simple and quick test that involves growing a susceptible plant in a medium that has been dosed with the suspect material.  A control sample is grown alongside it, using the uncontaminated medium (see Organic Authority: Hold the Herbicides: Testing for Soil Contamination for one description of this test.  You can find much more rigorous descriptions of the same basic test on the Internet).

Test peas showing damage by herbicide in mulch

Test peas showing damage by herbicide in mulch

[photo, above, shows control pea on left, contaminated pea on right, with yellowing leaves].

Last year I bought some wheat straw from CHS, which had been grown by a local farmer.  I inquired as to what the wheat had been sprayed with, and was told that 2,4-D was among the chemicals used on that crop. (2,4-D is the ingredient in Weed-be-Gone, and is commonly used for broadleaf weed control on wheat.)  However, I tested a sample of this straw using the above-mentioned test, and found no ill effects.  I used it on my garden.

Contaminated wheat straw

This year, I bought some more.  Same vendor, same farmer.  I ran the test, expecting a null result, and was surprised to see that there clearly was some residue of some ag-chemical in the straw.  I don’t know what it is, but suspect 2,4-D. (see photo. Contaminated sample on right).  The amount of contamination here does not seem to be fatal, but it certainly is enough to significantly affect plant growth.  I’ll let these grow further, and see what happens.  I am now tasked with the chore of removing and discarding the straw that I prematurely mulched my garden with.

The moral of this story is that we all need to be very vigilant about what we let into our gardens.

Some herbicides are worse than others

While Milestone is a quite persistent herbicide, 2,4-D (Weed-Be-Gone) and glyphosphate (Round-Up) both break down fairly quickly in the presence of soil microbes, so their long term consequences are not nearly so great.

References, and For More Information

  1. Organic Authority article: Hold the Herbicides: Testing for Soil Contamination (www.organicauthority.com/organic-gardening/organic-gardening/hold-the-herbicides-testing-for-soil-contamination.html)
  2. Washington State Univ. Extension: Bioassay Test for Herbicide Residue in Compost (www.puyallup.wsu.edu/soilmgmt/Pubs/CloBioassay.pdf)
  3. The EssentiaList: Alert: Poisoned Soil, Compost, Manure

6 Responses to “Test for hidden poisons in compost, manure, wheat straw”

  1. Jeffrey Funk says:


    I have been aware of this problem for some time, and might have had some slightly contaminated manure at one point, but am not sure. A couple of comments….

    First, the testing you refer to is certainly a good idea.

    Also, in order to minimize the possibility of inadvertently poisoning your garden, using mulches that include broadleafed plants pretty much guarantees that these toxics will not be present. [The herbicides kill broadleaved plants, so if they are present in the mulch, it is not likely that the herbicides have been used].

    In using manures, if the feed hay contains alfalfa, then you are going to be safe for the same reason [the herbicides also kill legumes like alfalfa].

    In our garden we make significant use of mulch made from [leaves of] maple, ash, cottonwood, etc. Unless these have been mixed with lawn clippings (which very likely do contain broad leaf herbicides), they are not only safe, but break down faster and with a much lower nitrogen requirement than straw or wood based mulches. These leaves are available in essentially unlimited quantities for free from the city of Kalispell.

    This year we are mixing them with cow manure to accelerate the composting of both materials, expecting to get finished compost in the course of a summer. Thanks for your alert to the widespread use and danger of herbicides in mulch and manure.

  2. Catherine says:

    Thanks, Jeffrey for the great comment.
    Do you know whom to contact (and telephone number) at the City of Kalispell, regarding the leaves, should anyone want to use them? I can add that to the post, or better yet, create a new post with that info.

  3. Don Bates says:

    Good points, but I think your point about using mulches containing alfalfa is obsolete. Monsanto’s “Round-up Ready Alfalfa” now makes it possible to treat alfalfa with Round-up, without killing it. It is now very likely that purchased alfalfa will contain this residue. Chalk up another one for agri-business.

  4. Catherine says:

    Another good point. This is what is so wonderful about having our own community website where we can share knowledge and experience with others in our community, toward living a healthful life while working toward sustaining the health of our planet.

  5. Jeffrey Funk says:


    I am aware that Monsanto has developed this alfalfa, but I am not sure it is permitted in Montana at this point. I will try to find this out. It is a scary world out there when corporations like Monsanto have license to poison the soil.

  6. Elizabeth Roycroft says:

    Thanks for this very useful info. Wish I had read it a year ago, since I used wheat mulch from Lowes this year and had germination failure and poor growth in transplants which I couldn’t understand. Finally figured out the only thing I was doing differently was getting my mulch from a different source.

    Puzzle solved.