How to test your compost (for persistent herbicides)


Hello everyone!

It’s that time of the year again – soil amendment time, and you know what my favorite amendment is, right? It’s compost! Many people are blessed with enough space to have a compost pile in their yard, but even they probably don’t make enough for all their garden needs. One simply can not have too much compost, is my opinion. So we do end up purchasing it from commercial sources, and it’s great that we have that option. The problem is, not all compost suppliers are created equal, and we don’t always know how strict they are about what inputs go into their compost heaps. For example, if they accept grass clippings, do they make sure that the lawn has not been treated with herbicides? Same goes for hay – did the grower use any chemicals?

The herbicides used today, especially the ones that favor grasses (thus used on lawns and hay fields), are so stable that they persist in the dead plants even after commercial composting process, which gets pretty hot (up to 150F). They also are very potent. So if a fraction of treated grass or hay (even completely composted) ends up in your garden bed or planter, your plants are doomed (unless they are grasses, of course). The US Composting Council is a wonderful organization that monitors select compost suppliers and ensures the safety of their product, so if you can get your hands on some certified compost – lucky you! Fortunately for the rest of us, who have to buy bags or bulk piles of non-certified compost, there is a simple (and free) two-step test, that I described last year. Since I don’t have anything new to add, I’m just going to repeat it here:

  1. Take a look (and a sniff). A good batch of compost should look like very dark (brown-to-black) soil and smell like soil (no foul or funny smells). It should also have a certain texture, once again, resembling rich, fluffy soil.

    My two test subjects: compost from The Dirt Factory (wonderful place, but had to close last spring, hopefully, temporarily) on the left, and Harvest Mushroom Compost on the right.
  2. The grow test.
    1. Choose some seeds: beans are a great choice, because they sprout so fast, and they are very sensitive to herbicides, so you will be able to detect even low traces of growth-retardant residues.
    2. Fill the pots with your compost and sow the seeds.day1
    3. Wait for the seeds to sprout.
    4. Get your answer! In my case, both kinds of compost performed well, with the seeds growing a bit faster in the mushroom compost.

      And the winner is…

Did you purchase any commercial compost this year?

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