I was recently copied on an email to a community garden regarding the safety of city compost. The email pretty much said “Don’t worry, the city says that the compost is OMRI (the Organic Review Materials Institute) approved, therefore it’s safe.”
But it turns out that OMRI doesn’t require testing for herbicides or pesticides! So you could have city compost with nasty stuff in it, even though it has passed all OMRI requirements.
And this isn’t hypothetical problem: a quick search of the internet turns up a number of incidents where nasty persistent chemicals, both pesticides and herbicides, have been found in city compost and have caused big problems for backyard gardeners. Persistent chemicals are ones that survive the composting process and remain in the soil for years.
I found a very relevant article on the topic that shares important information and perspective:
When you think about it, it’s not surprising that city compost contains these products. Practically every garden store has a full assortment of herbicides and pesticides, and people buy them and use them. Even if vegetable gardeners are less likely to use them, you’ve got the lawn industry, which is probably 100 to 1000 times larger than the vegetable gardening industry.
All of this yard waste, and the chemical residue on them, go into the collection bin, which goes into the shared city compost. Many of these chemicals are not broken down by the composting process, and they can go on to cause terrible problems for the vegetable gardener.
It’s critical to understand, as well, that OMRI doesn’t require testing for pesticides:
The nonprofit Organic Materials Review Institute, which reviews organic inputs, requires approved composts to be tested for heavy metals and pathogens every five years. The US Composting Council operates a voluntary accreditation program in which the largest participants must submit to similar metals and pathogen tests once a month. Neither program requires testing for pesticide residues.
This is NOT a hypothetical concern! From Mother Jones:
Composting Council executive director Stuart Buckner believes that the EPA still hasn’t learned its lesson from the early aughts, when the herbicide Clopyralid was banned for lawn use after turning up in damaging levels in compost.
Here’s a study of what Clopyralid does to plants:
To me it’s just common sense: given where the raw materials comes from, I assume that city compost contains harmful pesticides and herbicides, unless proven otherwise. Why take the very real risk on city compost, when you can make your own compost from known good materials?