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What is ‘safe’ compost?

Meleah Maynard

Meleah Maynard

May 22, 2013

A few weeks back I did a short presentation in Pine City, Minn., on how to build good, healthy soil, and a woman in the audience asked: “How do I know if the compost I’m using is safe?” I’ve been wondering that same thing, I told her, explaining that I’ve been researching the topic so I have some answers, but many more questions, too. This prompted more people to weigh in on the subject, asking: Was it important to use organic compost, especially when growing edibles? How do you know that even organic compost is safe?

Dairy cow Photo Kabsik Park

Dairy cow Photo Kabsik Park

Does composted manure from conventional farmers contain pesticide and herbicide residue that could cause problems in their gardens? Should you have compost tested to find out what’s in it before you use it on food crops and, if so, where? And what about GMOs? Is it safe to use composted manure produced on conventional farms on which cows eat things like Monsanto’s genetically modified Roundup Ready corn and alfalfa?

Complex questions like these are difficult to answer definitively for a lot of reasons. Sometimes, there aren’t many studies, if any, on a particular topic. Or maybe there are numerous seemingly reputable studies, but many of them conflict with one another. For example, as a journalist who interviews people for a living, I can tell you that for every scientist I’ve talked to who dismisses the French study that came out last year linking a genetically-modified strain of maize to huge tumors in rats, I’ve got another scientist saying the study should be given serious consideration.


Compost may sound like a simple enough topic, but like so many things it is complicated by money, politics and personal biases of all persuasions. And so, fellow gardeners, I think the best we can do here is take a look at the science that’s available and make the best informed decisions we can. This is the first of several columns I plan to write on compost in an attempt to answer some of the questions above, and others that will no doubt come up. Let’s start with a little background.

Look for an OMRI label

Organic compost is probably not necessary for all of your gardening needs, but a lot of gardeners, including me, like to use it for edible crops. But what does organic mean, exactly? Compost is by its very nature organic, so an “organic” label doesn’t necessarily tell you a whole lot.


If you’re looking for organic compost, for example, that doesn’t contain sewage sludge and other things that are precluded by regulators, look for an OMRI label on the packaging. This label, from the Organic Materials Review Institute, means that the product has been reviewed and approved for use in certified organic production. You’ll find a list of OMRI-approved soil amendments here.

Does it meet NOP standards?

Compost does not necessarily need to have OMRI approval to be used by organic farmers. In the U.S., the National Organics Programs (NOP) sets the standards for compost. Those standards basically cover what the compost can and cannot contain (like, no sewage sludge or urea), carbon to nitrogen ratio, how often piles must be aerated and how many days the pile must be turned while temperatures are kept between 130 and 170 degrees Fahrenheit (which kills weed seeds and disease-causing pathogens).

Certified organic farmers can use compost that meets NOP standards even if that compost was produced on a conventional farm. That’s because NOP regulations do allow low levels of synthetic substances, including synthetic pesticides and herbicides. The explanation being that, realistically, we live in a polluted world and some level of environmental contamination is unavoidable. (Go here for a list of what can be used on organic farms in Minnesota.)


Curious about what types of compost local organic farms use, I called Northfield-based Gardens of Eagan, one of the biggest suppliers of local, certified organic produce in the region for more than three decades. They told me they make a lot of their own compost, but they also use Cowsmo, a compost product John Rosenow and his wife Nettie have been selling for more than 20 years. Their business is an offshoot of their fifth-generation conventional dairy farmer in Cochrane, Wis.

Rosenow kindly took the time to explain what he has explained to countless others over the years. No, Cowsmo compost is not certified organic, but it does meet NOP standards and is widely used by certified organic growers in the region. Yes, it really is composted manure from their dairy cows. The cows do eat Roundup Ready corn and Bt-Corn, but the alfalfa they eat is not GMO. (Here is their website’s FAQ.)


Rosenow, who also sells four types of compost-based potting mix used by organic growers, says he hears the disappointment in people’s voices when they call him looking for organic compost. They’d prefer not to buy from a conventional farm, and he gets that. But, once they’ve called around, folks usually call back to order a load because organic compost from an organic farm is in short supply around here. If organic beef farmers have it, they usually need most of it for themselves, he explains. John Middleton, Gardens of Eagan’s operations and field manager, agrees that availability and cost make it difficult to get organic compost from an organic farm in great quantity. So compromises have to be made.

In addition to Cowsmo, Middleton likes Purple Cow Organics’ compost, which is available locally in bags at many garden centers. Cowsmo compost and potting soil can be purchased locally in bulk, totes or bags at several Twin Cities’ locations, including the Wedge Co-op, Mother Earth Gardens and Mississippi Market. If you shop for other types of compost, he advises checking the labels carefully. “You’re looking at what’s in it,” he says. “Don’t buy something that contains things you can’t pronounce or has numbers in it, which would indicate things like dyes and fungicides.”

Stay tuned for Part Two on all things compost. In addition to other things, I’ll be talking about more quality compost products offered by Kern Landscape Resources in St. Paul, Mississippi Topsoils in Cold Spring, Minn., and more.

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