Is Our Compost Safe?

Sightline recently highlighted a story about a scientist in British Columbia who measured the residue on leaves along his bike route to work to determine the quality of the air he was breathing as he rode. The quality was so bad that he opted for a longer route with less cars. This type of concentrated pollution associated with heavy traffic–before it disperses into the atmosphere –is what people commonly refer to as ‘hot spots’ (major highways, etc).

When one of our readers shared the story, they received an interesting reply: ”It occurred to me that the magnetic leaf story provides one more reason not to use urban yard-waste compost processed by well-meaning cities, especially on food crops, until it is shown contaminants are removed by composting. It’s not the iron compounds one worries about so much as concentrating toxic metals and organics.”

Here at Urban Farm Hub we thought it would be worth investigating the impacts of using urban yard-waste on food crops. We sent the question on to Kristen McIvor, who is working on a PhD at the University of Washington and has a background in soil science and community gardening and this is what she had to say:

It has been shown that the vast majority of potential contaminants in compost are destroyed or broken down in the composting process. The very few that do not are (clopyralid, for example) constantly monitored for by composters.

Heavy metals do not ‘break down’, but compost is regularly tested.  There are usually low concentrations of heavy metals in most compost, but not at levels that would be of concern.

The urban environment is not clean and will not be clean in our lifetimes, but that does not mean we cannot have healthy, organic agriculture in urban areas – even areas that are “contaminated.”  If the ultimate goal is to grow food, then what really matters is what plants take up and store.

Metals–it depends somewhat on which metal you are talking about, but as a rule, they do not concentrate in plants. Lead is the one to be most concerned about in urban areas, as it was ubiquitous for many years in paint and gasoline, but it tends to bind strongly to soil particles (meaning it stays in the soil) and does not get taken up into plants.

‘Organic Contaminants’ – and this is a really broad term for any compound that contains carbon – will either get broken down in the composting process or bind tightly in the soil. They also are large molecules, as a rule, that are not going to be taken up into a plant.  (Don’t eat the dirt!)

If your urban soil has elevated levels of lead (or anything else), the very best thing you can do to remediate is add compost or biosolids. Even with some lead in these amendments, it is at such low levels that you are effectively cutting the amount of contamination in half just by diluting it and the compost will add life to the soil which will 1) break things down, and 2) bind tightly with the remaining contaminants.

Marra Farm probably has elevated levels of lead from its proximity to major roads, but all the compost they’ve added over the years certainly isn’t the reason. And although I don’t know if it’s ever been tested, I would be very surprised if it was elevated enough to be of concern. And then that concern would really be for children in the children’s garden eating the soil, not consuming the produce.

Also, ‘Organics’ as a category are generally not included in testing as they have been shown to, in general, break down rapidly. ‘Organics’ is also a category of things that people are concerned about, but do not understand the range of compounds included in this category – if you have a particular type of organic compound you are concerned about, we can give you a little more information on its fate in the environment.

In summary – adding organic matter in the form of compost and biosolids is the best thing that urban gardeners can do for their soil. The composting process is highly managed and routinely tested for heavy metals and monitored for the persistent herbicide clorpyralid. Healthy soil is amazing and can break down many compounds. The urban environment is not clean or pure but that doesn’t mean we can’t grow healthy food.

Related posts:

1. The Bounty of Biosolids: What Safety Issues?
2. Vermiculture: The Economic Benefits of Creepy Crawlies
3. pH is pHundamental for Gardening

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