Composting and Hazardous Materials

As far as this idealist is concerned, anything with a name that ends in -icide shouldn't be used on your lawn or garden. I know that my yard is pesticide free, and so is my compost, but what happens when pesticides wind up in backyard or municipal compost piles? Here are the facts, according to Whatcom County in Washington, USA.

Is it safe to use compost from yard wastes that have come in contact with pesticides, or other toxic chemicals? The major route of breakdown of pesticides is through microbial degradation, which is the process of decomposition. Any pesticide a homeowner can buy without a license will be broken down in the compost pile before the end of the process. The one exception to this is clopyralid, which is contained in certain Dow products. Confront is the product that homeowners might use. This is a long-lasting herbicide, and vegetation that has been treated with this should NOT be composted, since the resulting compost can cause serious injury to sensitive crops.

Some typical home yard chemicals, and their reaction to composting:

Slug bait
Most commercial slug baits contain metaldehyde which, when exposed to water, quickly breaks down to a harmless alcohol. (Fresh metaldehyde is toxic to slugs, snails, birds, cats, dogs, raccoons, rabbits, and humans).

Herbicides
Some herbicides become harmless in a very short time in the soil and compost piles (such as Diquat, Paraquat). Others (such as 2,4-D and propanil) break down in compost piles, but only after thorough composting. Still others (such as arsenic, borate, picloram, simazine, sodium chlorate) are extremely long-lived and will probably survive most composting processes. Do not use organic matter in your compost pile if it was treated with long-lived herbicides, such as CONFRONT.

Insecticides
All contemporary insecticides will break down during the decomposition process. Old chlorinated hydrocarbon insecticides such as DDT (which has been banned for a long time) may survive.

Fungicides
Vegetation that has been just sprayed with a fungicide may suppress the development of decomposing fungi if it is added to the compost pile. A few weeks will degrade the fungicide enough so that it will not affect the decomposition process. Currently, one turf fungicide, PMA, contains mercury and may only be used by commercial pesticide operators. This should not be used.

Do not use pressure-treated wood to construct compost bins. It is now well demonstrated that chemical components of the pesticide do leach from treated lumber. The compost may retain a good share of those chemicals, and some would be carried with water into the soil or drains below. This could affect the compost's quality, as well as safety and performance.

In summary, as long as you avoid the really nasty stuff (which you usually need a license for anyway) the products will generally break down. This is not meant to encourage you to spray the evil dandelions instead of digging them out (or, heaven forbid, to enjoy them!). There are many reasons to avoid chemical pesticides and herbicides. For more information visit the Saskatchewan Environmental Society's website at: www.environmentalsociety.ca They have all sorts of tips for natural pesticide reduction.

For tips on how to get your compost 'hot' see our website: www.saskwastereduction.ca. A steamy compost pile will do more than breakdown any rogue pesticides, it will also kill weed seeds and pathogens. Spring is just around the corner folks, happy composting!

Adapted from the Washington State University WHATCOM extension division website. Authors: John VanMiert and Joyce Jimmerson. Reprinted with permission. whatcom.wsu.edu/ag/compost/fundamentals/consideration_pesticides.htm

(Source: February 2008 WasteWatch)

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