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 effect the decomposition process. Currently, one turf fungicide, PMA, contains mercury. This may only be used by commercial pesticide operators. This should not be used.

Composting bins are often made of pressure treated wood to prevent rot from destroying the bin. Contrary to intuitive expectations, it's safer to use wood treated with arsenic than wood treated with either creosote or pentachlorophenol. Several studies found no evidence that arsenic migrates from treated wood into garden plants growing in planter boxes of arsenically-treated wood. It seems reasonable to assume that arsenic would not migrate into compost either.