Destruction of Harmful Elements

The destruction of harmful elements is a most important problem of compost containing weeds, grass seed and other plant pathogenic organisms. 

An analysis of the typical temperature and of the thermal death points of a number ofpathogenic micro-organisms and parasites indicates the unlikelihood of survival of some of the common disease-bearing organisms. The magnitude and duration of the high temperatures, results in very few pathogens or parasites surviving the aerobic composting process.

As previously described, the high temperature zone usually extends only to within 4 to 8 inches of the surface. Turning materials is therefore necessary, for ensuring pathogen and parasite destruction, particularly if a composting period under six months is used. In some composting operations the material is turned only once or not at all. A thermophilic temperature is developed after the initial aerobic stacking. This is considered to be sufficient to destroy the pathogens and parasites. 

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 also the process of decomposition. Most pesticides will be broken down in the compost pile before the end of the process. The general belief is that most turf and garden pesticides that are currently permitted, and applied at label rates will be broken down and degraded more quickly in a compost pile than they will be in most other environments. 

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: Composting, an accepted decomposition process, biodegrades many compounds faster than soil degradation. Therefore, if organic materials are composted at least one year, pesticide residues should not be a problem when compost is used. (See Table 1.)

Insecticides: All contemporary insecticides will break down during the decomposition process. 

Fungicides: Vegetation recently sprayed with a fungicide may suppress the development of decomposing fungi if added to the compost pile. A few weeks of composting will degrade the fungicide enough so that it will not affect the decomposition process. 

Composting bins are often made of pressure treated wood to prevent rot from destroying the compost bin. Contrary to what you might intuitively expect, it’s apparently safer to use wood treated with CCA than wood treated with either creosote or pentachlorophenol. Several studies have found no evidence that CCA migrates from treated wood into garden plants growing in planter boxes of CCA treated wood. It seems reasonable to assume that CCA would not migrate into compost either.