Composting Part Four

What NOT to put in your compost bin...

CHEMICALLY-TREATED WOOD PRODUCTS

Sawdust is often available from constructions sites, friends, or your own building projects. If you are considering composting sawdust, be sure of the origin of the sawdust. Sawdust from chemically-treated wood products can be bad stuff to compost. For example, take pressure-treated wood (sometimes called CCA), which usually has a greenish tint to it (I have also seen it in other colors). It contains arsenic, a highly toxic element, as well as chromium and copper. There is evidence to suggest that arsenic is leached into the soil from these products when they are used to make compost bins or raised beds, so composting the sawdust would certainly be a mistake. You may wish to read the 'Letters' section of Organic Gardening, April 1994 and July/August 1992, for more information. Avoid other chemically-treated wood products and sawdust as well, such as wood treated with creosote or 'penta' preservative.
DISEASED PLANTS

Many plant disease organisms are killed by consistent hot composting, but it's difficult to make sure that every speck of the diseased material gets fully composted. It's best not to compost diseased plant material at all, to avoid reinfecting next year's garden.
HUMAN WASTES

Human feces can contain disease organisms that will make people very sick. Composting human feces safely requires that the compost pile reach high (thermophilic) temperatures over a period of time. It isn't necessarily that difficult to reach these temperatures in a home compost pile, but the potential health costs of improper composting are high. Composting of human feces should not be attempted, except by experienced 'hot pile' composters who are well informed of the temperatures and times required to kill pathogens, and who are willing to take 100% responsibility for the process and product. If you would like to learn more about composting humanure, I recommend The Humanure Handbook, listed in the resources section of the Rot Web.
MEAT, BONES, AND FATTY FOOD WASTES

These materials are very attractive to pests (in an urban setting, this could mean rats...). In addition, fatty food wastes can be very slow to break down, because the fat can exclude the air that composting microbes need to do their work.

PERNICIOUS WEEDS

Morning glory/bindweed, sheep sorrel, ivy, several kinds of grasses, and some other plants can resprout from their roots and/or stems in the compost pile. Just when you thought you had them all chopped up, you'd actually helped them to multiply! Don't compost these weeds unless they are completely dead and dry (you may want to leave them in a sunny place for a couple of weeks before composting). Remember also that composting weeds that have gone to seed will create weeds in next year's garden, unless a very hot pile temperature can be maintained to kill the seeds.
PET WASTES

Dog and cat feces may carry diseases that can infect humans. It is best NEVER to use them in compost piles. Some people do bury them 8" deep in the soil, but ONLY in areas where food crops are never grown.