The term “heavy metal” has no precise chemical definition. It has come to designate a variety of toxic elements, most of them metals or metalloids (metal-like elements) heavier than carbon that are a source of serious environmental concern.
While compost can break down complex pesticide molecules, it cannot perform similar wonders on heavy metals, all of which are elements, the simplest form of coherent matter, composed of atoms. They can sometimes be transformed into different (even less toxic) forms of themselves, but they cannot be further broken down. The most that compost can do is to offer binding sites to which these metals can attach. If firmly enough incorporated into a complex molecule, even heavy metals can be rendered harmless.
This is one reason why it’s important to use only mature compost (see When is it Finished?). In unfinished compost, compounds are still undergoing quite rapid change, so no bond can be considered firm. A molecule captured at one minute may be discarded the next.
Solid, Unmoving, Unchanging — Not So Much
Movement at the molecular level is a constant. This includes the shuffling and trading of particles, ions, atoms and molecules. The amount of activity that goes on in compost, soil or your dining room table would astonish most non-chemists and seriously call into question our ideas of a solid surface or stable entity.
While the particles in your dining room table are in constant motion, their relative distribution is stable and their activity is limited, not in frequency, but in range. An atom may be released a hundred times in a minute, but it will recombine with the same type of molecule every time because that’s the only sort available. The table, therefore, remains unchanged.
Active compost, however, is a very different kettle of leaves. Microbes, enzymes and so on promote a constantly changing environment in which not just a vast number but a vast range of chemical combinations and re-combinations take place at every second.
The best chance for immobilizing heavy metals is to wait until the microbial storm has passed, the heap has cooled, and the compost has matured. Yes, the compost continues to decay even after it’s cured but change slows drastically.
Heavy metals, like pesticides, can reach compost through any of several routes. But unlike most current (third generation) pesticides, they can bioaccumulate or biomagnify. These are poorly defined terms (along with bioconcentrate), but in substance they refer to the fact that certain materials cannot be efficiently or completely eliminated by biological organisms. They tend to accumulate in individuals (bioaccumulations) and to be passed up the food chain (biomagnification). The second generation pesticide DDT is perhaps the best-known example of a substance that functions this way.
Mercury, copper, cadmium, chromium, lead, nickel, arsenic, zinc and tin are all toxic heavy metals that bioaccumulate because they cannot be easily or efficiently eliminated from the body. Many are essential in small doses, but toxic in large ones. Since the body lacks enzymes to break them down quickly, they can accumulate slowly, eventually reaching toxic doses. Chronic, long-term exposure even at low levels can be dangerous.
Some of these elements, such as arsenic, are found in trace amounts in all soils. Others remain locked deep in the earth until mining removes them and smelting, manufacturing and waste disposal disperses them. They can end up in your soil if you garden where any kind of factory, dump or landfill used to be, or you inadvertently add them, usually in a soil amendment such as compost.
Coal ashes, manure, processed feeds, and contaminated soils can all add heavy metals to your compost. For a variety of reasons, manures and litters produced on large-scale industrial feed-lots all contain heavy metals. Some (arsenic, copper) are added to feeds. Some (copper and zinc) are used in hoof disinfectants. Some enter through other routes.
Improper or incomplete sorting can contaminate municipal composting facilities, as when leaking batteries end up in the compost. Wood ashes contain only trace amounts of heavy metals. Most coal by-products contain significant levels. Recycling processed feed by composting it could also introduce metals into the compost.