Compost Science

Biology, chemistry and physics come together in the making of compost. Here’s how.

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The science of composting is complex. The distinction of physical, chemical and biological processes is to a large extent artificial. All overlap and all affect each other. Virtually any effect of composting can be traced to activity in more than one of these categories. Take, for example, the well-documented fact that the composting process kills many disease-causing pathogens. According to a 2003 review of the literature (PDF) by R. Noble and S. J. Roberts, no fewer than seven mechanisms are involved:

The best known and most widely studied mechanism is the heat produced by thermophilic bacteria during the hottest stage of composting.
Much earlier in the process, when pH drops, pathogens die because the compost is producing organic acids, ammonia, and similar substances toxic to many them.
Enzymes produced by some microbes have the ability to destroy pathogenic cells by breaching or breaking their outer membrane, a process known as lysis.
Some chemicals produced during home composting stimulate certain pathogens to germinate prematurely.
Micro-organisms directly attack pathogens through the manufacture of antibiotics and though parasitism.
Beneficial microbes can frequently out-compete pathogens for nutrients.
Over time, many pathogens simply lose their vitality or viability: they become less able to infect a host or even to reproduce.

The first on the list — heat — is a physical property produced by biological processes. Numbers two, three and four involve processes that are primarily chemical though with strong biological overtones. The remaining three are primarily biological with strong whiffs of chemicals.

None of these three areas — the physical, the chemical and the biological — stands alone, in compost or anywhere. These divisions, while convenient, reside not in nature but in our desire for categories. It is important to remember that they remain primarily a convenience.

Related Questions

What products to compost
Hello,
You will be able to source all of the essential elements in order to build a great compost pile without having to look too far! As long as your carbon to nitrogen ratio is optimal (25-30:1) your compost pile will be breaking down properly. Here are some lists of acceptable additions:
Carbon Rich Material "Browns"
Cardboard (free of dyes)
Corn stalks
Fruit waste
Leaves
Newspaper
Peat Moss
Saw dust
Stems & twigs
Straw
Nitrogen Rich Material "Greens"
Alfalfa/Clover/Hay
Algae
Coffee grounds
Kitchen food waste
Garden waste
Grass clippings
Hedge clippings
Manures
Vegetable scraps
Weeds (that have NOT gone to seed)
Things to Avoid
Meats
Bones
Fats/oils/grease
Diseased plant material
Colored paper
Coal/charcoal
Cat/dog waste
Manures from carnivorous animals
Onions
Garlic
Citrus peels
As for the rhododendron and holly leaves, you can definitely put them in your compost pile. However, it is a good idea to really chop or shred them up, as they take much longer to break down due to their fibrous and waxy make up. It really depends on how quickly you are trying to create usable compost. It might be a good idea to have a separate pile going that you incorporate those leaves into and another pile that you do not. That way you can have a pile you know will rapidly break down into garden goodness and have yet another ready to use later on. Good luck!