It’s More Than Dirt!

What is compost? Soil amendment? Decayed organic matter? Fertilizer? The answers might surprise you.

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Decayed Vegetable Matter

Most gardeners think of compost as what’s left when vegetable matter decays. That’s pretty close. But animal matter will also compost: (a number of states now compost animals killed on highways). Manures can also be composted but they are still called manure, not compost.

The decay of organic matter involves more than the physical shredding and chopping into microscopic pieces. It’s also the chemical and biological transformation. Most of this transformative work is done by micro-organisms, though some larger organisms including various insects and earthworms can also be involved at different stages.

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In the simplest terms, decay occurs when these organisms eat organic matter — literally in the case of earthworms, more figuratively in the case of micro-organisms — and excrete it. Most molecules may pass through the systems of numerous organisms, sometimes emerging unchanged, but eventually undergoing numerous chemical changes. The result is rich, dark and crumbly like good earth which is what it smells like. But it contains a much higher concentration of beneficial microbes than does ordinary soil.

There’s a problem, albeit a small one, with the definition above. One minute you’re calling compost “decayed vegetable matter,” the next it’s “what’s left when vegetable matter decays,” and pretty soon you’re saying “it’s the end-product of decay.” At this point, the fastidious amongst us will start to look nervous because there really is no such thing as an end product. (The closest candidate might be oil; its formation requires thousands of years, not a few weeks or months.)

If compost truly were an end product, it would not change after being added to soil. It would go on performing its function forever. In fact, it continues to decay after it’s added to the garden. This is why more needs to be added every year. Its further broken down and incorporated into the earth as its components are used by plants, microbes, insects and other denizens of the soil.

This distinction between “end product” and “decayed vegetable matter” might seem like splitting hairs, but the confusion created by ignoring it leads to at least one frequently repeated myth, that “compost = humus.” Compost contains humus, but it is far from pure humus. If it were, you would not need to add more to your soil each year, because humus takes hundreds — even thousands — of years to break down.

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Yet for practical purposes, it makes sense to speak of compost as an “end product.” Certainly some sort of process goes on in the bin that comes to an approximate end. This is why it is possible to say that the stuff is done. You gather up materials, it heats up then cools. You turn it, it heats back up; this goes on several times, but eventually, when you turn it, nothing happens. It no longer heats up. After a few weeks curing it’s done, it’s finished, it’s ready to use — that particular round of decay cycles has reached its end.

Though a cool pile does not offer such dramatic evidence that the process is over, the experienced composter can recognize when the pile had reached a point of relative stasis. Nothing more is happening here; it’s time to put the stuff to work in the garden.

A Soil Amendment

Though most people think that compost is a fertilizer, it is actually a soil amendment. Fertilizers add nutrients to soil; amendments improve the soil so that plants can make use of those nutrients. Other familiar amendments include lime, added to raise the pH of alkaline soils, and sulfur, added to lower the pH of acidic soils. Compost does contain low levels of plant nutrients, but its primary role is not to feed plants but to improve the soil so that plants can feed themselves.

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!