It’s More Than Dirt!

What is Compost?

If you start talking to gardeners and reading about compost, you’ll encounter several overlapping ideas about what compost actually is: It’s decayed vegetable matter. It’s a soil amendment. No, it’s a fertilizer. It’s humus. Only the first two of these are pretty much right; the others, being more wrong than right, are confined to the Composting Myths section.

Decayed Vegetable Matter

Composting WormsMost 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.

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.

Yet for practical purposes, it makes sense to speak of compost as an “end product.” Certainly some sort of process goes on in a compost heap that comes to an approximate end. This is why it is possible to say that the stuff is done. You build a compost heap, 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 compost 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.