What Is Compost?

Compost with composted earth

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Compost is decomposed organic matter rich in nutrients that can be used to fortify soils for gardening, horticulture, and agriculture. Also known as "black gold," compost is made by the natural process that occurs after combining water with brown materials (like dead leaves, twigs, and branches) and green materials (like grass clippings, and fruit and veggie scraps). It's the end process of biodegradation that naturally takes place when these materials are combined.

Whether you compost at home or your town does large-scale or industrial composting, the end result is an incredibly useful material that has a host of environmental, economic, and social benefits.

What Happens During the Composting Process?

Composting is simply a more concentrated (and usually faster) version of the natural process of degradation and recycling that has been going on for millions of years on planet Earth.

Microorganisms including bacteria, actinomycetes, and fungi work together to decompose plant material into compost. Bacteria do most of the heavy lifting by using a wide variety of enzymes to chemically break down organic materials. Worms, sow bugs, nematodes, and other invertebrates and insects also contribute to the process by physically breaking down those materials.

To better understand the final result, let's consider what happens at every stage of the composting process. Imagine you have just tossed a bucket of food scraps (greens) into a compost bin and topped them with leaves (browns). What happens next?

The first stage lasts a couple of days and involves microorganisms that start pulling apart the biodegradable stuff in your pile. These organisms are mesophilic, which means that they like temperatures between 68 F and 113 F (20 C and 45 C).

Mesophilic organisms create heat as they do their work, which is when the next set of microorganisms comes in. Over the next few days or weeks, thermophilic organisms, which like even higher temperatures, move in and break down the materials even more — these organisms can break down complex carbohydrates, proteins, and fats, too.

Plant and human pathogens are killed when temperatures rise above 131 F (55 C), so professional and industrial composters always ensure that this level is met.

Because you don't want the compost to get too hot and kill the thermophilic organisms, however, it's important to aerate your pile, which also guarantees that enough oxygen gets into the system. You should aim to keep temperatures below 149 F (65 C) in your compost pile.

The last part of the process is the cooling and maturation phase. As the high-energy fuel that keeps the compost hot enough for the thermophilic organisms to thrive gets depleted, the compost cools and the mesophilic organisms move back in.

You can tell compost is ready to use when it looks like the black gold composters are famous for: a soil-like material that is dark and rich-looking, feels crumbly, and has a smooth texture, without any recognizable pieces of what you originally put into it. It should smell like rich earth, not ammonia or anything sour. It will be about 1/3 smaller than the original pile and it won't be much warmer than the outside air.

What Is in Compost?

Hands holding Compost
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After the original mix of compost materials — the carbon-rich brown stuff and nitrogen-rich green waste — goes through the composting process, the resulting material will have plenty of the main nutrients needed for fertilizing plants: nitrogen, phosphorous, and potassium.

These nutrients will be in a more diluted form and will be released over a longer period of time than a chemical fertilizer. That's why compost is often referred to as a soil conditioner — it improves the overall quality of the soil, it doesn't just feed plants.

In addition to the "big three" nutrients, which are also typically found in chemical fertilizers, compost provides a host of micronutrients and trace minerals not available in commercial formulas. The exact combination of those additional nutrients and minerals depends on what you put into the compost bin to begin with. Those materials will leave behind the nutrients that are typically part of their nutrition profile; for example, apples and bananas will provide boron, while beans and nuts will degrade and provide molybdenum to the compost. Other important micronutrients found in compost include sulfur, carbon, magnesium, calcium, copper, iron, iodine, manganese, and zinc.

There is always a possibility that your compost could become contaminated with heavy metals or chemicals if they are present on the material you put into your compost bin (say, pesticide-treated hedge trimmings). However, in most cases, heavy metals make their way into composts via industrial processes that involve sewage sludge and aren't much of a concern for the home gardener or community compost program. Harmful bacteria and pathogens will be killed by the heat from the compost process.