What compost is and how it benefits your plants

What is Compost?

The compost definition is simply the decomposition of organic material in the presence of oxygen

Compost defined

More technically stated to it is the breakdown of any material are carbon based (as opposed to inorganic materials that lack carbon such as plastics, etc).

Composting occurs naturally. Just take a walk in the forest in the spring time. Remember all the leaves that fell to the ground? We’ll they’ve been broken down over the winter. That’s why we’re not hiking in 10 foot high leaf piles thanks to the composting process that occurs naturally.

What is Humus?

As part of the compost definition, you might hear the word “humus” referred to.  Humus is considered part of the compost definition, but is the decomposed organic material that is at the soil surface, also known as the “O Horizon” if you are a soil geek like me.

It’s part of the topsoil that has a lot of organic matter in it.  If you have every experienced walking around in a swamp and pull your shoes out and it has a really dark slick material, this is the ultimate in soil humus. Here’s a photo of my friend Alan in a NC swamp, with lots of dark, rich organic matter around.

Soil humus is the thick black organic-matter rich surface horizon often found in swamps

Most of us interested in organic gardening want high quality compost for the garden.  The best is that which is hot composted, thus decomposed in the presence of oxygen and heat, and turned on a regular basis to maintain oxygen levels.

Done right, it will be free of weeds and diseases since the heat and turning will kill the bad guys. It’s a faster process than what nature would normally do, and superior to any static or cold composting process.

What Materials Can Be Composted?

Compost can be made of a variety of materials, typically divided into how they are digested by the microbes which are responsible for decomposition. 

This includes materials that have a narrow carbon to nitrogen ratio, commonly called “green materials” such as manure, and those that have a wide carbon to nitrogen ratio, called “brown materials”, such as straw and leaves.

If you don’t know what a carbon to nitrogen ratio is then just focus on greens VS browns.

Composting materials

Green Composting Materials

Further breaking down when starting a compost pile, the green materials include easy to break down plant matter such as grass clipping, garden weeds, and hay.  If you are confused on what constitutes a green material – just thing of it this way – a “green” material is any material that is cut green.

Check out other composting tips if you are just getting started.

So both your cut grass clippings, and when you pull the garden weeds if they are still alive, constitute a “green” material.  Manures (chicken, horse, llama, cow, etc) are often used as a green material, or as a starter material, since manures typically have a very narrow carbon to nitrogen ratio and can really heat up a compost pile quickly.

Green Materials

Brown Composting Materials

The other part of the compost definition is the “brown materials”. These are the hard-to-digest materials such as wood chips, bark, straw, leaves and other plants that have a wide carbon to nitrogen ratio. This requires our native fungi to be present to break down these materials with their digestive enzymes.

So if you are composting leaves, our fungi are responsible for breaking it down into compost.  Fungi are pretty amazing organisms. Mushroom compost (aka fungi) is a really powerful type for shrubs and plants or for revitalizing disturbed or degraded soils.

Straw and leaves are two brown materials used in composting
Brown Materials

Why Do Plants Need Compost?

Your plants NEED organic matter. By adding compost to your lawn and garden you incorporate the organic matter that provides a slow release nutrient source. So if you are growing tomato varieties or purple potatoes, your plants will love you for it.

The benefits of compost go well beyond just a nutrient source for your garden, such as adding air to the soil, feeding beneficial soil microbes, break down harmful chemicals, and holding onto water.

By not adding compost, you are starving your plants and the beneficial soil microorganisms that help plants share resources, leaving plants more susceptible to disease.  So if you are growing tomatoes or other plants, add the “Black Gold”!

Advantages of Compost

There is a huge advantage of using compost as an organic fertilizer. Unlike synthetic fertilizers such as Miracle Grow, which give the plant only certain nutrients, and provide them all at once (whether they need it or not!), compost provides the nutrients throughout the season.

The plants can harvest these nutrients, with the aid of mycorrhizal fungi, when they need them.

Compost Tea

Beneficial microbes attached to compost, can be extracted and added as liquid form, called compost tea. It doesn’t provide the organic matter, but does extract all the beneficial microbes off the surface of the compost.  Apply it in liquid form to plant surfaces and soil.

Think of it as Probiotics for Plants. It’s commonly used in organic farms, on orchards and in habitat restoration projects.


The compost definition encompasses decomposition of organic material, and so, it’s simply organic matter that has broken down. What specifically is compost made of will depend upon the materials or ingredients used in the composting process.

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What is Compost and why not using it could hurt your plants.
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4 thoughts on “What is Compost?”

  1. Pingback: Diane J

  2. Pingback: Good explaination

  3. Pingback: Beginning composter here

  4. Pingback: Tom

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