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Compost is by far the most important ingredient in a healthy garden. Experienced gardeners swear by an annual addition of compost for rich, dark soil.
But, where does all of that compost go?
Compost does not become soil, but it does become an important ingredient in healthy soil. Compost is classified as active soil organic matter because it is still decomposing. As compost decomposes, it turns into stable soil organic matter, which can survive in the soil for decades.
Compost and humus each have positive effects on the soil, but they serve different purposes.
So, what makes them different, and what’s keeping them from becoming soil?
Let’s dig in.
What Is The Difference Between Compost & Humus?
There are two types of plant organic matter in soil:
- Active: plant matter that is still decomposing
- Stable: humus
A basic compost pile is made from green, or nitrogen-rich ingredients, and brown, or carbon-rich ingredients. These ingredients are layered together in a pile and kept moist, which creates the perfect environment for bacteria to live and reproduce.
As the bacteria feed on the plant material, they break down the fibrous plant structure and excrete nutrients and minerals in a plant-soluble form.
As the center of the compost pile heats up, the bacteria become more active, until eventually the heat kills them off. Then, the compost pile is turned to introduce new plant material into the center of the pile, and the process repeats.
Each time the pile is turned, it looks less like leaves and twigs and banana peels and more like a dark, crumbly garden soil. Once the pile stops heating up in the center, and the plant material is no longer recognizable, the compost is finished.
Under ideal conditions, plant material can decompose into compost in a few weeks, although it usually takes 3-6 months.
As the bacteria work through the plant material, they separate the nutrients and minerals from the tough, fibrous plant structures. The nutrients and minerals are either used by plants or incorporated into the soil. The small pieces of plant structure left behind are what we call humus.
Compost provides an obvious benefit for the soil. As the plant material continues to decompose, more and more nutrients are released into the soil. This is what makes compost the ultimate slow-release fertilizer.
However, perhaps the more important benefit of compost is the spongey, crumbly texture. Finished compost contains lots of tiny pieces of raw material that are too small to see.
These small pieces give compost its spongey texture, which helps to aerate compacted soils, improve drainage, and give sandy soils more structure.
As the small pieces of plant material decompose, the compost essentially disappears. One of the two huge benefits from compost is the light, fluffy nature of partially decomposed plant material.
Once compost is completely decomposed and it is no longer light and airy, it is no longer compost; it has fully transformed into humus.
What is Soil?
Soil is made up of 5 ingredients:
- Parent material: sand, silt, clay
- Living organisms: worms, bacteria, beetles, etc.
- Organic matter: active (decomposing) & stable (humus)
Soil is also made up of layers, or horizons:
- Organic layer: leaf littler, grass clippings, etc.
- Topsoil: primary root zone
- Subsoil: root zone for large plants
- Parent rock
However, not all layers of all soils contain all ingredients. The top layers of soil will have considerably more organic matter than deeper layers.
Not all soils have an organic layer. Some soils, like a sandy desert landscape, may not have any organic matter at all. The amount of organic matter in a soil depends on the amount of dead or dying plant material that falls onto the soil each year, and whether or not the environment is suitable for decomposition. (Learn the difference between Soil vs. Dirt).
The amount of living organisms, gas, water, and organic matter can fluctuate in each soil, and some may not be present at all. However, all soils must have parent material in order to be a true soil.
For more information, see What Is Soil (And Why Does It Matter?)
Growing Media vs. Soil
So, if soils have parent material, living organisms, and organic matter, what about seed starting mixes or raised bed mixtures? Are they not soil?
Growing media is the term for any material that holds a plant’s roots while it grows.
Soil is one kind of growing media, but there are many other types of growing medias for different growing systems.
Seed starting mixes, coconut coir, rockwool blocks, raised bed mixtures, and many other mixes and materials can be classified as a growing media even though they are not classified as a soil.
Can Compost Turn Into Soil?
Compost can not become soil because compost does not have a parent material or bedrock that provides the foundational sand, silt, or clay texture.
When compost is added into the soil, it becomes part of the soil organic matter, which makes up less than 5% of most soils. As compost matures and transforms into humus, it shrinks and makes up less than 2% of the soil organic matter.
So, while compost and humus do become an important part of soil, they cannot become true soil on their own.
Can compost be used as a growing media?
However, compost is going to continue to decompose, which means it will eventually lose volume.
Fresh compost is a potent soil amendment with lots of plant-available nutrients. If you use 100% compost in a container or raised bed, the plants may get nitrogen burn.
Plants that may do well in compost are heavy feeders, like annual fruits and fruiting vegetables:
These plants need a lot of nutrients, and they usually only live 3-5 months, so they will be able to take advantage of the spongey texture of the compost before it matures and loses volume.
Although it is possible to grow in pure compost, it is not recommended, and there are much better ways to use compost in a growing media.
Compost and other plant materials, like wood chips or leaf litter, should make up no more than 1/3rd of a growing media, and it should only be used in mixes that are used outdoors.
Compost is still actively decomposing, and it can cause problems in indoor containers.
Refresh compost in containers each year to maintain the volume.
Compost does add nutrients to the soil, but it may not replace the nutrients plants use each year. Plants may still show nutrient deficiencies in containers with compost-based mixes.
Compost is an amazing soil amendment, and it can have a supporting role in an outdoor container mix, but compost is not soil. And yes, you can have too much of a good thing.