Does Compost Turn Into Soil? Here’s All You Need To Know

The mango or banana peels in your waste bin will eventually decompose, as will many forms of organic matter thanks to helpful microorganisms. Composting is simply a process that helps speed up the natural decay of organic matter by offering the ideal conditions for detritus-eating microorganisms to thrive.

The end-product of this decomposition process is nutrient-rich compost that can help your crops, garden plants, and trees to grow. But after use, where does compost go after providing the essential nutrients to your garden plants? Does it turn into soil?

One common misconception is that compost eventually turns into soil. Compost and soil are strikingly similar to one another, and therefore, it can sometimes be very difficult for people to differentiate them. It is imperative to note that compost does not become soil. However, it remains an essential ingredient in healthy soil.

Successful gardening largely depends on fertile soil, and one way to boost soil fertility is to add compost. Adding a layer of compost to your garden soil will not only help fuel your plants, but will also boost the overall structure of the soil.

Difference Between Compost and Hummus

There are two forms of organic matter in the soil: active organic matter that is still decomposing, and stable organic matter that is inactive.

Compost starts out as being active organic matter before it eventually turns into stable organic matter (also called hummus) and it can survive in the soil for hundreds of years.

Since active organic matter is in decomposition, it’s great food for microbes.

It plays a critical role in the overall health of the soil. It stabilizes soil aggregates, releases vital nutrients through mineralization, and fuels microbial activity, which can also result in the suppression of plant diseases and enhanced plant growth.

Stable organic soil matter is biologically inactive, so it provides very little food for soil organisms. Experts sometimes refer to stable organic soil matter as humus. It has less influence on overall soil fertility because it’s inactive.

Regardless, it is still very important for good soil management because it contributes to soil tilth, soil structure, and cation exchange capacity. Stable soil organic matter is also what darkens the color of the soil.

Bottom line: Compost is decomposing organic matter and humus is stable organic matter, the final product of decomposition. That’s it.

Breaking Down Compost

compost heap

A compost pile contains nitrogen-rich (greens) and carbon-rich (browns) matter. Greens include things like grass clippings, vegetable and fruit waste, and coffee grounds. Browns are things like dead leaves, twigs, branches, and yard clippings.

We mix these two vital ingredients to help create the ideal conditions for bacteria to thrive, reproduce, and multiply. According to the United States Environmental Protection Agency, we need a precise balance between browns and greens to create an environment where decomposition can occur.

Studies show that a carbon-to-nitrogen ratio between 25 to 1 and 30 to 1 is ideal for optimal decomposition. The browns provide bacteria with energy, much of which is released as heat and carbon dioxide, whereas greens provide additional nutrition for them to continue to reproduce and grow.

This well-balanced mixture results in compost, a natural fertilizer that benefits both soil and plants.

Other Key Considerations

If there is excess carbon in your compost, decomposition will occur at a slower pace as bacteria will produce less heat from the lack of greens that allow them to reproduce and grow. In other words, they’re unable to efficiently break down carbon.

When there is excess nitrogen in the compost, it might lead to the production of ammonia gas, which has an unpleasant odor. It also increases the acidity of the compost, which can be highly toxic for some species of bacteria.

Adequate moisture is also important for the health of the bacteria. Moisture content between forty and sixty percent suffices to keep the bacteria active.

The amount of oxygen in the compost is crucial, otherwise it may end in a scenario where you have a foul-smelling and rodent-prone pile. You can easily avoid that by routinely turning the pile using a shovel or a pitchfork.

As the microorganisms feed on the organic material, they break down the fibrous plant structure and eventually excrete minerals and nutrients in plant-soluble form.

Finally, don’t forget to pay attention to the temperature of the compost.

Even though microorganisms are more active with warm temperatures, too much heat might kill them. So if you live in a tropical climate, build your pile where there is shadow and introduce oxygen by regularly turning the compost repeatedly. In due time, the compost will turn into something that looks like dark, crumbly soil.

How Long Does It Take To Have Finished Compost?

In perfect conditions, organic matter can take between three and six months to decompose. As I’ve explained earlier, as microorganisms decompose plant waste, they gradually separate both minerals and nutrients from the hard, fibrous plant materials.

Plants absorb a portion of those minerals and nutrients, while the soil gets another fraction. The small pieces of organic matter left behind make up the hummus.

Compost offers many benefits. However, one of the most important components in compost is its spongy and crumbly texture. Mature compost features tiny pieces of raw materials that aren’t visible to the naked eye.

These tiny pieces give compost its spongy texture, a key ingredient that that helps aerate clay soil (compacted soil), boost the structure of sandy soils (less compacted soils), and enhances soil’s overall drainage.

What is Soil?


The basic definition of soil is that it is a thin layer of material that covers the earth’s surface that gets formed because of the weathering of rocks.

Soil is made up of organic matter, mineral particles, living organisms, air, and water, which interact with one another.

As soils develop with time, horizons or layers form a soil profile. Soil layers are layers are perceivable when you navigate the soil profile from top to bottom. Dig down deep into any type of soil, and you’ll notice its different layers. When those layers are put together, they form a soil profile.

The five layers of soil include:

  • Organic horizon that features grass clippings, leaf litter, among others.
  • Topsoil, which is the major root zone.
  • The subsoil, which is the root zone for large plants.
  • Parent rock.
  • Bedrock.

There are lots of factors that affect the formation of soils, including parent material, time, topography, and living organisms. These interacting processes are usually very slow and take time to form. Let’s take some definitions out of the way:

  • Parent material: Soil minerals form the basis of soil, and they are generated from rocks via weathering. Temperature variations, water, chemical interaction, gravity, living organisms, and pressure differences all help break down the parent rock.
  • Time: Soil properties will ultimately vary based on how long the soil has been exposed to weathering.
  • Topography: The grade, shape and the length of a slope impact drainage. The features of a slope determine the type of vegetation and also show the amount of rainfall received. All these factors have a direct impact on how the soils form.
  • Organisms: Different organisms hugely influence the formation of soil. These organisms can be plants, humans, animals, burrowing insects and microorganisms like fungi and bacteria.

However, not all the soil horizons feature every ingredient. The uppermost layers contain relatively more organic matter.

Also, not all soil types feature an organic horizon. For instance, sandy desert soils rarely feature any organic matter at all. The amount of organic matter available in the soil depends on the amount of dying or dead plant matter that falls on that soil annually. The prevailing environmental conditions should support decomposition.

Difference Between Soil And Growing Medium

Also known as potting soil or substrate, a growing medium is a material, other than soil on the spot, in which plants are grown.

Both consumers and the horticulture sector use potting soils to promote the development of plants. The growing medium allows the plants to grow healthily because it offers a range of helpful benefits such as:

  • Ample storage of oxygen for the roots.
  • An optimum rooting platform for enhanced physical stability.
  • Constant supply of moisture.
  • Constant supply of essential nutrients.

Coconut coir, starting mixes, raised bed mixes, and rock-wool blocks are all typical examples of growing media.

Will Compost Become Soil At Some Point?

Compost won’t turn into soil.

It doesn’t have a parent rock that gives it either the foundational clay, silt, or sandy texture. When you add compost into the soil, it becomes part of the soil’s organic structure. Soil organic material makes at most five percent of most soils.

Through the process of decomposition, compost eventually turns into humus, decreases in volume, and ends up making less than 2% of the total soil organic material. So, even though both compost and humus are important soil components, they can never become real soil.

Can You Use Compost as Potting Soil?

Compost is a type of growing media.

However, compost will eventually decrease in volume thanks to the decomposition process. If you use compost as your growing media, your plants may suffer nitrogen burn. Plants that thrive in a compost growing medium are heavy feeders such as fruits and vegetables, including melons, tomatoes, peppers, and squash.

These plants are not only nutrient-hungry but also have a relatively shorter lifespan, so they take full advantage of the compost before it loses its volume.

While you can grow plants in compost, it is an idea that most experts don’t recommend.

It adds essential nutrients to the soil, but it may never replenish the nutrients that plants consume every year. And because compost continues to undergo decomposition, it might have some incompatibilities with indoor plants.

I recommend that when using compost as growing media, other plant material, such as leaf litter and wood chips, should make up less than 33% of a growing media. Also, it’s best used outdoors than indoors.

Bottom Line

While amending your garden soil with compost can be a good thing because it adds vital nutrients to the soil and improves soil structure, the fact is that compost is not soil and doesn’t include enough of the nutrients your plants require to be healthy!

For this reason, you must use compost alongside soil to ensure your plants get the type of nutrition they need to stay healthy at all times!