Knowledge from Olle Garden Bed: Will Compost Become Soil? Misunderstood Facts

Compost is by far the most important ingredient in a healthy garden. Experienced gardeners vowed to add compost every year to get rich dark soil. The following content also has some reference value for raised garden beds.

But where did all this compost go?

Compost does not become soil, but it does become an important component of healthy soil. Compost is classified as active soil organic matter because it is still decomposing. As compost decomposes, it becomes stable soil organic matter, which can survive in the soil for decades.

Compost and humus have positive effects on soil, but they have different uses.

So, what makes them different, and what prevents them from becoming soil?

Let's take a closer look.

What is the difference between compost and humus?

There are two types of plant organic matter in the soil:

  • Activity: plant substances that are still decomposing
  • Stable: humus

The basic compost heap is made of green or nitrogen rich ingredients and brown or carbon rich ingredients. These ingredients are stacked together and kept moist, creating a perfect environment for the survival and reproduction of bacteria.

When bacteria feed on plant materials, they will decompose fibrous plant structures and expel nutrients and minerals in plant soluble form.

As the center of the compost pile heats up, bacteria become more active until eventually the heat kills them. Then, rotate the compost pile to introduce new plant material into the center of the pile and repeat the process.

Each time the pile is turned over, it looks more like dark, fragile garden soil than leaves, branches and banana peel. Once the pile is stopped heating in the center and the plant material is no longer recognizable, composting is complete.

Under ideal conditions, plant material can be decomposed into compost in a few weeks, although it usually takes 3-6 months.

When bacteria work through plant materials, they separate nutrients and minerals from tough fibrous plant structures. Nutrients and minerals are either used by plants or incorporated into the soil. The small plant structure left behind is what we call humus.

Composting provides significant benefits to the soil. With the continuous decomposition of plant materials, more and more nutrients are released into the soil. This is why compost is the ultimate slow-release fertilizer.

However, perhaps the more important benefit of composting is its spongy, friable texture. The finished compost contains many small pieces of raw materials that are too small to see.

These small pieces give the compost a spongy texture, help aerate the compacted soil, improve drainage, and make the sandy soil more structured.

With the decomposition of small plant materials, compost basically disappeared. One of the two great benefits of composting is the light and fluffy nature of partially decomposed plant materials.

Once the compost is completely decomposed and no longer light and ventilated, it will no longer be compost; It has been completely transformed into humus.

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What is soil?

The soil consists of five components:

  • Masterbatch: sand, silt, clay
  • Organisms: worms, bacteria, beetles, etc.
  • Gas
  • Water
  • Organic matter: active (decomposition) and stable (humus)

Soil also consists of layers or horizon:

  • Organic layer: small leaves, grass chips, etc.
  • Topsoil: main root zone
  • Subsoil: root zone of large plants
  • Parent rock
  • Bedrock

However, not all layers of all soils contain all components. The top layer of the soil will contain more organic matter than the deep layer.

Not all soils have organic layers. Some soils, such as sandy desert landscapes, may not have any organic matter at all. The content of organic matter in soil depends on the amount of dead or dying plant matter that falls on the soil every year, and whether the environment is suitable for decomposition. (Understand the difference between soil and dirt).

The amount of organisms, gases, water and organic matter can fluctuate in each soil, and some may not exist at all. However, all soils must have parent materials to become real soils.

Growth medium and soil

So, if the soil has parent material, organism and organic matter, what about the seed starting mixture or elevated bed mixture? Aren't they soil?

Growth medium refers to any material that maintains the roots of plants during their growth.

Soil is a growth medium, but for different growth systems, there are many other types of growth medium.

Seed initiation mixtures, coconut shells, rock wool lumps, elevated bed mixtures, and many other mixtures and materials can be classified as growth media, even if they are not classified as soils.

Will compost become soil?


Compost cannot become soil because it does not provide parent material or bedrock for foundation sand, silt or clayey soil.

When compost is added to soil, it becomes a part of soil organic matter, accounting for less than 5% of most soil. As compost matures and becomes humus, it shrinks and accounts for less than 2% of soil organic matter.

Therefore, although compost and humus do become an important part of the soil, they cannot themselves become real soil.

Can compost be used as a growth medium?


However, the compost will continue to decompose, which means that it will eventually lose volume.

Fresh compost is an effective soil conditioner with a lot of nutrients available to plants. If you use 100% compost in containers or elevated beds, plants may be burned by nitrogen.

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Plants that perform well in composting are heavy eaters, such as annual fruits and fruits and vegetables:

  • Tomatoes
  • Peppers
  • Eggplant
  • Pumpkin
  • Melons

These plants need a lot of nutrients, and they usually only live for 3-5 months, so they will be able to use the spongy texture of the compost before it matures and loses volume.

Although it can grow in pure compost, it is not recommended to do so, and there is a better way to use compost in the growth medium.

Compost and other plant materials, such as sawdust or fallen leaves, should not exceed 1/3RD growth medium and should only be used in mixtures used outdoors.

Compost is still actively decomposing and may cause problems in indoor containers.

The compost in the container is renewed annually to maintain volume.

Composting does add nutrients to the soil, but it may not replace the nutrients that plants use every year. Plants may still show nutritional deficiencies in containers containing compost mixtures.

Compost is an amazing soil conditioner that can support outdoor container mixtures, but compost is not soil. Yes, you can have too many good things.