Does Compost Turn Into Soil?

  • By: composthq
  • Date: April 1, 2022
  • Time to read: 7 min.

If you’re taking your first steps into the world of composting, you might be feeling a little confused about a key piece of information – does compost turn into soil?

After all, it’s used in the garden, it looks very much like earth, so what is it?

Compost is not the same as the soil you might dig out of a hole in your garden – and you shouldn’t use it in the same way.

It’s important to recognize the difference and know how to use both in order to grow healthy, happy plants.

What Is Compost?

Many people seem confused about what compost is and isn’t, but put simply, compost is a way of adding nutrient-rich components to your soil.

It is something that you can add to soil, and it becomes a part of the soil, but it isn’t, by itself, soil.

Compost is a mixture of decaying earth that is packed with nutrients and still active.

It is full of microbes, and has been created with a mix of nitrogen-rich “green” products (such as grass clippings and food scraps) and carbon-rich “brown” products (such as twigs and straw).

Compost will continue to break down, because it’s still active.

If you were to top a bed with compost, you would find that the level dropped relatively quickly, compared with topping the same bed using soil.

The two are very similar, and there is certainly a lot of overlap, but generally, the term isn’t synonymous and adding compost to your garden is not the same as adding soil.

Compost is strictly organic matter, while soil contains inorganic materials such as rocks.

What Is Soil?

The word soil is usually used to refer to the top layer of earth, the stuff you see directly below the surface plants. There are lots of different kinds of soil, such as clay, sandy, etc., depending on the makeup of the soil.

Soil contains rocks, old root structures, sand, water, decaying plant matter, worms, bacteria, and lots of other things.

It’s not generally considered active anymore, and after a couple of years of growing plants in it, you will probably want to top it up with fresh nutrients – especially if you’re growing nutrient-hungry plants.

Soil tends to be layered, and these layers will usually follow roughly this order (with some variations):

  • Organic layer, at the very top (made up of grass, plants, leaves, etc.)
  • Topsoil, the first real layer of earth (with plant roots and lots of organic matter)
  • Subsoil (with roots of large plants, and slightly less decomposing organic matter)
  • Parent rock
  • Bedrock

To count as soil, the material must have parent material, which is why compost is not really considered soil, despite the fact it clearly shares similar properties.

Parent material usually relates to how the soil originally formed when it settled on the bedrock.

Compost VS Soil In Terms Of Use

So, when do you use soil, and when do you use compost? Well, soil is all over your garden anyway, so you’ll always be “using” it (and its various layers) in some way or another, unless you’re planting in pots.

However, topsoil is a common addition to the garden, and often seems similar to compost.

Topsoil differs in that it is made of harder, more root-based ingredients, and it is weathered over a much longer period of time than compost is.

It tends to have more minerals, but like compost, it’s high in nutrients.

We’ll now look at comparing these two growing mediums and their different advantages and disadvantages.

There are times when you will want to add topsoil to your garden, and times when you will want to add compost – and some situations that will call for the addition of both.

Knowing which to use in which circumstances will help you grow healthy, happy plants.

When To Use Topsoil

Topsoil is a very good way to give your lawn and other garden plants a long-term boost. It will help add back depleted nutrients, and serves as a sort of slow-release fertilizer for many of your plants.

It can be a great way to smooth over lumps, and will encourage lush, even lawn growth.

It’s perfect if you need to create a new flowerbed or extend a patch of lawn, because it gives a secure, relatively stable structure for your plants to root into.

It also encourages decomposition in dead materials, as it is still slowly decomposing itself.

Topsoil improves soil structure; it is made of rooty organic matter, and will provide structure and support, allowing for good aeration and solidity.

It won’t break down and disappear the way compost will, so it’s a good choice when you want to build new flowerbeds or create lasting structures.

However, it doesn’t add a big burst of nutrients to the soil, so plants will need to spread their roots further to get everything they need.

This makes it unsuitable for planting into pots; the plants will not have the nutrients to grow healthily.

Although topsoil contains many nutrients, these tend to be released more slowly, so topsoil isn’t suitable for growing plants that want fast access to lots of nutrients.

mixture of compost and soil

When To Use Compost

Compost is a great way to improve the overall quality of the soil. It puts nutrients back into the ground, where plants can tap into them and use them to grow.

Compost is a very important part of growing nutrient-hungry plants like food crops; these will not thrive without it.

Compost has other advantages too.

It adds beneficial microbes to your soil, and these will promote good health throughout the soil, breaking down any remnants of waste matter, and releasing nutrients into the soil structure.

Compost can also help keep the soil healthy, and can help to trap moisture in the soil, keeping it cool and damp for plants to root into, ensuring they don’t dry out too readily.

Compost is key to growing food crops, especially when combined with liquid feeds that will boost the plants’ nutrient levels further.

You may choose to mix compost with potting soil or topsoil to create a good blend of all the best qualities, but there’s no doubt that compost is crucial for gardening.

Plants like tomatoes and peppers will thrive in soil that has a high ratio of compost added, because they are short lived and very nutrient-hungry.

They don’t survive long enough to suffer from the compost breaking down, but can take full advantage of the boost that it offers.

When You Shouldn’t Use Topsoil

Topsoil isn’t good if you are potting plants. While it does contain a lot of nutrients and can boost the health of your garden, it won’t provide the same nutrient hit that compost will.

Your plants will try to stretch their roots out to find more food, and will end up nutrient-deficient and potentially pot-bound as well.

Avoid using topsoil on very hungry plants, or at least mix it with compost or well-rotted fertilizer so that your plants will have access to the food they need as well as the structural benefits of topsoil.

If you plant food crops in pure topsoil, you are unlikely to see very good results.

When You Shouldn’t Use Compost

Compost is very versatile and you can use it for almost anything, but it’s important to note that it will keep breaking down, and will massively reduce in volume over time.

Just as it decreases in size from your original organic waste – going from a huge pile of food scraps and garden clippings to a small bag of compost – it will keep decreasing as the plants take nutrients from it, and the microbes continue breaking it down.

If you’re building new flowerbeds or trying to fill in spaces in your garden, compost is not the ideal material.

However, compost can be added to old topsoil to restore some of its nutrients, or mixed with fresh topsoil to make a nutrient-rich bed with the structural benefits that topsoil provides.

You shouldn’t use compost to fill in under large plants or shrubs, either.

Although it has sometimes been recommended because it encourages the plants to root deeply and reach for the nutrients, using too much compost could cause the plant to sink as the soil subsides when the compost breaks down.

Compost is great for planting in pots, where you can keep replenishing if the surface of the soil drops too low, but outside of this application, it should be mixed with other soil in order to give your plants soil structure and support as well as food.

Conclusion

Knowing the difference between soil and compost is key to successful gardening.

It will help you to choose the most suitable option, or know when a mixture will be necessary, meaning that your plants will have both the required accessible nutrients and the structure and slow-release nutrients they need.

Treating compost as different to soil is more likely to produce the desired results in your garden, and will prevent you from having to redo whole sections when the compost decomposes.

Treat compost as a “top up,” and topsoil as a foundation, and your garden will thrive.

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