Compost vs Soil: What’s the Difference?

by Matt Gibson

Compost and soil are relatively similar to each other, and therefore it can sometimes be hard for people to distinguish between the two mediums. Adding a layer of compost or a layer of topsoil to your garden soil mix will help to add nutrients to depleted soils and will improve the overall quality of your soil. There are many benefits to adding either medium to your soil.

In this article we will define compost and soil, respectively. We will also discuss the advantages and disadvantages of adding either compost, soil, or both to your garden beds. Since there is a handful of different types of soil, we will explain the difference between each type.

We also illustrate the commonalities and differences between the two and let you know when it’s best to use each of them to boost the nutrient levels of your soil, as well as to change the composition of your soil between growing seasons. 

What Is Compost?

Compost is a mix of decaying nutrient-rich soil with medium density that is naturally made using oxygen, bacteria, water, and organic materials. Compost combines green matter, such as food products and lawn clippings, with brown matter, such as twigs and dry leaves. This combination then begins to deteriorate in the composting process. The materials break down into a rich soil, which is predominantly used to refresh depleted soils in the springtime just before planting a new set of crops.

Uses of Compost

There are many ways to use compost in the garden once it has completed maturing and has become ready to use. You can spread it as mulch, mix it in with your soil to improve quality, water down compost tea to use as a liquid fertilizer, or spread it directly onto an aerated lawn.

To use compost as mulch, treat the compost the same way you would any other type of mulch and create a two to three inch layer on the top of your garden beds once or twice per year.

If your soil needs improving, just dig the compost through your soil down to about four inches deep. You can also speed up the composting process by adding worms or vermicomposting, or by composting in an insulated bin which is designed to trap in heat, allowing food scraps and other organic materials to break down quickly in the composter.

Compost tea is water that has been soaking in composted materials. During the soaking, some of the compost’s nutrients, microorganisms, and humates are leached into the liquid. Humates assist plants in their ability to better use nutrients that are available in the soil and provide a host of other benefits as well. Adding compost tea to your soil instead of water will help to boost the nutrient levels in your soil.

Advantages of Using Compost

The advantages of using compost are as follows:

• Works as a nutrient booster for the entire soil
• Keeps the natural health of your soil at a high level
• Helps for the proper growth of beneficial microbes
• Provides the soil with sufficient nutritional elements
• Helps the soil retain moisture
• Promotes proper growth of plants
• Helps the soil fight against disease
• Helps to control and limit weeds
• Works to feed soil directly
• Because you can make your own compost, it is much cheaper and more environmentally friendly than organic or non-organic fertilizers.

Compost sounds like a winner for multiple reasons, but using compost in your garden also has a few drawbacks.

Disadvantages of Using Compost

The disadvantages of using compost are:

• When using compost, make sure that none of your garden plants have soil-borne pathogens
• Compost takes more time to create an effect in the soil than other additives
• Composting can be a very time-consuming process, taking months to break down completely.
• Making compost requires more physical labor than digging up a bit of topsoil
• As compost is made from decaying organic matter, sometimes compost spreads can be kind of smelly.
• Compost takes more space to store and process.

When to Use Compost

If you are simply looking to improve the quality of the soil, compost is your best friend. Thanks to years of constant construction and agriculture in the Southeastern US, our soils are longing for essential nutrients. Incorporating compost into the soil helps to get some of the nutrients back into the ground for plants to use for optimal growth.

Also incorporate compost into your soil when planting shrubs, adding flower beds, installing sod, or sowing seeds. You should mix it into your soil by raking or tilling it in to apply nutrients into your topsoil.

When planting shrubs, flower beds, installing sod, or sowing seeds, you should try to incorporate compost into your soil by raking or tilling it in to apply nutrients.

There are organic compost mixes that you can use just like potting soil with certain plants, however. Soil3 organic compost is one of these all purpose mixes. Whether constructing raised garden beds, or just filling a few flower pots, you can plant directly into Soil3 organic compost. It’s a nutritious blend that gives your container plants all the boost that they’ll need to maintain a healthy growing season. 

Different Types of Compost

If you are using pure compost to top-dress your law, you should strain it so that it will work the best, incorporate into the soil more quickly, and better blend into the topsoil of your home instead of just sitting on the top level of your beds.

Thoroughly sieve your garden soil before applying heavier mediums so that the compost can be a top-dress that works its way through the deep veins, never to be heard from again. The most popular types of compost are:

Composted Wood Waste – this organic pure compost is made from wood chippings and cuttings from trees that may have been composted.

Composted Green Waste – This is made from garden waste and kitchen food and is commonly made in a compost bin. 

Composted Manure – This is raw manure which has been composted and usually contains some straw particles.

Sterilized Loam Compost – This is a combination of sand, silt and clay that has been treated to insure that there are not any undesirable chemicals or organisms growing in it. Typically, loam has a higher percentage of sand and silt than it does clay.

What Is Soil?

Soil is the top layer of the earth. Soils are made from various proportions of sand, silt, clay, and small amounts of organic matter in and on the earth as well as years service past me… The natural medium in which to grow plants is indoors. A publication from the EPA said there are more than 70,000 different types of soil out there.

Soil formation occurs when many things happen to interact over a period of up to 1,000 years. Air, water, decaying plant life, rock, animal life, and chemicals all interacting, as well as plant roots and lichens wearing down rocks, creating new soil, and roots and oxygen help to loosen and aerate it.

After several growing seasons using this soil, you will need to amend the soil and add soil to get the nutrient levels back to a suitable level. That is where compost becomes important.

Advantages of Using Soil

Using topsoil to topdress your lawn is a healthy way to boost the quality of your soil and should be done as often as possible. Not only will top-dressing make your grass much healthier, but it will help increase its drought resistance and make your soil less prone to infestations.

• When you add a layer of topsoil, all the nutrients in this fresh layer will get absorbed into the root system where it will have the most benefit.
• As your garden’s plants feed off the nutrients in your soil, the nutrients need to be replaced, which you can do with fertilizer. However, when you add a layer of rich soil to the topsoil, you do not have to use fertilizer, or can reduce usage, as you are already replacing  .
• Adding layers of organic soil to your topsoil starts a chain reaction of beneficial events as the soil starts to fully decompose, encouraging the thatch in your lawn or garden to also start to decompose.
• If you have an uneven lawn then adding topsoil is a wonderful solution, as you can use the added topsoil as an opportunity to even out the surface level of your lawn, while also improving your soil.
• Topsoil is great for use when you are creating a new flower bed or raised bed. It’s also perfect for topdressing weak lawn areas. It can be used as a base for improving existing lawns, or for creating new lawn areas.

Disadvantages of Using Soil

So when is using topsoil not the best idea?

• When growing plants in pots, you aren’t going to have the best results using only topsoil, as it doesn’t really provide enough nutrients for container gardening, unless it is mixed with a potting soil medium.
• Adding topsoil to your garden beds is fine, as your plants are able to spread out their roots and reach for more nutrients, but in a pot, especially when several plants are planted together in the same pot, using topsoil isn’t the best choice.
• In the summer, when you are planting bedding plants into containers to move them indoors, it is a better choice to use a potting soil medium, or compost, which will be better suited to the task, as you need more organic matter, more nutrients, and maybe even a slow-release fertilizer for the job.

Different Types of Soil

The name topsoil pretty much sums up what topsoil really is, which is the layer of soil that’s at the top where the roots of the plants grow. Having a healthy layer of topsoil is vitally important to successful gardening.

When adding topsoil to your garden beds, purchase the highest quality topsoil that you can get. This is especially important when topdressing your lawn or garden beds to add nutrients back into the soil, as it will lead to healthier growth for the upcoming growing seasons.

There are three different types of topsoil:

Clay soil – Clay soil is composed of tiny particles that hold water well but do not drain sufficiently. If you have a clay heavy topsoil, you should space out waterings as your soil will need ample time to drain. On the plus side, soil that holds water well also holds on to nutrients for longer periods, meaning you won’t have to amend as regularly to keep nutrient levels high.

Loam soil – Loam soil is composed of a mix of sand, clay, silt, and organic materials, and it is the ideal type of soil when it comes to nutrient levels. Loam soil is perfect for adding topsoil to your lawn or garden, as it is rich in nutrients and organic matter.

Sandy soil – Sandy soil is the opposite of clay soil when it comes to water and nutrient retention. If you have a sandy soil, you will need to water and fertilize it more regularly.

When amending your soil, balance is key. If you have a soil that isn’t draining well enough, adding sandy soil will improve drainage. If your soil isn’t holding water or nutrients very well, adding a clay-based medium will help to improve retention. If your soil is too sandy or too clay heavy, adding a loamy soil will help to improve its makeup.

Potting soil is also available in many different forms, and it too can be used to amend the soil already present in your garden beds.

Common Questions and Answers About Compost Versus Soil

Are soil and compost the same thing?

Compost and soil are not the same thing, although compost can be added to soil and become part of its makeup. Compost is made of organic material, while soil also includes elements that are not organic, such as rock particles or minerals. The word “soil” also tends to be used to describe the top layer of the earth’s crust, a natural material, while compost is made by humans. Gardeners mix compost into the naturally occurring soil in their gardens as a soil amendment to increase the nutrition available for their plants.

Can I compost directly in my garden?

While it’s not the most common technique, composting directly in your garden is possible. This simple approach to composting is called the “trench compost method.”

To compost directly in the garden, dig a trench in an unused section of the garden where you’ll be planting soon or in between the rows of your plants. Drop your compost materials into the hole, then cover them with soil to fill in the trench. As the compost materials decompose, the nutrients they contain will become part of the soil. Wait six weeks before planting on top of the trench to give the compost time to break down.

Trench composting is an easy way to amend your soil with compost. The only drawback to this technique is that it cannot be used in winter if your region gets cold enough for the ground to freeze. For instructions and more information, read our article “Trench Composting: Just Dig a Hole and Forget It!”

Can I plant vegetables in compost?

Most of the time, vegetables can’t be grown in compost alone; the compost must be mixed with soil for vegetable plants to grow in it. That’s because compost is a soil amendment that is meant to improve the quality of soil and add to the nutrition it contains. However, you can grow plants that don’t need a lot of structural support from their soil in compost alone.

Plants that you can grow in just compost without other soil added include cherry tomatoes, baby carrots, spring onions (also called green onions or scallions) and small herb plants. For success when growing these plants in just compost, you’ll want to make sure the pH level of the compost is between 6.0 and 7.0. If you aren’t sure of your compost’s pH level, you can read our article titled “How to Test pH in Your Soil” to learn how you can find out what your compost’s pH level is.

Can I use compost instead of topsoil?

It’s possible to use compost instead of topsoil if you need to, like when topsoil has eroded and must be replaced with compost. To use your compost as topsoil, make sure to grind the compost down to a fine texture. Getting your compost to a fine consistency helps the compost to mimic topsoil by allowing it to be packed more tightly and making it easier to mix with fertilizer. That said, you don’t want to go overboard and grind your compost until the particles are as fine as dust. Shoot for the same texture as regular soil when you’re preparing the compost.

You will also need to add fertilizer to the compost so you can grow healthy plants in it. Although compost is often thought of as a fertilizer, it does not contain enough of certain nutrients that plants need to flourish, such as nitrogen, phosphorous, and potassium. When you’re replacing topsoil with compost, use a layer that’s between six and 12 inches thick so plants have enough room to develop strong root systems.

Finally, you’ll need to check the pH level of your soil to ensure it meets the needs of the plants you’ll be growing. Check out our guide to testing the pH level of your soil for more information.

Can you add too much compost?

Yes, it’s possible to add too much compost to your garden soil, and doing so can have negative results for your plants. Too much compost means too many nutrients, so once plants have taken in what they need to thrive, the excess nutrients (like nitrogen and phosphorus) stay in the soil. While an excess of nutrients doesn’t sound like something that can cause problems for your plants, as the excess accumulates over time, the nutrients that are healthy in small doses can reach levels that are toxic for your plants. Limit your compost application to one or two inches on the surface of flower or landscape gardens and three inches on the surface of vegetable gardens.

When there’s too much phosphorus present in soil, that makes it difficult for plants to take in the manganese and iron they need, leading to malnutrition that can cause interveinal chlorosis. Too much phosphorus can also wipe out the population of beneficial mycorrhizal fungi, forcing plants to put energy into expanding the size of their root systems instead of using that energy for flowering, fruiting, and growing stronger and healthier. Having too much of certain nutrients because there’s too much compost in the soil can also make plants grow too quickly. When they grow too quickly, plants miss out on producing the natural pesticides that protect them from infestation by garden pests and the diseases those pests can carry.

Can you fill a raised bed with just compost?

Most plants won’t grow well if they’re planted in just compost without other soil added. (Exceptions include cherry tomatoes, baby carrots, spring onions, and small herb plants.) Ideally, the soil you grow plants in should contain no more than five percent of organic matter like compost. A good rule of thumb to make sure your compost doesn’t exceed the beneficial level is to limit your application to an inch or two on the surface of flower beds or landscape gardens and three inches on the surface of vegetable gardens.

Can you put dead plants in compost?

Most of the time, dead plant material is an excellent addition to your compost heap. You should avoid adding plants that have gone to seed or plants that are affected by disease to compost so they don’t start growing in the compost pile or spread the disease they are affected by. Large branches should also be left out of compost, as well as any plants that were heavily treated with pesticides or other poisons.

Can you plant grass seed on compost?

You wouldn’t want to plant grass seed in just compost without any soil added, but you can use compost to help in the process of planting your grass seed. Simply mix the seeds with compost and apply this mixture on top of the soil where you want to plant the grass seed. Combining your seeds with compost like this gives them some weight and prevents the seeds from lying exposed on the surface of the soil, making them less likely to be blown away by the wind, carried off by birds or other wildlife, or washed away by rainfall or irrigation.

Do you need fertilizer if you use compost?

Although people sometimes think of fertilizer and compost as being in the same category, compost is not a fertilizer and the two should be used together. You need fertilizer even if you’re already using compost because compost alone doesn’t contain enough of the nutrients your plants need to flourish, like nitrogen, potassium, and phosphorus.

Does compost go on top of soil?

You can either mix your compost into the soil you’ll use in the garden or apply the compost on top of the soil—either way works. To use compost on top of soil, just spread a layer over the surface as you would mulch. Use no more than an inch or two on flower beds or for landscape plants and three inches on the surface of vegetable gardens.

Does compost turn into soil?

Compost is a soil amendment, so it is mixed into topsoil in the garden and becomes a part of it. Compost is made of rotted organic material, while soil contains other substances as well, like minerals and rock particles. Whereas compost is created by humans, soil is the naturally occurring top layer of the earth’s crust. When compost has decomposed, broken down, and become a part of the soil, it is called “humus.”

How do you mix compost into soil?

You can amend your soil with compost either by adding compost in a layer over the surface of the soil or by combining the compost and soil and working them in together. To mix compost into your soil, start by layering the compost over the soil’s surface. Use one or two inches of compost over flowerbeds and landscape plants or three inches of compost over vegetable gardens. You can stop here if you want to leave the compost on the surface of the ground like mulch, or to mix it into the soil, till your garden area to work your compost into the soil to a depth of six to eight inches. This process is best completed before you sow seeds or add transplants to the garden at the beginning of the growing season. Alternatively, you can mix compost into your garden soil at the end of the previous growing season.

How long can you keep compost?

It’s best to use compost as soon as you can, but when it’s not possible to use compost quickly, you can store it until the following season. However, compost that is not used by the following season should either be mixed into compost that is almost finished or revitalized by adding more materials to feed the microorganisms compost contains.

How long does it take for compost to turn into soil?

The amount of time it takes for compost to break down into soil you can use in your garden varies depending on the size of your compost pile, what materials go into the compost, and how the compost is tended. For compost to transform into soil can take anywhere from three months to two years.

How thick should topsoil be?

Topsoil should be at least six inches deep so that there is enough room for the root system of your lawn or other plants to develop.

Is compost enough fertilizer?

While amending your soil with compost does add some nutrients, compost does not include enough of the nutrients your plants need to be healthy (like nitrogen, potassium, and phosphorus). For this reason, you should use fertilizer in conjunction with compost to ensure your plants have access to the nutrition they require.

Should I mix sand with topsoil?

Mixing sand into your topsoil has several benefits for your garden or lawn. Combining your topsoil with sand aerates the soil, creating small pockets of air throughout the topsoil that make it easy for the root systems of your plants to reach nutrients, water, and oxygen.

Adding sand to your topsoil also improves the soil’s overall ability to drain water quickly, preventing oversaturated or compacted soil. Good drainage is vital for many of the plants you may be growing in your garden. Incorporating sand in your topsoil loosens the soil up as well, making it less dense and heavy, which makes it easier for roots to grow and expand.

Adding coarse sand to your topsoil can also improve the germination rate of plants that have small seeds. The more substantial granules of sand prevent the seeds from falling down through cracks between soil particles and getting lost under the surface where they won’t get the sunlight that’s essential for germination. Make sure that when you add sand to your topsoil, you create a balanced mixture—you don’t want to end up with more sand than soil.

What is the best ratio of compost to soil?

The best ratio of compost to soil is dependent on the type of garden the soil is intended for. As a general rule, five percent compost is a good place to start for most types of gardening. This ratio increases for container gardening (20 to 50 percent), flower gardens (20 percent), and planting new trees or shrubs (10 percent). A guideline that’s easy to follow is to add one or two inches of compost over the surface of flower gardens or landscape plants and three inches over the surface of vegetable gardens.

What should you not compost?

Materials that should not be composted include the following:

• Alliums like onions or garlic
• Ashes
• Bread or bread products (like pastries, pasta, or baked goods)
• Cat litter
• Cooking oil, grease, or anything saturated in oil or grease
• Cloth
• Dairy products
• Dead animals
• Debris from diseased or insect-infested plants
• Debris from plants that have been exposed to insecticide or other chemical treatments
• Debris from plants that have gone to seed
• Debris from weeds or other invasive plants
• Diapers or other items soiled with bodily fluids
• Feminine hygiene materials (like pads or tampons)
• Foil
• Large branches or chunks of wood
• Lime
• Manure from cats, dogs, humans, or any sick animal
• Meat scraps (including fat, blood, and bones)
• Paper that is colored, glossy, heavily coated, or has been printed upon
• Peels from citrus fruits
• Poisonous or toxic items of any kind
• Produce labels or stickers
• Raw or cooked rice
• Sawdust
• Synthetic fertilizer
• Synthetic materials or anything nonbiodegradable (acrylic, plastic, polyester, rubber, and the like)
• Tea or coffee bags (but coffee grounds or tea leaves that are not in bags are fine to include)
• Walnuts or debris from black walnut trees

Where should compost be—in sun or shade?

Compost can be kept either in the sun or in the shade, but the sunnier the spot is where you position your compost, the quicker the materials will decompose into humus. That means if you put your compost pile in a sunny spot, the compost will be finished and ready to use faster than if it were located in the shade.

Want to learn more about compost versus soil?

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