Soil vs compost — what is the difference? While soil and compost are indeed different types of organic material, each with a specific definition, use, and purpose, there are also some myths around this subject we need to tackle and dispel.
Soil vs Compost
Soil is the natural medium where your garden takes root, while compost is the organic matter that enhances soil fertility and structure.
The primary difference between topsoil and compost is that topsoil is mostly composed of minerals, whereas compost is primarily organic and made from things that were once alive.
Soil needs compost mixed in as an amendment to be the best growing medium, but compost can be used instead of soil (with some caveats) to grow a thriving garden.
What Is Compost?
Compost is a type of organic matter that is created through the decomposition of various plant materials, such as leaves, grass clippings, vegetable scraps, animal manure, and other organic waste.
It undergoes a natural process called composting, where microorganisms like bacteria break down the materials into a nutrient-rich and dark, crumbly substance that can be added to soil to improve its fertility and overall health.
Once compost has fully matured and reached a usable state, there are numerous ways to incorporate it into your garden.
You can apply it as a protective layer of mulch, blend it into your soil for enhanced quality, create compost tea to serve as a nourishing liquid fertilizer, or even utilize it as a direct planting medium.
Adding compost into poor quality soil can also transform it into something where your vegetables thrive.
- Enhances soil fertility by providing essential nutrients.
- Maintains the natural vitality and health of the soil.
- Supports the growth of beneficial microorganisms.
- Supplies the soil with a wide range of essential nutrients.
- Improves moisture retention in the soil.
- Facilitates optimal plant growth and development.
- Boosts the soil's resilience against diseases.
- Helps control and minimize weed growth.
- Nourishes the soil directly, promoting long-term soil health.
- Compost can be produced at home, offering a cost-effective and eco-friendly alternative to commercial fertilizers.
What Is Soil?
Soil is the natural medium consisting of minerals, organic matter, air, water, and living organisms, where plants grow and thrive in a garden.
Soil is composed of a mixture of minerals (such as sand, silt, and clay), organic matter (such as decomposed plants and animals), air, water, and living organisms (including bacteria, fungi, insects, and worms).
But when it comes to soil for our gardens we want to specific and say "topsoil."
What Is Topsoil?
Topsoil refers to the uppermost layer of soil, typically found within the top few inches to several inches of the earth's surface. It is rich in minerals and frequently organic matter, nutrients, and microorganisms too, making it more fertile and ideal for plant growth.
Topsoil is what gardeners are typically looking for when we're buying soil to mix with compost or to add into our garden beds.
If your source does not specify topsoil specifically, ask them to clarify, as 'soil' can refer to many things and unscrupulous companies can sell you something that you really don't want — like fill or subsoil.
What Is Mulch?
Mulch refers to a layer of material that is spread on the surface of the soil in gardens or landscapes.
The primary purpose of mulch is to cover the soil, providing a range of benefits including moisture retention, weed suppression, temperature regulation, and erosion control.
Mulch helps to create a favorable environment for plant growth by conserving moisture, reducing weed competition, and moderating soil temperature fluctuations.
And you can use compost as mulch too.
I recommend you check out my article on the benefits of mulch for more information and resources on this important topic.
What Is Dirt?
A not easily-definable term to be honest.
There is a wide variety of soil, compost, and mulch available in the market, each designed to meet specific gardening and landscaping requirements.
However, when it comes to dirt, there is essentially only one type that can be purchased: fill dirt.
Fill dirt refers to unscreened subsoil with minimal organic content, potential presence of rocks and boulders, and commonly sourced from construction or mining sites located on hillsides.
This stuff can be rehabilitated (through a long process) and made useful in gardening but it is absolutely not recommended unless you want to garden weeds for some reason.
The Role of Soil & Compost In The Garden
For home gardeners and homesteaders focused on growing their own food, this subject holds significant importance, yet it is riddled with misinformation and confusion.
Navigating through the plethora of subpar soil mixes and compost options can be frustrating due to untrustworthy companies peddling ineffective products.
Naturally, we desire the optimal growing medium for our plants.
Superior soil not only yields more nutritious crops but also cultivates resilient plants capable of withstanding pests, diseases, and fluctuating weather conditions.
Understanding soil health and its intricate microbiology is paramount when planning our gardens, but unfortunately, it remains widely misconstrued.
For most gardeners, they are purchasing something called "triple mix" which will be a mixture of topsoil, compost, and something like peat moss or similar — but you need to ask some important questions.
Firstly, what is the ratio of each material?
When we set out to fill our raised beds, we wanted to get as close as possible to 100% compost, but this was not possible.
So we found a supplier that had triple-mix which was 60% compost with the rest being topsoil and (unfortunately as it is not a sustainable product) peat moss.
Call around and ask questions.
Furthermore, ask them the source of the compost too.
In an ideal world you will have prepared by producing a lot of your own and then using it as a mulch in your newly filled raised beds.
And if you are only using the triple-mix, try to get one that has at least 30% compost.
Growing In Soil Alone
Growing in topsoil alone without any other amendments like compost is not recommended.
Your vegetable garden is a workhorse that is growing your food and those plants are heavy feeders that require tons of nutrition.
While nitrogen-fixing plants like corn and beans can do fine, your fruits, tomatoes, peppers, squashes will simply not thrive or be as productive and hardy.
As the seasons go on, you will continue to be dependant on fertilizers and other inputs.
Do not grow in just soil that has not been amended with compost.
Can You Just Use Compost As Soil?
Absolutely you can.
With some cautions.
It is a MYTH that you cannot grow in compost.
Firstly, you want to make sure that the compost is finished and that you source or make high-quality composts.
Secondly, you ideally want composts from multiple sources for best results — animal manures and bedding, kitchen scraps, spent mushroom substrate, leaves and so on.
Mixed compost tends to be best in general (see my article on the best compost for raised beds) as it will carry a plethora of nutrients in a wide array.
The no-dig/no-till movement has been talking about this for decades — with results to back it up — and yet the myth that you can't plant directly in compost persists.
Do Plants Grow Better In Topsoil Or Compost?
Amending topsoil with compost enhances plant growth compared to using soil alone, particularly in cases of poor soil quality. While planting directly in compost is possible and has great potential results, there are considerations to keep in mind.
The recommended approach is "no-dig or no-till gardening," (what we do here) where you start with 6-inches of mature, finished compost (preferably from various sources) and then reapply an inch or so of compost as mulch each new gardening seasoning.
I won't declare one a winner over the other, as people are clearly gardening successfully with both approaches, but I prefer and recommend as much compost as possible.
Soil & Compost Mixture Ratio Examples
This is taken from my article on the best compost for raised beds and is very relevant to this discussion.
If you're wondering what ratios of soil vs compost to use in your gardens or raised beds, here are some examples:
- Fill your raised beds or gardens 100% with compost: as mentioned this is entirely a legitimate and fantastic choice. The downsides are that buying compost can be expensive and/or you may not be able to produce enough on your own if you don't have a lot of land or any animals. Technically speaking, you only need 6-inches of compost to start planting, so you can fill the bottom raised beds with lesser quality soil, leaves, logs and branches (hugelkultur), etc.
- Triple mix: a term used in the gardening industry for a mixture of compost, topsoil, and usually peat moss that might be an equal mixture of the three materials. However, sometimes this is not the case so ask for the ingredients and ratios before committing. A more sustainable alternative to peat moss such as leaf mold or coconut coir is also gaining in popularity.
- The 6:3:1 ratio: 60% topsoil, 30% compost, and 10% peat moss (or leaf mold or coconut coir).
- 50/50: Half compost and half potting soil or top soil works too.
- Square Foot Gardening Mel's Mix: an equal mixture of ⅓ compost from mixed sources, ⅓ vermiculite, and ⅓ peat moss (or again — leaf mold or coconut coir).
It's not really a soil vs compost fight. While these terms carry different meanings, they are an integral and complementary part of gardening. Growing in soil alone is challenging and not recommending whereas you can freely grow in compost alone as long as you follow some simple guidelines. Ideally, you will be adding compost back into your garden beds each spring.