I’ve heard experienced gardeners say that compost and topsoil are just the same things!
I think that statement sums up the level of confusion about these two substances.
After all, they look pretty similar.
And you grow plants in both of them… Right?
So what’s the big deal? Suppose you’re serious about growing healthy, disease-free plants and improving crop yields. In that case, understanding the difference between compost and topsoil is essential.
In this article, I’ll explain all you need to know…
Topsoil vs compost
Topsoil is the upper layer of soil that varies in depth between 2 to 10 inches, depending on location. Compost is made from decayed waste materials. Topsoil is mostly mineral, whereas compost is organic.
You need to be careful about generalizations like “soil and compost are the same things.”
It simply isn’t true!
Soil and compost behave very differently and have distinct characteristics. However, both substances have their roles to play and are essential for your garden.
But don’t mix them up. Actually, yes… you should mix them up 🙂
But you know what I mean?
A little bit of knowledge about “soil science” can help you get better results in your garden. Let’s take a closer look at each of these substances.
Is compost soil?
Compost is not soil. It doesn’t contain the same quantity of minerals that plants need for healthy growth. However, compost is a “soil amendment” that vastly improves soil quality.
So is topsoil the same as compost? No… They’re really not the same thing.
Of course, both vary considerably from one location to another or how it was made in the case of compost.
Topsoil in Florida is very different from topsoil in Texas! And farm compost made from cow manure will differ from backyard compost made with grass clippings.
But as a general rule, it’s pretty easy to see the difference when you understand the underlying qualities of each.
Difference between topsoil and compost vol 60 KD 8
Compost is organic. In other words, it comes from living organisms. Soil is primarily made up of minerals such as sand, silt, and clay. Minerals are inert and non-living substances, whereas compost contains a vast array of biological activity.
I think the fundamental difference is that soil provides the structure and a source of mineral nutrients. In contrast, compost brings useful active life to the earth.
How to define compost:
A dictionary-style definition of compost would be something like this:
“Compost is decayed organic matter used for fertilizing and helping grow plants.”
This is a bit of a narrow way to describe all the goodness that compost does for your plants.
Compost is made from things that were once living. That’s why we call it “organic.” The natural decomposition process breaks down the dead materials thanks to a remarkable ecosystem of microorganisms, all feeding off the remains.
This biological activity is incredibly helpful for soil when you add compost. The microbial life makes nutrients available for your plants and helps pull minerals from the earth.
Organic materials slowly decompose to become humus. This fibrous substance is also incredibly useful for improving the soil structure and holding on to nutrients and moisture. It also improves drainage to avoid roots from getting waterlogged.
Definition of soil:
“Soil is the upper layer of the ground consisting of clay, sand, silt, rocks, and some organic matter.”
Again, this doesn’t tell us much about how soil works or how plants grow.
Soil comprises several layers (the fancy jargon for these layers is “horizons”).
- Litter layer: This upper layer includes decaying matter, often called the “litter layer” or “humus layer.” This level is full of bacteria, fungi, and insects. It closely resembles compost if the litter is left to decompose over time. This surface layer is not very deep.
- Topsoil: Next comes the topsoil layer. This is the part that most interests gardeners. It’s made of mineral materials (sand, silt, and clay) and some organic material. This layer has biological activity thanks to any remaining organic substances and humus. The topsoil layer can vary in depth between 2 to 10 inches.
- Subsoil: This layer is rich in minerals but low in humus content. It provides a nice stable layer for plant roots. We never dig down as far as the subsoil level most of the time.
- Parent layer: Made up of broken, weathered rock.
- Bedrock: This is the foundation layer of rock.
Good quality topsoil has more organic matter. This attracts and promotes living organisms. These can be microbes, fungi, and microorganisms like worms. All this biological activity improves the soil’s structure, aeration, drainage, and nutrient availability.
Poor quality topsoil has low organic content.
It’s the quantity and quality of organic matter that’s important.
Soil that is poor in organic matter will not have as much soil life. Topsoils can vary in organic content, from only small trace amounts up to about 30% content.
You can tell if topsoil has more organic content (carbon) by its darker appearance.
Soil biology is essential for maintaining good soil structure.
This has nothing to do with nutrients. Most gardeners know the famous NPK (nitrogen, phosphorus, and potassium), considered to be the primary nutrients needed for plant growth. A soil that has a good nutrient status is not necessarily healthy! If it has little or no organic content, its biological activity will be poor, and plant fertility will be low.
Active soil organisms help to improve aeration in the soil. This helps create a matrix of soil aggregates which plant roots navigate in search of moisture and food.
As they eat and digest organic matter, microorganisms excrete substances that improve fertility. Worm castings are a classic example of this. They are rich in enzymes, useful bacteria, and nutrients. But similar processes happen at a microscopic level.
“bacterial activity converts organic matter into nutrients.”
This is why it’s important to add compost to the soil to improve the quality of topsoil.
Thanks to the humus in compost, it holds nutrients in a stable, insoluble form, so they do not leach away and remain available to roots over a long period.
But you need to keep adding organic matter to topsoil to renew and enhance it.
Topsoil remains constant while compost changes over time. This is because compost becomes more compact as the materials break down, losing up to ⅔ of its volume, whereas soil doesn’t.
A quick top-up of compost once a year does wonders for soil fertility!
Table of comparison: compost vs topsoil
Organic:100 % organic materials from decomposing once-living substances.
Mineral and mostly Inorganic: Made up of sand, silt, and clay. Includes varying amounts of organic matter.
Microbial activity: Compost has a high level of biological activity. Teeming with bacteria and fungal life.
Microbial activity: Compared to compost, topsoil has less biological activity, depending on the quantity of organic matter.
Fibrous structure: The organic materials (mostly carbon) provide a structure that improves the soil’s capacity to drain, or retain moisture and nutrients.
Can lack structure: Clay soils retain water and drain badly and have low workability. But sandy soil drains too easily.
Nutrients: Compost contains more nutrients than most soils, thanks also to the variety of ingredients used to make it, providing an array of micro-nutrients.
Nutrients: Clay soil holds on to nutrients whereas most nutrients wash out of sandy soils too easily. Most soils contain fewer nutrients than compost.
Appearance: Finished compost is dark and crumbly in texture.
Appearance: Soils vary in color depending on organic content. Darker soils contain more organic substances than lighter-colored topsoil.
Workability: Compost is light and easily worked.
Workability: Clay soils are heavy and “sticky” compared to compost. Sandy topsoils are easier to work with.
FAQ: A few questions answered
Is compost better than soil?
Neither substance is better than the other. Both soil and compost are needed for healthy plant growth.
Soil provides minerals and a stable medium for plants.
Compost provides friendly microbial life.
By the way, if you’re thinking of buying some topsoil, I recommend mixing it with a healthy amount of compost before use. Purchased topsoil is “dead” soil. Some of it comes from building sites and includes topsoil and subsoil, with very little organic content. Soil like this is usually piled up before being shipped. This essentially kills most or all of the microbial activity in the soil.
Do plants grow better in soil or compost?
Topsoil that has been amended with compost will help plants grow better than planting in soil alone, especially if you have poor soil quality. It’s possible to plant in compost, but there are several caveats. The best method is known as “no-dig gardening,” where compost is applied as mulch on top of the soil and used 6 months later when the compost has matured.
Growing only in the soil can be challenging if you have poor-quality topsoil. This can happen if you continually grow crops, remove dead plants, then continue to plant into the ground. The organic content slowly diminishes.
Compost gives back what we take away!
If you remove all your dead plants and annuals, turn them into compost and give it back to the soil, you create an environment that is higher infertility.