One of the most frequent questions that I get for a landscaping project is, “Should I use compost or topsoil for my garden?”
In speaking with people about their projects, I also often find that people ask for topsoil but they would be better off using compost for their gardens.
I love this general rule of thumb. If you’re looking to increase the level of your garden by 2 or 3 inches or more, use topsoil. If you’re just looking to improve your soil’s health, however, you want to use compost.
If you’re first learning about topsoil or compost, I think it’s helpful to think of it like this: to grow anything, you need to have nutrients (to feed the plants) and structure (a place for roots to grow so that they get water, air, and don’t blow over).
Compost provides nutrients; soil provides structure. This is absolutely an oversimplification, but it is an easy way to remember the general rule of thumb and leads to the right answer.
Here are a few helpful notes about compost and topsoil that are slightly more in-depth.
Compost is organic matter that has decomposed and is rich in nutrients, making it an excellent addition to soil for plants.
It improves soil structure, texture, and water retention while also promoting healthy microbial activity in the soil.
Compost is typically used as a soil amendment rather than a replacement for topsoil.
It is ideal for adding to existing garden beds or mixing with topsoil before planting.
Topsoil is the uppermost layer of soil—the top of a soil. In general, the top portion of our soil should be good for growing things—like plants or grass—and be 6-12’’ deep.
Topsoil is generally used to fill in areas or build up garden beds that are low on soil, and it provides a good base for planting.
If you need to create a new garden bed, topsoil may be a good choice to start with.
Which do I use: Compost vs Topsoil? Examples.
For example, when creating a new raised bed, I need something to grow in. Do I use topsoil or compost? Topsoil! First, I need to raise the absolute level by more than 2 or 3’’ (inches). Second, I don’t have structure (soil) or nutrients (compost). Therefore, I need healthy topsoil.
When there is already soil in place for a garden, I recommend people use compost (not topsoil) to amend the existing soil. First, I’m not trying to raise the absolute level. Second, I have existing soil (structure), so I just need nutrients (compost).
Why should I use compost instead of new topsoil?
It is possible to take out the existing soil and replace it with new soil. I hear of a lot of landscape contractors trying to do this but generally recommend against it.
It’s generally cheaper, easier, and of equal (or better!) quality to incorporate certified compost into the existing soil.
Cheaper: Compost is typically cheaper than a good topsoil. This is because a good topsoil actually has compost in it!
Easier: It’s less work and faster to add compost into the existing soil than trying to dig up, dispose of, and replace a garden’s soil.
Quality: There is a lot of bad soil that is sold in our Northern Virginia area. Just in Manassas, VA, there are a few places that only sell really bad soil--it is more like a fill dirt, rather than something that will grow plants, trees, and shrubs! Thus, using your existing soil and adding a certified compost is probably a lot better.
When should I replace my topsoil?
The general exception to this ‘amend the soil, don’t replace it’ guidance is if the existing soil has a lot of junk in it. If it’s so rocky and filled with plastic that you can’t put a shovel into it consistently across the garden, it’s probably better to get new soil. Since this is expensive and time-consuming to do, I’d check with a professional that this is the right approach.
I have created buying guides to help you find good, bulk compost as well as good, bulk topsoil. Since there are a lot of bad products out there across Northern VA, please check these out! This bulk and bagged compost calculator can also help you figure out how much compost, topsoil, or mulch you may need for your project.
In summary, if you're looking to add nutrients and improve soil health in an existing garden bed, compost is a great choice. If you need to build a new garden bed or fill in an area with soil, topsoil may be a better option. Additionally, using both together can provide the benefits of both, so consider using a mix of topsoil and compost for your specific needs.
Freestate Farms makes premium landscaping products—compost, topsoil, and mulch—by recycling food and yard waste in Manassas, VA. Our composts, topsoil, and mulches are specially designed to increase the health and productivity of local soils, and with our focus on sustainable practices, this lets the environment and your garden grow good, together. We sell bulk compost, bagged compost, bulk natural mulches, bulk dyed mulches, bagged dyed mulches, and bulk topsoil.