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What’s the difference between topsoil and potting soil? The choices can get overwhelming at the gardening center, but it simply boils down to what you’re using it for. Do you want fill in a hole? Do you have clay soil? Are you growing in a container? Learn more.
We run a greenhouse business. This past spring, due to supply chain issues, we could no longer get the excellent compost-based potting soil we have been using for years. We went from buying it by the bag to getting it by the truckload and our plants thrived on it. We called it the not-so-secret secret of our success.
One trip to the garden center and we couldn’t believe the range of options. Suddenly, we found ourselves with the dilemma faced by many gardeners. What’s the difference?
What is Topsoil?
First, let’s talk topsoil. It’s very name says it all. Topsoil is the soil scraped from the top of the ground. It’s what you find naturally in your yard.
Topsoil is not sterile so it often has viable weed seeds and can harbor disease pathogens and fungal spores.
On the plus side, it contains naturally occurring nutrients and minerals, and beneficial soil microbes.
It’s very heavy, often containing a large amount of clay which makes it hold water. This can be a good thing, lessening the frequency of watering or a bad thing since the soil stays soggy and doesn’t drain well. It tends to pack down, making it difficult for seedlings to emerge.
High-quality topsoil should have a loose texture that crumbles like cake between your fingers and not clump up. If you have poor-quality topsoil (often too much clay), then buy enriched topsoil. This is a mix of topsoil with organic matter such as compost, sand, or composted manure.
Not that topsoil is only only for outdoor use such as spreading on your lawn when planting grass seed to get grass off to a good start. Use it to fill in and level low spots and add it to outdoor planting beds.
Also, buy topsoil for raised beds if your own topsoil is poor. Mix in organic matter such as compost to improve drainage at a 2:1 ratio of topsoil to compost. Or, purchase the enriched topsoil already mixed with compost. Over time the quality of the soil below the raised bed will improve.
What is Potting Soil?
Potting soil (also called potting mix) often contains no soil at all, hence the name soil-less mix on many bags. It is usually based on peat moss or some other quick draining material and sometimes has compost or biochar mixed in.
Potting soil is lightweight, has lots of air in it so water runs through quickly and it will not pack down. Perlite and vermiculite add even more air space, making it light and fluffy. Ingredients are listed on the bag.
Soilless potting soil and potting mix are best for containers since they need good drainage. Look for even fluffier seed-starting mixes when planting seeds indoors.
We tried 6 different kinds of potting soil and liked this one the best.
Choosing Between Topsoil and Potting Soil
The choice between topsoil or potting soil comes down to what you’re using it for:
Topsoil is for landscaping and outside uses. It is heavier and cheaper and is not sterile. If you are filling a hole or leveling out the ground, just get regular topsoil. (Or, sometimes you can find “fill dirt” from construction projects but only use if you are reasonably certain it is free of soil-borne diseases, pests, and contamination.)
Enriched topsoil (topsoil + compost) is for actively growing plants. Garden beds, shrubs, and trees all need a rich layer of minerals and organic matter. Plan on 6 to 8 inches of topsoil. Grass lawns also need topsoil though grass is shallow-rooted so 4 to 6 inches of topsoil is ample.
Soilless potting soil aka potting mix is for containers and indoor use. It is sterile, lightweight and more expensive. If you’re planting in pots, use potting soil.
Can You Mix Topsoil With Your Potting Soil?
Potting soil can be mixed with topsoil for particular cases such as raised beds (though we’d recommend 50% topsoil/compost for raised beds), but topsoil is a bad idea for containers. This defeats the entire purpose; potting soil is specifically formulated to be “soil-less” and provide the light mix for drainage and root development in pots. Adding topsoil makes the soil dense and heavy, and not good for watering which is critical for containers.
Hopefully, next time you visit the garden center for soil, it won’t be so overwhelming!