If you do any container gardening or keep indoor plants, you’ve likely made use of potting soil — sometimes called potting mix or growing medium. It beats digging up dirt in your backyard to fill your pots. However, potting soil isn’t quite as innocent as it sounds. Most potting soil is actually soil-less and contains at least one rather unsustainable component. Fortunately, it’s pretty easy to make your own potting soil at home.
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What’s in Potting Soil?
You’ll notice that most potting soil is very lightweight compared to soil in the ground. This helps it retain water and makes it easier for tender roots to easily break through the growing medium. Many varieties also contain fertilizers to help plants grow quickly. Additionally, it is usually sterilized so it does not contain insects or diseases, or spread seeds unintentionally.
What is in this stuff, you ask? Not all potting media are the same but most contain some common ingredients.
This neutral medium absorbs water and adds some aeration and drainage. Unfortunately, sand mining is not well regulated and is often environmentally destructive, impacting low-income communities and destroying habitats.
A mined mineral that expands into lightweight particles when heated, vermiculite increases the porosity, drainage, and aeration of potting soil. It also adds beneficial calcium and magnesium to the soil and increases the soil’s ability to hold water. While the material is considered organic, all mining negatively impacts the land, water, plants, and animals in the vicinity.
Perlite is volcanic rock, also mined, with associated environmental impacts. When it’s heated, it expands and looks like small, white balls of Styrofoam. Perlite is lightweight and adds porosity, aeration, and drainage to potting soil.
Sphagnum Peat Moss
Not to be confused with sphagnum moss, sphagnum peat moss is a natural fungicide and is very lightweight and retains water extremely well. At the same time, it allows excess water to drain off. It’s naturally made in wetlands over centuries of decomposing plants. But unexpectedly, peat does not contain many nutrients for the plants to absorb. Like coal, peat is “natural” and “replenishable”- but not at the rate that we use it. And like coal, the extraction of peat has harmful environmental consequences.
Wetlands are important ecological landforms that act as habitat nurseries, migration stopovers, and help filter out water pollution. The peat harvesting process requires draining the wetlands before scraping the peat off of the soil. It’s then dried, screened, and compacted. The wetland and its inhabitants are very negatively impacted. Wetlands are federally protected in the United States; much of the peat that’s used in potting soil comes from Canada and sometimes Russia. Additionally, harvesting peat releases carbon dioxide, which is a major contributor to climate change.
Making DYI Potting Soil
When it’s time to fill your garden pots or house plant containers, why not mix up some homemade potting soil and skip the peat (and wetland destruction)? There are many easy-to-obtain ingredients to make a quality potting soil mix.
Compost is a great addition to your DIY potting soil, especially if it’s compost that you rotted in your backyard from kitchen and garden scraps. Compost contains lots of nutrients and is not as heavy as soil from the ground. You can use it to replace the fertilizers in commercial potting soil mixes.
Coconut coir is a byproduct of the coconut processing industry. This lightweight fiber is an ideal sustainable substitute for peat because of its ability to hold water and add drainage to DIY potting soil. Coir is usually sold in compressed blocks that expand when moistened.
Made from recycled paper, PittMoss is an environmentally friendly substitute for sphagnum peat moss. Its lightweight fibers evenly distribute water throughout the mix, discourage runoff, and encourage root growth.
Wood chips or pine needles are natural substitutes for perlite or vermiculite. Both woody alternatives will break down eventually but will add nutrients to your soil as they do.
Mixing the Soil
Mix equal parts compost and coir for your soil — 6 gallons of each is a good starting place. Then add 3 gallons of wood chips (or perlite or vermiculite). Experiment with the mixture to obtain the texture and quantity you need for your gardening. You might want to mix in some sand to get the desired texture.
Better Store-Bought Bagged Soil
If you are only potting two small indoor house plants, it won’t make any sense to buy all of these ingredients to make just a few cups of potting medium. Or perhaps you don’t have the capacity or time to make potting soil. No guilt here; it’s fine to buy potting soil, just avoid bags that contain sphagnum peat moss. Look for brands that contain PittMoss, compost, worm castings, or even bat guano instead of peat.
Originally published on May 16, 2022, this article was updated in July 2023.