Potting soil is made in a variety of ways, and with a variety of ingredients. Each ingredient serves a specific purpose in the mixture. A common additive to potting soil is vermiculite. You will find it in commercial potting soils as well as in recipes for homemade potting soil. The purpose it serves is to lighten soil. Vermiculite expands into an accordion like shape, allowing it to hold moisture and nutrients in the soil, but also its expansion allows for better air circulation, making soil lighter and providing better drainage.
Vermiculite is actually a mineral, not a type of soil. It closely resembles mica, and it made of many layered sheets. When the mineral is heated, it expands, making it a valuable soil additive. Its natural shape once heated, makes it able to add proper aeration to soil and aid in proper drainage. It is normally mixed with peat moss and/or compost, as well as various other minerals and fertilizers to create potting soil.
It makes a good additive because it is odorless, non-toxic, and is not susceptible to mold or rot. It mixes well with other soil additives, and because of its unique shape, will hold on to nutrients and other minerals that plants need, like potassium, calcium and magnesium.
If you want to make your own potting soil for organic reasons, or just to save money, vermiculite can be purchased by itself from garden centers. Here are two recipes to mix your own soil using vermiculite.
Step 1—Making Recipe #1
In your plastic container, begin adding equal amounts of compost, peat moss and vermiculite. Use the shovel to mix it all together. It is best to add just a little of each ingredient at a time, like a bucket of each. Spray the mixture with the spray bottle from time to time. This will make it easier to mix and helps to rehydrate the dry ingredients. Continue adding and mixing until you either have as much as you want, or your container is full. Once it is mixed, it is ready for use. If you won't be using it right away, seal the container with the airtight lid and store it in a dry place.
Step 2—Making Recipe #2
The night before you plan to make your potting soil, get the peat moss damp. Using your large container, add 8 gallons of shredded peat moss, 8 gallons of vermiculite, 1/2 cup of ground limestone, 1/3 cup 18-9-18 fertilizer. Use the shovel to mix the ingredients together. To make it easier, you can add a little bit at a time, mix that up, and then add more.
If you live in a hot climate, consider adding hydro-gels to your mix. The hydro-gels act like sponges and will soak up and retain water for longer periods of time.
Recipe #1 makes the more organic potting soil.