Potting soil self-sufficiency is good for your budget, your plants and the environment, and you gain convenience by always having potting soil ready when you need it. If you have soil and compost, you’ve got the basic ingredients for making your own potting soil. In place of peat moss, perlite and vermiculite (the three leading ingredients in bagged potting soil), you can simply combine your best soil with cured compost, leaf mold, or rotted sawdust from untreated wood.
Most commercial potting soils are based on some combination of peat moss, perlite and vermiculite. Peat moss comes from wetland bogs in Canada or Michigan, however, the best peat alternatives are well rotted leaves (leaf mold), rotted sawdust or a mixture of both. A 4-by-5-foot pile of chopped leaves will take about two years to decompose into leaf mold.
Rotted sawdust, leaf mold or an abundance of organic matter in garden waste compost will compensate for the absence of vermiculite. Vermiculite is a mined mineral with as many environmental issues as peat moss. Deposits in Montana and Virginia have been found to contain asbestos.
Perlite ore, mined from mountain plateaus from New Mexico to Oregon, does a good job lightening texture and improving drainage. You can get the same results by using clean sand. When you want a light mix, a handful of sand per quart will do the trick as not much is needed.
Prepare some small batches, then mix it with store-bought potting soil. You have instantly saved money by stretching your supply. You can do 50/50 or 60/40, it does not need to be exact. Also, sometimes a gradual transition is less intimidating.
Using compost or biologically active garden soil in your potting mixes does require two extra steps — screening and then heat-treating or pasteurizing the material at 160 degrees to 180 degrees Fahrenheit.The nature of compost makes it an ideal primary food source for your garden’s soil food web, but teeming colonies of random fungi and bacteria are the last thing you want in containers. Many of the microbes in compost specialize in breaking down dead plant matter, but if they are deprived of food they often find ways to invade live plant tissues. When let loose in a flat of seedlings with little or no appropriate food or competition, the fungi will go after tender new roots and stems rather than starve.
Numerous studies have shown that pasteurization, which involves heating compost or soil to 160 degrees for an hour, or 180 degrees for 30 minutes, kills a high percentage of all fungi and bacteria, while preserving the biological integrity of the material — and its ability to suppress other diseases. Pasteurization kills persistent insects such as fungus gnats, too, along with most weed seeds.
In summer, a solar cooker can be made from a cardboard box, a 3 gallon black plastic pot filled with compost and a plastic bag. Enclose the plastic pot inside the plastic bag and place in the sun. After a few hours it will be pasteurized. Allow to cool before using.
The rest of the year, use your oven and a big heat-retaining Pyrex baking dish to pasteurize compost or soil. When done correctly, you will smell an earthy fragrance. Here’s the basic method.
- Preheat the oven to 200 degrees.
- Place 3 to 4 quarts of screened, mature compost (or screened rich garden soil) in a bucket or pail, and mix in enough water to make it lightly moist.
- Spread the moistened material in a large pan, but don’t pack it down because steam needs to be able to circulate.
- Cover tightly with aluminum foil.
- Poke a meat or oven thermometer into the center of the cover at a diagonal angle.
- Place the pan in the oven, and check the temperature at 15 minute intervals.
- Turn the oven off when the thermometer shows 150 degrees.
- Compost heats up quickly, while denser soil can take up to 30 minutes to hit 150 degrees.
- After the oven is turned off, the temperature should rise to 170 degrees; vent the oven if it goes to 180 degrees.
- Strong odors indicate that things in the oven have gotten too hot.
- Allow the pan to sit in the warm oven for at least 30 minutes.
- When the pan is cool, dump the pasteurized compost or soil into a clean container with a lid.
- Let it rest until you need it.
Until 30 years ago, most gardeners made their own potting soil by combining their best garden soil with rotted manure from the barn or buckets of leaf mold hauled home from damp streams. North American gardeners now spend more than $500 million each year on potting mixes and specialty soils. Just think how your money can be better spent.