How to Make Your Own Potting Soil
(Page 2 of 4)
December 2008/January 2009
By Barbara Pleasant
Perlite ore, mined from mountain plateaus from New Mexico to Oregon, travels a long way to your garden, too, and its main contributions to potting soil — lightening texture and improving drainage — often can be matched by clean sand. When you want a light mix, a handful of sand per quart will do the trick (you don’t need much). As a hedge against slow drainage in the bottoms of seedling flats or containers, use a thin layer of rotted leaves, sawdust or sand.
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Using live compost or biologically active garden soil in your potting mixes often requires two extra steps — screening and then heat-treating or pasteurizing the material at 160 degrees to 180 degrees Fahrenheit. I like to use the compost made from garden waste to make potting soil, so whenever a batch looks good, I screen some and put it in plastic pails, bins or bags (such as those saved from purchased potting soil). My compost screen is a piece of half-inch mesh plastic fencing stapled to two pieces of scrap lumber. Many people use a similar version, with the screen attached to a sturdy wood frame. Stored where it can stay lightly moist, screened compost continues to cure and improves with age.
When you have excellent-quality, cured compost, and you’re not working with light-stressed little seedlings (the most disease-prone of all green beings), it’s fine to go ahead and mix up a 50:50 mixture of compost and good soil, and try it out. Or use more compost and less soil. My garden waste compost often contains quite a bit of soil already, mostly from the roots of pulled plants, so I often go with three parts compost to one part soil when potting plants that are ready for a rich, outdoorsy mix that gives them a nice taste of their future.
Now for the risks. The bioactive 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. Good examples are the cadre of fungi that cause seedlings to rot off at the soil’s surface, called “damping off.” These fungi are usually present in compost, but when forced to compete with other microorganisms in open soil, they stick to a dead-plant diet. But when let loose in a flat of tomatoes, with little or no appropriate food or competition, they 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 (the good and the bad), 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 all but a few heat-resistant weed seeds. The temperature must not go above 190 degrees, which can result in the formation of compounds that hinder plant growth.
