Seed-Starting Soil, From Scratch

A little dirt never hurt anyone. We acquire immunities early on from making mud pies, as health experts now know and gardeners have known all along.

The same goes for plants. This time of year, gardeners yearn to get their hands dirty and start seeds indoors. But how often is their seed-starting mix homemade, with compost and real soil?

For centuries gardeners eager to give tiny seeds a good start in life have improved on garden soil, which is cold and soggy in early spring. A seed-starting medium should be lighter and better aerated. Soil in little pots, blocks or cell trays dries out fast, so the mix must hold moisture, yet be well drained. It should be fertile. In the old days, vegetable growers stockpiled upside-down chunks of pasture loam, in which the roots of the grasses kept those blocks intact, aerated and suitable for growing seedlings. Before peat became a popular ingredient, growers used mixes based on leaf mold combined with sand and nutritional amendments such as hoof and horn meal, blood meal for nitrogen, bone meal for phosphorous, greensand for potassium, and limestone for calcium. Another practice some followed, to avoid the sudden collapse of seedlings and other soil-borne diseases, was to sterilize the soil and render it devoid of life.

Research by Professor Harry Hoitink at Ohio State University has shown that good compost introduces disease-suppressing microflora into a potting mix, so that it does not need to be sterilized. And I've found that giving seedlings good air circulation, plenty of light and, above all, good drainage helps keep them healthy. Bottom watering rather than surface watering reduces risk of disease. Over-fertilizing can lead to damping off, but if compost is included in the mix from the start, no further feeding is needed.

I tend to garden just the way I cook: without recipes. I'm apt to throw a bunch of ingredients into a tub or wheelbarrow and mix until it all looks right. So I have experimented with potting soil by starting out with a simple ratio (one third peat, one third compost, one third coarse sand or perlite) and then varying these ratios and the amounts of nutritional amendments. This has taught me to use a mix with more peat and no nitrogen for germinating seeds, and a more high-test, compost-rich one for growing transplants. The cabbage family seems to benefit when I add about 15 percent garden soil, preferably from a bed where onions were grown the year before. If the peat I buy is rough, with sticks in it, I sift those out.

Although all these ingredients can be purchased, the soil and the compost will be truly alive and kicking if they come from your own garden.