Gardening Articles: Care :: Seeds & Propagation

Soil for Seed Starting

by Jack Ruttle


It is one of the many paradoxes in gardening that the best soils for starting seedlings indoors contain no real soil at all. Ordinarily, weed seedlings sprout and grow in profusion in any speck of open ground. So what's the big deal with using some topsoil for starting seedlings-- Why can't we just dig up a little good earth and set it aside for our winter work.

As simple and as natural as that answer might seem, the truth is that using real topsoil creates more problems than it solves, even if your garden soil is a perfect loam.

Outdoor soil is too heavy and too compact to provide sufficient air circulation in the restricted space of a container. More important, it carries too many disease organisms.

Today's soilless mixes are mostly sphagnum peat moss, plus vermiculite and often a little perlite. The sphagnum peat used in good seed-starting and growing mixes is a very stable organic material that holds a great deal of water and air and does not decompose quickly. Both vermiculite and perlite are natural minerals that at very high temperatures pop like popcorn. Once expanded or popped, they are very lightweight and porous. In a soil mix, they improve both air circulation and water drainage. Perlite retains no water itself, vermiculite a little. All three of these basic ingredients are naturally disease-free.

The movement away from real soil in potting mixes began about 60 years ago, in an attempt to eliminate soil diseases that were plaguing the nursery industry. The first alternatives were mixtures of sand (or very sandy soils) with ground-up (or milled) sphagnum peat. But these still needed to be heat-treated to kill disease organisms that came in on the sand. The effort culminated in the early 1960's in research at Cornell University that produced the "peat-lite" formula.

Classic peat-lite is half milled sphagnum peat and half vermiculite, though sometimes a small portion of the vermiculite is replaced by perlite. Although you can buy true peat-lite mixes from many suppliers, most of their formulations will contain a higher proportion of peat, around 75 percent, simply because most growing-mix producers are peat bog owners, and they have more of it than anything else.

Firms that serve the nursery industry, such as Pro-Mix, Fafard or Baccto, may offer as many as 10 slightly different formulations, all essentially slight variations on the peat-lite formula. Most variations have arisen to suit local preferences of commercial growers or to allow the producers to make use of the resources at hand. These high-peat mixes work just fine, both for starting most kinds of seed and for growing the transplants on in larger containers.

All good growing mixes also contain a very small amount of ground limestone (usually dolomite) to correct the acidity of the peat and to help buffer against the varying acidity and alkalinity of local water supplies. Dolomite supplies some calcium and magnesium to plants. The mixes also include a wetting agent (tested to be safe for plant growth) to help wet naturally water-resistant dry peat. Most mixes contain a very small amount of fertilizer as well. Most of this will leach out within two weeks after irrigation starts. In fact, many suppliers recommend that liquid fertilization begin as soon the first true leaves appear on seedlings.

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