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.
You could easily get confused with all the choices on the shelf in garden centers and nurseries. The root of the confusion lies in a fine old gardening term "potting soil." Many mixes good for seed starting are labeled potting soil. But other things called potting soil are not very good at all for seed starting and are sometimes inappropriate even for container growing.
You can be most sure of what you are getting in mixes labeled "seed starting" or "germinating." The ingredients of germinating mixes are the same as in peat-lite: high-quality sphagnum peat, fine vermiculite and often perlite, a very small quantity of limestone, a wetting agent and enough fertilizer to last through two or three waterings. What defines a germinating mix, beyond these ingredients, is that the mix has been screened to be very fine. Germinating mixes are designed for very small seeds like petunias or impatiens and for filling small-sized containers and plug cells quickly.
But a germinating mix is not the only product offered that will serve well to start your vegetable and flower seeds. You might want a growing mix with larger particle sizes. For one thing, germinating mixes are more expensive. For another, they are not suitable for filling larger containers for growing through the season. Because they are so fine, they hold more water and eventually compact more than coarser mixes do. They stay too wet and hold too little air for good long-term root growth. (When starting seed in a germinating mix, the container should be deep enough to allow for at least one and a half inches of soil, deep enough to keep the topmost layer dry and aerated and lower layers moist.)
Unfortunately, some products traditionally labeled potting mix can include a multitude of ingredients, not all of which are particularly good for seed germination and root growth. You must read the label to know what's really in the bag. If the label doesn't say, stay away. The primary ingredients in the kind of mix you want should be milled sphagnum peat (typically from 50 to 80 percent), vermiculite and perhaps perlite. Avoid mixes containing manure, topsoil or muck peat. These materials are too heavy and do not have the water- and air-holding properties of a good germinating or growing mix.
Fortunately, the trend toward clear and useful labeling recently has come to the growing-mix industry. Though neither mandatory nor completely uniform, the labels of most reliable suppliers describe the content of their various mixes clearly. The most common ingredient is listed first (often with the percentage), followed by the lesser ingredients in descending order. Labels do not always indicate whether the mix contains a starter fertilizer, because doing so would subject the mixes to regulations for fertilizers, which clearly the mixes are not.
Soilless growing or germinating mixes are sold in plastic bags. The material will be either loose (in a broad range of sizes from one quart up to a 40-pound bag) or in compressed bales (about 70 pounds). Compressed bales yield almost twice the volume on the label when you dig out the amount you need and fluff it up.
You can buy mixes in three very different places. Superstores with gardening sections will have a wide choice of brand names and will offer smaller packages. At local nurseries and garden centers, the choice in brand names will narrow, but there will be a full line of soils, some of them in larger sizes.
At a retail nursery that produces at least some of its own plants from seed or cuttings, you have the narrowest but perhaps best choice of all: the mixes that professional growers use themselves. Very often the owner will sell a bale or large bag. These materials cost from $8 (for a 40-pound bag) to $14 (for a 70-pound bale) wholesale. So even if you pay twice that, you are getting a tremendous bargain if you can find a grower willing to sell some, and increasingly many growers will do that.
If you keep the bag tightly closed and protected from the rain, the material will keep well for more than one season. The plastic covering on commercial-sized bags and bales is usually treated with ultraviolet light inhibitors, giving the material about a one-year life when stored in the open. But all materials should be kept closed tight to keep out disease organisms and maintain the moisture level of the material in the bag, which should be just very slightly moist.
The quality of professional seed-starting and growing mixes is so high that there is little reason for anyone to bother with the dusty job of mixing their own. Still, if you somehow can't find a good one to buy, the recipe for a peat-lite mix is very simple: Make the blend between half and three-quarters milled sphagnum peat moss and the rest horticultural-grade vermiculite. For maximum air circulation and water drainage, substitute perlite for half the vermiculite.
If the peat is coarse or lumpy, break up clods and take out large pieces with your hands, or use a 1/4-inch screen. Mix in some dolomitic limestone, at the rate of five pounds per cubic yard of mix. It's important to incorporate the lime thoroughly through the mix. At least one day before you plan to plant, sprinkle the mix with water to allow it time to permeate the peat. Rather than trying to mix in fertilizer, it's better to wait until seedlings are up and then begin feeding with a nutrient solution right away.
Jack Ruttle is a former senior editor at the National Gardening Association.
Photography by John Goodman
Article published on June 23, 2008.