Gardening Articles: Care :: Soil, Water, & Fertilizer
Soil for Seed Starting (page 2 of 3)
by Jack Ruttle
What to Buy
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.