Soil Mixes For Plants
Soil mixes and other growing media The medium used for starting seed should be loose, well drained and fine-textured. Prepared mixes are available commercially, or materials can be mixed at home.
Vermiculite. This material when used alone provides good seed germination. It is clean, and if not contaminated during handling, will not need sterilization. If other seeding mixes are used, it is useful for covering seeds since it does not form a crust and provides easy emergence for the seedlings. Vermiculite is available in several size grades. For seeding, a fine grade is best.
Synthetic mixtures. Mixes that contain no soil are available for growing seeds. These contain either a combination of peat moss and vermiculite or peat moss and perlite. They may be purchased ready-made or can be mixed at home. To prepare such a mix, use 4 quarts of vermiculite, 4 quarts of peat moss, 1 tablespoon of superphosphate and 2 tablespoons of pulverized limestone. These mixes, as well as vermiculite used alone, have little fertility. Seedlings must be watered with a diluted fertilizer solution soon after they emerge.
Soil-vermiculite mix. Seeds may be started in a mixture of about one-third good garden soil and two-thirds vermiculite. Since good soil contains some fertility, prompt fertilization after germination is not so essential. When garden soil is used, the mix must be sterilized before seeds are planted in it.
Soil-peat-sand-mix. Large seeds or vigorous-growing seeds may be planted in a mix of two parts good garden soil, one part shredded peat moss, and two parts either perlite, vermiculite or sharp sand. Soil sterilization is necessary.
Milled sphagnum moss. This is a ground sphagnum moss sometimes used for starting seeds since it appears to have an ability to inhibit the seedling disease "damping off." It should be well moistened before use. Since it contains no fertility, prompt fertilization is essential after seeds have germinated.
Layered mixes. Another technique used for germinating seeds is to partially fill a flat or pot with a sterilized soil mix and then top it with a layer of vermiculite or milled sphagnum moss in which the seeds are planted. After germination, roots of seedlings move from this top area into the soil mix, which provides fertility, so adding liquid fertilizer is less critical.
Sterilizing mixes and containers To guard against plant diseases carried in soil and on containers, cleanliness and sterilization of materials is important. Place the moist, but not wet, soil mix in a container that can be covered to keep the soil from drying rapidly. If a cover is not available, cover with aluminum foil, and seal at the edges. The mixes may be sterilized directly in the pots or flats in which they are to be used. In addition to killing disease organisms, many weed seeds are also killed by sterilization.
Pinch a small hole in the center of the foil and insert the bulb end of a meat or candy thermometer into the soil so that the bulb is about at the center of the soil mass. Place the pan in an oven at 200 to 250 degrees F. Keep the soil in the oven until the thermometer shows a temperature of 160 to 180 degrees F. Remove the pan and allow it to cool. Baking soil will give off a strong odor, so some ventilation may be desirable. The length of time necessary for sterilization depends on the volume of soil, as well as its moisture content. Dry soil cannot be sterilized well.
After the soil has been sterilized, make sure that containers, tools and working area are also clean and sterile. Clean soil can be easily reinfected by careless techniques. In sterile soil, reintroduced disease may spread faster than it would in unsterilized soil.
Sterilizing containers. Wash thoroughly in soapy water all used containers to remove all debris. Don't put wooden flats or plastic containers in the oven. Clean wood and plastic items by rinsing them in a solution of one part chlorine bleach and 10 parts water. Let them dry before filling with soil.
Copyright 2000 University of Missouri. Published by
University Extension, University of Missouri-Columbia. UExtension
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