Potting soils are specially formulated soil mixes designed to produce optimal plant growth. The majority of potting soils consist of peat moss, bark and tiny white pellets known as perlite. Perlite is heat-inflated particles of volcanic glass, which keep the potting mix from getting too dense. Potting soil is heat sterilized in order to destroy all disease-causing microorganisms. Potting soils also contain a number of chemicals that support seed germination and plant health.
Most of the potting soils are sterilized to prevent the growth of disease-causing microbes. However, plants cannot grow in an entirely sterile environment, since this increases plant susceptibility to pests and diseases. Desirable microbes are added to potting soil that work to eliminate the pathogens from the soil. These microbial inoculants increase soil resistant to compaction, increases the soil's nutritional level, improves soil porosity and structure, and maximizes the conversion of the organic matter to soil humus.
Metam-sodium is added to potting soils for its sterilizing properties. Metam-sodium decomposes very quickly and leads to the formation of MITC, hydrogen sulfide and other volatile gasses, when provided with moisture and warmth in the soil. The gases then move upward through the space in the soil and account for the fumigation.
Dazomet is added to potting soil to control of nematodes, weeds and common diseases of many ornamental plants, including root rot, damping off and wilt diseases. Dazomet is effective in controlling in a number of soil-borne fungi and soil-borne bacteria including Meloiodgyn sp., Fusarium sp., Phytophthora cactorum, and Verticillium albo-atrum.