Potting soil fresh out of the bag often fulfills all requirements for a growing medium: good texture that allows good root penetration, water retention, and good drainage, a stable pH between 6 and 7, and a minimum supply of nutrients.
Premium fast-draining soils with good texture that will not break down quickly are the best choice. Organic potting soils are very popular. These soils are often fortified with organic nutrients including readily available high-nitrogen worm castings. Potting soils are very heavy, and transportation costs tend to keep them somewhat localized. There are many good brands of high-quality potting soil.
Stay away from discount brands of low-quality potting soil. These soils can be full of week seed and diseases, hold water evenly, and drain poorly. Ultimately, saving a few pennies on soil will cost many headaches an a low yield later.
Many potting soils supply seedling transplants and clones with enough fertilizer for the first two to four weeks of growth. After that, supplemental fertilization is necessary to retain rapid, robust growth. Add fine-grade dolomite lime to buffer and stabilize the pH. Trace elements in fortified soil and soilless mixes can leach out and should be replenished with chelated nutrients, if deficiency signs occur. Organic growers often add their own blends of trace elements in mixes that contain seaweed, guanos, and manures.
Although some growers reuse their potting soil, I do not recommend it. If used for more than one crop, undesirable microorganisms, insects and fungi start growing; nutrients are depleted; water and air retention are poor, causing compaction and poor drainage. Some growers mix their old potting soil with new potting soil to stretch their mix. Cutting corners this way most often costs more in production than is saved in soil.
Potting soil or soilless mix that contains more than 30 percent lightweight pumice or perlite may float and stratify when saturated with water before planting. Mix water-saturated soil thoroughly with your hands until it is evenly mixed before planting or transplanting, if necessary.
Mushroom Compost
Mushroom compost is an inexpensive potting soil and soil amendment that is packed with organic goodies and is sterilized chemically to provide a clean medium for mushroom growth. After serving its purpose as a mushroom growing medium, it is discarded. Laws usually require that it sit fallow for two years or more to allow all the harmful sterilants to leach out. After lying fallow for several years, mushroom compost is very fertile and packed with beneficial microorganisms. The high-power compost could also foster antifungal and antibacterial properties in foliage and below the soil line, which helps guard against disease. Mushroom compost is loaded with beneficial bacteria that hasten nutrient uptake. The texture, water holding ability, and drainage in some mushroom compost should be amended with perlite to promote better drainage. Check your local nursery or extension service for a good source of mushroom compost.