Horticultural crops are grown under a wide range of environmental conditions. Horticulturists generally attempt to control as many environmental factors as possible that influence plant growth, such as light, temperature, moisture, and nutrients. More of these factors can be controlled in the greenhouse than in the field. The assumption is that these are the main factors that influence plant growth, but one of the most critical ones, soil, often is overlooked. Even when soilless mixes are used, little thought may be given to controlling more than moisture and nutrients in the medium, although most researchers and growers recognize the need for mixes to have good aeration and drainage, capacity to hold nutrients, and freedom from insects, diseases, and weeds as well as harmful chemicals. The latter viewpoint is largely correct, but overlooks the role played by nonpathogenic soil microorganisms in the growth and development of plants.