Plant pathogens are often killed in propagation soils by applying steam or chemicals. The treatment given to soil to free it of pathogens and pests is called sterilization or pasteurization. Soil sterilization kills all organisms in the soil, while pasteurization kills harmful microorganisms and weed seeds. The most common used methods are steam pasteurization, electrical pasteurization, or chemical fumigation.
The recommended method of steaming soil is to bring the temperature in the coldest part of the soil to 180 degree Fahrenheit. This temperature is maintained for at least one-half hour. Oversteaming may cause a buildup of harmful substances in the soil. For example, manganese is a nutrient commonly used by plants in small amounts. Oversteaming causes manganese to be released by the soil in quantities that are toxic to the plant. Also, soils with high amounts of organic matter produce toxic levels of salt when exposed to oversteaming.
In the horticulture industry, soil technicians prefer using aerated steam at a temperature of 140 degree Fahrenheit for 30 minutes to treat propagation soil. Because higher temperatures (180 degrees Fahrenheit) kill all microorganisms, diseases may spread very rapidly with the absence of their natural enemies. On the other hand, when soil is treated with aerated steam at 140 degrees Fahrenheit, most of the beneficial microorgamism survive. Their presence stops or slows the growth of plant diseases.
Steam and air are mixed and injected into a soil bed or trailer. The aerated steam flows through the vents and into the soil, elevating the temperature to a recommended level to eliminate diseases.
Some plant propagators treat a soil with chemicals to eliminate harmful pathogens. Because fumigant gases diffuse slowly at low temperatures, soil fumigants are most effective if the soil is at the proper temperature.
Vapam is a liquid carbamate soil fumigant that kills nematodes, fungi, and pest weeds. It is effective at temperatures between 30 and 90 degrees Fahrenheit. At a rate of one quart per one hundred square feet, Vapam is often applied from a watering can or proportioner attached to a water hose. After application, Vapam must be watered to spread it thoroughly into the soil. Because Vapam treated soil is toxic to plants, it must be turned 2 or 3 times before use in propagation or growing soils.
Other soil fumigants used in the horticulture industry include chloropicrin (tear gas), methyl bromide, ethylene bromide, and formalin (40% formaldehyde). These chemicals are poisonous to microorganisms as well as animals, humans and plants. Because inappropriate use of fumigants may cause negative side effects to plant growth, the manufacturer’s directions must be followed exactly when applying these chemicals. Moreover, caution and common sense also are applied when using fumigants.