Soil Pasteurization, Fumigation, and Solarization
Soil Pasteurization
Soil pasteurization kills pathogenic organisms and weed seeds using aerated steam. It is customary to apply steam for 30 minutes beyond the time when the coldest spot in the batch of root substrate being pasteurized reaches 140 degrees F (60°C), although many growers pasteurize at a temperature of 160 degrees F (71°C). Most plant pathogens are killed by exposure to aerated steam at 140 degrees F (60°C) for a minimum of 30 minutes while higher temperatures are required to kill weed seeds (See Figure 12.1). Some growers, however, over-steam the soil by increasing the temperature to 212 degrees F (100°C) and hold it at this level for several hours. Over-steaming can cause a build of harmful substances in the soil—manganese toxicity and ammonium toxicity. Steam is provided by a portable steam generator (See Figure 12.1) or main steam line in the greenhouse. Soilless root media do not usually requiresteaming because the ingredients do not harbor pathogens and other harmfulorganisms. However, if soil-mixing equipment is exposed to dust and debris, or soilless media ingredients become contaminated (bags torn open and contents exposed), it is a good idea to pasteurize even soilless root media.
Freeflow versus Aerated Steam Treatment
There are two common methods of steam treatment: freeflowing and aerated. When steam leaves the boiler it is under slight pressure (5 to 15 psi). As soon as it is released into the growing mix it drops to atmospheric pressure giving up its heat. At this point it is considered freeflowing. As the temperature drops, it creates a heat zone that advances as the surrounding soil is heated.
Soil Preparation
Soils must be thoroughly mixed before they are steamed given that steam does not penetrate large lumps of soil to bring the temperature to the necessary level. The large pores in loose root substrates facilitate the movement of steam and thereby cut down the length of time required to pasteurize the soil as well as more effective in controlling soil-borne diseases, insects, nematodes, and weeds.
Ammonium Toxicity
High levels of ammonium can be released by soils or substrates high in organic matter after pasteurization. When the soil is heated to 180 degrees F (83°C), most of the bacteria that convert organic matter to nitrate are killed. However, the bacteria that convert organic matter to ammonia are hardier and often survive. Therefore, several weeks after steaming it is not unusual that high ammonia nitrogen is found in the soil. Often levels are high enough to burn roots.
Types of Steam Application
Surface Steaming
The easiest system to set up but the least effective is to lay perforated pipe on top of the bed. Perforated metal pipe is placed on top of the soil. A porous canvas hose is often used because it is easier to handle than pipe and works as well.
Buried Pipe Steaming
A better system uses perforated poly pipe buried 12 inches (30.5 cm) or deeper under the top of the bed.
Chamber (Vault) Steaming
Chamber steaming is commonly used by propagators and some potted plant growers. Metal containers, flats, wooden boxes, and clay pots are filled with the growing medium.
Click on the following topics for more information on greenhouse soil pasteurization, fumigation, and solariazation.
