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. Some growers, however, over-steam the soil by increasing the temperature to 212 degrees F (100°C) and holding 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 or main steam line in the greenhouse from the boiler. Steam generators generate heat by burning petroleum fuel. They can be easily moved into place and can provide enough heat in a short period of time. Soilless root media does not usually require steaming because the ingredients do not harbor pathogens and other harmful organisms. 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: free-flowing and aerated. When steam leaves the boiler, it is under slight pressure—5 to 15 pounds per square inch (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 free-flowing. As the temperature drops, it creates a heat zone that advances as the surrounding soil is heated. When one soil particle reaches 212 degrees F (100°C), then the steam moves past it to heat the next particle until the whole area is brought up to temperature.
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 and be more effective in controlling soil-borne diseases, insects, nematodes, and weeds. The substrate should not be dry.
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.
Steam Application
Soil steam sterilization is a farming technique that sterilizes soil with steam in greenhouses. Pests of plants, such as weeds, bacteria, fungi, and viruses, are killed through induced hot steam. The length of time and temperature for killing the pests may vary according to the pest and the type of soil.
Surface Steaming
The easiest system to set up but the least effective is to lay perforated pipe on top of the soil bed. A porous canvas hose is often used because it is easier to handle than pipe and works as well. Place a tarp or other covering over the treated area. If you choose to use steam heat, the tarp must be able to withstand the temperature of the steam. Using continuous sheets is best for disease and nematode control because the entire area is disinfested. Seal the edges of the tarp to prevent loss of heat.
Mobile Steam Application
A new approach to soil pasteurization is the use of mobile low-pressure steam units for disinfecting ground soil in the greenhouse (Figure 13.1). Typically, temperatures in these units do not exceed 195 degrees F (90°C). The labor cost is almost negligible since the steaming unit is pulled by a tractor and only needs to be turned at the end of each bed row.
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.
