Greenhouse growing media that contain field soil or artificial media that have been exposed or previously used can contain undesirable microorganisms, insects, and weeds. The elimination of these undesirables is termed pasteurization. Unlike sterilization, which kills all life in the soil, pasteurization kills just the harmful elements. Soil pasteurization is preferred to sterilization and, fortunately, is easier to accomplish.
Pasteurization is usually accomplished either with steam or with chemical fumigants. The fumigants are more expensive than steam, which is usually available from the greenhouse heating system. However, some weeds are not killed by the 180° F soil temperature of steam treatment and require fumigation. Small quantities of soil may be pasteurized with electric heat, but it is not a common method for commercial use.
Steam is applied to the bench, pots, or ground beds in different ways:
• from the surface
• through pipes buried in the soil •in a closed container
For bench soil pasteurization, benches must have evenly spaced openings in the bottom to permit air to escape as steam enters.
Surface steaming, like all steaming, is done after the soil is conditioned and of uniform consistency. All nutrients and pH adjustors should be mixed evenly into the soil before steaming except those that would be harmed by the treatment, such as time-release fertilizers formulated with plastic coatings.
The soil is then leveled in the bench and a perforated pipe or hose is placed on top to distribute the steam evenly down the bench. One pipe or hose is used per three feet or less of bench width. Next, at intervals of four to five feet down the center of the bench, inverted flats or concrete blocks are set into place. Finally, a solid cover, such as a canvas tarpaulin or sheet of heavy plastic, is placed over the bench and weighted down around the outside edge. The flats or blocks prevent the cover from adhering to the soil (Figure 20-1).
Steam is then introduced into the perforated pipe or hose from the boiler. Probe thermometers placed into the medium at intervals around the bench record the soil temperature as the steam permeates it. When the most distant parts of the bench attain a soil temperature of 180° F
figure 20-1. This greenhouse ground bed is being steam pasteurized. A tarpaulin held down by clips prevents the steam from escaping. (Delmar/Cengage Learning. Photo by Steven Newman.)
(or 140° F if air is being injected into the steam) for thirty minutes or longer, pasteurization is complete. The tarp, blocks, and pipes can be removed and the soil permitted to cool for planting.
Buried pipes are a preferred method of steaming if ground beds are being pasteurized, although the technique is also suitable for benches. In this method, perforated pipes are placed in the center of the bench and then buried at one-half or more of the soil’s depth. The bench is covered and the temperature measured as in surface steaming.
Closed Container Steaming
Closed container steaming is used for pots and other containers, with or without soil in them. It may also be used for tools and other production materials that could be contaminated. A thermometer must be outside the container to permit temperature reading.
Where growers do not have the facilities for steaming soil, they must use chemical fumigants. The fumigation should be done outside the greenhouse to help avoid injury to greenhouse crops or workers since many of the products are toxic to plants or humans. Benches can be disinfested with a dilute antiseptic. The fumigants are in two forms: pressurized gas canisters and liquids that turn to gas after application. The pile of soil being fumigated must be covered with a gas-impermeable canopy to hold in the gas and allow it to permeate the soil. The toxicity of some of the products is one of the disadvantages of fumigation. Another disadvantage is the waiting time required before the soil can be planted without injury to the crop. Up to three weeks may be needed for safety.
The fumigants currently used are chloropicrin (tear gas), Vapam, and Formalin.