Chapter 10

Growing Media for Greenhouse Crops

Soil Pasteurization and Fumigation

Soil pasteurization or fumigation can be used to control soil-borne diseases, insects, nematodes and weeds. It is important to take steps to prevent pests from being re-introduced to treated soil. Steam is the most common form of heat used for soil pasteurization. Soil fumigation involves the use of are volatile chemicals that produce a toxic gas when incorporated into the soil. Other growing media such as peat moss, pine bark, and sand can be pasteurized or fumigated but often are not. Peat moss and barks are not technically sterile since they contain various microorganisms. However, these microbes are usually not pathogenic. Occasionally, grower problems, especially in propagation media are traced to materials which were not pasteurized. Many growers consider sand and other aggregates inert and free of organisms. However, sand can and often does contain weed seed and other pathogens and pasteurization or fumigation may be the best policy. Vermiculite and perlite are heated to very high temperatures to produce them and so are initially free of pathogenic organisms. Composted materials reach temperatures as high as 160 degrees F (71 °C) and are therefore relatively pathogen free.

Soil Pasteurization

Soil pasteurization kills pathogenic organisms and weed seeds using aerated steam. Soil steaming is a method of applying heat to the soil to destroy harmful organisms.  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).

Aerated Steam

The preferred steaming method is the use of aerated steam at a temperature 140 degrees F (60°C) for 30 minutes.

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. Root substrate should not be dry. Dry root substrate acts very much as an insulator, resisting the conduction of heat and causing the substrate to warm up slowly.

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.

Soil Fumigation

Plant growers who do not have a way of steaming their growing media may use chemicals to do the job. Although chemical fumigants are not as effective as steam sterilization, they are a satisfactory substitute. Soil fumigation is effective only if the media are at the proper temperature. The gases do not diffuse easily (spread throughout the media) in cold soils. None of the chemicals presently in use will kill all of the organisms in growing media. If high dosage rates of fumigant are needed to destroy a particularly organism, there may be unwanted side effects. A few of the most common used fumigants are described in the following sections.

Methyl Bromide

Methyl bromide is a colorless, nonflammable, and odorless gas at ambient temperatures and pressures. Since methyl bromide is very toxic and is virtually odorless, all formulations for use in soil must contain at least 2 percent chloropicrin as a warning agent because of chloropicrin’s strong, teargas effect. Most methyl bromide formulations contain a higher percentage of chloropicrin to provide better control of target pests.

Chloropicrin

Chloropicrin is a broad-spectrum fumigant that controls some soil-borne insects, fungi, and bacteria. It provides limited control of some weed seeds and nematodes. Although chloropicrin is often added to other fumigants in low concentrations as a warning agent, it is also added at higher concentrations (up to 75%) to increase the overall spectrum of pest control.

Methyl Iodide

The effectiveness of methyl iodide is similar to methyl bromide, rendering it a potential replacement. Methyl iodide is more environmentally acceptable because it is degraded by UV light much faster than methyl bromide-that is, 1 to 4 days versus 0.7 years.

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