Steaming can be a viable alternative to methyl bromide for soil and growth media in greenhouses and some small-scale field nurseries. Steam effectively kills pathogens by heating the soil to levels that cause protein coagulation or enzyme inactivation (Langhans 1990). Soil steam sterilization was first discovered in 1888 (by Frank in Germany) and was first commercially used in the United States (by Rudd) in 1893 (Baker 1962). Since then, a wide variety of steam machines have been built to disinfest both commercial greenhouse and nursery field soils (Grossman and Liebman 1995). In the 1950s, for example, steam sterilization technologies expanded from disinfestation of potting soil and greenhouse mixes to commercial production of steam rakes and tractor-drawn steam blades for fumigating small acres of cut flowers and other high-value field crops (Langedijk 1959). Today, even more effective steam technologies are being developed.
The advantages of steam sterilization are that it can be a highly efficient, cost effective technology for the control of soil-borne pathogens, pests, and weeds; it eliminates the need of tarps and fumigants; it can be a neat, clean, and easy-to-use control technology, leaving no toxic residues or fumes and therefore less harmful to other greenhouse crops and growers (with no toxic fumes, workers can harvest or plant new cuttings in adjacent fields). In addition, it is non-selective (lethal to all pests). Steam requires little aeration time (steamed soils can be planted as soon as they cool, whereas chemically treated soils can have a relatively long treatment and aeration period). Steam can also be used to disinfest non-soil substances such as perlite, peat, and compost (Szmidt et al. 1989) and can be adaptable to many situations (i.e., most types of boilers used to heat greenhouses can be adapted to supply steam for sterilizing benches or soil bins).
However, it should be noted that while there are a number of positives aspects to using steam as a pest control tool, there are potential pitfalls and shortcomings. This method does not currently appear to be operationally feasible for large outdoor nursery crops due slow application speed as well as high energy and capital investment costs. Due to limited steam penetration in the field, surface application may not reach pests in deep rooted trees or crops. It is a very inefficient methods when soils are very wet (similar with most other fumigants, including methyl bromide). In addition, steam that is too hot (85oC to 100oC) may increase soil aggregation and destroy soil structure. A number of these issues may well be resolvable, and efforts by researchers to refine this technology should continue.
If deemed necessary, steam sterilization can be used in combination with other control mechanisms, such as nematicides, botanicals, soil amendments, and biological control agents (Stephens et al. 1983). For example, one possible biocontrol agent is the fungal antagonist, Trichoderma, which has been shown to increase biological control and horticultural productivity. Tricoderma spp. can hasten flowering of periwinkle, increase the number of blooms in petunias and chrysanthemums, and increase dry weights of flowers and vegetables such as tomato, pepper, and cucumber (Baker 1992, Chang et al. 1986, Locke et al. 1985, Horst and Lawson 1982).
Nursery crops account for 20 percent of the worldwide use of methyl bromide for soil fumigation (Anonymous 1995a). Steam sterilization can be an effective pest control method for many nursery crops, including ornamental bedding plants, potted foliage and flowering house plants, fresh cut flowers and greens, bulbs, container perennials, propagating material, vegetable starts, greenhouse grown vegetables, garden seeds, and sod. Soil used to grow cut Christmas trees and seedlings for orchards, vine-yards, and forests can also be effectively treated with steam technologies, depending upon crop value and size of area to be treated. In 1991, the value of crops produced by floriculture and on environmental horticulture farms was $8.7 billion, or 11 percent of total cash receipt from all crops (Johnson and Johnson 1993). In California, these industries are worth $2 billion in gross receipts, or about 10 percent of the total value of California's agricultural commodities (Anonymous 1993).
To effectively steam treat soils, soil temperatures of at least 70C must be achieved for 30 minutes. Temperatures below 70C will not kill all soil-borne pathogens and steaming for periods exceeding 30 minutes after the desired temperature has been reached does not further benefit the soil (Horst and Lawson 1982). Sterilization does not guarantee that disease causing organisms will not recontaminate the soil, therefore if the soil is not used immediately after it is treated it should be protected from reintroduced pathogens (i.e., by avoiding contact with non-sterilized soil and practicing standard sanitation procedures in the greenhouse) (Horst and Lawson 1982).
Steaming Greenhouse and Potting Soils
The use of steam for greenhouse and potting soil mixes is quick and efficient. Boilers used to heat greenhouses can often be adapted to supply steam for sterilizing greenhouse benches or soil bins. Bulk and container soils can be easily loaded into steam boxes with removable fronts and steam pipe grids for treatment. Alternatively, forklifts can load pallets of soil into pressurized autoclaves for steaming. Another way of disinfesting greenhouse and nursery soils is to cover perforated steam pipes with soil to be treated (Newhall 1955). Bed or bench treatments are most effective when perforated pipes are laid in the bottom of the bed because steam supplied from the top of the bed has limited penetration to about 8 inches depth (Bartok 1993). More recently, small portable steam generators have been developed and used for greenhouse benches in the U.S. and the Netherlands (Grossman and Liebman 1995).
Open Fields and Steam
In addition to greenhouses, it is also possible to use steam technologies on small nursery fields. For example, movable steam applicators, such as the steam rake and the steam blade have been used extensively in nursery fields. Both are pulled through the soil either by a winch or by a self-propelled unit containing a boiler to produce steam. In Florida, several small steam machines have been developed for field use. Using these machines can be less expensive and in some conditions may be more effective than methyl bromide fumigation (Grossman and Liebman 1995) and can disinfest a quarter of an acre of planting bed per work shift. Steam cultivation is also used in the Netherlands, where methyl bromide soil fumigation has been banned for several years and where large mobile boilers (that can be moved from farm-to-farm on trucks) have been developed and used in fields (Grossman and Liebman 1995). To aid steam penetration, soil is cultivated as deeply as possible. Typically steam is blown under a sheet covering the soil and left to penetrate. Clay is very easy to disinfest with this steam system, while slightly more energy is required to achieve high enough soil temperatures in sand, loams, and peat soils because of their water retaining capacity. To raise the temperature in these soils or in deeper soil layers, steaming sheets are sometimes covered with nylon nets or bubble foil , so that the pressure under the sheets can be increased and heat loss can be kept at a minimum. For high value Dutch crops such as carnations and cut flowers, field soils have also been disinfested by embedded steam pipes directly in the field. Though fuel costs for steam systems using embedded pipes are less than sheet steaming, material costs are often higher (Runia 1983).
Negative Pressure Steaming
Negative pressure steaming, the most recent advance in applied steam technologies for soils, was introduced to the Netherlands in 1981. Using this method, steam is introduced under the steam sheet and pulled into the soil by negative pressures created by a fan. Specifically, the fan draws air out of the soil through buried perforated polypropylene pipes (Runia 1983). The fans continue to move heat from the upper to lower soil layers for several hours after steam treatment. Deep soil temperatures achieved with negative pressure steaming are considerably higher than those obtained with sheet steaming, averaging 85 to 100C (185F to 212F) down to 35 cm deep (sheet steaming produces an average temperature of only 26C (78F) at the same depth). This method was found to be more energy efficient, economical (by up to 50 percent), and more reliable for the cultivation of some crops (i.e., chrysanthemums) than the conventional steaming methods used to disinfest soil in the Netherlands (Anonymous 1992). By 1982, over 100 nurseries in the Netherlands were using negative pressure steaming (Runia 1983, Banks 1995).
Although high-temperature negative pressure steaming has its advocates, some researchers believe that steam at 85C to 100C (185F to 212F) kills too many beneficial soil organisms (i.e., mycrorhizal fungi) along with the pathogens and can lead to the production of phytotoxic compounds harmful to crop plants. As a result, these researchers advocate the use of lower temperatures (70C) (152F)or cool steam, which does not kill beneficial organisms (i.e., nitrifying bacteria) and is less phytotoxic (Langhans 1990, Grossman and Liebman 1995, Baker 1970). To cool steam to the desired temperature (i.e., typically 70C for 30 minutes), it is mixed with a stream of air. Since lower temperatures are required, aerated steam is faster and approximately 40 percent cheaper than hot steam (Baker 1962, Bartok 1993). Likewise, Baker (1957) calculated that the cost of aerated steaming is 30 to 50 percent cheaper than methyl bromide (including the boiler costs).
Steam sterilization can be an economically viable alternative to methyl bromide fumigation in a number of crops (i.e., ornamental bedding plants, potted foliage and flowering house plants, fresh cut flowers and greens, bulbs, container perennials, vegetable starts, greenhouse grown vegetables, and garden seeds). Tables 1 and 2 present a cost analysis for steam compared to methyl bromide for cucumbers and chrysanthemums, respectively. The formula for calculating the cost of soil steam sterilization vs. methyl bromide takes into account soil volume and permeability, soil heat exchange efficiency, boiler efficiency, units of fuel required, the BTU constraints of the fuel, and water prices (Lawson and Horst 1982). Total steam sterilization costs ($/kg yield or $/bench) were comparable to that of methyl bromide fumigation for both crops analyzed (Anonymous 1995a). Furthermore, steaming has the extra advantage of allowing growers to replant up to three weeks sooner than methyl bromide treated fields (an important economic advantage in cool climates) (Grossman and Liebman 1995).
Since large steam boilers can cost up to $150,000, it is likely not practical for growers not currently using boilers to heat their greenhouses to buy new boilers to steam soil once a year. Instead, outside contractors can be hired for steam treatments (Grossman and Liebman 1995). This is especially common in the Netherlands, where in addition to stationary on-site boilers, growers commonly rent or contract for truck-mounted steam generators on an as need basis (Anonymous 1992 and Anonymous 1995b). As a result, the capital cost to purchase a boiler was not included in the cost estimates presented in the tables below. Likewise, in Table 2, the cost of tarps, plastic, canvas, metal pipes, and labor were excluded from analysis since these costs were found to vary significantly from greenhouse to greenhouse depending on the current material prices and labor rates (Lawson and Horst 1982).
Steam costs are expected to decrease as these systems become more commercialized and less expensive energy/water sources are utilized. For example, greenhouse heating costs can be kept at a minimum by tapping alternative fuels such as sawdust, rubber from old tires, methane from landfills, wind, hot water from electric power plants, and geothermal vests (Davis 1994).
Table 1. Cucumbers: Annual Cost of Steam vs. Methyl Bromide as a Preplant Fumigant.
Adjusted Cost ($/kg)
Source: Banks 1995.
Table 2. Chrysanthemums: Annual Cost of Steam vs. Methyl Bromide as a Preplant Fumigant.
Note: One bench equals 200 square feet.
Source: Lawson and Horst 1982.
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