Hot Water Technology

Background

Methyl bromide is the primary chemical of choice by vegetable growers to control soil-borne pathogens such as nematodes. Because of the impending ban on methyl bromide treatments, investigations have begun on hot water treatments as a potential alternative for large acreages of vegetable and nursery stock fields. In late 1993, Aqua Heat began designing and evaluating a hot water treatment/tillage system that could potentially control a wide range of soil pathogens by heat treatment rather than through the application of chemicals such as preplant fumigants.

Technology Development and Application

Successful sterilization of the soil using hot water requires proper soil conditions, tillage, and dosage to ensure uniform heating of soil to lethal temperatures. Aqua Heat's system works by pumping water through a flexible irrigation hose from a well to a mobile heating system mounted on the back of a tractor. The water heating system is a 25 million BTU diesel fired mobile boiler. The boiler is mounted on a flat bed with modified flotation tires to minimize soil compaction. The prototype boiler can heat between 250 and 300 gallons (946.3 to 1,135.5 liters) of water per minute continuously to temperatures of 200°F (93°C) to 230°F (110°C). To ensure uniform distribution of the scalding water within the soil profile, Aqua Heat has designed an application system in which the hot water is both sprayed onto and injected into the soil, and then immediately mixed into the soil during pre-plant tillage via a separate rotary tillage operation.

The precise volume of water that is required to treat most fields is dependent upon soil type, ambient soil temperature, the depth of soil to be treated, and the treatment area (i.e., the entire field or seedling beds). Current estimates suggest that approximately 25,000 to 50,000 gallons (94,625 to 189,250 liters) of hot water are required per acre for effective nematode control. Because the system is designed to prepare the seedor plantbed while the soil is being tilled and treated, and because pesticides are not used, growers should be able to plant their fields within hours after the soil has cooled. In addition, because this technology is used for preplant soil sterilization and raises the soil temperature to levels in excess of temperatures lethal to nematodes, it's application potential for crops is broad (Noling 1995).

Trials on tomato fields in Florida, performed by Aqua Heat and Dr. Noling, an Associate Professor of Nematology at the University of Florida, targeted the effectiveness of hot water treatments for controlling nematodes, specifically the southern root-knot nematode, Meloidogyne incognita, in sandy soils. Field tests were conducted using several different prototype hot water application systems in an effort to test potential equipment design and operating parameters. The treatment deemed to be most effective coupled a soil rotovating system with hot water injection.

Testing was performed on strips of bare soil that were 6 feet (1.8 meters) wide and 12 inches (30 cm) deep. The application apparatus was operated at speeds of 15.75 to 103.6 feet/minute (4.8 to 31.6 meters/minute). Water flow rate and temperature were maintained at 75 gallons/minute (284 liters/minute) and 219°F (104°C), respectively. Immediately following treatment, raised beds were formed. Temperatures were monitored at 2, 8 and 14 inches (5, 20, and 36 cm). All results were compared to an untreated control.

The results of preliminary field tests suggest that by treating soil with hot water during soil cultivation with a rotovator, it may possible to raise soil temperatures to levels that control soil-borne nematodes. At speeds of 15.75 feet/minute (4.8 meters/minute), the maximum temperatures achieved were 127°F (53°C) at 12 inches (30 cm), 135°F (57°C) at 8 inches (20 cm), and 116°F (47°C) at 4 inches (10 cm).

For most species of nematodes, an exposure to temperatures of 130°F (54°C) for a few minutes will ensure death (Mayo 1995a, Noling et al. 1995). Following heat treatment, reinoculation of the soil with beneficial bacteria as well as bacteria with known antagonistic properties to nematodes has shown to be effective in ensuring a healthy soil environment for crop production (Mayo 1995a). Other potential options include utilizing the synergistic effects of mixing the nematicide, Vapam , with pre-plant hot water treatments to improve its efficacy against nematodes.

Hotwater experiments performed in Florida have indicated that irrespective of soil depth, soil temperatures increase linearly with the amount of hot water used (Noling et al. 1995). The amount of hot water required to achieve temperatures for nematode control under plastic mulch in fine sandy soil (96% sand, <2% silt, clay and organic matter) is 30,000 to 70,000 gallons per acre (113,550 to 264,950 liters per acre) (Noling et al. 1995). Since fields are typically irrigated prior to planting, water requirements generally do not limit the adoption of this technology (Daar 1995).

These preliminary studies suggest that hot water holds potential as a practical, non-chemical alternative to methyl bromide fumigation for nematode control. At present, the method is slow in operation and uniform temperature elevations to a depth of 14 inches (35 cm) have not yet been achieved (Noling et al. 1995). However, Aqua Heat is currently developing a prototype to address and resolve these problems. More specifically to operate at commercially viable speeds and also refine the system design to perfect a more uniform distribution of heated soil (Mayo 1995b, Noling et al. 1995).

Costs

Aqua Heat's hot water treatment system does not require any capital outlay by growers. By the end of 1995, Aqua Heat intends to begin small scale regional commercial applicator services to growers. Approximately 300 gallons (1,135.5 liters) of diesel fuel is required per acre to heat water to raise soil temperature to 129°F (54°C). The estimated total cost to growers of contracting applicator services for hot water treatment is expected to range from $1,000 to $1,500 per acre (Mayo 1995b). Costs will vary depending on soil type, temperature, moisture, and depth. Methyl bromide treatments currently cost approximately $1200 to $1500 per acre (Mayo 1995b, Tjosvold 1995). The table below presents cost comparison data for preplant soil treatments.

Comparison of Estimated Total Costs of Preplant Soil Treatment/Acre

Aqua Heat Treatment

Estimated Cost to Grower: $1000 to $1500

Methyl Bromide Fumigation

Estimated Cost to Grower: $1200 to $1500

Aqua Heat technology will be applied via contract services. Methyl bromide is applied under contract or by the grower. The cost to the grower represents estimates provided by the contractors. Average actual cost figures are not yet available for Aqua Heat treatment as this system is not currently on the market. Costs will vary depending on soil type, temperature, and moisture. Only the major cost components are presented. This list does not include costs for depreciation on equipment, overhead, or contractor profits, etc. Costs were calculated using $1.12/lb methyl bromide for a 98:2 formulation at application rates of 180 240 lb/acre. Labor costs associated with performing heat treatments are separate from costs attributable to soil and bed preparation and tarping. Methyl bromide fumigation is accomplished during soil and bed preparation and tarping.

Sources: Asgrow 1995, Great Lakes 1995, Keene 1995, Mayo 1995a, 1995b, Tjosvald 1995

References

Asgrow. 1995. Price Quote: Active ingredient for methyl bromide. Asgrow Chemical, Immokalee, FL, February 1, 1995.
Daar. 1995 (January). Personal communication with Sheila Daar, Bio-Integral Resource Center. Berkeley, California.
Great Lakes. 1995. Specimen Label for Terr-O-Gas 98 (EPA Registration # 5785-22) and Terro-O-Gas 67 (EPA Registration # 5785-24). Great Lakes Chemical Corporation. West Lafayette, Indiana.
Keene. 1995 (April 18). Personal Communication with M. Keene, North Carolina Cooperative Extension, Lenoir County. North Carolina.
Mayo. 1995a. (April 14, 17, 21). Personal communication and faxes. Mr. Chapman Mayo, Aqua Heat Technology, Inc. Minneapolis, Minnesota.
Mayo. 1995b. (January). Personal communication with Mr. Chapman Mayo, Aqua Heat Technology, Inc. Minneapolis, Minnesota.
Noling. 1994 (November 13-16). "Mobile methods of in field soil sterilization using hot water." 1994 International Conference on Methyl Bromide Alternatives and Emissions Reductions. Kissimmee, FL.
Noling. 1995 (April). Personal Communication. J.W. Noling. Extension Nematologist, University of Florida, Citrus Research and Education Center. Lake Alfred, Florida.
Noling et al. 1995 (April). Mobile methods of in-field soil sterilization for soil pest control using hot water. Noling JW, Rajamanan, Mayo C, and Thompson J. Citrus and Vegetable.
Tjosvald. 1995. (April) Personal Communication with S. Tjosvald, California Cooperative Extension. Watsonville, California.

Please note that this publication discusses specific proprietary products and pest control methods. Some of these alternatives are now commercially available, while others are in an advanced stage of development. In all cases, the information presented does not constitute a recommendation or an endorsement of these products or methods by the Environmental Protection Agency (EPA) or other involved parties. Neither should the absence of an item or pest control method necessarily be interpreted as EPA disapproval.

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