Soil steaming is regaining popularity to control weeds, soil-borne diseases and insect pests in agriculture. It is an effective and somewhat sustainable alternative (the drawback is the use of fossil fuel) to chemicals and fumigants to disinfect soil in greenhouses, high tunnels and open fields, and therefore useful in conventional and organic production systems. Hot steam heats up the substrate to temperatures that kill or inactivate weed seeds, nematodes, fungi, bacteria, and viruses by destroying cell structure and proteins. Steaming can also disinfect compost, potting soil, pots, tools, etc.
Frank in Germany developed soil steaming for agricultural purposes in 1888. In the U.S., however, agricultural steamers were first commercialized in 1893 and many steamer designs were developed to disinfect soils in greenhouses and nursery fields. Among the steam application tools were steam rakes, and tractor-drawn steam blades for small acres of high value crops. However, chemicals (pesticides) and fumigants such as methyl-bromide got on the market in the 1950's replacing soil steaming in soil pest management. Nowadays and since the phasing out of methyl-bromide after the Montreal Protocol in 1987, steaming is regaining popularity to disinfecting soil to manage soil-borne diseases, weeds and other pests.
Soil steaming is the transfer of energy from burning fuel through water steam to heat up the soil or substrate to pasteurization or sterilization temperatures. Steam temperature at low pressure is above 212°F, but steam releases large amount of energy when condensates into water heating up the soil with minimal moisture. Soil pasteurization occurs at 160-182°F, but soil sterilization is at the water boiling temperature (212°F). Soil steaming in agriculture is considered a pasteurization process since temperature recommendations are 160°F for 30 minutes to kill most pathogenic fungi, bacteria, insects and nematodes, and 182°F for 30 minutes to kill resistant weed seeds. However, time and energy are necessary to reach those temperatures at the desired soil depth.
Soil revitalization with beneficial microorganisms (soil activator, compost, etc.) may be necessary after soil steaming. When heating to the highest temperatures for sterilization deep into the soil, practically all organisms die, including beneficial ones. Killing soil microorganisms diminishes if not eliminates the soil biological activity, which will affect soil health and nutrient cycling and availability for subsequent crops. In addition, steam killed microorganisms release significant amount of nutrients that were tied up in the living phase of the soil. Therefore, an increase in nutrient availability may enhance vegetative growth in the first crop after treatment, but may require additional amendments or fertilizer for subsequent crops. Consequently, reintroduction of soil beneficial organisms becomes necessary to reactivate and/or maintain the soil biological processes for a healthy and productive soil. Use of quality compost or other type of soil activator will reintroduce beneficial organisms. In the case of compost, plant material infected with soil-borne diseases should not be added and adequate composting temperature must be reached. Otherwise, soil-borne diseases will be reintroduced into the steamed soil because pathogens have resistant structures that will survive organic matter decomposition if killing temperatures are not reached.
There are practically three low pressure steam application types: surface steaming, deep soil steaming and container/stack steaming. There are also several variations within each type as well as applicator systems/designs. Examples are the area sheet steaming, steaming hood, steaming harrow, steaming plough, steam injectors with vacuums, and others.
Soil surface steaming or sheet steaming is the most economical system and used in high tunnels, greenhouses and field. Steam is injected through a perforated pipe or hose of adequate material laid on the soil surface, so covering the area is necessary to force soil penetration. This steaming type is effective to treat the top 2 to 6 inches of the soil. It is a shallow treatment and the effective treatment depth depends on the time the steaming application last. The longer the application time, the deeper the soil reaches the recommended treatment temperature. Because of the shallow heat treatment, beds/rows should be prepared before steaming and maintained undisturbed after treatment to avoid bringing up pests from soil at depths where killing temperatures were not reached.
Deep soil steaming is more effective and last longer, several years, because it reaches soil depth of 1 to 1.5ft. It reduces the risk of bringing up pests from soil at depths where temperatures were not high enough for killing when working the ground. However, it is more expensive because the increase in energy consumption and treatment time. There are many variations to this technique, but the main ones are depth steaming with vacuum (negative pressure) and the combination of surface and depth steaming (sandwich) system. In the depth steaming with vacuum, surface applied steam is suctioned via pipes installed at the specified depth for this purpose. This is a permanent installation, so the initial cost is significant. Drainage pipes are also used for this purpose. In contrast, mobile systems inject steam at the desire depth and the vacuum pipes are on the surface covered with a hood of aluminum or other corrosion resistant material. The sandwich steaming system, developed in Europe, uses specialized equipment because of the high steam pressure and the necessary hood to force soil penetration. It is more efficient because steam simultaneously penetrates the soil from both the surface and at a specified depth, so it reduces the application time and the total fuel used.
Container/stack steaming is used for potting soil, compost, pots, tools, etc. The use of large containers, boxes, piles, or even dump trailers depends on the amount of substrate. High pressure chambers are also available for specific sterilization purposes. In addition, steam injection via manifolds and using suction systems can heat up large amounts of substrate more evenly. Steamed compost reduces the risk of reintroducing soil-borne diseases back into disinfected soil, but soil revitalization with beneficial microorganisms may be necessary.
Cost of steaming depends on equipment (capacity), area and soil depth to treat, which determine time and fuel needed. Many steamer designs are available, so renting cost may vary depending on the model. However, a low-pressure steamer designed to treat a 300sqft section down to 2inch deep would take approximately 2.5h and 10gal of Diesel. Finally, make sure to monitor temperature when steaming to confirm appropriate disinfection throughout the treated area.
REVISED: November 24, 2020