Everything old is new again. Soil steaming, popular at the turn of the 20th century, fell out of use when chemical-based farming became the norm. Soil fumigants such as methyl bromide – popular with strawberry growers and recently prohibited in the U.S. except in extenuating circumstances – offered weed, pest and disease control. But these are toxic substances, causing real dangers during application, as well as causing environmental harm. (The EPA offers information on chemical soil fumigants and their appropriate use at epa.gov/soil-fumigants/soil-fumigant-chemicals.)

Soil fumigation with steam is an environmentally friendly alternative. Soil steaming involves no toxins, no chemicals and poses no threat to the ozone layer. Unlike pesticides used to control diseases and pests, resistance does not become a concern with soil steaming.

Steaming Process

The basics are the same as they were 120 years ago. Fuel is burned to create heat, which is used to boil water, creating steam. The steam is then injected into soil in one of several methods, reducing pathogen and weed seed loads. Today, farmers can be more targeted in their use of soil steaming and better control the outcome without a negative impact on overall soil health.

Steve Fennimore of UC-Davis has been researching soil steaming for more than a decade. “Soil steaming involves a lot of physics,” Fennimore said. “There’s a lot of ways to do this.” He recently presented a webinar on soil steaming for New Hampshire’s Cheshire County Conservation District.

While steaming does affect beneficial microbes, when properly done it does not sterilize the soil. It’s a pasteurization process, which leaves good microbial populations intact while dealing a stronger blow to the pathogenic ones.

“What is the objective of soil steaming? It is to raise the soil temperature to between 150º and 160º Fahrenheit for 20 minutes. It’s called the dwell time,” Fennimore said. “The reason steam works, and the reason we can get away with pasteurizing the soil, is that the bad guys are easier to kill than the good guys. The beneficials are more tolerant … You want to hit it just right: 158º for 20 minutes.”

Higher temperatures kill microbes quickly, while lower temperatures require more kill time. While there are some plant viruses, as well as very heat-resistant weed seeds, which won’t be destroyed unless soil temperatures reach 212º, most plant viruses and all pathogenic plant bacteria cannot withstand temperatures in the 160º – 170º range. Insects are controlled at about 150º. Molds, nematodes and most plant pathogenic fungi require even less extreme soil temperatures – about 140º – to succumb. Fusarium can be killed, but it returns and is challenging to control, Fennimore said.

Techniques & Results

The University of Vermont has been assisting growers with sheet steaming methods for high tunnels, focusing on effectiveness and cost for producers.

Grower Becky Maden of Singing Cedars Farmstead in Orwell, VT, is also a UVM Extension agent specializing in vegetable nutrient management. Maden is experimenting with soil steaming in high tunnels. Weeds are an issue in high tunnel vegetables, and sheet steaming can be a low-cost, effective method of control, particularly for growers not using pesticides. Chickweed in winter greens has been an issue on her farm.

Scott Greene of Singing Cedars Farmstead demonstrates how sheet steaming works. Photo courtesy of Rebecca Maden

Maden was featured on UVM Extension’s “The Ag Engineering Podcast,” where she talked about sheet steaming and demonstrated the process in a step-by-step explanation. The following steps will prepare the area for treatment: Prepare the growing bed; water it normally to help the heat move through the soil; put down steam delivery hoses (aka “socks”); apply a tarp or fabric over the area to be steamed and weigh down the sides; then connect the hoses to the steamer. The steamer tank needs to be filled with water and should be leveled.

The steamer output hoses can be closed to build up some pressure, and about 3 PSI is ideal for steam delivery. Reaching about 160º at a two-inch depth is recommended by Maden. Steaming can also be used for potting media, compost or even greenhouse shelving or tools. “It’s a great alternative for sterilizing things,” she said.

Maden is far from the only experienced grower using soil steaming. The equipment is expensive, so a sharing service is being explored. She and other Extension agents were awarded a Specialty Crop Block Grant from Vermont to focus on establishing best practices including steaming temperatures, studying the economics, researching microbe levels pre- and post-steaming and collecting data on the nitrate levels of the soil.

“The trial plots very quickly paid for themselves,” Maden said. The cost of hand-weeding for chickweed control would have been significantly more. There were very good crop seeding rates of 90% germination with excellent growth after soil steaming. They also didn’t need to do any hand-weeding that year, despite severe chickweed issues.

Moving ahead, she’d like to explore how to “feed back the soil microbes after steaming” and to better understand how microbes recolonize the soil. Providing beneficials a jump-start by exploring various soil inoculants is also a goal.

While the sheet steaming method is useful, it’s not the easiest or most efficient way to steam soil, Fennimore said. “When you blend steam with soil, it’s a lot easier to do it. When you physically mix steam with soil, you can heat up the soil very quickly,” he said. “If you don’t move the soil, if you just put a sheet on the soil and inject steam under it … it moves slowly.”

When soil is blended with steam, the steam has less distance to travel, so the soil heats up quickly compared to static soil. Soil moisture is important and should be present at a level similar to what would be ideal for planting. Moist soil conducts heat better than dry but takes longer to heat up. Weed control is better with moist soils. However, it’s easier to steam dry soil. Finding the ideal soil moisture level for applications can be a compromise, Fennimore said. Soils should not have clumps, as the temperature in the clot will remain cooler, and steam takes time to penetrate the mass.

Supplements can be added to steaming to reduce fuel use and increase efficacy. Mustard seed meal, which contains pest-suppression elements, as well as quicklime, which causes an exothermic reaction, can be added to enhance steaming and make the process more efficient and effective, Fennimore said.

Injecting steam via burying pipes under a sheet steaming setup can increase efficiency. Using an auger to prepare holes and injecting steam through them can also introduce steam more effectively into the soil. Sandwich steaming machines are mobile, steaming soil as they move across fields. Other machines utilize rototillers to inject steam.

Fennimore recommended utilizing methods which get steam into soil quickly, mechanically blend soil with steam and insulate soil for a brief period of time to ensure the dwell time is met. The depth at which steam needs to penetrate will vary with crop and with problem. Nematodes and Fusarium go deep into soils, but many weeds and pests live near the soil surface.

“Steam works,” Fennimore said, and does not harm beneficial soil microbes or do any other damage to the soil, as long as best practices are followed.

by Tamara Scully