Treatment of soil with steam is preferred over chemicals because it is very effective and safe. All pathogens will be killed by steam and only a few of the hardiest weed seeds will survive. There are portable steam generators available.

Ideally, the temperature of the entire soil mass should be raised to 160° to 180°F for 30 minutes. It is important to use several accurate thermometers placed in one or more corners and the center of the soil. Since it is difficult to avoid the soil temperature rising to 212°F, bring it up to this level and then reduce the pressure to allow the temperature to drop to 180°F in the coldest part of the soil mass.

The soil moisture content prior to steaming is important. Overly wet soil will take a long time to reach proper temperature. However, some moisture is necessary for the effective killing of microorganisms as well as the conductance of heat. Proper soil moisture for steaming is approximately the same for good planting conditions; soil squeezed in the hand should crumble easily. If possible, the soil mass should be moistened evenly two to three weeks prior to treatment. This will germinate difficult-to-control weed seeds, such as oxalis and clover, making them susceptible to heat.

Prolonged steaming of soil at temperatures higher than 180°F can result in undesirable side effects such as overkill of beneficial soil microflora and accumulation of ammonium and manganese. Soil high in organic matter should be tested for ammonium after steaming. Several weeks may be necessary to allow for the dissipation or conversion of the ammonium. The incorporation of dolomitic lime and superphosphate, based on a soil test, may reduce ammonium levels.