Commercial greenhouse soils are commonly sterilized by high-pressure steam released to penetrate downwards into the soil, which is covered by heat resistant plastic sheeting (sheet steaming). The steam condenses on contact with soil particles, and moves deeper only when that layer of soil has reached steam temperature. Some active soil pests, such as symphilids, may move downwards ahead of the steam 'front'.
The temperatures required to kill most nematodes, insects, weed seeds and fungi are 45°C, 55°C, 55°C and 60°C respectively. Beneficial bacterial spores are not killed below 82°C, and therefore growers attempt to reach, but not exceed, this soil temperature. Most mycorrhizal fungi are unfortunately killed by this process.
In this way, organisms difficult to sterilize, such as fungal sclerotia, Meloidogyne and Verticillium in root debris, may be killed. Sheet steaming is effective only to depths of about 15 cm, and its effect is reduced when soil aggregates (see p311) are large and hard to penetrate, or when soils are wet and hard to heat up. When soil pests and diseases occur deep in the soil, heating pipes may be placed below the soil surface, as grids or spikes, to achieve a more thorough effect. The 'steam-plough' achieves a similar result, as it is winched along the greenhouse. If soil is to be used in growing composts it should be sterilized (see sterilizing equipment). The clear advantage of soil sterilization may occasionally be lost if a serious soil fungus (such as Pythium) is accidentally introduced into a crop where it may quickly spread in the absence of fungal competition.
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