Steam
Steam is also beneficial for large scale greenhouse growth, effectively kills pathogens by heating the soil, leaves no toxic residue and requires little aeration time as compared to chemically treated soil. It is not economically feasible everywhere due to the need of appropriate machinery for steaming as well as water and fuel.
Steaming has been used for more than a century as a highly effective way of soil
disinfection and is under certain conditions a permitted practice in organic husbandry. Its application prior to crop establishment can be a very effective method of preventing weed seedling emergence on raised beds in vegetable cropping.
For efficient soil steaming, the soil should have a good tilth and be neither too wet nor too dry. Measuring soil temperature to ensure that the soil is heated to over 80EC (180EF) for 30 minutes is important. When this temperature or steaming time is exceeded, the farmers may face problems of waterlogging, ammonia and others may be encountered.
Soil treatment with steam at 80-100°C effectively controls most soil-borne pathogens
and weeds. Aerated steam (air-steam mixture) selectively kills plant pathogens at 50-60°C in 30 minutes and is used in some nurseries as an alternative to soil fumigation
Steaming at 100°C is in the Netherlands by far the most employed method owing to its small demand of labour.
Steaming was applied first using a mobile soil steriliser in 1908. Its application required all the soil to be dug up and wheeled out of the greenhouses to be steamed. A more practical method came later, in the 1920s, and for the next 50 years, when the “loco” “loco-type” boilers were used moved by man.
New and more effective steam application methods, such as negative pressure steaming, have been developed for greenhouse soil disinfestations.
Steam is introduced under a sheet and forced into the deeper soil layers by negative pressure created in the soil by a fan, which sucks air out of the soil through buried perforated polypropene pipes. The method is more energy efficient, economical, and more reliable for the cultivation of some ornamentals in glasshouses than the conventional steaming method used for soil disinfestation.
The disadvantage of soil steaming is that it leaves a biological "vacuum" suitable for re-infestation by plant pathogens, a common characteristic of many soil fumigants. In some cases, plant growth can be affected, possibly due to the release of toxic ammonia, manganese, and soluble salts or because of killing beneficial microorganisms, such as the mycorrhizal fungi. Pre-planting irrigation ring reduces toxicity after steaming.
Both steam and aerated steam are very expensive and only practical and economical
under greenhouse conditions.
Another promising way of steaming is the so-called “sandwich steaming” method consisting of steaming soil surface and simultaneously from the depth by a steam plough. In such a way, the soil is treated with steam from two sides. Results of this method show, that the energy for steaming can be reduced by 30% in comparison to the conventional steaming-methods (e.g. sheet - steaming). This method is highly effective for the nematode control.
