Articles
STEAMING METHODS FOR SOILS AND SUBSTRATES
A temperature of 70°C should be maintained for at least half an hour; soil viruses are eliminated between 85 and 100°C.
In the past soil disinfestation was performed with grids, which were dug in or spiked grids, which were trodden into the soil.
Both methods required much and heavy labour.
Nowadays more manageable spiked grids are used on a small scale.
The more convenient system of sheet steaming was developed, in which steam is blown under a sheet and left to penetrate the soil.
Clay soils can be disinfested very well but on sand, loam and peat soils the results are insufficient.
This method is still used, mainly for clay soils.
To improve the temperature range in the deeper soil layers a permanent steam system was developed, in which steam was introduced in drainpipes, after which the steam moved to the soil surface.
The method was an improvement on sandy and loam soils and was applied until the development of the negative pressure steaming method.
Steam is introduced under a steaming sheet and pulled into the soil by a negative pressure, created in the soil by a fan, which sucks air out of the soil through buried perforated polypropene tubes.
The temperatures achieved with negative pressure steaming were better than when sheet or drain steaming was applied.
Lower material costs and fuel savings in comparison with drain steaming favoured the application of negative pressure steaming, which is still widely used nowadays.
All types of substrates can be steamed, providing an adequate drainage of condensation water is guaranteed.
Substrates infested with vascular or internal root diseases however should be replaced.