Methods of Sterilizing Soil with Steam
Methods of applying steam to
“Soil steam sterilization (soil steaming) is a farming technique that sterilizes soil with steam in open fields or .”
Hoddesdon grid method
Named after the town of Hoddesdon, Herts, where the system was originally developed at the then existing Cheshunt Research Station, this process is used for borders in commercial greenhouses and involves the leading of steam under pressure into perforated steel pipes laid below the soil surface, which has previously been rendered moist and friable by deep cultivation.
Mobile steam systems of various designs have been developed for commercial growers and there are gardeners who have improvised, but it should be pointed out that steam can be a very dangerous thing and must be treated with respect.
This process was developed originally in the Netherlands and involves leading pressurized steam into either a coffin type box with outlet holes or perforated hosepipe under a heavy grade PVC sheet secured at the edges by long sandbags or other means. With a boiler output of 454kg (1,000lb) steam per hour it is possible to sheet steam an area of approximately 75 — 93m2 (800 — 1,000sq ft) in one operation.
One of the obvious advantages of sheet steaming over the grid method is the tremendous saving in labour, whereas its main disadvantage lies in the difficulty of achieving adequate penetration of the steam, which may take several hours to reach sufficient depth.
Low pressure steam sterilization
This has much more application for smaller growers, professional and amateur gardeners, although it is doubtful whether the setting up of the apparatus can now be merited in view of ready and reasonably cheap availability of prepared composts. When the John Innes composts were originally formulated and the specifications laid down, the John Innes low pressure sterilizers were built in their thousands all over Britain.
Here shredded loam was evenly spread in a 25-30cm (10-12in) layer on perforated steel sheeting over a shallow tray of water kept boiling by a coal fire, the steam passing slowly through the soil, being trapped by a tarpaulin or sack covering (now PVC). It took several hours for the soil at the top of the layer to reach the necessary 82-88°C (180-190°F) and careful checking was necessary to ensure that the whole batch of soil was evenly sterilized. Various models were available, most of which are no longer manufactured; low pressure sterilizers are now usually improvised.
Another form of steam sterilization is to drench shallow layers of soil on a clean surface with boiling water, covering with clean sacks or PVC sheets to retain the heat. On a still smaller scale suspending small sacks of soil in the steam given off by a water boiler can be remarkably successful. The efficiency of all these methods depends much upon the care which is exercised in ensuring even distribution of temperature.
Dry heat sterilization
By the application of dry heat to the soil at a sufficiently high temperature, its moisture content is converted to steam. The crudest form of this method is the use of a fire under a sheet of metal on which is placed a shallow layer of soil. The soil should be wet and care must be taken not to over-sterilize it. It is usual to stir the soil constantly to keep it from ‘burning’. The careful use of a flame gun can achieve the same result if the wet soil is spread in a shallow layer on a clean surface.
Rotary drum sterilizers rely on sterilization at ‘flash point’, the soil being rotated round a metal drum and falling through a fierce flame. The angle of the drum determines the number of times the soil is subjected to `flash point’ heat before being ejected out of the other end of the drum. The efficiency of rotary drum sterilizers depends not only on the correct adjustment of the burner and the angle of the rotating drum, but very much on the moisture content of the soil, which if too wet will ‘ball’ badly. Conversely if the soil is too dry it tends to `burn’, which destroys its structure. The ejected soil must be carefully checked to ensure that it has reached a temperature of at least 71-76°C (160-170°F) and that this is maintained in the heap for a reasonable period. Where potato cyst eelworm or virus is a problem, higher sterilization temperatures are advisable.
Electric soil sterilizers are available in different sizes, the soil being heated by panel type elements to a uniform 82°C (180°F); although the soil in the vicinity of the panels may be overheated and rendered sterile, it is soon reconstituted by the bulk of soil.