Steam power beats the bugs
Steam power beats the bugs
French lettuce growers are
turning to steam instead of
chemicals to rid the soil of
harmful organisms, and the
same technique could soon
be available in the UK,
reports Mike Williams
INTEREST in soil treatment by steam has been sparked by the increasing demand for crops grown either organically, or with low levels of pesticide application.
Steam is an alternative to using chemicals such as methyl bromide for controlling harmful organisms in the topsoil, and the process is used for a wide range of high value vegetables, and particularly salad crops.
Equipment for producing steam for soil treatment has been developed and produced by Regero SA of Nantes for more than 20 years. The company also makes planters and other machinery for vegetable growers and has clearly drawn on its experience in this field to perfect its steam treatment equipment.
Regero now claims to have more than 500 customers for its steam generators, and the converts include Frances biggest salad crop producer with more than 400ha (1000 acres) of lettuces near Lyons.
Using steam to kill harmful organisms in soil is not new, but the equipment from Regero is very different from the old approach to steam sterilisation which was popular 50 years ago before it was outdated by pesticides.
The old idea was to use steam at a high enough temperature to sterilise the soil, killing beneficial organisms as well as the soil borne pests. The modern version is called soil disinfection rather than sterilisation, and the maximum recommended temperature is normally 90 C (194F).
This is said to be sufficient to kill seedlings and ungerminated seeds, nematodes and the harmful bacteria, viruses and fungi in the soil, but excludes a number of useful organisms including nitrogen fixing bacteria.
Another big difference is that steam sterilisation was normally a static process restricted to small areas under glass. Regero equipment is available in a wide range of sizes and versions, including large tractor operated and self-propelled units suitable for field-scale operation.
Disinfection
Steam disinfection is carried out on cultivated soil, usually as the final operation before seeding or planting. The process can be used on any type of soil, but it works faster on a medium to light soil, while a high moisture content also slows the work rate.
The biggest units cover three standard bed widths and carry a diesel engine to power a generator producing electricity to heat the boiler, producing 600kg/hour of steam. Water is supplied to the boiler through a hose, using an automatic reel to wind the hose in as the unit moves towards the headland.
Steam from the boiler is released under three large aluminium shields where it is trapped to allow the heat to penetrate into the soil, with each shield spanning one bed. The work rate is slow, with a self-propelled unit taking up to 89 hours to treat 1ha on heavy land, and 56 hours/ha on light soils.
The slow work-rate is one reason why the driverless self-propelled unit, which operates automatically, is popular for large areas. One or more units can be set up on the headland and left to drive themselves slowly forwards until they come to a halt at the opposite headland. They are driven forwards by a 4 kW electric motor with two-wheel drive through a hydrostatic transmission. This motor also raises the three aluminium shields after each patch of soil is treated, and lower them again over the next area of untreated soil.
Before the steam units can be used, it is important to have a proper analysis of the water supply, according to Ernest Parsons, managing director of Plumtree Nursery Equipment, Plumtree, Nottingham, the UK distributor for Regero.
"This is important because some water supplies contain chemicals which could damage the boiler, and it may be necessary to treat the water before it is used to produce steam," he says.
Regero soil disinfection units are already working in most European countries including Denmark, Greece, Italy, Spain and the Netherlands, and the big French lettuce grower has 10 of the self-propelled units. Mr Parsons says some UK salad crop growers are considering a switch from pesticides to steam for soil treatment, but they are having to wait for HSE approval.
Three-phase supply
The delay may be due to uncertainty over the three-phase electric supply for the water heater, which is approved in other EC countries but not in the UK. "This is causing a problem at present, but I am sure it will be sorted out soon," he says. "Meanwhile there is one self-propelled Regero machine in the UK for a trials programme at Aberdeen University.
"I think there could be a big demand for steam generators to disinfect soil producing salad crops. The use of chemicals for food crop production has become a sensitive area, and there is increasing pressure from the supermarkets to reduce the amount of pesticides we use. If you stop using chemicals to treat the soil for crops such as lettuces, you need an alternative, and steam is the obvious option."
Mr Parsons price estimate for a self-propelled Regero steam generator suitable for treating three beds is about £45,000. *