Direct heat treatment kills soil-borne disease

27 August 1999

Direct heat treatment kills soil-borne disease

A NEW machine which heats soil to kill harmful pests and weed seeds can bring big savings in the use of crop protection chemicals, writes Mike Williams.

The soil heating equipment was developed in Norfolk and is designed mainly for treating land used for high value root and vegetable crops. A horticultural version is now available and an agricultural model for field scale treatment is due early next year with a 250hp engine to provide the power.

Behind the soil treatment process is UK Sterilizers of Southery, Downham Market, a company formed by Nicholson Machinery and local farmer Maurice Chapman, who have been working on the project for over decade.

Diesel-fired burner

While other treatment systems use steam to transfer heat to the soil, UK Sterilizers chose a direct heat process. Before treatment starts the soil is cultivated and set up in ridges – the machine is designed to lift the ridges with the soil passing through a chamber heated to between 68C and 70C by a diesel-fired burner. It is then deposited in ridges back on the ground.

David Nicholson, a partner in Nicholson Machinery and in UK Sterilizers, says that maintaining the correct temperature is important. Tests by ADAS, which provided technical help during the development process, have shown that 92% of weed seeds and most of the nematodes and other pathogens were destroyed at 62C, but 68C to 70C gave virtually a complete kill, including 100% of eggs present in nematode cysts.

Most of the beneficial organisms in the soil, including the bacteria, survive the treatment, but earthworms are an exception. Those exposed to the heat are killed, but previously ridging the soil encourages some of the worms to move down to undisturbed layers where there is more moisture, and these escape the process, Mr Nicholson says.

Moving and heating large quantities of soil is inevitably slow and costly, but Mr Nicholson believes it is an attractive alternative for farmers and growers who want to cut back on their use of crop protection chemicals – although sprays may still be needed to combat aphids and other air-borne organisms.

"We believe it will interest those who grow vegetables and root crops," he says. "Particularly on land with a heavy infestation of harmful organisms including some of the nematodes. There is a lot of pressure from supermarkets to reduce chemical inputs, and heating the soil to the correct temperature is much more efficient than using chemicals to control weeds and soil-borne pests.

"During our development work we have used the heat process for a wide range of crops, and we usually record a yield increase of about 10% compared with conventional chemical treatments. This is because heating the soil is a more efficient method of pest and weed control than using chemicals. It is also better than organic production because we can achieve the high quality that you cant get from organically grown produce."

Routine de-stoning

Heat treatment is suitable for most soils except the heaviest clays, and avoiding high moisture levels reduces fuel costs. Large quantities of stone also reduce efficiency, but for many of the crops likely to be grown after heat treatment the soil is already routinely de-stoned. Depth of soil treated depends on the crop, and ranges from about 10cm (4in) for shallow rooted crops to 25cm (10in) for potatoes.

The UKS750 machine for horticultural use has an 18hp diesel engine to power two rubber tracks, and it also drives a dynamo to supply power for electrical and electronic equipment, including the computer controlling the temperature. The transmission is hydrostatic, and the machine is self-steering, using a diablo to follow a soil ridge and signal corrections to the steering mechanism.

With a 75cm working width, the work rate is about 40 sq m an hour in medium soil and the price is £37,000. Price estimate for the 250hp agricultural version with a 2.0m working width is about £240,000, and the work rate at 15cm depth will be 2-4 acres per day depending on soil type. &#42

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