Grower improves lettuce crop by sterilising soil against diseases and weeds

A Norwich-based salad grower has cut nearly all of his lettuce crop this year thanks to the beneficial effects of soil sterilisation.

David Frost, who grows tomatoes, cucumbers, lettuce and celery crops under glass, has found that "cleansing" his soils during the autumn months when they are out of crop has helped to tackle weeds and diseases.

He cleansed the soils using the product Basamid and applied it every other year. Basamid's active ingredient dazomet releases methyl isothiocyanate gas when in contact with moist soil - a broad-spectrum sterilant effective at controlling a wide range of fungi, weeds and soil pests.

Frost said: "It's fantastic - it does a really good job. Our subsequent lettuce crops have done really well. We have cut almost 100 per cent of the lettuce crop this time.

"As any grower will know, the potential for weeds to build up in intensive production systems can be a very real challenge. Growing lettuce crop after lettuce crop, with as many as five crops a year, inevitably gives way to a build up of weeds. Groundsel was a particular problem for us before we started sterilising."

Frost added that he had to prepare the glasshouse thoroughly before sterilising in order to get the most out of the treatment. "My biggest word of advice to growers would be that attention to detail and thorough preparation are vital. Spend the time getting ready - the process itself doesn't take long, but it's well worth spending that little bit of time and effort beforehand to get the most from Basamid."

He added: "Before anything else, to ensure a thorough clean down, I sweep the lower parts of the glasshouse walls and glass as well as any other surfaces where weed seeds may have got stuck.

"We also dig a trench around the edge of the glasshouse and around the posts, turning out the soil into the centre so the spading machine doesn't leave any area of soil untreated. Wetting the soil before treating helps encourage any weed seeds to germinate and fungi to break their dormancy - making them more susceptible to the sterilising gas."

Frost said the soil's moisture content should be at an optimum 60-70 per cent water-holding capacity to breakdown the prills of Basamid - necessary to deliver a high enough concentration of gas and therefore effective treatment.

He added that soil temperature was also important. It should be at a minimum of 10 degsC for at least 48 hours to ensure the optimum release pattern of the sterilant gas and the physiological state of the target organisms is appropriate.

"We apply Basamid at the label rate to give 76g/m2 using a spading machine to incorporate the granules. Using a spading machine I can treat deep into the soil and achieve an even distribution through the soil profile. We then use polythene sheeting to seal the soil surface and ensure a thorough treatment and prevent any release of the gas."

The treatment can take between one to six weeks, depending on soil temperature and water content. When it is finished the soil is aerated to allow any possible traces of the sterilant gas to escape.

Before planting the next crop, and to ensure that all the sterilant gas has been released, a "cress test" is conducted on treated and untreated soils. If the cress in both samples germinates at the same time then it is safe to plant.

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