Could sulphur be a soil panacea, easing potato scab and curing nutrient lock-up? Tom Allen-Stevens finds out.
SULPHUR is increasingly becoming a champion among nutrients for potato growers whose crops suffer from common scab. Trials carried out by Lincolnshire consultant Robert Boothman show that, as well as curing deficiencies, an application of elemental sulphur can reduce the incidence of the disease by as much as 80%.
There is no chemical means of controlling common scab, a fungal disease which infests soil and causes unsightly blemishes on the skin of tubers. On average Mr Boothman has found that large applications of sulphur can halve the incidence of the disease. "Were not talking about scab elimination, but it could make the difference between your crop being rejected or accepted for the premium first-class pre-packing or baking markets," he points out.
A total of 11 trials were carried out last year, with all but one returning a reduced incidence of the disease, compared to untreated. Mr Boothman believes this one rogue result was caused by using the wrong type of sulphur: "There are four or five different formulations of elemental sulphur on the market and its crucial to use the one thats right for the soil conditions."
Soil type is just one of the factors to take into account when choosing the formulation; others include presence of irrigation, historic scab levels, whether there is grassland in the rotation and soil pH. Scab prefers alkaline conditions, and Mr Boothman believes it is the acidifying effect of applied sulphur that reduces the incidence of the disease. "The acidity shouldnt cause the crop any harm: weve spoken to potato growers in Jersey whose soil pH is below 5, but whose crops are fine."
To assist in the battle against scab, he has included information and guidance on his website (www.boothman.co.uk). Growers can also sign up to Scab-Watch, and obtain a free appraisal of their individual growing conditions.
Mr Boothmans general advice to growers is to apply a continuous, narrow band of sulphur along the top of the ridge. The rate should be kept high – between 50 and 100 kg/ha. "If you want the sulphur to work, it must be applied correctly. Broadcasting 15kg/ha over the whole field will not bring a result," he says.
As far as type of sulphur is concerned, Mr Boothman is excited about a new formulation soon to be imported into the UK by Cambridge-based J&S Technical Services. Brimstone 80 is manufactured in a new plant in Saudi Arabia – one of the few sulphur products produced outside the United States. It contains 80% elemental sulphur and 10% calcium sulphate. "The elemental S will provide a slow release form of sulphur, while the calcium sulphate will be a quick and immediate burst for the crop. The calcium content should also help to reduce internal rust spot," he explains.
In some specific circumstances specialists believe the sulphur can reduce nutrient lock-up and even enhance physical characteristics of the soil.
"In certain high pH, calcium-rich soils, spring applications of sulphur, in the form of potassium sulphate, can help reduce the dominance of the calcium in the root zone. This allows the crop to take up other nutrients that would otherwise be locked up," claims independent agronomist Neil Fuller.
Elemental sulphur can have a similar effect, according to claims from Biotechnica Services, who is promoting a liquid form called Biosulphur. Mr Fuller prefers to be cautious about these assertions: "The advantage of using the Biosulphur is that it can be applied with a sprayer and the effect is very quick. But at a rate of 40 litres/ha, the application will cost around three times the alternatives, and you must be careful how you treat the soil afterwards. It may be suitable where growers are experiencing these sorts of compaction problems in minimal tillage situations, as long as the high cost can be justified."
But expert opinion is split over the validity of solving such problems through addressing nutrient imbalance. John Hollies from the Potash Development Association believes such methods have not been proven for UK conditions: "This is based on ideas brought over from the US where there are some problems of high magnesium levels and very little weathering on some soils. Such extreme situations very rarely exist in the UK and there has been no proof that magnesium binds the soil up, for example."
While he accepts that higher pH soils may lock up potassium to a greater degree, he has seen no practical evidence that autumn applications are any less effective than those made in spring. "It is naive to believe you can release tonnes of locked-up nutrients simply by adding a magic ingredient," concludes Mr Hollies .