Rising risk of sulphur deficiency
ANY grower who still believes the need for sulphur on modern crops is being over-hyped should think again, according to two leading researchers.
Sulphur deficiency came to the fore in Scotland in the early 1980s when trials correcting deficiencies gave some dramatic results, says the SACs Kerr Walker. "On oilseed rape, a crop noted for its heavy sulphur demand, we sometimes saw a trebling of yield in response to added sulphur."
Since then, atmospheric S deposits have fallen so low that some areas receive less than 5kg S/ha/year. "Thats next to nothing, and it usually comes at the wrong time," says Dr Walker.
As a result, sulphur recommendations based on risk are commonplace north of the border. "On high risk sites 30kg S/ha is recommended for oilseed rape or grass with 20kg/ha for cereals. On second-cut silage the results can be as dramatic as applying nitrogen."
On moderate risk sites the advice is for 20 and 10kg/ha respectively, says Dr Walker.
England and Wales are about 10 years behind Scotland in terms of the development of sulphur deficiency, according to Steve McGrath of IACR-Rothamsted. "But sulphur deficiency has increased rapidly in the past 10 years and is now widespread, particularly in oilseed rape but also more recently in cereals."
Since 1995, the IACR model predicts that the area at high risk of S-deficiency has risen from 33% to 50% for oilseed rape and from 11% to 23% for wheat, he says.
"These high risk areas are mostly the light textured soils. However responses are now being detected on medium, and in the case of oilseed rape even on heavy textured soils.
"It is now probably necessary to add to this the area of medium risk for both crops. This means that 70% of rape and up to half the wheat would probably benefit from sulphur."
It is not just yield that can be jeopardised, he notes. Low sulphur in wheat can lead to poor bread-making quality. "We have seen quality very much improved by sulphur fertiliser."
However, the British Survey of Fertiliser Practice suggests that farmers are lagging behind both in terms of the percentage using sulphur and the amount applied, says Prof McGrath.
• The US-based Sulphur Institute estimates that in oilseed rape the return from spending £1 on sulphur can be as high as £15. Even in wheat the value/cost ratio can be 6:1. *
• Long-term problem in Scotland.
• England and Wales catching up.
• 70% OSR & 50% wheat may need S.
• Favourable cost benefit ratio.