23 February 1996

Certain West Country crops could need sulphur – now

WEST Country growers with milling wheat, oilseed rape and intensively-cut grass should ask "Am I sure I dont need to include sulphur in my fertiliser programme?," says York-based consultant Chris Dawson.

The point has been reached where some sulphate should be applied with at least the early nitrogen dressings, rather than waiting to see if a deficiency developed, he told a Cargill conference at Blandford, Dorset.

"The reduction in atmospheric deposition of sulphur is now so significant it is likely these crops already require fertiliser sulphate throughout much of the country."

Significant amounts of sulphur could only be supplied via the roots, he stressed. There is virtually no scope for "fire brigade" foliar applications after a deficiency is detected.

Requirements should be estimated by methods similar to those for nitrogen, he suggested. Where it was believed sulphur was not yet limiting yields or crop quality, routine tissue analysis could indicate whether the nitrogen to sulphur ratio was still within normal limits.

Otherwise it was a matter of guessing the local atmospheric deposition of sulphur and estimating the amount of sulphur supplied in dung or slurry, allowing for the fact that it would not all be available to crops immediately.

Soil reserves of sulphur were mainly in organic matter, with light and chalky soils tending to have less sulphur than heavier ones. The rate of removal of sulphur could be estimated according to leaching and published information on crop removal.

Mr Dawson warned that information on fertiliser bags was expressed as S03, not elemental sulphur. About 2.5kg of S03 are required to supply 1kg of sulphur.

When questioned about local trials which had shown grass yield responses only one year in three, Mr Dawson said the difficulty lay in not knowing how much atmospheric sulphur was deposited each year. Regular applications of sulphur should boost soil reserves which would be there if and when they were needed.

Sulphur deficiency was inevitable in the long run, he believed. "So why not start building up reserves now?" A "stop-go" policy based on annual leaf tissue tests was not appropriate, he warned.

&#8226 West Country crops at risk.

&#8226 Plan rate using balance sheet of inputs and outputs.

&#8226 Apply as fertiliser not "fire-brigade" foliar feed.

&#8226 Bags quote sulphate content – crops need 2.5kg of this to supply 1kg of S.