Beware sulphur muddle to keep yields on target
Beware sulphur muddle to keep yields on target
By Andrew Blake
DO you understand the difference between sulphur fertiliser rates expressed as kg/ha of elemental sulphur (S), and those given as kg/ha of sulphur trioxide (SO3)?
Confusing the two could have big yield implications.
Crops take up sulphur as sulphate (SO4), but to comply with EC regulations, fertiliser packaging shows the content in the trioxide form (SO3).
Oilseed rape and grass known to be at high risk of deficiency need 30kg/ha of sulphur and cereals 20kg/ha. But simply applying fertiliser as those rates, instead of making the 2.5 times conversion from the sulphur rate, could mean far too little nutrient is applied.
At last weeks meeting of the US-based Sulphur Institute, SACs Kerr Walker urged growers to use the 2.5 conversion factor to ensure correct applications (see box). Failing to do so can have significant consequences.
Delegates were told of an incident where a farmer thought he had applied 40kg/ha of sulphur. On investigation it was discovered he had actually only used half that rate, and that was of SO3 not sulphur. So in effect he had applied only 8kg/ha of S.
Dr Kerr suggested some of the early problems with Synergy oilseed rape could have been due to husbandry guidelines, which confused S and SO3.
Such mistakes also lead to inefficient use of applied nitrogen, which risks environmental pollution, as the unused proportion becomes more at risk from leaching, added Rothamsteds Steve McGrath.
Another common error is to apply extra N in an attempt to correct misdiagnosed sulphur deficiency.
"The worst thing you can do in those cases is to apply more N," said Dr Walker. In one Scottish oilseed rape trial on a sulphur-deficient site 150kg/ha of N delivered 2.36t/ha (19cwt/acre). Increasing the N dose to 250kg/ha slashed output to 1.05t/ha (8.5cwt/acre) by creating even more imbalance between the two nutrients. "It made the sulphur deficiency even worse."
A new free booklet Sulphur – ahead of the field from the US-based Sulphur Institute highlights the importance of terminology when evaluating fertilisers. It is available by email: agmarkt@sulphurinstitute.org *