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Whatever happened to Sulphur

In the early years of this decade sulphur was heralded as the fourth major nutrient, but today little mention is made of its benefits. So what's happened?

"There's plenty of independent evidence to show that sulphur deficiency results in poor crop
growth and adversely affects both yield and quality," says GrowHow's Allison Grundy.
 
This is because sulphur plays a vital role in converting nitrogen into protein. It improves the efficiency of N uptake and therefore increases yield and improves quality. It is also instrumental in the synthesis of oils in crops such as oilseed rape. In milling wheat it is
needed to produce the glutenins which give wheat flour its elasticity and in malting
barley it helps to keep grain N content down.
    
"Because it improves N uptake sulphur brings an added bonus," she adds. "It reduces the risk of leaching and losses to the environment and that's something which shouldn't be forgotten as farmers try to produce more and impact less."

POSITIVE EFFECT

Trials with sulphur fertilisers invariably result in a yield increase. Typically these are around 10% in cereals and 20% in oilseed rape. In malting barley yield increases of almost 50% have been achieved. In fact in sulphur deficient situations, yield losses of up to 80% in OSR have been recorded on very light soils. Up until the late 1980s, however,sulphur deficiency was rarely seen. Industrial processes produced huge amounts of sulphur dioxide so that crops received all the sulphur they needed and more. But times have changed.

"With the mean deposition of sulphur across the UK now around 15 kg SO3 /ha (6kg S/ha)
and even high deposition areas getting no more than 25kg SO3 (10 kg S/ha), the rule of thumb for 2010 should be, use sulphur routinely unless evidence suggests otherwise," advises Miss Grundy.

So why is no-one talking about sulphur? Perhaps it has dropped off the agenda because everyone knows about it. "There's a good deal of truth in that statement," she suggests. "But, just because awareness is high doesn't mean that the problem has been solved.
There are two key questions to ask. Are all the crops that would benefit from sulphur receiving it and is it being applied at the correct rate?"

TRIALS DATA

Evidence from the recently issued British Survey of Fertiliser Practice (BSFP) 2009 indicates that the use of sulphur is static at best and if anything declining in wheat and oilseed
rape with 39% and 60% respectively of these crops receiving a sulphur dressing. However, this leaves a lot of crops which get no sulphur. Even in Scotland, which has led the way in sulphur applications many crops still do not get any sulphur at all. The BSFP data on field rate also suggests that that the average rates being used are falling short of crop requirement. For example, high yielding oilseed rape crops need up to 80-100kg S03/ha (32-40kg S/ha). With no more than 10kg/ha coming from deposition, this means
that fertiliser and other sources of sulphur must supply the main crop requirement. High yielding cereals crops, including spring barley, all require around the same quantity of sulphur - around 50kg SO3/ha (20kg S/ha). The BSFP table below suggests under application on spring barley which will impact on yields and quality premiums.

The form in which sulphur is applied is also important. Before elemental sulphur can be taken up by the plants it has to be mineralised to a soluble sulphate (SO4) form in soil solution.
As this is a natural process, the rate at which it occurs is difficult to predict. It is also important to remember that once sulphur is in its sulphate form it is mobile in the soil like nitrogen and so subject to leaching risk. Of course organic manures also contain sulphur. But Miss Grundy warns: "The actual amount available to the crop is much less than the 50% quoted in current RB209. Somewhere between 5% and 10% availability is a much more realistic estimate.

To complicate matters further EU legislation insists that fertiliser manufacturers quote the
sulphur content of their fertilisers as SO3 even though what is in the bag is SO4 (see
Table 2).

Reproduced by kind permission of Farm Business Agronomist