EARLY results from new soils research point to a rethink on the best way to apply sulphur to crops.
Sulphur deficiency in crops is an unwelcome but necessary by-product of cleaner heavy industries. Total emissions of sulphur dioxide in the UK have decreased by more than 50% since 1970, and are set to fall further still through the next decade. By 2003, the area of cereals in the UK susceptible to sulphur deficiency is expected to increase by 22%.
None of this will come as much surprise. Its all been well publicised and, despite having to shoulder the cost, growers uptake of sulphur fertilisers has been an exercise in successful communication. Almost too successful, was the suggestion from some quarters when the HGCA quality survey of about 1,200 grain samples taken from last years wheat and barley harvest showed virtually nil deficiency.
But this doesnt mean weve cracked the problem, suggests Steve McGrath, soils expert at IACR-Rothamsted. It just goes to show how little we understand about the sulphur cycle, especially when analysis of this years grain shows deficiency is back on an upward track at 16%.
One explanation for the very low level of sulphur deficiency in 1995/6 was reduced leaching from the cold dry spring of 1996. Theres logic in the theory, reckons Professor McGrath, but very little science.
Not surprisingly for such a modern phenomenon, there are holes in our knowledge and theyre at quite a fundamental level. "We have a broad understanding of the processes involved in the sulphur cycle," he explains. "Theyre similar to nitrogen, except that losses to air are negligible, which makes it a little easier to study.
"We know that sulphate is very mobile, but weve never been able to put numbers against the losses."
Now new work at IACR-Rothamsted, funded by Stefes Plant Protection and the HGCA, is beginning to do just that. Nicky Riley of the soil science department is 18 months into a three-year project to assess leaching losses from different forms of sulphur fertilisers.
"For the first time weve been able to see how much is leached, how much is taken up by the crop – in this case ryegrass – and how much stays in the soil," she says. "Until now research into sulphur fertiliser characteristics has been confined to small columns of disturbed soil in the laboratory."
This study makes use of lysimeters which mimic field conditions far more closely. So-called soil monoliths – 80cm in diameter and 55cm deep – are carved intact from established pasture, encased and reinstalled adjacent to a covered pit. Here the drainage water is contained ready for collection and analysis.
On sandy loam soil at Woburn in Bedfordshire, three treatments plus a control were compared in two replicates. An elemental sulphur plus bentonite clay mixture (T90), dry flowable elemental sulphur (DFES) and ammonium sulphate (AS) were each applied in September 1996 at a rate equivalent to 50kgS/ha higher than current recommendations.
After a year, the most startling finding is the scale of leaching losses from ammonium sulphate – 70% of this fertiliser ended up in the drainage water, more than double the typical losses of nitrogen from ammonium nitrate. The elemental sulphur fertilisers, on the other hand, were less susceptible to leaching losses, seemingly because of their slow release characteristics.
Some sulphate also leached from the untreated controls, which indicates theres still some contribution from atmospheric deposition. But detailed sulphur balances, which take account of measured deposition and plant uptake, show that the control is losing more sulphur than its gaining from the air.
While the elemental sulphur/bentonite clay mixture, sold as Tiger 90, and the dry flowable elemental sulphur each retained about 30 to 40kgS/ha in the soil, only 10kgS/ha remained from the ammonium sulphate (see fig 1).
The patterns of leaching losses through the season are similar for each treatment, and as expected, says Prof McGrath, closely reflect the amount of rainfall and therefore drainage volume (see fig 2). "Timing wise, losses peak just before crops really take off in the spring.
"For ammonium sulphate users, theres little option but to apply it in March and accept that a large proportion will be lost if it rains, though not as much as from an autumn application.
"Splitting the sulphur application might in theory reduce late winter losses, but there would be problems trying this in practice. Growers are too constrained by the N:S ratios in compound fertilisers."
The next research stage is to take soil cores from within the existing lysimeters to establish the whereabouts of sulphur in the vertical soil profile and the form it takes. "Although we have demonstrated that oxidation of elemental forms of sulphur is slow enough to reduce leaching losses, we also need to be sure that its fast enough to match crop requirements."
The oxidation rate is determined by many factors, especially fertiliser particle size, soil temperature and soil moisture. Another project with the University of Newcastle is quantifying these influences, with the ultimate aim of building a computer model to provide decision support on the use of sulphur fertilisers.
New research shows it is not
what sulphur you apply, but how you apply it that is important.
Tia Rund reports.