9 January 1998

More work needed to evaluate elemental S

KEEP to readily available forms of sulphur when treating crops this spring and treat elemental sulphur products with caution until more is known about them.

"We do not really know the conditions in which elemental sulphur products are effective," says Paul Withers of ADAS Bridgets. "In our work they have always been second best to soluble sulphates."

Elemental sulphur degrades to the sulphate form – which plants take up – slowly and in line with soil temperature and moisture, he says. That means it may not be very available to plants in a dry spring. Autumn application could cause leaching, especially if the winter is warm and wet, he adds.

Using ammonium sulphate, available in sulphur-containing nitrogen fertiliser is a better bet, he reckons. "Apply 10-20kg/ha of S in the first top dressing, but do not go too early or you may lose it through leaching. Mid-March is ideal, just before stem extension."

Light sandy soils in areas of low atmospheric deposition are most likely to suffer deficiency. Field experiments show losses of 21% in grain yield are possible.

Soil analysis will indicate likely shortage; readings of less than 10mg/kg of soil suggest risk, above 20mg/kg is safe. Plant analysis will help confirm status, although it is not totally reliable, because uptake may vary later in the season, he says.

Treating half a field with S fertiliser to assess response is possible. But that should be done each year, as S shortages may not always occur due to variation in winter rainfall and rate of mineralisation.

lBread-making wheat is especially vulnerable to sulphur deficiency, because S is needed to provide the high quality protein associated with good loaf volume. Testing the strength of the resulting gel protein is a good indication of protein quality, says Sue Salmon of the Campden and Chorleywood Food Research Association. Such an approach could be developed as an intake test for millers. &#42

Degradable prills most economical?

Stoller claims its S90 degradable elemental sulphur prills are the most economical available, costing as little as £7.50/ha (£3/acre). The dust-free particles contain 90% elemental sulphur and Bentonite clays, which swell with soil moisture. That allows the prill to quickly disintegrate into fine particles that are gradually converted to sulphate sulphur, the form plants need for nutritional purposes.

Although ammonium sulphate provides such sulphate more readily, recent trials at Rothamsted show degradable elemental sulphur with Bentonite clays provide a slow release of SO4, which can be taken up by plants before leaching occurs, says Stoller.

S90 does not contain expensive "carrier" nutrients and can be applied by itself or blended with other nutrients in a planned nutritional programme, adds Stoller.