Which sulphur scotches spud scab?
ACIDIFICATION of soil with sulphur is recognised by many as one way to reduce common scab in potatoes.
But which product is best is open to debate. Claims by Boothman Agriculture that the Brimstone 80 elemental sulphur plus calcium sulphate combination it sells drops the pH faster than pure sulphur have been challenged by Omex.
"As far as we can see all the sulphur has to be is sulphur to get acidification," says Omex agronomist Andy Eccles. "We also feel acidifying the whole ridge is important, not just the top or bottom."
The Brimstone pellets or "pastilles" are applied to the top of the ridge post-planting, usually by contractor. In contrast, Omex mixes a finely ground elemental sulphur with suspension fertiliser and incorporates it into the top 10cm of soil before planting and ridging.
"As the soil warms the sulphur oxidises, acidifying the soil for about three months. The initial drop takes two to three weeks," says Mr Eccles.
The small particle size of the sulphur is the key to speeding up the acidification process, he says. "Our sulphur has 700,000 times the surface area of the pastilles. The average particle size is just six microns."
Another drawback with the pastilles is that acidification occurs in two phases, he says.
"In the first month after application the pastille swells and some sulphur is oxidised. But the biggest effect is when the soil is worked and the pastilles are broken up."
That second phase is too late for the potatoes to benefit and can cause problems for acid sensitive following crops such as barley, he suggests.
But Boothman stands by the Brimstone 80 brand. Elemental sulphur may work season long but it does not get away quick enough, argues Robert Boothman, especially in late sown crops that emerge and reach tuber initiation rapidly.
His brother David says that adding calcium sulphate to a pH7 solution drops the pH, hence the initial pH drop seen in the field.
But independent crop nutrition consultant Ian Richards, formerly of Levington, questions the importance of the sulphate component.
"I am not sure why calcium sulphate should have an acidifying effect on soil pH." But there is a lack of independent work with sulphur, he adds.
Both companies are at pains to point out that the sulphur must be applied as a crop nutrient and cannot be recommended as a pesticide.
"Potatoes grow best at pH5.5-6, so even if there is no risk of scab it is worth getting the pH down to increase phosphate and trace element availability," says Mr Eccles.
Omex reckons 50kg/ha of its suspension sulphur is sufficient, costing £25/ha. The cost/kg of the sulphur in Brimstone is comparable, but 75-100kg/ha is recommended and an extra application cost is incurred, he notes.
Other forms of sulphur such as Thiovit do a similar job to Omexs suspension, acknowledges Mr Eccles.
"It is the pastilles we really feel strongly about due to the lack of surface area and lack of rapid breakdown." *
• Suspension v pastilles.
• Role of CaSO4 questioned.
• Max surface area key?
• Lack of independent work.