SulFer 85 and Potato Scab
SULPHUR AND SCAB
The acidification effect of elemental sulphur products is created when S is oxidised to SO4. In order to do this, the prill, and then the particles within the prill, must be broken down. The smaller the particle size within the prill the more rapidly this will occur. Since tuber initiation usually starts around 30 days after emergence, and since this is the time when scab attack can be most severe, timescale is of vital importance.
Some of the inconsistencies with elemental S products arise because prills based on bentonite, i.e Tiger and Brimstone, do not break down quickly enough. Products such as 'SulFer 85' have a smaller particle size and bound in a faster releasing lignin. Heavier soils require more S to reduce pH than light soils.
Dr Noble, the Technical Director of crop nutrition specialists Demeter Technology comments that often no mention is made of the effects of other nutrients on scab. "At high pH copper, manganese, zinc and phosporus all become less available to the plant. Research has shown that all of these elements may reduce levels of scab in tubers. Copper in particular is involved in cell wall lignification and low levels of this element reduce the plants ability to withstand scab and other fungal diseases which affect skin finish. Acidifying soil makes all of these elements more available to the plant, but this can only help if there is sufficient present in the soil - many soils are now showing low levels of trace elements.
Dry soils also render these elements less accessible to the plant but will also reduce the breakdown of elemental sulphur prills.
Calcium is also a vital element to ensure plants have strong cell walls and hence good skin finish. Although many soils are high in calcium this is not readily available to the soil. Calcium applied as lime or calcium sulphate (gypsum) is not easily available to the plant - both are mined and are only slightly soluble. Calcium sulphate applied to potatoes will not have an effect on soil pH, nor will it provide instantly available sulphate which some suggest. However it will become available during the season, as conditions permit.
Scab levels in potatoes can be affected by plant nutrition but for best results it is essential to look at the whole nutrient status rather than at single elements in isolation.