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.