Sulphur: the scab beater
Sulphur: the scab beater
Can sulphur reduce common scab by lowering the pH around the tuber? Yes, says a clutch of companies – but they disagree on the best way of delivering this element. On 17 Feb, Crops carried an article looking at the slow release sulphur pastilles, Brimstone 80. Heres the response from other manufacturers
The case for sulphate fertiliser
Id like to make three points:
1 The effect of sulphur on scab depends on the starting soil pH. Sulphur can reduce scab, but only if it takes the pH in the soil surrounding the growing tubers below the optimum range for scab attack, which is 5.4-7.4. If the starting soil pH is high then applying sulphur might simply bring the pH to within the optimum range and so, paradoxically, could actually increase the incidence of scab. But if the starting pH is moderate, then using sulphur may well reduce the pH to outside the optimum and so alleviate the scab problem. Either way, sulphur will only alter soil pH when in its sulphate form. If products containing elemental sulphur are used they must first be oxidised by soil microbes to the sulphate form.
2Sulphate leaches in autumn and winter. Sulphate applied in spring will not significantly leach out of the soil during the growing season. Like nitrate, it leaches over the autumn and winter period but, of course, only the sulphate that has not been used by the crop nor incorporated into the soil organic matter will be lost.
3Because the beneficial effects of sulphur come from the sulphate form, sulphate-containing fertilisers can be just as effective. Indeed, there is plenty of evidence to suggest that, since elemental sulphur products require oxidation to sulphate before they become effective, the benefits they can bring are both fewer and more slowly gained.
Dr George Fisher
agronomy and research manager, Kemira Agriculture
Ground sulphur in suspension
Although it is well documented that sulphur can reduce potato scab, the method of application and the type of sulphur used have to be right. I dont believe that the best form of application is in pastille form.
Theres an alternative approach: mixing finely ground elemental sulphur throughout the ridge, and I would propose that applying sulphur in this way offers a more effective and less costly method of temporary soil acidulation.
Elemental sulphur is an excellent soil acidifier, and has the added advantage of allowing the soil pH to buffer back close to the original levels, minimising the risk of acidity in following crops.
Finely ground sulphur – similar in particle size to talcum powder – is an ideal source since it is rapidly oxidised by soil microbes, and when mixed evenly through the ridge it causes acidification of the soil in the vicinity of the developing tubers. There are also significant savings to be made when mixing the sulphur in suspension fertiliser, as it requires just one pass, and because it is in liquid form, is applied much more evenly throughout the seedbed.
Work has shown that sulphur pastilles require redistributing in the soil some months after application to expose the sulphur for microbial oxidation, and that the greatest acidulation can therefore occur in the crop following the potatoes. The use of sulphur pastilles often requires an additional application to the crop, and the pastilles are usually placed in the top of the potato ridge, where the soil can dry quickly, further delaying the oxidation of the sulphur.
Andy Eccles
agronomist, Omex Agriculture, Kings Lynn, Norfolk
Quick release sulphur pastilles
The acidification effect of elemental sulphur products is created when S is oxidised to sulphate. 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, such as Tiger and Brimstone, do not break down quickly enough. Products such as New-Triton "SulFer 95" have smaller particle size and are bound in a faster releasing lignin. Heavier soils require more sulphur to reduce pH than light soils.
Other nutrients also have an effect on scab. At high pH copper, manganese, zinc and phosphorus 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.
Dr Anne Noble
New-Trition Ltd, Shropshire