With grapes on clay soils, it is advantageous to apply potassium in a band to reduce fixation. Potash can injure roots and trunks if applied too closely to the trunk. In dry growing seasons potassium is not readily available to the plant. Where a potassium deficiency has been confirmed, foliar applications of potassium may be beneficial. Foliar potassium applied at veraison may improve fruit yield and quality.
Magnesium deficiency can lead to premature fruit drop at harvest.
Magnesium deficient vines have older leaves that are pale in colour, as magnesium is a part of the chlorophyll molecule. Petiole analysis is the best way to evaluate Mg levels. Foliar sprays of magnesium have been effective in correcting this deficiency for the current year only. For more permanent correction, soil applications of Mg may be required. Fruit or foliage injury is possible from a mixture of pesti-cides with magnesium sulfate.