potassium, one of the macronutrients required by the vine for healthy growth, along with nitrogen and phosphorus. It constitutes about 1–3% of the vine’s dry weight, and is an important component of grape juice. Potassium deficiencies show up first in older leaves as chlorosis, which may become a marginal burn when severe. Potassium-deficient leaves are often shiny. Severe deficiencies can inhibit growth, yield, and sugar content. They can be confirmed easily by analysing for potassium levels in leaves or petioles, and 1 to 1.5% potassium is the optimal range. Potassium deficiency is more evident during drought or in cold soils in spring, both of which reduce the roots’ uptake of potassium. However, soils containing a significant proportion of mica-type clay minerals can supply too much potassium, as occurs in parts of Sunraysia and the Riverland, Australia.
