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If the chlorosis (yellowing) happened on the older growth first then it is either magnesium deficiency (apply epsom salts) or manganese deficiency (spray with manganese sulphate). Both of these are common on acid soils.If the chlorosis developed on the new growth first then it is iron deficiency (add sequestered iron) This is dues to an alkaline soil.Chlorosis can also be caused by waterlogging, low temps or weedkiller contamination.