Bug watch: Magnesium deficiency
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When plants are low in magnesium, they move what they have from the old leaves to feed the new. Consequently older leaves begin to yellow from the sides to the centre. The yellowing pattern often creates an ‘arrow’ of green in the centre of the leaf, with green veins. Magnesium deficiency is easily fixed by feeding the plant a dose of Epsom salts, which is magnesium sulphate. Dissolve 20g of crystals in a 9l watering can and water over the plant.