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Magnesium is needed to give leaves their green colour, so when there's a deficiency, yellow breaks through between the veins and around the leaf edges instead. Other colours, such as purple, brown or red, might also appear. Older leaves suffer first, and will die if they're not given any treatment. The nutrient can be leached out of light, sandy, acidic soil by high rainfall. Or if there is too much potassium in the soil, the plants absorb that instead of the magnesium.
Solution
Organic
For a long-term solution, apply a yearly mulch of home-made compost. This will conserve moisture, prevent the leaching of nutrients during heavy rainfall, and provide the soil with sufficient quantities of magnesium to keep your plants healthy.
Chemical
Use a magnesium leaf spray, such as Epsom salts, on potatoes for a quick, temporary solution in summer. Apply Epsom salts or calcium-magnesium carbonate to the soil in autumn or winter to remedy the deficiency for next year.
