Answer: What actually causes this condition is not fully understood, but it seems to be related to high iron and acid levels in tubers caused by high nitrogen and low potassium levels in the soil, says Joe Sowokinos, a University of Minnesota potato specialistat the USDA Potato Research Laboratory in East Grand Falls. And the higher the pH, the more severe the darkening. The condition is most obvious on boiled potatoes as they cool and is less evident on baked or fried potatoes. Despite the discoloration, the potatoes are edible. To lessen the amount of darkening in this year's potatoes, boil the potatoes in a solution of three quarts water and 1/2 teaspoon citric acid powder. The citric acid lowers the pH and prevents the discoloration, explains Sowokinos. For next year's crop, lower the soil pH to 6.2 before planting and avoid adding high-nitrogen fertilizers such as manures. Instead, add a high-potassium fertilizer - muriate of potash, for example - at levels recommended in a soil test. Also, try growing varieties such as Russett Burbank and Kennebec that are less prone to this condition.
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