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WWW.POTATOGROWER.COM 17 POTASSIUM Potatoes take up significant quantities of potassium, and this nutrient plays important roles in tuber yield, size and quality. High potassium is necessary to prevent blackspot bruising and shattering and to attain good storage quality. However, specific gravity may be reduced if potassium fertilization is too high because it increases tuber water absorption. In-season applications have a greater effect on specific gravity than pre-plant or planting applications, and potassium chloride (0-0-60) can have more of an effect than potassium sulfate (0-0-50) at equivalent potassium rates. Applying significant amounts of potassium during the tuber bulking phase can reduce yields. Potassium is a relatively immobile nutrient in medium- and fine-textured soils but it does leach in sandy soils, particularly when they are acidic and low in organic matter. CALCIUM Calcium deficiency is rare in many agricultural soils, because they have high native calcium levels or are periodically limed to maintain soil pH. Sandy soils, however, do not maintain high calcium reserves, and the practice of growing potatoes at low pH to reduce scab means they are rarely limed. Under these conditions, soil calcium can fall to levels that reduce tuber quality and tuber yield. Calcium plays an important role in maintaining tuber quality in storage and reducing internal tuber disorders such as brown spot and hollow heart. Low calcium in tubers is often due to inadequate transport of calcium to the tuber, caused by water or temperature stress. This may be a localized calcim deficiency with adequate calcium levels occurring in leaves and the soil testing high in calcium. MAGNESIUM Similar to calcium, inadequate magnesium can occur on acid sandy soils that are not periodically limed. High rates of potassium fertilizer, which are often required for potatoes, can also induce magnesium deficiencies, since the two compete for uptake. Magnesium sulfate or potassium-magnesium sulfate are the most common magnesium sources available. They can be broadcast and incorporated prior to planting or banded in the row at planting. Another alternative is to apply low rates of lime during a non-potato year in the rotation. An application of 1,000 pounds of dolomite per acre will meet both the magnesium and calcium recommendations for low-testing soils. SULFUR In many soils, sulfur requirements are met from soil organic matter breakdown. Rainwater and irrigation water contain some sulfate and can also provide a significant proportion of the sulfur needed for growth. Sulfate readily leaches through sandy soils, so yield reductions from sulfur deficiency are most common on sandy, low organic matter soils. Ammonium sulfate, potassium sulfate, magnesium sulfate and calcium sulfate are common sources used to supply sulfur. They can be broadcast and incorporated prior to planting or banded in the row at planting. When ammonium sulfate is used, be sure to account for the nitrogen it contains in meeting the crop's nitrogen requirement. Elemental sulfur is not an immediately plant-available form and must be oxidized by soil bacteria to sulfate before it can be used by plants. The oxidation to sulfate has an acidifying effect on the soil, but the effect is small at the rates required to meet recommendations. POTASSIUM SULFUR In many soils, sulfur requirements are met from soil organic matter breakdown. Rainwater and irrigation water contain some sulfate and can also provide a significant proportion of the sulfur needed for growth. Sulfate readily leaches through sandy soils, so yield reductions from sulfur deficiency are most common on sandy, low organic matter soils. Ammonium sulfate, potassium sulfate, magnesium sulfate and calcium sulfate are common sources used to supply sulfur. They can be broadcast and incorporated prior to planting or banded in the row at planting. When ammonium sulfate is used, be sure to account for the nitrogen it contains in meeting the crop's nitrogen requirement. Elemental sulfur is not an immediately plant-available form and must be oxidized by soil bacteria to sulfate before it can be used by plants. The oxidation to sulfate has an acidifying effect on the soil, but the effect is small at the rates required to meet recommendations. Potatoes take up significant quantities of potassium, and this CALCIUM Calcium deficiency is rare in many agricultural soils, because they have high native calcium levels or are periodically limed to maintain soil pH. Sandy soils, however, do not maintain high calcium reserves, and the practice of Calcium plays an important role in maintaining tuber quality in storage and reducing internal tuber disorders such as brown spot and hollow heart. Low calcium in tubers is often due to inadequate transport of calcium to the tuber, caused by water or temperature stress. This may be a localized calcim deficiency with adequate calcium levels occurring in leaves and the soil testing high in calcium. In many soils, sulfur requirements are met from soil organic matter breakdown. Rainwater and irrigation water contain some