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Topics : Soil Fertility for Forage Crops : Pre-Establishment



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Calcium & Magnesium

Calcium and magnesium are essential secondary nutrients required by crops. Legumes in particular have a high demand for calcium and magnesium. A soil test is the best guide for determining the calcium and magnesium needs of a soil. Almost all calcium and magnesium are supplied to crops in limestone. Generally, in Pennsylvania soils, if the soil pH is in the optimum range there will be adequate calcium for the crop. Depending on the soil origin and the source of limestone that has been used in the past, however, magnesium may not be adequate. A soil test level less than 5 percent saturation of the soil CEC or less than about 200 pounds of MgO per acre is usually considered deficient in magnesium. When this occurs, a limestone should be used that contains magnesium to build the magnesium level in the soil into the optimum range. The amount of magnesium required will depend on the CEC of the soil and the magnesium level in the soil. Soil test recommendations for magnesium are give in several ways. The recommendation is sometimes given as the amount of magnesium to apply, or the recommendation may be given as a minimum magnesium content in the recommended limestone. Recommendations on Penn State soil tests are given in both of these ways. Sometimes the recommendation is simply "Use a dolomitic limestone". This is less desirable because there is no set definition or minimum magnesium content for a limestone to be labeled as dolomitic. If the soil is optimum or high in magnesium, either a calcitic or dolomitic limestone can be used. The decision should be based the availability and the cost of limestone.

High soil potassium can have a negative effect on the uptake of calcium and magnesium by crops. This antagonism does not usually affect the yield of the crop but it can have a negative effect on crop quality. In particular, low magnesium in grasses, when they are the primary feed for cattle, can result in hypomagnesimia or grass tetany. This is mainly a problem in pasture systems where cattle get most of their forage from the pasture. Pastures that have not been limed with a dolomitic limestone and have had potassium fertilizer applied are the most susceptible to this problem, particularly early in the spring.