Calcium, Magnesium, and Potassium In Food.

Calcium, magnesium, and potassium are essential macro mineral nutrients for animals and humans.  Animals and humans require much larger amounts of calcium than do plants, owing to its role in skeletal growth and maintenance.  Because of this difference, calcium probably should be considered a micro mineral nutrient for plants, although it is most often classified as a secondary macro mineral.  Calcium, magnesium, and potassium have major metabolic functions throughout animals and plants, and thus are consistently found in food.  The amounts of calcium, magnesium and potassium in foods of plant origin are partly dependent upon the availability of these minerals from soil.  Increasing the amount of calcium to the root increases the amount of calcium in plants.  However, plant species variation in the uptake of calcium is the most significant factor in the amount of calcium provided by foods of plant origin.  Some legumes and vegetables are fair sources of calcium, and cereal grains, especially without the bran component, and fruits are poor sources of calcium.  Magnesium preferentially accumulates in grain when soil availability is low, but when magnesium supplies approach adequacy and above, vegetative structures become storage sinks for magnesium.  As a result, magnesium in foods from plants, especially vegetative foods, can vary depending upon the environment in which they were grown.  Increasing potassium to roots increases the potassium content of all organs of plants except seeds and grains, which maintain relatively constant potassium content; soybeans may be considered an exception.  Thus, increasing soil potassium may increase the potassium content of fruits and vegetables but not cereal grains.  Plant nutrition influences the amount of calcium, magnesium, and potassium, and thus the amount of these minerals provided for human requirements, in foods of plant origin.