Relation of soil and plant magnesium to nutrition of animals and man.
The importance of Mg for the production and quality of crops grown on soils, as well as the health and productivity of animals and humans that consume these crops, is the focus of this paper. The Mg requirements and resulting concentrations of most crops are sufficiently high that primary Mg deficiency in animals consuming these crops is rare, providing the crops receive adequate Mg from soil solution for normal growth. Hypomagnesemic grass tetany or conditioned Mg deficiency occurs in mature ruminant lactating females grazing cool season forages. Grass tetany occurrence is seasonal, and is often related to heavy lactation demand for Mg. Another factor is reduced bioavailability of Mg to the ruminant associated with high concentrations of crude protein and K, as well as associated changes in plant constituents such as lowered soluble carbohydrate content and increased levels of higher fatty acids and organic acids. The high water content in the immature grass tissue being grazed may also result in less efficient Mg absorption by animals. Grass tetany studies at Watkinsville, Ga., revealed that high N fertilization of tall fescue pastures from poultry manure, or inorganic fertilizer increased incidence of hypomagnesemic grass tetany, and hypomagnesemia. Regression analyses indicate highly significant correlations between Mg levels in blood serum and forage Mg, K and crude protein. The results between years and experimental pastures clearly suggested that individual years and experimental pasture relationships were different. The following was concluded with regard to the relationship of soil and plant Mg to human nutrition: first, surveys of agricultural raw products indicate that genetic factors (species and cultivars) appear to have more effect on plant Mg composition than do soil and environmental factors. Second, patterns of Mg intake by humans in the USA suggest that about 50% of their intake is from foods of plant origin. About half of this intake is from vegetables, where increases in Mg concentration may be achieved by changing cultural practices. While direct supplementation with Mg appears more efficacious in prevention of Mg deficiency in humans, research with susceptible ruminants indicates that complete protection can only be assured by adequate daily intake of bioavailable Mg in their consumed food. Similar situations would be expected to prevail in humans.