Soil factors affecting magnesium availability in plant-animal systems: A review

Soils provide the support, water and most of the nutrient elements, including Mg, needed for plant growth. Magnesium uptake by plants depends largely on the amount, concentration and activity of Mg in the soil solution and the capacity of the soil to replenish Mg in the soil solution. The availability of Mg depends on the activity or proportion of Mg relative to soluble and exchangeable amounts of K, Ca, Na, Al and Mn. In humid regions, Mg losses from leaching are often greatest from agroecosystems receiving heavy N fertilization. Cool-season grasses produce nearly maximum growth at herbage concentrations of 1 to 1.5 g Mg/kg, 25 g K/kg and 30 g N/kg of dry matter. At these concentrations of N and K, herbage should contain about 2.5 g Mg/kg to avoid inducing hypomagnesemic grass tetany in ruminants. To increase herbage Mg concentration to this level often requires, except on sandy soils, an uneconomically large addition of Mg fertilizer. Adjusting soil conditions to produce grasses with a low-tetany potential may not always be possible physically. The risk of tetany can be reduced by a judicious program of well-timed N, K and Mg fertilizer applications. However, direct Mg supplementation of grazing ruminants is considered more cost-effective than is Mg fertilization to prevent grass tetany