Biological function:

• The most familiar function of Mg, as the central ion in chlorophyll molecule, accounts for <25% of total plant Mg
• Mg in the cytoplasm is related to enzyme activation: for example, the substrate for ATPases is MgATP.
• Additional Mg is present in the cell wall

Magnesium deficiencies in soils are relatively rare as far as plants are concerned, but animals fed on Mg-deficient forage may develop hypomagesameia (grass tetany). "Excess" Mg is known almost only in soils derived from serpentine or olivene, both Mg-rich rocks, in which the exchangeable Ca/Mg ratio is ~1/5 instead of the more common 4/1 to 1/1; such soils often also contain toxic, high levels of bioavailable metals, among them Ni, which together with high Mg limit the distribution of "normal" vegetation, leading to "serpentine barrens".

Cycle, Forms, Tests:

Very similar to Ca , except Mg weathers out of a differ set of minerals than Ca, but otherwise is present in soluble, exchangeable, and mineral forms, but not organic. As for Ca and K, exchangeable Mg is measured by displacement into 1 M ammonium acetate.

Mg Fertilizers:

• Dolomitic agricultural limestone, CaMg(CO₃)₂, contains up to 13% Mg. Even calcitic limestone will contain some Mg as Mg substitution for Ca (magnesian calcite).
• Potassium magnesium sulfate, K₂SO₄^.2MgSO₄
• Magnesium sulfate, Mg(SO₄)^.7H₂O, Epsom salt
• Magnesium nitrate, Mg(NO₃)₂, often applied as foliar spray.

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This page was last modified by Phillip Barak, Univ. of Wisconsin, on 5 Jan 1999. All rights reserved.