© 2007 Donald G. McGahan (aka soilman) All Rights Reserved

Magnesium is a macronutrient essential for all plants. The ability of a soil to supply magnesium to plants is intimately tied to soil acidity (and for agricultural soils, to liming).

  1. The deficiency of Mg is much more common than that of Ca
  2. The most common symptom of Mg deficiency is interveinal chlorosis on the older leaves. Magnesium is mobile in the plant.
  3. The main source of plant-available Mg in most soils is the pool of exchangeable Mg on the clay-humus complex.
    • Variable amounts of Mg are made available by the breakdown of plant residues and soil organic matter.
  4. Removal (losses) is by plants and leaching
  5. Mg availability to the soil solution is from the exchangeable pool
  6. Replenishment of the exchangeable pool is by Mg weathered from minerals (such as dolomite, hornblende and serpentine).
    • In some soils, replenishment also takes place from a pool of slowly available Mg in the interlayers of certain 2:1 clays.