Role of Magnesium in Plant Nutrition

The concentration of magnesium in soils generally lies in the range between 0.5 g/kg for sandy soils and 5 g/kg for clay soils. The levels of magnesium are higher in clay soils due to the presence of weatherable ferromagnesian minerals, such as biotite, serpentine, and olivine and also the carbonate mineral dolomite. It is also present in secondary clay minerals, such as chlorite and vermiculite. 

Magnesium ions are more readily leached from the upper layers of soil than Ca2+ owing to the more extensive hydration sphere surrounding Mg2+, which causes it to be less strongly bound to soil colloids. Also, 

The Chemistry and Technology of Magnesia, by Mark A. Shand Copyright 2006 John Wiley Sons, Inc. 

Mg2+ is not specifically bound to clay minerals as is the case for K+. Magnesium is, therefore, very prone to leaching and leaching rates are on the order of 2-30 kg Mg/ha/yr. 

---