Assessment of Magnesium in Soils

Forms of Magnesium in Soils

Approximately 1.3, 4.7, and 4.3% of the earth's continental upper layer, lower layer, and the ocean crust is made up of magnesium, respectively (202). However, in surface soils, magnesium concentrations usually range from 0.03 to 0.84%, with sandy soils typically having the lowest magnesium concentrations (≈0.05%), and clay soils containing the highest magnesium concentrations (≈0.50%) (10,29). Like other metallic elements, the soil magnesium pool consists of three fractions: nonexchangeable, exchangeable, and water-soluble fractions. The nonexchangeable fraction consists of the magnesium present in the primary minerals and many of the secondary clay minerals (Table 6.2) (29). In many cases these compounds may be hydrated with one to several water molecules. The exchangeable fraction may make up approximately 5% of the total magnesium in the soil, accounting for 4 to 20% of the cation-exchange capacity of the soil (29). Magnesium concentrations in the soil solution typically range from 0.7 to 7.0mM, but may be as high as 100mM, with the soil solutions of acid soils generally having a lower magnesium concentration (about 2.0 mM) than soil solutions derived from neutral soils (about 5.0 mM) (29,203-207).