Cations Retained by Soil Organic Matter

The soft Lewis acids (Section 3.5)—Cd, Cu, Zn, Hg, Pb2+, and to a lesser extent Fe2+ and Mn2+—tend to be more affected by soil organic matter and sulfide. Some generalities about these cations are the following  

Soil retention is less pH sensitive than for hard Lewis acids. 

Fraction retained, and strength of retention, decreases as concentration increases. 

The ion activity products of the cation s hydroxyoxide are less than their hy-droxyoxide solubility products. 

Although soft Lewis bases are associated with reducing (anaerobic, low oxygen) conditions, normal (aerobic) soils contain sufficient amounts of organic matter to retain the low amounts of these trace metal ions in soils. 

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