Variable charge soils : mineralogy and chemistry

Soils rich in particles with amphoteric surface properties in the oxisols, ultisols, alfisols, spodosols, and andisols orders are considered to be variable charge soils. They have developed under intensive weathering in subtropical and tropical regions or from volcanic ash parent material. The term “variable charge” is used to describe organic and inorganic soil constituents with reactive surface groups whose charge varies with pH, and ionic concentration and composition of the soil solution. The variable charge is developed on the surface groups as a result of adsorption or desorption of solid-like ions that are constituents of the solid phase, i.e., H+, and the adsorption or desorption of solid-unlike ions that are not constituents of the solid phase. Variable charge soils are heterogeneous charge systems. Variations in relative proportions and chemical composition of the major mineral soil constituents of highly weathered soils and volcanic ash soils influence the surface reactivity, which is the key factor controlling nutrient availability. The coexistence and interactions of soil particles and colloids with net opposite surface charges confer interesting properties with complex attributes with respect to physical and chemical behavior, compared to homogeneously charged soil systems of temperate regions. In this entry, the mineralogy, chemistry and management of variable charge soils are reviewed and discussed, focusing on the chemistry of these fascinating soil systems. Because surface charge properties are of central importance in the management of variable charge soils, different management practices that may manipulate charge characteristics and retention capacities are discussed