Aluminum Bonding in Soils: a Unifying Principle in Soil Science

The aluminum ion bonds through oxygen to form a variety of functional groups underlying diverse properties of soils. One aluminum-bond functional group provides the cation exchange site of soil layer silicate clays. Pauling's m-rule of acid strength for oxy-acids, (HO)ₙOₘC, applied to the 2:1 layer silicate clay formula of beidellite, considering all structural cations, yields m = 1.6, characteristic of a medium to strong acid strength (on an m-scale: 1 = weak; 2 = strong). The solubility product of Al(OH)₃, k₁ of Al(OH₂)₆³⁺, and the Pauling k₁/k₂ ratio of 10⁵ are applied to calculate concentrations of aluminohydronium monomeric cation species of valence of 3, 2, and 1. These concentrations relate to cation exchange and interlayer aluminum polymerization reactions in soils during chemical weathering. The latter and mineral colloid composition changes are summarized in the generalized bonding equation: Al - O ⇌ Al - OH, which provides the ultimate buffer, setting the lower and upper limits of the pH range of soils except in the presence of strong acidforming compounds such as S and FeS₂. Aluminum bonding is central to soil acidity, through not only the acidic aluminohexahydronium monomeric cations, but also through the weakly acid Al-OH₂ … OH pair at edges of polymerized (“precipitated”) hydroxy aluminum structures. The aluminum toxicity of soil acidity may involve aluminum bonding and solubility product relations at the soil-root interface and in solutions in soil and sap. Retention by soils of anions such as phosphate and sulfate is closely related to aluminum bonding of these anions (OH replacement). Some aspects of soil aggregate structure involve hydroxy aluminum bonding; loss of aggregates follows intensive cheluviation in the A2 horizon. Aluminum bonding in a real sense supplies a unifying principle for understanding many properties of the soil system, much as hydrogen bonding has served to unify understanding of many properties of water and of aqueous organic systems including living matter.