Mineralogy and chemistry of some variable charge subsoils

Abstract

Colloidal mineralogy is one of the main characteristics of the steady state reached in developed soils. Surface charge and other chemical and physical properties of the soil depend on colloidal mineralogy. It is, therefore, very important to further investigate the clay mineralogy of acidic variable charge subsoils in order to understand better their unusual chemical behavior. The objective of this investigation was to characterize the inorganic colloid mineralogy, chemical (subsoil solution pH and electrical conductivity), and charge properties (PZNC and PZSE) in some variable charge subsoils. Subsoil materials were collected from the southeastern United States and other tropical and subtropical areas around the world. The clay fraction mineralogy in the majority of the subsoils was dominated by the quartet kaolinite, gibbsite, goethite, and hematite. They manifested, however, a significant diversity in their charge and other chemical characteristics because the proportions and contents of mineralogical constituents, particle size distributions, and specific surface areas were very different. The pH_KCl values ranged from 3.69 to 5.91. Under such conditions, pure kaolinite and aluminum/iron (Al/Fe) oxides have opposite net surface charges, and acidic subsoils are mixed charge colloidal systems. They have extremely low EC values, varying from 9.9 to 132 μS cm^‐1, with corresponding ionic strengths between 0.14 and 1.86 mmol L^‐1. They develop towards a “no or little charge state”; and the native pH is near the PZNC or PZSE. The overall charge characteristics and adsorption properties in these heterogenous colloidal systems are clearly a direct function of the relative contents, interactions, and surface reactivity of mineralogical soil constituents in the subsoils.