ABSTRACT
The nature and extent of ion adsorption, exchange, and fixation largely determine the size and composition of the ion pool of the soil solution and affect plant growth and environmental pollution. The cation exchange capacity (CEC) of soils has traditionally been determined at pH of 7.0 or 8.2, depending on method. This allows valid comparisons of CEC between soils. The CEC determined at the soil's current, or natural pH, is called the effective CEC. Exchangeable ions in dry soils are adsorbed directly onto charged surfaces. When dry soil is wetted, polar water molecules are attracted by both the ions and charged surfaces, forming hydrated ions and charged particle surfaces with water films. Orisons with net positive charged subsoil layers are in the anionic subgroups. Organic matter contributes to the CEC but does not contribute significantly to the anion exchange capacity of soils. The positive charge in soils is related to the nature of the clay minerals and soil pH.
