Base Saturation and Cation Exchange in Forest Soils: A History of Concepts and Problems.

Cation exchange was one of the first areas of research in soil chemistry, dating from the mid-1800s. In forest soils, there has been a continued focus on the availability of exchangeable nutrient cations such as Ca and potentially toxic cations such as Al. A number of our current concepts of cation exchange and base-forming cation saturation have not change in well over a century. In those times, neither exchangeable Al nor variable charge was generally recognized. The early concepts, and the methods developed to measure them, are not readily transferrable to acid, high-carbon forest soils. The primary source of charge in these soils is derived from organic carbon and the retention of cations, especially Al, cannot be easily modeled by simple exchange phenomena. Although the effective cation exchange capacity in an individual forest soil sample will vary with the addition of strong acid or base, a pH effect in a population of different acid forest soil samples is difficult to find. In the highly acidic pH range below 4.5, lower pH soils will generally have lower concentrations of adsorbed Al. This is due to a low availability of weatherable Al-containing minerals and a high concentration of weak, organic acidity. Base cation saturation calculations in this pH range do not provide a meaningful metric and, in fact, pH is better modeled if Al is considered a base cation. Extraction of exchangeable Al with a neutral salt solution results in an ill-defined but usually repeatable portion of organically-complexed Al. Future research should recognize these challenges and focus on redefining our concepts of cation retention in these important soils.