Cation Exchange Capacity: Its Context As an Integral Component of Soil Characterisation.

Abstract:

The chemical fertility of a soil is related to the concentration and availability of key cations and anions in the soil system. The soil system is a complex ensemble of solid, aqueous and gaseous fluxes that are in dynamic equilibrium. The total amount of cations that can be retained electrostatically to soil component surfaces is termed the cation exchange capacity (CEC). The higher the exchange capacity, the more effective the soil is at retaining plant nutrients and other elements which is differentially impacted by pH and texture. A measurement of CEC is one of the few techniques available to understand the solid interface of soil and its likely relationship to solutions. A compulsive exchange method (using either NH₄OAC or barium chloride) for determining CEC is precise, repeatable and a direct measure. A key problem in any extraction method though, is that the process causes artefacts in the resultant analysis.

The relationship between soil pH, soil texture and cation exchange capacity (CEC) across a range of soils (32 different soils) was studied. Extraction by NH₄OAC and BaCl₂ were performed both by batch and column techniques using ambient and buffered (non-ambient) condition. Across the soils tested batch performed more consistently than column techniques. The CEC measured was greater and more consistent using NH₄OAC, but for calcareous soils BaCl₂was more suitable. The CEC was greater in buffered solutions when compared with ambient. The CEC increased commensurate with an increase in soil pH.

Having established an optimised procedure this was then applied to two large scale experiments; one considering the effectiveness of soil amendments in enhancing the CEC of marginally arable soils and the other studying temporal changes in metal binding of contaminated soil.

Soil amendments including biochar, bentonite and biosolids are acknowledged to enhance inherent CEC but their performance changes with time, as the amendment becomes an integral part of the study. Biochar only negligibly raised the CEC, while a comparable mass of clay or biosolids raised it significantly. The performance of biosolids declined with time.

The NH₄OAC extraction using ambient and buffered systems was very informative with respect to understanding the solid phase of soils and its relative binding efficiencies. Indeed the step was complementary to more traditional sequential extraction techniques revealing that the Kd of soil could be related both to the form and strength of binding and the relative saturation of the readily exchangeable fraction.

CEC, a factor often overlooked in current soil science investigations underpins fundamental processes both in agronomy, soil fertility and soil pollution science.

Keywords: Cation exchange capacity, Ammonium acetate, Barium chloride, Texture, Ambient, Buffered, , pH, soil metal pollution, soil amendments.