ARS | Publication request: PREDICTING SOIL CATION SELECTIVITY CONSTANTS

Submitted to: Agronomy Abstracts
Publication Type: Abstract
Publication Acceptance Date: July 24, 2003
Publication Date: November 5, 2003
Citation: Suarez, D.L. 2003. Predicting soil cation selectivity constants. Soil Science Society of America. Paper No. S02-suarez532703-O.

Technical Abstract: Various cation exchange equations have been developed utilizing different assumptions. Evaluation of these equations has generally been made using theoretical considerations but utilization has usually been based on practical considerations, such as ease of use and extent to which the selectivity coefficient is constant over a range of concentrations. Non-ideal behavior, when the exchange selectivity coefficients are not constant, has required utilization of activity coefficients. Expressing the solution phase composition in terms of free ion activities instead of total concentration is generally not sufficient to obtain constant selectivity values. Solid phase activity coefficients have been rarely utilized, as they are soil specific and obtained by fitting of the specific exchange data. An alternative approach is to consider that the soil is not a homogeneous phase but rather a mixture of exchanger phases all with different exchange selectivities. Examples are provided for Ca- Na and Ca-Mg exchange, considering organic matter and specific clay mineralogy. Various exchange equations and activity assumptions are examined using these experimental data.