Premium
Estimating Calcium‐Magnesium Selectivity in Smectitic Soils from Organic Matter and Texture
Author(s) -
Curtin D.,
Selles F.,
Steppuhn H.
Publication year - 1998
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1998.03615995006200050019x
Subject(s) - selectivity , chemistry , organic matter , soil water , cation exchange capacity , magnesium , soil texture , soil ph , soil organic matter , anhydrous , environmental chemistry , mineralogy , inorganic chemistry , soil science , geology , organic chemistry , catalysis
Selectivity coefficients are essential to describe the distribution of cations between the exchange and solution phases of soil. Because the factors influencing Ca‐Mg exchange have not been adequately defined, models commonly incorporate the dubious assumption that soils do not discriminate between these two cations (i.e., the selectivity coefficient is 1). The purpose of this study was to identify key variables influencing Ca‐Mg selectivity in smectite‐dominated soils of the Canadian prairies. Selectivity coefficients ( K ) were measured by reacting soils that differed in texture (50–500 g clay kg ‐1 ), organic matter (4–96 g C kg ‐1 ), and pH (4.1–7.3) with solutions containing mixtures of CaCl 2 and MgCl 2 . The value of K for the reaction Ca + Mg‐soil ⇌ Mg + Ca‐soil ranged from 1.1 to 3.4, depending on soil type and the composition of the reacting solution. Preference for Ca increased as the equivalent fraction of Ca in solution decreased from 0.8 (average K = 1.7) to 0.05 (average K = 2.9). We identified organic matter as the major source of Ca‐preferring sites. The ratio of organic matter to clay was the best single indicator of Ca‐Mg selectivity. Measurements on soils whose pH had been altered by laboratory addition of Ca(OH) 2 or field application of anhydrous NH 3 indicated that selectivity was independent of pH, even though the proportion of organic sites increased as pH increased. A simple two‐site model with K of 3.9 for organic matter and K of 1.3 for clay described our data reasonably well, although selectivity for Ca tended to be underpredicted at low Ca loading.