HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Curtin, D. Articles by Steppuhn, H. Search for Related Content PubMed Articles by Curtin, D. Articles by Steppuhn, H. GeoRef GeoRef Citation Agricola Articles by Curtin, D. Articles by Steppuhn, H.

Estimating Calcium-Magnesium Selectivity in Smectitic Soils from Organic Matter and Texture

New Zealand Inst. for Crop & Food Research Ltd., Private Bag 4704, Christchurch, New Zealand

F. Selles and H. Steppuhn

Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, P.O. Box 1030, Swift Current, SK, Canada S9H 3X2

*Corresponding author (curtind{at}crop.cri.nz).

ABSTRACT

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 CaCl₂ and MgCl₂. 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 NH₃ 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.

Received for publication January 24, 1997.

This article has been cited by other articles:

				T. M. DeSutter, G. M. Pierzynski, and L. R. Baker Flow-Through and Batch Methods for Determining Calcium-Magnesium and Magnesium-Calcium Selectivity Soil Sci. Soc. Am. J., February 27, 2006; 70(2): 550 - 554. [Abstract] [Full Text] [PDF]