Effect of ionic strength and clay mineralogy on Na-Ca exchange and the SAR-ESP relationship

The relationship between sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP) for all soils has traditionally been assumed to be similar to that developed by the United States Salinity Laboratory (USSL) in 1954. However, under certain conditions, this relationship has been shown no...

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Bibliographic Details
Published in:European journal of soil science 2006-10, Vol.57 (5), p.626-633
Main Authors: So, H B, Menzies, N W
Format: Article
Language:English
Online Access:Get full text
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Summary:The relationship between sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP) for all soils has traditionally been assumed to be similar to that developed by the United States Salinity Laboratory (USSL) in 1954. However, under certain conditions, this relationship has been shown not to be constant, but to vary with both ionic strength and clay mineralogy. We conducted a detailed experiment to determine the effect of ionic strength on the Na(+)-Ca(2+) exchange of four clay minerals (kaolinite, illite, pyrophyllite, and montmorillonite), with results related to the diffuse double-layer (DDL) model. Clays in which external exchange sites dominated (kaolinite and pyrophyllite) tended to show an overall preference for Na(+), with the magnitude of this preference increasing with decreasing ESP. For these external surfaces, increases in ionic strength were found to increase preference for Na(+). Although illite (2:1 non-expanding mineral) was expected to be dominated by external surfaces, this clay displayed an overall preference for Ca(2+), possibly indicating the opening of quasicrystals and the formation of internal exchange surfaces. For the expanding 2:1 clay, montmorillonite, Na(+)-Ca(2+) exchange varied due to the formation of quasicrystals (and internal exchange surfaces) from individual clay platelets. At small ionic strength and large ESP, the clay platelets dispersed and were dominated by external exchange surfaces (displaying preference for Na(+)). However, as ionic strength increased and ESP decreased, quasicrystals (and internal exchange surfaces) formed, and preference for Ca(2+) increased. Therefore, the relationship between SAR and ESP is not constant and should be determined directly for the soil of interest.
ISSN:1351-0754
DOI:10.1111/j.1365-2389.2006.00753.x