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

Abstract

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 þ -Ca 2þ 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 þ , 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 þ . Although illite (2:1 non-expanding mineral) was expected to be dominated by external surfaces, this clay displayed an overall preference for Ca 2þ , possibly indicating the opening of quasicrystals and the formation of internal exchange surfaces. For the expanding 2:1 clay, montmorillonite, Na þ -Ca 2þ 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 þ ). However, as ionic strength increased and ESP decreased, quasicrystals (and internal exchange surfaces) formed, and preference for Ca 2þ increased. Therefore, the relationship between SAR and ESP is not constant and should be determined directly for the soil of interest.Correspondence: P. Kopittke.

Applying HYDRUS to Flow in a Sodic Clay Soil with Solution Composition–Dependent Hydraulic Conductivity

“…Cation exchange coefficients are often assumed to be constant for a given soil, but there is evidence that these coefficients can vary with both ionic strength and clay mineralogy (Kopittke et al, 2006). In HYDRUS, the selectivity coefficients are assumed to be independent of solution conditions, and as such they remain constant.…”

Inorganic Constituents in Soil

“…As widely different K G values have been reported for different pure clay minerals, the almost constant values for many soils might be due to mixed clay mineral compositions (Shainberg et al 1980;Miller et al 1990;Kopittke et al 2006;Endo et al 2002). Because the average SAR of the river water used for irrigation of the tsunami-affected area is 0.6 (mmol L À1 ) 0.5 and the calculated ESP value using Eq.…”

Role of Inorganic Soil Constituents in Selected Topics

“…The clay mineral composition of the area is a mixture of smectite, vermiculite, and kaolin minerals. As widely different K G values have been reported for different pure clay minerals, the almost constant values for many soils might be due to mixed clay mineral compositions (Shainberg et al 1980;Miller et al 1990;Kopittke et al 2006;Endo et al 2002). Because the average SAR of the river water used for irrigation of the tsunami-affected area is 0.6 (mmol L À1 ) 0.5 and the calculated ESP value using Eq.…”