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Effects of Magnesium on Cation Selectivity and Structural Stability of Sodic Soils
Author(s) -
Curtin D.,
Steppuhn H.,
Selles F.
Publication year - 1994
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/sssaj1994.03615995005800030013x
Subject(s) - chemistry , magnesium , soil water , sodium adsorption ratio , sodium , cation exchange capacity , dispersion (optics) , electrolyte , zoology , mineralogy , geology , agronomy , soil science , physics , organic chemistry , optics , drip irrigation , electrode , irrigation , biology
Abstract This study was conducted to evaluate the effects of Mg on the cation‐exchange relationships and structural stability of sodic soils. Samples of prairie soils (Haploborolls) from Saskatchewan, Canada, were equilibrated with sodic solutions [sodium adsorption ratio (SAR = Na/(Ca + Mg) 0.5 , where concentrations are expressed in millimoles per liter) ranging from 5 to 40] with different Ca/Mg ratios. Exchangeable Na increased as the Mg/Ca ratio increased. Averaged from all SAR treatments, the Mg‐Na systems had between 17 and 37% more exchangeable Na than did the Ca‐Na systems. The Mg‐induced increase in exchangeable Na was directly related to a preference by the soils for Ca over Mg, which makes Na more competitive against Mg than against Ca. The influence of Mg on structural stability was examined by measuring saturated hydraulic conductivity ( K ) using Mg‐Ca‐Na solutions (Mg/Ca ratios ranging from 0:1–1:0) with SARs of 10 and 20. The Mg‐Na system developed considerably lower K than did the Ca‐Na system when leached with solutions with total electrolyte concentration ≤ 100 mmol c L −1 . Systems with intermediate Mg/Ca ratios (i.e., Mg/Ca of 1:2 and 2:1) behaved more like the Ca‐Na than the Mg‐Na system. Clay concentration in leachate from the soil columns increased as Mg/Ca ratio increased, suggesting that a major effect of exchangeable Mg was that it increased the tendency for clay dispersion. Dispersion tests confirmed that the effect of Mg was greater than could be explained by the higher exchangeable Na level in the Mg system, i.e., Mg had a specific effect on clay dispersion. Our results suggest that exchangeable Mg is about 5% as dispersive as exchangeable Na.