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Soil Pore Structural Stability and Irrigation Water Quality: II. Sodium Stability Data¹

ABSTRACT

The effect of mixed sodium/calcium solutions on the hydraulic conductivity (HC) of a range of soils was measured at fixed sodium absorption values and a range of cation concentrations decreasing from 800 to about 1 meq/liter. A similar series of solutions was used to measure water uptake (macroscopic swelling) of extracted soil clays. Using these data, sodium sensitivity and threshold swelling models were computed. The slope of the sodium sensitivity model was related to the smectite content, cation exchange capacity, and specific surface of pedal, nonvertisol soils through log-log relationships having correlation coefficients of 0.67*, 0.61**, and 0.66**. It was also related to swelling by a log-linear relationship (r = 0.64*). Some vertisols did not obey these relationships, possibly because of the effect of marked alteration to flowbed geometry during swelling. Such alteration maintained HC at higher values than would have been inferred from soil properties alone. Apedal soils containing large amounts of smectite also displayed greater stability than expected, possibly because of the stabilizing effect of sesquioxides on clay swelling. Soils with low specific surface, low CEC, and little smectite and/or sufficient sesquioxide to inhibit swelling (red, apedal) tend to be most stable in the presence of mixed Na/Ca solutions.

¹ Contribution from the Dep. of Soil Science and Agrometeorology, Univ. of Natal, Pietermaritzburg, South Africa, and Dep. of Agron., Univ. of Georgia, Athens, GA 30602.

² Senior Lecturer in Soil Science, University of Natal; and Professor of Agronomy, University of Georgia.

Received for publication April 3, 1981. Accepted for publication January 22, 1982.