Response of soils to sodic and saline conditions

Abstract

The interaction between soil particles and the soil solution depends on the types and amounts of soil clays. Montmorillonite is the most reactive clay. Ca-clay forms tactoids (quasi crystals) and has limited swelling and dispersion. Na-clay forms single platelets and disperses freely. In a mixed Na/Ca system, “demixing” of the cations occurs as the Na ions concentrate on the external and the Ca ions on the internal surfaces of the tactoids. The demixing explains why a small percentage of exchangeable Na sharply increases the zeta potential and dispersion of montmorillonite clay. The hydraulic conductivity of a soil depends on both Na and the total salt concentration of the percolating solution. High hydraulic conductivity may be maintained, even at high exchangeable sodium percentage (ESP) values, if the solution concentration is above a critical (threshold) level. When waters of very low salt content are used, decreases in hydraulic conductivity and clay dispersion occur even in soils with low ESP values (<10). The main mechanism for hydraulic conductivity reduction in waters of medium to high salinity (replacecodegt0.5 dS/m) is clay swelling. High content of expansible clays increases the susceptibility of soils to intermediate (10<ESP<20) sodicity. The effect of low sodicity (ESP < 10) on clay swelling is minimal and the hydraulic conductivity of these soils is maintained at high values. When waters having low salt concentrations (salt concentration below the flocculation value of the clay) are used, clay dispersion, movement, and lodgement in the conducting pores comprise the main mechanism responsible for hydraulic conductivity reduction. These results occur even at low ESP. The most important soil property which determines its susceptibility to sodic conditions, when the soil is leached with rain or snow water, is the potential of the soil to release electrolytes from primary minerals and/or CaCO3 dissolution. Soils which release sufficient electrolytes to maintain the concentration of the soil solution above the flocculation value of the clay will not disperse, and thus will not be sensitive to low sodicity.