Permeability of Salt Affected Soils

The rate of infiltration, which refers to the rate of water entry into the soil, generally increases with increasing salinity and decreases with either decreasing salinity or increasing sodium content relative to calcium and magnesium (SAR). A column study on the effect of water quality on saturation hydraulic conductivity was conducted in the laboratory. Silty clay that contained 40% clay from the B horizon of the experimental plot was tested. Soil samples were packed into plastic cylinders with a length of 50 mm and an internal diameter of 50 mm at the bulk density of 1.4 g/cm3. Columns were first saturated with a solution that contained 0.5 mo^/l (moles of charge per liter) Cl- (CaC12 andNaCl) at SAR = 10. After saturation, the column was leached consecutively with solutions of the same SAR but diluted concentrations (0.05, 0.01 molc/l) and distilled water. EC of the solution was 50 dS/m for 0.5 molc/l, 5 dS/m for 0.05 molc/l and 1 dS/m for 0.01 molc/l solution, respectively. The effluent was collected with a fraction collector and saturation hydraulic conductivity (HC) was measured.

Figure 9.2 shows the relative HC as influenced by water quality and the cumulative effluent volume. The relative HC is the ratio of treatment HC and HC at 0.5 molc/l concentration. HC of the soil at 0.05 mo^/l concentration was not changed with the increase of effluent volume, and kept constant at 10 mm/ hr. HC was only 60% and 40% a with leaching solution of 0.01 molc/l and distilled water compared with the 0.5 molc/l solution, respectively. This decrease in the soil HC resulted from clay dispersion and/or swelling. When the concentration of soil solution becomes lower than a certain value, clay swelling and/or dispersion occur, causing soil pores to be narrowed or clogged, and HC of the soil will decrease. Clay swelling is the major cause for the decrease of soil HC when soil ESP (Exchangeable Sodium Percentage) >15,

Fig. 9.2. Effects of concentration and SAR of water on the relative hydraulic conductivity. The lines represent SAR of 0.05 molc/l, 0.01 molc/l and distilled water (D).

whereas clay dispersion is responsible for the decrease of soil HC when ESP <15.4 As leaching continues, the soil ESP decreases, owing to desodification of adsorbed salts. Thus, the decrease in soil HC was considered to be due to clay dispersion. Since the decrease in soil permeability affects the irrigation efficiency as well as the leaching efficiency, use of flocculant as a soil conditioner to improve the soil permeability may be promising for water and soil management.

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