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Fig. 18.26 a Hydraulic conductivity volumetric soil content relations for a soil sample (load
13.3 g/cm 2 ) subjected to a series of solutions of decreasing concentration at SAR = 40 (Dane
and Klute 1977 ). b Bulk density during conditions of ponded solution for a soil sample (load
13.3 g/cm 2 ) subjected to series of solutions of varying concentration at SAR = 40. Arrows
indicate the sequence of solution application (modified after Dane and Klute 1977 ). Copyright
1977 Soil Science Society of America. Reprinted with permission
solutions at constant SAR values of 0-40, respectively. When the SAR of the
solutions was 40, the bulk density decreased with a decrease in the concentration of
percolating solution.
Dane and Klute ( 1977 ) considered that a decrease in the electrolyte concen-
tration of the surrounding solution caused clay platelets to increase their distance
of separation. Under these conditions, the repulsive forces begin to dominate over
the attractive forces, with the clay particles forming an unstable arrangement.
A partial collapse of the clay particle configuration causes the bulk volume of the
soil to decrease, explaining the increase in bulk density when solution concen-
tration decreases. Repercolating the soil samples with an electrolyte solution of an
increasing concentration does not enable a return to the initial state.
The effect of pH on clay dispersion in Na-saturated soils was reported by
Schofield and Samson ( 1954 ), who showed that kaolinite edge faces become
negatively charged at high pH and positively charged at low pH. An example of
the effect of pH on kaolinitic shortlands soil clay in Na solution is found in Frenkel
et al. ( 1992 ), where the critical flocculation value, defined as the NaCl concen-
tration required to sediment 8 % of the clay after standing for 24 h, was measured.
The critical flocculation values determined at pH values of 6.6, 7.3, and 8.5 are
shown in Fig. 18.27 .
18.2.3.2 Crust Formation and Infiltration Rate
Sealing the soil surface by crust formation, and subsequent decreases in the water
infiltration rate, may be caused by the presence of sodium in the incoming water
(see also Sect. 18.1.1 ). The extent of crust formation is affected by the texture and
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