Stability of Soil Aggregates as Inferred from Optical Transmission of Soil Suspensions
Stability of the semiarid Zezia soil aggregates was evaluated by measuring the optical transmission (%T) of suspensions of two size fractions (<2 and <5 µm) dispersed at decreasing electrolyte concentration of constant sodium adsorption ratio (SAR). At a given SAR, the %T decreased linearly with decreasing electrolyte concentration. Slopes of the equations decreased with increasing SAR; indicating a decreasing dependence of %T on electrolyte concentration as SAR increased. Such slopes were called stability indices (SIs). The clay-SI was on the average 5.6 times greater than that of the <5-µm fraction. At a given SAR, electrolyte concentrations corresponding to maximum clay dispersion (minimum %T values of 10) or very little clay dispersion (%T values of 20) were called critical concentrations (CCs) and threshold concentrations (TCs), respectively. At SAR values of 0, 5, 10, 25, and ∞, the CCs were 1.0, 3.7, 5.4, 10.3, 10.0, and 13.0 mol m−3, respectively, and the TCs were 2.2, 5.5, 9.2, 16.7, 17.8, and 32.6 mol m−3, respectively. Both CCs and TCs increased linearly with increasing SAR to a limiting value of 20. The TC for the <5 µm dispersed size fraction also increased linearly with increasing SAR. The TCs were 11.8, 40.4, 49.0, and 61.7 mol m−3 at SAR 0, 10, 15, and 20, respectively. These relatively high values imply that slaking of soil aggregates into sizes >2 µm can take place without a significant clay dispersion.
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