4.6: Chemical dispersion and flocculation

In addition to these physical properties, chemical properties of the soil and the solution flowing through the soil can also impact the saturated hydraulic conductivity. These chemical effects arise when soil and solution characteristics promote swelling and chemical dispersion of clay present in the soil. Chemical dispersion here is the process in which soil particles, which previously were held together in close contact within soil aggregates, respond to a changed chemical environment by expanding and separating from one another, breaking down the soil aggregates. Swelling and dispersion can reduce the saturated hydraulic conductivity of a soil by a factor of 100 or more[8]. Swelling and dispersion can be promoted by any of the following conditions: irrigation with sodic water [8]; wetting of soil with a high content of 2:1 clays, particularly montmorillonite [9]; low electrical conductivity of the flowing solution [10]; or high exchangeable sodium percentage (ESP) in the soil. The ESP is defined based on the amount of exchangeable sodium in a soil divided by the sum of the exchangeable calcium, magnesium, potassium, and sodium. For example, this excellent time-lapse video demonstrates the dramatic swelling behavior of a dry clay loam upon wetting [Website]. Coarser-textured soils can also be impacted by swelling and dispersion. As illustrated in the following figure, leaching a column of sandy loam soil having an initial ESP of 10% with distilled water caused a 90% reduction in the K_s value (Fig. 4-6). Thus, distilled or deionized water should typically not be used when measuring soil physical properties, unless the goal is to determine the impacts of chemical dispersion on those properties.

Figure 4-6. Relative hydraulic conductivity (K_rel) and effluent electrical conductivity (EC) and pH changes produced by leaching a column of Fallbrook sandy loam with distilled water. The soil had 10% of primarily kaolinitic clay, an initial exchangeable sodium percentage (ESP) of 10 and a bulk density of 1.68 g cm^-3. Reproduced from Frenkel et al. (1978).

Chemical dispersion of soil can sometimes be reversed, and the reverse process is called flocculation. Flocculation is the process in which dispersed soil particles come together, often due to a change in the chemical environment. A high proportion of polyvalent cations, such as Ca²⁺, Mg²⁺, and Al³⁺, promotes flocculation, while a high proportion of monovalent cations, particularly Na⁺, promotes chemical dispersion. Gypsum (CaSO₄) has been successfully used to remediate chemical dispersion in sodic and saline-sodic soils because it provides Ca²⁺ to displace Na+1 on the soil’s cation exchange sites [11]. Sulfuric acid (H₂SO₄) is another good remediation option for chemical dispersion in calcareous sodic soils because the acid dissolves calcium carbonate (CaCO₃) present in the soil, and the Ca²⁺ released displaces Na_⁺[11].