Sodic soils dispersion

Hydration swelling, 108-115 Layer charge and CEC, 113-115 Structure, 111 Sodic soils, 407 Dispersion, 414 Exchangeable Na+, 409, 412 Toxicity to plants, 407-408 Sodium, 411... 

Sodic (black alkali) soils are a particularly difficult management problem. The water permeability of these soils to water is very slow. The pH of sodic soils is commonly greater than 9 or 9,5, and the clay and organic fractions are dispersed. Dispersed organic matter accumulates at the surface of poorly drained areas as water evaporates and imparts a black color to the surface, hence the name black alkali. Sodic soils are found in many parts of the western United States. In some locations they occur in small patches, slick spots, less than 0.5 ha in extent. Such patches occupy slight depressions, which become accentuated as surface soil particles disperse and are blown away by wind erosion. The percolation of insufficient water to satisfy plants and to control salinity is the main problem associated with sodic soils. In addition, their relatively low soluble-salt concentrations and high pH values can result in direct Na toxicities to the most sensitive plants. 

Remediation of sodic soil is far more difficult than reclaiming a saline soil. The condition of sodicity (ESP > 15%) requires the addition of calcium amendments, time for them to react with the soil, and percolation of water through the soil to remove excess sodium. Whether or not this can even be done depends on the degree of dispersion and swelling. 

At about one-quarter of the threshold concentration,for a given sodicity, dispersed particles appear in the percolate, indicating the start of the dismantling of clay domains. It is noteworthy that this concentration is almost ten times lower, or even more if natural dispersants are present [e.g. organic compounds], than that obtained for the flocculation of a suspension of the soil.This reflects the fact that it is harder to release the crystals from within the clay domains, than to simply flocculate the free crystals. 

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