Soil Crusts in the Amudarya River Delta: Properties and Formation

Particle size distribution, salt content and composition, mineralogy and micromorphology of a solonchak salt crust and a takyr crust from the Amudarya River Delta (ADRD) were determined. In the (salt-free) salt crust, 100-μm particles dominate, while silt and fine sand are present in minor quantities only and clay is absent. The soil below the crust has a similar particle size distribution. In the takyr crust, fine particles dominate. In both crusts, quartz dominates, and is accompanied by calcite and mica. Clay minerals (chlorite and kaolinite) appear in minor amounts only. Cl is the major anion among the soluble salts, followed closely by sulfate. The salt crust contains four times more salt than the takyr crust. The salt in the salt crust appears in the form of crystallites of halite and Na, Mg sulfates such as thenardite and epsomite, in addition to gypsum. The size of the interlocking crystallites is 5–10μm. The takyr crust is highly porous, with fine pores on the upper surface, and coarse pores in the bottom part of the crust. It is proposed that particle size distribution of the sediment and groundwater are among the important factors in determining the development pattern of Amudarya River delta soils. A fine particle size composition and absence of groundwater (including drainage water) close to the surface will lead to takyr formation. A relatively coarser particle size distribution and the presence of groundwater close to the surface will lead to solonchak (+salt crust) formation.