عنوان مقاله [English]
Background and objectives: Loess derived soils, in both arid and humid areas are the most common soils that are formed in eolian sediments. Lamellae as thin bands are observed in both alluvial and eolian soils in various landscapes. Each lamella separates from the other one by a band with less clay content. The lamellae play a role in increasing water and nutrient holding capacitis of the coarse textured soils. Also in archaeological and geomorphological studies, indicate the approximate age of the soils. The objective of this research has been to study the macro-morphology, micro- morphology, physical and chemical characteristics of loess- derived soils with lamella.
Materials and methods: The study area is located twenty kilometers east of Zavareh, Ardestān city, north-east of Isfahan province. This area has an average annual rainfall of 110 mm and an average annual temperature of 19.5oC. Its parent materials are loess and its land form is piedmont plain. In the study soils, eight soil profiles were excavated, described and disturbed soil samples were collected from all of their horizons to investigate about the morphological, physical and chemical characteristics of the soils. Of the eight studied soil profiles, in two profiles, bands with different color and texture (lamella and the eluvial part) were observed. Micropedological studies including preparation and description of soil thin sections were carried out for these soil profiles. All of the studied soil profiles were classified at phase of soil series level.
Results: The studied loess –derived soils that all of them are Aridisols were classified in three suborders, four great groups, four subgroups, five families, eight series and eight phases of series. The results showed that in some horizons of the loess-derived soils, there are lamellae that meet argillic horizon requirements. In geogenic stage of lamella formation, with decreasing wind speed, finer particles precipitate in the form of very thin horizontal bands, parallel with the bands of coarse particles. In the pedogenic stage, with stop of a wetting front at the border between the layer of fine-grained and the underlying coarse-grained layer and before saturation of the fine-grained layer, the wetting front loses its moisture through evaporation or absorption by the roots. Clay particles suspended in the wetting front, after evaporation of the front moisture, form clay film on wall of the pores or on the surface of the sand and silt grains. Over time, the film thickens and the fine pores diameter of the fine textured bands is reduced until the lamella finally is formed. Occurrence of lithologic discontinuity at a depth of 140 cm and filling the channels by pedogenic gypsum crystals accelerate pedogenic formation of the lamella. Due to weathering of the primary iron bearing minerals, present in the lamellae forming materials, iron is released from the minerals and after oxidation, precipitates on the lamellae surfaces.
Conclusion: When wind speed decreases, finer particles precipitate in the form of very thin horizontal bands, parallel with the bands of coarse particles. This stage of the lamella formation is geogenic.
Pedogenic stage begins when a wetting front stops at the border between the layer of fine-grained and the underlying coarse-grained layer, before saturation of the fine-grained layer and the wetting front loses its moisture through evaporation or absorption by the roots. Suspended clay particles, present in the wetting front, after evaporation of the front moisture, form clay coatings on wall of the pores or on the surface of the sand and silt grains. Over time, this coating thickens and the fine pores diameter of the fine textured bands is reduced until the lamella finally is formed.