Palygorskite Dominated Vertisols of Southern Iran

Abstract

Vertisols are generally known as smectite-dominated soils with prominent shrink-swell properties, although Vertisols with mixed, kaolinitic, illitic or other non-smectitic mineralogical compositions have also been reported. Palygorskite-dominated Vertisols have not yet been described in detail. From about 70,000ha of cultivated Vertisols in Iran, 16,000ha is located in the Fars Province (southern Iran). These soils formed on calcareous sediments in the lowlands, are classified as Sodic Haplusterts, Aridic Gypsiusterts, Aridic Haplusterts, Halic Haplusterts and Aridic Haploxererts. X-ray diffraction and electron microscopy indicate that the smectite content is very low to almost absent in most soils, while palygorskite and chlorite are dominant silicate clay minerals, especially in the fine clay fraction. A good relationship was observed between COLE and fine clay/total clay ratio. Strongly developed angular blocky to prismatic structure with wedge-shaped peds is dominant in the subsurface horizons. Due to the high micritic carbonate content of the groundmass, the b-fabric is mostly crystallitic, but decalcified zones (after HCl treatment) show a stipple-speckled b-fabric. Fe/Mn hydroxide impregnations are omnipresent, which is an indication of seasonal hydromorphic conditions. In some part of the profiles prismatic and lenticular gypsum crystals occur as loose continuous infillings of channels, reflecting mineral formation in brine-filled macropores.