Coarse, medium, and fine-textured soils were studied in three different arid regions of Iran, namely Gazvin, Karaj, and Kooshkak alluvial plains, respectively. Six representative pedons including 33 samples were tested.
X-ray analyses indicated the clay mineral components of soils consist of some discrete vermiculite, chlorite, clay mica (illite), kaolinite, and interstratified clay mica-vermiculite or chlorite.
Quantitatively, vermiculite and clay mica are the major clay mineral components in these soils. The average vermiculite contents of coarse, medium, and fine-textured soils are calculated as 41.7, 41.4, 42.4%; clay mica contents are 26.3, 38.0, and 20.0%, respectively. The amount of clay mica decreases significantly with depth in most of the soils studied.
Physico-chemical determinations revealed the soils are calcareous throughout and the carbonates increase from coarse- to fine-textured soils. The morphogenetic sequences of carbonate accumulation are divided into three morphogenetypes. Most organic matter contents are < 1%. They decrease with depth but increase from coarse- to fine-textured soils.
Soils with poor structural development contain > 20% CaCO3, and have considerable amounts of vermiculite but lower amounts of clay mica. The slight accumulation of silt, clay, and carbonates at the surface of some soils may indicate the influence of atmospheric addition in the genesis of the soils studied. Two of the soils are classified Typic Camborthids, two of them Xerollic Camborthids, and the other two, Xerollic Calciorthids.