Gypsum is deposited in soils by capillary rise from water tables (per ascensum), or by movement downwards following incomplete wetting (per descensum). Both these situations were reproduced in artificial columns using a wide range of soil materials. Two soil profiles formed per ascensum on Bahrain Island and two per descensum in Iran were also studied. The size, shape and arrangement of gypsum crystals were determined by optical examination of specimens and thin sections and by using the scanning electrod microscope. A comprehensive scheme for their description is outlined. Tabular, prismatic, lenticular, acicular, fibrous, tubular and granular forms were found, often modified by features produced by partial solution and subsequent resumption of crystallization. The form of gypsum crystallization changes intricately in space, and is often ephemeral in time, making it difficult to draw firm conclusions linking crystal shape to particular environments or soil types.
The potassium pools of five major physiographic units of the Urmia region situated in western Azerbaijan province, north-west Iran, were studied to determine the distribution of K forms as functions of clay mineralogy and physiographic units. Soil samples from horizons of ten pedons were selected and analysed for physiochemical properties, clay mineralogy and forms of K. X-ray diffraction patterns revealed that the soils were similar in clay-mineral compositions, consisting of illite, smectite, chlorite, and kaolinite, for the different physiographic units, but vary in the relative amounts of these minerals. The illite content was highest in piedmont plain (P.P) followed by plateau (Pl), river alluvial plain (R.A.P), colluvial alluvial plain (C.A.R) and lowland (L.L) units. Smectite content was highest in Pl followed by P.P, L.L, R.A.P and C.A.P units. Several processes, such as the diversity of weathering rate, biocyclying processes of K accumulation, geomorphologic conditions and soil formation processes, caused significant differences in most K forms in the soils. A wide variation in total K (HF-extractable K) (0.54–1.1%), non-exchangeable K (280–450 mg kg–1) and exchangeable K (217–330 mg kg–1) occurred among the physiographic units, corresponding to variations in their mineralogical compositions, mainly the abundance of illite. Significant differences (P ⩽ 0.05) were found for K, HNO3-extractable K and non-exchangeable K between the soils with large illite contents (30–50%) and with small illite contents (10–30%). A significant positive relationship existed between mineral K and illite content (r2 = 0.85, P ⩽ 0.001) and non-exchangeable K and illite content (r2 = 0.84, P ⩽ 0.001). The results indicated that these pools of K are mainly released from the frayed edges and wedge zones of illite.
The iron oxides fractions of four major physiographic units obtained from a transect of calcareous materials were studied to assess the effects of key pedogenic processes and local hydrology conditions as well as physiographic units in controlling iron oxides forms in the north-west of Iran. Samples from different horizons belonging to six pedons were selected and analyzed for soil physicochemical properties, clay minerals, and Fe oxides forms (Fe d , Fe o , Fe p ). In general, the soils indicated some variation in the concentration of iron oxides that could be related to rate of weathering, pedogenic accumulations, geomorphologic conditions (as results of different in physiographic units), wet and dry cycle, and organic matter. A wide relative variation in mean values of Fe d (6.4-9.9 g kg 71 ), Fe o (2.9-4 g kg 71 ), and Fe p (0.68-1.3 g kg 71 ) was observed among physiographic units. On the plateau unit, the presence of the most stable geomorphologic conditions and high rate in situ weathering (reflected in clay content), coupled with minor deposition of sediment suggest that the soils have more dynamic conditions than other units, reflecting in the greatest amount Fe d and the lowest Fe o /Fe d ratio. Fe d content of the soils containing less clay content (15-25%) was significantly different from those with greater clay content (25-35%).
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