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Potassium (K) is known as one of the essential nutrients for the growth of plant species. The relationship between K and clay minerals can be used to understand the K cycling, and assess the plant uptake and potential of soil K fertility. This study was conducted to analyze the K forms (soluble, exchangeable, non-exchangeable and structural) and the relationship of K forms with clay minerals of calcareous soils in Kohgiluyeh and Boyer-Ahmad Province, Southwest Iran. The climate is hotter and drier in the west and south of the province than in the east and north of the province. A total of 54 pedons were dug in the study area and 32 representative pedons were selected. The studied pedons were mostly located on calcareous deposits. The soils in the study area can be classified into 5 orders including Entisols, Inceptisols, Mollisols, Alfisols and Vertisols. The main soil clay minerals in the west and south of the study area were illite, chlorite and palygorskite, whereas they were smectite, vermiculite and illite in the north and east of the province. Due to large amount of smectite and high content of organic carbon in soil surface, the exchangeable K in surface soils was higher than that in subsurface soils. It seems that organic matter plays a more important role than smectite mineral in retaining exchangeable K in the studied soils. Non-exchangeable K exhibited close relationships with clay content, illite, vermiculite and smectite. Although the amount of illite was the same in almost all pedons, amounts of structural and non-exchangeable K were higher in humid regions than in arid and semi-arid regions. This difference may be related to the poor reservoir of K + minerals like palygorskite and chlorite together with illite in arid and semi-arid regions. In humid areas, illite was accompanied by vermiculite and smectite as the K + reservoir. Moreover, the mean cumulative non-exchangeable K released by CaCl 2 was higher than that released by oxalic acid, which may be due to the high buffering capacity resulting from high carbonates in soils.

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Sequential extraction is used effectively to determine the chemical forms of heavy metals. Since few studies have been conducted in calcareous soils of Iran, the current research has been carried out to evaluate the effect of moisture regimes and incubation time on the chemical forms of cadmium in two calcareous soils. Treatments included three levels of cadmium (0, 30 and 60 mg/kg of soil as CdSo 4), three incubation times (2, 4 and 8 weeks), two soils (clay and sandy clay loam) and two moisture regimes (Waterlogged and Field capacity). The experiment was performed as factorial on the basis of randomized complete blocks design with two replications. At 2, 4, and 8 weeks after treatment, the Tessier et al. (1979) sequential extraction method was applied to measure the cadmium concentration in water soluble-exchangeable (WsEx.), iron and manganese oxide (Fe-MnOx.), carbonate (Car.), organic matter (Om.) and residual forms (Res.). The results showed depending on the soil texture, 68 to 72 percent of the cadmium was in water soluble-exchangeable and iron-manganese oxide forms. Flooding condition decreased the concentration of cadmium in water soluble + exchangeable form and increased the concentration of cadmium in the forms of Fe-Mn oxides, carbonate and organic matter. Concentration of cadmium in residual form was higher in clay soil. In sandy clay loam soil, water soluble-exchangeable, carbonate and organic matter forms were higher than clay soil. The iron-manganese oxides form showed no significant difference in two soils at field capacity regime. In flooding conditions, the concentration of the water soluble-exchangeable form decreased and the concentration of other forms increased. In contrast to the oxidizing conditions at the reduction conditions, no significant difference was observed in the residual and organic matter forms in two soils.