The relationships between different pools of K, i.e. exchangeable, HN03-extractable, mineral and total, were investigated as a function of clay mineralogy and soil development in soils of Kharkeh Research Station, Kurdestan Province, western Iran. Samples from different horizons often pedons were selected and analysed for clay mineralogy and K fractionation. X-ray diffraction patterns revealed that clay minerals in the soils studied were similar in type, while their abundances were different. The smectite content was significantly greater in Vertisols than in the other soils. The results of K fractionation showed that mineral K and HN03-extractable K (exchangeable and nonexchangeable, respectively) and the clay content of the soils containing lesser illite (10–30%) were significantly different from those with more illite (30–50%). Moreover, the regression slopes between water-soluble and NE4OAc-extractable K were lower in soils with more smectite due mainly to the presence of larger specific surface areas for K sorption in smectitic soils. Based on soil evolution and different forms of K, the soils studied were grouped in two major categories: (1) Vertisols and (2) Entisols, Inceptisols and Mollisols. There were greater contents of all forms of K in Vertisols than in the other soil orders. This was mainly related to the greater clay content and the dominance of smectite in the clay fraction of Vertisols.

Abstract:Thirty-two surface soil samples with different physical and chemical properties were collected from calcareous soils of arid regions of Khuzestan Province, Southwestern Iran. Actual and potential cadmium (Cd) bioavailability was assessed by means of a greenhouse study, two different single extractions: 0.005 DTPA and 0.05 EDTA and two sequential extraction procedures: modified five-step Tessier method and the three-step BCR method. Extraction results were correlated with grain Cd uptake by Durum wheat (Triticum durum L. ) grown under greenhouse conditions. Cadmium in the durum grain ranged from 0.064 to 0.273 mg kg -1 . The results showed better suitability of DTPA extraction in these highly carbonated soils than EDTA extraction for predicting Cd phytoavailability. DTPA -extractable Cd was positively related to soil electrical conductivity (p < 0.01, r = 0.65). Cadmium was predominantly associated with carbonate fraction in these soils (Tessier procedure 40% and BCR 44%).1 M NH 4 OAc which extracted the soluble + exchangeable fraction in the Tessier scheme may also provide useful information on Cd availability in studied soils.

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