بررسی ظرفیت تثبیت پتاسیم در خاک های تحت کشت گندم

This research was conducted to verify the effects of soil mineralogical composition on potassium fixation and release capacity. At first, 50-soil samples were collected from selected farms under wheat cultivation from different provinces. 18 soil samples were selected among them for detailed analysis. Soil characteristics measured included: Available potassium (Kava), Potassium fixation capacity (PFC), Cation exchange capacity (CEC), Organic matter (OC), CaCO3, Texture, Reaction, Electrical conductivity, soluble potassium, Exchangeable potassium and Nonexchangable potassium. Soils were categorized into three different groups؛ Low (lower than 100 mg/kg), Medium (100-200 mg/kg) and High (more than 250 mg/kg) according their plant available potassium. Potassium fixation capacity (PFC) were determind by 10 times wetting and drying method. Clay mineralogical composition was determined by x-ray diffractometer.Comparison of X-ray diffractograms prepared for low, medium, and high Kava soil groups showed that soil Kava. content is influenced by simultaneous presence of illite and other 14 Ao minerals (smectite, vermiculite, and chlorite). Collective effects of clay content, proportion of each clay minerals and degree of development of crystallographic structure of clay minerals controls soil Ka content. Diffractograms provided for soils in each Ka group, showed that soil Ka increases with structural enhancement of illite and its proportion against 14 Ao minerals in soil composition. Determination of soil potassium fixation capacity with wetting and drying method, showed that swelling and shrinkage induced by soil wetting and drying did not fix the added potassium to the soil. In some soils, these processes released considerable amount of potassium. Mineralogical studies of clay fraction of these soils showed that potassium fixation and release take place in each two groups notwithstanding to soil initial Kava and clay content, soil mineralogical composition and illite predominance or presence of 14 Ao minerals. The effect of wetting and drying on potassium fixation and release seems not to be straightforward. In a soil with certain mineralogical composition wetting and drying may contribute to potassium fixation and in another soil with the same mineralogical composition may increase potassium release. In case of soils, which depicted fixation capacity, the process of drying probably causes phyllosilicate layers to collapse. Consequently, potassium ions located on the edge sights of clay particles may be retained between the phyllosilicate layers. On the other hand, in the soils with ability to release, exfoliation of phyllosilicate layers may trigger potassium release into the soil solution. Key words: Potassium fixation capacity, Available potassium, Potassium release, Wetting and drying of clay minerals.