The effects of mineralogical composition of soils on potassium availability in areas under wheat.

This research was conducted to verify the effects of soil mineralogical composition on potassium fixation and release capacity of soil. In total, 324-soil samples were collected from selected places in different areas of Iran where wheat cultivation is practiced. Soil characteristics measured included: available potassium (Ka), potassium fixation capacity (PFC), cation exchange capacity (CEC), organic matter (OC), CaCO3, gypsum and soil texture. These characteristics were determined by acetate ammonium, Jackson, Bawer, Walky & Black, titration, acetone and hydrometer methods, respectively. Soil mineralogical composition determined by x-ray diffractometer (Diano 8000). Potassium adsorption isotherms prepared by Pal procedure. Soils were categorized into Very low, Low, Moderately low, Moderate, Slightly high, High, Very high and Extremely high based on their Ka content. Each group was then subdivided into four groups (Low, Moderate, High, very high) according to their CEC and clay content. The results showed that there is a positive significant correlation between Ka and soil CEC, OC, and soil clay content and a negative significant correlation between Ka, soil silt, and CaCO3 content when considering all soils as whole. Soils in Ka groups and cation exchange capacity subgroups did not show any significant relation between Ka as a dependent variable and other measured soil characteristics as independent variables. Comparison of Xray diffractograms prepared for moderately low, moderate, slightly high, and high Ka soil groups showed that soil Ka 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. In all CEC subgroups, CEC increases with increased proportion of 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 dos not fixe the add potassium to the soil. In many cases, these processes release considerable amount of potassium to soil solution. Statistical analysis in soils with fixing ability is showed a positive significant relation between Ka and soil properties such as CEC, OC, silt and clay content and a negative relation between Ka ii and soil CaCO3 content and sand. Soils having capacity to release potassium showed positive relation between Ka and CEC, OC, silt and clay and a negative relation between Ka and soil CaCO3 content and sand. No significant relation between Ka and potassium release and other soil variables was not observed in Ka different soil groups. Mineralogical studies of the clay fraction of these soils showed that potassium fixation and release take place in each two groups notwithstanding to soil initial Ka content, clay amount, 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 the case of the 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 high release ability, exfoliation of phyllosilicate layers may trigger potassium release into the soil solution. Separate studies showed that, however, there is not any clear relationship between soluble k ion and the amount of clay content of the soil never eless when the size of soil particles decreases, the soluble k ion content of the soil increases. Potassium adsorption increases with increased soil CEC and clay content. There is a positive significant relation between Ka and adsorption energy, and clay. This means that with increasing adsorption energy and amount of clay, soil Ka content increases. There is a negative relation between amount of Ka and adsorption of potassium. With increasing adsorption power, soil Ka content decreases. Adsorption capacity of soil and adsorption energy of soils having illite as predominant mineral is less than soils containing vermiculite and smectite as predominant minerals.