Mineralogical Characterization of Potassium-Bearing Clay Minerals and Their Implications on K Availability in Calcareous Soils of Sulaymaniyah, Kurdistan

Mineralogical Characterization of Potassium-Bearing Clay Minerals and Their Implications on K Availability in Calcareous Soils

Abstract

The mineralogical composition and properties of potassium-bearing clay minerals are important to determine the availability of potassium (K) in the calcareous soils, but there is limited knowledge about the agricultural soils in the Kurdistan Region of Iraq. This study characterized the prevailing potassium-bearing clay minerals within agricultural soils of three representative sites in the Sulaymaniyah Governorate (Bakrajo, Pshdar, and Bitwen) and determined their effects on the K availability. Soil samples were collected from the plow layer (0 - 30 cm) and analyzed to determine the physicochemical characteristics, K fractions, and clay mineralogy by X-ray diffraction (XRD). The findings revealed that soils are slightly alkaline pH (7.41-7.82), low electrical conductivity (0.26-0.41 dS/m), and variable cation exchange capacity (16.38-48.88 cmol_c/kg). Soluble K was 11.5-32.07 mg/kg and exchangeable K was quite variable 191.35-448.98 mg/kg, indicating that there was a lot of spatial heterogeneity in K status. The XRD analysis identified illite was the major mineral and mixture of illite-smectite, chlorite, and expandable components of clay with varying percentages were detected across all sites. The soils of bakrajo showed mixed-layers of illite-smectite assemblages with large CEC, and moderate exchangeable K, which represented intermediate weathering. Bitwen soils exhibited a high content of expandable clay with exceptionally high exchangeable K (373.8 - 448.98 mg/kg) that could be attributed to the high levels of illite weathering and progressive release of K.  Pshdar soils were highly crystalline, well-ordered illite with minimal expandable components hence the lowest exchangeable K (191.35 – 210 mg/kg) despite abundant illitic mineral content, indicating that crystallinity and structural order limits K release from interlayer positions. The results prove that K retention and availability in such calcareous soils depend on clay mineral composition, the degree of crystallinity, the weathering degree, rather than overall clay content. The nature of Pshdar soils being in need of intensive fertilization with K due to limited mineral releases of K, whereas Bitwen soils are partially able to depend on natural sources of K through continual weathering. The research will be of critical mineralogical importance in designing site-specific K management plans and achieving optimal efficiency on the fertilizer use in the semi-arid farming systems of the Kurdistan Region.