Impact of Clay Mineralogy and Landscape on Potassium Forms in Calcareous Soils, Urmia Region

The potassium pools of five major physiographic units of Urmia region in Western Azarbaijan Province were studied to investigate the distribution of K forms as a function of clay mineralogy and physiographic units. For this, soil samples from different horizons of ten pedons were selected and analyzed for physiochemical properties, mineralogy of clay fraction, and forms of K. Analyses of XRD revealed that the soils are similar in their clay mineralogy, consisting of illite, smectite, chlorite, and kaolinite, but with different in contents. Illite was higher in the Piedmont Plains (PP) followed by Plateaux (Pl), River Alluvial Plains (RAP), Colluvial Alluvial Plains (CAP), and Lowlands (LL) units, whereas a higher content of smectite was observed available in the Pl followed by PP, LL, RAP, and CAP units. Several such simultaneous processes as mineral weathering and soil forming, biocycling processes, and geomorphologic conditions had resulted in significant difference in K forms in the soils studied. A wide variation in total K (HF-extractable K) (0.54-1.1%), non-exchangeable K (280-450 mg kg-1), and exchangeable K (217-330 mg kg-1) were recorded among different physiographic units. Statistically significant differences (P≤ 0.05) were observed in the levels of mineral K, HNO3-extractable K, and non-exchangeable K in soils of high illite (30-50%) as compared with those of low illite contents (10-30%). A highly significant positive relationship was recorded between total K and illite content (r2= 0.85, P 0.001) and as well between non-exchangeable K and illite contents (r2= 0.84, P 0.001) suggesting that these pools of K are mainly released from the frayed edges and wedge zones of illite. The soils were categorized into two major groups based on the physiographic unit's assertion and soil solution K content: first group comprised of Pl and PP units while the second group of RAP, CAP, and LL units.