Effects of Long-Term Continuous Cropping of Sunflower on K Forms in Calcareous Soils of Western Azerbaijan Province Iran

Potassium forms and clay mineral composition of soils under sunflower cropping were compared to those adjacent virgin soils. For this purpose forty surface soil samples belonging to 10 soil series in Western Azerbaijan province, Iran were selected to determine the changes in K forms and K adsorption behaviour of the cultivated soils after longterm cropping. The samples were analyzed for soil physical and chemical properties, mineralogy of clay fraction, different forms of K, and K adsorption characteristics. The soils studied were alkaline and calcareous. Illite, illitesmectite and chlorite- kaolinite were the dominant clay minerals in Typic Xerocrepts, Typic Xerofluventsand Fluventic Xerocrepts, respectively. No changes in K-bearing minerals (illite) were detected due to cropping and K depletion. Soil solution K (So-K) constituted 1.65% of exchangeable K (Ex-K) and 4.35 % of non-exchangeable (NEx-K) for the cultivated soils and 2.54 % of Ex-K and 4.35 % of NEx-K for the adjacent virgin land. Significant declines in Ex-K content from 464 to 241 mg kg^-1 (48 %, on average), from 488 to 264 mg kg^-1 (46%, on average), were observed for Fluventic Xerocrepts and Typic Xerofluvents (P ≤ 0.01) soil series, respectively. Significant changes in the NEx-K content were observed after long-term cropping of sunflowers in Fluventic Xerocrepts (P≤0.05) but no changes in Typic Xerofluventsand Typic Xerocrepts of the soils. A highly significant positive relationship (r^2 = 0.70, P≤0.01) was observed between NEx-K and illite contents, indicating that this form of K is mainly released from the frayed edges of illite. Paired t-test revealed that in Fluventic Xerocrepts and Typic xerofluvents, K adsorption significantly was increased (P≤0.01) and exchangeable K was decreased as a result of exhausting cropping of sunflower.