Effect of continuous cropping on potassium forms and potassium adsorption characteristics in calcareous soils of Iran

The potassium (K) status of 5 major soil types, under sugar beet cultivation for 35-40 years, and adjoining virgin lands were compared. Fifty-nine paired surface soil samples belonging to 24 soil series from the major sugar beet growing areas in Western Azarbaijan province located in the north-west of Iran were studied to ascertain whether relative K form sizes and K adsorption behaviour of the soils are being changed by long-term cropping. The samples were analysed for soil physical and chemical properties, mineralogy of clay fraction, different forms of K, and K adsorption characteristics. The soils are alkaline and calcareous. The K-bearing mineral illite is the dominant clay mineral in Typic calcixerepts, Typic haploxerepts, and Typic endoaquepts, whereas in Vertic calcixerepts and Vertic endoquepts, the clay mineralogy is dominated by smectite. A highly significant relationship (r² = 77, P [less-than or equal to] 0.001) was observed between CEC and smectite content. No changes in K-bearing minerals were detected due to cropping and K depletion. Soil solution K (So-K) constituted 1.7% of exchangeable K (Ex-K) and 1.4% of non-exchangeable K (NEx-K) for the cultivated soils and 1.8% of Ex-K and 2% of NEx-K for the adjacent virgin land. A pronounced significant decline (P [less-than or equal to] 0.001) in the Ex-K content from 507 to 270 mg/kg (a drop of 45%) and a less significant degree (P [less-than or equal to] 0.05) in the So-K content from 28 to 12 mg/L (a drop of 55%) were detected in the soil series of Typic calcixerepts as a result of long-term cropping. Continuous sugar beet cropping did not result in changes in the NEx-K contents in any soils except in Vertic endoaquepts containing low illite. A highly significant positive relationship (r² = 0.79, P [less-than or equal to] 0.001) was observed between values of 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 continuous sugar beet cultivation increased significantly (P [less-than or equal to] 0.01) K adsorption in Typic calcixerepts, Typic haploxerepts, and Vertic endoaquepts, where exchangeable K was decreased as a result of intensive cultivation.