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

Abstract. The potassium (K) status of 5major 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 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 0.001) in the Ex-K content from 507 to 270mg/kg (a drop of 45%) and a less significant degree (P 0.05) in the So-K content from 28 to 12mg/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 highl