The erubáz volcanic soil of Hungary: Mineralogy and classification

Abstract

Abstract In this study the erubaz soil, a shallow soil influenced by volcanic parent rocks is examined with special emphasis on its clay content and clay-mineral-composition. Fifteen profiles at seven study areas and with four different parent material types are analysed. Bulk soil mineralogical and clay-mineralogical properties are examined by X-ray diffractometry (XRD). According to the mineralogical analysis the presence of the unstable mafic silicates indicates that the composition of the parent rock is still well reflected in the examined soil profiles. Illite is the most common clay mineral followed by kaolinite and smectite. Kaolinite and illite contain ca. 15–20% and 10% interlayered of smectite, respectively. Poor crystallisation and composition of secondary minerals suggest that these soils are in an early stage of their pedogenic evolution, but are more developed than the typical Andosols. Presence of opal-C and cristobalite in the andesitic samples is characteristic. Our study revealed that the erubaz soil type is not as homogeneous as it was suggested before. According to our analysis, mainly on the basis of their mineralogy, these soils can be divided into two well-defined groups. These two groups (erubaz soils formed on intermediate-acidic and on basic lithology) can be regarded as subtypes of the erubaz soil type. Moreover, two varieties (black and brown), mostly based on colour and SOM content, are distinguished. In these varieties the conditions of the decomposition of organic material are very different due to the moisture content, temperature and pH-conditions of the soil. The studied soil profiles were included into the WRB-system. On the basis of threshold-values of their diagnostic horizons, the studied profiles did not fulfil the criteria of the Andosol group and were inserted into the assemblage of Phaeozems.