Magnetic and chemical parameters of andic soils and their relation to selected pedogenesis factors

• Grain-size sensitive magnetic parameters show positive correlation with precipitation.
• Even low values of frequency-dependent susceptibility show influence of pedogenesis.
• Magnetic S-ratio and Fe extraction parameters determine the alu-andic subtype.

In the case of andic soils − developed on volcanic rocks − permanently humid conditions are essential for andosolisation processes. Although volcanic rocks are usually rich in highly magnetic iron oxides, knowledge about these minerals in andic soils with respect to the soil development is missing. The aim of this contribution is to investigate the relationship between magnetic and basic chemical properties of soils developed on volcanic basement and parameters related to the pedogenesis factors. The studied sites are located on basaltic lava flows in the eastern part of the French Massif Central. Investigated pedons with alu-andic, sil-andic, and vitric properties were pedologically described and analysed by a set of magnetic and soil-chemical methods. Magnetic parameters include low-field volume-specific and mass-specific magnetic susceptibility, frequency-dependent magnetic susceptibility, remanent properties (including S-ratio) and coercivities. Chemical parameters included the pHH2O, soil organic matter, total cation exchange capacity, base saturation, and content of Fe and Al extracted in dithionite-citrate, acid-ammonium oxalate, and pyrophosphate solutions. Micro-morphology of Andosols and parent material was observed by scanning electron microscopy, completed with energy dispersive spectrometry. The pedogenesis factors are represented by parent-rock age, annual precipitation, and soil thickness. Our findings suggest that (1) precipitation is the most important pedogenesis factor with a strong relationship to the relative magnetic grain-size, concentration of ferrimagnetic minerals, aluminium dissolved in pyrophosphate and acid-ammonium oxalate, and content of organic matter; (2) parent-material age shows a well-pronounced relationship with magnetic grain-size parameters represented by the frequency-dependent susceptibility and ratio of saturation remanent to saturation induced magnetisation (Mrs/Ms), selective dissolution parameters, and pHH2O; (3) thickness of soil profile shows a link to Mrs/Ms, pHH2O, and content of organic matter.