Andic properties in soils developed from nonvolcanic materials in Central Bhutan

A number of soils are described in the literature as having andic and spodic soil properties, but have developed in nonvolcanic and nonallophanic materials and lack typical Podzol eluvial and illuvial horizons. They cover a wide range of parent materials and different types of climate. They have always been regarded as restricted to small areas. They were assigned to Andisols/Andosols, Podzols/Spodosols, or andic Inceptisols in the WRB and Soil Taxonomy and sometimes also named Cryptopodzols or Lockerbraunerden. Recent soil surveys in Bhutan, E Himalayas, show these soils are widespread at altitudes between 2200–3500 m asl and are spanning several bioclimatic zones. The aim of this study is the detailed characterization of specific properties and processes of formation by physical and chemical analyses, NMR spectroscopy, column experiments, SEM, XRD, and ¹⁴C dating in one of these soils in E central Bhutan. The results indicate advanced soil development with high amounts of oxidic Fe and Al compounds, low bulk densities (partly <0.5 g cm^–3), P retention >85%, and a dominance of Al‐hydroxy‐interlayered phyllosilicates. Scanning electron microscopy of sand fractions indicate microaggregates highly resistant to dispersion. Column experiments show podzolization with mobilization and translocation of DOM, Fe, and Al. Nuclear‐magnetic resonance spectroscopy and ¹⁴C ages of 16,000 BP indicate stabilization of DOM. Applying classification criteria, these soils appear to have andic and spodic features, but are neither Andosols nor Podzols senso strictu. Especially the role of Fe seems to be underestimated with regard to the specific soil‐forming processes. Because of their widespread occurrence and distinct properties, we suggest either a simplification of the criteria for existing soil types or a clearly defined separation of volcanic and nonvolcanic/nonallophanic Andosols.