Pedogenic Pathways in Andic Soils of the Northern Rocky Mountains (USA)

Core Ideas Andic soils are extensive in the Northern Rocky Mountains. Andic soils form via three primary pathways. Several decades of research on these soils are reviewed and summarized. Landscapes of the Northern Rocky Mountains Major Land Resource Area (MLRA 43A) are dominated by andic soils, which occupy more than 4.7 million ha in the region. This paper reviews research conducted on these soils over the past three decades and describes three primary pathways by which these soils form. Two of these pathways—allophanic and non‐allophanic—are similar in that they are variations of the general process of andisolization in which in situ weathering leads to the formation of various nanoscale minerals and metal‐organic complexes that impart andic soil properties. The third pathway is defined by the dominance of podzolization, in which distinct translocation of the weathering products has occurred and andisolization is a subsidiary process. The majority of andic soils in the region have silandic properties and have formed via the allophanic pathway. In areas where forest vegetation has been replaced by bracken fern communities, non‐allophanic soils form and develop aluandic properties in which the majority of secondary Al is contained in organic complexes. At higher elevations and sites with greater snowpack and effective precipitation, podzolization replaces andisolization as the dominant pedogenic process, resulting in the formation of Spodosols and spodic intergrades. Each of these three pedogenic pathways can be altered by disturbance. When disturbance is followed by erosion, diminishment or in extreme cases, complete loss of andic properties can occur.