Because of their occurrence, high specific surface area and reactivity, poorly ordered secondary minerals are key actors controlling a lot of biogeochemical processes in soils. Among them, short-range ordered aluminosilicates form in volcanic ash soils and control in part the larger scale properties of these soils in term of hydraulic properties, ion retention and organic matter cycling. This chapter aims at revisiting the mineralogy and formation understanding of these imogolite-type materials in regard of their occurrence (soils sensus-stricto vs weathered tephra layers). Finally, their reactivity and their effects on soil properties are examined.