Mobilities and Related Factors of Chemical Elements in the Topsoils of Andosols in Tohoku, Japan

We conducted detailed mineralogical and chemical investigations into the clay, silt, and sand fractions separated from the topsoils of Andosols formed from felsic parent ash under a humid temperate climate. We concluded that the primary factors influencing the mobility sequence CaO, Na2O > SiO>2 > MgO > A12O3, Fe2O3, K2O in these soils are as follows: (1) Al2O3 and Fe2O3, very low solubility in soil solution; (2) K2O, preferential retention by volcanic glass and formation of a randomly interstratified chloritized 2:1 mineral-illite (formation of mica structure); (3) MgO, formation of 2:1 minerals (incorporation of magnesium into octahedral sheets); (4) SiO2, formation of 2: 1 minerals and possibly quartz; and (5) CaO and Na2O, facility in release from the parent ash, high solubility in soil solution, and absence of clay minerals containing significant amounts of both elements. Volcanic glass (silicic) appears to be the most important parent mineral, and its weathering and abundant formation of 2:1 minerals are strongly related to the mobility sequence of chemical elements we obtained, which is unique compared with other geochemical studies.