Effects of tephra addition on soil processes in Spodosols in the Cascade Range, Washington, U.S.A.

Abstract A study was conducted in the Cascade Range of Washington State, U.S.A., to examine the effects of tephra additions on soil processes occurring in Spodosols. Unleached tephra from the 18 May 1980 Mt. St. Helens eruption was applied in 5 or 15 cm depths to soils that had escaped airfall tephra deposition in 1980 but had received periodic additions of tephra during the Holocene. Soil solutions were collected from the major genetic horizons of tephra-treated and non-treated profiles. Interpretation of soil solution composition provided an instantaneous record of the response of soil processes to the added tephra. Soluble salts, composed of basic cations (Ca 2+ = Na + > Mg 2+ > K + ) and SO 4 2− , were initially leached from the tephra. Basic cations in the leachate displaced H + and Al 3+ from the exchange sites in the original upper horizons. The soluble salts and Al 3+ migrated through the soil profile where their concentrations were greatly attenuated by interaction with the solid phase. Calcium, K + , Al 3+ and SO 4 2− were strongly retained in the B horizons whereas Na + , Mg 2+ and Cl − showed little affinity for the solid phase. Biological uptake, anion adsorption, exchange reactions and neoformation of minerals may all be responsible for the differential movement of solutes through the soil profile. No enhancement of elemental leaching was measured from the C horizon during the first four months of monitoring. Podzolization, the prevailing process in these mountains, was relatively unaffected by the addition of tephra.