Pedogenesis and surface charge of some Andic soils in Washington,U.S.A.

Chemical and surface charge characteristics were determined for three soils that were formed in a mantle of volcanic tephra along a bio-climatic gradient in forest in northcentral Washington state. The soils are Nevine silt loam (Typic Vitrandept), Manley silt loam (Entic Cryandept), and Moses silt loam (Andic Cryochrept). Selective dissolution analyses for Al, Fe, and Si suggests that the weathering products in the ash mantle are poorly crystalline aluminosilicates, hydrous and crystalline Fe-oxides, and Al-humus complexes in decreasing order of abundance. The chemical and physical data indicate that Moses silt loam has undergone a greater degree of weathering than either Nevine silt loam or Manley silt loam, and we attribute this to differences in the weathering environment. The measured points of zero charge parameters are typical for soils that are dominated by amphoteric surfaces: the soils exhibit charge reversal, and charge magnitude is dependent upon pH and electrolyte concentration. The point of zero salt effect ranged from 5.1 to 5.3, and the point of zero net charge ranged from 4.3 in Nevine silt loam to 5.4 in Moses silt loam. Pedogenesis has not caused large enough differentiation among the soils to allow us to use zero charge parameters as indicators of the degree of pedogenesis.