Evolution of structure in a chronosequence of andesitic forest soils

Using scanning electron microscopy, westudied changes in soil structure in achronosequence of four andesitic forestsoils at Mt. Shasta. The soils (Xeropsam-ments) were 55 yr old ( flow A), —300 yrold ( flow B), and an estimated severalthousand years old ( flows D and E).
Soil skeleton in flow A consisted ofcoarse particles whose accessible surfacewas partially or entirely covered by silt-and clay-size particles. Weathering prod-ucts and organic materials were adsorbedon accessible surfaces in isolated thinpatches. An abrupt boundary at the 10-cmdepth separated a friable surface horizonfrom a cemented substratum. The bound-ary was deeper and more diffuse in flowB. Both flows appear to have been ce-mented initially, probably by interstitialsedimentation and deposition of fine par-ticles around coarse grains as waterdrained from the soil. The soils were ap-parently further stabilized by precipita-tion of solutes at zones of interparticle con-tact as water evaporated from storagepores. Soil in flows D and E was friablethroughout the profile. The main struc-tural units were large grains entirely cov-ered with silt- and clay-size particles andamorphous coatings stabilized by weath-ering products and organic matter. Dis-crete microaggregates were relatively un-common. Such formation and stabilizationof structure may be characteristic of an-desitic parent material, which weathersmainly to allophane, halloysite, and chlor-itic intergrades, rather than to high-charge phyllosilicate clays.