LASER DIFFRACTION GRAIN-SIZE CHARACTERISTICS OF ANDISOLS IN PERHUMID COSTA-RICA - THE AGGREGATE SIZE OF ALLOPHANE

Grain sizes of eight profiles from a soil catena (630-3160 m) on andes itic ash in perhumid atlantic Costa Rica were investigated by laser di ffraction grain-sizing. The samples were (1) shaken with water, (2) tr eated with peroxide, and (3) treated with peroxide-oxalate. Allophane is absent above 2000 m; it appears around 2000 m and increases downwar ds. Peroxide treatment and oxalate extraction indicate that the alloph ane fraction occurs in aggregates of well-defined sizes. The size of t he allophane aggregates is between 2 and 20 mu m, and it increases wit h increasing allophane content. Considerable crystalline clay fraction is only obtained upon oxalate extraction. Repeated oxalate extraction does not remove all allophane in high-allophane profiles. The method further allows the recognition of a large number of ash deposits and o f recent ash additions, thus facilitating the recognition of stratifie d profiles.