LASER GRAIN-SIZE DETERMINATION IN SOIL GENETIC-STUDIES .2. CLAY CONTENT, CLAY FORMATION, AND AGGREGATION IN SOME BRAZILIAN OXISOLS

Grain-size determination by laser diffraction can be a very useful too l in soil genetic studies. In three deeply weathered soil profiles fro m Brazil, we have been able to identify changes in clay content, grain -size distribution of the clay fraction, and disappearance of silt fra ctions as a result of weathering. The grain-sizer is also a useful too l in studies of soil aggregation. In the studied soils we found a clea r change of aggregation with depth, Sequential removal of organic matt er and free iron indicated that the aggregation was attributable to or ganic matter rather than to sesquioxides. Good correlations were found between pipette-clay and laser-clay, but there was never a 1:1 correl ation. For very clayey samples, maximum information is obtained by mea suring at two optical densities of the suspended material. The laser g rain-sizer provides much more detailed information than classical meth ods and is very useful for identifying particle size shifts, especiall y in the smallest size fractions.