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 tool in soil genetic studies. In three deeply weathered soil profiles from Brazil, we have been able to identify changes in clay content, grain-size distribution of the clay fraction, and disappearance of silt fractions as a result of weathering. The grain-sizer is also a useful tool in studies of soil aggregation. In the studied soils we found a clear change of aggregation with depth, Sequential removal of organic matter and free iron indicated that the aggregation was attributable to organic matter rather than to sesquioxides. Good correlations were found between pipette-clay and laser-clay, but there was never a 1:1 correlation. For very clayey samples, maximum information is obtained by measuring at two optical densities of the suspended material. The laser grain-sizer provides much more detailed information than classical methods and is very useful for identifying particle size shifts, especially in the smallest size fractions.