Thickness and size distribution of clay-sized soil particles measured through atomic force microscopy

Soil particle size distribution is a very important soil characteristic because it is related with many physical, chemical and biological processes occurring in soils. However, at the sub-micron range, where many important sorption and transformation processes occur, there are few techniques available for particle size characterization. Among the available methods are transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The classical TEM and SEM, however, are sometimes unable to clearly differentiate between agglomerates, particles and grains; and sample preparation is very difficult and tedious. To overcome these limitations, we have applied the atomic force microscopy (AFM) technique to analyze the particle size distribution of an oxisol from São Carlos site, in Brazil, at the sub-micron range. Thickness and diameter of soil particles deposited in freshly cleaved mica were measured for each individual particle, allowing to determine their particle size distribution. Assuming cylindrical shaped particles and a constant particle density of 2.7 g cm−3, the mass-based soil particle distribution at the sub-micron range is obtainable.