Three main stages are involved in particle-size analysis: pretreatment of samples to remove cementing and flocculating agents, sample dispersion and soil fraction quantification. This quantification requires the previous separation of the soil fractions. According to the size, coarse and fine sand fractions are separated by sieving, while silt and clay are separated by sedimentation. Some methods propose to calculate the silt proportion by subtracting the other soil separates from the original sample. In the Soil Physics Laboratory at the Federal University of Viçosa, Brazil, an additional suspension (silt + clay) is collected to increase the accuracy in the particle-size analysis. Methods that do not take this aspect into account overestimate the silt proportion by accumulating all experimental errors in this fraction. It can be concluded that the silt + clay determination and resulting calculations lead to better results in the soil particle-size analysis as they distribute the experimental error among the four granulometric fractions.
soil separates; soil fraction; mechanical analysis; pipet method
