Particle Size Analysis by Hydrometer: A Routine Method for Determining Clay Fraction

The Day hydrometer method has a sound theoretical base and is widely accepted by soil laboratories as a reliable means of determining soil particle size distribution. To evaluate methods of routinely determining the clay fraction (< 0.002 mm), 288 soil samples, representing a broad range of texture and organic matter content, were analyzed by the Day method. Models were developed to correct the percentage clay determined by the Bouyoucos method (2 hour), and 4, and 6 hours of sedimentation to the Day method of determining percentage clay. To validate the models, results from the Simplified Day method and the Bouyoucos, 4‐, and 6‐hour correction equations were compared to the percentage clay from the Day method for an independent data set of 127 samples. The Simplified Day and corrected Bouyoucos methods proved most useful for routine analysis of the clay fraction, with respective r 2 values of 0.96 and 0.98. For rapid, routine work involving few hydrometer readings and calculations, the Simplified Day and the corrected Bouyoucos methods provide adequate approximations of the clay fraction. These methods will be useful in quantifying clay estimates in soil survey field offices and in converting existing hydrometer estimates to a similar datum. The methods described are not meant to replace the more rigorous pipet procedure.