This method determines the approximate proportion of sand (2000-50um), silt (50-2.0um) and clay(<2.0um) particles in a soil. Forty grams of air dried (crushed to <2mm) soil is shaken for 16 hr with 100 ml of 5% sodium hexametaphosphate. The suspension is quantitatively transferred to a sedimentation cylinder and brought to a total volume of 1L with deionized water. After a 2 hr temperature equilibration, the suspension is stirred vigorously for one minute to re-suspend the particles. An ASTM No. 152H hydrometer is carefully placed in the suspension and used to take two readings, one at 40 sec. and another at 6-8 hours (exact time depends on the temperature of the suspension). The percentage of sand, silt and clay in the soil is calculated from the resulting hydrometer readings.
(See Method Reference No. 64)