Comparison of hydrometer settling times in soil particle size analysis

Abstract

Although soil texture is important to plant growth, cultivation, hydraulic conductivity, and soil strength, laboratory procedures for determining particle size distribution can be confusing. A number of settling times have been proposed for the hydrometer method used to analyze the fine earth fraction of soils. To separate sand and silt, hydrometer readings at 30 and 60 seconds, 35 seconds, or at 40 seconds have been recommended. To distinguish between silt and clay, recommendations have been made for readings at 6-8 hours and 12-15 hours, 1.5 and 24 hours, 2 and 24 hours or at 8 hours. In this study, no significant differences in estimates of sand content were found between readings made at 30 and 60 seconds and at 40 seconds. However, estimates from readings on both sides of the silt-clay separation (at 6 hours and 12 hours) showed a significant variation of clay content within the sample probably due to an inadequate method of splitting the soil samples into subsamples. Clay estimates from 2-hours readings differed significantly from the average estimate of the split sample 6/12-hours readings. Numerical differences were seen among particle size estimates from various methods; if the soil texture is near a division between 2 classes, these differences may result in different textures being assigned.