Measuring soil particle density using water pycnometer: Influencing factors, errors, and correction

The water pycnometer method, which has been widely used for measuring soil particle density (ρ_s), is subject to errors due to interactions between water molecules and clay minerals. In this study, we evaluated the influences of soil mineralogy and organic matter content on ρ_s and quantified the measurement errors of the water pycnometer by comparing with gas (He) pycnometer data obtained in 35 soils from Northeast China. The ρ_s results showed a significant positive correlation with the content of secondary minerals (including illite-smectite mixture, illite, kaolinite, and chlorite), displayed a negative correlation only with the primary mineral of plagioclase, and exhibited statistically insignificant correlations with soil organic matter content and other primary minerals (quartz and potassium feldspar). A mean ρ_s value of 2.644 g cm⁻³ was obtained from the He pycnometry measurements. The ρ_s data from the water pycnometry were generally higher than those from the He pycnometry, which was mainly attributed to the increased density of adsorbed water on clay minerals. A linear relationship between ρ_s and clay content was developed and verified, which could be applied to correct errors in water pycnometer data.