Measuring soil water content through volume/mass replacement using a constant volume container

• A CVC is designed for accurately measuring volumes of liquid and solid mixture.
• Procedure and algorithm are detailed for soil water, particle density measurements.
• Laboratory experiments are made to demonstrate the method.
• CVC volume replacement can replace oven-drying method and be used in field.

Accurate and rapid measurement of soil water content is important in research and applications, such as hydrological processes, farmland water management, and ecological issues. Soil particle density (SPD) and water content were measured using a proposed constant volume container (CVC) through volume/mass replacement (VMR). The CVC consisted of a vessel body and a lid to limit the water surface to a constant level, with 3-mm-diameter inlet to fill up the container with water to the full level as well as an outlet orifice to drain excess water. The mathematical algorithms for partitioning water and soil particles of the soil sample were detailed. Measurements and computational procedures were performed on two soils with varied water contents. The unique structure of the CVC effectively eliminated volume measurement errors caused by liquid surface tension, resulting in 50% lower measurement standard deviation of SPD compared with those obtained using a pycnometer or a volumetric flask. The SPD values of loam and clay loam soil were 2.64 and 2.66 Mg m− 3, respectively. The soil water contents of the tested soils determined with the measured SPDs by using the CVC exhibited equivalent accuracies to those measured through oven-drying, with R2 = 0.98. Moreover, soil water content (5–35%) measured with an assumed SPD of 2.65 through VMR using the CVC showed an equivalent accuracy to that determined through oven-drying. This finding indicated the feasibility of measuring soil water content through VMR using the SPD of 2.65 Mg m− 3 in these two experimental soils. Theoretical analysis and computation showed that soil organic matter content (SOM) affected SPD, thereby indirectly influenced the accuracy of soil water content measurements through VMR. The VMR method with the CVC could achieve acceptable accuracy with SOM values lower than 2% and soil water content higher than 5%. This approach can be used as an alternative to oven-drying with appropriate accuracy and efficiency, and it is potentially useful for measuring soil water content or bulk density in the field.