Volume replacement method for direct measurement of soil moisture and bulk density

Direct measurement of soil moisture and bulk density is the foundation of related research and applications, which is of great importance in such studies and applications as soil mechanics, crop production, irrigation and ecological environments. This paper presents a new method for direct measurement of soil moisture and soil bulk density based on volume replacement, which is equivalent to the conventional oven-dry method. The measurement principle is based on the model of soil three-phase composition, soil particles, water and air. Its volume and mass are partitioned into three fractions of these substances. The initial soil mass is approximately determined by the soil particles and water when the air mass is neglected. The measurement principles are as the follows. The samples used for measurements of moisture content and bulk density were taken by corers of known volumes and were weighted to determine their initial masses before they were filled with water to saturation state to replace all the air-filled spaces in the soil samples. The initial/original weight of the soil sample and that after saturation were used to compute the water mass required to replace the air-filled volume. Under the known soil particle density, the given volume as defined by the corer and the determined air-filled space volume, the original soil water content and soil bulk density can be calculated. An experimental system and the related algorithm procedures were proposed for the direct measurement of water content and soil bulk density. The standard soil corers were used to prepare the soil samples under designed bulk densities. The saturators were used to pre-saturate the soil samples. The samples were dripped with water to ensure their saturation during the measurement. Three soil materials, a clay loam from Yangling of Shaanxi province, a silt loam from Beijing and a red clay from Jiangxi province, were used for the demonstrational experiments to illustrate the measurement procedures and to verify the method. Seven different initial soil water contents, about 2%-3%, 5%, 10%, 15%, 25%, 30% and saturated water content, and three bulk densities of about 1.25, 1.35 and 1.45 g/cm3 were adopted in the experiments. These samples of the different initial water contents and soil bulk densities were used to measure the soil moisture by the volume replacement method. Detailed experimental procedures were presented to illustrate the experimental procedures. Conventional oven-dry method was used to determine parallelly the moisture contents of the soil samples similarly prepared. The water content changes when samples were oven-dried for 8, 12, 24, 48 h were measured to determine the influences of oven-dry period. The measured data showed that the soil water contents measured by the volume replacement were 2%-3% higher than those by the oven-dry method for a drying period of 8 h and were about 1% higher than those by the conventional oven-dry method for a drying period of 48 h. In addition, the traditional drying method is more time-consuming and not suitable for the field work. This indicates oven-dry method needs extended period of time to dry the soil sample. The results verify the newly-suggested measurement method and the experimental procedures. In general, the experimental operation of the volume replacement method is simple, time efficient, low energy consumption and high accuracy.