Wang et al. (2011) Soil Sci Soc Amer J

Wang, Lu, Ren and Li
Bound water content of air-dry soils measured by thermal analysis
Soil Sci Soc Amer J

Conventional oven-drying method may not give accurate information of bound water content in soils. The objective of this study was to determine the fraction of soil water that is bound to soil colloids using the thermogravimetry (TG) technique. A heating program from room temperature (RT) to 200°C was developed to partition bound water into loosely bound water (equivalent to water content determined by conventional oven-drying method) and tightly bound water that was not accounted by conventional oven-drying method on original and organic matter (OM) removed soil samples. Nine air-dry soils, with varying amounts of clay and OM contents, were tested. For the original air-dry samples, bound water content ranged from 0.54 to 5.22%, and the fractions of loosely and tightly bound water were about 80 and 20%, respectively. On a mass fraction basis, the specific water adsorption capacity of soil OM was 10 to 40 times that of soil minerals. For most mineral soils, however, the contribution of soil minerals to bound water (>70%) was much larger than that of soil OM (<30%), since the mineral fraction usually dominates over the OM fraction. In a soil with high OM concentration but a relatively low clay content, OM contribution to bound water exceeded 50%. Soil specific surface area (SA) showed a strong influence on bound water content, and a linear relationship between SA and bound water content was established.