SSSAJ: Volume 75: Number 2 • March–April 2011 481 Soil Sci. Soc. Am. J. 75:481–487 Posted online 3 Jan. 2011 doi:10.2136/sssaj2010.0065 This research is supported by the Natural Science Foundation of China under Grant No. 41071155 and the Basic Research Development Program of China (973 Program, no. 2009CB118607). Received 5 Feb. 2010. *Corresponding author (tsren@cau.edu.cn). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Bound Water Content of Air-Dry Soils Measured by Thermal Analysis Soil Physics T he structure, energy, and properties (e.g., speciic heat capacity and dielectric permittivity) of soil water vary with the distance from solid surface (Etzler, 1983; Yamaguchi, 1959). Water bounded in the irst few layers is assumed to have stronger bonding energy (Panchev et al., 2005) and therefore diferent structure than the water in bulk (Bogdan et al., 1996; Kleinberg and Griin, 2005). Soil water content is usually referred to the quantity of water that is evaporated from soil by heating the sample at 100 to 110°C (average 105°C) until there is no further weight loss. Some research has implied that the drying temperature of porous me- dium perhaps should be higher than 105°C. For example, thermo-analytical stud- ies of water on activated alumina showed that the drying temperature for this ma- terial should be 120 to 125°C (Hampson and Bleam, 1996). Bogdan et al. (1996) measured water content of pyrogenic silica powder by drying at 200°C. Gardner et al. (2001) pointed out that heating at temperature range of 110 to 160°C was required to remove bound water from clay surfaces. Logsdon and Laird (2004) dried clay samples at 190°C. Mitchell and Soga (2005) stated that depending on soil texture, the drying temperature for mineral soils should be in the range of 100 to 300°C. hus it is questionable if the conventional method of oven-drying soil samples at 105°C is suicient for determining the amount of bound water in soils. Several methods have been applied to determine water content in solid surface, such as the cryoscopic method, the calorimeter method, the centrifugation method, the dilatometric method (Smith and Vesilind, 1995), the heat capacity Yajing Wang Dep. of Soil and Water Sciences China Agricultural Univ. Beijing, China 100193 Sen Lu Research Institute of Forestry Chinese Academy of Forestry Beijing, China 100091 Tusheng Ren* Baoguo Li Dep. of Soil and Water Sciences China Agricultural Univ. Beijing, China 100193 Conventional oven-drying method may not give accurate information of bound water content in soils. he 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 speciic 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 speciic surface area (SA) showed a strong inluence on bound water content, and a linear relationship between SA and bound water content was established. Abbreviations: DSC, diferential scanning calorimetry; DTA, diferential thermal analysis; EGD, evolved gas detection; MS, mass spectrometry; OM, organic matter; RT, room temperature; SA, speciic surface area; SAR, speciic adsorption ratio; TG, thermogravimetry.