492 SSSAJ: Volume 74: Number 2 • March–April 2010 Soil Sci. Soc. Am. J. 74:492–494 doi:10.2136/sssaj2009.0047N Published Online 8 Jan. 2010 Received 5 Feb. 2009 *Corresponding author (Budiman.minasny@sydney.edu.au). © Soil Science Society of America 677 S. Segoe 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. The soil water characteristic curve (SWCC) is a funda- mental soil physical property for assessing soil water availability, water transport, and mechanical strength. Soil water characteristic curves can also be used to derive the unsaturated hydraulic conductivity and shear strength characteristics (Vanapalli et al., 1996). he standard method for determining the SWCC in the labo- ratory involves equilibrating soil samples at prescribed pressures us- ing a tension table or pressure plate. hese laboratory methods are time consuming because they require the soil to attain equilibrium with the applied pressure (Dane and Hopmans, 2002). To determine the SWCC more eiciently, a variety of sensors have been developed during past decades to simultaneously measure the volumetric water content (θ) and soil matric potential (ψ). An early design used a series of time domain relectometry (TDR) mini- probes together with a series of tensiometers inserted in the soil cylin- der at diferent depths (Malicki et al., 1992). Following that, several conigurations of combined tensiometer–TDR probes were proposed (Noborio et al., 1999; Or and Wraith, 1999; Vaz et al., 2002; Lungal and Si, 2008). With a similar intention, a TDR–pressure cell was re- cently tested across a matric potential range of 0 to −0.5 MPa (Moret- Fernández et al., 2008). Apart from the TDR sensor, frequency do- main (FD) and electric resistance sensors have also been embedded within porous materials to measure the SWCC (Rassam and Williams, 2000; Whalley et al., 2007; Xin et al., 2007). hese methods measure the water potential indirectly by determining the water content of the porous material, assuming that the porous material and soil samples have reached hydraulic equilibrium. In this study, a new sensor design that allows combined soil water content and potential measurement was developed. he sen- sor contains a single-electrode FD sensor integrated into a conven- tional water-illed tensiometer. he performance of the sensor for determining the SWCC was evaluated by comparing the results with those from a tension table and pressure plate apparatus on three soils of diferent textures. MATERIALS AND METHODS Probe Design Figure 1 shows schematically the structure and dimensions of the dual sensor. We used a commercial laboratory tensiometer from UGT, Model Tensio 130 (Müncheberg, Germany). he ceramic cup (23 mm long) of the tensiometer, with an air-entry value about 80 kPa, is attached to a stainless tube (37 mm long) that serves as the water reservoir. he tensiometer body is illed with water and a pres- sure transducer is attached at the end of the tube to measure the pres- sure within the system. Rather than building and attaching an additional moisture sen- sor to the ceramic cup or tensiometer, the stainless steel tube was used as an electrode for the water content sensor. herefore, in this design, no modiication was made to the tensiometer. he conventional FD sensor for measuring soil moisture con- tent is based on the impedance method, using two or more elec- trodes (Gaskin and Miller, 1996; Sun et al., 2005). In this study, however, we used the stainless tube of the tensiometer as a single electrode for measuring the radiation impedance. To this end, a radio frequency oscillator (100 MHz) is connected to the stainless steel tube via a coaxial cable. Based on the antenna theory ( Jordan and Balmain, 1968), the tube can be viewed as a microelement of a wave emitter with radiation impedance R rad of 2 2 rad 80 for L R d L ⎛ ⎞ = π << << λ ⎜ ⎟ λ ⎝ ⎠ [1] SOIL PHYSICS NOTE There is increasing interest in the development of a technique that can simultaneously measure soil matric potential and water content for rapidly determining a soil water characteristic curve. In this study, we developed a new combined soil water content and potential sensor. The new sensor adapted the stainless steel tube of a conventional water-filled tensiometer into a single sensing electrode for measuring soil water content. This novel design has the advantage of utilizing the whole length of the tensiometer without adding additional components to the sensor. To verify its feasibility, the sensor was tested on three soils (sand, sandy loam, and clay loam) while they were drying under laboratory conditions to produce the soil water characteristic curves in the matric potential range of 0 to -80 kPa. Water characteristic curves of the three soils were also obtained using standard laboratory techniques (tension table and pressure plate). The results show that water characteristic curves of the three soils from the new sensor are in good agreement with those obtained using the standard methods. A Combined Frequency Domain and Tensiometer Sensor for Determining Soil Water Characteristic Curves Yurui Sun Shujuan Ren College of Information and Electrical Engineering China Agricultural Univ. Beijing, P.R. China Tusheng Ren College of Resource and Environment China Agricultural Univ. Beijing, P.R. China Budiman Minasny* Australian Centre for Precision Agriculture The University of Sydney Sydney, NSW 2006, Australia Abbreviations: FD, frequency domain; SWCC, soil water characteristic curve; TDR, time domain reflectometry.