New Polymer Tensiometers: Measuring Matric Pressures Down to the Wilting Point

Tensiometers are commonly used for measuring soil water matric pressures. Unfortunately, the water-filled reservoir of conventional tensiometers limits their applicability to soil water matric pressures above approximately20.085 MPa. Tensiometers filled with a polymer solution instead of water are able to measure a larger range of soil water matric pressures. We designed and constructed six prototype polymer tensiometers (previously called osmotic tensiometers) consisting of a wide-range pressure transducer with a temperature sensor, a stainless steel casing, and a ceramic plate with a membrane preventing polymer leakage. A polymer chamber (0.1–2.2 cm) was located between the pressure transducer and the plate. We tested the polymer tensiometers for long-term operation, the effects of temperature, response times, and performance in a repacked sandy loam under laboratory conditions. Several months of continuous operation caused a gradual drop in the osmotic pressure, for which we developed a suitable correction. The osmotic potential of polymer solutions is temperature dependent, and requires calibration before installation. The response times to sudden and gradual changes in ambient temperature were found to be affected by polymer chamber height and polymer type. Practically useful response times (,0.2 d) are feasible, particularly for chambers shorter than 0.20 cm. We demonstrated the ability of the instrument to measure the range of soil water pressures in which plant roots are able to take up water (from 0 to 21.6 MPa), to regain pressure without user interference and to function properly for time periods of up to 1 yr. THE DRIVING FORCE for water flow in soils is the gradient in the total water potential, ct (J kg). This potential is defined as the amount of useful work per unit mass of pure water that must be done by means of externally applied forces to transfer reversibly and isothermally an infinitesimal amount of water from a reference reservoir to the soil liquid phase at the point under consideration (Bolt, 1976). The total water potential is generally written as the sum of constituent potentials, most commonly the gravitational (cg), matric (cm), and osmotic (co) potentials, as follows: ct 5 cg 1 cm 1 co [1] Equation [1] assumes that the soil liquid phase is dilute enough so that the osmotic potential can be added to the other potentials (Corey and Klute, 1985; Hillel, 1998). In this study, the soil water potential is expressed in terms of energy per volume (pressure equivalent). Tensiometers are widely used for measuring soil water matric pressures under field and laboratory conditions. Its measurement principle is based on the use of a waterfilled reservoir enclosed by a pressure transducer and a water-saturated ceramic tip that is in contact with the soil. Equilibrium between the liquid phase in the soil and in the tensiometer makes it possible to determine the soil water matric pressure. Direct measurement of matric pressures with conventional tensiometers (CTs) is restricted to values greater than approximately20.085MPa (Cassel and Klute, 1986; Koorevaar et al., 1983; Young and Sisson, 2002). Most plants can take up water down to matric pressures of about 21.6 MPa. To determine the matric pressures below the tensiometer range, thermocouple psychrometers and relative humidity sensors can be used; however, thermocouple psychrometers have a slow response and are subject to significant measurement errors above 21.0 MPa, while relative humidity sensors seem to be more suitable for measurements below 22.0 MPa (Andraski and Scanlon, 2002; Agus and Schanz, 2005). Other field methods measure the volumetric water content and infer the matric pressure from the soil water characteristic (Klute, 1986; Dane and Hopmans, 2002). Recognizing the desirability of direct matric pressure measurements throughout an extended range, Peck and Rabbidge (1966, 1969) proposed an osmotic tensiometer. Its principle is based on the osmotic potential of a highly concentrated hydrophilic polymer solution. The soluble polymer molecules are retained inside the osmotic tensiometer by a ceramic membrane that is permeable to the soil solution, but impermeable to the polymers. The osmotic potential of the polymer solution strongly reduces the total water potential inside the osmotic tensiometer (Hillel, 1998). The reduction in the total water potential then causes buildup of a positive pressure inside the tensiometer. Consequently, water in an osmotic tensiometer at equilibrium with a drying soil will cavitate at a much lower soil water pressure than essentially pure water in a CT. The positive pressure inside the osmotic tensiometer can thus be related to the negative matric pressure. Although Peck and Rabbidge (1966, 1969) were able to construct an osmotic tensiometer capable of measuring matric pressures in the range of 0 to 21.5 MPa, their instrument suffered from slow equilibration times G. Bakker, ALTERRA, Wageningen Univ. and Research Centre, Droevendaalsesteeg 4, 6708 PB, Wageningen, the Netherlands; M.J. van der Ploeg, G.H. de Rooij, and H.P.A. Gooren, Dep. of Environmental Sciences, Soil Physics, Ecohydrology and Groundwater Management Group, Wageningen Univ. and Research Centre, Droevendaalsesteeg 4, 6708 PB, Wageningen, the Netherlands; C.W. Hoogendam and L.K. Koopal, Lab. for Physical Chemistry and Colloid Science, Wageningen Univ. and Research Centre, Dreijenplein 6, 6703 HB, Wageningen, the Netherlands; C. Huiskes and H. Kruidhof, Faculty of Science and Technology, Twente Univ., Langezijds LA 1727, P.O. Box 217, 7500 AE, Enschede, the Netherlands. Received 10 Aug. 2006. *Corresponding author ([email protected]). Published in Vadose Zone Journal 6:196–202 (2007). Original Research doi:10.2136/vzj2006.0110 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: CT, conventional (water-filled) tensiometer; POT, polymer tensiometer; TDR, time domain reflectometer. R e p ro d u c e d fr o m V a d o s e Z o n e J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 196 Published online February 27, 2007