2 Matric Potential Chris E. Mullins University of Aberdeen, Aberdeen, Scotland I. INTRODUCTION The total potential c t of soil water refers to the potential energy of water in the soil with respect to a defined reference state. Various components of this potential control water flow in the soil (Chaps. 4, 5, and 6), from the soil into roots, and through plants. Matric potential refers to the tenacity with which water is held by the soil matrix (Marshall, 1959). In the absence of high concentrations of solutes, it is the major factor that determines the availability of water to plants. After al- lowing for differences in elevation, differences in matric potential between differ- ent parts of the soil drive the unsaturated flow of soil water (Chap. 5). A. Definition The soil physics terminology committee of the ISSS provided agreed-upon defi- nitions for total potential and its various components (Aslyng, 1963), which were slightly modified in 1976 (Bolt, 1976). A brief summary is given here. More de- tailed discussions of the meaning and significance of these definitions are given in soil physics books such as those of Marshall et al. (1996) and Hillel (1998). Total potential of soil water can be divided into three components: c ϭ c ϩ c ϩ c (1) tpgo The pressure potential c p is defined as ‘‘the amount of useful work that must be done per unit quantity of pure water to transfer reversibly and isothermally to the soil water an infinitesimal quantity of water from a pool at standard atmospheric pressure that contains a solution identical in composition to the soil water and is Copyright © 2000 Marcel Dekker, Inc. at the elevation of the point under consideration’’ (Marshall et al., 1996). Similar definitions have been given for gravitational potential, c g , and osmotic poten- tial, c o , which refer to the effects of elevation (i.e., position in earth’s gravita- tional field) and of solutes on the energy status of soil water. The sum of gravi- tational and pressure potential is called the hydraulic potential c h . Differences between the hydraulic potential at different places in the soil drive the move- ment of soil water. Matric potential c m is a subcomponent of pressure potential and is defined as the value of c p where there is no difference between the gas pressure on the water in the reference state and that of gas in the soil. The above definition of pressure potential includes (1) the positive hydro- static pressure that exists below a water table, (2) the potential difference experi- enced by soil that is under a gas pressure different from that of the water in the reference state, and (3) the negative pressure (i.e., suction) experienced by soil water as a result of its affinity for the soil matrix. In the past, some authors (Taylor and Ashcroft, 1972; Hanks and Ashcroft, 1980) have used the term ‘‘pressure potential’’ to refer only to subcomponents 1 and 2. However, all authors use equivalent definitions for matric potential, which is subcomponent 3. Matric po- tential can have only a zero or negative value. As water becomes more tightly held by the soil its matric potential decreases (becomes more negative). Matric or soil water suction or tension refers to the same property but takes the opposite sign to matric potential. In a swelling soil, overburden pressure can cause a slight error in applications where it is intended to relate matric potential to soil water content (Towner, 1981). The sum of matric and osmotic potential is called the water potential c w and is directly related to the relative humidity of water vapor in equilibrium with the liquid phase in soils and plants. c w is an important indicator of plant water status and is also important in saline soils, where the osmotic potential of the soil solution is sufficient to influence plant water uptake. B. Units Since potentials are defined as energy per unit mass, they have units of joules per kilogram. However, it is also possible to define potentials as energy per unit vol- ume or per unit weight. Thus, since the dimensions of energy per unit volume are identical to those of pressure, the appropriate unit is the pascal (1 bar ϭ 100 kPa). Similarly, the dimensions of energy per unit weight are identical to those of length, so the appropriate unit is the meter. Because it is common to refer to the pressure due to a height h of a column of water as a pressure head (or simply head) h,this term is often used to describe the potential energy per unit weight. The relation c (m) Ϫ1 c (J kg ) Ϫ gc (Pa) ϭ (2) g 66 Mullins Copyright © 2000 Marcel Dekker, Inc. where g is the density of water and g is the acceleration due to gravity (ϳ 1000 kg m Ϫ3 and 9.81 m s Ϫ2 , respectively), is used to convert potentials from one set of dimensions to another. A logarithmic (pF) scale (Schofield, 1935), where pF ϭ log (negative pressure head in cm of water) (3) 10 has also been used. II. AN OVERVIEW OF METHODS FOR MEASURING MATRIC POTENTIAL The main features of methods for measuring matric potential and the addresses of some manufacturers and suppliers are given in Table 1. The web sites for many of the manufacturers list their suppliers in many countries. In considering the cost of instruments, it is important to decide whether a data logger is required, and to consider the cost of the logger or meter as well as the cost of the sensor, since some sensors are more easily logged than others and some are available with cheap loggers. Consequently Table 1 should be treated only as an initial guide to purchase, because of the pace of development in the choice of loggers and meters. There are many earlier reviews of the design and use of such methods (Marshall, 1959; Rawlins, 1976; Cassell and Klute, 1986; Rawlins and Campbell, 1986). Methods have been classified according to the measurement principle involved and are discussed in detail in the following sections. Tensiometers (Sec. III) con- sist of a porous vessel attached via a liquid-filled column to a manometer. Porous material sensors (Sec. IV) consist of a porous material whose water content varies with matric potential in a reproducible manner; a physical property of the material that varies with its water content is measured and related to matric potential using a calibration curve. Psychrometers (Sec. V) measure the relative humidity of water vapor in equilibrium with the soil solution. Because they measure the sum of matric and osmotic potentials, they are also readily applicable for measurements in various parts of plants. There have been large improvements in the performance and availability of data loggers over the past ten years, some improvements in methods for measur- ing potential, and a growing use and awareness of the importance of measure- ments of potential. Despite this, there is still a need for a single sensor that can log matric potential to a field accuracy that is sufficient for understanding water move- ment and soil aeration under wet conditions (e.g. 0 to Ϫ100 Ϯ 0.2 kPa) while being able to measure to a reasonable accuracy (say Ϯ 5%) down to ϽϪ1.5 MPa. This is a tall order, but it explains the continuing interest in the osmotic tensiom- eter and improved porous material sensors. Matric Potential 67 Copyright © 2000 Marcel Dekker, Inc. III. TENSIOMETERS A tensiometer consists of a porous vessel connected to a manometer, with all parts of the system water filled (Fig. 1). When the cup is in contact with the soil, films of water make a hydraulic connection between soil water and the water within the cup via the pores in its walls. Water then moves into or out of the cup until the (negative) pressure inside the cup equals the matric potential of the soil water. The following equations are used to obtain matric and hydraulic potential from the mercury manometer readings shown in Fig. 1. h Ϫ 12.6b Ϫ c c ϭ m g Ϫ(12.6b ϩ c) c ϭ (4) h g The factor of 12.6 is the difference between the relative densities of mercury and water. c is a factor to correct for the capillary depression that occurs at the mercury–water interface. If g is omitted from these two equations, they will give the potentials in head units. 68 Mullins Fig. 1 Mercury manometer tensiometer. Copyright © 2000 Marcel Dekker, Inc. Tensiometers are also available with Bourdon vacuum gauges, with pressure transducers (for data logging), and for portable use. Cassell and Klute (1986) pro- vide a good discussion of methods for installing and maintaining tensiometers. I have discussed limitations common to most designs before considering each type of tensiometer. A. Design Limitations 1. Trapped Air All water-filled tensiometers have a lower measuring limit of about Ϫ85 kPa be- cause, at more negative potentials, there is a tendency for air bubbles to nucleate at microscopic irregularities within the instrument. At such a low pressure relative to atmospheric pressure these bubbles expand, augmented by dissolved air coming out of solution, and can eventually block the tubing, making further readings un- reliable. Filling with deaired water, which has had some of its dissolved air re- moved by boiling or by leaving it for some hours under a vacuum, is done to counteract this effect. Despite this, because dissolved air tends to move into the porous cup and come out of solution, tensiometers often incorporate an air trap that allows air to collect without blocking the instrument (Fig. 1). However, since this air causes the reponse time to increase (become slower), it is usual to ‘‘purge’’ tensiometers at regular intervals (ca. weekly or less often under cool wet condi- tions) by replacing the trapped air with deaired water (Cassell and Klute, 1986). The temporary release of suction during purging allows some water to pass into the surrounding soil so that readings are not reliable for some time after purging. 2. Response Time Because any change in matric potential will cause a change in the volume of liq- uid in the tensiometer, time is required for this water to move into or out of the instrument and hence for it to respond. The conductance of the porous cup and the unsaturated hydraulic conductivity of the soil control the response time as well as the amount of water movement required for a given change in potential (the ‘‘gauge’’ sensitivity). Mercury manometers and Bourdon vacuum gauges are much less sensitive than pressure transducers. However, since most tensiometers operate with some trapped air within them, and since their tubing is not com- pletely rigid, differences in response time between pressure transducers and other tensiometer types are much less than would be expected from the sensitivity of the gauges. A tensiometer is said to be tensiometer limited if its response time is not influenced by soil properties, but only by the cup conductance and gauge sensi- tivity; otherwise it is soil limited. Tensiometer-limited response time is inversely proportional to cup conductance and gauge sensitivity (Richards, 1949), and cups Matric Potential 69 Copyright © 2000 Marcel Dekker, Inc. with 100 times greater conductivity than normal cups are available for specialized applications. It is not difficult to obtain tensiometer-limited conditions, although in some soils tensiometers may be soil limited in drier soils (Towner, 1980). Tensiometer-limited conditions are advantageous because instrument be- havior is reproducible and not dependent on variable soil conditions (Klute and Gardner, 1962). This is particularly important when the potential is changing fast. However, obtaining a tensiometer-limited response is not the main consideration when tensiometers are used to monitor field conditions over periods of weeks or months and are read at infrequent intervals. Furthermore, too high a sensitivity can cause problems if the tensiometer is then too sensitive to other factors that can cause a change in the liquid-filled volume such as temperature changes (Watson and Jackson, 1967) and bending of the tubing. In field use, all tensiometer tubing should be shaded from direct sunlight where possible. Otherwise, sudden expo- sure to the sun can cause the tubing (and any air it contains) to expand and tem- porarily perturb the readings. High sensitivity/fast response tensiometers require careful handling and operate better under laboratory conditions. Porous cups are usually made of a ceramic and must have pores that are small enough to prevent air from entering the cup when it is saturated. The cup must also have a reasonably high conductance. Ceramic tensiometer cups for field use have a conductance of about 3 · 10 Ϫ9 m 2 s Ϫ1 , and even a mercury-manometer tensiometer with such a cup will have a (tensiometer-limited) response time of about one minute in the absence of trapped air (Cassell and Klute, 1986), more than adequate for most field use. B. Mercury Manometer and Bourdon Gauge Tensiometers A manometer scale can easily be read to the nearest millimeter, so that mercury tensiometers have a scale resolution of Ϯ 0.1 kPa. However, with the smallest (1.7 mm diameter) nylon tubing commonly used for the manometer, there is a significant capillary correction (ϳ 0.8 kPa) and hysteresis, caused by the mercury meniscus sticking to the walls of the tube. If the tube is agitated, to cause a small fluctuation in the mercury level, an accuracy of Ϯ 0.25 kPa can be achieved; otherwise much larger errors can occur (Mullins et al., 1986). Bourdon vacuum gauges are less accurate, typically with a scale division of 2 kPa, but friction in the gauge mechanism and the difficulty of setting an accurate zero further limit their accuracy. Mercury tensiometers suffer from the environmental hazard of mercury and require a 1 m manometer post but are preferable if high accuracy is required (e.g., when measuring vertical gradients in hydraulic potential). Mercury tensiometers can be constructed very cheaply, without the need for workshop facilities (Webster, 1966; Cassell and Klute, 1986). Where several ten- siometers are used in the same vicinity, it is common to share a single mercury 70 Mullins Copyright © 2000 Marcel Dekker, Inc. reservoir among 6 –30 tensiometers. Because the mercury withdrawn from the reservoir will cause a slight drop in its level, for high accuracy, the level should be measured each time a reading is taken, or the reservoir should have a cross-section many times greater than the sum of the cross-sections of the tubes that dip into it. It is also advisable to check each tensiometer for air leaks before installation. This is done by soaking the cup in water, then applying an air pressure of 100 kPa to the inside of the tensiometer while it is immersed in water (Cassell and Klute, 1986). To minimize thermal effects, the manometer tubing should be shielded from direct sunlight (e.g., by facing the manometer post away from the midday sun). With prolonged outside use, some plasticizer may come out of the nylon tubing and collect as a white deposit, which can eventually block the tube. We have not found this to be a problem over a single season, but 1.7 mm tubing may need to be occasionally replaced over longer periods. C. Pressure Transducer and Automatic Logging Systems Because pressure transducers have a high gauge sensitivity, they are particularly useful when a short response time is important. They can also be used with data loggers. Transducers (e.g., piezoresistive silicon types) that are not temperature sensitive and have a precision of Ϯ 0.2 kPa can be bought for ϳ $140. Types that are vented to the atmosphere should be used so that changes in atmospheric pres- sure have no effect. In the unusual case that matric potentials are required at a considerable depth (say 10 m), a pressure transducer located close to the measuring depth is essential because a hanging water column will break once the tension in it ap- proaches 100 kPa. 1. Automatic Logging Systems Automatic logging systems are required at remote sites, when measurements are required more often than the site can be visited, and to study laboratory or field situations in which many measurements are required over a period of hours or days (e.g., drainage studies). In the former case a provision for automatic purging may also be necessary if weekly visits (or less frequently in wet conditions) are not possible. Systems that use a motor-driven fluid-scanning switch allow a num- ber of tensiometers to be connected each in turn to a single pressure transducer (Anderson and Burt, 1977; Lee-Williams, 1978; Blackwell and Elsworth, 1980). It is necessary to have a transducer attached to each tensiometer if very short measurement intervals are required because re-equilibration, when a trans- ducer is switched between tensiometers at different potentials, can take 2 minutes (Blackwell and Elsworth, 1980) or more (Rice, 1969). The effect of temperature Matric Potential 71 Copyright © 2000 Marcel Dekker, Inc. 72 Mullins Table 1 Methods, Range, Accuracy, Typical Cost, and Suppliers for Measuring Matric (c m )or (Where Indicated) Water (c m ) Potential Method, range, and accuracy a Unit cost (U.S.$) Manufacturers/suppliers and References Tensiometers (0 to ؁85 kPa) Bourdon gauge, Ϯ 2 kPa 150 C, D, F b Mercury manometer, Յ Ϯ 0.25 kPa 30 ϩ post &Hg Homemade with commercial cups (Webster, 1966; Cas- sell and Klute, 1986) Ceramic cups for tensiometers 15 E, F Pressure transducer: normal, miniature, c Ϯ 0.2 kPa 250, 450 B, G, H Portable Bourdon gauge, Ϯ 2 kPa, but see text 1,000 C, D, F (Mullins et al., 1986) Puncture tensiometer, Ն ϩ 0.7 kPa (system- atic) ϩ portable readout 40 each ϩ 1,000 G, H Filter paper (c m /c w )(Ϫ1kPatoϪ100 MPa), 0toϪ50 kPa Ϯ 150%, Ϫ50 kPa to Ϫ2.5 MPa Ϯ 180% 1 All suppliers of Whatman filter paper (Deka et al., 1995) Electrical resistance, c Watermark (Ϫ10 to Ϫ400 kPa) Ϯ 10%, Gypsum block (Ϫ50 to Ϫ1500 kPa) 50, 25 F, G, H, I Heat dissipation c (Ϫ10 kPa to Ϫ100 MPa) Ϯ 10% 200 ϩ 2,500 A Equitensiometer c (0 to Ϫ100 kPa) Ϯ 5 kPa 800 ϩ 500 B (Ϫ100 to Ϫ1000 kPa) Ϯ 5% ϩ portable d meter Psychrometers (c w ), all for disturbed samples except the Spanner psychrometer Isopiestic (0 to ϽϪ40 MPa) Ϯ 10 kPa 15,000 (see text) (Boyer, 1995) Dew point (0 to Ϫ40 MPa) Ϯ 100 kPa 4,500 A Richards (0 to Ϫ300 MPa) Ϯ 5 –10% ϩ meter 2,500 ϩ 2,500 A (but may no longer be available) Spanner (0 to Ϫ7MPa)Ϯ 5–10% ϩ meter 40 ϩ 2,600 I (field/in situ measurement) a Accuracy represents the best reliable reported values or manufacturers’ figures, but see text for details, since accuracy can be limited by a number of factors. b Key (many web sites list local suppliers): A, Decagon Devices Inc., U.S.A. (http://www.decagon.com). B, Delta T, U.K. (http://www .delta-t.co.uk). C, Eijkelkamp, The Netherlands (http://www.eijkelkamp.com). D, ELE In- ternational Ltd., U.K. (http://www .eleint.co.uk). E, Fairey Industrial Ceramics Ltd., Filleybrook, Stone, Staffs., ST15 0PU, U.K. F, Soilmoisture Equipment Corp., U.S.A. (http://www .soilmoisture.com). G, Skye Instruments Ltd. (http://www .skyeinstruments.com). H, UMS GmbH, Germany (http://www.ums-muc.de). I, Wescor Inc., U.S.A. (http://www .wescor.com). c Can be used with data loggers ($1000 –3000). Copyright © 2000 Marcel Dekker, Inc. fluctuations on readings, which is most notable where nylon tubing is exposed above ground (Watson and Jackson, 1967; Rice, 1969), is also minimized with the transducer attached directly to the tensiometer. Such tensiometers and loggers are commercially available (Table 1). 2. Systems with Portable Transducers (Puncture Tensiometers) A puncture tensiometer consists of a portable pressure transducer attached to a hypodermic needle that can be used to puncture a septum at the top of a perma- nently installed tensiometer and hence measure the pressure inside it (Fig. 2) (Marthaler et al., 1983; Frede et al., 1984). In this way, one transducer and readout unit can be used to measure the pressure in a large number of tensiometers. Each tensiometer simply consists of a porous cup attached to the base of a water-filled tube topped by a rubber or plastic septum that reseals each time the needle is removed. A small air pocket is deliberately left at the top of each tensiometer to reduce any thermal effects on the reading and the small pressure change caused Matric Potential 73 Fig. 2 Various tensiometers. From left to right: data logger attached to a pressure trans- ducer tensiometer (only the top part with cover removed to reveal transducer); Webster (1966) type mercury manometer tensiometer; ‘‘quick draw’’ portable tensiometer (case, auger, and tensiometer); portable tensiometer with a pressure transducer and readout; punc- ture tensiometer without, and with, portable meter attached. Copyright © 2000 Marcel Dekker, Inc. by the introduction of the needle. The needle and sensor are designed to have a very small dead volume to minimize this. However, Marthaler et al. reported sys- tematic errors of ϳ 0.7 kPa in potentials close to zero (Ϫ2toϪ3.6 kPa) but a good overall relation between mercury manometer and puncture tensiometer read- ings. Eventually the septum needs to be replaced, and careful insertion is required to ensure that there is no leak into the system. Consequently, these devices are not as accurate as systems with an in situ manometer or pressure sensor. D. Portable Tensiometers Portable tensiometers with Bourdon vacuum gauges (Table 1) and ones with a pressure transducer (available from UMS, Table 1) that can be read to Ϯ 0.1 kPa are commercially available. These can be stored with their tips in water when not in use so that there is little accumulation of air within them, and they rarely need to be refilled. They can be used when single or occasional measurements are re- quired. However, they cannot usually give a reliable reading quickly after insertion because of the effect of soil deformation during insertion. Mullins et al. (1986) found that re-equilibration of the disturbed soil with that surrounding it took only a few minutes in soil at ϾϪ5 kPa but Ͼ 2hinsoilatϽϪ30 kPa (irrespective of the use of the null-point device supplied on one model). E. Osmotic Tensiometers Peck and Rabbidge (1969) described the design and performance of an osmotic tensiometer. It consists of a cell containing a high molecular weight (20,000) polyethylene glycol solution confined between a pressure transducer and a semi- permeable membrane supported behind a porous ceramic. The cell is pressurized so that it registers 1.5 MPa when immersed in pure water, allowing the tensiometer to measure matric potentials between 0 and Ϫ1.5 MPa. However, there were prob- lems due to polymer leakage and sensitivity to temperature changes (Bocking and Fredlund, 1979). Biesheuvel et al. (1999) have used an improved membrane to prevent leakage and have shown how readings can be corrected for temperature effects. Their tensiometer had an accuracy of Ͻ 10% at potentials ϽϪ100 kPa. The technique is promising but requires further development and testing in soil to demonstrate that it has long-term stability and acceptable accuracy and re- sponse time. IV. POROUS MATERIAL SENSORS These sensors are made of a porous material whose water content varies with matric potential in a reproducible manner. A physical property of the material 74 Mullins Copyright © 2000 Marcel Dekker, Inc. [...]... J., and G L Ashcroft 1980 Applied Soil Physics Berlin: Springer-Verlag Hillel, D 1998 Environmental Soil Physics New York: Academic Press King, J A., K A Smith, and D G Pyatt 1986 Water and oxygen regimes under conifer plantations and native vegetation on upland peaty gley soil and deep peat soils J Soil Sci 37 : 485 – 497 Klute, A., and W R Gardner 19 62 Tensiometer response time Soil Sci 93 : 20 4 20 7... Isotopes and Radiation Techniques in Soil Physics and Irrigation Studies Vienna: IAEA, pp 435 – 445 McQueen, I S., and R F Miller 1968 Calibration and evaluation of a wide-range gravimetric method for measuring stress Soil Sci 106 : 22 5 23 1 Merrill, S D., and S L Rawlins 19 72 Field measurement of soil water potential with thermocouple psychrometers Soil Sci 113 : 1 02 –109 Mullins, C E., O T Mandiringana,... 1985 Spatial variability of field-measured soil- water characteristics Soil Sci Soc Am J 49 : 1075 –10 82 Hagan, R M., H R Haise, and T W Edminster, eds 1967 Irrigation of Agricultural Lands Madison, WI: Am Soc Agron Haise, H R., and O J Kelly 1946 Relation of moisture tension and electrical resistance in plaster of Paris blocks Soil Sci 61 : 411– 422 Hamblin, A P 1981 Filter-paper method for routine measurement... in deep soils J Soil Sci 2 : 21 2 22 3 Perrier, E R., and A W Marsh 1958 Performance characteristics of various electrical resistance units and gypsum materials Soil Sci 86 : 140 –147 Phene, C J., and D W Beale 1976 High-frequency irrigation for water nutrient management in humid regions Soil Sci Soc Am J 40 : 430 – 436 Phene, C J., G J Hoffman, and S L Rawlins 1971a Measuring soil matric potential in... within a porous body I Theory and sensor construction Soil Sci Soc Am Proc 35 : 27 –33 Phene, C J., S L Rawlins, and G J Hoffman 1971b Measuring soil matric potential in situ by sensing heat dissipation within a porous body II Experimental results Soil Sci Soc Am Proc 35 : 22 5 22 9 Rawlins, S L 1976 Measurement of water content and the state of water in soils In: Water Deficits and Plant Growth, Vol 4 (T... Water Status of Plants and Soil San Diego, CA: Academic Press Bruini, O., and G W Thurtell 19 82 An improved thermocouple hygrometer for in situ measurements of soil water potential Soil Sci Soc Am J 46 : 900 –904 Campbell, G S., and W H Gardner 1971 Psychrometric measurement of soil water potential: Temperature and bulk density effects Soil Sci Soc Am Proc 35 : 8 – 12 Campbell, G S., and G W Gee 1986 Water... J Appl Sci 2 : 56 –75 Anderson, M G., and T P Burt 1977 Automatic monitoring of soil moisture conditions in a hillslope spur and hollow J Hydrol 33 : 27 –36 Arya, L M., D A Farrell, and G R Blake 1975 A field study of soil water depletion patterns in presence of growing soybean roots: I Determination of hydraulic properties of the soil Soil Sci Soc Am Proc 39 : 424 – 430 Aslyng, H C 1963 Soil physics... © 20 00 Marcel Dekker, Inc Matric Potential 93 Rawlins, S L., and G S Campbell 1986 Water potential: Thermocouple psychrometry In: Methods of Soil Analysis, Part 1 (A Klute, ed.) Madison, WI: Am Soc Agron., pp 597– 618 Richards, L A 1949 Methods of measuring soil moisture tension Soil Sci 68 : 95 –1 12 Richards, L A., and G Ogata 1958 Thermocouple for vapor-pressure measurement in biological and soil. .. Soil Sci 46 : 23 3 23 8 Fawcett, R G., and N Collis-George 1967 A filter-paper method for determining the moisture characteristics of soils Aust J Exp Agric Animal Husb 7 : 1 62 –167 Fourt, D F., and W H Hinton 1970 Water relations of tree crops A comparison between Corsican pine and Douglas fir in south-east England J Appl Ecol 7 : 29 5 –309 Frede, H G., W Weinzerl, and B Meyer 1984 A portable electronic... capillary tension of soil moisture over a wide moisture range Soil Sci 43 : 27 7 29 3 Goltz, S M., G Benoit, and H Schimmelpfennig 1981 New circuitry for measuring soil water matric potential with moisture blocks Agric Meteorol 24 : 75 – 82 Goodman, D 1983 A portable tensiometer for the measurement of water tension in peat blocks J Agric Eng Res 28 : 179 –1 82 Greminger, P J., Y K Sud, and D R Neilsen 1985 . a motor-driven fluid-scanning switch allow a num- ber of tensiometers to be connected each in turn to a single pressure transducer (Anderson and Burt, 1977; Lee-Williams, 1978; Blackwell and Elsworth,. potentials and from between the fibers at high potentials. With calibrated batches of filter papers, accuracies of Ϯ150% and Ϯ180% can be expected between 0 and Ϫ50 kPa, and Ϫ50 kPa and 2. 5 MPa, respec- tively. is possible in the range Ϫ10 to 20 0 kPa with individually calibrated sensors (Wes- cor web site). The gypsum block sensor is 32 mm long and 22 mm in diameter and covers the range Ϫ50 to Ϫ1500