TY - JOUR
AU - Jenkins, M.
AB - Abstract The availability of soil water, and the ability of plants to extract it, are important variables in plant research. The matric potential has been a useful way to describe water status in a soil–plant system. In soil it is the potential that is derived from the surface tension of water menisci between soil particles. The magnitude of matric potential depends on the soil water content, the size of the soil pores, the surface properties of the soil particles, and the surface tension of the soil water. Of all the measures of soil water, matric potential is perhaps the most useful for plant scientists. In this review, the relationship between matric potential and soil water content is explored. It is shown that for any given soil type, this relationship is not unique and therefore both soil water content and matric potential need to be measured for the soil water status to be fully described. However, in comparison with water content, approaches for measuring matric potential have received less attention until recently. In this review, a critique of current methods to measure matric potential is presented, together with their limitations as well as underexploited opportunities. The relative merits of both direct and indirect methods to measure matric potential are discussed. The different approaches needed in wet and dry soil are outlined. In the final part of the paper, the emerging technologies are discussed in so far as our current imagination allows. The review draws upon current developments in the field of civil engineering where the measurement of matric potential is also important. The approaches made by civil engineers have been more imaginative than those of plant and soil scientists. Matric potential, measurement, porous matrix sensors, sensors, tensiometer, water release characteristic. Introduction A quantitative description of the water status of soils and plant tissue is a necessary component of most experiments in plant biology. However, perhaps because the importance of the issue is not appreciated, or that methods are poorly understood, it is not uncommon that such measurements—if made at all—are inadequate, making it difficult to replicate the experiments under similar conditions. Knowledge of the water available to plants in the growth medium is often critical to understanding growth, development, physiology, and gene expression patterns of plants in any system. This review attempts to explain concepts of soil water and some ways in which it can be measured, with particular emphasis on soil matric potential sensors. Comprehensive treatment of methods for measuring plant and soil water status are found elsewhere (e.g. Boyer, 1995). The water-filled tensiometer is synonymous with the measurement of matric potential. Tensiometers tend to be the preferred sensor because, when working correctly, they give a direct measurement of matric potential. The working principles of the water-filled tensiometer have been understood for >100 years following the early description by Livingstone (1908). A helpful summary of the history of the development of the tensiometer as an instrument is given by Or (2001). Despite the long history of the water-filled tensiometer, those used by plant and soil scientists have not been particularly improved in comparison with the early descriptions, except for the use of improved pressure transducers and data logging systems. The perceived limitation of a narrow measurement range is widely reported in text books (e.g. Marshall et al., 1999). It is because of this that there is a widely held view that the water-filled tensiometer can only be used to measure matric potentials greater than approximately –90 kPa, due to cavitation of water at lower pressures. We will describe the basis of this belief and show how water-filled tensiometers can actually be used to measure much lower matric potentials. Civil engineers, by improved saturation procedures, have in the last 20 years considerably extended the range of matric potentials over which the water-filled tensiometer can be used, to matric potentials as low as –1500 kPa (Ridley and Burland, 1993). Tensiometers designed for this purpose have become known as ‘high-capacity tensiometers’ (Take and Bolton, 2003; Marinho et al., 2008). Careful saturation of commercially available tensiometers can allow matric potentials as low as –200 kPa to be measured (Whalley et al., 2009). In this review, we explore the possibility of using the high-capacity tensiometer in plant sciences. An alternative to the high-capacity tensiometer is described by Bakker et al, (2007), where the water in the ceramic cup is replaced by an osmoticum. This has the effect of the hydrostatic pressure in the tensiometer cup being zero when the soil is at a matric potential equal to the osmotic potentials of the osmoticum. This has the advantage of a positive pressure in the tensiometer cup even in dry soils; thus, cavitation is no longer an issue. A significant effort has been made to develop sensors which can reliably measure matric potentials which are much lower than –90 kPa. The most common design is the porous matrix sensor. Here a porous material is allowed to equilibrate with the soil water, then the water content of the porous matrix is measured and converted to a matric potential using a calibration curve. The original sensors of this type were made using plaster of Paris and monitored by measuring electrical resistance (Bourget et al., 1958). The use of electrical resistance measurements to measure soil water content was described in 1887 by Professor Milton Whitney, who became one of the first to develop approaches for the in situ determination of soil water content based on the measurement of the electrical conductivity of soil (see Freeland, 1989). The encasement of the resistance sensor in plaster of Paris provided some stabilization of resistance measurement between soil types because of its buffering capacity against variable electrical conductivity of the soil water. These sensors are best suited to dry soils (matric potential less than −500 kPa), where they work well, in part, because hysteresis in the sensor is small at these matric potentials. However, with careful calibration, taking into account the logger calibration and temperature, gypsum blocks can be used at higher matric potentials (Johnston, 2000). Recently, they have been modified by increasing the pore size of the porous matrix to obtain a sensor that will work at matric potentials between 0 and −200 kPa. However, in making this adaptation, the sensor became unreliable, as the calibration varied when the same sensor was repeatedly calibrated in the same soil (see Spaans and Baker, 1992). Whalley et al. (2001) analysed this adapted design and concluded that it could never work well because (i) performance in wet soils is likely to be a function of the soil rather than the sensor; and (ii) even in drier soils, hysteresis is not taken into account in the calibration. However, Whalley et al. (2001) showed how hysteresis could be taken into account in the calibration of a ceramic-based porous matrix sensor between matric potentials of 0 and −60 kPa. Recently, Whalley et al. (2007, 2009) have designed dielectric tensiometers that work over a much wider range of matric potentials. In the introductory part of this review, we have introduced the more common approaches to measuring matric potential and highlighted some of the issues to be considered. Before considering these in more detail, the nature of the water release curve is explored. The purpose is to emphasize the point that both water content and matric potential need to be measured if soil water status is to be fully described. It is not the purpose of the review to catalogue a number of sensors; instead, we wish to outline the concepts and principles that underpin the different methods of measuring matric potential. To rationalize our discussion we first consider direct measurements of matric potential, including the measurement of relative humidity with psychrometers. Secondly, we explore the use of indirect measurements of matric potential. Finally, we attempt to identify some new approaches that are not yet used, but which may fill gaps left by the current technology. Matric potential and the water release characteristic What is matric potential? Passioura (1980) provides an excellent account of matric potential both in the soil and in the plant. He reminds us that the matric potential was originally introduced by T.J. Marshall in 1959 and that two instruments, the tensiometer and pressure plate apparatus, measure or generate a matric potential consistent with the operational definition: ‘matric potential is the difference in water potential between a system and its equilibrium dialysate when both are at the same height, temperature and are subjected to the same external pressure’. The ‘equilibrium dialysate’ is a solution in equilibrium with the soil solution, separated by a semi-permeable membrane or barrier, one which allows the movement of water but not solutes or soil particles. Both the tensiometer (see below) and the pressure plate apparatus depend, respectively, on the measurement or creation of a pressure difference across a membrane which is permeable to soil water solution, but not the soil particles. Passioura (1980) explains that the attractive forces between liquids and solids consist of the short-range London–van der Waal’s forces that act over a few molecular layers and the longer range electrostatic forces present when the surfaces of the soil particles are charged. In the case of an uncharged hydrophilic soil, which is an approximation of sand, the short-range forces and cohesive force between the water molecules interact to produce a concave meniscus at the air–water interface. This is recorded by the tensiometer, in equilibrium with the soil water, as a negative hydrostatic pressure, which is called ‘matric potential’. The capillary pressure is frequently written with the Young–Laplace equation as  (1) where γ is the surface tension of water, θ is the angle of contact between the water and soil at the air–soil–water interface, and r is the radius of the pore. Recently there has been much interest in hydrophobic soils (Hallett et al., 2011), and here the contact angle is given by the Young equation  (2) where the interfacial tensions are annotated in Fig. 1. In hydrophilic soil, Cos(θ)=1 and it becomes smaller in more hydrophobic soils. In agricultural soils, hydrophobic behaviour is often associated with higher clay content where organic molecules are adsorbed to the charged clay surfaces (Matthews et al., 2008) or with plant waxes adsorbed to siliceous sands (Roper, 2005). In clay-rich soils, the charges on the particle surfaces need to be balanced by counter ions. As Passioura (1980) explains, these counter ions are not free to diffuse but are constrained, and this has the effect of creating a localized osmotic pressure, analogous to that generated by a semi-permeable membrane. In these soils, matric potential, ψm, is a combination of the effects of hydrostatic pressure and osmotic pressure, which cannot be dissected experimentally, and is defined by the equation Fig. 1. View largeDownload slide Interfacial tensions of a partly saturated pore. Fig. 1. View largeDownload slide Interfacial tensions of a partly saturated pore.  mD (3) where P is the hydrostatic pressure and πD is the osmotic pressure of the dialysate. In sands with a negligible surface charge, P=ψm. In saturated clay pastes, a tensiometer can record an apparent matric potential due to πD. The water release characteristic The relationship between soil water content and matric potential is called the ‘water release characteristic’. In an idealized form, the water release characteristic of a rigid soil (a soil that does not shrink) can be divided into three regions (Fig. 2). As the matric potential becomes more negative, initially no water will drain and the soil is said to be ‘tension saturated’. Eventually, as the matric potential becomes more negative, air invades the soil matrix at the ‘air entry potential’ and this region is called the ‘capillary fringe’. At very low matric potentials, the water menisci become disconnected and the soil is said to be at the ‘residual’ water content. These idealized phases can only be easily observed in sand (see Whalley et al., 2012) where matric potential is usually plotted on a linear scale. In agricultural soils, the water release characteristic is sigmoidal, highly non-linear, and commonly fitted to the ‘van Genuchten function’ (van Genuchten, 1980). This particular function is popular because fitted parameters can be used to estimate the relative conductivity of unsaturated soil. The van Genuchten function has also been modified to allow the hysteretic nature of the water release curve to be described (Kool and Parker, 1987). An additional complication is that the temperature dependence of surface tension makes the water release characteristic itself temperature dependent (Haridasan and Jensen, 1972), which affects both the wetting and drying limbs. Fig. 2. View largeDownload slide Ideal water release curve. Fig. 2. View largeDownload slide Ideal water release curve. The water release curves of six soils from Gregory et al. (2010), which have been subjected to two compaction treatments and de-structuring by shear deformation, are shown in Fig. 3. These data show that the water release characteristic is greatly affected by soil type and changes in soil structure induced by various types of soil damage. Thus, matric potential cannot be easily estimated from measurements of the volumetric soil water content of soil which is the output of most soil moisture sensors. An additional complication is that the non-linear nature of the relationship makes the estimation of low water potentials from measurements of soil water content inaccurate. The difficulty in predicting matric potential from soil water content makes the use of sensors to estimate matric potential an important part of most experimental studies into soil–plant relationships, although water content should still be measured. Fig. 3. View largeDownload slide Soil water release characteristics of the six soils which had been lightly compressed with a 50 kPa axial pressure (L), moderately compressed with a 200 kPa axial pressure (H), or shear deformed (S). The data are fitted to the van Genuchten function (lines). The error bar gives the standard error of the difference [P < 0.001 except for Broadbalk (P)KMg (P=0.341); df=30] for the structural state×matric potential interaction within a soil. Redrawn from Gregory et al. (2010). Used by permission of the Soil Science Society of America, Inc. Fig. 3. View largeDownload slide Soil water release characteristics of the six soils which had been lightly compressed with a 50 kPa axial pressure (L), moderately compressed with a 200 kPa axial pressure (H), or shear deformed (S). The data are fitted to the van Genuchten function (lines). The error bar gives the standard error of the difference [P < 0.001 except for Broadbalk (P)KMg (P=0.341); df=30] for the structural state×matric potential interaction within a soil. Redrawn from Gregory et al. (2010). Used by permission of the Soil Science Society of America, Inc. Direct measurement of matric potential The water-filled tensiometer The most commonly used sensor for soil water matric potential is the water-filled hydraulic tensiometer, described in its modern form by Richards (1949) and more recently by Mullins et al. (1986). The concept of the water-filled tensiometer (Fig. 4) has been outlined above. There is a common view that these tensiometers have a narrow measurement range and will not record matric potentials smaller than –90 kPa. Figure 5 shows four sets of previously unpublished data from the work described by Whalley et al. (2008). One of the tensiometers records matric potentials smaller than –200 kPa, while the others record matric potentials no smaller than –95 kPa. Marinho et al. (2008) explain that it is entirely possible for water-filled tensiometers to record matric potentials much smaller than –90 kPa, and they present a comprehensive thermodynamic analysis of the issue. Pure water has a very high tensile strength of ~100MPa (Marinho et al., 2008), so this is not the limitation to the measurement range of a tensiometer (Take and Bolton, 2003). Marinho et al. (2008) comment that in measurements of tensile strength, water has never been successfully subjected to its theoretical limit. Instead, water cavitates at much smaller tensile loads due to imperfections in the water or in the walls of the tensiometer. Cavitation within the water is called ‘homogeneous nucleation’ and it arises because the thermal vibration of molecules can cause microscopic voids in the bulk liquid. Marinho et al. (2008) calculated the pressure needed to expand such a void in pure water at 20 °C to be –823MPa, which has never been achieved experimentally. Impurities in the water probably lead to cavitation at a smaller tension. However, it is the cavitation at the interface between the water and the tensiometer cup that is most likely to be the primary limitation to the measurement range of the tensiometer. This is called ‘heterogeneous cavitation’. Fig. 4. View largeDownload slide Water-filled tensiometer. Fig. 4. View largeDownload slide Water-filled tensiometer. Fig. 5. View largeDownload slide Matric potential data measured with four water-filled tensiometers used by Whalley et al. (2008). Note that one of the tensiometers recorded matric potentials as low as –200 kPa (set 3) and it cavitates at its most negative matric potential. Fig. 5. View largeDownload slide Matric potential data measured with four water-filled tensiometers used by Whalley et al. (2008). Note that one of the tensiometers recorded matric potentials as low as –200 kPa (set 3) and it cavitates at its most negative matric potential. The progression to heterogeneous cavitation is illustrated in Fig. 6. In dry soil, as the water in the tensiometer cup equilibrates with the soil water, the pressure becomes negative. Eventually, air/gas/vapour trapped in the ceramic in microscopic bubbles expands. Air may also diffuse from the water in the cup into the bubbles according to Henry’s law. This situation proceeds until the bubble escapes from the wall of the ceramic and enters the tensiometer cup. Since absolute vapour pressure cannot be negative, the bubble expands rapidly until a pressure is reached that is determined by the phase relationships, usually approximately –100 kPa (relative to atmospheric pressure). In the tensiometer that recorded a matric potential of –200 kPa in Fig. 5, the point of cavitation is marked. Following cavitation the matric potentials recorded by all the tensiometers were similar (between –80 kPa and –95 kPa) until eventually they reported a matric potential of approximately zero kPa. However, following cavitation, measurements of soil water content showed that the soil continued to dry. The point at which the tensiometers returned to zero kPa corresponds to the point at which air permeates throughout the ceramic (see Fig. 6) and the tensiometers have collapsed completely. Between cavitation and complete collapse (see Fig. 5), up to a week elapsed when tensiometers falsely reported that the soil was at a matric potential between –80 kPa and –90 kPa. Such data should always be viewed with suspicion, since in the field a matric potential that does not change with time is unlikely. Fig. 6. View largeDownload slide Failure stages of the water-filled tensiometer. Fig. 6. View largeDownload slide Failure stages of the water-filled tensiometer. A key target for civil engineers has been to minimize or prevent cavitation events, which has been achieved by improved saturation methods. The primary goal of civil engineers has been to extend the range of matric potentials that can be measured, and the high capacity tensiometer allows the direct measurement of matric potentials as low as –1500 kPa. Take and Bolton (2003) describe the design of a device for saturating tensiometers by the application of a vacuum (0.05 kPa) to the ceramic, which can then be immersed in water and degassed under the same vacuum. This is followed by the application of a high positive pressure to force water into any of the remaining crevices or unsaturated pores. This approach was developed by Ridley and Burland (1993). A theoretical justification of the two-stage saturation process is given by the equation  (4) where ΔP is the increase the pressure needed to achieve a final saturation, S (here S=1); Pi is the initial pressure; Si is the initial saturation, and H is Henry’s constant. According to Equation 4, ΔP can be minimized by a low value of Pi and a high value of Si, which is what is achieved in vacuum saturation. Suction tests on a fully saturated high capacity tensiometer are shown in Fig. 7a–e. When fully saturated (Fig, 7a), the tensiometer accurately follows the applied negative pressure. When the saturated tensiometer was exposed to low water potentials by allowing water to evaporate from the surface of the ceramic, it cavitated (point E, Fig. 7b) and reported a pressure of about –90 kPa until it was returned to water. The cavitated tensiometer was not able to track the applied pressure accurately (compare Fig. 7a and c). At point N in Fig. 7d, the tensiometer failed completely and the recorded pressure returned to zero kPa as air permeated the ceramic cup. Interestingly, when the completely failed tensiometer (from Fig. 7d) was tested (Fig. 7e) it gave an appearance of a ‘sensible’ result, although here it was known to be erroneous. There are similarities between these laboratory data and our field data (Fig. 5) that allow us to identify the failure of the fully saturated tensiometer (compare Fig. 7b with trace 3), and that three of the tensiometers had cavitated, by comparison with (Fig. 7c). In contrast to the ephemeral measurement of low matric potential (approximately –200 kPa; Fig. 5), Cunningham et al. (2003) report a measurement of –850 kPa for 8 d. Fig. 7. View largeDownload slide Laboratory studies of tensiometer failure using a high capacity tensiometer (taken from Whalley et al., 2009; used by permission of the Soil Science Society of America, Inc.) The tensiometer had a ceramic with an air entry pressure of 300 kPa. In these panels, the applied pressure is always shown by the dashed line and the output recorded by the tensiometer is shown by the solid line. In (A), a fully saturated tensiometer is able to follow the allied pressure and the dashed and solid lines are coincident. In (B), the tensiometer is allowed to cavitate (point E) by letting water evaporate from the surface of the ceramic in air. The tensiometer is returned to water at point G in (B). In (C), the cavitated tensiometer is tested and it can no longer follow the applied pressure. In (D), the tensiometer is allowed to collapse completely at point N by allowing water to evaporate from the ceramic. Finally in (E), the results of a test on the ‘completely collapsed’ tensiometer are shown. Fig. 7. View largeDownload slide Laboratory studies of tensiometer failure using a high capacity tensiometer (taken from Whalley et al., 2009; used by permission of the Soil Science Society of America, Inc.) The tensiometer had a ceramic with an air entry pressure of 300 kPa. In these panels, the applied pressure is always shown by the dashed line and the output recorded by the tensiometer is shown by the solid line. In (A), a fully saturated tensiometer is able to follow the allied pressure and the dashed and solid lines are coincident. In (B), the tensiometer is allowed to cavitate (point E) by letting water evaporate from the surface of the ceramic in air. The tensiometer is returned to water at point G in (B). In (C), the cavitated tensiometer is tested and it can no longer follow the applied pressure. In (D), the tensiometer is allowed to collapse completely at point N by allowing water to evaporate from the ceramic. Finally in (E), the results of a test on the ‘completely collapsed’ tensiometer are shown. It appears that the primarily limitation of the tensiometer is not the restricted measurement range, but uncertainty over the saturation state and hence the accuracy of the recorded pressures. To put it in another way, it may not be obvious if your tensiometer is working reliably. While we have shown that commercial tensiometers can record low matric potentials, this is not the norm. However, we note that one commercial tensiometer has a lower measurement limit of –200 kPa (UMS, Munich, Germany): this will be shown in a following section when the comparisons between water-filled and porous matrix sensors are explored. From the preceding discussion, it seems likely that the UMS tensiometer has achieved the extended measurement range by using a tensiometer design that minimizes heterogeneous cavitation. As far we know, the designs of tensiometers used by soil and plant scientists cannot be exposed to the very high pressures that have been reported in the civil engineering community (e.g. 1000 kPa) that are used to saturate their tensiometers. Even the civil engineers have challenges to overcome, summarized by Marinho et al. (2008). Those most relevant to soil and plant science are equilibrium time, air diffusion, long-term measurement, and tensiometer saturation. The osmotic tensiometer An alternative to decreasing the pressure at which tensiometers cavitate is to add osmoticum to the water in the tensiometer cup in order to increase the hydrostatic pressure when the matric potential is zero. Thus, on soil drying, the pressure in the tensiometer cup remains positive (Peck and Rabbidge, 1966, 1969). The idea of Peck and Rabbidge is a good example of lateral thinking; however, their sensor was prone to measurement errors primarily due to temperature fluctuations and a drift in the pressure corresponding to a zero matric potential. The response of an osmotic tensiometer to changing temperature and water potentials has been modelled (Biesheuvel et al., 1999, 2000), and the tensiometer designs that arose from this work have been tested more recently by Bakker et al. (2007). They showed that it was possible to use the osmotic tensiometer reliably for an extended period of time (Fig. 8). In our view this recent work on osmotic tensiometers in The Netherlands is an encouraging technological advance (van der Ploeg et al., 2010). Fig. 8. View largeDownload slide A comparison of soil drying measured with a conventional water-filled tensiometer and an osmotic tensiometer. The matric potential was also deduced from measurements of soil water content. Also shown are the limit to water extraction by plant roots (approximately –1.5MPa) and the usual limit of water-filled tensiometers (approximately –90 kPa). This is redrawn from Bakker et al. (2007). Used by permission of the Soil Science Society of America, Inc. Fig. 8. View largeDownload slide A comparison of soil drying measured with a conventional water-filled tensiometer and an osmotic tensiometer. The matric potential was also deduced from measurements of soil water content. Also shown are the limit to water extraction by plant roots (approximately –1.5MPa) and the usual limit of water-filled tensiometers (approximately –90 kPa). This is redrawn from Bakker et al. (2007). Used by permission of the Soil Science Society of America, Inc. Measurement of relative humidity The water potential (ψw) of soils is determined by the osmotic and matric forces on water attached to the soil particles (see earlier discussion). The water potential of soil water—or Gibbs energy of a system in thermodynamic terms—determines how much work plant roots must expend in order to extract this water. The Kelvin equation describes the principle:  (5) where R is the gas constant; T, the temperature in Kelvin; , the partial molal volume of water; ew, the partial vapour pressure of the water in the system; and e0, the saturated vapour pressure. Because ψw is energy per unit volume of water, which is force per area, or pressure, the units to express ψw are kPa, or MPa. Using ψw to describe the water within a system can indicate the ability of water to do work (relative to pure, unrestrained water) and which direction water will flow: usually in the direction of more negative potentials within an isothermal system. Psychrometry is one method of measuring soil ψw, and there are several variants, depending on the kind of instrument that is used. These methods have been reviewed and described in detail (Boyer, 1995; Campbell, 1990; Oosterhuis, 2007a). Details of the theory and construction of psychrometers have been published for some time (Boyer, 1966; Campbell, 1979); however, their use can be problematic if researchers are not aware of the basic principles, and particularly the issues that can introduce errors into measurements. The three types of instruments that are commonly used are the dewpoint hygrometer, Peltier (wet-bulb) psychrometer, and isopiestic thermocouple psychrometer. As water in the sample itself is difficult to measure without disturbing the original state of the water, all methods depend on the establishment of water vapour pressure equilibrium between the water in the sample and the air above the sample under isothermal conditions. The relative humidity of the air is then sensed in various ways. One instrument that uses the dewpoint method cools a mirror until water begins to condense onto the mirror surface when the dewpoint is reached (Decagon WP4, Pullman, WA, USA). Accurate measurement of the sample temperature and the mirror temperature allows calculation of the relative humidity, which is then related to sample ψw using the above relationship. The dewpoint temperature changes by –0.12 °C per MPa. The electronics of the instrument allow the mirror temperature to be cycled above and back to the dewpoint many times to improve the accuracy. The precision of the method is ±0.1MPa. Thermocouple psychrometers make use of the Peltier effect, which describes the phenomenon when two dissimilar metals are joined within a circuit: a voltage difference is produced that is proportional to the temperature difference between the two junctions. One junction can be cooled relative to the other junction by passing a current in one direction, or warmed by reversing the direction of current flow. Based on this principle, the Peltier (wet-bulb) type psychrometer comprises a thermocouple junction held within the air space of a sealed chamber containing the soil sample [e.g. Wescor C-52/HR-33T, Logan, UT, USA, which can operate in either hygrometric (dew point) or psychrometric (wet-bulb) mode]. Alternatively, pre-calibrated in situ thermocouples can be buried directly in the soil. The thermocouple is protected by a porous ceramic cup or stainless steel screen that allows water vapour equilibrium with the surrounding bulk soil (Fig. 9). Current is passed through the circuit, causing the junction to cool to a temperature lower than the dewpoint, which causes water to condense on the thermocouple junction surface. The current flow is then stopped and the condensed water begins to evaporate back into the sample chamber air. The change in temperature caused by the rate of evaporation, which is a function of the humidity of the sample chamber air, is measured as a voltage produced between the measurement and reference junctions. Because humidity in the chamber is always high (even soils as dry as –4MPa equilibrate with a relative humidity of 97% at 25 °C), the measured temperature changes are small, as are the voltages produced, so it is vital that the system is thermally and electrically insulated. The output of thermocouples is typically 5 μV per MPa, so circuitry designed with minimal contact potentials and a sensitive and stable microvoltmeter is required. Each thermocouple has unique electrical output characteristics that require individual calibration with solutions of known ψw. The accuracy can range between ±0.1MPa and ±0.01MPa, depending on thermal stability and other factors, detailed below. Fig. 9. View largeDownload slide An isopiestic thermocouple psychrometer (left) and an in situ soil psychrometer (right; from Wescor, USA). On the isopiestic thermocouple psychrometer, note the thermocouple junction centred in the loop, which holds a droplet of solution of known water potential. The sample chamber may contain plant tissue, soil, or a solution such as plant sap. Fig. 9. View largeDownload slide An isopiestic thermocouple psychrometer (left) and an in situ soil psychrometer (right; from Wescor, USA). On the isopiestic thermocouple psychrometer, note the thermocouple junction centred in the loop, which holds a droplet of solution of known water potential. The sample chamber may contain plant tissue, soil, or a solution such as plant sap. A system can be measured most accurately when the system itself is minimally disturbed. This can be achieved by maintaining a state of equilibrium: for instance, pressure can be quantified by applying a measured counterbalancing pressure so that no flow takes place, no work is done by the elements that are desired to be measured, and the system remains relatively unchanged. The isopiestic (‘equal pressure’) psychrometer uses such an equilibrium method. Instead of a droplet of pure water on the thermocouple junction, a droplet of solution nearly matching the ψw of the sample is applied to the junction, minimizing water vapour flux from the sample (Boyer, 1966; Fig. 9). If the solution applied to the junction has a ψw slightly more positive than the sample, the junction cools slightly as water evaporates; a more negative solution ψw results in condensation and the junction warms. The isopiestic value (equivalent to the sample ψw) is determined by extrapolating to the zero value (no condensation or evaporation) between the two solutions. The accuracy of this technique is ±0.01MPa over the entire range of soil water contents. Psychrometric methods rely on using solutions of known ψw, either for measurement (the isopiestic method) or for calibration of thermocouple output. Solutions of KCl can be prepared, although sucrose solutions are potentially less corrosive. Although the van’t Hoff equation describes the relationship between solute concentration and ψw, this is derived from gas laws and applies only to ideal (i.e. weak) solutions. The solutions needed to achieve low ψw are concentrated to the extent that the dissolved solute molecules no longer behave like gases, but interact in a complex way. Thus, empirical relationships are needed to correct for this, and each solute has a unique osmotic coefficient. For example, tables are available that give the ψw for sucrose solutions of known molality and temperature (Michel, 1972). To make accurate measurements of soil ψw using psychrometric methods, it is important to understand major sources of error so as to minimize their effect (Brown and Shouse, 1992; Boyer, 1995; Oosterhuis, 2007b). Water vapour equilibrium between the sample chamber air and water within the sample depends on resistances in the diffusion path. The further the solution on the thermocouple is from the isopiestic point, the greater the influence of the resistance to water vapour flux from the sample to the air; errors can vary 8–16% (Boyer, 1995). Isothermal conditions are critical and perhaps most difficult to verify. Small temperature differences (>0.1 °C) between the sample and the measurement junction that are not accounted for will result in error, and differences <0.001 °C are required for an accuracy of 0.01MPa. Maintaining the instrument in a thermally stable environment helps, but usually a housing with large thermal mass (e.g. an aluminium block or water bath) is required to dampen changes during measurements. The path of water vapour flux between sample and thermocouple junction can be complicated if other surfaces (e.g. sample cup walls) allow water adsorption. Clean surfaces made of, or coated with, minimally sorptive material reduces this error and speeds attainment of vapour equilibrium. In preparing soil samples, loss of moisture to dry air, or absorption of moisture from the air into very dry samples can be minimized by working quickly and taking appropriate caution, for instance by using a humid box to handle and transfer samples. In summary, isopiestic thermocouple psychrometry is the most precise and accurate method to measure soil ψw, but cost, process time per sample, and availability of instrumentation may be other factors to consider. Indirect measurement The filter paper method When a relatively large number of soil samples need to be measured, and urgency is not an issue, a simple and cost-effective method of estimating soil ψw is the filter paper technique (Deka et al., 1995). A filter paper disk is placed in contact with the soil sample in a sealed container until the paper is in equilibrium with the soil (7 d may be required). The gravimetric water content of the paper is determined by careful weighing, and related to ψw via a calibration curve determined for the particular type and batch of filter paper. Soil ψw ranging from –10 kPa to –10MPa could be measured, although an accuracy of ±7% (compared with psychrometric and tension table-derived values) was greatest between –250 kPa and –2.5MPa, and coefficients of variation were 1–3% (Deka et al., 1995). Johnston (2000) shows that good comparisons can be obtained between matric potential estimated with the filter paper method and carefully calibrated gypsum blocks. Porous matrix sensors The concept of the porous matrix sensor has been introduced. Developments in porous matrix sensors have focused on the identification of more appropriate porous materials and the use of improved methods to measure their water content (e.g. Whalley et al., 2001, 2007, 2009; Malazian et al., 2011), but this has produced mixed results. Wraith and Or (2001) described the use of calibrated soil as a reference material for use with TDR (time domain reflectrometry) probes to infer the matric potential of soil from its water content. The most well-known attempt to alter the measurement range of the gypsum block sensors is probably the ‘Water Mark®’ sensor which is currently widely used in irrigation control. This was achieved by increasing the pore size of the porous matrix and had the desired effect of making the sensor sensitive to higher matric potentials (i.e. it could be used in wetter soils), which are helpful in root growth studies. However, the accuracy of these sensors is not sufficient for most scientific purposes without calibration. Unfortunately, the repeatability of calibrations is poor even when the same sensor is repeatedly recalibrated in the same soil (Spaans and Baker, 1992). Whalley et al. (2001), based on some idealized experiments (Fig. 10), showed that a porous matrix could remain saturated at matric potentials much lower than its air entry potential if the surrounding soil had pores small enough to remain tension saturated. It was proposed that an ‘air entry pipe’ should be used with porous matrix sensors to improve their reliability (e.g. Whalley et al., 2009). The non-linear nature of electrical resistance measurement used in ‘Water Mark’® sensors to measure the water content of the porous matrix is an additional source of variability. Recently, Decagon Inc. have developed a new commercial porous matrix sensor which uses dielectric measurement to monitor the water content of a ceramic material. This is a relatively low cost sensor but its accuracy is poor (Malazian et al., 2011). With individually constructed porous matrix and calibrated sensors, it is possible to achieve good agreement with the water-filled tensiometers (see the following section and Fig. 11; Whalley et al., 2009; Malazian et al., 2011). Fig. 10. View largeDownload slide This shows the drainage of a ceramic with an air entry potential of –4 kPa which is surrounded by silica paste with an air-entry pressure of –100 kPa. The ceramics were put on a tension plate set at a matric potential of –4 kPa surrounded by different amounts of silica paste, and the degree of saturation following a period of equilibrium is indicated. A saturation of 44% corresponds to the expected water release characteristic, measured with the same silica paste boundary. When the coverage of silica paste increases, the drainage of the ceramic decreases, until eventually the porous ceramic does not drain at all. Taken from Whalley et al. (2001) Reprinted from European Journal of Soil Science, 52, Whalley WR, Watts CW, Hilhorst MA, Bird NRA, Balendonck J, Longstaff DJ. 2001. The design of porous material sensors to measure matric potential of water in soil. pp. 511–519, Copyright© 2001, with permission from John Wiley and Sons. Fig. 10. View largeDownload slide This shows the drainage of a ceramic with an air entry potential of –4 kPa which is surrounded by silica paste with an air-entry pressure of –100 kPa. The ceramics were put on a tension plate set at a matric potential of –4 kPa surrounded by different amounts of silica paste, and the degree of saturation following a period of equilibrium is indicated. A saturation of 44% corresponds to the expected water release characteristic, measured with the same silica paste boundary. When the coverage of silica paste increases, the drainage of the ceramic decreases, until eventually the porous ceramic does not drain at all. Taken from Whalley et al. (2001) Reprinted from European Journal of Soil Science, 52, Whalley WR, Watts CW, Hilhorst MA, Bird NRA, Balendonck J, Longstaff DJ. 2001. The design of porous material sensors to measure matric potential of water in soil. pp. 511–519, Copyright© 2001, with permission from John Wiley and Sons. Fig. 11. View largeDownload slide A comparison between water-filled tensiometers and porous matrix sensors in the laboratory (A) and in the field (B). In the laboratory comparison, T5× tensiometers were used and in two cases they recorded matric potentials more negative than –90 kPa (sensors 1 and 3). In one case, the water-filled tensiometer and porous matrix sensor were in good agreement until approximately –200 kPa (sensor 3). While the porous matrix senor continued to show the soil drying, the water-filled tensiometers failed to track the decreasing matric potential. One of the water-filled tensiometers indicated that the soil dried to a matric potential of only –90 kPa and then failed (sensor 2). In the field data (B), comparison of the water-filled tensiometer with sensor 2 (A) indicated that it had cavitated. For both laboratory and field data, complete tensiometer collapse occurred when the porous matric sensor indicated a matric potential of approximately –350 kPa. Note that the porous matric sensor used in both laboratory and field experiments had been previously calibrated. Fig. 11. View largeDownload slide A comparison between water-filled tensiometers and porous matrix sensors in the laboratory (A) and in the field (B). In the laboratory comparison, T5× tensiometers were used and in two cases they recorded matric potentials more negative than –90 kPa (sensors 1 and 3). In one case, the water-filled tensiometer and porous matrix sensor were in good agreement until approximately –200 kPa (sensor 3). While the porous matrix senor continued to show the soil drying, the water-filled tensiometers failed to track the decreasing matric potential. One of the water-filled tensiometers indicated that the soil dried to a matric potential of only –90 kPa and then failed (sensor 2). In the field data (B), comparison of the water-filled tensiometer with sensor 2 (A) indicated that it had cavitated. For both laboratory and field data, complete tensiometer collapse occurred when the porous matric sensor indicated a matric potential of approximately –350 kPa. Note that the porous matric sensor used in both laboratory and field experiments had been previously calibrated. Although much effort has been made to increase the matric potentials at which gypsum blocks can be used, they are in fact very useful for measuring low matric potentials. Typically plaster of Paris drains at matric potentials below –100 kPa and a plot of log matric potential against log resistance is generally linear. The use of different mixtures of plaster of Paris allows these sensors to be adapted to different ranges of matric potential (Perrier and Marsh, 1958). The use of plaster of Paris ensures that the electrodes are surrounded by calcium-saturated water, which buffers the sensor’s calibration against changes in the electrical conductivity of the soil water. Hysteresis in the relationship between matric potential and water content within the sensor presents a complication in the use of resistance blocks. However, Bouget et al. (1958) suggested that it could be ignored, provided that a range of matric potentials is identified where the hysteresis is low. Modern computers and microprocessors allow the effects of hysteresis to be taken into account (Whalley et al., 2009), although this may add a layer of unwanted complexity for some applications of the sensors. Lessons from the conjunctive use of direct and indirect measurements In Fig. 11, two lessons on the use of water-filled tensiometers are clearly illustrated. First, as we have discussed in Fig. 5, a steady output from a water-filled tensiometer must be viewed with caution. In Fig. 11a, the calibrated porous matrix sensor clearly shows that the soil continues to dry, which accurately reflects the experiment. Similar conclusions can be arrived at from comparisons between the output from tensiometers in the field and adjacent porous matrix sensors (see data for the end of June in Fig. 11b). Secondly, the point of complete tensiometer collapse corresponds to a matric potential determined by the porous matrix sensor, which is very close to the expected air entry point of the ceramic of the water-filled tensiometer. Thus, if cavitated tensiometers are allowed to continue in drying soil, they will provide information at the point of complete collapse. We do not recommend the common practice of ‘topping-up’ tensiometers with water. If they need topping up they have cavitated. Discussion In this review we have presented the various options for measuring matric potential. The most common sensor is the water-filled tensiometer. We wish to emphasize that even this instrument is commonly used incorrectly. The experimenter will often ask, ‘When should I refill?’ We hope to have shown that if you need to ask this question then your data may not be reliable. Resisting the temptation to refill, in a drying soil, will allow the identification of a point in time when the soil has dried to the air entry potential of the ceramic used to make the tensiometer bulb (typically –350 kPa). The civil engineering community has made significant advances in the use of water-filled tensiometers at very low matric potentials. It is important that plant scientists are aware of this work because it will allow them to use currently available tensiometers in a more reliable way, specifically by paying more attention to saturating the tensiometer, and by identifying and making use of data traces that appear to come from a cavitated tensiometer. An alternative to the water-filled tensiometer is the porous matric sensor, but presently its accuracy depends on calibrating individual sensors before use. A significant advantage of some porous matrix sensors is that they are relatively inexpensive. For many applications such as irrigation scheduling, this is important. Irrigation rates can be adjusted according to the response of the crop, and highly accurate matric potential data may not be needed. For more accurate estimation of matric potential with inexpensive sensors, calibration is recommended. However, the experience of Spaans and Baker (1992) suggests that this is not without difficulties. Laboratory data suggest that the use of a simple pipe to allow air to enter the porous matrix may improve the reliability of porous matrix sensors, but this remains to be tested in the field. The psychrometer can be used to measure water potential across a range of soil water contents, and the isopiestic method is highly precise, but in practice it is not often used because the measurement is more technically demanding to make. However, for some applications, such as monitoring soil during seed germination, it is the only sensible approach. There is a need for simple measurement methods that can cover a wide range of matric potentials. One emerging method is measurement of shear wave velocity. In drying soils, the decreasing matric potential increases the rigidity of the soil fabric which results in a higher shear wave velocity. Yang et al. (2008) have shown that this method compares favourably with the filter paper method; however, we believe that the shear wave method needs further development. Specifically, the effect of soil type and condition on the calibration needs to be understood. Nevertheless, Whalley et al. (2012) have shown that the shear wave velocity does provide a useful calibration with a wide range of matric potentials (see Fig. 12). Fig. 12. View largeDownload slide Matric potential plotted as a function of the velocity of shear waves using data redrawn from Whalley et al. (2012). Reprinted from Soil and Tillage Research, 125, Whalley WR, Jenkins M, Attenborough K, The velocity of shear waves in unsaturated soil. pp. 30–37, Copyright© 2012, with permission from Elsevier. Fig. 12. View largeDownload slide Matric potential plotted as a function of the velocity of shear waves using data redrawn from Whalley et al. (2012). Reprinted from Soil and Tillage Research, 125, Whalley WR, Jenkins M, Attenborough K, The velocity of shear waves in unsaturated soil. pp. 30–37, Copyright© 2012, with permission from Elsevier. An important issue to consider is whether the measurements are made at the appropriate scale. In this respect, we are constrained by the dimensions of the sensor, which is typically a few centimetres in size. However, a significant advantage of matric potential, compared with soil water content measurements, is that interpolation between point measurements can produce a contour map showing lines of equipotential, which has a physical significance. This is not true for water content measurements, where a similar plot would be empirical. Recently, neutron radiographs have allowed steep gradients in water content in adjacent roots to be visualized (Carminati et al., 2010). Presently, as far as we know, it is not possible to measure matric potential at the millimetre scale, which would be required for direct measurement. This remains a challenge (Chapman et al., 2012). In order to understand plant behaviour, whether at a molecular or field level, it is necessary to quantify the availability of water in the soil–plant system. In reporting experiments, it is helpful to describe the plant growth medium or the soil texture class and to measure its water content, but because the water release characteristic can change depending on how soil is handled, these measurements are not sufficient. Current advances in sensor technology and understanding of sensor behaviour should make quantification of soil matric potential standard practice. 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TI - Measurement of the matric potential of soil water in the rhizosphere
JF - Journal of Experimental Botany
DO - 10.1093/jxb/ert044
DA - 2013-03-22
UR - https://www.deepdyve.com/lp/oxford-university-press/measurement-of-the-matric-potential-of-soil-water-in-the-rhizosphere-VEe04ZH31c
SP - 3951
EP - 3963
VL - 64
IS - 13
DP - DeepDyve
ER -