Reviewer: Robinson DA2
Measurement unit: soil water potential – total soil water potential: m in J kg–1 (energy per unit mass), y in N m–2 (energy per unit volume) or h in m (energy per unit weight) v. volumetric water content (m3 m-3). Although only m has the unit of potential in the strict sense, the various expressions are equivalent under the assumption of constant density of water and all expressions are generically used in soil and plant sciences; Measurement scale: plot; Equipment costs: €€€; Running costs: none; Installation effort: moderate; Maintenance effort: low; Knowledge need: moderate; Measurement mode: data logger
Whereas soil moisture indicates the amount of water in the soil, the soil water potential provides information on the force with which that water is held by the soil matric forces (soil suction) in addition to chemical forces at play (e.g. salts): in other words how soil water is retained. Root water uptake by vegetation is determined by the difference in the total water potential between soil and root xylem (Steudle & Peterson, 1998). While the soil water potential may consist of several terms (e.g. Jury et al., 1991), the dominant term in unsaturated soil is the matric potential which describes water retention as a result of capillary forces in plants and soil, in addition to molecular imbibition forces associated with cell walls in plants and colloidal surfaces binding some of the soil water. Although soil water retention is harder to measure than soil water content, it is essential in dry soils where soil moisture sensors may fail to pick up small changes in soil moisture. While only small changes in soil moisture occur in dry soil, these reflect a large change in the soil water potential and thus have large consequences for the ability of microbes and vegetation to take up water. Root-sourced signals in response to soil water availability appear to play a key role in regulating stomatal aperture (e.g. Bacon, 2004). Moreover, as plants may exhibit adaptivity to changing environmental conditions (Rodriguez et al., 2008; von Arx et al., 2012) and exhibit differences in drought sensitivity (Engelbrecht et al., 2007; Harnett et al., 2013), the protocol can also be of use in other study types considering climate change and land-use change.
3.4.1 What and how to measure?
Soil water potential measurement
Tensiometers filled with a polymer solution (called osmotic, polymer, or full-range tensiometers) are currently the only instruments capable of measuring soil water matric potential directly with adequate accuracy and low maintenance under field conditions (Bakker et al., 2007; van der Ploeg et al., 2010). The measuring range is between 0 and 2 MPa (1 Pa = 1 Nm–2) soil suction with 0.1% full scale accuracy for the pressure sensor and 0 to 40 °C with 0.01 °C accuracy for the temperature sensor. The osmotic properties of the polymer ensure that the tensiometer recovers negative pressure measurement ability automatically after drying below -2 MPa followed by rewetting of the soil. The measurement volume is around the volume of the employed ceramic interface between soil and polymer solution. The sensors can be installed as stand-alone with a data logger and lithium battery, or employed as part of a multi-port data logger with battery.
Alternatively, water-filled tensiometers are widely used to monitor the soil water matric potential and have been used for almost 100 years (Or, 2001; Young & Sisson, 2002). However, measurements are limited to about 1 atmosphere (e.g. 0.1 MPa) soil suction under field conditions at sea level. Porous matrix sensor methods are available that employ dielectric methods to measure water content in a ceramic material with known water retention characteristics and in equilibrium with the surrounding soil (e.g. Whalley et al., 2007). Comparison under laboratory conditions of a porous matric sensor type with a measurement range of 0.01 to 0.5 MPa soil suction with polymer tensiometers shows good reliability, although possible temperature effects on performance are unknown (Degré et al., 2017).
Where to start
Bakker et al. (2007) describe the principles of the polymer tensiometer; Degré et al. (2017) present a comparison of polymer tensiometers with two alternative porous metric sensor types; Or (2001) provides a historic overview of tensiometry; van der Ploeg et al. (2010) show how polymer tensiometers perform in two soil types; Whalley et al. (2007) explain the principle of a dielectric porous matrix sensor; Young & Sisson (2002) provide a technical overview of conventional tensiometry.
3.4.2 Special cases, emerging issues, and challenges
Sensors considering measurement of the soil water potential are installed below the soil surface, preferably inserted horizontally to prevent preferential flow of water along the data cables that lead to the data logger. The sensors need to be in good contact with the surrounding soil. Compensating for temperature effects in the shallow subsoil is necessary for all soil water potential sensors, but especially for polymer tensiometers, which therefore generally have an integrated temperature sensor.
Theory, significance, and large datasets
Bolt et al. (1976), Corey & Klute (1985), Grant & Backmann (2002), Groenevelt & Bolt (1969), Nitao & Bear (1996)
More on methods and existing protocols
Young & Sisson (2002)
Bakker, G., van der Ploeg, M. J., de Rooij, G. H., Hoogendam, C. W., Gooren, H., Huiskes, C., … Kruidhof, H. (2007). New polymer tensiometers: Measuring matric pressures down to the wilting point. Vadose Zone Journal, 6(1), 196-202.
Bacon, M. A. (2004). Water use efficiency in plant biology. In M. A. Bacon (Ed.), Water Use Efficiency in Plant Biology. Oxford: Blackwell.
Bolt, G. H., Iwata, S., Peck, A. J., Raats, P. A. C., Rode, A. A., Vachaud, G., & Voronin, A. D. (1976). Soil physics terminology. Bulletin of the International Society of Soil Science, 49, 26-36.
Corey, A. T., & Klute, A. (1985). Application of the potential concept to soil water equilibrium and transport. Soil Science Society of America Journal, 49(1), 3-11.
Degré, A., van der Ploeg, M. J., Caldwell, T., & Gooren, H. (2017). Comparison of soil water potential sensors: a drying experiment. Vadose Zone Journal, 16(4), 0067.
Engelbrecht, B. M. J., Comita, L. S., Condit, R., Kursar, T. A., Tyree, M. T., Turner, B. L., & Hubbell, S. P. (2007). Drought sensitivity shapes species distribution patterns in tropical forests. Nature, 447, 80-82.
Grant, S. A., & Backmann, J. (2002). Effect of temperature on capillary pressure. In P. A. C. Raats, D. Smiles, & A. W. Warrick (Eds.), Environmental Mechanics: Water, Mass and Energy Transfer in the Biosphere (pp. 199-212). Washington: AGU Geophysical Monograph 129.
Groenevelt, P. H., & Bolt, G. H. (1969). Non-equilibrium thermodynamics of the soil-water system: review paper. Journal of Hydrology, 7(4), 358-388.
Hartnett, D. C., Wilson, G. W., Ott, J. P., & Setshogo, M. (2013). Variation in root system traits among African semi‐arid savanna grasses: Implications for drought tolerance. Austral Ecology, 38(4), 383-392.
Jury, W. A., Gardner, W. R., & Gardner, W. H. (1991). Soil Physics. New York: John Wiley & Sons.
Nitao, J. J., & Bear, J. (1996). Potentials and their role in transport in porous media. Water Resources Research, 32(2), 225-250.
Or, D. (2001). Who invented the tensiometer? Soil Science Society of America Journal, 65(1), 1-3.
Rodriguez, R. J., Henson, J., Van Volkenburgh, E., & Hoy, M. (2008). Stress tolerance in plants via habitat-adapted symbiosis. ISME Journal, 2, 404-416.
Steudle, E., & Peterson, C. A. (1998). How does water get through roots? Journal of Experimental Botany 49(322), 775-788.
Van der Ploeg, M. J., Gooren, H. P. A., Bakker, G., Hoogendam, C. W., Huiskes, C., Koopal, L. K., … de Rooij, G. H. (2010). Polymer tensiometers with ceramic cones: Direct observations of matric pressures in drying soils. Hydrology and Earth System Sciences, 14(10), 1787-1799.
von Arx, G., Archer, S. R., & Hughes, M. K. (2012). Long-term functional plasticity in plant hydraulic architecture in response to supplemental moisture. Annals of Botany, 109(6), 1091-1100.
Whalley, W. R., Clark, L. J., Take, W. A., Bird, N. R. A., Leech, P. K., Cope, R. E., & Watts, C. W. (2007). A porous‐matrix sensor to measure the matric potential of soil water in the field. European Journal of Soil Science, 58(1), 18-25.
Young, M. H., & Sisson, J. B. (2002). Tensiometry. In J. H. Dane, & C. G. Topp (Eds.), Methods of Soil Analysis, Part 4—Physical Methods (pp. 575-678). Madison: Soil Science Society of America.
Author: van der Ploeg M1
Reviewer: Robinson DA2
1 Soil Physics and Land Management, Wageningen University, Wageningen, the Netherlands
2 Centre for Ecology & Hydrology, Environment Centre Wales, Bangor, UK