Moisture release curve (Pressure-volume curves) of substrate/soil



Tamir Klein1, Maria A. Lo Gullo2, Andrea Nardini3,  Elisabetta Oddo4, Fabio Raimondo2, Tadeja Savi5, Patrizia Trifilò2

Author affiliations

1Institute of Botany, University of Basel, Switzerland; Department of Earth and Planetary Sciences, Weizmann Institute of Science, Israel

2Dipartimento di Scienze Biologiche e Ambientali, Università di Messina, Salita F. Stagno D’Alcontres 31, 98166 Messina, Italy

3Dipartimento di Scienze della Vita, Università di Trieste, Via L. Giorgieri 10, 34127 Trieste, Italy

4Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche, Università di Palermo, via Archirafi 38, 90123, Palermo, Italy

5University of Natural Resources and Life Sciences, Vienna, Department of Integrative Biology and Biodiversity Research, Institute of Botany, Gregor-Mendel-Straße 33, 1180 Vienna, Austria


This protocol describes the procedure for the elaboration of a moisture release curve of soil/substrate using a psychrometer (dewpoint hygrometer).


A quantitative description of the water status of soil/substrate is essential for deep understanding of plants behavior. Soil water potential can be measured with several approaches i.e. tensiometers, pressure-plate apparatus, psychrometers, porous matrix sensors etc. Psychrometers can measure water potentials across a wide range of soil water content and with high precision (Whalley et al., 2013).


  • pot (1.5 l)
  • filter membrane
  • psychrometer (e.g. Dewpoint Hygrometer, WP4, Decagon Devices)
  • laboratory spoon
  • sampling holders
  • balance (precision 0.0001 g)
  • oven

Units, terms, definitions

The Moisture release curve or Soil pressure-volume curve (PV curve) is the relationship between the relative water content (RWC, g) and the water potential (Ψsoil, MPa) of a soil/substrate. It provides information on the soil water holding capacity and on the theoretical amount of water available to vegetation.


  1. A homogeneous sample of soil/substrate (approximately 1 liter) is placed in a pot containing a piece of filter membrane which prevents the loss of fine soil particles during wetting.
  2. The substrate is slowly and gently wetted with abundant tap water to full saturation.
  3. The pot is left for 5-20 minutes on the bench to allow drainage of excess water.
  4. The soil is carefully mixed with a laboratory spoon and approximately 20-25 small sub-samples, weighing a few grams each, are placed in sampling holders.
  5. The fresh weight (FW) of 3-4 saturated samples (Ψsoil = 0 MPa) is immediately recorded with a balance.
  6. The soil samples are left dehydrating on the bench and at different time intervals their water potential (Ψsoil) is measured using a psychrometer (Whalley et al., 2013). Samples are dehydrated until water potentials of -5/-6 are reached.
  7. After each Ψsoil measurement the sample FW is immediately recorded.
  8. All samples are oven-dried for 24 h at 70°C in order to get their dry weight (DW).
  9. The relative water content of samples is calculated as: RWC = (FW – DW) /DW. The average of the highest values of RWC, measured immediately after saturation of the soil sample, is considered as water content at saturation (SWC).
  10. All Ψsoil recorded during sub-samples dehydration are plotted versus the corresponding RWC values. The regression curve is used to extrapolate the RWC at Ψsoil = -1.5 MPa which is considered as a reference permanent wilting point (Kramer and Boyer, 1995).
  11. The theoretical amount of available water to vegetation (AWC) is calculated as the difference between SWC and the water content at Ψsoil = -1.5 MPa.

Notes and trouble shooting tips

If the exact turgor loss point of species of interest is known, than AWC can be calculated as the difference between SWC and the water content at Ysoil = Ytlp.

The psychrometer can be used for spot measurements of soil water potential. The soil is sampled in the field, placed in sampling holders which are covered with lids, and wrapped in cling film. The sampling holders are inserted in a plastic bag containing a piece of wet filter paper and transported to the laboratory using a cool bag. Water potential of samples have to be measured within 3 h.

Several psychrometers can be found in commerce. The Dewpoint Hygrometer, WP4, Decagon Devices has the advantage of the relatively bigger sample volume (approximately 6.5 cm3) that can be measured if compared to the other psychrometers.

Literature references

Kramer PJ, Boyer JS (1995) Water relations of plant and soils. Academic Press Inc, New York.

Whalley WR, Ober ES, Jenkins M (2013) Measurement of the matric potential of soil water in the rhizosphere. Journal of Experimental Botany 64, 3951-3963.