The primary adsorption and desorption curves describe limits to equilibrium states. Hasan is right that there is no one true equilibrium, but a response area for equilibria which will depend on the adsorption/desorption history. The question how long you have to wait to attain equilibrium is independent of the above discussion: infinitely long.

The answer is that there is no true equilibrium condition. Because of the porous medium characteristics, two different curves are obtained in the mid of the pF curve. during emptying the ca pillar pores or filling up them, soil holds different amount of water even in same suction/ tension levels. To understand the process I recommend to search for ' ink bottle' theory . To overcome the confusion better to mention which of the methodology ( drying or wetting the soil) is fallowed to get the curve.

Hasan Öztürk

Ankara University,Turkey

Thanks for the information Hasan. I think what you refer to (the ink bottle effect) mostly applies to wet end of the retention curve (pF1 to around pF4.8) where capillarity plays a major role. I am more interested in the dry part of the retention curve (pF5 to pF6.6) which is often measured using dewpoint potentiameters. In this region, one can have true equilibrium in theory as long as one is willing to wait.

The methodology we used was the chilled-mirror dewpoint technique with automated drying and wetting of the samples (pls see the methodology section and Fig. 1 of the following OPEN ACCESS paper for details: https://www.soils.org/publications/vzj/articles/13/1/vzj2013.10.0185 )

Looking forward to hearing more opinions on this

The primary adsorption and desorption curves describe limits to equilibrium states. Hasan is right that there is no one true equilibrium, but a response area for equilibria which will depend on the adsorption/desorption history. The question how long you have to wait to attain equilibrium is independent of the above discussion: infinitely long.

It's also worth noting that it may be difficult to wholly exclude hysteresis from a desorption curve as any entrapped air in the saturation process will prevent complete saturation, although it is very difficult to determine whether that has occured. Theoretically, any desorption curve produced in the lab may in reality be a scanning curve if entrapped air prevented total saturation. However, I think there is a general consensus in the literature that where adequate precautions have been taken, i.e. gradual saturation under vacuum, that laboratory desorption curves are taken to be 'true' desorption cuves.

Thanks a lot Sara for the insightful input. Would you be kind to give me one or two papers (or sources) which mention that a carefully obtained desorption curve may be close to the true curves?

Secondly, our work focuses on the dry part of the retention curve (-10 MPa to -480 MPa; considering that wilting point is just -1.5 MPa) so i am sure entrapped air may not apply in this case

Sure. Dane and Topp, 2002 and Nimmo 1990 suggest using deaerated CaSO4 solution as a saturation fluid to limit entrapped air. Reatto, 2008 also saturated the samples from below under vacuum. I am using the centrifuge method to determine the desorption curve and these are some of the steps I am taking to ensure thorough saturation.

Dane, J.H. & Topp, G.C. (2002) Methods of soil analysis. Part 4: Physical methods. Soil Science Society of America Book Series No. 3

Nimmo, J.R. (1990) Experimental testing of transient unsaturated flow theory at low water content in a centrifugal field. Water Resources Research. 1951-1960

Reatto, A., da Silva, E.M., Bruand, A., Martins, E.S. and Lima, J.E.F.W. (2008) Validity of the centrifuge method for determining the water retention properties of tropical soils. Soil Science Society of America Journal. 72(6): 1547-1553.

It may also be worth considering whether you are using disturbed or undisturbed samples, as the intact structural porosity of undisturbed samples may effect the likelyhood of entrapped air.

Thanks Sara