If I get the point, you want to measure the diffusion rate within soils sample ? If so I can propose a protocol I used to measure water diffusion through thin erodable polymer films. I used ATR FTIR spectroscopy. Simply I layed a polymer film on the ATR crystal (you can use a layer of soil to test) and I record other about 2 hours the IR spectra. Progressively the strong O-H water band increases; Plotting the water band area as a function of time allows accessing  Fick diffusion coefficents and so compare different soils.

M. Thouvenin, I. Linossier, O. Sire, J.-J. Péron, K. Vallée-Réhel

« Structural and Dynamic Approach of Early Hydration Steps in Erodable Polymers by ATR-FTIR and Fluorescence Spectroscopies ».

Macromolecules (2002), 35, 489-498

Good Luck !

Dear Gustavo

Have you ever make an attempt to measure contact angle for soil and water sample, using imaging method?

Cheer

I am afraid that contact angle will be hard to perform since the spatial heterogenity of soil particles is large as compared to the drop size and it needs a perfectly flat surface to measure the angle.

Dr. Olivier,

Do your method can be used to test the hydrophobicity of sphagnum (calcined and not calcined ) ?

Quite a tricky question ! Just consider that the method allows following a z axis water diffusion front. If you can apply the matter to probe as a film on a flat surface, it should be right. In my memory, sphagnum has a huge ability to trap water (like a sponge). This buffering effect must be taken into account. Intuitively, I should prepare sphagnum "films" by pressing them while allow to dessicate. You would be hence able to monitor the rehydration rate of calcined (or not) samples. Consider also that the water diffusion may avoid hydrophobic domains and percolate through interleaved domains. May e an alternative procole I used to bacterial exopolysaccharides would perform better : lay as described a dehydrated sample on the ATR crystal. Then cover it (without direct contact) by a moistened filter paper (a paper and above a moistened piece of sponge allowing a water vapour saturation). Then monitor the IR signal.

E. Lahaye, T. Aubry, N. Kervarec, P. Douzenel, O. Sire

Does water activity rule P. mirabilis periodic swarming? I. Biochemical and functional properties of the extracellular matrix.

Biomacromolecules (2007) 8, 1218-1227.

Tanks Dr. Sire for your answer and the reference. I will try your method and adapt it if needed.

Greetings.

We have studied the effects on soil properties of planting trees (eucalyptus and pinus) on soils previously under their natural grassland vegetation in Uruguay. We fund that the water desorption curves of undisturbed soil samples of medium and sandy textured soils, taken from the A horizon at different depths, and also from the upper part of the B horizon, hold less water under trees than under pastures, at the same tension (or water potential), at 0,1 and 0,33 Atm. At the same time, we didn´t find bulk density or porosity differences. We believe that this was due to the hidrofobicity caused by the leaves of the trees decomposition products infiltrating into the forested soil. You can consult  one of our publicatios with this and other data (in spanish, but with an abstract in english: Delgado, S.; Alliaume, F.;García Préchac, F. y J. Hernández (2006). Efecto de las plantaciones de Eucalyptus sp. sobre los recursos naturales en Uruguay. Parte II: Suelos, AGROCIENCIA., Vol. X Nº 2 , 95 - 107; you can get it from the site www.fagro.edu.uy, where the publications of this journal are available free or ask me for a copy by e-mail:[email protected]).

But, regarding your question, hydrofobicity could be detected by comparing the water holding capacity of the soil under the condition in which this effect could be present, compared with the soil without this effect.

Interesting discussion . How do you differentiate between hydrophobicity  ( often it is  soil–water contact angle >0° ) and water repellency..?We have observed this phenomenon in crop like citrus..?

Here are some concerned literature...?