Most growing media have a more or less pronounced hydrophobic character in relation to their moisture level (Michel et al., 2001). To remedy this, the addition of mineral materials or wetting agents has been proposed. However, the influence of these additives is often evaluated in terms of their effects on physical properties such as water-holding or air-filled capacity (Bunt, 1983; Martinez et al., 1997) or indirectly during the growing (Rivière et al., 1995) and rarely in terms of water repellency or hydrophobicity. Furthermore, the effects of theses additives on the relationship between water repellency and water content have never been investigated.
Water repellency (or wettability) of a material is defined by the ability of a liquid to spread on a solid surface and is then classically characterized by measuring the contact angle of a drop of liquid on the surface of the material (Letey et al., 1962) (Fig. 1). However, this method of measurement requires a smooth surface and, in the case of porous materials such as growing media, it thus requires the material to be compacted. As a consequence, this technique can only be used on dried materials (Jouany et al., 1992). However, the development of a methodology already described by Michel et al. (2001) and based on the capillary rise principle now allows us to characterize the wettability of porous materials in relation to their water content. The change in contact angles from the hydrophilic to the hydrophobic character of peat samples during desiccation and the decrease in wettability related to the degree of peat decomposition have already been shown using this method (Michel et al., 2001). Both methods of contact angle measurements were coupled to the qualitative water drop penetration time (WDPT) test (McGhie and Posner, 1980), most often used to estimate water repellency of reconstituted soils (Dekker and Ritsema, 1994, 2000; Dekker et al., 2000; Doerr, 1998; Letey, 1969; McKissock et al., 2000).
On the basis of these studies, we questioned the efficiency of clay addition to improve the wettability of organic growing media. Clay and more or less decomposed peats were chosen as models because of the ability of peats to become hydrophobic during desiccation and because of the lack of studies concerning the direct effect of clay addition on water repellency in growing media compared with wetting agents. The objective was then to determine whether the incorporation of additives in growing media leads not only to a change in water retention characteristics resulting from solid phase reorganization, but to a change in wettability as well, improving the ability and speed of the growing medium to rewet.
Bunt, A.C. 1983 Physical properties of mixtures of peats and minerals of different particle size and bulk density for potting substrates Acta Hort. 150 143 160
Dekker, L.W., Ritsema, C.J. & Oostindie, K. 2000 Wettability and wetting rate of Sphagnum peat and turf on dune sand effected by surfactant treatments 566 574 Rochefort L. & Daigle J.-Y. Proc. 11th Int. Peat Cong 6–12 August 2000 Quebec, Canada
Doerr, S.H. 1998 On standardizing the ‘WDPT’ and the ‘Molarity of an ethanol droplet’ techniques to classify soil hydrophobicity: A case study using medium textured soils Earth Surf. Process. Landf. 23 663 668
Jouany, C., Chenu, C. & Chassin, P. 1992 Détermination de la mouillabilité des constituants du sol à partir des mesures d'angles de contact: Revue bibliographique Science Sol 30 33 47
Letey, J. 1969 Measurement of contact angle, water drop penetration time, and critical surface tension 43 47 DeBano L.F. & Letey J. Proc. Symp. Water-repellent Soils 6–10 May 1968 University of California Riverside, CA
Martinez, F.X., Sepo, N. & Valero, J. 1997 Physical and physicochemical properties of peat-coir mixes and the effects of clay-material addition Acta Hort. 450 39 46
McKissock, I., Walker, E.L., Gilkes, R.J. & Carter, D.J. 2000 The influence of clay type on reduction of water repellency by applied clays: A review of some West Australian work J. Hydrol. 231–232 323 332
Michel, J.C. 1998 Etude de la mouillabilité de matériaux organiques utilisés comme support de culture Doctorat 3ème cycle, ENSA Rennes France
Michel, J.C., Rivière, L.M. & Bellon-Fontaine, M.N. 2001 Measurement of the wettability of organic materials in relation to water content by capillary rise method Eur. J. Soil Sci. 52 459 467
NF EN 13041 2000 Amendements du sol et supports de culture—Détermination des propriétés physiques—Masse volumique apparente sèche, volume d'air, volume d'eau, valeur de rétraction et porosité totale Association Française de Normalisation Paris, France