Because of their organic matter content, the surface tension of water of soils is about 8 to 9 erg cm-' (8 to 9 x lo-' J cm -* ) less than that of pure water. It is estimated that the surface tension of soil solution is 63-64 erg cm -2 in the surface soils studied.In troducrion THE influence of the solid phase of soils on the properties of the liquid phase arises from a) the surface forces of the solid, and b) soluble compounds contained within the solution. As result of (a) the water changes its properties only at the interface of the solid and water, whereas as a consequence of (b) the properties of the water in bulk are changed.While many studies have been devoted to the first phenomenon, comparatively little has been done on the second, possibly because the influence of soluble substances has been thought to be insignificant or not detectable. Such opinion is not correct as w i l l be shown below.The soil contains inorganic and organic soluble compounds. Most of the first group are polar, whereas the second group of molecules, for example alcohols and acids and their derivatives, may contain polar and nonpolar groups. To the latter, asymmetrical groups of molecules with polar (OH, NHz , etc.) and nonpolar (CH3, CH2, etc.) groups, the term 'amphiphilic' has been applied (Joly, 1972). This term signifies that a molecule has a tendency to become adsorbed on hydrophilic (with polar end) and on hydrophobic (with nonpolar end) surfaces.The addition of electrolyte, with very few exceptions such as NH,OH, H N 0 3 , HCI, increases the surface tension of water by approximately 1.2 erg cm-' per unit increase in normality. (Surface tension is conventionally expressed in erg cm-' . In SI units 1 erg = lo-' joule). There are indications that the surface tension of dilute solutions ( l O -'~/ l ) is slightly (0.01-0.02 erg cm-') less than that of pure water, but increases at higher concentrations (Harned and Owen, 1950). Thus, in saline (Solonchak) soils the surface tension should be > 7 2 erg cm -' , but as will be shown below, due to the influence of organic matter, which is greater than that of inorganic salts, the surface tension of soil water is actually always less than 72 erg cm-' .There is no doubt that soil humus and humic acid are in part surface active. As shown by Tschapek and Wasowski (1976) this surface activity is sufficient for a 5 per cent solution of peptized humic acid to lower the surface tension of water from 72 to 45 erg cm-'.As a result of biological activity there might therefore be present in soil surface active substances which can decrease the surface tension of soil water. Characteristically, these amphiphilic substances are of medium molecular weight and have asymmetric molecules.