The surface tension of aqueous solutions of soil humic substances

Surface tensions of the following solutions were measured: (a) fulvic acid (FA) and humic acid (HA) at various pH and concentration levels; (b) FAs and HAs from widely differing origins; (c) pyrolyzed FA; (d) leaf extracts; and (e) saturated soil extracts. Both FA and HA solutions lowered the surface tension of water significantly. The surface tension of HA solutions decreased almost linearly with increasing concentration, whereas a surface tension vs. concentration plot for FA had a hyperbolic shape. Surface tensions of HA and FA solutions also decreased with increase in pH. The lowest surface tensions measured were 43.2 and 44.2 dynes/cm for FA (at pH 12.0, 3 percent w/v) and HA (at pH 12.7, 2 percent w/v), respectively. The surface tension of water is 72.0 dynes/cm at the same temperature and pressure. Numbers of molecules per 100Ųof liquid-gas interface were 1.04 and 2.47 for FA and HA, respectively. Surface tensions of FA and HA solutions at pH 7 (0.6 percent w/v), prepared from humic materials from widely differing origins ranged from 46.1 to 69.2 dynes/cm. Pyrolyzed FA, which had lost its functional groups, lowered the surface tension of water only slightly. The total acidity of the FAs and HAs showed a linear, inverse, significant correlation with surface tension. Leaf extracts depressed the surface tension more than did saturated soil extracts. From the results it appears that if there is a deficiency of FA in the soil solution, water repellence will result; but if there is an ample supply of FA, soil wettability increases.