Molecular size distribution and hydrophilic and hydrophobic properties of humic acids isolated from forest soil

The aim of the present paper was to determine hydrophilic and hydrophobic properties and the degree of polydispersity of humic acids depending on their degree of maturity and the properties of the plant material participating in the process of humification. The study involved humic acids isolated from samples taken from organic and mineral horizons of forest soils in forest stands as follows: 1) pine with an admixture of hardbeam, 2) European beech, 3) elm, 4) fir, 5) spruce, 6) thuja. As shown by our results, the properties of humic acids in both horizons studied were determined by the kind of plant litter, i.e by the dominant tree species in the stand. The humification process was characterized by an increase of the proportion of humic acids of low molecular size as well as by an increase of hydrophilic fractions and a decrease of hydrophobic fractions. The degree of polydispersity of humic acids molecules was related to their hydrophilic and hydrophobic properties. Humic acids with a large proportion of high-molecular fractions had also a high proportion of hydrophobic fractions.