Properties of soil humic substances in fractions obtained by sequential extraction with pyrophosphate solutions at different pHs

Abstract

Humic (HAs) and fulvic acids (F As) were obtained from A horizons of four different soils (a Fibric Histosol, Dystric Cambisol, Umbric Andosol, and buried Umbric Andosol) by the sequential extraction with pyrophosphate solutions at different pHs (i.e. at pH 3, 5, 7, 9, 11, and 13). The yield of the FAs from the four different soils decreased with the increase in pH values from 3 to 7, and then, increased or remained constant with pH values from 9 to 13. The distribution pattern of the yield of HAs differed among the four soils, and the total yield for HAs extracted at pH values extending from the acid to neutral region (3–7) was equal to or higher than that in the alkaline region (9–13), except for the Histosol. In particular, the yield of buried Andosol HAs at pH values of 3,5, and 7 was remarkably high, ca. 25–30% of total HAs with pyrophosphate extractants. Sequential extraction with the pyrophosphate solutions gave sufficient amounts of HAs extracted for wide-ranging pH values extending from the acid to alkaline region to compare the properties of individual HAs based on the difference in pH values. Particle size distribution analysis of each HA determined by gel permeation chromatography (GPC) suggested that the proportion of components with a larger particle size differed among the HAs extracted from the four soils for the same pH value. It is, however, interesting to note that the proportion of components of HAs with a larger particle size in the four different soils increased gradually and regularly with the increase of the pH value. These results suggest that this sequential extraction technique is suitable for obtaining a series of HAs differing in particle size.