Composition of humic acids with respect to the degree of humification in cultivated soils with and without manure application as assessed by fractional precipitation

Abstract

A fractional precipitation method with 0.01 mol L⁻¹ NaOH–ethanol solutions was used to assess the temporal change in the composition of humic acids (HAs) in upland field soils with respect to the degree of humification as expressed by the degree of darkening. A Typic Hapludult (Togo) and Pachic Melanudand (Kuriyagawa) with and without the application of cattle manure were examined. Humic acids were fractionated into five to eight fractions, and a fraction precipitated at a lower ethanol concentration tended to have a larger degree of humification in each soil. The slight increase in the degree of humification of Togo HAs in both the manured (40 Mg ha⁻¹ year⁻¹) and non-manured plots after 10 years was exhibited as a shift in the dominant fraction. The distribution pattern in the fractionation of Kuriyagawa HAs was similar between the initial soil and the manured (80 Mg ha⁻¹ year⁻¹) and non-manured plot soils after 19 years. However, the degree of humification of the dominant fraction that was precipitated at the lowest ethanol concentration (200 mL L⁻¹) was smaller in the manured plot soil than in the initial and non-manured plot soils. In another Kuriyagawa soil with manure applied at 160 Mg ha⁻¹ year⁻¹ for 19 years, the amounts of three HA fractions with smaller degrees of humification were larger than those in the other Kuriyagawa soils, and the dominant fraction was observed at a higher ethanol concentration (400 mL L⁻¹). Thus, fractional precipitation was effective in assessing changes in the degree of humification of HAs induced by agricultural practices, including manure application.