Direct expression of an index for the degree of humification of humic acids using organic carbon concentration

One of the indices for the degree of humification of humic acids, RF, was evaluated in terms of organic C concentration. Thirty-six humic acid samples varying in the degree of humification were analyzed for comparison of the volumes of 0.02 M KMnO4 consumed by humic acid solutions, with their organic C concentration determined using a dissolved C analyzer. A significant correlation (r = 0.981***) was found between them regardless of the degree of humrncation, and RF could be replaced with A 600/C, the absorbance at 600 nm per mg C mL−1 in 0.1 M NaOH, by multiplying by a factor of 0.0648 (r = 0.994***). Accuracy of the organic C concentration of the humic acid solution that was determined using the dissolved C analyzer was confirmed by comparing with the C content determined by the dry-combustion method. Although the organic C concentrations in the humic acid solutions estimated by a colorimetric method with potassium dichromate-sulfuric acid as reagent were significantly lower than those determined using the dissolved C analyzer, the high correlation between them (r = 0.918***) enabled to estimate A 600/C by the colorimetry. In the A 600/C-log (A 400/A 600) (= Δlog K) diagram, A 600/C values of 2.5 and 5.0 could be applied instead of RF values of 40 and 80 as boarders to classify humic acids into lYPes A, B, and Rp. Significant correlations were also found between E 4/E 6 , the ratio of absorbances at 465 and 665 run in 0.025 M NaHC03 , and A 400/A 600 or Δlog K except for a sample containing a high proportion of Pg, the green fraction, which indicated the interchangeability between them with exception. Shortening of the period for extracting humic and fulvic acid fractions without changes in quantity and quality was also examined using four typical Japanese soils. Modification of extraction with standing and intermittent shaking with 0.1 M NaOH at 25°C for 48 h in the present NAGOYA method (Kuwatsuka et al. 1992: Soil Sci. Plant Nutr., 38, 23–30) to continuous shaking under the same conditions enabled to shorten the period of extraction to 24 h. Shorter periods of 4 hand 16 h (overnight) were considered to be insufficient for a Fluvisol containing humic acids with a low degree of humification.