Humic acids (HA) were precipitated by the acidification of the HLS
extract to pH 1.0 with concentrated HCl. Precipitated
HAs were separated from fulvic acids (FA) by centrifugation
at 4000 rpm for 20 min and redissolved in a sodium
hydroxide solution (0.1 M). Fulvic acids were obtained
from the supernatant solution recovered after the precipitation
of the humic acids. The total organic carbon content
of HA and FA solutions was measured by the dichromate
oxidation method (Walkley and Black, 1934). Spectrophotometric
measurements were carried out with a Perkin
Elmer Lambda 20 spectrometer using quartz cells. The
E4/E6 ratios were calculated as the ratio of absorbance at
465 and at 665 nm according to Schnitzer and Khan
(1972).