the
recommendation of the International Humic Substances
Society (Swift, 1996). The humic substances
were extracted with 0.1 M NaOH, under N2 atmosphere,
with HA being purified with a mixture of HF
and HCl and FA passed through a column of XAD-
8 resin. Total acidity, –COOH and phenolic–OH
groups of these fractions were measured by the
methods of Schnitzer and Gupta (1965). Barium hydroxide
was used to measure total acidity and calcium
acetate to determine carboxylic groups; phenolic–OH
groups were obtained by the difference between total
acidity and carboxylic groups. All results are presented
on an oven dry ash-free basis.