For the determination of humic acids (HA) and fulvic acids (FA)
contents a method combining the procedures proposed by ISO 5073
(1985) and the International Humic Substances Society (Swift, 1996;
IHSS, 2007) was followed. From each sample a mass equivalent to 2 g
of organic matter on a dry and ash-free basis was placed in a 250 ml
Erlenmeyer flask, and 100ml of 0.1ΜNaOHwere added. The flaskwas
sealed and shaken for 24 h at room temperature. Afterwards the alkaline mixture (dark colored) was centrifuged in order to separate
the supernatant from the solid residue. The whole procedure (NaOH
addition, shaking and centrifugation) was repeated until a translucent
suspensionwas obtained. The final solid residue represents the humin
(HU) fraction plus the residual mineral matter. The suspension
collected from all the centrifugations represents the HS fraction; this
was acidified with HCl until pH 1. HA (in colloidal form, although some
inorganic matter might also contribute) were separated from the FA
(in solution) by centrifugation. The HA fractionwas dried at ∼70 °C for
48 h and weighed. Accordingly the mineral matter content of the HA
was determined after combustion at 550 °C, and subtracted. The FA
were extracted from the residual solution using the XAD-8 resin and a
cation exchange resin (CER), and finally recovered in solid form after
lyophilization. Both HA and FA contents were calculated as weight
percentages on a dry basis.