Humic substances (humic acid and fulvic acid) from the air-dried compost samples were extracted using the procedure of Huang et al. (2006) and analyzed using the KBr pellet method (Wei et al., 2007) with the Nicolet 550 Magna-IR spectrometer (Nicolet Instrument Corporation, Madison,WI) using a detection range between 350 and 4000 cm1.