The effects of some humic substances and vermicompost on phosphorus transformation rate and forms in a calcareous soil

The role of some humic substances (HS) on phosphorus (P) bioavailability and fractions was investigated in a greenhouse study using a calcareous soil from Eastern, Iran.Soils were treated with two levels (2 and 4%) of Humic Acid (HA), Fulvic acid (FA), Vermicompost (Verm) and their mixtures including FA+HA and FA+HA+Verm, and incubated for 24, 360, 720 and 1080 h.The Olsen-P was determined in amended and nonamended soils after incubation time.The P release was modeled by Parabolic, Power and Elovich equations to determine the transformation rates.Since the maximum P release was found in vermicompost treated soil, P was sequentially extracted with H 2 O, NaHCO 3 , NaOH and HCl at 24 and 1080h after incubation.The highest released P was measured in vermicompost treatment and decreased as follows: HA+FA+Verm>HA+FA>HA>FA>CO. The decrease of P release data was best described by Parabolic equation (r 2 =0.83-0.93,SE=0.51-2.91).Higher transformation indices of vermicomopost amended soil than other treatments showed more efficiency of vermicompost to release of phosphorus in soil.H 2 O-P, NaHCO 3 -P and NaOH-P increased after vermicompost application and converted to HCl-P fraction so that at the end of the experiment, calcium phosphates comprised 69 and 43% of inorganic P (Pi) at 2 and 4% vermicomopost treatments, respectively.Results of the study showed that humic substances could increase P extractability and availability in soil.