Comparative efficiencies of liquid humic substances extracted from raw and composted agro-wastes on phosphorus release from phosphate rock
The study compared the relative effectiveness of liquid humic substances (LHS) extracted from raw and composted agro-wastes on Phosphorus (P) released from solubilization of Ogun phosphate rock (OPR). Liquid humic substances were extracted from dried raw and composted agro-wastes (swine dung, cocoa pod husk and poultry dropping) following the International Humic Substance Society (IHSS) standard procedure. Humic acid (HA) were precipitated by adjusting the LHS extracts to pH < 2 with concentrated H2SO4. The HAs were characterized using High Performance Liquid Chromatography (HPLC) and Fourier Transform Infrared (FTIR). Kinetics of P released from the dissolution of PR was carried out in duplicates by shaking 10 g OPR in 20 mL dilute LHS (5% volume basis) extracted from the raw and composted agro-wastes. The experiment was repeated with 10 g of raw and composted solid agro-waste (SAW) and 20 mL distilled water. The mixtures were shaken at regular intervals and then filtered. The filtrates were collected over time (2, 4, 6, 8, 10… 48 hrs) and were analyzed for available P using iron oxide-impregnated paper method. An incubation experiment was carried out to further monitor the release of P from OPR (0, 30 and 60 kg P ha-1) added to 100 g of air-dried and sieved soil sample (Iwo soil series) using 20 mL LHS and 10 g SAW separately. The HPLC chromatograms revealed the major peaks for all the samples at retention time(TR1) of 1.43 -1.47 minutes with a larger peak area in all the raw than in the composted samples for example, HA in raw cocoa pod covered 67.25% while the composted cocoa pod was 46.57%. Analysis of the FTIR spectra showed that HAs from the raw samples had stronger absorption within the hydrophobic (except for poultry droppings) and hydrophilic signal regions than composted samples, for example, absorbances of the raw and composted swine dung hydrophobic regions were 0.93 and 0.65 while the hydrophilic regions gave absorbances of 0.70 and 0.50 respectively. Similarly, the HA from raw samples showed stronger absorption for the acidic functional groups than composted samples (raw and composted poultry manure - 3.96, 3.69, cocoa pod - 3.90, 2.71 and swine dung - 2.37, 1.73, respectively). The kinetic results showed initial gradual increase in P concentration in all the treatments before a decline, followed by an increase and the data did not fit into any of the considered kinetic models. The incubation experiment showed higher concentrations of P released with SAW than LHS, there was no significant difference in P concentrations from raw LHS and composted LHS. Therefore, LHS extracted from raw agro-wastes is recommended for OPR solubilization in order to save time and energy that would have been exerted into composting.