Humic substances and their complexation with phosphorus and calcium during composting of pressmud and other biodegradables

Humus production is the end result of composting. Humic substances behave like weak acid polyelectrolytes and the occurrence of anionic charged sites accounts for the ability to retain cations. The release of humic substances and their chelating capacity for phosphorus and calcium during composting of pressmud and other biodegradables was studied. The C:N ratios of the compost mixtures were decreased substantially up to the 90th day, stabilized later on, and reached the optimal value of 15 in 120 days. The humic acids consistently increased throughout the decomposing period, whereas fulvic acid contents in compost mixtures increased significantly up to the 60th day; later, it decreased continuously in all the composts. The composts, which received inorganic additives, enhanced the formation of both humic and fulvic acids compared with compost without additives. A clear tendency was observed in relation to humic and fulvic acids in which a gradual increase in humic and a decrease in fulvic acid with an increase in the polymerization rate (HA:FA) and PQ value were noticed as the decomposition proceeded from the 90th day. The amount of phosphorus (P) and calcium (Ca) held by fulvic acids was significantly higher than humic acid fractions, and the retention was higher in composts that received inorganic additives than those without inorganic additives.