• liquid fertilizer;
  • compost;
  • humic-like substances;
  • elemental analysis;
  • fluorescence spectroscopy

Two potential liquid fertilizers were produced from two mature “alperujo” composts by using an alkaline extraction (1M KOH at 70°C). The characteristics of the resulting humic-like substances were compared to those extracted according to the international humic substance society protocol by using elemental analysis, FTIR and fluorescence spectroscopy in both excitation-emission matrix and synchronous modes. The results obtained confirmed that the hot alkaline extraction dissolved more polymerized compounds, probably including lignin degradation by-products and/or newly formed aromatic materials. On the other hand, based on fluorescence excitation-emission matrix results, different Δλ (18, 66, and 100 nm) were selected to provide more information about humic substances in synchronous mode. The results showed that the extracted humic-like substances presented characteristics close to those of standard humic materials. As a consequence, the alkaline extraction of “alperujo” compost with 1M KOH at 70°C can be used to produce liquid organic fertilizers, thus diversifying the marketable products obtained from “alperujo,” the most abundant by-product of the Spanish olive-oil extraction industry. © 2012 American Institute of Chemical Engineers Environ Prog, 32: 1170–1178, 2013