The characterization of pig sludge liquid fraction (PSLF) was conducted to achieve a better understanding of its chemical and physical chemical characteristics. Total dissolved organic carbon (TDOC) from pig sludge was separated into two fractions: acid insoluble dissolved organic carbon (AIDOC) and acid soluble dissolved organic carbon (ASDOC). Elemental analysis, acidic functional group determination, E4/E6 ratio, Gel‐Filtration Cromatography and FTIR Spectroscopy of ASDOC and AIDOC showed a similarity with humic and fulvic acids, isolated from soils and organic wastes. Phosphorus‐31 Nuclear Magnetic Resonance spectra demonstrated that the organic P in PSLF was present as P‐monoester. The complexing capacity of DOC vs. trace metals was demonstrated by Gel Filtration Chromatography and FTIR Spectroscopy. The interactions of DOC, Cu, and Zn, present in PSLF, with a calcareous soil were evaluated by adsorption isotherms that indicated that Zn was completely adsorbed by the soil, while Cu followed the same behavior as DOC, with only partial adsorption. Therefore, both Cu and DOC should be taken into consideration as a possible groundwater pollution hazard after pig sludge application to calcareous soil.