Little is known regarding how to discard the sludge accumulated in vertical flow constructed wetlands (VF-CWs) and what the potential impacts could be. The objective of this paper was to evaluate the mineralization of organic matter (OM) in soil and productivity of tifton 85 grass (Cynodon spp.) after incorporating sludge collected at different depths from a VF-CW (used to treat septic tank sludge), to a tropical soil (Oxisol). Sludge samples were collected at depths of 0–5, 5–10 and 10–15 cm from a VF-CW that was used over a period of three years. The sludge collected at each depth was incorporated into the soil at a dose equivalent to 30 g m–2 year–1 of total nitrogen, and the experimental area was planted. During a period of 215 days, total and easily oxidizable carbon, total, ammonia, nitric and organic nitrogen in the residue-soil mixtures were analyzed. Based on the data obtained, the mineralization fractions (MF) were estimated for the specific monitoring period and annually considering first order and two-phase kinetic equations. The annual MF of the OM were higher than 96% and the sludge-amended soil resulted in an increase in grass yield.