Over a period of 2 yr, the effects of dehydrated septic tank sludge application on the chemical properties of a severely disturbed forest clayey soil were assessed and compared with application of native forest floor (i.e., from neighboring forest). Six treatments [fresh and mature sludges × two depths (15 and 25 cm), forest floor, and a control] were replicated three times according to a complete random design. Total organic C and N concentrations of amendments and their chemical structure, based on13 C nuclear magnetic resonance (NMR) spectroscopy, were determined. Mineral soil C and N concentrations and C mineralization rates were monitored as well as nutrient supply rates using Plant Root Simulator™ probes. White spruce [Picea glauca (Moench) Voss] seedling foliar nutrition and growth were also monitored. NMR spectroscopy revealed differences among amendments, with the forest floor spectra displaying lower O-alkyl C and higher alkyl C and carbonyl C proportions relative to sludge. Neither soil C concentrations nor mineralization were significantly improved in the mineral soil under any treatment, even at application rates exceeding 700 t sludge ha?1 (dry mass). The sludges supplied more NO3 and P, and less NH4 and K to the mineral soil than the forest floor and control. Increased nutrient availability under sludge and forest floor generally resulted in improved foliar nutrition and growth of white spruce seedlings. Despite differences in organic matter quality and mineral N form supplied by sludge and forest floor, sludge application is a valid restoration approach. © 2018, Agricultural Institute of Canada. All rights reserved.