A field study was conducted to investigate the potential of residues from renewable energy production for fertilizing pastures. Six treatments including manure or biogas sludge, each combined with wood ash at 0, 1 and 3 t ha(-1) were applied at the same N equivalent (190 kg N ha(-1)). Three soil samplings and two cuttings were carried out within one growing season (4 months). Plots amended with biogas sludge had a higher EC and total N, those amended with wood ash at 3 t ha(-1) scored higher soil pH, EC and C/N ratio. K concentrations increased in plots amended with biogas sludge. Basal respiration, microbial biomass, metabolic quotient and C-mic/C-org from the differently treated soils were not influenced neither by the nature of the organic waste nor the amount of wood ash added. The time of sampling (seasonal effects) was found to influence the soil pH, EC and all the microbial parameters. A higher forage yield and lower proportion of leguminous plants was observed for soil treated with anaerobic sludge. It may be concluded that the combination of anaerobic sludge and wood ash has a double positive effect, both on some selected soil chemical parameters as well as on total forage yield. The results obtained need to be confirmed in long term experiments, and the best practice for a combined application still needs to be worked out. (C) 2012 Elsevier Ltd. All rights reserved.