Field experiments were undertaken during 1998–2000 at Askov Experimental Station and Lundgård Experimental Site, Denmark, to investigate the fertilizer value of anaerobic and aerobic sewage sludges (SS1 and SS2), composted household waste (Compost) and farmyard manure (FYM). The organic residues were applied at two rates with or without supplementary mineral nitrogen (N). The effects of residue application on spring cereal dry matter (DM) yield and N-offtake were related to reference treatments with increasing rates of calcium ammonium nitrate (CAN). DM yields at the high application rates of aerobic sewage sludge (198 kg N/ha/year) and farmyard manure (300 kg N/ha/year) were comparable to the reference treatments receiving 90 kg mineral-N/ha/year. The comparable high application rate of anaerobic sewage sludge was 138 kg N/ha/year only and the DM yield was less. In contrast, the yield effect of Compost was very poor; even the high application rate of 321 kg N/ha/year yielded less than the low application rates of aerobic sewage sludge and farmyard manure, 66 and 100 kg N/ha/year, respectively.

The nitrogen fertilizer replacement value (NFRV) of the organic residues was estimated for all four combinations of the response variables DM-yield and N-offtake and the independent variables of total-N and ammonium-N applied. NFRV was in the range 49–68% for the sewage sludges and FYM based on the DM-yield[ratio ]total-N relation, and slightly smaller for the N-offtake[ratio ]total-N relation, 29–53%. The highest values were obtained for the aerobic sewage sludge, having a potential manurial value comparable to FYM. In contrast, the NFRV of Compost was low, about 10%.

The concentration of ammonium-N in organic residues and manures is often used for predicting the fertilizer value. Ammonium-N based NFRV of 160–210% for the sewage sludges indicate that water extractable ammonium-N underestimates the NFRV, probably due to the content of easily degradable organic matter in digested sewage sludge. The estimated NFRV for Compost was above 100% but connected with high uncertainty. In contrast, the ammonium-N based NFRV for FYM was only 70–87%, probably due to ammonia volatilization caused by incomplete incorporation of large quantities and the high ammonium-N[ratio ]total-N ratio in this residue.