Field experiments which consisted of the incubation of soil within polyvinylchloride (PVC) tubes inserted in glass jars previously buried in potato crop rows were conducted in order to measure the contribution of soil organic matter and organic amendments to the N nutrition of the crop. The experiments were carried out in Bragança, NE Portugal, in the summer seasons of 1996–1998. Five treatments including farmyard manure, poultry manure, municipal solid wastes, urea and the control were used. Manures and urea were applied in rates which correspond to 100 kg N/ha. Nitrogen released from soil organic matter and manures was checked through crop N recovered and petiole nitrate concentrations. In the plots of urea treatments 80.1, 68.4 and 98.8 mg NO3-N kg–1 were released during the entire growing seasons of 1996, 1997 and 1998, respectively. In the control treatments 64.1, 41.5 and 55.4 mg NO3-N kg–1 were recorded. The mean values of NO3-N yielded from amended plots were not statistically different than control, excluding the plots of poultry manure on the initial sampling dates at the start of incubation. The measured amounts of inorganic N mineralized in the field from native soil organic matter were five to eight times higher than previous laboratory estimates from soil analysis. These results demonstrated that soils with low levels of organic matter could release significant amounts of inorganic N during a cropping season. Soil N availability estimated from the incubation technique was confirmed by petiole nitrate concentrations and closely related to the crop N recovered. However, in the plots of urea treatments some inconsistencies were found, probably originating from the leaching of urea as a molecular form to layers below the limit of soil coring. Soil N balance showed that more than 82% of inorganic N released from organic matter came from the 14-cm soil surface layer.