Efficiency of organic nitrogen fertilization of potato in northeast Portugal

The study was conducted over a 4 year period with the purpose of measuring N mineralization using an in situ incubation technique, following the application of farmyard manure, poultry manure or municipal solid waste. The results varied from -10 % (net immobilization) to +28 % (net mineralization) in relation to the total organic N applied. N fluxes were assessed through plant N analysis during the growing season and through determination of potato tuber dry weight (TDW), N uptake by tubers and the apparent N recovery (ANR) of added N at harvest. Organic material applications slightly increased TDW above the control treatment (9 to 25%), although the results indicate a shortage of N, where no N was added to the crop. The effect of organic fertilization on N uptake by tubers was also limited. Petiole nitrate concentration and estimated net N mineralization were in agreement with trends in TDW and N uptake by tubers, with no differences in soil N availability between the control and organic treatments. Values of ANR were particularly low, ranging from 4 to 23 %. The results suggest that caution is needed when judging the contribution of organic fertilization to N nutrition of crops, which have a short growing season. Synchronization between N release and plant uptake is discussed.