Abstract
In order to provide nitrogen to the soil, in organic farming only organic fertilizers and soil conditioners are allowed. The mineralization of these products is slow, and therefore problems in nitrogen supply may arise exactly when the plants require high amounts of nitrogen. In this study, different organic fertilizers and soil conditioners, application timing and rate, and soil management techniques were compared in the open field. The soil mineral nitrogen (SMN), the nitrogen content in leaves, the increase in tree trunk circumference, and the total yield was measured for 15 different treatments including untreated control over a period of 5 years. Among the different fertilizers, best results in terms of total yield (kg/tree) over the entire study period were obtained with Azocor 105, a commercially available organic fertilizer, followed by the combined use of compost + Nutristart (liquid vinasse) and the conventional reference fertilizer ammonium sulfate. In the trials, in which different application timing and rate of Azocor 105 were compared, one single early spring application at the recommended field rate resulted in highest yield. Applying higher total amounts of fertilizer, but splitted over time (3 spring applications), did not provide any additional value in terms of yield. Among the different soil management techniques, the highest yield was achieved by using ploughing and brushing.