Soil applied nitrogen and foliar fertilizers together with biostimulators increase productivity but decrease antioxidative potential in hazelnut A. Solar 1 , J. Jakopic 2 , F. Stampar 2 and R. Veberic 2 1 Department of Agronomy, Biotechnical Faculty, University of Ljubljana, Experimental Field for Nut Crops Maribor, Vinarska 14, 2000 Maribor, Slovenia; 2 Department of Agronomy, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia. Abstract A combination of soil applied nitrogen (N) together with foliar applied macro- and micro-nutrients and biostimulators were used in a production orchard of ‘Istrska dolgoplodna leska’ during two consecutive years in order to improve quantity and quality of the yield. Calcium ammonium nitrate (KAN) and calcium cyanamide (CC) were applied three times and once, respectively during spring time. Foliar program (FP) included both aminoacids and nutrients, such as Fe, Zn, B, Co, Cu, Mn, Mb, P, K, S and N within products Delfan plus, Tradecorp AZ plus, Calitech, Trafos K, Folur and Tytanit. Nutritional sprays were combined with bacterial blight and hazelnut weevil control, performed from leafing-out to postharvest period. Four treatments i.e., KAN, CC, KAN-FP and CC-FP were compared with untreated control (C). In two-year average, the KAN-FP combination caused an increase of the yields from 8.2 (CC-FP) up to 34% (C). The same treatment resulted in the highest kernel percentage, but the lowest percentage of marketable kernels. When only KAN was soil applied, the heaviest in- shell and kernel weight, as well as the highest increase of number of the nuts per fruiting in one-year-old shoot (nuts shoot -1 ) were observed. The applied CC resulted in an increased shoot length and basal diameter, as well as the nuts shoot -1 , compared to CC-FP combination, where the increased number of the nuts shoot -1 was observed on shorter and thinner fruiting shoots. Neither N alone nor N together with foliar fertilizers led to higher nutraceutical value of the kernels, measured as the total phenolic content and antioxidative potential. Keywords: Corylus avellana L., fertilization, yield, nut and kernel quality, shoot growth INTRODUCTION Environmental conditions and orchard management practices are the key factors of the profitable hazelnut production. Among cultural measures, fertilization is one of the most important. The demands of the hazelnut tree related to macro- and micronutrients should be fulfilled in order to obtain regular yield of high quality hazelnuts. The role of nitrogen, potassium, phosphorous and boron in hazelnut nutrition was explained by Bergougnoux et al. (1978). Nitrogen application significantly increases the length of annual growth, as well as hazelnut tree production. It also increases the size and weight of the nuts, but reduces kernel percentage and the number of blanks. The input of potassium increases the yields and has a positive effects on the size and weight of the nuts, as well as the kernel percentage. There is also a marked reduction in the number of blanks that resulted from potassium intakes. Phosphoric acid does not show a great effect in French experiments, while the testing carried out in Italy (Bergougnoux et al., 1978) shows a synergy between phosphorous and potassium and nitrogen. The soil’s intake of boron had no marked effect on the yield and size of the nuts. It appears that boron has a slightly favorable effect on kernel percentage but no influence on the percentage of blanks. Soil fertilization is the main method of supplying nutrients to the hazelnut, taking up the nutrients by the plant’s roots. It comprises potassium and phosphorous application within a period from fall to early spring, followed by two or three nitrogen spring Acta Hortic. 1226. ISHS 2018. DOI 10.17660/ActaHortic.2018.1226.42 Proc. IX International Congress on Hazelnut Eds.: C. Tuncer and V. Erdoğan 281