Effect of colemanite boron fertilization on hazelnut (Corylus avellana L.) yield and nutrient concentration of leaf F. OÖ zkutlu 1,a , A. Aygü n 2 , K. Korkmaz 1 , M. Akgü n 1 , OÖ . Ete 1 , OÖ . Şahin 3 , B. OÖ zcan 1 and B. Taşkin 3 1 Department of Soil Science and Plant Nutrition, Faculty of Agriculture, University of Ordu, 52200, Cumhuriyet Yerleşkesi, Ordu, Turkey; 2 Kocaeli University, Faculty of Arts and Science, Department of Biology, Kocaeli, Turkey; 3 Department of Soil Science and Plant Nutrition, Faculty of Agriculture, University of Ankara, Dışkapı Campus, Ankara, Turkey. Abstract In this research, the effect of boron fertilization on leaf mineral concentration, nut yield and percent kernel in ‘Tombul’ hazelnut was determined. The experiment in a randomized complete block design was carried out in a farm orchard with soil B concentration of 0.02 mg kg -1 in Çarşamba-Samsun location in 2013 and 2014 using boron fertilizer of colemanite source. Boron fertilization was carried out as 6 g B ocak -1 (T1) and 12 g B ocak -1 (T2), along with the control (T0). Supplying doses of colemanite (T1 and T2) resulted in an increase in nut yield in both years, in comparison to the control. The yield increase due to boron application in the first year was statistically nonsignificant. In the second year, boron fertilization caused a significant yield increase from 55.5 kg da -1 in the control plot to 69.2 and 72.4 kg da -1 in T1 and T2 applications, with a 24.7 and 30.4% yield increase, respectively. As a result of increasing doses (T1 and T2) of B fertilization of colemanite source, increases in nut yield were determined. 6 g B ocak -1 soil colemanite application produced significant yield increases in both years as compared to the control and percent kernel, these increased from 48 to 52%, and from 46 to 50% in the first and second year with 8.3 and 8.6% increases, respectively. In hazelnut orchards with boron deficiency, boron fertilization increased the cluster formation, nut yield and percent kernel, decreasing empty hazelnuts. Keywords: hazelnut (Corylus avellana L.), boron, yield, percent kernel INTRODUCTION Boron (B) deficiency in agricultural soils directly affects the plant growth and development negatively. Among mineral element stresses, the most common one is growing retardation resulted from boron deficiency (Brown et al., 2002). Boron deficiency is the most common nutritional disorder. Boron is an easily leachable element in sandy soils with low organic matter. In areas with a high amount of rain and in sandy and acidic soils, boron can easily be leached from the soil as B(OH)3. It is known that, in rainy regions, liming application carried to increase low soil pH also increases boron deficiency in the soil and further in plants. In a study including 30 different countries, Sillanpaa (1982) declared that the B content of Turkey’s soil varied between 0.06 and 9.99 mg kg -1 with a mean of 1.6 mg kg -1 , the lowest B content was found in the Black Sea Region. Hazelnut yield was affected negatively because boron deficiency is common in the Black Sea Region. In the last 70 years, as a result of B fertilizer applications in crop production in over 80 countries in the world, positive effects were determined in 132 plant species (Shorrocks, 1997). Boron applications in hazelnut and some other fruit species showed positive effects on the fruit cluster formation (Hanson, 1991; Brown and Shelp, 1997; Ferran et al., 1997). Economical hazelnut production depends on the number of total flowers formed in a single ocak and fruit clusters a E-mail: fozkutlu@odu.edu.tr Acta Hortic. 1226. ISHS 2018. DOI 10.17660/ActaHortic.2018.1226.40 Proc. IX International Congress on Hazelnut Eds.: C. Tuncer and V. Erdoğan 265