V. G. Kachova1*, I. D. Atanassova2 1Forest Research Institute, Kliment Ohridski Blvd. 132, 1756 Sofia, Bulgaria 2Nikola Poushkarov Institute of Soil Science, Agrotechnology and Plant Protection, 1080 Sofia, Bulgaria (Manuscript received 4 April 2017; accepted for publication 30 May 20