G. Panayotova1*, S. Kostadinova2, S. Stefanova-Dobreva3, A. Muhova3 1Department of Plant production, Faculty of Agriculture, Trakia University, 6000 Stara Zagora, Bulgaria 2Department of Agrochemistry and Soil Science, Faculty of Agronomy, Agricutural University, 4000 Plovdiv, Bulgari