1. a Department of Agrochemistry , Debrecen Agricultural University , P.O. Box 36, Debrecen , H‐4015 , Hungary E-mail:
2. b Central Laboratory , Debrecen Agricultural University , P.O.Box 36, Debrecen , H‐4015 , Hungary E-mail:
3. c Central Laboratory , Debrecen Agricultural University , P.O.Box 36, Debrecen , H‐4015 , Hungary
4. d Department of Agrochemistry , Debrecen Agricultural University , P.O. Box 36, Debrecen , H‐4015 , Hungary