TY JOUR TI KW seed productivity KW meadow clover KW nanopowders KW microelements KW presowing treatment KW economic efficiency JO MOSCOW ECONOMIC JOURNAL AU Ponomarev, Y.O. AU Prudnikova, A.G. AU Prudnikov, A.D. AU Semchenkova, S.V. PY 2017 IS 2 PB Electronic science