TY JOUR TI POTATO PRODUCTIVITY OF DIFFERENT GROUPS OF RIPENESS, DEPENDING ON METHODS OF AGRO-MACHINERY CULTIVATION ON GRAY FOREST SOILS OF THE MIDDLE VOLGA STEPPE KW potato KW variety KW productivity KW calculated doses of fertilizers KW quality indicators KW starch KW vitamin C KW nitrates. JO Vestnik of Kazan State Agrarian University AU Chekmarev, P.. AU Gareev , I.. AU Vladimirov, V.P. PY 2015 IS 10 PB Kazan state agrarian University