FIELD CHLOROPHYLL AND PETIOLE SAP NITRATE INDICATE THE NITROGEN STATUS OF POTATO VARIETIES

We report field experiments, under N limiting conditions, where applied N was varied between 0 and 252 Kg/ha. Two varieties (Castile and Allegheny) were planted in 1990 and 3 varieties (Katahdin, Monona and Superior) were planted in 1991. A curvilinear tuber yield response to N rate was obtained both years for all varieties with maximum yields at about 168 Kg/ha of N. Weekly field measurements with a hand-held chlorophyll meter (Minolta SPAD 502) were highly correlated to yield, to fresh petiole sap nitrate, to dry petiole nitrate and to the total N of whole leaves, all of which were generally highly correlated to each other and markedly affected by applied N. As plants matured the chlorophyll readings, and concentrations of nitrate and N decreased. In general variety differences or variety × N rate interactions were not significant which encourages the use of such tests to avoid unnecessary N sidedressings.