HAND-HELD CHLOROPHYLL METER - A PROMISING TOOL TO ASSESS THE NITROGEN STATUS OF POTATO FOLIAGE

A field experiment with potato (Solanum tuberosum L., cv. Vebeca) was conducted on a sandy soil near Wageningen (52-degrees North) in 1992. The treatments included a zero-nitrogen control and combinations of three amounts of nitrogen, viz. 110, 180 and 250 kg N ha-1, and splitting of the N dose in one (early May), two (early May and June) or three (early May, June, July) applications. The chlorophyll content of the uppermost fully grown leaves was assessed with a SPAD-502 chlorophyll meter (Minolta, Osaka, Japan) throughout the season. The pattern of change with time in SPAD-values differed between treatments. SPAD-502 readings correlated well with laboratory measurements of the chlorophyll content and with the nitrogen concentration in leaves (r2 >0.95). Data on the nitrate concentration in petiole sap (included as a reference) showed that this variable responded much more to split nitrogen applications than the SPAD-value. Future research will need to consider other factors which may affect the chlorophyll content of the foliage.