Rapid Diagnosis of the Nitrogen Status of the Wild Lowbush Blueberry

Optical instruments were proposed to assess the nitrogen (N) concentration of the leaves of wild lowbush blueberries (Vaccinium angustifolium Ait.). The objectives were to assess the effects of NPK fertilizer rates on leaf N concentrations and optical measurement values and to examine the relationship between leaf N concentration and optical measurements. Four rates of ammonium sulfate (0 to 90 kg N ha(-1)), two rates of P (0 and 20 kg P2O5 ha(-1)) and four rates of K (0 to 90 kg K2O ha(-1)) were applied in the spring of the sprout year. Leaf sampling and optical measurements were conducted four times during the sprout year. Optical measurements were made with a SPAD-502 meter (Minolta) and a Dualex(3) meter (Force-A). Leaf N concentrations decreased during the growing season while SPAD and Dualex(3) values increased, although SPAD values dropped at the end of the season. N rates increased leaf N concentrations and SPAD values on all sampling dates while decreasing Dualex(3) values. The best estimate of leaf N concentration was obtained with SPAD measurements and with the SPAD/Dualex(3) ratio carried out at the end of July (r(2) > 0.73). However, as optical measurements depend on the sites, reference plots are needed to standardize the values.