The development of critical levels for sap nitrate and chlorophyll meter reading (SPAD
test) in the case of various crops is of great importance for growers in characterizing
a plant’s N status. A field experiment with spring oat (Avena sativa L.) was carried
out on loamy soil in Debrecen, Hungary, using a small-plot design. Ammonium nitrate
was broadcast at rates of 0, 30, 60, and 90 kg N/ha in three replicates. The total
N content of the plant, sap nitrate content, and SPAD values were measured at jointing
when the first node appeared above the soil surface (Feekes 6) and at boot stage (Feekes
10). Regression analysis between total N content and sap nitrate showed cubic and
linear relationships with r2 = 0.7982 (Feekes 6, whole plant) and 0.9625 (Feekes 10,
upper developed leaves), respectively. Optimal grain yield was obtained when sap nitrate
exceeded 650 mg/L and 540 mg/L at Feekes 6 and Feekes 10, respectively. There were
linear and logarithmic relationships between total N content and SPAD values with
r2 = 0.8058 and 0.6258 at Feekes 6 and 10. Optimal grain yield occurred over SPAD
values of 43 and 48 at Feekes 6 and 10, respectively. Optimal N rate was 60 kg N/ha
on the experimental site.
