Evaluating the impacts of nitrogen on the growth stages of cucumber

Monitoring physiological parameters of plant and fertilizer requirements are the basic principles in precision farming. Non-destructive and accurate remote sensors make this process feasible and light-handed. The present study evaluates the efficiency of GreenSeeker (GS) and Soil-Plant Analyses Development (SPAD) in Nitrogen (N) fertilizer management. Normalized difference vegetation index (NDVI) was measured with GS and compared to SPAD. Fertigation with 5 different N treatments was applied to 100 pots. The first treatment (N1) had no N concentration, while the other treatments (i.e., N2, N3, N4, and N5) received 2, 9, 22, and 37 mmol.L-1 weekly, respectively. Next, the effect of volumetric fertilizing was investigated by adding supplemental fertilizer to N1 to N3 pots 71 days after planting. Nitrogen concentration in the leaf and first growing fruit was tested using the Kjeldahl method. The results of applied sensors confirmed with visible-near infrared spectroscopy at 200-1100 nm wavelength. NDVI, soil-adjusted vegetation index, and chlorophyll index were calculated from the available spectra and compared to the sensor outputs. Strong correlations were obtained between NDVI and all indices derived from spectra, especially in the vegetative phase. The results showed a strong correlation of NDVI with N rate, especially after supplemental fertilizing. Since the vegetation indices from spectra almost correlated well with NDVI and SPAD in all treatments, spectroscopy monitoring of cucumber could be a precise alternative technique. Linear and nonlinear regressions were applied to model variations of NDVI and SPAD. This study demonstrated the feasibility of using GS for N management according to its sensitivity to cucumber N status.