Leaf chlorophyll readings as an indicator for spinach yield and nutritional quality with different nitrogen fertilizer applications

The objective of this study was to investigate the possibility of predicting the concentrations of total nitrogen (N), nitrate-nitrogen, and ascorbic acid in spinach (Spinacia oleracea) leaves using the pocket chlorophyll meter SPAD-502 (Minolta, Japan) in a pot experiment in a greenhouse. Spinach plants were grown in plastic pots filled with 0.5 kg of brown soil per pot with urea as N fertilizer at 0, 30, 60, 120, and 240 mg N/kg soil. SPAD readings of the two uppermost fully expanded leaves were recorded 18, 25, and 32 d after sowing and at harvesting (34 d). Dry-matter biomass and total N concentrations in leaves and roots, and NO3-N, and ascorbic acid concentrations in leaves, were measured after harvesting. SPAD readings showed continuous reduction with increasing growth period irrespective of N applications. SPAD readings at harvest were significantly correlated with total N, leaf dry weight (DW), and NO3-N concentration. However, this correlation did not exist between SPAD readings and ascorbic acid concentrations in leaves. The above results suggest that it is possible to apply SPAD readings to estimate NO3-N concentrations in spinach plants, and that they may be applied for field assessments in decision-making and operational nutrient-management programs for the plant. Furthermore, the SPAD method may also be useful for ascertaining the harvest time. The results suggest that treatment with 120 mg N/kg significantly improved both leaf yields and leaf quality (i.e., leaf nitrate-N concentration and ascorbic acid). Too little and too much N fertilizer was not good for yield or spinach quality.