High levels of N fertilizers are generally applied in intensive leafy vegetable production in the peri-urban agriculture of Southeast Asia. This study was conducted to develop a simple and rapid method of determining nitrate-N accumulation in selected leafy vegetables. Five leafy vegetables, including amaranth (Amaranthus tricolor), kangkong (Ipomoea aquatica), Ethiopian kale (Brassica carinata), choysum (Brassica campestris sp. parachinensis), and leafy lettuce (Lactuca sativa) were grown on raised beds in 32-mesh nethouse in randomized complete-block design with four replications. The crops were fertilized with eight N levels: 0, 50, 100, 150, 200, 250, 300, and 350 kg·ha-1 in three splits. At harvest, nitrate contents were determined in tissue sap of fully expanded leaf blades, petioles and whole plant using a Cardy nitrate meter. Chlorophyll content readings were also measured on the same leaf using a chlorophyll meter. Nitrate accumulations varied with vegetable species. Significant correlations (P< 0.001) existed between N fertilizer rate and nitrate content as well as leaf chlorophyll and yield. Using N application rate of 200 kg·ha-1, nitrate accumulation in Ethiopian kale was highest (7000 ppm), followed by kangkong (4000 ppm), amaranth (3500 ppm), and leafy lettuce (1200 ppm). The correlation between leaf chlorophyll meter reading (LCMR) and nitrate content was also significant (P< 0.001), suggesting the feasibility of using Cardy nitrate meter test and LCMR for monitoring production of low-nitrate and safe vegetables. The Cardy meter was also sensitive in detecting soil nitrate-N below 20 ppm and is a rapid and reliable alternative to conventional distillation method.
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