On‐farm monitoring of soil and crop nitrogen status by nitrate‐selective electrode

Nitrate‐nitrogen concentration in fresh petiole sap, as measured by a portable, battery‐operated, nitrate‐selective electrode, was highly correlated with NO3‐N concentration in dry petiole tissue of broccoli [Brassica oleracea L. (Italica group), r2 = 0.84], celery [Apium graveolens L. var. dulce (Mill.) Pers., r2 = 0.88], lettuce (Lacluca saliva L., r2 = 0.77), pepper (Capsicum annuum var. annuum L., r2 = 0.89), tomato (Lycopersicon esculentum Mill., r2 = 0.83), and watermelon [Citrulius lanatus (Thunb.) Matsum. & Nakai, r2 = 0.88]. This relationship was linear over a wide range of NO3‐N values and was generally unaffected by site, crop, cultivar, or growth stage, provided that petiole tissue analyzed was from recently matured leaves. Sap was analyzed directly without dilution or filtration. The slope of the regression equation differed widely among crops. Selective electrode analysis of NO3‐N concentration of soil solution samples obtained by suction lysimetry was also highly correlated with conventional laboratory technique (r2 = 0.87). The nitrate‐selective electrode appeared to be a useful tool for on‐farm monitoring of soil and crop N status.