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 co rrelated with NO3-N concentration in dry petiole tissue of broccoli [B rassica oleracea L. (Italica group), r(2) = 0.84], celery [Apium grave olens L. var. dulce (Mill.) Pers., r(2) = 0.88], lettuce (Lacluca sali va L., r(2) = 0.77), pepper (Capsicum annuum var. annuum L., r(2) = 0. 89), tomato (Lycopersicon esculentum Mill., r(2) = 0.83), and watermel on [Citrulius lanatus (Thunb.) Matsum. and Nakai, r(2) = 0.88]. This r elationship was linear over a wide range of NO3-N values and was gener ally unaffected by site, crop, cultivar, or growth stage, provided tha t petiole tissue analyzed was from recently matured leaves. Sap was an alyzed directly without dilution or filtration. The slope of the regre ssion equation differed widely among crops. Selective electrode analys is of NO3-N concentration of soil solution samples obtained by suction lysimetry was also highly correlated with conventional laboratory tec hnique (r(2) = 0.87). The nitrate-selective electrode appeared to be a useful tool for on-farm monitoring of soil and crop N status.