Nitrogen

Abstract

Drip irrigated ‘Celebrity’ tomatoes were grown at 8 rates of nitrogen (N) on a well‐drained Oxisol on Central Oahu during summer 1985. Weekly fertigation was simulated by applying a small volume of solution containing nitrate N to the base of each plant each week. One thirteenth of the total amount of N was applied weekly. Petiole sap nitrate levels were monitored biweekly with paper test strips. Marketable yields of table‐ripe fruit over a 4‐week harvest period were related by a quadratic function to total N applied, with an estimated maximum yield of 35.1 t/ha occurring at a N application rate of 219 kg/ha. Sap nitrate‐N readings associated with 90 percent of maximum yield or more (desirable values‐DV) were 517–733 mg L‐1 at flowering, 478–915 mg L‐1 when the plants had small fruit, and 167–912 mg L‐1 when the plants had large green fruit. These DVs were somewhat lower than previously reported. Sap nitrate values that support maximum yields may be lower when N is supplied frequently than when...