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 N is supplied in fewer, more discrete, split applications.

Key words:

• Fertilization
• Lycopersicon esculentum
• nitrogen
• quick tests