Sampling Considerations for Nitrate Quick Tests of Greenhouse-grown Tomatoes

Abstract

Tomato plants were grown in two greenhouse experiments to determine sampling guidelines for using semiquantitative quick tests of petiole sap nitrate to monitor crop N status. In Expt. 1, a 40-µg NO 3 -N/ml liquid feed rate produced maximum marketable yields of ‘Celebrity’ tomatoes (Lycopersicon esculentum Mill.) (3.4 kg/plant) with estimated petiole sap concentrations of 2138, 1091, and 636 µg NO 3 -N/ml at early bloom, small fruit, and full-ripe fruit stages, respectively. In Expt. 2, a stable minimum level of petiole sap NO 3 -N of about 800 µg·ml −1 was found during fruit production in maximally yielding (3.1 kg/plant) ‘Tropic’ tomato plants. These plants received 60 µg NO 3 − N/ml in liquid feed solutions during fruit production. Liquid NO 3 -N feed rates producing maximum yields in these experiments were about five times lower than those typically used for growing tomatoes. Sap nitrate content did not fluctuate greatly between 0803 hr and 1430 hr . Sap nitrate levels rose sharply in response to increased N feed concentrations and as a result of lower leaf pruning. Because of high day-to-day and plant-to-plant variation in petiole sap nitrate readings, strategies for obtaining results with sufficient diagnostic value would appear to involve at least weekly samplings of five to 15 plants, and restricting diagnoses of deficient crop N to instances where readings < 800 µg NO 3 -N/ml for two or more consecutive samplings have been obtained.