Petiole sap nitrate-N quick test for determining nitrogen status of tomato

Most vegetable farmers use nitrogen fertilizers to increase crop yield and quality. However, applying more nitrogen than the crop can take up increases production costs and may contribute to environmental pollution. To meet the crop’s need for optimal productivity and to minimize fertilizer cost and loss, plant nitrogen status needs to be evaluated and fertilizer application rate needs to be calculated accurately (Lang et al. 1999). The nitrogen level in fresh petiole sap is a very sensitive indicator of the current crop’s nitrogen status and is especially useful for scheduling nitrogen fertilizer application. Fresh petiole sap can be measured by a few different methods, such as the colorimetric method using a meter (HACH; Loveland, CO), the nitrate-N test using test strips (Merckoquant, Merck; Darmstadt, Germany), or a Cardy meter (Horiba Scientific; Edison, New Jersey). The meters or test strips for these methods are available in the market and are affordable; however, for the first two methods, it is necessary to dilute the fresh petiole sap with distilled water (1:19 ratio). In contrast, the Cardy meter directly measures the fresh sap—a few drops of the petiole sap are placed on the sensor, and the result is displayed on the digital meter. Thus, the nitrate test with a Cardy meter can be performed easily and in a few minutes in the field. The objective of this publication is to provide information on how to test petiole sap of tomato to determine the plant nitrogen level. Instructions are provided on how to use a Cardy meter for this test and also how to interpret the petiole sap nitrate-nitrogen (N) test results