Washing and/or cutting petioles reduces nitrate nitrogen and potassium sap concentrations in vegetables

Petiole sap testing using ion-specific electrodes is a simple method that can be used to guide in-season applications of nitrogen (N) and potassium (K) to vegetable crops. This method requires petiole sampling and sap extraction using a sap press. Because some vegetables are grown with foliar applications of N and/ or K and because some crops have large petioles, petioles may need to be washed and/ or cut before being pressed. Limited information is currently available on the effect of washing/ cutting on sap-testing results. Hence, muskmelon (Cucumis melo L.), bell pepper (Capsicum annum L.) and tomato (Lycopersicon esculentum Mill) petioles were used to test whether washing/ cutting reduced NO3 (nitrate)-N and K concentrations and changed the subsequent interpretation of plant nutritional status. Washing for 30, 60, or 120 sec in distilled water and cutting petioles before or after washing significantly reduced sap concentrations (p = 0.01 and p = 0.04 for NO3-N and K, respectively) in seven of 12 tests when compared with the control method (petioles cut and not washed). The average concentration reductions between the control and the lowest value among all the washing/ cutting treatments were 30% for NO3-N and 19% for K. These losses due to washing/ cutting are likely often to change the diagnosis of nutritional status from "sufficient" to "less than sufficient" and, therefore, may suggest the need for unnecessary fertilizer applications.