Testing petiole sap for nitrate and potassium: A comparison of several analytical procedures

An important aspect of establishing critical sap nutrient concentrations for diagnostic purposes is to determine the accuracy of the analytical method used. We compared a Cardy flat membrane NO₃ electrode, a Hach portable NO₃ electrode, and a Wescan N analyzer for their ability to determine NO₃ concentrations in sap of potato (Solanum tuberosum L.) petioles. The Hach and Wescan instruments require diluted sap, while nondiluted sap can be used with the Cardy. Nitrate-N concentrations in nondiluted petiole sap measured with the Cardy electrode were 90 to 120 mg·L^-1 higher than the other two methods. Using sap diluted with 0.075 m aluminum sulfate tended to lower Cardy NO₃ readings to concentrations closer to the other methods, but made the procedure more complicated for practical use. We also compared a Cardy K electrode with flame emission spectroscopy for determining K concentrations in sap. Using nondiluted sap with the Cardy procedure resulted in K concentrations 200 to 2500 mg·L^-1 lower than those determined by flame emission, depending on K concentration of the sap. Diluting sap with 0.075 M aluminum sulfate or deionized water for use with the Cardy electrode resulted in K concentrations similar to those determined by flame emission.