Testing Petiole Sap for Nitrate and Potassium: A Comparison of Different Analytical Techniques

An important aspect of establishing critical sap nutrient concentrations for diagnostic purposes is to determine the accuracy and precision 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 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 100 to 200 mg·liter–1 higher than the other two methods. Using sap diluted with 0.15 M Al2(SO4)3 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 1000 to 1700 mg·liter–1 lower than those determined by flame emission. Diluting sap with 0.15 M Al2(SO4)3 for use with the Cardy electrode resulted K concentrations similar to those determined by flame emission. Implications for using the electrodes for diagnostic purposes will be discussed.