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.
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