TESTING PETIOLE SAP FOR NITRATE AND POTASSIUM - A COMPARISON OF SEVERAL ANALYTICAL PROCEDURES

An important aspect of establishing critical sap nutrient concentratio ns for diagnostic purposes is to determine the accuracy of the analyti cal method used. We compared a Cardy flat membrane NO3 electrode, a Ha ch portable NO3 electrode, and a Wescan N analyzer for their ability t o 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 1 29 mg . L(-1) higher than the other two methods. Using sap diluted wit h 0.075 M aluminum sulfate tended to lower Cardy NO3 readings to conce ntrations closer to the other methods, but made the procedure more com plicated for practical use. We also compared a Cardy K electrode with flame emission spectroscopy for determining K concentrations in sap. U sing nondiluted sap with the Cardy procedure resulted in K concentrati ons 200 to 2500 mg . L(-1) lower than those determined by name emissio n, depending on K concentration of the sap. Diluting sap with 0.075 M aluminum sulfate or deionized mater for use with the Cardy electrode r esulted in K concentrations similar to those determined by flame emiss ion.