Field Use of an Ion-Specific Electrode to Measure the Potassium Concentration in Alfalfa

Abstract

Methods for determining K deficiencies in alfalfa (Medicago sativa L.) are either imprecise, time-consuming, or expensive. New to the market are portable and inexpensive meters that use a K-specific electrode (KSE) to measure the K concentration of plant sap. Little research has examined the precision or accuracy of these meters for determining the K concentration in alfalfa shoots. The objective was to determine if the KSE meter could precisely and accurately estimate the K concentration in alfalfa shoots. Laboratory, greenhouse, and field experiments were conducted to test the KSE meter's ability to measure the concentration of K in aqueous solution and in alfalfa sap. Laboratory results showed that KSE meter readings are subject to interference from Cl−, SO2−4 and NO−3 ions. In greenhouse and field experiments, the KSE meter precisely estimated the K concentration in alfalfa shoots (R2s > 0.74) if readings were included in a multiple regression equation that also included sample moisture content or mean stage count and air temperature. Readings taken from fresh plant sap using the KSE meter were highly biased, with the readings being four to six times lower than those obtained by flame photometry. Readings from the KSE meter, if used with other variables in a multiple regression equation, would allow users to quickly assess the K status of alfalfa shoots for a relatively small investment.