MONITORING POTASSIUM IN ALFALFA USING A HAND-HELD ION-SPECIFIC ELECTRODE

A reliable method to rapidly assess the potassium (K) status of alfalf a (Medicago sativa L.) would enhance the ability of producers and crop consultants to make timely decisions regarding K fertilization. Recen tly introduced to the agricultural market are portable, hand-held, ion -specific electrodes, that allow direct measurement of K in plant sap. The objective of this study was to compare the potassium ion-specific electrode (KSE) meter to flame photometry as a tool to estimate the K status of alfalfa. A total of 75 fields in the low deserts of Califor nia and Arizona were sampled in June and again in November-December of 1995. Seventy-five mid-stems from each field were collected and chopp ed into 1 cm pieces. Sap was extracted from half the sample and analyz ed for K concentration with the KSE meter. Sap readings were taken eit her immediately in the field(method 1) or after transport to the labor atory (method 2). The other half of the sample was oven dried at 60 de grees C and analyzed for K concentration via flame photometry. Sap K c oncentration by method 1 was well correlated (r(2)=0.68) with flame ph otometry; field pressed, but laboratory analyzed samples (method 2) we re not as well correlated. In conclusion, the KSE meter could be used as a quick and inexpensive tool to monitor the K concentration of alfa lfa mid-stems if samples are extracted and read immediately in the fie ld.