Use Diluted Tomato Petiole Sap for K Measurement with the Cardy Meter

Abstract

Undiluted tomato petiole sap from a variety by K rate experiment (48 treatment rep combinations) was used to measure K concentration via the battery operated portable Cardy meter and ICP laboratory instrumentation. Three sample 1998 dates, 16 July., 21 Aug., and 8 Sept., resulted in K sap readings by ICP of 3917, 2612, and 2297 ppm, respectively. At sap levels below 3000 ppm the linear Cardy:ICP correlation was r = 0.04, but above 3000 ppm only 0.53. From 3500 to 5000 ppm K the Cardy meter under estimated actual sap K by 200 to 900 ppm. For the years 1999 and 2001, tomato petiole sap at each sample date (4) was diluted 1:1 with deionized water. The linear regression equation describing the relationship between ICP and Cardy meter measurements was: Cardy K ppm = 0.733 * ICP + 685 (r = 0.92, n = 190). The Cardy meter error over the 2000 to 5000 ppm K range was 8 to 12%. Petiole sap K, measured by either Cardy or ICP, was highly correlated to whole leaf K concentration both years. But even though the slope of the regression lines was similar the intercepts were significantly different (P≤.01). The significant 0.32% K difference in whole leaf between years precluded the development of a common regression line to predict whole leaf K from Cardy petiole sap determinations. The Cardy meter can be reliably used for tomato petiole K determination provided the sap is diluted and the usual handling precautions are taken to prevent petiole moisture loss.