Use of diluted tomato petiole sap for potassium measurement with the cardy electrode meter

Tomato petiole sap from potassium (K) rate experiments conducted on prairie loam and river coarse sand soils from 1998 to 2002 was used to measure K concentration via the battery-operated portable Cardy flat membrane electrode meter and inductive coupled plasma (ICP) laboratory instrumentation. In 1998, for undiluted sap levels less than 3000 mg kg(-1) the linear Cardy-ICP correlation was r = 0.94, but when K was more than 3000 mg kg(-1) was only 0.53. Diluting the sap with water (1: 1) improved the linear range to 6000 mg K L-1 (r = 0.87, n = 190). Petiole sap K, measured by either Cardy or ICP, was highly correlated to whole-leaf K concentration all years, r = 0.8 to 0.91. Even through the slope of the regression lines were similar for the years, the intercepts were significantly different (P < 0.01). The Cardy electrode 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 and maintain stability in electrode readings.