Potassium Fertilizer Effects of Potato Yield and Petiole Sap Potassium Concentrations

A 2-year field study was conducted on a low- to medium-K testing sandy soil 1) to evaluate the effects of various K management strategies on potato (cv. Russet Burbank) yield and quality and 2) to calibrate a petiole sap test for determining plant K status. Treatments included banded applications of potassium chloride fertilizer at planting with K ranging from 0 to 300 kg·ha–1 in 75 kg·ha–1 increments. Comparisons of preplant broadcast + banded applications and evaluation of in-season applications of potassium nitrate also were made. In both years, tuber yield increased with increasing banded K fertilizer up to 150 kg ha-1 K the first year and 225 kg ha-1 K the second year. In-season applications of potassium nitrate increased tissue K levels, but at equivalent K application rates, timing of K application had no effect on yield. Petiole K concentrations, measured on a dry weight and sap basis, increased with increasing K fertilizer application. Potassium concentrations in nondiluted sap determined with the Cardy K electrode were ≈200 to 2500 ppm lower than those determined by flame emission. The greatest discrepancy occurred at the higher K sap concentrations. Potassium concentrations determined with the Cardy electrode in sap diluted with aluminum sulfate or deionized water were much closer to those determined by flame emission. These results suggest that dilution of the sap is necessary to obtain accurate K concentrations in petiole sap.