Comparison of Nitrate-Nitrogen Levels in Grape Petioles as Determined by Phenoldisulfonic Acid and Specific-Ion Electrode Methods

Inconsistencies have been found in nitrate-nitrogen results obtained by the phenoldisulfonic acid (PDS) and the specific ion electrode methods of analysis with various plant tissues. This study was undertaken to determine if the inherent errors in the phenoldisulfonic acid method produce erratic results in grape petiole tissue analysis. Leaf petiole samples were taken in two `Thompson Seedless9 and one `Muscat Canelli9 vineyard at various stages of vine development during the growing season of 1974. Survey samplings were also made in 25 other `Thompson Seedless9 vineyards at bloom. A comparison was made of NO 3 -N levels as determined by the PDS and the nitrate specific ion electrode procedure. The values obtained by PDS were most often greater than those by the specific ion electrode, particularly at the more critical lower NO 3 levels. The potential errors produced by PDS in this study are not found in the specific ion electrode procedure.