Can Late Season N Applications to Irrigated Vineyards in Eastern Washington Increase Yan without Adversely Affecting Vine Growth?.

Wine grapes in eastern Washington are historically in low yeast assimilable nitrogen (YAN), despite the recommended application of 33.6 kg/ha/yr of nitrogen (N) fertilizer for ideal canopy development.  As a result, during the winemaking process, additional N is added to the must, usually in the form of diammonium phosphate (DAP), to attain the recommended 140-150 mg/L/N necessary for Saccharomyces cerevisiae to complete fermentation. This study was designed to evaluate the possibility of increasing YAN in the field through supplemental late season foliar and soil applications of N. Increasing YAN in the field has the potential to significantly impact winemaking-processes, particularly those producing organic wines where there are limited options of N supplements at the winery level.  Applications of 0, 17.25, or 33.6 kg/ha of N, in both soil and foliar forms, were conducted on mature, irrigated, Vitis vinifera L. Riesling wine grapes in eastern Washington at 50% véraison (color change and berry softening; defined as when 50% of the clusters had 50% of the berries transitioning). Both conventional and certified organic N sources were used.   The following measurements were made on replicated plots: shoot length, soil NO3-N from 0 - 45 cm, plant root simulator (PRS) NO3-N, yield (both mass and cluster number), and pruning weights. Preliminary results are still being analyzed, and further information applicable to the wine and fruit quality will be discussed.