Defining Phosphorus Requirements for Pinot noir Grapevines

A study to examine the phosphorus (P) requirements of Pinot noir was carried out using a microplot vineyard with carefully controlled P inputs. Pinot noir grafted onto 101-14 rootstock was exposed to four levels of P supply delivered via fertigation beginning in their fourth growing season. Vine nutrient status, productivity, and must chemistry were studied over four years (2012 to 2015), and fermentation dynamics were evaluated over three years (2012 to 2014). P supply primarily influenced vine productivity by reducing leaf area at veraison and by reducing yield, which occurred after three years in vines that received no P fertilizer. Flowering and fruit set were not altered by low P status. P supply had the most significant impact on must P levels, where the two lowest P supply treatments had reduced must P within the first year that P was altered. However, must P concentrations as low as 32 mg P/L did not affect the time needed for yeast to complete alcoholic fermentation. These findings suggest that limiting P reduces Pinot noir canopy size and yield before it alters flowering parameters or reduces must P concentrations sufficiently to alter fermentation. A P concentration of 1.0 g P/kg dry weight (DW) in leaf blades at veraison is proposed as the critical level below which growth and yield of Pinot noir are reduced when vines are cropped at levels typical for premium wine production in the region. Growers should closely monitor vine P status when leaf blade P at veraison approaches 1.2 g P/kg DW in western Oregon Pinot noir vineyards to account for sampling and laboratory error.