Nitrogen-deficient vines may lack aromatic compounds.
Nitrogen management is important in all wine grapes, but even more so in white varieties because of its association with fruit flavor compounds. But how much nitrogen is needed for adequate levels in fruit, and when should it be applied?
Research has shown that nitrogen plays a key role in the grapevine's canopy growth and development and is a significant component of fruit, said Dr. Joan Davenport during a discussion at the annual meeting of the Washington State Grape Society last fall. But there is also a strong relationship between nitrogen and fruit flavor compounds, particularly compounds found in white grape cultivars, where most of the aromatic work has been done, she said. White wines are more dependent on aromas and are not oaked as heavily as red wines.
Davenport, a soil scientist for Washington State University, cited a study done by the University of Bordeaux that examined flavor compounds of Sauvignon Blanc grapes. Of the primary flavor compounds that were identified, some were desirable, like mercaptohexan-1-ol, which gives a tropical fruit aroma and mercapto-4methylpentan-2-ol, a citrus peel smell. But others, like 4-mercapto-4-methylpertan-2-one, contributed a cat urine smell to the grapes and would not be desirable.
The French researchers found that Sauvignon Blanc grapes deficient in nitrogen had very low levels of the desirable tropical aroma compound. When the grapes were fertilized with nitrogen, they observed a four times increase in the levels.
"You do need to supply enough nitrogen to the plants to ensure that you're going to get the proper aromatic compounds, and if you have them, particularly in grapes like Sauvignon Blanc, you'll have the kinds of flavors that you want that are ideally associated with that grape," Davenport said, adding that the Bordeaux scientists also found similar results with Riesling and Chenin Blanc grapes.
Davenport also shared findings of another study done in California that focused on water and nitrogen management in Riesling grapes and analyzed sensory components. Riesling grapes with an underdeveloped canopy that exposed clusters to direct sunlight had a very strong diesel flavor. When the canopy was moderate, with sprawling shoots and dappled sunlight exposure, the diesel flavor was either reduced or negligible.
"Nitrogen in white, aromatic grapes is really important," she said. "You need to have enough nitrogen to make sure that the plant is not stressed and doesn't make undesirable aromatic compounds."
Several studies by W.J. Conradie of South Africa and the U.S. Department of Agriculture's Paul Schreiner in Corvallis, Oregon, have examined grapevine nitrogen uptake and distribution of nitrogen in the plant. Davenport explained that Conradie's research with two-year-old, potted, Chenin Blanc grapevines found very little nitrogen uptake from the soil from bud break to bloom, with most of the nitrogen at that time moving from the plant's reserves into plant tissue. However, at bloom, most of the nitrogen came from the soil, with less from the woody tissue.
Schreiner found similar patterns in 20-year-old, own-rooted Pinot Noir grapes, she said. He found that 90 percent of the uptake of nitrogen by the vines occurred between bud break and bloom, with 75 percent of the nitrogen going to bloom and canopy tissue. "We see that in Concords as well," she noted.
Although Conradie found that about 20 percent of the nitrogen uptake by the vine happened at postharvest, other studies done in the Pacific Northwest and California have not shown the same results. In Concords, WSU researchers found no postharvest uptake, with little occurring after veraison.
Davenport believes that the difference in climate between South Africa and the Pacific Northwest could be the main reason why researchers have not observed postharvest uptake. South Africa has longer autumns, with more moderate temperatures and less frost likely than Washington, she said, adding that Washington also has different daylight periods.
"Our soil temperatures get pretty cool and are not actively taking up nitrogen in the fall," she said. "We also have a much shorter interval between harvest and leaf fall."
Most of the nitrogen that is taken up by the vine goes to the fruit. Conradie's study on Chenin Blanc found that 45 percent of nitrogen taken up is in the fruit. "It really tells us that nitrogen in fruit is a very significant component," she said. "We're taking 45 percent off every time we harvest."
For Concords, Davenport said that they found slightly less nitrogen going to the fruit because Concords have a larger canopy and more leaves than vinifera grapes.
The tricky part, she said, is getting adequate amounts in the fruit when it's delivered to the winery. "If we don't have enough nitrogen in the fruit, it leads to sluggish or stuck fermentations."
Davenport and her research team will be exploring the use of late-season foliar nitrogen applications to see if they can increase nitrogen levels in the fruit for fermentation purposes. Cornell University researcher Wayne Wilcox has been successful in raising nitrogen levels in fruit from late-season applications, although there may be adverse effects (increased disease and pest pressure and increased canopy vigor) from such actions.
Timing and amount
Northwest growers should avoid making postharvest nitrogen applications, she advises, noting that such timing can result in unwanted leaching. The ideal time for application—which will aid in canopy development—is after three to four leaves have developed but before bloom. If additional nitrogen is still needed, small amounts can be applied again between bloom and veraison.
Research conducted by WSU suggests that Riesling grapes need between 20 to 40 pounds of nitrogen per acre, she said. "But that doesn't mean you run out and apply that amount." The amount that is applied will depend on what's already in the soil.
She recommends growers use a soil test in the spring to learn their soil nitrogen supply, taking soil texture into consideration. Sandy soils have higher nitrogen needs than silty or sandy loam soils.
Growers should also look at their canopy. "If your canopy is out of control, it's telling you that you have more than adequate amounts," Davenport noted.