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Catherine Jones, middle, puts Merlot grapes through a destemmer under the watchful eye of her advisor Joan Davenport.

Catherine Jones, middle, puts Merlot grapes through a destemmer under the watchful eye of her advisor Joan Davenport.

For both grape growers and winemakers, the status of nitrogen in the vineyard can make big differences in fruit and wine quality. It’s a balance of not too much and not too little.

Catherine Jones, vineyard and winery technology grant director at Yakima Valley Community College, likens the nitrogen status to Goldilocks’s porridge in the nursery tale of the three bears. Jones, who is working on her master’s degree in soil science at Washington State University, says that growers must find the “just right” nitrogen levels that balance the vine’s nutrient needs for growth and plant development with the winemaker’s needs for adequate nitrogen in the juice for fermentation.

Like the porridge that’s too hot, Jones said that too much nitrogen in the vine results in vigorous canopy growth, increased leaf density, reduced fruit quality, shading of fruiting sites, which can reduce fruitfulness the next year, and leaching into the soil that can contaminate groundwater. But too little can result in poor vine growth, low vigor, small leaf area, chlorosis, and low crop yields.

In the fruit must or juice, the yeast needs nitrogen (ammonia and amino acids known as yeast assimilable nitrogen or YAN) for fermentation. Too much nitrogen causes increased cellular mass and fermentation rates, and can result in microbial instability, a haze to the wine, high volatile acidity, and an increase in the formation of methyl carbamate. Slow or stagnant fermentations, off flavors, and high levels of hydrogen sulfide are the results of too little nitrogen in the must.

Finding that “just right” amount of nitrogen in the must leads to steady and complete primary and secondary ­fermentation and good flavors in the wine, she said.

Past surveys of wine must analyzed by Washington State University scientists showed that up to 50 percent of the samples collected were deficient in YAN, and were below the 140 to 150 milligrams per liter nitrogen level that’s recommended for complete fermentation. YAN musts with less than 100 milligrams of nitrogen per liter are considered low, moderate YAN musts contain 250 to 350 milligrams of nitrogen per liter, and, high YAN musts have greater than 600 milligrams.

Low YAN must levels are a problem around the world, Jones said, adding that winemakers often add small amounts of nitrogen in the form of diammonium phosphate, yeast hulls, or other commercial products to “fix” the YAN levels in the must.

Late-season nitrogen

Jones set out to learn if late-season foliar nitrogen sprays, applied after veraison, could result in a more rapid nitrogen uptake than soil applications and improve YAN must levels. “The soil-applied vineyard nitrogen is not giving us enough nitrogen in the must,” she stated during a presentation of her master’s research at winter grape industry meetings.

Under the guidance of WSU soil scientist Dr. Joan ­Davenport, Jones compared foliar applications of 15 pounds of nitrogen per acre split over five weeks (a rate of 3 pounds per week) of conventional nitrogen, “UAN-32,” and an organic nitrogen. The two-year trial was conducted in Merlot and Riesling grapes, with applications beginning at veraison. She studied the effects on yield, yield components, ­vegetative growth, and flavor ­compounds.

A previous Cornell University study showed success in increasing YAN levels with a foliar application of 30 pounds of nitrogen per acre, Jones said. The Cornell study significantly increased YAN levels and reduced off flavors associated with atypical aging. Jones was concerned that a late-season application of 30 pounds of nitrogen in eastern Washington would be nearly as much as some growers apply all year.

“Rates that growers are applying to the soil may be ­efficient for plant growth, but they are not enough for yeast growth,” she said.

She included an organic nitrogen because winemakers are limited in what they can use to increase YAN must levels in organic wine.

She found no differences in vine growth—pruning weights, shoot emergence, shoot length, and leaf area—in the two years. She is still analyzing berry and wine data collected in 2010, the second year of the project.

In year one, the 15-pound nitrogen rate did not affect YAN levels in the control, conventional, and organic treatments. YAN for the treatments ranged from 98.7 to 100.8 milligrams of nitrogen per liter. But the nitrogen applications did change the flavor profile of the wines. A trained sensory panel found differences between the wines made from the treatments, although brix, pH, and titratable acidity of the berries were not statistically different. Analysis of berry and wine components, using gas ­chromatography, will be used to quantify flavor differences.

Although the foliar nitrogen applications didn’t increase the YAN levels, Jones noted that they didn’t affect canopy growth. A higher rate of nitrogen would likely have increased YAN levels, but could have also produced the unwanted effect of increased canopy growth, she said.

However, she’s encouraged by the difference in flavors she found from the nitrogen applications. Follow-up research is being considered that would apply the nitrogen through a drip irrigation system.