By Luke Haggerty, Dr. Terry Bates, and Dr. Cain Hickey
Nitrogen is the mineral nutrient that is needed most by Concord grapevines. It is also the most widely applied fertilizer. Nitrogen is the backbone of amino acids and, as such, nitrogen is the building block for compounds such as proteins and chlorophyll, the latter, the pigment most responsible for photosynthesis. Nitrogen-deficient vines are often characterized by reduced vigor, crop yield, and photosynthesis. However, excessive nitrogen can cause overly vigorous shoot growth that results in shaded fruiting buds and reduced yields. Nitrogen is a very important nutrient for grapevines, and calculating nitrogen needs is a complicated decision. As such, nitrogen studies have been conducted in Concord for many years.
Figure1. The relationship between pruning weight and crop yield as affected by viticulture practices and nitrogen application.
Evaluation of Concord nitrogen needs started with Dr. Nelson Shaulis in the “West Tier” experiment. In this experiment, three nitrogen rates (0, 50, and 100 pounds of actual nitrogen per acre) and several viticultural practices were evaluated for their impact on vine size and crop yield potential. Vines with high nutrient and water uptake had increased vine size, pruning weight and yield potential (Figure 1). It was shown that 50 pounds of nitrogen per acre increased pruning weights (vines size) and crop yield, but adding an additional 50 pounds of nitrogen per acre offered no further benefit (Figure 2). Concord vines in the Lake Erie region typically have less than three pounds of pruning weight, and average less than eight tons per acre. Therefore, Dr. Shaulis concluded that healthy Concord vines required somewhere between 0 and 50 pounds of nitrogen per acre to increase and maintain vine growth and crop yield. (Bates et al. 2002)
Figure 2. Concord yields compared to pruning weights using three N rates (0, 50, and 100 lbs/acre).
The West Tier experiment was conducted on well drained gravel loam soils with low organic matter. Years later, Dr. Bates conducted the same study, but on heavy clay soils with relatively high organic matter; this experiment was called the “Betts’ Nitrogen Trial”. Vines were again given 0, 50, and 100 pounds of nitrogen per acre over seven consecutive years. Similar to the West Tier experiment, it was observed that crop yield was greater when 50 compared to 0 pounds of nitrogen per acre was applied, but that applying 100 pounds per acre did not further increase crop yield. Figure 3). It was concluded that soils with high organic matter release more nitrogen and, thus, vines needed less supplemental nitrogen.
In other works, Dr. Lailiang Cheng and Dr. Bates investigated the ability of Concord vines to take up nitrogen, as well as their annual nitrogen demand. It was found that only 24% (24 pounds for every 100 applied) of 50 applied pounds of nitrogen was incorporated into the vine. In other words, vines were only able to uptake ¼ of the applied nitrogen. It was also found that about 50 pounds of nitrogen per acre were required in Concord vineyards. Other research found that only about 10 of 100 applied pounds of nitrogen per acre were incorporated into the vines. (Randall et al. 2004)
Figure 3. Seven year cumulative Concord yield using three different nitrogen rates (0, 50, and 100 pound of N per acre).
So where does the excess nitrogen go? Some nitrogen is used by macro and microorganisms in the soil, such as weeds, worms, and bacteria. This nitrogen enters the living portion of organic matter and can eventually be used by the vines in the future (remember from above that nitrogen can be supplied by soil organic matter) A very small amount of nitrogen is absorbed into soil particles by cation and anion exchange. The rest of the nitrogen can be lost through leaching, erosion, and denitrification. Nitrogen loss, especially leaching, is getting more attention these days because of its documented negative impact on the environment, such as algal blooms in bodies of water.
Concord vines rely mostly on stored starches and nutrients during the transition from dormancy to bloom. Approximately 80% of the reserved starches and nutrients are used for pre-bloom shoot and root growth. Relatively little nitrogen (around 5%) is taken up prior to bloom when compared to what is taken up after bloom. However, this 5% has potential to be important if weak vines have inadequate nitrogen storage reserves. The 5% is less important in healthy vineyards where nitrogen reserves support strong early season growth; this includes new root growth that absorbs additional soil nitrogen. Reserves are depleted after bloom. Thereafter, Concord vines rely on nutrient uptake from the roots, and assimilated carbon via photosynthesis. The amount of time it takes for supplemental nitrogen to reach the rooting zone is dependent on several factors, including rainfall and, consequently, soil moisture. Thus, it is typical for nitrogen applications to occur around or before bloom, when soil moisture is relatively high. Nitrogen should be applied in split applications in vineyards needing more than 40 pounds of nitrogen per acre; these sites may be characterized by large vines and/or low soil organic matter. The general rule for split application timing is at two weeks pre-bloom, and at two weeks post-bloom.
There are basically three nitrogen sources for grapevine growth: mineralization of nitrogen from soil organic matter, remobilization of stored nitrogen from perennial tissues, and inorganic nitrogen fertilizers. By far, the largest pool of N for grapevine growth comes from the mineralization of soil organic matter. While there are many biological, chemical, and environmental processes working in concert, the basic rule of thumb is that about 15-20 pounds of nitrogen is released for every 1% of soil organic matter in your vineyard (table 1). Storage nitrogen reserves are likely the most easily used by vines because it is already in the vine – it just needs to be converted and remobilized in the spring. Inorganic nitrogen fertilizers are primarily used to supplement nitrogen remobilization and mineralization sources during periods of peak nitrogen demand. Peak vine nitrogen demand occurs during rapid canopy and fruit development, which starts a few weeks before bloom, and lasts until about 40 days after bloom. During this timeframe, the release of nitrogen from organic sources may not be enough to keep up with vine demand. Therefore, supplemental nitrogen fertilizers should be applied just before rapid vine growth, even in vineyards with relatively high organic matter soils. In the Lake Erie region, this critical period of nitrogen fertilization is typically in late May or early June.
References:
Much of the information used for this article has not yet been published.
Bates, Terence R., Richard M. Dunst, and Paula Joy. “Seasonal dry matter, starch, and nutrient distribution in’Concord’grapevine roots.” HortScience 37.2 (2002): 313-316.
Randall J., Thomas J. Zabadal, and Eric J. Hanson. “Effect of nitrogen application timing on N uptake by Vitis labrusca in a short-season region.” American journal of enology and viticulture 55.3 (2004): 246-252.