The Science of Grapes

Photo by: German Wine Institute

A Guide to Viticulture

Climate's Impact on Winegrapes

By: Brent Nakano

Sunlight

Rainfall and Irrigation

Soil

Ripeness/HarvesT

“Great wine is made in a vineyard” is an adage and borderline cliché that refers to the need for “great grapes” to make great wine. Given: Wine = Grapes + Yeast and that yeast has no flavor but rather rearranges what’s already present, there is at least some truth to that statement. However, what constitutes “great grapes” and how to produce them is highly subjective and evolves daily with advancements in viticulture (the agricultural science of grape growing) and oenology/enology (the science of winemaking). As the science of both has exponentially progressed, with the ease of access to digital tools to collect data that was formerly cost prohibitive or even impossible to gather, as well as the ability to share data rapidly via the internet with multinational collaborators, how to influence the aroma of wine is no longer as dependent on conjecture as it once was. As Hawaii Beverage Guide progresses, the goal is to organize much of this information into a format that can help a purchaser focus on a smaller group of products that fit a desired criteria, rather than sifting through a myriad of similar bottles. It is common practice in the wine world, after selecting red, white, rosé or sparkling, to consider region when choosing a wine. That is because, as much as humans can manipulate their environment, they have limited ability to change the weather, and grapes are dependent on weather.

In the premiere viticultural school in the United States, the University of California, Davis, Ronald Jackson’s Viticulture and Enology textbook, Wine Science, is required reading in Winemaking 01. In it, he writes: “The view that the vine needs to “suffer” to produce fine quality fruit is long established in wine folklore. If interpreted as restrained grapevine vigor, open-canopy development, and fruit yield consistent with capacity, the concept of vine suffering has more than just an element of truth.”

In the text, Jackson highlights the multitude of factors that a winegrower and vintner must take into account when producing a bottle. These include the biology and chemistry of winegrapes, the geographical and geological factors that influence their growth, and the science of winemaking. Unlike many wine

books which are based upon a series of “I feel statements of the writer”, these insights are backed by viticultural and oenological research. This has quickly made this book one of our favorite wine resources, and we feel like it has exponentially accelerated our understanding of wine which may have otherwise taken years of talking to winemakers to piece the information together for ourselves. As a wine professional, the acceleration of this knowledge base is well worth the investment of $122 - $175 spent on the book, as it will help one better understand what they are buying. The only drawback is that it can be difficult, but by no means impossible, to read if one has not taken a college chemistry class as there are many organic chemistry references.

To purchase a copy of Wine Science in Digital or Print visit:
www.elsevier.com/books/wine-science/jackson/978-0-12-816118-0
or from your favorite ecommerece book retailer.

The following are our notes from the text about growing grapes that are paraphrased from text with additional citations and suggested readings. They do not require any background in chemistry or biology to read or understand.

Overarching trends that impact
When talking about plant growth, the general trends are:
Photosynthesis, which supplies the plant with energy through the conversion of water (H2O) + carbon dioxide (CO2) + photons of light into C6H12O6 (glucose).
The grapevines' ability to find, absorb, then combine essential nutrients into cells and other biological compounds.

These general trends can be examined through the lens of:

Macroclimate: Climate for the entire region
Mesoclimate: Climate for a particular vineyard
Microclimate: Climate for a row of vines

Environmental Factors Affecting Photosynthesis

Sunlight

Temperature

Influence on Wine
Measuring Temperature
Macroclimate Factors
Mesoclimate Factors

Sunlight and Temperature
Sunlight is composed of two types of energy: photons, which are visible, and infrared radiation (heat) which is “invisible.” However, since photons are difficult to measure, and the average energy per photon is proportional to the temperature, heat is a useful measurement of light.

Temperature
The physical temperature of the vine and berries is what ultimately dictates grapevine growth. This is impacted in a multitude of ways, the most significant is ambient air temperature. This small, quantifiable snippet of information, by definition, is adjusted for a multitude of geographical and geological factors including latitude and elevation, making it a valuable statistic for inferences about the climate.

Impact of Temperature

Grape Cultivation

Temperature of a region is important to viticulture because grapevines generally only photosynthesize when the average temperature is above 50ºF (10ºC). However, as the temperature rises, the energy available from solar radiation also rises, and plant growth accelerates. The viticulture problem is that some parts of the plant grow at faster rates than others, and not all of that accelerated growth has positive impacts on wine flavor.
Cultivating grapevines where temperatures are typically below 50ºF (10ºC), regardless of whether the temperature is caused by higher latitude or altitude, increases the risks of underdevelopment of grapes and/or grapevines, and frost damage.
When the temperature drops below 28° F, active green foliage of the grapevine is killed1.
Grape cultivars (the technical term for the difference between Merlot and Cabernet Sauvignon), colloquially known in the wine world as grape varietals, express different characteristics when grown in different parts of their growing temperature range.

Grape Aroma Compounds

Impact: Temperate/cooler conditions appear to favor the development and retention of grape aroma compounds.
Biological Process: There are a multitude of biological processes involved in the development of grape aroma compounds. The following have done some insightful research and reviews on wine aroma Compounds:

Cohen, Seth & Tarara, Julie & Kennedy, James. (2008). Assessing the impact of temperature on grape phenolic metabolism. Analytica chimica acta. 621. 57-67. 10.1016/j.aca.2007.11.029.
ir.library.oregonstate.edu/downloads/gf06g719s

Gambetta, Joanna & Bastian, Susan & Cozzolino, Daniel & Jeffery, David. (2014). Factors Influencing the Aroma Composition of Chardonnay Wines. Journal of agricultural and food chemistry.
https://doi.org/10.1021/jf501945s.
Alternative Source: digital.library.adelaide.edu.au/dspace/bitstream/2440/119623/4/Gambetta%20Maggioncalda2017_PhD.pdf#page=12

Teixeira, A., Eiras-Dias, J., Castellarin, S. D., & Gerós, H. (2013). Berry phenolics of grapevine under challenging environments. International journal of molecular sciences, 14(9), 18711–18739. https://doi.org/10.3390/ijms140918711

Acidity [2]

Impact: Relative to the grape cultivar, cool conditions can retain fruit acidity, which improves the microbial and color stability of wines. Similarly, warm temperatures decrease acidity.
Biological Process: Before veraison, which is the process of when grapes soften and change color, the optimal temperature for malic acid accumulation for V. vinifera grapes is between 68ºF (20ºC) and 77ºF (25ºC), but sharply declines at temperatures above 38ºC. After veraison, the speed of degradation of malic acid and tartaric acids accelerates in proportion to temperature because malate (malic acid) is converted in respiration (the process of converting glucose to “energy”) to pyruvate. Pyruvate is then used in the Krebs cycle for energy. [3]

For more on grape acidity, read:

Haggerty, Luke LeMay. (2013). Ripening profile of grape berry acids and sugars in University of Minnesota wine grape cultivars, select vitis vinifera, and other hybrid cultivars. Retrieved from the University of Minnesota Digital Conservancy,
https://hdl.handle.net/11299/160115.

Impact on Winemaking

Impact:
Temperature, if no refrigeration system is used, can have various effects on fermentation.
Increased temperatures can also increase the rate of aging in oak barrels.
Biological Process:
The yeast used to ferment wine produces different flavors based upon the temperature of the fermentation. For example red wine is typically fermented between 68-86°F (20-30°C) whereas white wine is typically fermented below 59°F (15°C ).
The wine molecules in oak have more kinetic energy at warmer temperatures therefore interact with the oak at a faster rate.

Impact of Sunlight
Sunlight provides the photons of energy required for breaking the bonds of carbon dioxide (CO2) and water (H2O) to create glucose during photosynthesis and also influences other phytochrome-induced metabolic reactions.

Impact of Sunlight on Grapes

Some studies show that sunlight increases anthocyanin [7], phenol and flavanol synthesis [8], whereas others have conflicting findings. According to Jackson, “Some of these differences may relate to macroclimatic differences, where increased exposure (and temperature) favors anthocyanin and phenolic synthesis in cool climates, but does not favor synthesis (or enhances degradation) in hot climates.”
One aroma compound that has been conclusively tested for macroclimate differential is sunlight’s ability to decrease the methoxypyrazine content in grapes. [9], [10] For reference, methoxypyrazine has a grassy, bell pepper or vegetative aroma common in Cabernet Sauvignon and Sauvignon Blanc grapes.
Excessive sunlight on berries can cause sunburn. Symptoms range from brown or necrotic spots on the grape’s skin to the complete desiccation (shriveling) of the berries. [11]

Measuring the Impact of Temperature of a Macroclimate:
The Winkler Index

As grapes grow above 50ºF (10ºC), the first question is: How often is the temperature above that threshold? Once that answer is found, the next question is: What grapes grow “best” when the temperature is above 50ºF at that frequency? This is important as wine becomes less "complex" outside of a particular temperature range.

Background on the Winkler Scale
M.A Amerine and A.J. Winkler, in their 1944 study, the Composition and Quality of Musts and Wines of California Grapes, focused on heat-summation units called growing degree days (GDD). Previously, the selection of an ideal grape cultivar for a region was done by trial and error. However, the pair realized the inefficiency of this, and devised a study with the goal of scientifically creating a map of California that would show the ideal places to grow particular grapes based on both the grape’s temperature affinity and the geographical region in which those temperatures occurred. Since there is a general range of temperatures in which the grape will grow, the number of GDD was grouped into regions.

Why use the Winkler Index?

The Winkler Index has been used extensively throughout the wine world and in particular the United States and Australia. It provides a preexisting metric that is readily available.
The Winkler Index is better defined than “Cool Climate” vs “Moderate Climate” vs “Warm Climate” “vs “Hot Climate”. Additionally, when using Winkler Regions, we find that I to IV is an easier scale to discuss than Cool, Moderate, Warm and Hot, especially when referencing the temperature range of a grape cultivar.

Practical Approach to using the Winkler Index
Instead of comparing regional growing degree days, the range of the Winkler Index can be used to provide a buffer/margin of tolerance for calculation discrepancies. Then, additional data can be used to get a general idea of temperature differences.

Additional Macroclimate Temperature Indicators

Diurnal Shift/ Diurnal Range: The difference between the average daytime high temperature and the average evening low temperature.

Generally, the overall temperature is built into the Winkler Index. However, when comparing two regions with a similar climate, it can be used to make general inferences about grape ripening in the region.
The mechanism behind diurnal shift is that photosynthesis does not occur in the evening, but warm temperatures will continue to ripen the grapes by the metabolism of malic acid. This means the key statistic is not so much the size of the shift, but rather the evening temperature. Similarly, in cool regions where daytime temperatures are already low, smaller diurnal shifts may be more ideal to allow for additional sugar ripening through malic acid’s conversion.

Continentality [6]: Measured in Mean daily range (MDR), is the difference between the annual average maximum and minimum temperatures. Continentality refers to the amount of continental influence a region has, as opposed to maritime influence.

A site with high continentality has a high MDR and means more heat is available during the growing season to promote fruit maturation, however, there is also a greater potential for winter cold damage.
Continentality can help to determine the duration of the growing season and has a greater impact in places where the coldest temperature is below 50ºF and less impact in regions that are almost always above 50ºF.

For a more technical understanding of continentality, read:
Pool, R. (2002). Continentality in Relation to Vineyard Site Selection. Cornell University College of Agriculture and Life Sciences. Retrieved September 23, 2021, from: https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/0/7265/files/2017/01/Continentality-in-Relation-to-Vineyard-Site-Selection-1fxo5dn.pdf

Macroclimate Impact of Elevation
Though typically tied into the temperature of a region, mountainous regions with significant elevation change significantly vary in temperature. According to the standard lapse rate: For every 1,000 meter elevation rise, air temperature drops of 6.5°C (3.5°F or 2°C per thousand feet, up to 36,000 feet)12. Most viticultural regions with significant elevation change will be broken down into smaller sub-viticultural regions until the resulting region has marginal elevation gains. This allows for majority if not all of that sub-region to be within the same general Winkler Region. The major exception is the mountainous Mendoza region of Argentina, which has several thousand-foot elevation changes that are significant enough to change the Winkler Region within a sub-viticultural area.

Temperature and Sunlight Mesoclimates
Within a vineyard, there are temperature variations that are not accounted for in the regional temperature models. These include:

Solar Exposure by Slope and Aspect

Slope: The grade or incline of the vineyard.
Aspect: The orientation of the vineyard rows in relation to the sun.

Slope and aspect’s impact on the amount of solar radiation
Slope and aspect can increase a grapevine's duration of exposure to the sun by exposing the canopy’s sides (which have the largest surface area) to direct sunlight during the midmorning and afternoon hours. The ideal aspect is a north-south row orientation.

Rows oriented southwest can increase exposure to the early morning sun.
Rows oriented southeast can improve fruit heat accumulation in the autumn.
Conditions such as the prevailing wind direction also can influence optimal row orientation.
Slope and aspect mainly have an impact in cooler climates that are frost-prone and require every bit of help ripening grapes. For this reason, Germany and Austria, which are very cold northern wine countries, have many sloped vineyards.
In warmer regions where grapes do not need help ripening, slope and aspect can also be used to minimize sun exposure.

Additional Impacts of Slope and Aspect

Air circulation can be increased around the vineyard and ripening can be assisted by directing cold air away from the vines.
Drainage can be increased by increasing slope.
Sloped vineyards in cool climates use coarsely textured top soils to enable even faster drying. This minimizes the soil’s heat loss, since it takes at least twice as much heat to raise the temperature of a moist soil, compared to its dry equivalent.
Vineyards with fast drainage can become nutrient deficient because the nutrients can wash away.
Erosion risk is increased by steeper slopes. To minimize this risk, rows are planted perpendicular to the slope. However, this may cause rows to not be planted in the ideal north-south alignment which can counteract some of the increased sun exposure from being on a slope.

Cloud Cover and Fog
Cloud cover and fog create shade, which can decrease solar radiance and temperature. This is typically built into the temperature model unless there is non-uniformity within the macroclimate/region, and the weather data was not pulled from the fog-covered area.

Wind

Wind is influenced by a vineyard's topography and geography.
Wind typically has minimal impact on vineyards, as windbreaks or shelterbelts can be used. However, if a region has particularly strong winds, like the mistral of Southern France, different vine training systems can reduce impacts from winds.
Majority of the water a grapevine absorbs is used to regulate the plant’s temperature, which optimizes its metabolic processes. This is done through the evaporation of water from the leaves. Wind on the grapevines leaves increase evaporation and can increase the cooling effect of the plant if there is enough water.
For more on the impact of water, see the section: "Rainfall and Irrigation".

Heat-Retaining Geographical Features
Geological features moderate temperature, including:

Bodies of water, lakes in particular, can moderate temperature on the mesoclimate scale due to the high specific heat of water. In contrast, oceans affect the macroclimate through weather moderation. An example of mesoclimate temperature regulation by water: New York's Finger Lakes AVA, which is a few degrees warmer closer to the lakes than a mile away.
Rocks and soil can impact temperature. See the “Soil” section to learn more.

Microclimate Modification: Vine Training and Pruning

The goal of vine training, canopy management, and pruning is to manipulate the grapevine in order to:

Achieve an ideal microclimate with:
A particular leaf area/fruit (LF/A) ratio. However, leaf area is difficult to calculate because it is either time consuming or there are no tools to instantaneously do the calculation.
A specific fruit growth location for ideal sun exposure and ease of picking.
Adequate airflow around the grapes and leaves.
Manipulation of the height of the grapes for ease of harvest or soil warmth.
Redirect the vine’s energy into the grapes rather into vegetative growth by restricting vegatative growth and preventing overcropping.

Vine “training” systems are composed of the following elements:

Training (either head or cordon): The process of getting the trunk of the vine to grow in the desired position.
Pruning (either spur or cane): The process of getting the bearing wood to grow in the desired direction.

Training System [13]

Training: Cane Height
Training: Cane Positioning
Pruning

Low Trained

Principal Arm Height:

The principal arm is less than 0.6 meters (2 feet) above ground level.

Ideal Application:

In cool climates, the vine’s low height can use the residual soil warmth to minimize day–night temperature fluctuation, help grape maturation, provide frost protection in the fall and minimize the portion of the vine damaged by freezing.
The location of the renewal portion of the vine close to the ground may also provide protection from a snow cover.

Additional Benefits:

By limiting the need for vine trellising, stout, short trunks can lower vineyard operating expenses.
Can limit vine vigor on poor soils.

Medium to High Trained:

Principal Arm Height:

Medium: 0.6 to 1.2 meters (2–4 ft)
High Trained: above 1.2 meters (4 ft).
Arbors (decorative vines): Have trunks 2–2.5 meters (6–7 feet) high.

Ideal Application:

In training systems with trailing shoots, which reduces or eliminates the need for skirting the growth.
To improve exposure of the leaves and fruit to both direct and diffused sunlight.
To position the buds and shoots away from the ground in order to protect them from the cold air that can accumulate at ground level in areas where the soil does not maintain heat.
To allow herbicide application (by positioning buds and shoots away from the ground).
For trunks of moderate height 1–1.4 meters (4.59 feet) this locates the canopy at chest height, making many manual and mechanized vineyard practices easier.
To allow for greater nutrient storage by the vine, as the woody area of the trunk is where the vine stores much of its reserves.

Head Trained
Head training positions the canes or spurs that generate the fruit-bearing shoots radially around the trunk apex.

The Technique:
The bearing shoots of one season often provide the bearing wood for the next year’s crop. As the truck thickens, the vine becomes self-supporting. The technique avoids the complexity of trellising and shoot positioning

Additional Benefits

Historically, head training was a common method because it was easy to develop and did not require trellising (formerly cost prohibitive when the materials like wire were made by hand).
The lack of support wires allows for easier manual weed control.

Challenges

Head training can create shoot crowding in vines with medium to high vigor. This leads to decreased light exposure to the fruit, leaf, and canes, and higher canopy humidity. Additional pruning to avoid these problems can be done, but at the risk of over pruning, which would also negatively impact yield and fruit quality.
Head training has limited bud retention. This increases the risk of frost damage, making the technique rarely used in cool regions.

Cordon Trained
Cordon training positions the bearing wood along an angled portion of the trunk which is horizontal and parallel to the row, or vertical.

Technique: Horizontal cordon training

Systems have either one cordon (unilateral) or two cordons (bilateral).
Uniform spacing of the fruiting portions of the vine (bearing wood) along the cordon create a microclimate capable of high amounts of photosynthesis. This can generate higher yields while maintaining fruit quality and increasing the nutrient reserves in the vine's woody parts.

Technique: Quadrilateral cordon training

The trunk is divided into two horizontal trunks, directed at right angles (laterally) to the vine row. These subsequently branch into two or more cordons running in opposite directions, parallel to the row.
Vertical (upright) cordon systems are uncommon because of apical dominance (when the central stem of the plant is dominant over other side stems) and shading from the leaf growth at the top. Additionally, mechanized pruning and harvesting is difficult.

Spur Pruning
Benefits

Spur pruning is generally more adapted to mechanical harvesting than is cane pruning, because it positions fruit growth in a specific location.
It is easier for inexperienced pruners to learn than cane pruning.
It has the ability to limit productivity which can be beneficial or detrimental, depending on the vigor and capacity of the vine.
Berry size is generally reduced with spur pruning. This increases the skin-to-pulp ratio.
Yield restriction may be desirable in cool climates or under poor nutrient conditions, but is less important in warm climates on rich soils.
The delay in leaf and canopy production can create higher concentrations of methoxypyrazines when used with Cabernet Sauvignon.

Cane pruning
Benefits:
Ideal for small cluster cultivars that need the retention of extra buds.
Facilitates the development of wide-topped trellises, extending both along and perpendicular to the row.
Cane-pruned vines tend to develop their canopy sooner in the season.

Challenges

Expensive due to trellising and the labor expense of tying the shoots.
Requires manual pruning by skilled workers who are capable of selecting the “right” canes that will bear fruit. Damaging or destroying one cane can significantly reduce vine yield. Further complicating pruning is the required removal of the current year’s bearing wood, because the next season’s crop typically comes from shoots that develop from renewal spurs.
The length of the bearing wood can cause uneven shoot development and a nonuniform canopy due to apical dominance (the vertical shoot growing more than the horizontal ones). This can result in inconsistent fruit ripening. Arching or positioning the canes obliquely downward can often minimize apical dominance, but it places the bearing shoots and fruit in diverse environments.
Converting the pruning methodology from spur to cane pruning often results in temporary over cropping.

Training/Pruning Systems

Head Trained Spur Pruning
Head Training Cane Pruning
Cordon Trained Spur Pruned
Cordon Trained Cane Pruned

The System: Head Training Spur Pruning

Head training combined with spur pruning typically creates compact canopies.
When combined with short trunks, these bushy vines provide beneficial fruit shading, limiting leaf light exposure in order to minimize drought stress and fruit sunburn in hot dry climates like in the Mediterranean.
In cool, wet climates, this technique may increase disease and poor maturation.
Head trained, spur pruned systems are unsuited to mechanical harvesting.

Example: The Goblet training system, a sub-style used in Southern France is suitable only for varieties with medium-size fruit clusters that produce fruitful buds at the base of the cane.

The System: Head Training Cane Pruning

Head training and cane pruning is used in cooler, wet climates because it positions the fruit-bearing shoots away from the head of the trunk and permits more buds to be retained.
Improves yield for small-fruited cultivars, including Chardonnay and Pinot Noir. Conversely larger-clustered varieties, including Chenin blanc and Grenache can tend to overproduce with cane pruning.
In cane pruning, renewal spurs are selected because the canes of one season are typically not used as bearing wood the next season, as it would relocate the bearing wood away from the head. The same tendency occurs with spur pruning, but at a much slower rate. The conscious selection of renewal shoots may be reduced if water sprouts develop from the head.
Example: Guyot, Guyot Double (two canes instead of one cane), The Mosel Arch, Guyot in France and the Kniffin in eastern North America.

The System: Cordon Trained Spur Pruned
As cordon trained vines are typically more vigorous, they are commonly spur pruned to minimize over cropping. This feature limits the combined use of spur pruning and cordon training to varieties that bear fruit buds down to the base of the cane.

The System: Cordon Trained Cane Pruned
Cane pruning is combined with cordon training for strong vines capable of maturing heavy fruit crops.

Other Benefits

Well suited for mechanical harvesting because the location of the fruit is placed at a consistent height along the row. It also provides relatively homogeneous growing conditions, which favors uniform maturation.
Well suited for mechanical pruning because the bearing wood is placed in a narrow region above or below the cordon.
For these reasons and others, many new training systems are based upon cordon training.

Challenges
Cordon training has higher costs due to the required trellising and support wires, as well as the labor expense to install the system. There is also greater skill demanded in selecting and positioning the arms that bear the spurs or canes.

Examples: Cordon de Royal, Pergola, Geneva Double Curtain; Ruakura Twin Two Tier, Lyre, Scott Henry, Hudson River Umbrella, and Dragon.

For more insight into vine training, read:
lodiwine.com/blog/Head-vs--vertical-cordon-trained-old-vines--and-the-skilled-labor-behind-it

lodigrowers.com/vertical-shoot-positioned-trellis-systems-in-warm-regions/

winescholarguild.org/blog/vine-school-part-1-common-vine-training-systems

Yield Control: Fruit-cluster and flower-cluster thinning

Cluster thinning is a popular methodology for controlling yields. It can help growers avoid delays in ripening and improve sugar ripening by concentrating the energy of the vine into only the remaining grapes, but it reduces the quantity of harvestable fruit. The research into cluster thinning has found:

In a study of Finger Lakes Riesling by Dr. Justine Vanden Heuvel and Dr Trent Preszler of Cornell University, cluster thinned vines produced grapes with higher soluble solids (Brix) at harvest. However, a tasting panel did not believe that the wine make from cluster-thinned grapes was any better than wine made from vines that were not cluster thinned14.
Research conducted by Dr. Markus Keller et al on irrigated vines of Cabernet Sauvignon, Riesling and Chenin Blanc in the warm dry climate of Yakima Valley, Washington found “Cluster thinning and its timing had little or no influence on shoot growth, leaf area, pruning weight, berry number, berry weight, and fruit composition (soluble solids, titratable acidity, pH, color) in both the current and subsequent seasons”. [15]
Convers to Dr Keller's research, Esperanza Valdés et al on Tempranillo grapes in the warm dry Extemadura climate found that cluster-thinning accelerated ripening, created a different phenolic profile by improving the contribution of the anthocyanin-tannin combinations to wine color, and increased wine tannins but also increased wine pH (lowered acidity). [16]

There are other methods of pruning used to control yield including defoliation. More can be read about those methodologies here:

Martinson, T. (2016, November). How Defoliation, Defruiting, and Extreme Shoot Reduction Affected Clusters, Fruit Composition, and Bud Hardiness. Grapes 101 | Viticulture and Enology.
Retrieved September 24, 2021, from
grapesandwine.cals.cornell.edu/newsletters/appellation-cornell/2017-newsletters/issue-31-november-2017/grapes-101/.

Rainfall and Irrigation

The right amount of water in regards to vineyard cultivation for high quality wine is different from the right amount of water for maximum grape yield (in tonnage). However, what constitutes the “right amount” and “high quality wine” is subjective. The current agricultural techniques call one of the ideal irrigation strategies “Regulated Deficit Irrigation” (RDI), which restricts vine water use below that of a fully watered vine. In Europe, irrigation of vineyards is generally prohibited to “better express the terroir”. However, science has shown that irrigation can be used to improve yields while maintaining quality.

Ideal water for wine grapes:

Permits grape culture in arid and semiarid regions.
Can facilitate production of premium quality grapes in regions with variable rainfall.
Directs the energy of the grapevine into fruit development rather than shoot growth.
Optimizes phenol content, anthocyanin content and flavanol content, and produces wines with better aroma attributes. This is done by reducing the berry size, which increases the skin-to-pulp-ratio and other influences independent of size. [17]
Enhances the inherent resistance of vines to several pathogens by reducing berry size, which reduces the risk of bunch rot (as the berries are not so tightly packed together in the bunch).

Insufficient water for wine grapes:
Excessively reduces the quantity of fruit produced by the vine, the berry size, and the initiation and development of the flower clusters that compose a main branch (inflorescence), when grapevine is most sensitive to insufficient water, between flowering and fruit set. The degree to which these occur depends both on the timing and duration of the deficit.

Excessive water for wine grapes:

Undesirably effects cane maturation, causing fruit ripening issues, including large berries (reduced skin/juice ratio), reduces sugar and anthocyanin concentrations, compact clusters, and increases disease incidence
Causes high soil-moisture content which accentuates cracking of the berry skin and susceptibility to bunch rot

For more on water’s impact on wine aroma, read:
Chapman, Dawn, Roby, Gaspar & Ebeler, Susan & Guinard, Jean-Xavier & Matthews, Mark. (2008). Sensory attributes of Cabernet Sauvignon wines made from vines with different water status. Australian Journal of Grape and Wine Research. 11. 339 - 347. 10.1111/j.1755-0238.2005.tb00033.x.
citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.716.5158&rep=rep1&type=pdf

Macroclimate Factors

Timing of Irrigation and Vine Growth Stages [18]
In regions with adequate rainfall from both a timing and quantity perspective, irrigation is generally unnecessary, and often not religiously measured. What denotes the rare occasion, according to Joseph Hegele of Maui Wine, in a email response was simply: “If it hasn’t rained in a long time. 😊" He added, "figuring out when to irrigate is based upon visual cues like drooping of tendrils, or you can touch the leaves and find out as well, but you have to build up a reference for it. There are also tools that will actually measure the amount of moisture in a leaf.” Unlike other flowering plants, building up a reference to what a grape leaf looks like under ideal water conditions is important, as physiological disruption occurs long before wilting, making it too delayed for usage as a reference for water needs.

The quantifiable approach to irrigating grapevines uses a “pressure chamber” to measure water potential (Ψ) which is given in megapascals (MPa) or bar. This is the measure of the vine’s relative ability to extract water from the soil (average water deficit), or current water stress. [22]

March to May
May to June
June to July
August to harvest
September to November

March to May: Bud Break & Vegetative Growth
Dormancy ends and new leaves emerge from buds. If applicable, winegrowers replace damaged posts, broken trellis wires, and vines are attached to the trellising.

The branches which can grow between 5 an 15 cm a day must be lifted and trellised regularly before being pollarded (trimmed).

Amount of Water:
Water stress should be avoided until flowering

May to June: Flowering
Flowers bloom from the new vine shoots. Grapevines are self-pollinating and each has the potential of turning into a berry.

Winegrowers may work on disbudding, the selection of the branches for fruit growth and the removal of any unwanted buds or burgeoning branches. Removing buds or shoots from the base of the vine or the ground is called suckering.

During pollination, cold temperatures, precipitation can lead to frost damage, coulure - a failure to set fruit which decreases the number of berries per bunch, or millerandage - hindered berry growth.

The size of the canopy and the climate are the major dictators of water usage by the grapevine.

June to July: Fruit set
Flowering ends and the flowers become berries. The hot, humid weather is conducive to a number of pests. At fruit set, the leaves are thinned out on the vines on the side facing the sunrise and those vines too laden with grapes are thinned (green harvest).

At this point, winemakers already have a notion of the harvest date as it occurs 120 days after flowering (particularly for Merlot noir) and 45 days after mid version in August.

Amount of Water:
After flowering moderate water deficits can control vegetative growth, while allowing photosynthesis to continue. Additionally time-limited water availability can restrain undesired vegetative growth17 .

“Water deficit during the period after flowering resulted in the greatest berry weight reduction compared with that of well-watered vines especially in years with high temperature summation16 ".

August to harvest:
Véraison and Ripening

Vines slow or stop vegative growth, aroma compounds develop, the grapes change color and soften, and L’aoûtement, the hardening and color change of the shoots from green to brown, signals the winegrower to prepare for harvest. The exact length of this period depends on weather conditions.

Winemakers may defoliate or cluster thin.

Amount of Water:
After veraison, moderate water deficits are ideal to limit shoot growth. If an RDI strategy is used, berry size can be reduced (in a good way) when compared to a well-watered vine. [21] However, recent research supports reducing the magnitude of water deficit during the last few weeks prior to harvest to prevent the minimization of crop reduction while still maintaining desired fruit characteristics.

September to November:
Harvest
The harvest lasts one to three weeks, depending on the size of the vineyard and the weather

(Post Harvest)
November to February: Leaf-fall and Dormancy

The winemaker vinifies the grapes and the vineyard might be ploughed and fertilized. When leaf-fall ends, winemakers begin pruning unwanted wood. This period runs into mid-March.

Amount of Water:
After harvest, both drought and overwatering should be avoided to favor leaf function into the autumn, restrict late vegetative growth, promote cane maturation and encourage an autumn surge in root growth. Frost penetration during the winter will also be deeper in dry soils, potentially leading to more root damage.

For more on the technical aspect of calculating irrigation read:
Jovanovic, Z. and R. Stikić. “Partial Root-Zone Drying Technique: from Water Saving to the Improvement of a Fruit Quality.” Front. Sustain. Food Syst. (2018). https://doi.org/10.3389/fsufs.2017.00003

Mesoclimate Factors

Irrigation Stratigies
Drip Irrigation
Sprinkler Irrigation
Furrow Irrigation

Mesoclimate Factors That Influence Water Needs

The choice of vine training system will affect water needs, as increasing canopy surface area, air flow, and sun exposure will increase water demands through increasing evaporation.
Some varieties require less water, which will dictate vineyard water usage. Grenache, for example, requires much less water than Syrah/Shiraz. Rootstock will also influence the water needs.
The timing of water needs can also vary by cultivar and pruning technique, since minimally pruned vines, which have earlier canopy development, require more water earlier in the season.

Water Mesoclimates: Irrigation Strategies
Of the irrigation strategies we have found, regulated deficit irrigation (RDI), which implements drip irrigation, is the most common strategy. Partial root-zone drying is another strategy, in which one side of the plant’s roots are exposed to drought while, simultaneously, the other side is irrigated. To avoid drying of the roots, the wet/dry sides are rotated. The theory is that, while the plant is receiving enough water, the roots that are exposed to drought produce abscisic acid, which triggers the stomata of the leaves to close, thereby reducing water needs. [23]

For more on vineyard water strategies:
Water Management Parts i-iii from Lodi Wine Growers
www.lodigrowers.com/water-management-part-i/
www.lodigrowers.com/water-management-part-ii/
www.lodigrowers.com/water-management-part-iii

Water Management from the Australian Wine Research Institute
awri.com.au/industry_support/viticulture/water-management/

Mesoclimate: Mitigating Excessive Water
Typically, it is easier to add more water to a plant than it is to take it away. In regions with excessive amounts of water, mitigation strategies include:

Increasing drainage, utilizing slopes, and soil porosity. Unfortunately, both are predominantly geological factors with little manipulation that can be done.
Increasing the canopy surface area through the creation of a divided-canopy training system; basal leaf removal is another technique.
Maximizing air flow through the vineyard by adjusting the angle of the rows.

Drip Irrigation
How it works:

Drip irrigation emitters are spaced to produce a relatively uniform zone of irrigation along the length of each row.

Benefits

Can create uniform saturation.
Minimizes water loss due to evaporation or runoff.
Can be used on vineyards of any slope.
Can be used to efficiently apply fertilizer and nematicides to plants in a technique called fertigation which is similar to hydroponics. The technique is best suited for sandy soil because of its minimal nutrient retention.
Can be used to influence the area of root growth, as the root systems become focused within the moisture zone.
Uninfluenced by wind conditions.

Drawbacks

Can be expensive to install.

Good for vineyards that:

Are regularly in need of irrigation.
Do not have cheap and abundant access to water.
Have shallow soils, or those with saline water tables close to the soil surface (Jackson Winegrapes).

Sprinkler Irrigation
How it works:
Water sprinklers are used to saturate the soil.

Benefits

If the system is constructed correctly, the soil can be uniformly saturated.
Advantageous on sloping terrain, where runoff and erosion are potential problems with other systems.
Cheaper and less labor intensive to install than drip irrigation systems.
Can be used for frost control and freezing issues.

Drawbacks

Inefficient usage of water due to losses by evaporation.
Can be affected by wind.

Good for vineyards that:

Do not need irrigation regularly.

Furrow Irrigation
How it works:
Furrows are created between the rows of vines. Irrigation involves flooding the rows with water in order to water the entire row. Enough water is used to moisten the soil to field capacity at an effective rooting depth of 1–1.5 meters.

Benefits

Cheap to install, and the technique has been used for centuries.

Drawbacks

Inefficient usage of water due to losses by evaporation.
Can create non-uniform soil saturation.
Not feasible on hilly or sloped terrain.

Good for vineyards that:

Are flat and do not require regular irrigation.

Soil

Soils provide plants a place to anchor to, a supply of inorganic nutrients, and a source of water. As much as soils are talked about in the wine world, much of the academic research is of the opinion that soils have little to do with wine characteristics. [24] Or to quote Ronald Jackson, “Of climatic influences, soil type appears to be the least significant factor affecting grape and wine quality.”

This occurs because grapevines only uptake a specific set of nutrients, chiefly a set of macronutrients and micronutrients, which can be found in the section titled “Mesoclimate: Soil Chemistry.” Additionally, grapevines regulate the quantity of the nutrients that are absorbed. At first glance, this runs counter to the heralded correlations of soil and aroma touted by sommeliers, based upon their tasting experiences. However, this correlation may be simultaneously true and false. There can be a correlation between soil and aroma characteristics, because a particular grape cultivar will respond similarly if planted in soils with similar attributes, including microbial compositions, and heating and geological properties. Also, the soil may require similar canopy management techniques for water mitigation strategies. If wine is about creating a “sense of place,” then winemakers recreating similar experiences from similar terroirs may use similar winemaking techniques. This means the direct and indirect influences on the grape may provide properties that can be interpreted as being “from the terroir.” So, because correlation does not mean causation, while the wines may have similar aroma characteristics, they are NOT from direct absorption of the aroma characteristics of the soil.

Soil Composition Impact on Root Development

Heavy Clay Soils
The small diameter of heavy clay soils makes root penetration difficult and results in poorly aerated conditions when wet. As a consequence, roots remain at or near the surface, exposing vines to severe water stress under drought conditions.

Lighter Soils

Vines may experience less severe water deficit under drought conditions, if the soil is sufficiently deep to permit root access to groundwater.
Soil depth may offset the poor nutrient status of many light soils.

The negative effects of the small and large pores of heavy and light soils, respectively, may be counteracted by humus.

Impact of Tillage and Groundcover On Root Development

Under zero tillage, most root development occurs in the upper portion of the soil, whereas conventional cultivation limits root growth to deeper portions of the soil. [32]
Under grass cover, root distribution is relatively uniform in the top meter of the soil. Cultivated vineyards show lower levels of organic material. [33] This may result from enhanced aeration and solar heating. Both stimulate the microbial mineralization of the soil’s organic content.

Macroclimate: Soil Structure

Soil Texture
Soil Depth
Organic Matter
Soil Origin

Soils in Combination
Soils are rarely found in one texture. The United States Department of Agriculture has classified soil combinations.

For more insight into soil classifications:
The U.S. Department of Agriculture
Soil Mechanics Level I: USDA Textural Soil Classification Study Guide
www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1044818.pdf

Soil Depth and Soil Layers

Soil depth
The depth of soil before reaching a hardpan (made of densely packed clay), bedrock or another barrier that prevents root growth.

Soil Layers
In addition to depth, soil can also be composed of layers. The depth and composition of each layer can vary by microclimate. These layers are also known as the soil horizons (each layer is a horizon):

Hummus/Top Soil
As the primary root growth is within 24 inches of the surface, the composition of the soil up to that depth is of most importance.
Subsoil/Substratum
Below the top soil is the subsoil. In regions where vines have low accessibility to water or nutrient-deficient soils with no ability for fertilization, soil depth to 20 feet is more important.
Bedrock
Bedrock can vary in depth. Even in places where the bedrock is closer to the surface, access to the water table can provide a non-soil based water source for grapevines. This is found in St. Émilion, for example. [34]

Root Depth
Primary Root Depth
The primary root growth, including the fine roots which account for the majority of the absorption.
Radius: 4-8 meters around the trunk.
Depth: Typically 4 to 24 inches (~60cm) deep.

Deep Root Growth
Deep roots allow the vine to avoid serious water stress during drought and limits the development of nutrient deficiencies in poor soils.

Depth: Roots up to 1 centimeter in diameter can penetrate to a depth of more than 20 feet (~6m)

Humus: Soil’s Organic Material
Though humus in soil can be manipulated at the mesoclimate and microclimate level, it is still a significant part of soil composition, and typically referenced in some capacity on the macroclimate and regional level.

Influence on Water:
The organic content of soil improves water retention and permeability by enhancing its aggregate structure. In particular, humus modulates pore size, and facilitates the upward and lateral movement of water, increases water absorbency, and retains water at tensions that permit roots ready access to the water.

Influence on Heat:
Humus darkens the color of soil, which helps to absorb heat.

Influence on Nutrients:
Roots rarely absorb organic compounds from soil. Exceptions are systemic pesticides and highly volatile compounds such as ethylene (released by many soil microorganisms).

Soil Flora and Fauna
The flora and fauna found in vineyards' soils have major implications on the health of a vineyard. Of much benefit, they are decomposers that turn organic matter into minerals, polysaccharides and humus. Bacteria and fungal metabolism creates acids that are important for the extraction (and eventual solubilization) of inorganic nutrients from the mineral content of the soil. The decomposition and general soil activity occurs more readily in warm and damp soils than cool and dry soils.

Examples of Beneficial Contents

Mycorrhizal Fungi
Nitrogen-Fixing Bacteria

Examples of Detrimental Contents
Microbial Pathogens
Nematodes
Phylloxera (insect pest)

Origin of Soil
The origin of the soil is of debatable influence, as sommeliers will claim a correlation to particular aromas, whereas studies on soil composition and root soil absorption have not proven this. This is because clay particles are chemically and structurally transformed minerals that bear little resemblance to the parental material, and the minerals absorbed by the root systems are not derived from the rocks themselves. However, there may be an indirect correlation. The soil’s origin may influence or in conjunction with other external factors influence the soil’s microbiome. This in turn can influence wine aroma.

Types or Rock Origins
Igneous

Origin: Volcanic
Common Types: Granite, pumice, basalt, obsidian

Sedimentary

Origin: Rock formed by layers of sediment like sand, silt, dead plants, and animal skeletons, followed by cementation.
Common Types: Sandstone, limestone, shale, fossilized limestone, chalk, gypsum.

Metamorphic

Origin: Rocks formed from other rocks that were changed by heat and underground pressure.
Common Types: Slate, schist, marble

Ideal Soils
The ideal soils for growing wine grapes are those with enough fertility that the vine grows, but not so much fertility that there is excessive vegetative growth, which directs a higher proportion of photosynthate toward fruit maturation. Research has established that ideal soils are highly dependent on a multitude of factors, including the climatic conditions and access to irrigation. [22]

The Australian Wine Research Institute has a recommended set of indicators to assess soil health in vineyards, which we have used as a reference for the following soil attributes. The full document can be found here: White, R. (2018, June). Assessing soil health in a vineyard. Fact Sheet Viticulture. Retrieved September 23, 2021, www.awri.com.au/wp content/uploads/2018/06/assessing-soil-health-in-a-vineyard.pdf.

Water Retention
Heat Retention

Water Retention Abilities
Water’s accessibility to a plant is based on the soil's ability to retain water and its ability to release water to the roots. This is primarily influenced by the soil particle size (soil texture), textural class, and its humus content (organic material from decomposed plant and animal matter).

Key Concepts
Drainage and water retention are opposites by definition.

Beyond issues caused to grapes by excess water, waterlogged soils can cause the development of chlorosis (yellowing of normally green leaves due to a lack of chlorophyll) in lime soils, increase the likelihood of root pathogen problems, and hinder movement of machinery in the vineyard.
Larger particles allow water to both drain more easily and be more accessible to roots. Smaller particles hold more water, but can potentially do so to the extent that it is inaccessible. As soils are typically multi-textured, the proportion of each texture creates differing water retention characteristics.

Field Capacity: “The water remaining in a soil after it has been thoroughly saturated and allowed to drain freely, usually for one to two days”. [26]

Available Water Capacity: “Available water capacity is the amount of water that a soil can store that is available for use by plants. It is the water held between field capacity and the wilting point, adjusted downward for rock fragments and for salts in solution”. [27] Additionally, a soil moisture tension of less than 0.2 MPa is required to extract water from the soil through leaf transpiration and capillary action from the roots. The required pressure can change in water deficit conditions. [28]

Heat Retention
The soil’s ability to retain warmth will dictate plant growth, through direct heating or by regulating the activity of soil microbes. The soil attributes that influence temperature are:

Particle Size

Larger particles fluctuate in temperature less than soils of the same color with smaller particle size. For example, gravel soils will heat up during the day and cool down more gradually than the ambient temperature, thereby releasing heat to the grapes if they are close enough to the ground, and reducing the likelihood of frost damage. [29]
In very fine-textured soils, water has a significant influence on heat retention as much of the heat absorbed during sun exposure is transferred to water as it evaporates. This heat can be subsequently lost as the water evaporates.

Soil Color
The primary effect of soil color is its influence on the soil’s ability to absorb heat. In particular, dark soils absorb more heat than lighter soils. Coloration, however, is a minor contributor to heat, and therefore more important in regions where heat is problematic. The impact of soil color and moisture content on temperature are most significant during the spring and fall. This is because summer temperatures are typically adequate for ripening. Other factors of importance to soil color are:

Light colored soils can reflect photosynthetically active radiation (PAR) into the vine’s canopy and influence grape yield as well as sugar, anthocyanin, polyphenol, and free amino acid contents (Robin et al., 1996).
PAR is the light spectrum range (wave band) of solar radiation that is between 400 to 700 nanometers, the range photosynthetic organisms are able to use for photosynthesis.
White Soils: Typically high in calcium
Red Soils: Typically high in iron
Dark Brown to Black Soils: Typically high in humus, though soil needs only about 5% organic material to appear black when wet. Additionaly, dark soils with high moisture content absorb more solar radiation but warm more slowly than drier soils, due to the high specific heat of water. (4,184 Joules. Specific heat of water refers to the amount of energy required to raise the temperature of 1 kg of water by 1°C.)

Utilization of Groundcover

Reflective groundcovers can slow the rise in soil temperature during the spring, and moderate its decline in the autumn.
Plastic mulches often enhance early soil warming in vineyards. [30]

Rootstocks and Cover Crops

Rootstocks
Cover Crops

Soil Microclimate: Rootstocks (42)
The primary purpose of rootstocks is protection from soil borne pests, in particular Phylloxera. Most rootstocks in use have this trait. Most rootstocks have other attributes as well. These include:

Influence on a grapevines growth, vigor and mature vine size. Most rootstocks on deep, well-drained fertile soils are more vigorous and larger than ungrafted vines, a few are extremely invigorating, and some are somewhat devigorating.
Differing soil limitations. Some rootstocks can provide a degree of drought tolerance, or provide tolerance in waterlogged soils, while others are better adapted to acid soils, or alkaline soils or specific soil chemical conditions like high sodium, or high chloride, or high salinity, or serpentine soils.
Different interactions with the scion it is grafted to. This is based Ultimately, a rootstock needs to be able to graft with the scion (the actual cultivar) and that the interaction between rootstock and scion varies on scion cultivar and clone. Additionally, the carbohydrates and hormones produced in the top of vines are more important to overall vine function than the mineral nutrients taken up and the hormones synthesized in the bottom of vines.

Cover Cropping Systems (41)
Research has shown that cover cropping systems can be effectively used to reduce erosion, control weeds, and increase organic matter in the soil, including nitrogen, while minimizing or eliminating the need for the addition of introduced chemicals or fertilizers. There are a multitude of cover crops which can be generally divided into: grass; grass and legumes; legumes and spontaneous vegetation. In research, legumes can increase soil nitrogen, and all cover crops add organic matter to the soil. There are, however, no definitive conclusions as to their direct impact on wine aroma characteristics.

For more insight into cover crops, read:
Lodi Wine Growers Association.
Cover Cropping Systems for Organically Farmed Vineyards
www.lodigrowers.com/cover-cropping-systems-for-organically-farmed-vineyards/

Australian Wine Institute
Documents on Cover Crops
www.awri.com.au/industry_support/viticulture/vineyard-practices/cover-crops/

For additional insight into soils, read:
Lanyon, Dean & Cass, A & Hansen, D. (2004). The effect of soil properties on vine performance.
www.researchgate.net/publication/228433458_The_effect_of_soil_properties_on_vine_performance

Mesoclimate: Soil Chemistry

Soil’s nutrient content is derived from all of the aforementioned factors: parent material, particle size, soil composition, humus content, pH, water content, temperature, root-surface area, and flora and fauna composition. Soil nutrients can be divided into three categories:

Primary Macronutrients: nitrogen (N), phosphorus (P), and potassium (K)

Secondary Macronutrients: calcium (Ca), magnesium (Mg), and sulfur (S)

Micronutrients (required only in trace amounts): Boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn).

Most of the nutrients required for viticulture, especially the micronutrients, are needed in such trace amounts that they never need to be accounted for. As some manipulation of the soil is allowed in most wine regions, the nutrient content of the soil can be adjusted through fertilization. Deficiencies are typically measured by sampling plant tissue, or through the plant’s “symptoms” of deficiency, as a soil analysis may give a general trend but cannot be used to predict nutrient uptake.

Historical Fertilizers

Compost has been the most common means of increasing the humus content.
Manure has been historically used to enrich the soil’s organic content.
Straw has been used as a mulch, but requires earthworms to gradually incorporate it into the soil. Additionally, because of straw’s high carbon-to-nitrogen ratio, it can cause a temporary nitrogen deficiency which may need to be compensated for until humifaction is complete.

pH
Nitrogen
Phosphorus
Potassium

Soil Acidity: pH
The pH of soil influences the solubility, and therefore availability, of minerals to the grapevine as well as the activity of soil microbes. The uptake of minerals is regulated by the roots and mycorrhizal (microbial soil fungus) association.

Ideal Range: 6.0–8 (1:5 pH water)

What Influences pH

The chemical composition of the parental rocks.
The increased weathering of the rocks causes an increased acidity, lowering pH.
High rainfall areas have higher pH and are deficient in calcium, magnesium, potassium, phosophate (phosphorous) and Molybdate (molybdenum).
Soils are typically pH buffered, which is the capacity of the soil to resist pH change. Sandy soils acidify more quickly than clay soils because of the lower buffering capacity, but the pH can be recovered faster with the application of less lime, compared to clay soils. [35]

Mesoclimate: Vineyard pH
Measuring the pH of vineyard soils is helpful in understanding macrotrends on potential deficiencies. Due to the complex factors that regulate nutrient uptake, however, sampling plant tissue is the only way to actually analyze vine nutrition.

Adjustments to Acidic Soil

Acidic soils are often treated with crushed limestone to raise their pH, preferably before planting.

Adjustments to Alkaline Soil
(calcareous/high limestone content)

Lime-tolerant rootstocks are extensively used on calcareous soils.
Soils are treated with sulfur, which oxidises into sulfuric acid. Gypsum (calcium sulfate) may also be added to alkaline and sodic soils.

Nitrogen (36)
Ideal potentially mineralizable nitrogen:
6-11 mg N/kg soil/week

Plant Usage

Nitrogen is required by plants to build amino acids, nucleic acids, proteins, enzymes and other organic compounds. This makes its usage highly critical for growth.
A slight nitrogen deficiency can reduce vine vigor and improve the phenolic content in both grapes and wine. [37] However, inadequate nitrogen can severely impact growth. Due to its inability to remain in the soil for prolonged periods, nitrogen is typically used during bud break and fruit set, which is when the need is greatest.

Cause for Deficiency
Nitrogen is not retained in the soil easily, due to its usage by other plants and microbes if found in abundance. This is why it is the most used viticultural fertilizer.

Naturally Sourced Nitrogen
Nitrogen is typically added to the soil by nitrogen-fixing bacteria that live in the general soil or live in the nodules of legumes. As these bacteria are highly influenced by temperature, soil moisture, and soil aeration, they can be inconsistent.
Fertilizer Options

Manure is the historical method of adding nitrogen to vineyards.
Inorganic nitrogen fertilizer in the form of urea and ammonia salts.
Urea is easily soluble, but also easily leached through the soil. It also requires microbial transformation to be absorbed.
Ammonium compounds are easily absorbed by clay particles, making them less available to vines. This absorption also makes them less likely to be lost via leaching. For compound fertilizer to be used by the vine, it is generally transformed to nitrate before being taken up by the roots, although ammonium can be actively taken up by vines.
Organic nitrogen can be released more gradually than inorganic sources and can correspond more closely to the grapevines’ needs (Jackson pg 167).
Nitrogen fertilization, groundcovers that utilize legumes and/or mowing and tilling can be used to supply and store nitrogen.

Phosphorus(38)
Plant Usage

Beyond being a component of cell membranes and DNA, phosphorus is needed for photosynthesis, the movement of sugars, and carbohydrate storage within the vine.
Deficiencies impact the reproductive processes of the vine before vegetative growth is affected.

Cause of Deficiency
Phosphorus is easily leached from sandy soils, or may be lost from the soil when surface soil is eroded. It is also removed from the vineyard at harvest at a rate of approximately 0.6 kg per ton of grapes.

Naturally Occurring Phosphorus
Phosphorus develops in the soil via the slow breakdown of organic materials.

Fertilizer Options
Single superphosphate, double and triple strength superphosphate, monoammonium and diammonium phosphate (MAP and DAP), phosphoric acid.

Potassium (39)
Plant Usage

Potassium is used to facilitate a multitude of plant processes, including turgor in the nonwoody parts of the vine; ionic balance, a structural role neutralization of organic acids; electrochemical processes; regulation of stomatal function; enzyme activation; protein synthesis; cell division; and synthesis and translocation of sugars.
Deficiencies include: “leaf scorch” (the browning of the perimeter of the leaf), “black leaf” (the blackening of the leaf), or reduced drought and cold tolerance.
Excess potassium may disrupt the roots’ ability to uptake magnesium and raise the grapes’ pH too much. If the pH is raised (becomes less acidic), tartaric acid may need to be added during vinification, since insufficient fruit potassium may lead to slow or stuck fermentations at the winery. [40]

Cause of Deficiency
Potassium deficiency is common in manipulated (leveled, for example) or eroded soils where the topsoil has not been redistributed evenly, thereby exposing less fertile subsoil.
Sandy soils in high rainfall regions, or vineyards characterized by high calcium or magnesium contents may also be potassium deficient.

Naturally Occurring Potassium
Potassium is naturally found in soils from mineral weathering.

Fertilizer Options
Potassium chloride, potassium nitrate, potassium sulphate.

For additional trace elements of boron, copper, iron, zinc, and molybdenum, read
Trace Elements:
The Australian Wine Research Institute. “Grapevine Nutrition Trace Elements .” The Australian Wine Research Institute, 2010, www.awri.com.au/wp-content/uploads/4_nutrition_trace_elements.pdf.

Ripeness/Harvest

The determination on when to pick a grape is ultimately based upon the perceived flavors it will generate after vinification. However, this decision is anything but straight forward. Except for the skins, most grape varieties, except for muscat, have essentially odorless juice at any level of ripeness. [43] Additionally, predicting the complex chemical reactions that convert various compounds present in the grapes into perceivable aroma compounds in wine has improved in recent years, but remains far from definitive.

Ripeness Factors to Consider:
Wine style, specifically in terms of what flavors the winemaker wants to express, is a major factor in when to pick the grapes. For example, white grapes of intermediate maturity may produce fruitier, but less complex, wines than fully mature grapes. [44]

Measuring Ripeness
Sugar Ripeness and Acidity
Phenolic Ripeness

Measuring Ripeness
Historically, harvest date depended (assuming favorable weather conditions) on subjective visual, textural and flavor clues to fruit ripeness.

Currently the practice of basing grape payment is generally on:

The grape variety.
Weight of the total harvest (typically in tons).

Advancements in Understanding Ripeness
Though there have been significant advancements in the understanding of the potential precursors in “ripeness,” there is still difficulty in determining the final flavor profile through chemical analysis of the grape, due to the multitude of intertwined factors that make up wine aroma. However, as the ease of access to chemical analysis increases, we expect advances that give a better understanding of ideal ripeness.

Sugar ripeness:
The development of sugar in the grape measured in degrees Brix. This dictates alcohol potential.

Sugar content of a grape measured in degrees Brix or degrees Baumé

Brix also can be used to calculate the total alcohol potential of a wine. The formula is: Potential ABV = Degrees Brix x Alcohol conversion factor (Where alcohol conversion factor is 0.55 to 0.64).
In cool climates where sugar has difficulty accumulating to the desired level, this is a major harvest indicator.

Acidity measurements
Used globally but are particularly important in temperate climates that can experience a loss in acidity through malic acid conversion. These measurements are combined with the sugar content to figure out the sugar/acid ratio.

pH:
In warm regions where sugar accumulation is not an issue, maintaining acidity can become the priority and the catalyst for harvest.
pH values of greater than 3.3 for white wines and greater than 3.5 for red wines is ideal.
pH without adjustments can also dictate microbial composition in vinification.
Titratable Acidity
The total quantity of tartaric and malic acid measured in grams per liter (g/L).

Phenolic ripeness:
The development of phenols and other aroma compounds which dictate the aroma characteristics of the wine.

The point of “ripeness” means different things to different people, and is difficult to measure. Sometimes it is based on the grape’s color. Its correlation however, varies by varietal, region, and year. Additionally, what is desirable for one wine style will may be different for another.
Phenolic expression is also dependent on the acidity of the grape”.

Using Ripeness to Determine Wine Aroma
In the development of our data points, to organize wine into similar constructs, it seems like viticultural practice from a process perspective, in conjunction with the current methods of using sugar/acid balance, may be the best way to compare and understand what the final wine may taste like.

Resources and Suggested Reading

Appellation Cornell Topical Index: Grapes 101
Grapes 101 is a series of brief articles highlighting the fundamentals of cool climate grape and wine production. https://grapesandwine.cals.cornell.edu/newsletters/appellation-cornell/appellation-cornell-topical-index/

For additional insight into the nuances of grape development read:
KELLER, M. (2010), Managing grapevines to optimise fruit development in a challenging environment: a climate change primer for viticulturists. Australian Journal of Grape and Wine Research, 16: 56-69. https://doi.org/10.1111/j.1755-0238.2009.00077.x
*Professor Keller is the Chateau Ste. Michelle Distinguished Professor in Viticulture

Sources
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2. Haggerty, Luke LeMay. (2013). Ripening profile of grape berry acids and sugars in University of Minnesota wine grape cultivars, select vitis vinifera, and other hybrid cultivars. Retrieved from the University of Minnesota Digital Conservancy, https://hdl.handle.net/11299/160115.

3. Lakso, A. N., & Kliewer, W. M. (1975). The Influence of Temperature on Malic Acid Metabolism in Grape Berries: I. Enzyme Responses. Plant Physiology, 56(3), 370–372. http://www.jstor.org/stable/4264164

4. M.A Amerine and A.J. Winkler in their 1944 study the Composition and Quality of Musts and Wines of California Grapes (http://hilgardia.ucanr.edu/Abstract/?a=hilg.v15n06p493)

5. Jones, Gregory. “Climate, Grapes, and Wine Terroir and the Importance of Climate to Wine Grape Production.” GuildSomm, Aug. 2015
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6. Pool, R., 2002.

7. Ivanišević, D., Kalajdžić, M., Drenjančević, M., Puškaš, V., & Korać, N. . (2020). The impact of cluster thinning and leaf removal timing on the grape quality and concentration of monomeric anthocyanins in Cabernet-Sauvignon and Probus (Vitis vinifera L.) wines. OENO One, 54(1), 63–74. https://doi.org/10.20870/oeno-one.2020.54.1.2505

8. Flavour Development in the Vineyard: Impact of Viticultural Practices on Grape Monoterpenes and their Relationship to Wine Sensory Response, A.G. Reynolds and D.A. Wardle. https://pdfs.semanticscholar.org/6f03/b8fad901d74612a34684bbf66f9e69c68468.pdf

9. The Effect of Various Leaf Removal Treatments on the Aroma and Flavor of Sauvignon Blanc Wine, R. A. Arnold, A. M. Bledsoe Am J Enol Vitic. January 1990 41:74-76; published ahead of print January 01, 1990 https://www.ajevonline.org/content/41/1/74

10. Lacey, Michael. (1993). Methoxypyrazine grape flavour: Influence of climate, cultivar and viticulture. Die Wien-Wissenschaft. 48. 211-213 www.researchgate.net/publication/235432613_Methoxypyrazine_grape_flavour_Influence_of_climate_cultivar_and_viticulture

11. Gambetta, J. M., Holzapfel, B. P., Stoll, M., & Friedel, M. (2021). Sunburn in Grapes: A Review. Frontiers in plant science, 11, 604691. https://doi.org/10.3389/fpls.2020.604691

12. Wikimedia Foundation. (2021, July 13). Lapse rate. Wikipedia. Retrieved September 24, 2021, from https://en.wikipedia.org/wiki/Lapse_rate.

13. Roby, Gaspar & Harbertson, James & Adams, Douglas & Matthews, Mark. (2008). Berry size and vine water deficits as factors in winegrape composition: Anthocyanins and tannins. Australian Journal of Grape and Wine Research. 10. 100 - 107. 10.1111/j.1755-0238.2004.tb00012.x. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.716.4384&rep=rep1&type=pdf

14. University of California Davis. Irrigation Management of Winegrapes with a Limited Water Supply http://ucmanagedrought.ucdavis.edu/Agriculture/Crop_Irrigation_Strategies/Winegrapes/

15. MCCARTHY, M. (1997), The effect of transient water deficit on berry development of cv. Shiraz (Vitis vinifera L.). Australian Journal of Grape and Wine Research, 3: 2-8. https://doi.org/10.1111/j.1755-0238.1997.tb00128.x

16. Dry, Peter & Loveys, B. & Mccarthy, Michael & Stoll, Manfred. (2001). Strategic management in Australian vineyards. Journal International des Sciences de la Vigne et du Vin. 35. 129-139. 10.20870/oeno-one.2001.35.3.1699

19. Jovanovic, Z. and R. Stikić. “Partial Root-Zone Drying Technique: from Water Saving to the Improvement of a Fruit Quality.” Front. Sustain. Food Syst. (2018). https://doi.org/10.3389/fsufs.2017.00003

20. Moflat, R. & Asselin, Christian & Pages, P. & Robichet, Jacqueline & Leon, Huguette & Remoué, Michel & Salette, Jean & Monique, Caille. (1983). Caractérisation intégrée de quelques terroirs du Val de Loire. Influence sur la qualité des vins. OENO One. 17. 219. 10.20870/oeno-one.1983.17.4.1767.

21. Lanyon, Dean & Cass, A & Hansen, D. (2004). The effect of soil properties on vine performance. www.researchgate.net/publication/228433458_The_effect_of_soil_properties_on_vine_performance/

22. USDA Natural Resources Conservation Service. “Soil Quality Indicators.” www.nrcs.usda.gov, June 2008, www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_053288.pdf.

23. USDA Natural Resources Conservation Service. “Soil Quality Resource Concerns: Available Water Capacity.” Www.nrcs.usda.gov, Jan. 1998, www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_051279.pdf.

24. Düring, H. (1984). Evidence for osmotic adjustment to drought in grapevines ( Vitis vinifera L.). Vitis: Journal of Grapevine Research, 23, 1-10. https://doi.org/10.5073/vitis.1984.23.1-10

25. Verbrugghe, M., et al. “Influence De Différents Types De Sol De La BASSE VALLÉE Du Rhône Sur LES Températures De Surface DE Raisins Et De FEUILLES De Vitis Vinifera: Semantic Scholar.” Semantic Scholar, 1 Jan. 1990, www.semanticscholar.org/paper/Influence-de-diff%C3%A9rents-types-de-sol-de-la-basse-du-Verbrugghe-Guyot/61f626e0b8224f802fe706dbf7fce238a1bf9c8f.

26. United States Department of Agriculture. “Soil Survey of Howard County, Arkansas.” Nrcs.usda.gov, Oct. 1975, www.nrcs.usda.gov/Internet/FSE_MANUSCRIPTS/arkansas/howardAR1975/howard.pdf.

27. Hagan, R. (n.d.). XIVth International Horticultural Congress. In Factors Affecting Soil Moisture - Plant Growth Relations. Retrieved September 23, 2021, from https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.499.8192&rep=rep1&type=pdf.

28. Soyer, Jean-Pierre & Delas, J. & Molot, C. & Andral, P.. (1984). Techniques d'entretien du sol en vignoble bordelais. Consequences sur la vigne (Production, vigueur, enracinement, nutrition) et sur le sol apres 20 ans d'experimentation..

29. Pagliai, M & Lamarca, M & Lucamante, G & Genovese, L. (1984). Effects of zero and conventional tillage on the length and irregularity of elongated pores in a clay loam soil under viticulture. Soil & Tillage Research - Soil Till Res. 4. 433-444. 10.1016/0167-1987(84)90051-5. https://www.sciencedirect.com/science/article/pii/0167198784900515?via%3Dihub

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33. Choné, X. & van Leeuwen, Cornelis & Chéry, Philippe & Ribéreau-Gayon, P.. (2001). Terroir influence on water status and nitrogen status of non irrigated Cabernet Sauvignon (Vitis vinifera): Vegetative development, must and wine composition. S. Afr. J. Enol. Vitic. 22. 8-15.

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41 Esperanza Valdés, M., Moreno, D., Gamero, E., Uriarte, D., del Henar Prieto, M., Manzano, R., Picón, J., & Intrigliolo, D. S. (2009). Effects of cluster thinning and irrigation amount on water relations, growth, yield and fruit and wine composition of Tempranillo grapes in Extemadura (Spain). OENO One, 43(2), 67–76. https://doi.org/10.20870/oeno-one.2009.43.2.799

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A Guide to Viticulture

Sunlight

Temperature

Microclimate Modification: Vine Training and Pruning

Training System [13]

Training/Pruning Systems

Yield Control: Fruit-cluster and flower-cluster thinning

Rainfall and Irrigation

Macroclimate Factors

Mesoclimate Factors

Soil

Macroclimate: Soil Structure

Rootstocks and Cover Crops

Mesoclimate: Soil Chemistry

Ripeness/Harvest

Resources and Suggested Reading

MENU

SUBSCRIBE

DIGITAL
EDITION

BEVERAGE
GUIDE

NEws and
Events

ABOUT

CONTACT

A Guide to Viticulture

Sunlight

Temperature

Microclimate Modification: Vine Training and Pruning

Training System [13]

Training/Pruning Systems

Yield Control: Fruit-cluster and flower-cluster thinning

Rainfall and Irrigation

Macroclimate Factors

Mesoclimate Factors

Soil

Macroclimate: Soil Structure

Rootstocks and Cover Crops

Mesoclimate: Soil Chemistry

Ripeness/Harvest

Resources and Suggested Reading

MENU

SUBSCRIBE

DIGITAL ​EDITION

BEVERAGE​GUIDE

NEws and ​Events

ABOUT

CONTACT

DIGITAL
EDITION

BEVERAGE
GUIDE

NEws and
Events