By Michela Centinari
Potassium (K) plays a critical role in many plant physiology and biochemistry processes (e.g., photosynthesis, osmoregulation, enzyme activation, etc.). Inadequate supply of K can result in reduced shoot, root and fruit growth as a result of reduced xylem sap flow, and can also increase the risk of drought stress . Potassium deficiency leads to inhibition of photosynthesis and to sugar (sucrose) being “trapped” in the leaves which adversely affects yield, fruit ripening and berry soluble solid concentration .
While grape growers should monitor vine health to avoid K deficiency, at the same time they should also be on the lookout for excessive concentration of K in vine tissues (e.g., leaf, berry) because of its potential negative impact on vine health and wine quality.
When looking back at the past two years of inquires received from Pennsylvania wine grape growers related to vine nutrient or nutrient-related problems, we (Denise and I) found that the number of those concerning excessive concentration of K and related issues (e.g., high/unstable wine pH) were more common than inquiries related to K deficiency, which mostly occurred in young vineyards.
This short article will review problems related to high/luxury absorption of potassium, briefly discuss how soil mineralogy and pH can affect K uptake, why it is important to regularly monitor vine nutrient status, and what environmental and cultural factors may impact K uptake and accumulation in plant tissues. In-depth information on mode of uptake and transport of K in the plant, and functions of K can be found in “A review of potassium nutrition in grapevines with special emphasis on berry accumulation” by Mplelasoka et al. 2003 .
Also, you can refer (as I did) to the valuable web-resources recently published by Virginia Tech University (Dr. Tony Wolf and Russel Moss), which includes edited information to supplement Chapter 8 of The Wine Grape Production Guide of Eastern North America:
Potassium in Viticulture and Enology, May 2016
Why high/excessive concentration of K in the grape berries may negatively impact wine quality?
Grape berries are a strong sink for K during ripening. Potassium accumulates mainly in the berry skin tissues (Figure 1) and is the most abundant cation (K+, hereafter referred to as K) in grape juice . Mature grapes may have, indeed, almost twice as much K as nitrogen ; for example one ton of mature grapes contains about 11 lbs (5 kg) of K .High concentration of K in grape juice (e.g., > 50 mmol/L) may result in a high juice pH (e.g., > 3.8) and negatively impact wine quality . During winemaking, high concentration of K causes precipitation of free acids, mainly tartaric acid, leading to an increased wine pH . The high pH may reduce color stability of red wines due to a shift of anthocyanins to the non-colored forms . High concentration of K may also reduce respiration and the rate of degradation of malic acid and consequently increase malolactic fermentation .
While many studies linked high juice/wine pH to high juice/wine K concentrations (see for example Figure 2) it is also true the other factors, aside from K, can affect wine pH and that wines with low pH (e.g., < 3.25) may also have high K concentration . Moreover, there are varietal differences in the relationship between juice pH and K concentration (see Chardonnay vs. Shiraz, Figure 2), as well as between wine K and juice K .pH can be adjusted during winemaking through addition of tartaric acid, adding additional costs for the wineries. Ensuring adequate concentration of K in the grapes at harvest will not only help reduce winemaking costs but is also likely to improve wine quality .
In next week’s blog post, Denise Gardner will discuss options for dealing with high K concentrations in juice and wine. However, the first step is to determine if there is or there may be a potential problem with K in your vineyard (either deficiency or excess level).
Potassium availability in the soil
It is well known that concentration and availability of K vary with soil type and is greatly affected by the physical and chemical properties of the soil . Potassium in soil is classified into four groups in relation to its availability to the plants: 1) water-soluble (K dissolved in soil solution), 2) exchangeable (on cation exchange sites of surfaces of clay minerals and humic substances), 3) non-exchangeable and 4) structural forms .
The water-soluble and exchangeable pools (1 & 2) represent only » 0.1-0.2% and 1-2% of the total soil K, respectively . Both forms are readily available for plant root absorption. The majority of the K is bound in mineral structures, such as mica and feldspar, or it is part of secondary minerals such as vermiculite  and thus considered a (very) slowly-available source of K for plants. Clays also have different capabilities of binding K as well as different rates of K release . For example, micas release K at a remarkably faster rate than feldspar .
Since clay mineralogy impacts the release of K into the soil solution over time and the K supplying power of the soil, it is important to have detailed information concerning the soil structure and composition of your vineyard (i.e., does your site have high levels of exchangeable K?).
Other important factors affecting K availability are soil pH and relative concentration of K to that of the other cations, such as magnesium (Mg2+) and calcium (Ca2+). Low/acidic soil pH (<6) increase K availability and potentially increase its uptake while reducing uptake of Ca2+ and Mg2+ . Excess K can decrease concentrations of Ca2+ and Mg2+ in plant tissues and induce symptoms of Mg deficiency  (Figure 3).
Assessing K concentration in the vine
Conducting plant tissue (petiole) testing on a regular basis (annual or every two years) to monitor vine nutritional health (K and other essential nutrients) and promptly correcting problems related to nutrient imbalance is strongly recommended. Visual observations of foliar symptoms of nutrient deficiency or toxicity are important clues (Figure 4), but a nutrient management program should not be exclusively based on visual observations because: 1) it is possible to be misled by symptoms that are not nutrient related (e.g. mite injury, virus, etc.) and 2) to develop an appropriate nutrient management program it is crucial to understand the nutritional requirements of the vines .
Soil testing is an extremely useful tool in the pre-planting stage for determining the potential of a vineyard site and the amendments needed, and also for monitoring soil pH over the years (after the vines are planted). However, soil testing only tells one side of the story: what is potentially available to the vine. Again, the recommended and preferred method to assess vine nutritional health and to effectively identify potential nutrient (in this specific case K) deficiency or excess is plant tissues (petiole) testing.
Some limitations of the soil testing include:
- Soil samples are often limited to the first 10-20 inches. Roots of mature vines tend to be sparse and, in deep soils, they can grow much deeper than 10-20 inches. Thus soil testing may not be a good indicator of the soil/plant interaction.
- Soil testing may underestimate the reservoir of K available to the vines . The laboratory nutrient extraction analysis is run over minutes while vines have much longer to absorb/extract nutrients [6,9].
Thus, don’t be too surprised if results of K soil testing are poorly correlated with those of plant tissue testing [6,7,9].
When is the best time to conduct leaf petiole testing?
The preferred time for leaf petiole testing is bloom and late-summer (70 to 100 days after bloom). Assuming bloom was in June, you may still have time left this season to conduct the test. In case of suspected K or other nutrient deficiency the samples can be collected anytime during the season . It is important to have a standardized tissue-sampling procedure. For example, at bloom collect 60 to 100 petioles of healthy leaves opposite to the flower cluster (first or second) for each cultivar. Don’t collect more than one or two petioles per vine. Late summer samples should be collected from “the youngest fully expanded leaves of well-exposed shoots, usually located from 5 to 7 leaves back from the shoot tip”. If the shoot has been hedged, collect “primary leaves near the point of hedging” .
More information on how to collect leaf petiole samples and interpret the results can be found at Monitoring Grapevine Nutrition (eXtension.org) and in the Wine Grape Production Guide for Eastern North America, chapter 8 .
Reference values of K concentration in grape leaf petioles for bloom- and late summer-collected samples are reported in Table 1. (Note: the source used is the Wine Grape Production Guide for Eastern North America , sources from other regions may provide slightly different standards).
Table 1. Reference values for K concentration in grape leaf petioles for bloom- and late summer-collected samples.
In table 2, I included the results of a petiole test conducted at a research vineyard in south central Pennsylvania. At bloom, K concentrations in the leaf petiole of Cabernet Sauvignon and Merlot vines were slightly above the ‘excessive’ range. We repeated the analysis around veraison and found that at that time K concentrations were greatly above the 2% ‘excessive’ threshold. Not too surprisingly, at harvest, both varieties had high grape juice pH and high K concentration according to reference values reported by Mpelasoka et al. 2003  (Table2). Soil testing showed a high level of exchangeable K, and low pH (below 6), the vines were highly vigorous: all factors that can contribute to luxury uptake of K.
Table 2. Potassium concentrations in the petiole of Cabernet Sauvignon and Merlot leaves at bloom and veraison. Potassium concentration and pH of the grape juice was measured at harvest.
What to do next?
In case of K deficiency (petiole and soil testing and visual observations) (Figure 4) you can consult with an extension educator in your county or a viticulture consultant who can assist with the development of an appropriate fertilization program.
For those of you who may have missed it, Tony Wolf (professor and viticulture extension specialist at Virginia Tech university) recently issued an important update on K fertilization recommendations . The lower limit of optimal soil K reported in the “Wine Grape Production Guide for Eastern North America”  has changed from 75 ppm (150 lbs/ac) to 40 ppm (80 lb/acre) (Note: those values are based on Mehilch-3 extraction protocol which is the one used by Penn State Agricultural Analytical Services Lab but not by Virginia Tech). In the July issue of Viticulture Notes Tony Wolf wrote:
“Potassium fertilizer is not recommended pre-plant or to existing Virginia vineyards if the soil test results are at or above 40 ppm (80 lbs/acre) actual K as determined by Mehlich-3 test procedures, or 28 ppm (56 lbs/acre) actual K as determined by Mehlich-1 test procedures. However, young vines should be visually monitored and irrigated under drought conditions to avoid potential K deficiency on soils that are inherently low in exchangeable K.”
How can K concentration and uptake by the vines be reduced?
In the pre planting stage, if the soil selected for planting the vineyard has high exchangeable K levels, an option is to select rootstocks that accumulate low concentration of K. Rootstocks, and grapevine varieties in general, differ in their capacity of K uptake and translocation . For example, rootstocks with V. berlandieri genetic background tend to have reduced K uptake as compared to others, as those with V.champini parentage . In northern California, Chardonnay, Cabernet Sauvignon and Zinfandel vines grafted on 101-14 Mgt and 3309C (V. riparia x V. rupestris), two commonly-used rootstocks in the eastern US, consistently had leaf petiole K concentrations within the intermediate range compared to those of the same varieties grafted on V. berlandieri (lowest K concentrations) and V. champinii (highest K concentrations) crosses . A study conducted at Winchester, VA, by Tony Wolf research group found that the use of 420-A (V. berlandieri x V. riparia) rootstock reduced juice pH in Cab Sauvignon vines as compared to those grafted on 101-14 Mgt and Riparia .
However, it is important to consider that the performance of rootstocks in terms of K uptake varies depending on rootstock-scion combination (i.e., the same rootstock may have variable effects on different scion varieties), soil type, climate, and management practices.
Another aspect to consider when selecting rootstocks is their vigor or growth-potential. Vigorous rootstocks or rootstocks that convey high vegetative growth and yield potential to the scion may cause increased K uptake as a result of increasing vine demand.
Growth drives K uptake: Factors such as high vine vigor, leaf area, and extensive root system can enhance K uptake, translocation, and accumulation in the grape berries. Soil moisture increases the dissolution of K from clay particles, thus facilitates K supply and uptake by the roots. High soil water availability also leads to increase vegetative growth which may indirectly affect K uptake and its accumulation in the berries.
Shaded leaves are a source of K translocation to the grape berries: Canopy microclimate and mainly foliage shading can affect the accumulation of K in the berries. For example, artificial (shading cloths)  or natural (canopy) shading  was found to increase K concentration in berries and juice. We don’t know exactly why this happens yet, but it is possible that in conditions of low sugar accumulation, as under foliage shading, the increasing accumulation of K in the berries helps regulating osmotic potential, maintaining cell turgor and thus minimizing reduction in berry growth which may occur with low sugar content .
Can crop load be regulated to reduce K accumulation in the fruit?
Since berries after veraison are the primary sink for K, we would expect that regulation of crop load (commonly defined as the ratio of fruit weight to pruning weight or to leaf area) can affect K translocation and accumulation in the berries . However, results from previous studies are inconclusive. For example, in hot climate regions (Israel, California) cluster thinning reduced berry K in one study  while having no effect on juice K in another study . Factors such as grape variety, timing of thinning, and the amount of crop retained can greatly affect outcomes and explain why the effect of crop load on accumulation of K in the berries still remains unclear. Making matters more complicated, manipulation of crop load may also affect vegetative growth, and the degree of foliage shading, thus indirectly impacting K translocation into the berries.
Generally speaking, over cropping may result in a lower or insufficient amount of K in the vine tissues (K deficiency tends to be more pronounced on heavily cropped vines after veraison). At the same time if yield is very (too) low the shoots may become competitive sinks for K and as a result its accumulation in the berries may be reduced .
Vineyard management practices that decrease leaf shading may reduce K accumulation in the berries. Reducing canopy density and shading, either through a) the removal of lateral shoots, b) lateral and top-hedging, or c) basal leaf removal reduced K concentration and, in some cases, pH in juice and wine of Tannat vines in Uruguay (Figure 5) . The use of divided-trellis systems could also be a method to manage highly vigorous vines and decrease leaf shading .
In conclusion, if you suspect a problem with high vine K and/or high juice/wine pH, here a few things you can do:
- Plant tissue (petiole) analysis to assess the K level of your vines and to confirm that high/excessive K is the real issue. Again, don’t rely exclusively on soil testing, which is still useful to assess soil pH and other factors that may affect K uptake.
- If your vines are highly vegetative you could test the effect of reducing foliage shading on juice/wine pH and K. Reducing canopy density may also have additional benefits for the health of the grapes and quality. Canopy practices such as basal leaf removal, hedging, shoot positioning and thinning can be used. It is a good practice to leave a block of untreated vines (no additional canopy management) as a control. At harvest measure juice pH and K concentration in the treated (less shaded) and untreated (more shaded) vines to assess if the extra canopy management mitigate the K/pH level in the grape juice.
If you have tried or panning on trying to manage K levels in your vineyards we would be happy to hear about your experience and methods/results.
- Keller M. 2010. The Science of Grapevines: Anatomy and Physiology. Publisher: Academic Press.
- Mpelasoka BS, Schachtman BP, Treeby MT, Thomas MR. 2003. A review of potassium nutrition in grapevines with special emphasis on berry accumulation. Aust. J. Grape Wine Res. 9, 154–168.
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- Walker RR, Clingeleffer PR, Kerridge GH, Rühl EH, Nicholas PR, Blackmore DH. 1998. Effects of the rootstock Ramsey (Vitis champini) on ion and organic acid composition of grapes and wine, and on wine spectral characteristics. J. Grape Wine Res. 4, 100–110.
- Kodur 2011. Effects of juice pH and potassium on juice and wine quality, and regulation of potassium in grapevines through rootstocks (Vitis): a short review. Vitis 50, 1–
- Wolf TK. Viticulture Notes. Vol 31 No. 5. 23 July 2016. Virginia Tech University Cooperative Extension. Available at: http://www.arec.vaes.vt.edu/alson-h-smith/grapes/viticulture/extension/news/vit-notes-2016/vit_notes_july_2016.pdf
- Walker RR, Blackmore DH. 2012. Potassium concentration and pH inter-relationships in grape juice and wine of Chardonnay and Shiraz from a range of rootstocks in different environments. J. Grape Wine Res. 18, 183–193.
- Zörb C, Senbayram M, Peiter E. 2014. Potassium in agriculture – Status and perspectives. J. Plant Physiol. 171, 656–669.
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- Moss R. 2016. Potassium in viticulture and enology. Virginia Tech University Cooperative Extension. Available at: http://www.arec.vaes.vt.edu/alson-h-smith/grapes/viticulture/extension/news/vit-notes-2016/kinvitandeno.pdf
- Wolf TK. 2008. Wine grape production guide for Eastern North America. Natural Resource, Agriculture, and Engineering Service: Ithaca, NY USA.
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- Rojas-Lara BA, Morrison JC. 1989. Differential effects of shading fruit or foliage on the development and composition of grape berries. Vitis 28, 199–208.
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- Hepner Y, Bravdo B. 1985. Effect of crop level and drip irrigation scheduling on the potassium status of Cabernet Sauvignon and Carignane vines and its must and wine composition and quality. J.Enol. Vitic. 36, 140–147.
- Freeman BM, Kliewer WM. 1983. Effect of irrigation, crop level and potassium fertilization on Carignane vines II. Grape and wine quality. J.Enol. Vitic. 34, 197–207.
- Coniberti A, Ferrari V, Fariña L, Disegna E. 2012. Role of canopy management in controlling high pH in Tannat grapes and wines. Am. J.Enol. Vitic. 63:554-558.
By: Lee Stivers, Penn State Extension Educator – Washington County
June 2015 is certainly shaping up to be one of the rainiest on record for many parts of Pennsylvania. During growing seasons like this, when it seems that every day brings another shower or thunderstorm, most Pennsylvania crop producers start to think about applying supplemental nitrogen to their field, forage and vegetable crops. But what about grapes—should wine grape growers be concerned about nitrogen losses in a rainy year?
Nitrogen is a critically important nutrient for any plant to grow and thrive. However, managing nitrogen in the vineyard can be tricky because vine growth and fruit quality are affected when nitrogen is available in excess amounts as well as when it is deficient. Too much nitrogen can stimulate excessive vegetative growth, throwing the vines out of balance. This is especially true under rainy, wet conditions. That extra vine growth may need to be pruned out later in the season in order to get sufficient air and light penetration into the canopy for proper fruit ripening. Yields can also suffer from excessive nitrogen uptake.
We don’t want to provide too much nitrogen to wine grapes, nor do we want to provide too little. Insufficient nitrogen can reduce crop yield through a reduction in clusters, berries, or berry set. Nitrogen deficiency in wine grapes is not easily recognized, but a typical symptom is a uniform light green color of leaves, compared to the dark green of healthy grape leaves. Typically, soil organic matter decomposition supplies much of the nitrogen needed by grape vines. Nitrogen depletion will occur most rapidly in soils with low organic matter. If soils are not supplying enough nitrogen, then a supplemental fertilizer application may be needed. Soils with adequate organic matter will not necessarily lose much nitrogen during a rainy growing season since organic matter is not soluble.
How do you know if your vines need supplemental nitrogen fertilizer? There is no simple answer to this question, but Tony Wolf provides a very useful set of observations and measurements for assessing the nitrogen status of a vineyard in the “Wine Grape Production Guide for Eastern North America” (page 159-160). By making observations over time of canopy fill, leaf size and color, shoot growth rates and fruit quality, and also measuring yields, cane pruning weights, and bloom-time petiole nitrogen concentrations, you can determine if your vines have a nitrogen status of deficient, adequate, or excessive. In addition to these factors, it is also important to test your soil periodically to monitor soil organic matter levels, as well as other plant nutrients.
How to collect and submit a petiole sample for plant tissue analysis. It’s easy to collect and submit a petiole (leaf stem) sample to Penn State’s Agricultural Analytical Services Laboratory, and you don’t even have to visit an Extension office to purchase a kit. Full instructions can be found here: Penn State Plant Tissue Analysis Submission Instructions and you can download the proper form here: Penn State Plant Tissue Analysis Submission Form.
For routine analyses, petiole samples must be taken at bloom, or at veraison, at least 70 days after full bloom, as these are the only times of year when nutrient levels in the plant are relatively stable. If samples taken at other times of the year are submitted, faulty interpretations could result due to incorrect sampling technique.
To collect the sample, remove 50 to 75 first fully-expanded leaves. Use the higher leaf number for varieties which have small petioles. Samples should be taken from fruiting shoots located halfway between the ground and highest trellis wire. After removing leaves from vines, separate petioles from leaf blades. Send only petioles (leaf stems) for analysis. Loosely wrap the petioles in a dry paper towel. Fill out the form completely, and attach a check, payable to Penn State University, and mail to Penn State Agricultural Analytical Services Laboratory, Tower Road, University Park, PA 16802.
How to take a soil sample and submit it for nutrient analysis and recommendations. Penn State’s Analytical Laboratory also offers soil testing services for commercial growers. This important soil management tool aids in monitoring soil fertility and determining optimum lime and fertilizer requirements for all types of crops, including wine grapes. Full instructions can be found here: Penn State Soil Analysis Instructions and you can download the proper form here: Penn State Soil Analysis Submission Form.
To collect the sample, you will need a trowel, shovel or auger, and a clean bucket. Obtain thin slices or borings from 13-15 randomly selected places in the vineyard. Sample to approximately 12 inches in depth. Collect the samples in the bucket, and mix thoroughly into one composite sample. Spread the soil on newspaper in a warm room to air dry overnight. Do not heat. Take 1 cup of the sample and place it in a sturdy bag or plastic container.
Fill out the form completely and attach a check, payable to Penn State University. Note that there is an additional fee for the test for organic matter. Mail the sample and the form to Penn State Agricultural Analytical Services Laboratory, Tower Road, University Park, PA 16802.