Inadequate supplies 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 an 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 past inquiries received from Pennsylvania wine grape growers related to vine nutrient or nutrient-related problems, we 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. In-depth information on the 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 .
Why might a high/excessive concentration of K in the grape berries 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 .
Figure 1. Potassium concentrations in different berry tissues. Adapted from Mpelasoka et al. 2003 . Data from Walker et al. 1998 .
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, a high concentration of K causes precipitation of free acids, mainly tartaric acid, leading to an increased wine pH . The high pH may reduce the 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 .
Figure 2. The relation between grape juice pH and K concentration for Chardonnay and Shiraz vines at 4 Australian vineyards. Within each site, a data point represents a different rootstock. Data adapted from Walkers et al. 2012 .
The pH can be adjusted during winemaking through the addition of tartaric acid, adding additional costs for the wineries. Ensuring an adequate concentration of K in the grapes at harvest will not only help reduce winemaking costs but is also likely to improve wine quality . 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 and 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).
Figure 3. Leaf symptoms of magnesium deficiency. Photo credit: Andrew Harner (graduate research assistant, Penn State University).
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:
- it is possible to be misled by symptoms that are not nutrient related (e.g., mite injury, virus, etc.) and
- 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 the 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). 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 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.
|Stage||Deficient %||Marginal %||Adequate %||Excessive %|
|Bloom||< 1.00||1.00–1.50||1.50–2.50||> 2.50|
|Veraison||< 0.80||< 1.20||1.20–2.00||> 2.00|
In table 2, are 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  (Table 2). 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.
|Vine||Petiole K |
|Petiole K |
|Grape Juice K |
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.
Figure 4. Symptoms of potassium deficiency on Cabernet franc leaves.
Tony Wolf (professor and viticulture extension specialist at Virginia Tech University) 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 2016 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 the 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 to regulate 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, the timing of thinning, and the amount of crop retained can greatly affect outcomes and explain why the effect of crop load on the 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 the 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 .
Figure 5. Effect of canopy management treatments on K concentration and pH of Tannat wines over three years. Asterisks (*) indicate a significant difference with respect to the control (p ≤ 0.05) based on Tukey’s test.
In conclusion, if you suspect a problem with high vine K and 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 mitigates the K/pH level in the grape juice.
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