Besides that which is essential for photosynthesis, namely carbon dioxide, water, and sunlight, grapevines also require a range of nutrients in order to grow, survive and prosper. These nutrients are split into two groups depending on scale of requirement, macro and micronutrient, the former being those required in larger amounts. Potassium is the second most abundant mineral nutrient in plants and has a number of roles. It is associated with the movement of water, nutrients and carbohydrates whilst also helping to regulate stomata and supporting enzyme activation. A deficiency can reduce yields, fruit quality and increase susceptibility to disease. Too much can cause a finished wine to lose acidity. In this article I will explore potassium in viticulture from soil to bottle.
Potassium in the soil
Potassium (K) is the eighth most abundant element on earth and comprises about 2.1% of the earth’s crust. Elemental potassium does not occur in nature because of its high reactivity. In the periodic table, potassium is one of the alkali metals, all of which have a single valence electron in the outer electron shell. This single valence electron, through the process of chemical bonding, can be easily removed. Potassium loses (technically it shares) one electron when it reacts with chlorine (this electron is transferred to a chlorine atom to form a chloride ion)
As potassium loses this single valence electron (the number lost is the same as the number of positive charges) it becomes a positively charged ion, positively charged ions are called cations. This positively charged electrolyte (cation) potassium ion is known as K+ (which I will for the sake of this article refer to hereon as simply potassium) and is the potassium compound found in soils and that which is essential to plant life. This differentiation is important, more specifically because the positively charged nature of K+ (the potassium ion) is crucial to plants, this should be understood before proceeding.
Felspars and micas are the principle constituents of granites, schists, greywackes, sandstones and clay minerals. It is these feldspars (particularly orthoclase) and micas which serve as the principal sources of potassium. Montmorillinate, vermiculite, kaolinite and illite are also important clay minerals. Potassium available for use by plants is water-soluble and referred to as exchangeable. This pool of available potassium in soil develops overtime (extremely slowly) as a result of continuous weathering (including freezing, thawing, rainfall & biology) of parent materials such as felspars. Although mineral soils generally have large overall levels of potassium, the percentage which is immediately available to plants is relatively low, usually around 0.1- 0.2%.
Potassium exists in soil in a number of different pools, each more or less available to plants. Potassium moves from one pool to the next whenever there are removals or additions which change the balance within each pool. The ability of the soil to supply potassium is dependant upon the transformations between the various potassium pools. These pools can be categorised as:
|Available K||Tends to be the smallest K pool in the soil and contains water soluble K for plant uptake|
|Readily available K||pool replenishes the available K pool many times during the growing season as K is released from the surfaces of the clay particles|
|Less readily available K||Interchangeable with the readily available K pool|
|Very slowly available K||Made available over time through weathering (clay) and organic matter decomposition processes and this pool of K tends to be very stable.|
Once plants have taken up potassium it is leached from leaves to soil by rainfall, a further quantity of potassium is also returned to the soil via animal urine. The availability and cyclical nature of the potassium pool is better understood within the context of the potassium cycle. This cycle sees potassium move between pools within the cycle over varying periods of time.
Availability of potassium is not static across sites, studies have shown that there are a range of factors which influence the availability of potassium to plants. These factors promote or inhibit the ability of potassium to move effectively between these pools. Namely these factors are:
Soil type and CEC – Cation exchange capacity (CEC) is the total capacity of a soil to hold exchangeable cations like potassium. The higher the CEC, the more cations which can be supplied to the vine. This is called the soil’s buffer capacity, the term relates to the buffer supply held by the soil. CEC is an inherent soil characteristic and is extremely difficult, if not almost impossible, to alter significantly. Soils with a low CEC are more likely to develop deficiencies in potassium (K+).
Sandy soils tend to have a low CEC, as a result wines grown in sandy soils can be more prone to potassium deficiency than those not. Organic (peaty) soils also have a much lower capacity to retain potassium. In both cases, sandy and organic, the low CEC is due to a small amount of clay minerals in the soil. The structure of these clay minerals hold on to potassium in a crystal lattice structure. Different clay minerals hold on to potassium differently, some at the edges and some between layers, this affects the availability of readily available potassium. For example, soils dominated by illite or vermiculite will have good soil potassium reserves compared to soils dominated by kaolinite this is due to the clay mineral structure.
As an aside, both nitrogen and phosphorus are constituents of the soil organic matter, but potassium is not. Soil organisms have a much lower requirement for potassium than plants do. Consequently, as organic residues decompose, most of the potassium is quickly released. The behaviour of potassium in the soil is determined more by physical properties than by chemical or biological processes.
Soil pH – The cation exchange capacity of a soil depends on the pH of the soil. At low pH (acidic soil) the CEC decreases. This is because at low pH hydrogen ions displace other exchangeable ions from the soil (more competition for CEC sites). Once these hydrogen ions have displaced other exchangeable ions they are more easily leached from the soil, making them unavailable. At high pH (basic soils) the CEC increases. At high pH the OH- ions remove the hydrogen ions (which previously displaced other exchangeable ions) from the clay, creating a negative charge and less competition for CEC sites.
Soil moisture – Moisture is required to liberate fixed potassium and provide the soil solution necessary for plant intake. Higher soil moisture usually means greater potassium availability. Increasing soil moisture increases K’s movement to plant roots and enhances availability through diffusion. Studies have shown that increasing water content in control experiments from the 0.1 to 0.4 resulted in a rise of the effective diffusion coefficient by a factor of about 10. However, too much moisture restricts root growth and oxygen availability and slows potassium absorption by the roots.
Soil temperature – Where soil temperature is particularly low both potassium uptake and plant growth has been shown to be limited. Plants are shown to uptake potassium and other nutrients with greater efficiency when the soil temperature is between 16 °C and 27 °C.
Wet/dry cycles – Exchangeable potassium can increase or decrease when soil is dried dependent upon the type of clay minerals present. Some minerals when they dry (dependent upon their structure) will collapse around around the cations which are contained within their layers causing the clay mineral to hang on to the K+. Other mineral clays (again dependent upon structure) will open up when dry and release potassium. This is why soil sampling for potassium should be done at the same time of year or in similar conditions this allows for control of the variable.
Freezing – The impact of freezing cycles are in essence linked to soil type and/or structure. In soils with considerable mica fixed potassium will release much more abundantly due to the layered structure of mica. Soils with smaller amounts of mica and more exchangeable potassium are not so affected by freeze/thaw. An understanding of vineyard soil composition is important in order to understand whether your potassium schedule ought to be influenced by freeze/thaw cycles.
Leaching – Leaching is the loss of water-soluble plant nutrients from the soil, due to rain and irrigation. Soil structure, cover crop planting, type and application rates of fertilisers, and other factors are taken into account to avoid excessive nutrient loss
The reserve, transfer, availability and cycle of potassium in the soil is complex and multi-faceted. Availability of potassium is in the most part influenced by soil structure, which is of course difficult to influence. However, the amount of readily available potassium (which can be influenced in the vineyard) is indeed influenced by seasonal variation in weather, and over time, climate. A salient point to consider when exploring the concept of minerality in wine. A number of the most influential factors affecting K availability are inherent to site (soil type, drainage etc.) and as such difficult to manipulate, this adds weight to the importance of site selection in ensuring optimum growing conditions.
Function in the vine
Having explored potassium in the soil, we ought to now consider its function in the vine. Unlike most other nutrients, potassium is not metabolized to become part of the structural components of vines. It remains in its molecular ionic form, and plant membranes are highly permeable to it. Potassium is primarily phloem mobile, which means that it can be redistributed to different parts of the vine as needed (phloem flows bi-directionally). While a lot of potassium leaves the vineyard in the fruit every year, it also accumulates throughout the growing season and during post-harvest into storage in permanent woody structures.
Vines need potassium to complete a range of essential functions. In photosynthesis, potassium regulates the opening and closing of stomata, therefore regulating CO2 uptake. Potassium ions accumulate in the guard cells around each stomata and manage turgidity, thus regulating the stomata opening.
Potassium also triggers activation of enzymes and is essential for production of ATP. Plants rely on ATP (adenosine triphosphate) as it provides the necessary energy to photosynthesise. Potassium ions allow ATP to maintain an electrical charge. If a plant is deficient in potassium, this slows the rate of ATP production and affects the rate of photosynthesis. Potassium is also known to “activate” at least other 60 enzymes, such as those which synthesise starch and encourage plant growth. Furthermore, potassium works to stabilise pH levels necessary to bolster the efficacy enzymatic reactions at an optimum level between 7 and 8.
Potassium in viticulture also plays a fundamental role in the regulation of water in plants, both uptake and loss are influenced by potassium, thus playing a key role in drought resistance. As previously noted, potassium regulates stomatal activity, this stomatal activity is how a plant exchanges gasses. When potassium is present, the pores open. When it is removed, they close. In times where water levels are low, the plant will release potassium, which closes the stomata so the plant can retain more water. This helps protect the plant from the stress associated with drought.
In addition potassium also plays a part in transportation of sugars, nutrients and amino acids. Sugar transportation and metabolism is vital to healthy fruit and grain development. ATP requires all the energy it can muster to transport sugars produced by photosynthesis throughout the plant. When potassium is present, sugars flow freely. If there is a deficiency, fruit development will suffer as a result of restricted movement.
In addition to potassium, plants require a range of minerals in order to grow, each of these minerals serve varying functions in the vine. Mineral requirement is not static throughout the season, the plants requirement varies dependent upon the growing stage. This varying requirement of potassium, amongst other minerals, can be seen in the chart below.
|Budburst to Fruit Set||24||31||41||33||36|
|Fruit Set to Berry Softening||38||27||30||56||36|
|Berry Softening to Harvest||5||2||9||7||13|
Because the plant requires differing levels of potassium at varying stages of its grow onset of visible signs of potassium deficiency are likely to occur at particular stages. This should be considered in both the testing and supplementation schedule where necessary. I have touched on the key functions of potassium in the vine; however, ongoing research is exploring further, more intricate, functions. For a thorough scientific review of potassium function in the berry this resource is advised; however, for the majority this piece should provide an adequate overview.
Deficiency and excess in viticulture and winemaking
Now that we have established the importance of potassium in both proper function of the vine and generation of healthy yields, we will explore the impact of both deficient and excess potassium. Potassium deficiency usually appears early-to-mid-summer (referencing the above table, when it is most required) Inadequate supplies of potassium 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 being “trapped” in the leaves which adversely affects yield, fruit ripening, and berry soluble solid concentration
Typical symptoms of potassium deficiency in plants include brown scorching and curling of leaf tips as well as chlorosis between leaf veins. Purple spots may also appear on the leaf undersides. Plant growth, root development, and seed and fruit development are usually reduced in potassium-deficient plants. Often, potassium deficiency symptoms first appear on older (lower) leaves because potassium in viticulture is a mobile nutrient, meaning that a plant can allocate potassium (via the phloem) to younger leaves when it is potassium deficient. Deficient plants may also be more prone to frost damage and disease, and their symptoms can often be confused with wind scorch or drought.
Stephen Skelton notes that rootstock can be important in predicting and understanding potassium deficiency. The importance of rootstock is echoed by both anecdotal evidence and a number of of academic studies. Stephen notes in his book, Viticulture, that low-vigour rootstock (Riparia Gloire, 101-14 and 420A) tend to give juice and wine with lower potassium levels. Conversely he notes that high-vigour rootstocks (5BB, 110R, 5C and 125AA) tend to give high levels of potassium. This is likely a result of over-vigorous vines often having more shaded canopies, where canopies are shaded potassium migrates to the leaves and thence to the fruit.
While grape growers should monitor vine health to avoid potassium deficiency, they should also be mindful of excessive concentration of potassium in vine tissues. Excessive concentration can have potential negative impacts on vine health and more specifically wine quality. In the most part, having spoke to growers, the concerns relating to excess potassium in viticulture and winemaking are centred primarily in the winery. Randall Graham tells me has never experienced an issue with too much potassium in the vineyard.
Grape berries are a strong reservoir for potassium during ripening. Potassium accumulates mainly in the berry skin tissues and is the most abundant cation in grape juice. High concentration of potassium in grape juice (> 50 mmol/L) may result in a high juice pH (> 3.8) and could negatively impact wine quality. During winemaking, a high concentration of potassium can cause precipitation of free acids, mainly tartaric acid, leading to increased wine pH. The high pH tends to reduce the colour stability of red wines due to a shift of anthocyanins to the non-colored forms. High concentration of potassium may also reduce respiration and the rate of degradation of malic acid and consequently increase malolactic fermentation.
Management in the vineyard
Where deficiencies or excess exist, they must be managed, in order to effectively manage, vineyard managers must first test for plant nutrient status.
Conducting plant tissue, or petiole, testing on a regular basis to monitor vine nutritional health enables vineyard managers to promptly correct nutrient imbalance issues. Visual observations of foliar symptoms of nutrient deficiency or toxicity are important clues, but a nutrient management program should not be exclusively based on visual observations. This is because it is possible to be misled by symptoms that are not nutrient related and to develop an appropriate and robust nutrient management program it is crucial to understand the nutritional requirements of the vines.
Where the vineyard manager opts for plant tissue or leaf blade testing, the tables below provide a broad framework in which to quantify results in to categories which can broadly be considered deficient, normal, adequate and high, dependent on stage in the growing cycle.
Soil testing is a useful tool (some debate this) in the pre-planting stage for determining the potential of a vineyard site and the amendments needed. It is also helpful in monitoring soil pH over the years after the vines are planted. However, soil testing only tells one side of the story, it may only assess a small percentage of the potassium which is potentially available to the vine. The recommended and preferred method to assess vine nutritional health and to effectively identify potential potassium deficiency or excess is plant tissues testing.
There are further limitations to soil testing, namely these are that soil samples are often limited to the first 10-20 inches of topsoil. Roots of mature vines tend to be sparse and, in deep soils, can grow much deeper than 10-20 inches. Due to this soil testing may not indicate well the of the specific plant and its soil. Furthermore, soil testing often underestimates the reservoir of potassium available to the vines due to the nature of the various potassium pools.
In general, potassium values often fall between 100 and 400 ppm; however, as has already been established, soil potassium testing has not historically been a good criteria for evaluating the actual real world potassium status of grapevines. Thus, one should not be surprised if results of potassium soil testing are poorly correlated with those of plant tissue testing.
Management of potassium in viticulture should consider soil type, drainage and micro and mesoclimate. Where vineyard managers opt for soil tests these tests should of course be carried out with control variables (rain, temperature etc.) kept as static as possible, this will provide a data set which more accurately tells a story of soil potassium over time, thus more reliably indicating the nature of potassium availability in the site itself.
A number of fertiliser options are available, the below table provides rough guidance on application quantity dependent on deficiency severity and chosen fertiliser.
I spoke to Randall Graham about his experience with potassium deficiencies in the vineyard. He told me of the issues he had experienced in Soledad in the Salinas Valley. He and his team experimented with a number of different ways to get potassium into the soil, he applied potassium sulphate via both fertigation and shanking. Various methods of application are available, the chosen method should suit current soil potassium levels and soil type. Randall tells me he has not used granite dust systematically with compost application but did note its efficacy in getting potassium in to the soil.
This being said, establishing a fertiliser framework which achieves optimum potassium concentration in the berry, and consequently in the juice and finished wine, simply does not exist universally due to the impact of site variability on potassium uptake. This can be frustrating for vineyard managers, since management of potassium in the vineyard is the recommend method of influencing potassium in the fruit, juice, and finished wine. However, by understanding the many factors at play vineyard managers can to the best of their ability tailor a management practice suitable to a particular site. Keep in mind that potassium fertilisation impact on vine and berry potassium is influenced by: amount type, timing and frequency of fertiliser, soil characteristics, root distribution, canopy microclimate, irrigation, and rootstock and scion combination.
Considerations in winemaking
Where winemaking is concerned, more often than not it is excessive potassium which will present problems to a winemaker. Excessive potassium levels are associated with high juice pH. During winemaking, a high concentration of potassium causes precipitation of free acids (mainly tartaric acid) leading to an increased wine pH. This high pH may reduce the colour stability of red wines and may also reduce respiration and the rate of degradation of malic acid, consequently increasing malolactic fermentation.
Brad Greatrix of Nyetimber tells me that as a general rule the higher the pH the more quickly the wine will develop and the less effective So2 is. He also notes that high potassium can increase tartaric acid precipitation which further exacerbates the high pH. Importantly, he also notes that when it comes to pH and acidity, there is no absolute high or low, more that it should be considered on a spectrum, where you want to land is dependent upon desired style. He tells me that what he may consider high for Nyetimber may be perfectly desirable for someone making a fruity, soft sparkling destined to be drunk after one year in bottle.
I asked Brad whether high potassium can be adjusted in the winery, he tells me whilst winemakers can acidify they opt not to do this at Nyetimber due to the potential knock-on effects, he believes it is always best to address the problem in the vineyard. 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