MONDAY, AUGUST 26, 2024. BY STAN GRANT, VITICULTURIST.

Poor potassium. So vital and yet, so often misunderstood. Many times, potassium (K) is expected to behave like other macronutrients, that is similar to nitrogen, phosphorus, sulfur, magnesium, and calcium. But nature destined potassium to follow a different path. For vineyard managers, the uniqueness of potassium presents some challenges. In this article, in an effort to effectively manage potassium, we will attempt to deepen our understanding of it in grapevines and in vineyard root zones, as well as fertilization options.

Potassium in Grapevines – What It Does

Most macronutrients, which are mineral nutrients required in relatively large quantities in plants, serve mainly structural functions in grapevines. Nitrogen, phosphorus, sulfur, and magnesium are components of large molecules, like amino acids, nucleic acids and chlorophyll, while calcium is part of cell walls. Potassium, in contrast, remains a free and highly mobile positively charged ion (a cation) that readily moves within vines and accumulates in their cells. These traits contribute to potassium’s essential roles in vine tissues.

Potassium moves throughout grapevines within the sap of vascular tissues, as well as in and out of the fluid (cytoplasm) within cells. As it does, it performs four important functions. First, potassium activates over 50 enzymes, including some involved in carbohydrate metabolism and protein synthesis. Second, it helps regulate the pH within cells to between 7 and 8, which is optimum for physiological processes. Third, with its positive charge, it neutralizes the negative charge of anions (negatively charged ions), like nitrate as it moves from roots to shoots. Fourth, as a dissolved chemical ion (solute), potassium is an osmotic regulator, contributing to energy gradients across cell membranes. In this role, potassium helps to maintain cell turgidity, promotes cell expansion, and facilitates the loading of sugars into vascular tissues (phloem) and their translocation from source organs to sink organs. This it does effectively during fruit maturation, drawing sugars produced in leaves into berry cells.

Given the importance of these functions, a deficiency in potassium is understandably disruptive. It reduces carbohydrates, both decreasing the synthesis of new carbohydrates (photosynthesis) and increasing the consumption of existing carbohydrates (respiration). Potassium deficiency also impairs nitrogen metabolism in vines, decreasing protein production. These carbohydrate and nitrogen disruptions restrict vine growth, including the growth of roots, shoots and fruit. At the same time, a deficiency of potassium diminishes grapevine drought tolerance and increases its susceptibility to wilting and stress damage (Figure 1).

Figure 1. Leaf symptoms of ripening period heat stress associated with potassium deficiency in Chardonnay. (Photo Source: Progressive Viticulture, LLC©)

Visual symptoms of potassium deficiency include a faded-appearing yellowing (chlorosis), sometimes with dead specks or small patches, on leaves. Early in the growing season, potassium deficiency may be evident as reduced fruit set and later, as uneven fruit color and inhibited fruit maturation. Increased powdery mildew and botrytis susceptibility and premature leaf aging are other facets of potassium deficiency in vines. After harvest, potassium deficient juice is prone to browning and stuck fermentation. And because potassium deficiency inhibits cane ripening, affected vines are prone to cold damage.

Potassium in Soils – Where It Is

Very little potassium is accessible to grapevine roots in most California vineyard soils. Easily-available potassium in soils consists almost entirely of potassium dissolved in the soil solution, that is the liquid within soils. This pool of potassium is customarily called soluble potassium (Figure 2). As in grapevine tissues, potassium in the soil solution is not part of larger molecules but remains a free ion. As a result, depending on soil texture and other factors, soluble potassium can be leached from topsoils with downward percolating water.

In contrast to soluble potassium, most California vineyard soils contain abundant potassium inaccessible to vine roots. Actually, the largest pool of potassium resides within the structures of primary mineral particles (Figure 2). Structural mineral potassium very slowly becomes available as these minute rock fragments break down due to combined actions of weathering agents, including air, water and the organic products of microbes and other soil inhabitants. A few primary minerals (micas) can reabsorb small quantities of potassium from the soil solution. Notably, unlike nitrogen, phosphorus and sulfur, soil organic matter decomposition is an insignificant source of potassium for vines.

Figure 2. Potassium pools in soils: mineral, non-exchangeable exchangeable and soluble. (Graphic Source: Progressive Viticulture, LLC©)

When sufficiently abundant, exchangeable potassium will replenish depleted soil solution potassium (Figure 2). This pool consists of potassium adsorbed and loosely held, along with other positively charged ions, on or very near the surfaces of negatively charged soil particles. These are mainly clay and organic matter particles. Predictably, sandy soils in California, which are low in clay and organic matter, are low in exchangeable potassium. Still, even under the best circumstances, the combination of exchangeable and soluble potassium, the two most available pools of potassium, typically represent less than 2% of the total potassium in soils.

A fourth pool, non-exchangeable potassium, is potassium trapped within the layers of secondary clay minerals (Figure 2). This potassium pool is especially important in many clay soils in and around the Sacramento River Delta. When wetted, these active clay particles swell and expand, allowing potassium to enter the spaces between their plate-like layers, and when sufficiently dry, they shrink and collapse, fixing potassium within. Accordingly, potassium that migrated during the winter while soils were wet becomes mostly fixed and unavailable during the summer while soils are dry. Such fixation does not occur to any appreciable extent for other mineral nutrients and as such, it is another unique aspect of potassium.

In any given soil, the relative potassium contents of the four potassium pools – mineral, non-exchangeable, exchangeable, and soluble – depend on its mineral composition and degree of weathering, which also affect its ability to adsorb and fix potassium. Further, the four pools overlap and to varying degrees, potassium movement between pools is bidirectional. Because of that, some added potassium fertilizer will move out of the soil solution and into the other pools, thereby decreasing fertilization efficiency.

Potassium in Soils – How Roots Get It

Grapevines acquire potassium from soils mainly by two means. Neither of them is particularly rapid. The first involves potassium moving towards vine roots. Some mineral nutrients in soils readily flow to grapevine roots as they take up water and some people assume potassium does as well, but unless a large dose of soluble potassium fertilizer has been recently applied, it does not. Instead, potassium, by virtue of its low concentration in the soil solution, moves very short distances (typically less than 0.16 inch) in the soil primarily by random motion within the thin films of water adhering to soil particles; a process called diffusion.

A concentration gradient created by the depletion of solution potassium near the root surface is the driving force for diffusion. Such depletion is the result of active potassium uptake by grapevine root cells, which is an energy consuming physiological process. In this way, grapevines can maintain an internal potassium concentration that is greater than that of the soil solution. As uptake depletes soluble potassium in the soil, exchangeable potassium comes into solution to replenish it. Still, the rate of potassium uptake frequently outpaces the capacity of the soil to supply potassium.

While root uptake of potassium initiates and sustains diffusion, soil water content effects its rate. As such, low soil moisture, which is typical following fruit set under regulated deficit irrigation (RDI) schedules, limits the thickness of water films that serve as avenues for potassium movement in vineyard soils, intensifying the risk of potassium deficiency in grapevines. Soil compaction, which diminishes soil porosity, can similarly inhibit potassium movement.

The second way in which grapevines acquire potassium is by extending their roots into unoccupied, unmined soil. Accordingly, anything that limits grapevine root systems can induce potassium deficiency, including cool soil temperatures, soil saturation, soil compaction, a subsoil claypan or hardpan, soil borne pests and diseases, and other grapevine stresses (Figure 3).

Figure 3. Anything that limits the growth and extensiveness of root systems limits access to soil potassium, including (A) topsoil compaction and (B) a subsoil hardpan layer. (Photo Source: Progressive Viticulture, LLC©)

Potassium in Soils – Competition

Potassium competes to varying degrees with other positively-charged mineral nutrients in root zones. Both calcium and magnesium have greater affinities for exchange sites on soil particle surfaces than potassium. Consequently, both displace adsorbed potassium, thereby decreasing the supply of reserve potassium.

Further, both magnesium and calcium compete with potassium for uptake by grapevine roots. This effect is particularly a problem for California vineyard soils relatively high in absorbed magnesium and relatively low in absorbed potassium (magnesium base saturation ≥ 20% and potassium base saturation ≤ 3%). When sufficiently high, sodium and ammonium also compete with potassium for uptake.

Potassium in Grapevines – Demand and Competition

The demand for potassium within grapevines steadily increases early in the growing season and accelerates after bloom. Initially, potassium remobilized from reserves stored in woody tissues satisfies this internal demand, but as soils warm and root activity increases, potassium uptake from the soil becomes the prime source.

After fruit set, there is a surge in potassium demand that is roughly proportionate to increases in berry size and number. During this time, berries require potassium to fulfill their many functions for normal development, including enlargement and maturation. Hence, berries are principal sinks for potassium, drawing heavily on the potassium supply (Figure 4). When ripe, the fruit will contain about 5 pounds or more K per ton, which is greater than the content of any other mineral nutrient (Table 1). Just as important, after nitrogen, potassium is the second most plentiful mineral nutrient in grapevines.

Figure 4. Berries are the principal in-season sink for potassium and they contain more potassium than any other mineral nutrient. (Photo Source: Progressive Viticulture, LLC©)

Not surprisingly, given the short supply of potassium in most vineyard soils and the challenges for root systems in acquiring it, the potassium demand for berry growth and development is oftentimes less than fully satisfied. In instances of deficiency, berries draw potassium from other vine tissues. As a result of internal competition between vine organs for potassium, deficiency symptoms can appear beginning midseason on mid-shoot leaves and late in the season on apical leaves (Figure 5).

Figure 5. Leaf symptoms of potassium deficiency typically occur (A) midseason on mid shoot leaves on the afternoon sun sides of canopies and (B) late season on apical leaves. Symptomatic leaves may appear somewhat glossy and late in the growing season, may curl downward. (Photo Source: Progressive Viticulture, LLC©)

Managing Potassium in Winegrape Vineyards

Consistently effective potassium management in vineyards involves consideration of the supply, demand and acquisition factors presented in this article. It begins with a root zone managed for optimum function, including those functions affecting water and roots. It follows with supplemental potassium applied as fertilizer in proportion to the targeted fruit yield and corresponding crop demand.

Common organic amendments, manure and compost contain potassium. However, unlike nitrogen, phosphorus, and sulfur, the potassium content of these amendments is insufficient to meet grapevine needs when they are applied at typical rates. Consequently, with regard to grapevine potassium, the benefits of applied organic matter are mainly indirect and largely due to their effects on soil inhabitants, including vine roots.

The traditional method for supplying additional potassium to vineyards involves high-rate (more than 1000 lb fertilizer/acre) soil applications of dry potassium fertilizers. Traditional fertilizers are potassium sulfate (a slowly soluble salt) and potassium chloride, which is suitable only for well-drained soils due to concerns for the effects of excess chloride on vines (Table 2). Autumn is the common application timing, which allows winter rains to dissolve the fertilizer, making potassium available to vines the following growing season.

The objective of high application rates of dry fertilizers is to saturate the capacity of the vineyard soil to fix potassium, thereby ensuring some will remain available to vine roots. Accordingly, dry potassium fertilizers are usually banded on the soil surface or shanked into the soil adjacent to the vine rows. (Broadcast applications of dry potassium fertilizers are more effective for very sandy soils).

These concentrated potassium applications are often successful under surface irrigation, with beneficial effects sometimes lasting several years. They are, however, less effective under drip irrigation where active root zones recede away from fertilizer bands as moisture from winter rains is depleted. That being so, drip-irrigated vines fertilized in this way often become deficient in potassium during the growing season.

In fact, for most vineyards, total dependence of preseason fertilizer applications leads to in-season potassium shortages of varying degrees, which may be expected given this nutrient’s supply, demand and acquisition challenges.

The greatly diminished size of midseason root zones of drip-irrigated vineyards are areas of concentrated root activity, including potassium uptake. As a consequence, the supply of available potassium in these compressed root zones is rapidly consumed. These events coincide with increasing potassium demand in developing berries and correspondingly, increasing risk of potassium deficiency in vines. Fortunately, during the same time, vines rapidly respond to modest rates of potassium fertilizers applied to drip zones, especially liquid fertilizers containing potassium in a highly soluble form.

Potassium thiosulfate (KTS) and potassium bicarbonate (0-0-30) are perhaps the two most frequently used liquid potassium fertilizers for in-season fertigation (Table 2). Typical rates are 5 gallons per acre or less, which equates to about 12.5 pounds of potassium per application or less. Depending on crop level, two or more applications between fruit set and harvest may be required. Applying fertilizer potassium in combination with nitrogen phosphorus and sulfur, as in an NPK blend such as 3-12-14-4(S), normally enhances potassium uptake beyond what is possible when potassium is applied alone. Such blends are especially beneficial after harvest for promoting stress relief and potassium storage in woody vine tissues for use next season.

Foliar fertilization is another option for supplementing the vineyard potassium supply in-season. Several highly absorptive potassium fertilizers have been developed for this purpose, including formulations involving potassium chelated or complexed with amino acid, organic acid, flavonol or sugar molecules. For dry-farmed vineyards, foliar fertilizers are particularly useful for in-season supplementing of soil-applied dry fertilizers. Due to their comparatively short-term effects, foliar fertilizers may require multiple applications.

Conclusions

Maintaining adequate potassium in grapevines is essential for consistent yields of high quality fruit, as well as minimizing grapevine stresses. At the same time, the unique characteristics of potassium make it one of the most challenging mineral nutrients to manage in vineyards. Several factors contributing to limited availability in soils and difficulties in root acquisition, as well as high in-season demand for berry growth and development are among potassium management challenges. A basic understanding of these factors, as well as some familiarity with potassium fertilizers and their application, is fundamental for successful potassium management.

TOWARDS AN UNDERSTANDING OF VINEYARD POTASSIUM

Potassium in Grapevines – What It Does

Potassium in Soils – Where It Is

Potassium in Soils – How Roots Get It

Potassium in Soils – Competition

Potassium in Grapevines – Demand and Competition

Managing Potassium in Winegrape Vineyards

In fact, for most vineyards, total dependence of preseason fertilizer applications leads to in-season potassium shortages of varying degrees, which may be expected given this nutrient’s supply, demand and acquisition challenges.

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