Potassium in agriculture – Status and perspectives

Global Potassium Demand for Agriculture

Since the 1960s, the world population has doubled, increasing the demand for crop production to meet food and energy needs.
Climate models predict increasing drought and heat stress, negatively impacting major crops and food security.
Major challenges for agriculture include enhancing crop yields with resource efficiency and stabilizing plant development under stress.
Potassium (K) plays a crucial role in physiological processes vital to crop growth, yield, quality, and stress resistance.

Potassium in Soils

K constitutes about 2.1–2.3% of the earth’s crust, making it the seventh or eighth most abundant element.
Soil K reserves are generally large, but many agricultural areas are deficient in K availability.
These include ¾ of paddy soils in China and 2⁄3 of the wheat belt in Southern Australia.
Soils inherently low in K are often sandy, waterlogged, saline, or acidic.
In intensive agricultural production, K has become a limiting element, especially in coarse-textured or organic soils.
Lower fertilizer K application can deplete available soil K reserves, decreasing soil fertility.
K fertilizers are applied at a much lower rate than nitrogen (N) and phosphorus (P), with less than 50% of the K removed by crops being replenished.
An analysis of the nutrient balance in six Asian countries from 1961 to 1998 indicated an overall annual K deficit of about 11 Mt, 250% more than their current K fertilizer use.

Potassium in Soils: Content and Availability

Mineral soils contain 0.04–3% K, with the total K content of the upper 0.2 m of most agricultural soils ranging between 10 and 20 g kg−1.
Most soil K (90–98%) is incorporated in the crystal lattice structure of minerals and is not directly available for plant uptake.
Availability of K varies greatly with soil type and is affected by physico-chemical properties.
K in soil is classified into four groups based on availability to plants:
- Water-soluble:
  - Directly available for plants and microbes.
  - Subject to leaching.
  - Makes up about 0.1-0.2% of the total K in soil
- Exchangeable:
  - Electrostatically bound to the surfaces of clay minerals and humic substances.
  - Considered to be easily available for crops.
  - Makes up about 1-2% of the total K in soil
- Non-exchangeable:
  - Considered to be slowly available for plants.
  - May contribute to the plant supply in the long term.
- Structural:
  - Considered to be non-available K sources for plants
  - May contribute to the plant supply in the long term.
Dynamic equilibrium reactions exist between the different pools of soil K.
Soil physical and chemical properties, plant-soil interactions, and soil microbial activities affect K fixation and release.
Most K in soil is in the structural form, comprised of K-bearing primary minerals like muscovite, biotite, and feldspars.
K-feldspars may directly release K to the soil solution, while interlayer K of micas is held tightly by electrostatic forces.
Weathering of K-feldspars and micas produces secondary soil minerals which represent potential sources of plant-available K.
K in trioctahedral micas (biotite and phlogopite) is more readily released by weathering; application of biotite to K-deficient soils may enhance plant-available K content.
Formation of dioctahedral expansible 2:1 minerals from biotite may enhance the amount of K in soil solution.
Weathering of K-containing primary minerals is very slow; sole addition to soil may not be beneficial compared to soluble K fertilizers.
Addition of rock K materials may increase the long-term fertility of the soil by increasing K depots.
Plant species effective in K uptake and K-solubilizing microbial populations may control K release from soil minerals.

Effect of Potassium-Solubilizing Bacteria on Potassium Availability in Soils

Some soil microorganisms can release K from K-bearing minerals by excreting organic acids.
Examples include Pseudomonas spp., Burkholderia spp., Acidothiobacillicus ferrooxidans, Bacillus mucilaginosus, Bacillus edaphicus, and Bacillus megaterium.
Organic acids directly dissolve rock K or chelate the primary mineral’s silicon ions to bring K into solution.
Inoculation of K-solubilizing microorganisms in conjunction with application of rock-K to soil has gained attention.
Beneficial effects of inoculated-mica application to soil on plant K uptake have been reported in cotton, oilseed rape, pepper, cucumber, and sudan grass.
Exudates of these microorganisms can effectively enhance the release of K from clay minerals.
Application of inoculated feldspars into soil enhances about 40–60% of K solubility and plant K uptake.
Little information is available on the field application of such methods due to difficulties in soil inoculation under field conditions.
More field studies are needed to evaluate their effect on both soil properties and crop growth.

Effect of Root Exudates on Potassium Availability in Soils

The utilization of non-exchangeable K sources is an important factor for the K uptake efficiency of crops.
Plant species or genotypes within species differ in their capacity to use this resource.
Ryegrass and sugar beet are more efficient in mobilizing K than wheat and barley.
Sugar beet took up 3–6 times more K per unit of root length than wheat and barley grown on K-fixing soils.
Crop differences in K uptake are attributed to the mobilization of non-exchangeable K by root exudates.
Major compounds released are organic acids such as citric and oxalic acids by maize, tartaric acid by pak-choi and radish, and malic acid by oilseed rape.
Amino acids detected in root exudates of wheat and sugar beet enhance the release of K from clay minerals.
The depletion of K in rhizosphere soil solution below a threshold level (10–20 µM) activates the root exudation mechanism.
Organic acids facilitate the weathering of soil minerals through the formation of metal–organic complexes and by enhancing the exchange of H^+ for K^+.
A better understanding of the mechanisms behind the K release from soil minerals is key to developing new approaches for sustainable agriculture.
Crops have differences in transforming non-exchangeable K to soluble forms.
In K-limited areas, the selection of certain species or varieties that are efficient in solubilizing K via exudates should greatly increase resource use efficiency.

Potassium Fixation in Soil

Soil minerals can also fix K, significantly affecting K availability.
This involves the adsorption of K ions onto sites in the interlayers of weathered sheet silicates, such as illite and vermiculite.
The degree of K fixation in soils depends on the type of clay mineral and its charge density, moisture content, competing ions, and soil pH.
Montmorillonite, vermiculite, and weathered micas are the major clay minerals that tend to fix K.
Soil wetting and drying also significantly affects K fixation.
The fixation process of K is relatively fast, whereas the release of fixed K is very slow due to the strong binding force between K and clay minerals.
Whether a soil fixes or releases K highly depends on the K concentration in the soil solution.
In addition to organic acids, the H^+ concentration in soil solution (via soil pH) seems to play a key role in K release from clay minerals.
Optimization of soil pH may be a means of enhancing K release.
For optimized K fertilizer management practices, it is crucial to understand the factors that regulate K release from the soil non-exchangeable pool.

Effects of Potassium Fertilization on Soil Physical Properties

K fertilization enhances the water-holding capacity of soils and improves the structural stability of sandy soil.
The effect of K fertilization on soil stability is likely due to an increase in the electrolyte concentration in the soil solution, causing flocculation and precipitated salt crystals.
A higher concentration of K in soil solution may cause an increase in micro shear resistance that may explain the change in water retention.
Mg^{2+} and Ca^{2+} are more effective cations for stabilizing soil structure; relative flocculating power for K^+, Mg^{2+} and Ca^{2+} was 1.7, 27.0 and 43.0, respectively.
Higher water retention plays a key role in securing the soil productivity in water-limited areas.
More information is needed in order to understand the effect of K fertilization on the soil’s physical properties and soil water holding capacity.

Determination and Adjustment of the Potassium Supply of Crops

Fertilizer application is required to ensure and sustain an adequate supply of soluble K to crops.
Application rates and timing of organic or inorganic fertilizers are often based on an optimal N supply, but not on K requirements, leading to an excess or a shortage of K.

Soil Analysis

Monitoring of soil K reserves is extremely important in order to make precise fertilizer recommendations.
Simple soil extraction methods for measuring exchangeable K are widely used to estimate the K fertilizer demand of a crop.
An estimation of exchangeable soil K by extracting the soil with neutral ammonium acetate, ammonium chloride, calcium chloride, or ammonium fluoride (Mehlich 3) from air-dried or oven-dried soil samples is the most widely used soil test for K.
The preparation procedure includes the drying of soil samples to a maximum of 40 ◦C and crushing them to pass a 2-mm sieve to provide a homogenous mixture for analysis.
Drying of soil samples can influence the amount of K extracted by traditional extractants.
Some researchers have reported an increase in extractable K upon drying.
Others have reported that soils high in exchangeable K tend to fix K, and soils low in exchangeable K tend to release K upon drying.
The impact of sample drying on K extracted depends on the deviation from the equilibrium K concentration at sampling time and on soil mineralogy.
Illite appeared to be the source of K released by drying, while vermiculite and montmorillonite were associated with K fixation.
The wet-extraction-method (ammonium acetate-extraction of field-moist soil) was reported to yield a better correlation with crop K uptake than methods extracting K from air-dried soil.
Moist-extraction K tests had a superior capacity to predict crop responses to K fertilization, as compared to the commonly used dry extraction method.
Few laboratories have adopted wet extraction methods due to impractical procedures, such as sieving moist soil.
The extraction methods discussed above may provide sufficient information for fertilizer recommendations in light textured soils that do not contain 2:1 clay minerals.
In soils that contain 2:1 clay minerals, the non-exchangeable K pool usually contributes largely, sometimes to over 50%, to crop K supply.
Measuring plant-available soil K that is released from non-exchangeable K reserves is very difficult due to the complexity of the dynamic equilibrium among the various forms of soil K during crop growth.
No routine methods are available to measure this parameter.
Various methods have been established to assess the slowly or potentially available K in soils, e.g. extraction by 1 M HCl, boiling in 0.5 M or 1 M HNO_3, electro-ultrafiltration, exchange resins, Jackson’s test (sodium tetra-phenyl-boron; NaTPB), and field balances.
Common acid extraction methods only remove a proportion of the reserves of K present in the non-exchangeable pool.
Boiling of soil in 0.5 M HNO3 was a better predictor of K uptake than other acid-extraction methods in light-textured soils, but boiling in 1 M HNO3 extracts more K than would normally become available to a crop.

Potassium Fertilizers

In intensive agriculture, fertilizer application is mandatory in order to ensure and sustain an adequate supply of available K to crops.
Since 1980, there has been about a 25% increase in K fertilizer use.
World fertilizer K demand is projected to further increase from 23.8 Mt K in 2011 to 27.1 Mt K in 2015 (FAO, 2011) due to the targeted increase in global agricultural production.
China and the USA use almost 40% of the global K fertilizers, and developing countries are expected to drastically increase their fertilizer K consumption in the near future.
Cereal crops have the highest share (37%) in K fertilizer use followed by fruits and vegetables (22%), oilseeds (16%), sugar and cotton (11%), and others (14%).
Cereals are considered low K consumers, and the amount of K removed from the field with the grains is largely independent of K supply.
Potassium chloride (KCl), a natural mineral mined from deep deposits, is the major form of K fertilizer.
Potassium sulphate and potassium nitrate are also commercially available but are more expensive and are preferred for crops that are especially sensitive to chloride, such as potatoes and fruit crops (banana, citrus, grapes, and peach).
Organic fertilizers are also an important source of fertilizer K.
In most annual crops, the general practice is to apply K fertilizers at or before planting.
In light-textured soils that have very limited capacity to retain K from leaching it may be more efficient to make two or three split applications, and this practice may also be applied on soils that tend to fix K.

Potassium in Plants

K concentrations of crops vary widely with site, year, crop species, and fertilizer input; concentrations in the range of 0.4–4.3% have been reported.
Crop K concentrations are often well below (<2.5–3.5%).
For many crops, the critical K concentration is in the range 0.5 to 2% in dry matter.
The concentration of K in the cytoplasm is kept relatively constant at around 50–150 mM, while the concentration in the vacuole varies substantially depending on supply status.
Vacuolar K largely determines the osmotic potential of the cell sap.
High K concentrations in crops have often been termed “luxury consumption,” but high accumulation of K by crops during optimal growing conditions may be considered as an “insurance strategy” to enable the plant to better survive a sudden environmental stress.
The percentage of K in dry matter decreases significantly during growth, and thus it would be difficult to use % K in dry matter to diagnose K deficiency unless the optimum concentration would be defined accurately for each growth stage of each crop species.
Plant species are known to differ in their K requirement and in their ability to take up K due to variations in root structure, such as root density, rooting depth, and root hair length.
Positive correlations between K uptake efficiency and root hair length or density in K-depleted soils have been reported for maize, oilseed rape, tomato, pea, red clover, barley, rye, and perennial ryegrass.
Ryegrass competes for K more effectively than red clover due to its longer root hairs and denser root system, and both morphological parameters may also deplete the K in larger volumes of soil solution, initiating the release of non-exchangeable K.

Potassium Supply, Crop Yield Formation and Crop Quality in a Changing Environment

Potassium deficiency rarely results in the accumulation of starch, whereas accumulation of sugars is often observed in K-deficient leaves.
This is likely a consequence of impaired sucrose export from leaves, rather than a limitation of photosynthesis.
Increased leaf sucrose concentrations in K-deficient plants do not generally promote accelerated root growth.
K-deficient roots generally do not show an accumulation of sugars but rather have lower concentrations of sucrose and starch than their K-replete counterparts.
One reason for this is that sucrose export to the root is reduced in K-deficient plants attributed to a K requirement of phloem loading with sucrose.
In Vicia faba and maize, specific K channels are linked to sugar loading and unloading.
In intact plants, K uptake by leaves is stimulated by light.
Positive effects of K nutrition on the rate of photosynthesis only in crops subjected to some drought treatment have been shown.
Plants supplied with elevated K levels showed similar levels of photosynthetic rates, but when exposed to drought, rates of photosynthesis were positively correlated with application rates of K.
More research is needed to explain how K starvation or sub-optimal K nutrition down-regulate photosynthesis, e.g. under drought conditions using techniques such as chlorophyll fluorescence imaging.
Capturing critical threshold values for potassium deficiencies (and quantifying optimum or slight super-optimum potassium nutrition) under high light, drought stress or heat stress may be possible at early stages of the vegetation period.

The Role of Potassium in Phloem Transport

The translocation of photosynthates from leaves to roots and other sinks is mainly mediated by the phloem.
K transport in the phloem is largely directed from older to younger plant tissues, which ensures redistribution of this ion toward growing tissues.
Adequate K supply plays a crucial role in phloem translocation of assimilates.
K circulating in the phloem serves as a decentralized energy store that can be used to overcome local energy limitations.
Posttranslational modification of the phloem-expressed Arabidopsis K channel AKT2 taps this “K battery,” which then efficiently assists the plasma membrane H^+-ATPase in energizing the transmembrane phloem (re)loading processes.
K retranslocation from the shoot back into the root via the phloem and subsequent reloading back into the xylem occurs in the case of K delivery exceeding shoot requirements or under K deficiency.
In addition to its role in phloem loading, K has an important function in mass flow-driven solute transport in the sieve tubes of the phloem.
K accumulation is a prerequisite to establish and maintain a high osmotic potential in the sieve tubes for enabling high transport rates.
In experiments with sugar cane, about half of the ^{14}C-labeled photosynthates were exported from the source leaf to other organs within 90 min in K-sufficient plants, with 20% transported to the stalk as main storage organ; in K-deficient sugar cane plants, the export rates were much lower.
In grape berries, the wine quality correlated with the sink strength of the grape berries for K and other solutes during ripening and was controlled at the phloem unloading step.
K as the most abundant inorganic cation in the phloem vessels has an additional function in counterbalancing mobile anions in the phloem.
The accumulation of organic acid anions in plant tissues is often the consequence of K import without an accompanying inorganic anion into the cytoplasm.
The role of K in the cation–anion balance is also reflected in NO3^− metabolism, in which K is often the dominant counterion for NO3^− in long-distance transport in the xylem as well as for storage in vacuoles.

Potassium Nutrition of Cereals, Sugar Beet, and Grape

The roles of K and the specific requirement of K supply differ with crop types, here, wheat as a representative for cereal crops, sugar beet as a natrophilic root crop, and grape as a berry crop and a source for wine are reviewed.
After N, K is the nutrient required in the largest amounts by plants.
When plant-available native soil K concentrations are low, yields of wheat and sugar beet can benefit from K fertilization.
Both sugar beet and wheat have a high K demand, and uptake ranges from about 50 to 300 kgKha−1 per season, broadly similar to the uptake of N.
Most K uptake in wheat takes place as the shoot is undergoing its rapid phase of growth.
For cereals, more than 70% of K remains in the straw, but this value depends on cereal species and fertilization level.
How crop residues are handled is, therefore, very important when calculating fertilizer requirements.
Potassium deficiencies in cereal crops may restrict their ability to utilize N, resulting in an increased potential for nitrate leaching.
A lack of K can increase a crop’s susceptibility to diseases and hence the need for pesticide applications.
Adequate fertilization with K also improves the resistance of wheat to lodging.
Early visible symptoms of K deficiency are chloroses at the tip of the oldest leaves, showing a transfer of K from older to younger leaves to maintain growth.
Progression of K deficiency results in further symptoms, with chlorosis starting to evolve from the tip to the edge and finally turning into necrosis.
The symptoms are often accompanied by a bronzing due to an accumulation of putrescine in those leaves.
Under severe K-deficiency, plants also show wilting symptoms, which are due to a disturbed water balance as well as a limited lignification of cell walls leading to decreased quality.
The K requirement of natrophilic species, such as sugar beet (Beta vulgaris), is lower because these plant species can efficiently replace K by Na to some degree.
About 60% of the K in the cells of sugar beet is exchangeable by Na, whereas less than 15% of the K is exchangeable in wheat.
Especially chenopodiaceae, such as sugarbeet, have a high capacity to exchange K for Na for osmotic purposes in the vacuole, whereas the portion of K which is necessary to activate K-specific enzymes in the cytosol is not exchangeable.
Therefore, sugar beet is considered a natrophilic crop, and cheap fertilizers such as NaCl could be used efficiently and with an additional economic benefit.
The potential to fertilize wheat with NaCl is very limited, and such a practice would decrease wheat growth invariably because most crops are highly salt-sensitive.

Potassium Supply and Product Quality of Cereals, Potatoes, and Wine

The nutritional quality of plant products is a highly complex trait composed of appearance, nutritional, sensory, shelf live, and technological quality.
Product quantity is the principal function driving the producer’s revenue stream, but product quality is important for consumers and often serves as a criterion for the market price of the crop.
Many parameters influence product quality, such as genetic constitution, agronomic measures, including fertilization and pest management, as well as water supply and climatic factors.
As outlined below for wheat, potatoes, and grapes, a balanced fertilization regime including K is important for achieving high quality products.

Wheat

It is relatively easy to increase the K concentration of various plant organs by increasing the K supply to plant roots, but this does not apply to grains and seeds.
Mature grains contain a relatively low and constant K concentration of around 3 mg g^{−1} DW under a wide range of K supply levels.
K concentration in grains is constant across all K supply levels, while K levels in straw reflect the fertilization regime.
With increasing consumption of processed, K-depleted, food, combined with a reduction in the consumption of K-rich fruits and vegetables, there has been a large decrease in K intake which now averages around 70 mmol day^{−1}, only one third of evolutionary intake.
The K demand of humans lies on average at the level of 2–5.9 g day^{−1}.
Much evidence shows that an increased K intake has beneficial effects on human health (lowers blood pressure, reduces cardiovascular disease mortality, may prevent or slow down the progression of renal disease).
An effective way to increase K intake is to increase the consumption of fruits and vegetables.
To increase the K intake originating from cereal grains, it will be necessary to increase the K content in grains by physiological and molecular understanding of the processes regulating this parameter and a screening of possible genotypic differences.

Potato

Nutritional and technological quality varies with K nutrition.
In tomato fruits, the incidence of ripening disorders increases with inadequate K supply.
In potato tubers a whole range of quality criteria is affected by the K concentration of the tuber tissue.
The K-concentration of well-supplied potato lies in the range of 4 g kg^{−1} dry matter.
The concentrations of most minerals were higher in the skin than in the flesh of tubers because of direct uptake from the soil across the periderm.
Potassium nutrition substantially influences the concentration of reducing sugars in potato inverse relationship of reducing sugar concentration and K supply has been observed in most cases.
Tubers grown with high N and low K fertilization accumulated most precursors for acrylamide formation during frying.
Increasing K addition always raised the citrate and malate concentrations, but lowered the concentrations of reducing sugars.
Acrylamide is formed during processing from reducing sugars and free asparagine.
Reducing sugars such as glucose and fructose function as carbonyl group donors in the Maillard reaction.
Acrylamide is considered a carcinogen and is neurotoxic for humans.
Fried potato products can contain appreciable amounts of acrylamide (>2 mg g^{−1} dry matter).
Technological quality of potatoes is also influenced by the storage and handling of the tubers until processing.
Sugars are known to accumulate at low temperatures and have a role in osmoregulation and cryoprotection in many plants.
Potato tubers stored at low temperatures accumulate sugars, making them unsuitable for processing.
There is a need for further breeding efforts to increase the tuber quality with respect to reduced acrylamide formation but an uncompromised flavor and taste.

Grapevine

K plays an essential role for grapevine growth and yield as well as for wine quality.
Grape berries function as a strong sink for K, especially during the onset of ripening and volume increase.
Too high K concentration decreases free organic acids, increases overall pH, lowers the tartrate-to-malate ratio, increases the tartrate precipitation during wine-making and reduces the overall wine quality.
A sharp increase in berry K concentration is observed during the whole maturation period of the berries.
A voltage-gated inwardly rectifying K channel is stimulated upon acidification of the berry, mediating rapid K transport into the berry and contributing to the extensive re-organization of the translocation pathways and transport mechanisms that occur at interception of ripening.
The K concentration is generally higher in the skin of the berry, where anthocyanins are located, than in the fleshy pericarp.
An optimal pH of the red wine should be in the order of 3.3–3.7, requiring careful calibration of vineyard management options to manipulate the best K concentration.

Breeding for Better Potassium Efficiency

There is a necessity developing more efficient crop cultivars in terms of the nutrient efficiency or quality of harvest products for low-input agriculture, minimizing fertilizer costs, and achieving sustainability and quality on a high level.
Efficient genotypes may have specific physiological mechanisms or traits to gain access to sufficient quantities of a nutrient such as K (uptake efficiency) and to more effectively utilize amounts of K taken up (utilization efficiency).
Using K efficient genotypes in combination with optimized soil fertilization is the perfect nutrient management strategy for stable and sustainable farming systems.
Genotypic differences for K uptake and use efficiencies have been detected in diverse wheat varieties (El-Bassam, 1998).
Significant wheat genotypic differences in the K efficiency ratio (the ratio of growth at deficient and adequate K supply) have been detected.
A wheat mutant was identified that accumulated more K in the leaf tissue than the wild-type line.
Breeders can intercede in root morphology, formation of root hairs, root exudates, ability to release K from non-exchangeable pools, kinetics of K uptake, K translocation, K substitution, and harvest.
Enhanced K uptake by roots is a goal for breeding of efficient crops by manipulation of carbohydrate metabolism.
The increasing demand for crops as a source for biogas and biofuels also requires a breeding program for K-efficient plants.
An improved K-use efficiency is crucial for the conservation of K resources.

Potassium Nutrition and Crop Stress Resistance

The physiological functions of potassium are critical for yield formation and product quality under otherwise optimal and undisturbed growth conditions.
Crops are faced with a multitude of factors that affect metabolism, growth, and thus yield, and abiotic and biotic stresses reduce crop yield to a large extent.
These conditions will have a dramatic impact on agricultural production and farming practices and may lead to massive, often complete, crop failures.
K nutrition impacts on the resistance of crops to virtually all abiotic and biotic stresses, in both direct and indirect ways, and the agro- nomic aspects of this relationship are highlighted.

Potassium Nutrition and Abiotic Stress Resistance of Crops

K is a main determinant of cell turgor.
Maintenance of turgidity and water uptake from the soil requires a further reduction of the plant’s osmotic potential by an increase in cellular osmolyte concentration.
The uptake and storage of increased amounts of K is an energetically ‘cheaper’ alternative to the synthesis of compatible solutes.
An ample K supply will thus support osmotic adjustment and sustain cell expansion at low soil water potentials.
As N is most frequently the primary growth-limiting factor in agricultural crops, an increased N fertilization requires a further increase in K availability to maintain the plant’s water status, in particular in dry conditions.
K also plays a central role in regulating stomatal aperture, and hence in the limitation of water loss.
K is required for proper stomatal opening by providing the osmotic driving force for water influx into the guard cell vacuole.
Stomatal resistance is decreased, and CO_2 assimilation is enhanced by high K supply under environmental conditions that promote stomtal opening, such as high light.
Not only stomatal opening, but also stomatal closure is defective in K-limited plants, due to the fact that full stomatal closure requires the backpressure by fully turgid epidermal pavement cells, whose pressure potential is built by K accumulation.
An alternative explanation for the inhibition of stomatal closure in K-deficient plants is based on the increased production of ethylene induced by K starvation, negatively interfering with abscisic acid signaling.
A major share of the alleviation of drought stress by K is not due to an improved regulation of stomatal aperture, but due to non-stomatal effects on photosynthesis, CO_2 fixation, primary metabolism, phloem loading, as well as on the osmotic pressure in the sieve tubes and thus the flow rate of photosynthates into the sink organs.
Suboptimal K nutrition affects phloem loading, and thus causes an accumulation of sucrose in photosynthetically active tissues and a poor energy supply of roots.
A range of enzymes of primary metabolism, such as starch synthase, require K for functioning.
Both processes cause an accumulation of soluble sugars in the shoot, which feedback-inhibit photosynthetic carbon fixation.
The light reaction is also dependent on K providing charge balance.
Inhibition of photosynthesis by both, drought or K deficiency, results in a misallocation of electrons to O_2, thereby producing reactive oxygen species (ROS) that cause oxidative stress.
The requirement of a sufficient K supply by a crop is therefore higher under drought as compared to well-watered conditions.
The acquisition of K from the soil is impeded by drought conditions.
Preparing existing cultivars for periods of drought stress calls for K fertilization above the level required for optimum yield under non-stressed conditions, and foliar application of K has also been suggested.
A continued uptake from a soil drying out from the surface can also be ensured by a deep placement of the K fertilizer, in particular in combination with nutrients that trigger root proliferation, such as nitrate.

Salt Stress

Salinity affects nearly a third of the agricultural land area worldwide.
Salinity exerts an osmotic stress due to decreased soil water potential and an accumulation of salts in the plant cell walls, as well as an ionic stress on the plant due to phytotoxicity of the ions present in high concentrations in the soil solution.
In most monocotyledonous plants it is the Na load which is most critical, albeit some dicotyledonous crops, such as potatoes or field bean, are very sensitive to the commonly accompanying ion, Cl.
An accumulation of Na in the cytosol is toxic even to salt-tolerant plants, displacing K from K-dependent proteins, leading to their inactivation.
Resistant glycophytes exclude Na from the root cells by active transport mechanisms, causing an increased Na concentration in root cell walls, and an energy-dissipating futile cycling of Na across the plasma membrane.
Cytosolic Na may alternatively be sequestered into vacuoles of shoots and roots, providing the required osmotic adjustment in the vacuolar lumen.
Most salt-tolerant crop species, such as sugar beet or barley, have includer characteristics.
Introducing or enhancing includer characteristics may thus improve growth and reduce the K requirement of crops on salt-affected soils, if the Na can be safely ‘locked up’ in the vacuole.
A high K-to-Na ratio needs to be maintained in the cytosol to maintain essential enzymatic functions.
Na influx causes a depolarization of the plasma membrane, thereby diminishing the driving force for K uptake and competing for the same uptake sites, which are mainly nonspecific cation channels (NSCCs).
Besides NSCCs, there are other potential Na influx pathways, such as members of the above-mentioned HKT transporter family.
It is a common observation that high external K reduces Na influx.
High Na load elicits a massive leakage of K from root cells caused by the activation of outward-rectifying K channels, such as GORK1, by the decreased membrane voltage.
In addition, Na triggers an influx of Ca into the cytosol, activating NADPH oxidases that generate OH^*, which may be converted to H2O2, activating Ca and K channels.
The Na-induced K leakage can be ameliorated by ROS scavengers, such as proline, and genotypic differences in ROS- and Na-induced K leakage correlate well with salt tolerance.
An improved crop performance on Na-affected soils by K fertilization is frequently observed.

High Light

In K-deficient plants photosynthetic CO_2 assimilation is impaired, and a high light intensity will put an extra strain onto those processes, with the effect of a sub-optimal K supply exacerbated under high light, hence plants are more prone to high light damage.

Cold Stress and Frost

With decreasing temperature, enzymatic processes and transporters are slowed down, causing an enhanced generation of ROS because the incoming light energy cannot be effectively funneled into assimilatory processes, but is transferred onto O_2.
As in drought-stressed plants, high K supply is believed to reduce the ROS load of chilling-stressed plants.
An increased accumulation of K decreases the symplastic osmotic potential, and hence will limit the freezing-induced dehydration.
Sensitivity to frost damage is strongly negatively correlated with the availability of soil K and the K status of the plant.
Frost damage is often ameliorated by high K fertilization.

Ammonium Toxicity

An optimal K supply is able to improve the tolerance of plants against high concentrations of ammonium (NH_4^+).
In soils, NH4 + is generated from the degradation of biomass and also from the hydrolysis of urea, which is also the world’s quantitatively most important N fertilizer.
Such high NH4 + concentrations are potentially phytotoxic because they can offset membrane potentials and cytosolic pH.
The plant’s tolerance to a high NH4 + load correlates very well with its K supply.
The beneficial action of K appears to be due to an inhibition of the NH4 + fluxes by K, probably by and direct competition of K with NH4 + for uptake at high concentrations, and by the activity of enzymes involved in NH4 + assimilation strongly increased with increased K supply.
Ammonium causes a large reduction of K uptake from media with a low K concentration, due to the fact that NH4 + inhibits some high-affinity transporters that mediate K uptake