1. Activates enzymes:
More than 60 enzymes have been shown to require potassium. Potassium rises the activity of amylase, invertase and proteolytic enzymes. Consequently,  potassiumis responsible for synthesis of surose, starch  and proteins, that leads to .  higher yields and better crop quality.

2. Promotes photosynthesis:
Since, potassium is vital for the activity of many enzyme systems involved in the process of photosynthesis, an adequate supply of potassium is essential for plants. Research has shown that green leaves well supplied with potassium are able to assimilate twice as much CO2 as leaves with lower potassium content. Potassium promotes the better usage of iron in the process of chlorophyll synthesis. One can see this when there is lack of iron in the soil.

3. Expedites the flow of assimilates:
Assimilates are transported from leaves to storage organs or fruits. The flow of assimilates is faster when plants are well supplied with potassium.

4. Improves assimilation of nitrogen and facilitates synthesis of proteins:
Potassium doesn’t only increase the content of carbohydrates, but also changes the ratio among different groups, leading to transfer of monosaccharide to sucrose. Plants well supplied with potassium are able to take up more nitrogen and convert it more rapidly into protein.

 5. Regulates the reduction- oxidising processes:
This role of potassium depends on the enough quantity of nitrogen in the plant. When there are a lot of nitrates the number of organic acids decrease, while in the plant with a big amount of ammonium the content of organic acids rises.

 6. Improves water-use efficiency:
Plants abundantly supplied with potassium can utilise soil moisture more efficiently than potassium-deficient plants. Therefore, crops that are well supplied with potassium need less water for the production of a given yield than crops that are undersupplied with it.

7. Enhance the natural ability of plants to resist stress from drought and cold, pests and disease:
Potassium improves the ability to resist stress from cold in winter crops and perennial herbs. Potassium influences the quality of the products while storing. It is well known fact that root rot appears in the plants, that are not enough supplied with potassium. When there is a lack of potassium a sugar beet rots already in the field.

1.1. K deficiency symptoms

Potassium participates in many physiological processes, such as growth, movement, transmission of signals, the upkeep of turgor pressure, and reproduction. Therefore potassium deficiency causes a variety of dysfunctions in plants’ metabolic processes, which can result in decreased productivity and quality of the crop yield.
Potassium deficiency in plants can de difficult to identify at first. Often, at the initial stage, potassium deficiency only reduces the growth and development rate of the plant - a condition known as 'hidden hunger'. Visible symptoms appear at advanced stages in the forms of chlorosis and necrotic spots. In potassium-deficient plants, potassium is transported from older leaves and organs to younger ones, so potassium deficiency is first observed on older leaves.

Pic potash/rice
A number of visible symptoms can indicate Potash deficiency in rice, such as stunted plants with dark green leaves and short thin stems, rusty-brown spots that start at the tip and later spread over the entire area of the leaf, dry tips and margins of leaves, and a high percentage of rotten roots. In addition, under conditions of high temperature and low humidity, potash deficiency may cause premature senescence of older leaves and rolled-up younger leaves. Finally, the lack of potassium can result in a high percentage of sterile or unfilled grains. 

Soya 
Pic potash/soya
Soy plants growing in potassium-poor soils are often affected by potassium deficiency. First the margins of the leaves turn yellow, and then the yellowing spreads inwards. The centre and base of the leaf remain green. If the deficiency is not addressed in time, chlorotic areas of the leaves extend into interveinal areas, giving the leaf a “fishbone” pattern. Symptoms usually first appear on older leaves. If the deficiency is not addressed the leaf margins eventually die.

Wheat
Pic potash/p3, potash/p4
Symptoms of potassium deficiency in wheat crops first appear on lower leaves progressing towards the top as the severity of the deficiency progresses. A deficit of potassium results in discoloration of tips and margins of wheat leaves as they turn yellow and brown. As in other cereal crops, lack of potassium in wheat often causes the weakening of the straw, which may result in lodging.

Maize
Pic potash/p5, potash/p6
Leaves of maize plants suffering from potassium deficit turn yellow at tips and edges, then brown, then the discoloration affects the entire leaf and it dies. In sections, nodes are deep brown, different from internodes. The growth of maize plants diminishes, roots stop their activity, stems weaken, and the plant is ready to fall down, when there is not sufficient amount of potassium in the plant organism. The lack of potassium leads to malfunction of plant breathing, and the plant begins to use protein instead of carbohydrates. Nitrogen necessity of plant is tightly connected with potassium content:  potassium deficiency stops nitrogen assimilation. Potassium deficient plants soon show signs of wilting under dry conditions; grain formation is depressed and the tip of the ear may be without grain.  

Sugarcane
Pic potash/p7
Older leaves of potassium-deficient plants are affected with chlorosis and necrotic lesions at their tips and along their margins. The upper surface of the midrib discolours to red.

Sunflower
Pic potash/p8 
Sunflower plants suffering from potassium deficit have thinner, more brittle stems. The deficiency of potassium reduces yield and oil content, as well as negatively affecting the ratio of unsaturated/saturated fat acids. Plant growth is significantly impeded by potassium-poor soils. Younger leaves of potassium-deficient sunflower plants form a rosette-like pattern and eventually develop dark brown necrotic patches. Ample application of potassium fertilisers may prevent these problems from occurring. 

Potato
Pic potash/p9, potash/p10
Among the first visible symptoms of potassium deficiency in potato plants are reduced growth, reduced flowering, and bluish-green foliage. Over time, plant internocles become shortened, leaves bend downward and turn yellow, and eventually necrotic patches appear on their surface. The colour of potato crisps can be improved by potash fertilisers. Crisps made from potatoes grown using ample potash fertilisers (left) are bright golden brown.  

Cotton
Pic potash/p11
In plants the potassium ion is mobile. When it is deficient it tends to move from older leaves to newer ones, therefore the first visual symptoms of potassium deficiency can be identified by yellowish-white mottling of older leaves, which later changes to light yellowish-green. Leaves develop yellow spots between veins, their centres die, and numerous brown spots develop at tips and margins between veins. Tips and margins tend to curl downwards. Eventually, leaves become reddish-brown, dry out and fall off prematurely, cotton balls fail to develop properly and lint is of poor quality.
Pic potash/p12В 

1.2. K and crop quality

Potassium has been described as the 'quality element' for crop production. Potassium can also enhance crop quality indirectly provided it interacts effectively with other nutrients and production practices.
Examples of the role of potassium as the quality element include:

• Potassium facilitates the absorption of nitrogen, stimulating the translocation of amino-acids from vegetative shoots to the grain that promotes the synthesis of gluten and prolamines, as well as the formation of proteins that improve baking quality.
• Potassium application increases the starch content of rice, wheat, soybean, sesame, and some other forage crops.
• Potassium increases oil content in sesame, soybean, rape, groundnut, and cottonseeds.
• Potassium increases protein and vitamin C content in potatoes. In addition, it increases the yield of large and medium-sized tubers and decreases weight loss in tubers after harvest. Potassium deficiency causes accumulation of reducing sugars and decreases starch content in potato tubers. Potato crisps made of such tubers are dark in colour.
• Potassium increases the size of cotton balls, improves micronaire value and fibre strength, and increases the percentage of mature fibres.
• In citruses, potassium positively affects rind thickness and improves fruit colour. It also enhances the contents of citric and ascorbic acids (vitamin C) in the juice and positively affects other juice characteristics such as the acid/sugar ratio and the content of soluble solids.
• A poor supply of potassium in bananas results in thin fragile bunches with a shorter shelf life.
• Potassium increases the amounts of solids, sugars, acids, and carotene in fruits and vegetables, and extends their shelf life.

1.3. K and plant health

Use of potassium fertilisers is one of a number of factors that may affect plant health - others include climate, plant protection, cultivation techniques, and soil conditions.
Application of adequate amounts of potash fertilisers can be viewed as insurance against the risk of diseases and pest attacks, especially on soils that are not sufficiently supplied with potassium or in situations where conditions for plant protection are not optimal.

Potassium affects the reaction of plants to pests or diseases as it:

• helps reduce the number of pathogens and restrains their development, multiplication, survival, vigour, and life cycle.
• helps plants to adjust their internal metabolism, restrains food supply for pathogens, and modifies microclimate through changes in habit and density of growth.
• restrains the establishment of pathogens and their proliferation within the plant by adjusting plant structures such as cell wall and cuticle thickness and tissue firmness through the functioning of the stomata.
• improves plants' ability to recover from pest or disease attack by repairing damage.

The right balance of nutrients is particularly important for a plant’s health. Excessive application of nitrogen fertilisers can throw the plant out of balance. Potassium can counteract the unfavourable effects of nitrogen. It improves nitrogen efficiency by reducing its stimulating effect on pathogens, or by increasing its beneficial effect on yield.

1.4. K and stress resistance

Plants that receive an inadequate supply of potassium are often more susceptible to frost damage. Improved resistance to frost is attributed to a number of physiological and morphological factors such as healthy deep roots, large xylems vessels, high sugar and carbohydrate content, reduced transpiration and water loss. Potassium acts on most of these factors, thus reducing winter injuries.
Plants adequately supplied with potassium can utilise soil moisture more efficiently than potassium-deficient plants. Potassium boosts a plant’s tolerance to drought by stimulating enhanced water uptake by the plant’s roots and reducing the loss of water through transpiration.

1.5. K and lodging

Plant lodging is usually caused by insufficient mechanical strength, attacks of diseases or pests, or by a combination of both. The propensity of cereals such as wheat, rice, and corn to lodging is largely accounted for by potassium nutrition.
Large cereal yields require large application of nitrogen. Nitrogen fertilisation leads to lavish growth of vegetation and if potassium is not applied together with nitrogen, plants may lodge.