Potassium (K) is the 19th element of the Periodic table and the 8th most abundant element in Earth’s crust. Chemically, it belongs to the alkaline metal group. Elemental potassium is a soft, silvery metallic solid that vigorously reacts in contact with water, and burns with a light purplish flame. Due to its immense reactivity, elemental potassium does not occur naturally, but in combination with other elements. It can form various salts and minerals. Aside from sediments, potassium is also found in seawater, making up about 0.04% of its entire weight.
This interesting element is very important for all living organisms, as it plays the role of the main intercellular ion. However, its functions and concentrations can vary depending on the species. For example, in animal cells, potassium is a key part of the sodium-potassium pump which resides in the cell membrane. The Na-K pump maintains the adequate electrical charge of the cell membrane and regulates the transport and volume of the cell.
Plant cells can have an especially high potassium content, as they store this element in their vacuoles. Potassium activates many important enzymes and is a key player in many physiological processes during plant growth. That is why its availability in soil is so important during crucial developmental phases, especially in young plants. Together with nitrogen and phosphorus (P), potassium forms the “Holy Trinity” of plant nutrition – NPK.
A bit of history
The discovery of potassium dates back to the early 19th century. Its name is a Latinized version of the word potash, as potash was the material potassium was first isolated from. Sir Humphry Davy, the English chemist and inventor, was the first to discover this element. In 1807, Davy conducted an experiment taking caustic potash (KOH) and passing electricity through it with a help of a voltaic battery. This process, called electrolysis, breaks down compounds into their simpler constituents. The discovery of potassium was groundbreaking since it was the first time anyone found a new element using electricity. Uncovering the marvels of electrolysis led Davy to reveal the existence of other elements too – sodium, calcium, barium, boron, magnesium, and strontium.
Although we learned about potassium not long ago, we are familiar with potash for centuries. Potash is a term that encompasses many different salts with one unifying quality – they contain water-soluble potassium. These salts had and still have numerous practical uses, e.g., soap-making, dyeing, as well as the production of ceramics, fireworks, and explosive weapons. Interestingly, potash was the subject of the very first patent in US history. The patent was granted in 1790 to Samuel Hopkins, who found a new method to produce this important material.
The surge of chemical and agricultural industries during the 20th century significantly increased the demand for potassium salts. Today, most of the potash is mined to make fertilizers, detergents, pharmaceuticals, and various products of the chemical and materials industry.
Why is potassium important to plants?
Potassium is the element that plays a key role in water, nutrient, and carbohydrate transport in plant tissues. It activates many different enzymes crucial for the proper development of plants and indirectly affects photosynthesis by regulating ATP production. Since it affects the activity of enzymes crucial for plant growth, the availability of this element is especially important for young plants. In fact, the highest concentration of potassium in the plant is found in its youngest tissues, where the process of cell division is intensive. Although it makes up a humble 1-2% of the plant’s dry mass, potassium has numerous important physiological functions in its cells and tissues.
Summary of potassium’s most important functions in plants
- Maintains the rigidity of cells (turgor)
- Regulates the opening and closing of stomata on leaves, so it plays a role in transpiration
- Maintains the pH of the cell in a favorable range
- Creates membrane potential along with hydrogen ions (H+), regulating ion movement through the membrane
- Affects certain physical qualities of the cell, like viscosity
- Increases water use efficiency
- Enables phototropic and gravitropic reactions
- Affects plant’s reactions to certain stimuli
- Cofactor of more than 50 enzymes (dehydrogenases, oxidoreductases, synthetases, transferases, etc.)
- Essential for the production of nucleic acids and ATP
- Activates enzymes that affect membrane activity, which is significant for the transport of various substances, osmoregulation, cell elongation, and ion balance within the cell
- Affects carbohydrate and protein metabolism, as well as biosynthesis of high molecular weight compounds like cellulose, hemicellulose, starch, and lipids
- Improves plant’s resistance to drought and pathogen attacks
- Plays a role in nitrate reduction and nitrogen metabolism
- Affects photosynthesis and accelerates CO2 assimilation
- Plays a role in the absorption and transport of assimilates in the phloem
- Improves both the quality and the number of fruits and tubers
Potassium in the soil
Potassium in the soil is most widely present in its inorganic forms which include soluble salts like carbonates, sulfates, and chlorides. These salts provide three types of potassium – available, semi-available and unavailable. The first type includes exchangeable ions that are easily absorbed by plants. The second type includes potassium ions that are fixed to certain minerals in the soil but susceptible to degradation by water and microbes, releasing available ions over time. The last type is the most common one, and it refers to potassium ions that bond very tightly to soil minerals, becoming unavailable for plant absorption.
How do plants absorb and transport potassium?
Plants absorb potassium easily and very intensely during the early phases of development. They also transport it quite efficiently through the phloem, and it is one of the most represented elements within these tissues. Plants take up potassium from the soil solution in the form of ions. Usually, the concentration of potassium in the soil is significantly lower than the concentration of elements like calcium and magnesium. Because of this, plants need to invest considerable amounts of energy to absorb potassium ions and transport them to the place where they are most needed.
Plants developed two mechanisms of potassium adoption. Which one will take precedence depends on the K+ concentration in the soil solution. The mechanism I produces a high affinity towards K ions and is active when the concentration of available potassium in the soil is limited. Mechanism II is active when the soil solution is abundant in K+, activating a lesser affinity towards the ions.
Plants have potassium channels that enable the transport of this element from the cell wall and intracellular spaces to the living part of the cell. These channels are highly selective and cannot be used by other ions.
Potassium deficiency disrupts many important processes in plants. To compensate for the lack of this element, plants first start to use potassium from the vacuoles, then from the cytoplasm. If the problem continues, chloroplasts and mitochondria will start to degrade, as potassium is crucial for energy synthesis and transport in these organelles. The synthesis of complex compounds like proteins, lipids, and starch reduces, which is why this physiological disorder is highly detrimental to yields.
Another outcome of potassium deficiency is the thinning of the leaf cuticle. The cuticle is a waxy, protective layer that reduces the loss of water and makes it hard for pathogens to infect plants. Potassium-deficient plants are more susceptible to pathogens and dehydration.
Symptoms of potassium deficiency:
- The growth of the plant is slower or it ceased completely
- The plant loses turgor and starts to wilt
- Leaves turn yellow between the veins (chlorosis), starting from the edges
- Leaf edges curl and become brown
- Stems and leaf petioles become thinner and brittle
- Inhibited root growth – plants develop short roots with very few root hairs
Because potassium is a very mobile element, symptoms of deficiency first manifest on older leaves.
Crops that are sensitive to potassium deficiency include cereals, tomatoes, cabbages, potatoes, sugar beet, apples, and berries (raspberries, currants, gooseberries).
Why does K deficiency happen?
Potassium deficiency can have many different causes, but the most common ones usually have to do with too much water – i.e. improper watering and/or high water retention of the soil. Potassium ions are very water-soluble, so improper irrigation can cause leaching and loss of fertilizer. Avoid soaking the soil or dumping too much water in one go while watering to prevent this.
Compact soils are prone to water retention problems that can create unoxygenated patches with every watering. No oxygen means that the roots are drowning, and excess water brings in the rotting pathogens. Both the lack of oxygen and pathogen attacks negatively affect potassium uptake and general plant health.
Soil solution that is a bit too alkaline might be another reason why your plants are showing symptoms of deficiency. Maintaining good fertilization practices and monitoring pH levels
Potassium toxicity is very difficult to notice as the plants don’t display specific symptoms. This type of toxicity is likely to cause nitrogen and calcium deficiency, so look out for symptoms like interveinal chlorosis (leaf tissues turning yellow between the veins), and brownish spots. Too much potassium can also affect the uptake of micronutrients like manganese, zinc, iron, and magnesium.
The good news is that this type of disorder is extremely rare. Plants take up most of the potassium they need during the early phases of their development and are not greedy with it during maturity, so even if there are more than enough available ions in the soil solution the plants won’t have the temptation to grab it.
Fertilizing with potassium
Most commercial potassium fertilizers are various types of potash. The type depends on the elements that are in the mix. Given the nature of potassium salts and different rates of potassium availability, it is often necessary to apply a significant amount of fertilizer to deficient soils to provide enough plant-available ions. Substrates that are prone to potassium fixation, like vermiculite, require even more. This creates problems in outdoor plant cultivation, as the excess fertilizer can wash away into groundwaters, canals, and rivers. Increased concentration of potassium in the water can stimulate algal growth and create an anoxic environment that is very harmful to aquatic organisms.
Indoor plant cultivation doesn’t suffer from these problems, but it is nevertheless important to adjust application rates carefully to provide maximum benefits to plants. We will mention here the most common potash fertilizers, as well as the most favored organic sources of potassium.
Muriate of potash (potassium chloride)
Potassium chloride is the most popular choice of many farmers and plant growers when it comes to potassium fertilization due to its cheapness and efficiency. It is available in three different potassium concentrations – 50%, 41%, and 33%. Although providing a plentiful amount of available potassium, and thus having many beneficial effects on plants, this type of fertilizer should be applied with caution. Its high chloride content can have a toxic effect on certain plants and many soil microbes.
Sulfate of potash (potassium sulfate)
Potassium sulfate (K₂SO₄) is also one of the most common types of K fertilizers in agriculture. It is water-soluble, providing available nutrients soon after application. As its content indicates, potassium sulfate provides the plants with not only a hefty supply of potassium (43%) but also sulfur (17-18%). Another advantage of potassium sulfate is that it can be mixed with other fertilizers without having a toxic effect on plants. This type of fertilizer is especially useful for plants that are sensitive to chlorides, like potatoes, citruses, almonds, and flowers.
Sulfate of potash magnesia
Sulfate of potash magnesia (K2SO4 * MgSO4) is a powerful combination that offers 18-21% potassium, 11% magnesium, and 22% sulfur. It is most commonly applied in chloride-sensitive crops that need a boost of magnesium, but it can be just as beneficial to other plants too. Just like potassium sulfate, sulfate of potash magnesia is also eligible for bulk mixes with other fertilizers.
Potassium carbonate (K2CO3) is another excellent source of potassium for chloride-sensitive plants. It has a very high potassium content of about 56%. As opposed to the previous three types, calcium carbonate can be produced from plants. It is a fast-acting fertilizer that also helps improve the structure of the soil. Liquid formulations are available for hydroponics.
Wood ash is historically probably the oldest potassium fertilizer. The average NPK content of wood ash is about 0-1-3, but it also includes a variety of micronutrients necessary for a balanced plant diet. Depending on the species of wood, the nutrient content can highly vary. It is not recommended to use wood from species that are hyperaccumulators of heavy metals because their ash can be toxic to cultivated plants.
It is important to note that wood ash also has a considerable content of calcium, so its application can change the pH value of the soil.
Kelp is a term that encompasses many different species of brown algae one of the favorite fertilizers among organic farmers as it provides both direct and indirect benefits to plants. Don’t let its low NPK rate fool you – kelp is an amazing organic source of potassium and numerous micronutrients. It contains bioactivators that stimulate microbial activity and degradation of organic compounds, enabling better utilization of nutrients in the soil and improving its structure.
Compared to potassium salts, kelp is a significantly more sustainable resource since it can be grown and harvested. Commercially available kelp fertilizers also include liquid products suitable for hydroponics.
- Sherer, H.W. (2005). Fertilizers and fertilization.
- Potassium for crop production. The University of Minnesota Extension.
- Naylor, L.M., Schmidt E.J. (1986). Agricultural use of wood ash as a fertilizer and liming material.