Potassium, a chemical element, along with nitrogen and phosphorus, is the most important element of plant nutrition. Attempts to replace it with closely related elements (sodium, lithium, rubidium) proved unsuccessful.
The need for potassium for plants was first suggested by Sosur in 1804 on the basis of chemical analysis of plant ash, in which potassium was always present. Later, Liebich concluded that potassium fertilizer was necessary. The first experimental confirmation of the necessity of potassium for plants was obtained by Salm-Gorstmar in 1846.
Potassium content in the plant organism
Potassium in plants is mainly concentrated in the cytoplasm and cell vacuoles in ionic form. It is not part of organic compounds, but is involved in photosynthesis. About 80% of potassium is in plant cell sap and can be easily washed out by water, such as rain and especially from old leaves, while the remaining 20% is retained in the exchange-absorbed state by colloids of cytoplasm. It enhances the hydration of cytoplasmic colloids, increasing the water-holding capacity and drought tolerance of plants. About 1% is absorbed by mitochondria.
During the daytime, potassium, while remaining mobile, is retained in plant cells. At night, when photosynthesis stops, some potassium may be excreted through the root system, but when the first rays of the sun appear, it is absorbed by the plant again.
Young plant organs contain 3-5 times more potassium than old ones: its content is higher in organs and tissues, where metabolic processes and cell division are intense. Therefore, potassium is also called the element of youth. Potassium is contained in large amounts in pollen. For example, pollen ash from corn contains up to 35.5% of potassium, while calcium, magnesium, sulfur and phosphorus account for 24.7% in total. The mobility of potassium determines its reutilization by moving it from old leaves to young ones. Therefore, its distribution in plants is characterized by a basipeptal concentration gradient, that is, its content in leaves and stem per unit of dry matter increases from bottom to top.
In the cell sap, the content of potassium is much higher than other cations and exceeds the concentration in the soil solution by 100-1000 times.
Unlike nitrogen and phosphorus, potassium is concentrated in vegetative organs and not in reproductive ones. For example, in cereal straw potassium is 2 times more, and in corn stalks – 5 times more than in grain. Therefore, the removal of potassium with the nonreproductive part of the crop is usually greater than with the marketable part, with the exception of leguminous plants.
Potassium content in plants may vary depending on climatic conditions, the applied agricultural techniques, and soil fertility.
Grain crops contain 15% of the total amount of potassium in the crop, and straw contains 85%. Potato tubers contain up to 95% and haulm up to 5% of the total potassium content.
Таблица. Average content K2O in the yield of some crops, % on absolutely dry matter (by Peterburgsky)
| Winter cereals | Grain | Potatoes | Tubers | ||
| Straw | Above-ground part | ||||
| Spring cereals | Grain | White cabbage | Heads | ||
| Straw | Carrots | Roots | |||
| Corn | Grain | Cucumber | Fruits | ||
| Stems | Tomato | Fruits | |||
| Peas | Grain | Flax | Straw | ||
| Straw | Cottonwood | Fiber | |||
| Sugar beet | Roots | Meadow clover | Hay | ||
| Above-ground part | Lucerne | Hay | |||
| Fodder beet | Roots | Vicia | Hay | ||
| Above-ground part | Timothy | Hay |
Importance of potassium
Potassium regulates photosynthesis, increases outflow of carbohydrates from leaf lamina to other organs, participates in synthesis of sugars and high-molecular weight carbohydrates – starch, cellulose, pectin substances, xylans.
Potassium promotes the accumulation of monosaccharides in fruit and vegetable crops, sucrose in root crops, starch in potatoes, thickens cell walls of cereal straws, increases resistance to lodging, in flax and hemp it improves fiber quality.
Table. Effect of potassium on the content of reducing sugars, sucrose and starch in tomato leaves and petioles, % (by Bagaev)
| Reducing sugars | ||||
| Sucrose | ||||
| Starch and dextrins | ||||
By accumulating carbohydrates in plant cells, potassium increases the osmotic pressure of the cell sap, thereby increasing the cold tolerance and frost tolerance of plants.
Potassium accumulation in chloroplasts and mitochondria helps to stabilize their structure and formation of ATP. It increases the hydrophilicity of protoplasm colloids and decreases transpiration, which helps plants to better tolerate short-term droughts.
Potassium is involved in protein synthesis and metabolism. If it is lacking, synthesis is reduced with the simultaneous breakdown of old protein molecules. Amino acids accumulate in plants. Optimized potassium nutrition leads to an increase in the proportion of protein in wheat plants. Asparagine and glutamine synthesis is enhanced. The positive effect of potassium on protein synthesis is associated with its effect on the accumulation and transformation of carbohydrates (carbohydrates during respiration form keto acids, from which amino acids are synthesized), as well as with an increase in the enzymatic activity of protein synthesis.
Potassium catalyzes the synthesis of vitamins thiamin and riboflavin, regulates the functioning of the stomata closing cells of leaves.
Potassium is absorbed by plants as a cation and in this form remains in cells and is the main counterion of negatively charged cell anions. Potassium creates an electrical potential difference between the cell and the environment.
By participating in the most important biochemical processes, potassium increases resistance to various diseases during the growing season and in the post-harvest period, improves the storability of fruits and vegetables.
The critical period of potassium consumption by plants occurs in the first 15 days after sprouting. The period of maximum consumption often coincides with the period of intensive growth of biological mass. In some crops, such as flax, for example, the intake of potassium stops at the phase of full flowering or at flowering – the beginning of ripening, as in cereals and legumes. In other crops, the intake is more protracted and occurs throughout the growing season, as in potatoes, sugar beets, and cabbage.
In areas where the effect of potassium fertilizers is the most effective, their use provides an increase in yield for each kilogram of potassium fertilizer: 2-3 kg of grain, 20-33 kg of potatoes, 35-40 kg of sugar beets, 1-1.5 kg of flax fiber, 20-33 kg of hay of sown grass and 8-18 kg of hay of meadow grass.
Potassium deficiency
Potassium deficiency leads to decreased enzymatic activity, impaired carbohydrate and protein metabolism, and increased consumption of carbohydrates for respiration.
As a result, plant productivity and product quality decrease. Grain crops form puny grains, reduced germination and seed viability. Bread straws often lodge due to reduced strength. The starch content of potato tubers, sucrose content of sugar beet roots, pectin content of fruits and berries, and vitamins in products are reduced. The incidence of diseases increases. Fewer storability during storage.
Outwardly, potassium starvation is manifested on leaves of the lower layer: they turn yellow prematurely, starting from the edges; subsequently, the edges turn brown, then die off and are destroyed, so they look like burnt. This phenomenon is called “marginal burn”. Potassium deficiency leads to decreased turgor, and leaves wilt and droop. Most often, the lack of potassium occurs during intensive growth (in the middle of the growing season), when its content in the cells decreases by 3-5 times the norm.
Potassium deficiency is most strongly reacted by potassium-loving crops.
An excess of potassium also negatively affects growth and development. It manifests itself in the appearance of pale mosaic spots between leaf veins, with time they turn brown, and leaves fall off.
Potassium cycling and balance in agriculture
Potassium cycling in biocenoses is intense. The content of potassium in the biomass of biocenoses varies from 20 ka/ha for deserts to 2000 kg/ha for oak forests.
The closed cycle of the nutrient cycle in natural biocenoses due to the accumulating activity of plants leads to the accumulation of potassium within the root layer and the gradual enrichment of the upper horizons with this element.
In agrocenoses, the circulation and balance of potassium is mainly influenced by economic activities: the provision of fertilizers, the specialization of farms.
Gross reserves of potassium in soils are 5-50 times higher than the reserves of nitrogen and phosphorus. D.N. Pryanishnikov when estimating the balance of potassium for the country as a whole allowed its deficit of 20-22 kg/ha per year.
The main expense item of the balance of potassium in agriculture is the economic removal with the products.
With a crop of plants can be taken out annually from 40 to 310 kg/ha of potassium. These figures are calculated, in particular for cereals, for average yields, with increasing productivity they will naturally increase.
Table. The content of potassium in the yield of the most important crops[1] Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.
| Cereals | |||
| Buckwheat | |||
| Flax and hemp | |||
| Sunflower | |||
| Potatoes | |||
| Sugar beet | |||
| Cabbage | |||
| Leguminous | |||
| Clover (hay) | |||
| Alfalfa (hay) | |||
| Meadow grasses (hay) | |||
| Timothy (hay) | |||
For an objective assessment of the balance of potassium it is necessary to take into account the distribution of removal between marketable and non-marketable parts of the product. For example, the grain of wheat contains 15% of the total economic removal of potassium, and straw contains 85% of potassium. The less potassium is contained in the marketable, alienated from the farm part of the crop and more in the non-marketable, remaining in the field or farm, including feed, the less potassium is alienated from the on-farm cycle. Thus, farm specialization determines the on-farm potassium balance.
Part of potassium can be lost from the root layer of soil as a result of infiltration: on light soils about 5%, on heavy soils – 2% of the amount applied with fertilizers. The intensity of leaching is influenced by the granulometric composition of the soil, water regime, fertilizer doses, biological features of crops.
Part of potassium can be lost through water and wind erosion. According to average data, this value is 4-8 kg/ha. It is considered that the expenditure items of losses from erosion are compensated by the receipt with seeds (about 2 kg/ha) and precipitation (2-6 kg/ha).
Some part of exchangeable potassium can pass in the soil in the fixed (non-exchangeable-absorbed) state, becoming inaccessible to plants. Subsoil layers are also involved in supplying potassium to plants, thus reducing the consumption of potassium in the arable layer. Thus, in experiments on sod-podzolic soils, sunflower and lupine on average about 32% of the total potassium removal was consumed from the subsoil horizons.
Sources
Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.
Evtefeev Y.V., Kazantsev G.M., Bases of agronomy: textbook. – M.: FORUM, 2013. – 368 p.: ill.
Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – Moscow: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.