Importance of potassium for plants and its content in soil
Value of potassium for plants
Potassium is a major nutrient, along with nitrogen and phosphorus. The function of potassium in plants, as well as other elements necessary for them, is strictly specific.
The first assumptions about the need for potassium to plants were expressed by Saussure in 1804, based on an analysis of plant ash, in which potassium was always present. Then Liebig concluded that it was necessary to use potash fertilizers. The first experimental data on the absolute need of potassium for plants were obtained by Salm-Horstmar in 1846.
Unlike nitrogen and phosphorus, potassium is not included in the composition of organic compounds in plants, but is found in plant cells in ionic form in the form of soluble salts in cell sap and partially in the form of transient complexes with cytoplasmic colloids.
Potassium is much more in young viable parts and organs of plants than in old ones. About 80% of potassium is in the cell sap and can be easily washed out with water (rain and watering). Young plant organs contain potassium 3-5 times more than the old ones: it is more in those organs and tissues where metabolic processes and cell division are intensive. With a lack of potassium in the nutrient medium, its outflow from the oldest organs and tissues to young growing organs occurs, where it is subjected to repeated use (recycling).
The physiological functions of potassium in the plant body are diverse. It has a positive effect on the physical state of the cytoplasm colloids, increases their water content, swelling and viscosity, which is of great importance for normal metabolism in cells, as well as for increasing plant resistance to drought. With a lack of potassium and increased transpiration, plants lose turgor and fade more quickly.
Potassium has a positive effect on the intensity of photosynthesis, oxidative processes and the formation of organic acids in plants, and is involved in carbohydrate and nitrogen metabolism. With a lack of potassium in the plant, protein synthesis is inhibited, as a result, the entire nitrogen metabolism is disturbed.
The lack of potassium is particularly pronounced when the plants are fed with ammonium nitrogen. The introduction of high standards of ammonia nitrogen with a deficiency of potassium leads to the accumulation in plants of a large amount of not processed ammonia, which has a detrimental effect on the plant. With a lack of potassium, the conversion of simple carbohydrates into more complex (oligo- and polysaccharides) is delayed.
Potassium increases the activity of enzymes involved in carbohydrate metabolism, in particular sucrase and amylase. This explains the positive effect of potash fertilizers on the accumulation of starch in tubers of potatoes, sugar in sugar beets and other root crops. Under the influence of potassium, the frost resistance of plants increases, which is associated with a high content of sugars and an increase in osmotic pressure in the cells.
With sufficient potassium nutrition, plants are more resistant to various diseases, for example, in cereal grains – to powdery mildew and rust, in vegetable crops, potatoes and root crops – to pathogens of rot. Significantly improves the keeping quality of fruits and vegetables. Potassium has a positive effect on the strength of the stems and plant resistance to lodging, on the yield and quality of flax and hemp fiber.
Potassium content in plant cells is significantly higher than other cations. The intracellular concentration of potassium in plants is 100-1000 times higher than its concentration in the soil solution.
A critical period in potassium consumption by plants is in the first 15 days after germination. The period of maximum consumption coincides with the period of intensive biomass growth. Intake of potassium ends in flax in the flowering phase, in cereals and legumes – before flowering, milky ripeness. In other cultures, the period of receipt of potassium in plants is more extended, and passes during the entire growing season (potatoes, sugar beets, cabbage).
The relative content of elements of mineral nutrition in the main and by-products of various agricultural crops is determined primarily by their species characteristics, but also depends on the variety and growing conditions. The content of nitrogen and phosphorus is significantly higher in the economically valuable part of the crop – grain, root and tuber crops than in straw and tops. Potassium is more contained in straw and leaves than in the commodity part of the crop.
Calico-loving crops (sugar and fodder beets, potatoes, cabbage, and corn) consume this element much more than grain and leguminous crops, flax, and grass. Also consumes a lot of potassium sunflower.
The total removal of potassium from crop yields varies greatly. This is due to the peculiarities of the chemical composition of plants, fluctuations in the level of yield formation and changes in its structure.
Lack of potassium causes many metabolic disorders in plants. As a result, the productivity of the plant decreases, the quality of production decreases, the plants start to be more often affected by various diseases.
External signs of potassium starvation are manifested in the browning of the edges of leaf blades – `the edge fuse`. The edges and tips of the leaves acquire a “burnt” view; small rusty spots appear on the plates. With a lack of potassium, the cells grow unevenly, which causes corrugation, dome-shaped twisting of the leaves. In the potato on the leaves also appears characteristic bronze plaque. Especially often the lack of potassium is manifested when growing more demanding of this element potatoes, roots, cabbage, silage crops and perennial grasses. Cereals are less sensitive to potassium deficiency. But even with acute potassium deficiency, they do not grow well, the interstices of the stems are shortened, and the leaves, especially the lower ones, wither even with a sufficient amount of moisture in the soil.
Excessive potassium nutrition of plants also negatively affects their growth and development. It manifests itself in the occurrence between the veins of the leaves of pale musical spots, which eventually turn brown, and then the leaves fall off. Therefore, an optimally developed potassium nutrition plan will greatly affect the productivity and quality of the crop.
Potassium in the soil
The content of potassium (K2O) in various soils ranges from 0.5 to 3% and depends on the mechanical composition. More potassium is found in the clay fraction of the soil. Therefore, heavy clay and loamy soils are richer in potassium (2-3%) than sand and sandy (1.5-2%). Peaty soils very poor in potassium (0.03-0.05%). In most loamy soils, potassium contains 2-2.5%, that is, significantly more than nitrogen and phosphorus. Total (gross) potassium contains:
- in the composition of primary and secondary minerals (not less than 91%),
- in exchange-absorbed (0.5-2%) and non-exchange-absorbed (up to 9%) states,
- in the form of salts of soil solution (0.05-0.2%),
- in the composition of crop residues, microorganisms (up to 0.05%).
According to the degree of mobility and accessibility for plants, potassium compounds contained in the soil can be divided into the following main forms.
Unformed absorbed (fixed) potassium
It is part of strong aluminosilicate minerals, mainly feldspar (orthoclase, etc.) and mica (muscovite, biotite, etc.). Potassium feldspar is not readily available for plants. However, under the influence of water and carbon dioxide dissolved in it, changes in the temperature of the medium, and the activity of soil microorganisms, these minerals are gradually decomposed to form soluble potassium salts. Potassium muscovite and biotite available to plants.
Absorbed by soil colloids, accounts for 0.8-1.5% of the total potassium content in the soil. He plays the main role in plant nutrition. The good availability of exchangeable potassium for plants is due to its ability, when it is exchanged with other cations, to pass easily into the solution from which it is absorbed by plants. When potassium is absorbed by plants from a solution, its new portions are transferred from the absorbed state to the soil solution. As the use of exchangeable potassium, this process slows down more and more, and the residual potassium is increasingly held in the absorbed state. The content of exchangeable potassium can serve as an indicator of the degree of soil availability of assimilable potassium. The usual and powerful black soils and serozems are richer in exchangeable potassium than sod-podzolic soils, especially sandy and sandy.
Represented by various salts, soluble in soil moisture (nitrates, phosphates, sulfates, chlorides, carbonates), which are directly absorbed by plants. Its content in the soil is usually insignificant (about 1/10 of the exchange), since the potassium from the solution immediately goes into the absorbed state and is consumed by plants. In some soils, water-soluble potassium (and also potassium applied to the soil as a fertilizer) can be absorbed in non-exchanged form, as a result, its availability for plants is reduced. Non-exchange fixation of potassium, as well as ammonium ion, is most pronounced on chernozem and sierozem, especially when alternately moistened and dried.
There is a dynamic equilibrium between the various forms of potassium in the soil. The amount of water-soluble forms of potassium can be replenished by exchange-absorbed, a decrease which over time can be compensated for by a fixed form. It should be borne in mind that when water-soluble potash fertilizers are applied, their transformation can proceed in the opposite direction. Part of the potassium is lost from the root layer due to infiltration (impregnation and seepage process) from 2% in heavy and up to 5% on light soils from the amount of fertilizer applied. Losses can also occur from water or wind erosion.
So, the main condition for maintaining an optimal balance of nutrients in the soil, including potassium, is cost recovery through the use of mineral and organic fertilizers.