Phosphorus is a chemical element, known in several modifications: white, red, black and metallic, which are solid substances of corresponding color. It was first isolated by the Hamburg pharmacist Henning Brandt in 1669 from. Its role in plant life was first mentioned by Dendonald in 1795. The Swiss naturalist Sossure found calcium phosphate in the ash of all the plants he analyzed a little later.
Phosphorus content in plant organisms
Phosphorus consumption by plants is less than nitrogen; it accounts for 0.2-1.0% of dry matter mass. The distribution of phosphorus in plants is the same as that of nitrogen: most of it is accumulated in reproductive organs and organs, where the processes of synthesis of organic matter occur intensively. Nitrogen and phosphorus in plant organisms are characterized by a fairly stable ratio in the yield.
The ratio of nitrogen and phosphorus for grain, roots, tubers, and hay is approximately 1:0.3, whereas between nitrogen and potassium it can vary from 1:0.6 to 1:1.4. In vegetation experiments, changing the ratio of nitrogen and phosphorus in nutrient media can achieve different ratios of these elements in plants, but under field conditions this ratio is stable due to the property of soil to regulate plant nutrition.
Table. Average ratio of the main nutrients in the yield of plants, %[1]Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.
| Winter wheat, grain | |||
| Sugar beet, roots | |||
| Potatoes, tubers | |||
| Meadow clover, hay |
Phosphorus in plants is represented in the mineral (5-15%) and organic (85-95%) forms. Mineral phosphorus compounds are phosphates of potassium, calcium, magnesium and ammonium. Organic compounds: nucleic acids, nucleoproteins and phosphatoproteins, adenosine phosphates, sucrose phosphates, phosphatides, phytins.
Nucleic acids, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), are high-molecular-weight compounds shaped as helical strands (25 A in diameter) and composed of combinations of nucleotides. Nucleotides include nitrogenous bases, sugars, and phosphoric acid. The carbohydrate component of RNA is ribose; in DNA it is deoxyribose.
Connecting together in various combinations, nucleotides form nucleic acids. A single nucleic acid molecule can have thousands of combinations of nucleotides joined together by acidic phosphoric acid residues. The combinations of nucleotides in nucleic acids form a kind of code that records the hereditary properties of an organism. Thanks to an almost infinite number of combinations of nucleotides, a huge variety of species of all living things is created.
DNA is the molecule that stores all the information about the genetic properties of the organism, while RNA is directly involved in the synthesis of proteins. Phosphorus in nucleic acids accounts for about 20%. Nucleic acid molecules are present in all plant tissues and organs, in every plant cell. In plant leaves and stems nucleic acids account for 0.1-1.0% of dry weight, more in young leaves and shoot growth points, less in old leaves and stems. The highest content of nucleic acids is in pollen, seed germ, and root tips.
Nucleic acids can form complexes with proteins – nucleinoproteins that are part of cell nuclei.
Phosphorus is involved in energy metabolism of plant cells due to adenosine phosphates that can release energy during hydrolysis. Adenosine monophosphate (AMP), adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are distinguished by the number of phosphoric acid residues. An ATP molecule consists of a purine base (adenine), a sugar (ribose), and three orthophosphoric acid residues:
Energy-intensive phosphate macroergic bonds (wavy line) contain 50280 J of energy, and 31,425 J are released when they are broken. One acidic residue of phosphoric acid is lost, and ATP is converted to ADP. ADP can also participate in this circuit with the formation of AMP.
Adenosine phosphate compounds in the plant cell are energy accumulators that are used in many vital processes of the cell, such as biosynthesis of proteins, fats, carbohydrates, amino acids and other compounds. The formation of ATP in plants occurs through respiration processes. In addition to adenosine phosphate compounds, other macroergic compounds that include phosphorus are known.
Phosphatides, or phospholipids, are also found in any plant cell. They are esters of glycerol, high molecular weight fatty acids, and phosphoric acid. They are part of phospholipid membranes, regulate the permeability of cell organelles and plasmalemma. For example, the cytoplasm of plant cells contains lecithin – phosphatide, a fat-like substance derived from diglyceride-phosphoric acid.
Sucrose phosphates, or phosphorous esters of sugars, are present in plant tissues. More than ten such compounds are known. They are involved in plant respiration, conversion of simple carbohydrates into complex carbohydrates during photosynthesis, and mutual transformations. Phosphorylation is a reaction for the formation of saccharophosphates. The content of saccharophosphates in plants ranges from 0.1 to 1.0% of dry weight, depending on the age and nutritional conditions.
Phytin is calcium-magnesium salt of inositphosphoric acid. Phytin ranks first among other organophosphorus compounds by its content in plants.
Table. Forms of phosphorous compounds in plants, % P2O5 to dry matter[2]Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. — М.: Колос, 2002. — 584 с.: ил.
| Wheat, grain | |||||||||
| Clover, hay | |||||||||
Phytin is contained in young organs and tissues of plants, most of all in seeds. For example, in the seeds of legumes and oilseeds it accounts for 1-2% of dry weight, in the seeds of cereals – 0.5-1.0%. In seeds, phytin is a reserve of phosphorus for germination and emergence of young seedlings.
Most of it in plants is concentrated in reproductive organs and young growing parts. Phosphorus accelerates the formation of the root system. Maximum consumption of phosphorus falls on the first phases of growth and development. Later it is easily reutilized, i.e. it moves from old tissues to young tissues and is reused.
Importance of phosphorus
Phosphorus contributes to:
- economical consumption of moisture by plants;
- increased drought tolerance;
- improvement of carbohydrate metabolism, which contributes to sugar content of beets and starchiness of potatoes);
- increasing the content of sugars in the bush nodes of winter crops and tissues of perennial grasses, which increases frost-resistance and winter-hardiness;
- resistance to lodging of grain cereals;
- resistance to diseases;
- processes of flower fertilization, ovary formation, fruit formation and ripening.
Spinning crops produce a long thin and strong fiber.
Excess phosphorus leads to premature development and early fruiting, thereby reducing yields.
Lack of phosphorus causes slower growth and development of plants, reduced synthesis of protein and sugars, leaves form small and narrow, delayed flowering and fruit ripening. The lower leaves become dark green in color with a red-purple, purple, bluish or bronze tinge, the edges are curved upward.
There is a relationship between nitrogen and phosphorus nutrition of plants: lack of phosphorus slows down protein synthesis in tissues, while increasing nitrate content. This occurs most often in unbalanced plant nutrition, i.e. excessive doses of nitrogen.
Plants are most sensitive to phosphorus deficiency at a young age, when underdeveloped root system does not have sufficient absorption capacity. The deficit during this period can not be compensated later, even with optimal phosphorus nutrition.
The maximum absorption of phosphorus occurs during the period of intensive growth of vegetative mass.
Sources of phosphorus nutrition for plants
In natural conditions, the source of phosphorus nutrition of plants are salts of orthophosphoric acid – phosphates, as well as pyro-, poly- and metaphosphates after hydrolysis. The latter are not present in the soil, but can be a part of complex fertilizers.
Orthophosphoric acid during hydrolysis dissociates into anions H2PO4–, HPO42- and PO43-. According to calculations by B.P. Nikolsky, under conditions of weakly acidic soil reaction, the most common and available is H2PO4–, to a lesser extent – HPO42-, PO43- is almost not involved in the nutrition of most plants, except for lupin and buckwheat, to a lesser extent mustard, pea, melilot, sainfoin and hemp.
All salts of orthophosphoric acid and monovalent cations (NH4+, Na+, K+) found in the soil are well soluble in water. Also soluble are one-substituted salts of divalent calcium cations Ca(H2PO4)2 and magnesium Mg(H2PO4)2. Two-substituted salts of calcium CaНРO4 and magnesium MgНРO4 are poorly soluble in water, but soluble in weak acids, including acidic root excretions and organic acids formed during the activity of microorganisms. Therefore, dihydroorthophosphates (one-substituted) and hydroorthophosphates (two-substituted) are a source of phosphorus for plants.
Table. Forms of phosphorous compounds in plants, % P2O5 to dry matter[3]Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. — М.: Колос, 2002. — 584 с.: ил.
Three-substituted phosphates (orthophosphates) of divalent cations are insoluble in water and inaccessible to most. However, freshly deposited tri-substituted calcium phosphate, formed from mono- and divalent phosphates during chemical absorption by the soil, is slightly better absorbed by plants in its amorphous state. As they age, these amorphous triphosphates change to crystalline forms and lose their availability to plants.
Trivalent cations of orthophosphoric acid [AlPO4, Al(OH)3PO4, FePO4, Fe2(OH)3PO4, etc.] are not available to plants, account for most of the mineral phosphates of acidic soils.
As a source of phosphorus nutrition of plants is phosphate in the exchange-absorbed (adsorbed) soil colloids state. These anions are displaced by anions of mineral and organic acids (citric, malic, oxalic acids). The soil in the solid phase-solution system contains anions in sufficient quantities. In the process of breathing, roots emit carbon dioxide, which acidifies the reaction and forms hydrogen carbonate ions when dissolved. The latter displace the adsorbed phosphorus in solution from the PPC.
It has been experimentally confirmed that exchange-absorbed phosphoric acid anions are close to water-soluble phosphates in terms of availability to plants. However, the amount of the latter in the soil is small, so adsorbed phosphates are of great importance in the balance of phosphorus nutrition of plants.
Some plants have the ability to assimilate phosphate-ion organic compounds, such as phytin and glycerophosphates, due to root excretions containing the enzyme phosphatase. Under the action of phosphatase, the phosphoric acid anion is detached from the organic compounds and absorbed by the plant. Such plants include peas, corn, and beans. Phosphatase activity increases under conditions of phosphorus deficiency.
During phylogenesis, plants have adapted to nutrition from solutions with very low concentrations. In the studies of M.K. Domontovich, all experimental plants (oats, corn, wheat, peas, mustard and buckwheat) could absorb phosphorus from solutions with concentrations from 0.01 to 0.03 mg P2O5 per 1 L. It is generally accepted that the optimal concentration of phosphorus for plant nutrition is 1 mg/L.
Phosphorus absorbed by the roots is quickly included in the synthesis of complex organic compounds directly in the roots. In experiments with pumpkin, 30% of labeled phosphorus 32P was found in the composition of organic compounds after 30 minutes of absorption, and after 3-5 minutes – 70% of absorbed phosphorus. Phosphorus is primarily consumed for nucleotide synthesis. To transport phosphorus to other parts of the plant, phosphorus is again transformed into mineral compounds.
Phosphorus cycle and balance in agriculture
In natural biocenoses, phosphorus has no sources of recharge in the soil, while at the same time, its natural reserves in soils are significant. According to A.V. Sokolov, the one-meter layer of soil contains from 10 to 35 t/ha of various phosphorus compounds. Due to the fact that the roots of many field crops penetrate to a depth of 0.9 to 2.8 m, and perennial grasses – to 3-5 m, the mobile forms can be used by plants. The consumption of P2O5 by plants in subsoil horizons is experimentally confirmed, which can account for up to 30% of the total removal with the crop.
Phosphorus removal with agricultural products averages 25-40 kg/ha per year. Thus, the natural reserves in the soil significantly exceed the removal.
In natural biocenoses with their characteristic closed cycle of nutrients, phosphorus slowly accumulates in the upper layers of the soil due to its redistribution from the activities of plants.
Table. Content of gross phosphorus and organic phosphate in various soils, mg/100 g (according to generalized data of Ginzburg)
A feature of the phosphorus cycle in agrocenoses is that most of it is concentrated in the harvest, for example, up to 2/3 of all phosphorus absorbed by plants is concentrated in grain, the remaining 1/3 in the non-commodity part – straw. Given that only a small part of the grain remains in the farm, the alienation of phosphorus from farms is significant. In addition, phosphorus is also contained in livestock products, which should also be taken into account in the external balance.
Table. The content of phosphorus in the harvest[4]Fundamentals of agronomy: textbook / Yu.V. Evtefeev, G.M. Kazantsev. - MOSCOW: FORUM, 2013. - 368 p.: ill..
In agrocenoses, the phosphorus cycle is relatively easier than the nitrogen cycle.
Phosphorus losses can be associated with soil erosion in the form of losses of the solid part with wind erosion and runoff with water erosion. On average, losses can be up to 11 kg/ha per year. On soils of medium and heavy granulometric composition infiltration as a rule does not exceed 1 kg/ha per year, on light and peaty soils – up to 3-5 kg/ha.
An insignificant amount of phosphorus enters the soil with seeds of plants, atmospheric precipitation and dust.
For these reasons, compensation of expenditure items of the balance of phosphorus in agriculture is possible through the use of organic and mineral fertilizers.
In the 70-80s, a positive balance of phosphorus was formed in the USSR: in many regions there was an increase in its content in the soil. Thus, in the Central region of the Nonchernozem zone the amount of mobile phosphate in the soil increased from 5.3 to 12.5 mg/100 g, in the Moscow region – from 6.4 to 20.6 mg/100 g.
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.