Plant nutrition

Types of plant nutrition

Autotrophic type of nutrition — independent provision by a plant of its nutrient needs through the uptake of inorganic substances from soil and carbon dioxide from the atmosphere. It is characteristic of most plants. Organisms with an autotrophic type of nutrition also include some bacteria capable of photosynthetically or chemosynthetically assimilating carbon dioxide.

Symbiotrophic type of nutrition — provision by a plant of its nutrient needs at the expense of other organisms (symbionts). Symbiosis developed during evolutionary processes as a form of relationship beneficial for plants. In the symbiotrophic type of nutrition, mutual use of metabolic products for nutrition is observed. The boundaries of symbiosis cannot always be precisely determined, since it is difficult to determine the benefit or harm brought by one organism to another.

Mycotrophic type of nutrition — symbiosis of a higher plant with fungi. The mycorrhiza of the fungus ensures the entry into the higher plant of water and mineral salts and other substances dissolved in it; the fungi use organic compounds synthesized by the higher plant. The significance of fungal mycorrhiza lies in increasing the absorbing surface of plant roots through the fungal mycelium.

Mycorrhizal fungi have been discovered that improve plant nutrition with phosphorus. Further study of this symbiosis and its use in agricultural practice may be of great importance, since it allows reducing the application of phosphorus fertilizers. For example, in a field experiment conducted in Wales, with liming and phosphorus fertilization, the dry-matter yield of clover inoculated with mycorrhiza was 3 times higher, shoot formation increased 2 times, and rhizobium nodules 5 times. Similar data were obtained in Tropical Africa, Brazil, Australia, and Spain on soils poor in available phosphorus.

Bacteriotrophic type of nutrition — symbiosis of higher plants with bacteria. The most striking example is the symbiosis of nodule bacteria with legumes. Under conditions of intensification, chemicalization, and ecologization of agriculture, the importance of the ability of legumes and microorganisms to fix molecular nitrogen from the atmosphere increases. Annually, as a result of symbiosis between bacteria and legumes, 40–106 t of nitrogen is fixed.

Conditions of plant nutrition

Providing optimal nutrition conditions through the use of fertilizers allows more economical use of moisture for creating a unit of yield. The transpiration coefficient can decrease by 15–20%. On the other hand, the economic effectiveness of fertilizers through additional yield increases under the condition of good water supply to plants. Numerous cases of absence of a positive effect of fertilizers on acidic and solonetzic soils have been noted.

For a correct assessment of the effectiveness of fertilizer application, it is necessary to correctly evaluate all factors limiting yield. For example, in northern regions under conditions of sufficient moisture, factors of heat and soil nutrient supply become more important.

In southern regions, especially on ordinary southern chernozems and chestnut soils, characterized by high potential fertility, the limiting factor is more often a lack of moisture.

Kinds of plant nutrition

Main page: Air nutrition of plants

Main page: Mineral nutrition of plants

Air nutrition of plants — carbon nutrition of plants carried out through the assimilation of atmospheric carbon dioxide by green leaves during photosynthesis.

Foliar nutrition of plants — the process of nutrient entry into plants through aboveground organs. The discovery of this process led to the development of foliar top dressing, which allows increasing yield and its quality.

Root nutrition of plants — absorption from soil of water and mineral salts, as well as small amounts of certain organic substances.

According to research, the division into root and air nutrition is arbitrary, since the same substances can be absorbed by both roots and leaves. Thus, carbon dioxide enters the plant through roots to the same extent as through leaves. Sulfur enters the plant through roots in the form of sulfates. Later, using the sulfur radioisotope, the ability of plants to assimilate sulfur oxides from air through leaves was demonstrated.

Root and foliar nutrition of plants are interrelated. Thus, a lack of nutrients in soil delays the formation of organic compounds in leaves, which in turn inhibits plant development.

Chemical elements necessary for plants

According to modern data, plants require 20 elements, and 12 are considered conditionally essential:

• essential (biogenic, or biophilic): hydrogen, sodium, potassium, copper, magnesium, calcium, zinc, boron, carbon, nitrogen, phosphorus, vanadium, oxygen, sulfur, molybdenum, chlorine, iodine, manganese, iron, cobalt;
• conditionally essential: lithium, silver, strontium, cadmium, aluminum, silicon, titanium, lead, chromium, selenium, fluorine, nickel.

Essential elements include elements involved in plant life processes and that cannot be replaced by others. Conditionally essential elements include those that, according to research data, may have a positive effect on the development of some plants.

Micronutrients — elements whose content is expressed in thousandths and hundred-thousandths of a percent. The effectiveness of some micronutrients depends on natural-climatic conditions. For example, a positive effect of zinc, manganese, and iron is observed on neutral soils of the steppe zone, especially on calcareous chernozems, whereas on sod-podzolic soils plants often suffer from their excess. In the forest-steppe and steppe zones, a yield increase from the application of copper microfertilizers is rarely observed, except for maize in some cases. On the contrary, on drained bog peat soils, copper as a micronutrient fertilizer is a necessary condition for obtaining high yields of cereal crops.

Molybdenum almost everywhere has a positive effect on the yield of legumes, which is associated with its participation in the physiological-biochemical processes of atmospheric nitrogen fixation by nodule bacteria. However, the effectiveness of molybdenum under different soil-climatic conditions varies, which is explained by different contents of its mobile forms in soils.

Ultra-trace elements — elements whose content is less than one hundred-thousandth of a percent. Ultra-trace elements include gold, silver, chromium, nickel, tungsten, bromine, uranium, rubidium, cesium, and others. The physiological significance of these elements in plant life is poorly studied.

The division into macro-, micro-, and ultra-trace elements is arbitrary. For example, iron, by content in plants, belongs to macronutrients, but by functions performed, to micronutrients.

The content of micronutrients in different plant organs follows certain patterns. Thus, manganese and molybdenum are more often present in larger amounts in leaves, whereas zinc, boron, cobalt, and copper, with sufficient supply, accumulate in both vegetative and generative organs. For cereal crops, a higher boron content is characteristic in grain, and for legumes, in vegetative organs.

Different biological groups of plants differ in their requirements for optimal concentrations of micronutrients. Thus, maize and tobacco have a greater need for zinc, while cereal crops need manganese and molybdenum.

Plant requirements for nutrients

For their growth, development, and yield formation, plants use organic and mineral substances, which in photosynthesis are transformed into complex organic compounds.

In their composition, plants contain carbon, oxygen, hydrogen, nitrogen, and many other elements. Carbon, oxygen, and hydrogen together account for 94% of dry matter; by element: carbon 45%, oxygen 42%, hydrogen 7%. The remaining 6% of dry mass consists of nitrogen and mineral elements.

The main nutrient is carbon dioxide CO₂. Annually, plants absorb about 20 billion t of carbon from atmospheric air.

Practically all chemical elements have been found in various plant parts; the participation of 27 elements in biochemical processes has been proven, and 15 of them are essential for growth and development.

Humans, through the application of fertilizers, agrotechnologies, reclamation, and various species and varieties, have a significant impact on composition and soil processes.

In extensive agriculture, the only source of mineral substances for plants was their natural reserve in the soil. When natural fertility was depleted, people excluded these lands from cultivation and developed new ones. Abandoned plots restored fertility through natural processes over a long time. The most striking examples of this approach are shifting and fallow farming systems.

The transformational capacity of soil, that is, the ability to supply plants with nutrients and water introduced from outside, plays an important role in intensive farming systems. However, this capacity is also insufficient under conditions of modern intensive agriculture. Moreover, higher demands are placed on the soil’s phytosanitary state and agrotechnological properties. Consequently, improvement of the entire complex of soil properties is required through the use of the newest technologies for expanded reproduction of fertility. The possibility of solving this task lies in the nature of soil itself as a renewable resource. But improper use of soil can lead to loss of fertility.

Forms of compounds in which plants absorb nutrients

Plants assimilate the main amount of nutrients in ionic form through the root system. Amino acids, sugars, and sugar phosphates can be used in small amounts for plant nutrition.

After entering plants, amino acids undergo deamination, and the released ammonia is used in synthetic processes.

Nitrogen is absorbed in the form of nitrate NO₃^- and ammonium NH₄⁺. These ions are formed in soil from organic substances as a result of microbiological ammonification and nitrification. The nitrate form is reduced to ammonia under the action of enzymes.

The ammonium form of nitrogen is used in the substitution reaction of the oxygen atom of the carbonyl group of keto acids to form the corresponding amino acid:

In the nitrogen nutrition of plants, the process of nitrogen fixation of molecular nitrogen under the action of soil microorganisms plays a large role. Important functions in this process are performed by the enzymes nitrogenase, leghemoglobin, compounds of the vitamin B₁₂ group, iron, molybdenum, cobalt, copper, etc.

Sulfur is assimilated by plants in the form of sulfate SO₄^2-. In plants, sulfate is reduced to sulfite SO₃^2- and sulfide S^2-, which, by adding hydrogen, forms sulfhydryl groups (S—H), or, by losing a hydrogen atom, a disulfide (—S—S—) group. Sulfur is part of acetyl coenzyme A and the amino acids cysteine, cystine, and methionine.

Phosphorus is assimilated by plants in the form of phosphates H₂PO₄^-, HPO₄^2-, or PO₄^3-. In plants, phosphorus is part of nucleic acids, phospholipids—compounds responsible for the properties of cell membranes—coenzymes, including pyridine nucleotides and nucleoside phosphates. In energy metabolism, adenosine phosphates are important.

Primary phosphorus metabolism is associated with its involvement in nucleotide synthesis within milliseconds. At exposures up to 10 minutes, phosphorus is detected in nucleic acids. Exposure for more than 3 h, when the metabolic pool of phosphorus acceptors is saturated, shows phosphorus entry into the vacuole in inorganic form. In the absence of air, accumulation of phosphorus acceptors not used in respiratory metabolism occurs, which explains the intensive accumulation of phosphorus in roots under oxygen deficiency.

Chlorine enters plants in the form of chloride Cl^-. In many plants, chlorine can be present in high concentrations without exerting a negative effect. This primarily applies to halophytes—salt-tolerant plants.

Boron and molybdenum enter plants in the form of borates and molybdates.

Calcium, potassium, magnesium, copper, iron, and zinc enter plants in the form of cations; manganese in the form of cations and anions.

A high concentration of potassium ions, up to 50–100 mM, is a characteristic feature of all plant and animal cells. Only at a certain concentration of potassium ions in the cell can protein biosynthesis, photosynthesis, respiration, and synthesis of high-molecular compounds (starch, fats, carbohydrates) proceed normally.

Plant nutrition during different growing periods

Uptake of nutrients in ontogenesis, i.e., during the growing season, occurs unevenly; therefore, the fertilizer system must take into account plant needs in different periods of the life cycle. Insufficient nutrient supply during various periods of plant life leads to reduced yield and worsened quality of plant products.

It is especially important to provide plants with nutrients during the critical period, when nutrient deficiency at that time sharply worsens growth and development. The same applies to the period of maximum uptake.

High sensitivity to deficiency and excess of mineral nutrition is noted in plants during the initial growth period.

The high demand of young plants for mineral nutrition is explained by the high intensity of synthetic processes with a poorly developed root system. Thus, in cereal grasses, initiation and differentiation of reproductive organs begin during the unfolding of the first three to four leaves. Nitrogen deficiency during this period leads to a reduction in the number of spikelets and lower yield. Subsequent normal nutrition does not compensate for the nutrient deficiency at the initial stages of development.

The intensity of nutrient consumption in different crops changes depending on the development period. For example, sugar beet plants in the first month consume nitrogen, phosphorus, and potassium at 2 kg/ha each, and in the second month N 96 kg/ha, P₂O₅ 34 kg/ha, and K₂O 133 kg/ha.

Grasses and sugar beet are distinguished by a long period of nutrient consumption. Hemp, on the contrary, has a short period of intensive consumption—75% of the total amount of nutrients is consumed from the budding phase to the flowering phase.

Spring cereals consume the greatest amount of mineral nutrients during the period from stem elongation to heading. During heading, wheat consumes about 76% of its maximum nitrogen, phosphorus, and potassium; barley about 67%; and oats 47%.

Cereal crops are most demanding for nitrogen nutrition during the formation of the assimilation apparatus and during differentiation of reproductive organs. Sugar beet needs sufficient potassium supply during sugar accumulation.

Cucumber is demanding for nitrogen nutrition during formation of the assimilation apparatus, and for phosphorus before flowering. During fruiting, cucumber places increased requirements on nitrogen and potassium supply.

Strengthening nitrogen and partly phosphorus nutrition during budding and flowering increases the yield of cereals. Increased nitrogen nutrition during leaf mass formation and improved phosphorus-potassium nutrition subsequently increase the yield of root and tuber crops.

The need of most crops for nitrogen nutrition decreases by the beginning of fruit formation; the role of phosphorus and potassium, on the contrary, increases. Overall, the fruit formation period is characterized by reduced nutrient consumption, and vital processes in plants toward the end of the growing season occur mainly through reutilization of accumulated nutrients.

In the fertilizer system, the main fertilizer should provide plant nutrition throughout the entire growing season; therefore, before sowing, all organic and most mineral fertilizers are applied. To provide plants with nutrients in the initial period, a starter fertilizer is applied.

The quantity and quality of the yield can be regulated by top dressings during different growing periods. Top dressings improve plant nutrition during the most critical periods or when a deficiency of a nutrient is detected.

The need for nutrients also changes during the day. Daily periodicity has been noted for almost all plant life processes.

Under artificial nutrition (on nutrient media), the composition, concentration of the nutrient solution, and regime of its use during the growing season matter. For example, by creating a temporary deficiency of nutrients in the external environment during certain growing periods, root system development can be enhanced, and by replacing the nutrient solution with water, temporary starvation can be caused, thereby stimulating tuber formation in potato, fruit set in tomato, and achieving early ripening by this technique.

Daily periodicity of nutrient uptake manifests under variable and constant environmental conditions and has the character of an internal endogenous rhythm. Such regulated daily periodicity of processes allows plants to adapt to changing environmental conditions. Endogenous daily and circadian rhythms under constant artificial conditions tend to fade but are restored under changing conditions. The ability of plants to change circadian rhythm allows increasing their survival.

Plants have annual, seasonal, and daily rhythms. Impulse rhythms with periods from several seconds to hours are also noted. For example, such short-activity rhythms have been noted in the absorbing and excretory activity of roots.

Under artificial crop cultivation, the method of periodic nutrition is of interest, as it allows increasing plant productivity without increasing costs.

References

1. Mineev V. G., Sychev V. G., Gamzikov G. P., et al. (2017). Agrochemistry. Textbook. Ed. by Mineev V. G. Moscow: Publishing House of the VNIIA named after D. N. Pryanishnikov.
2. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. by B. A. Yagodin. Moscow: Kolos.