The effect of moisture regime and light intensity on plant mineral nutrition

The water content in the soil is an important factor affecting the intensity of root growth, as well as the availability and uptake of mineral elements by plants. Its influence on the intake of nutrients into plants is due to: 1) an intensification of physiological processes in plants, as normal hydration of tissues promotes improved metabolism, which largely determines the uptake of nutrients by roots; 2) an increase in root mass and, consequently, an increase in their total absorbing surface area; 3) the versatility of water as a medium for the diffusion of ions from the soil solution and the soil-absorbing complex to the roots. Excessively low or high water content in the soil has such a strong influence on root growth that it is difficult to distinguish the direct effect on mineral uptake from the indirect effect caused by changes in the rate of root growth and differentiation. In water-saturated soils, denitrification occurs, while in dry soils, microbial activity is reduced. In waterlogged soils, the concentration of reduced forms of iron and manganese increases sharply and may reach levels toxic to plants, while in dry soils, conversely, plants exhibit symptoms of deficiency in these elements. Plants growing in dry soil, as a rule, experience phosphorus and potassium deficiency because these elements are bound by the soil, and furthermore, root growth is reduced, decreasing their absorbing and uptake surface area. The degree of soil moisture saturation influences the accumulation of ions by roots by changing the quantity, composition, and mobility of the soil solution. The reduction in the uptake of mineral salt ions by roots at low soil moisture levels is caused by the disruption of the integrity of the soil water film. This sharply reduces the solvent capacity of soil water. As a result of the decrease in soil moisture levels, the completeness of cation accumulation around soil particles is reduced. A rational agrotechnical method for improving the water regime of the soil is irrigation.

One of the conditions for the existence of a plant organism is light. Despite the fact that plant roots themselves are not exposed to light, the periodic alternation of day and night affects them through the metabolism of the aerial organs. Changes in illumination are accompanied by consistent shifts in the physiological activity of roots and their uptake of nutrients from the soil solution. As a rule, light has a positive effect on these processes. It acts on the ion uptake processes in plants by influencing transpiration, cell membrane permeability, and photosynthetic activity. Furthermore, plant cells are capable of accumulating ions against a concentration gradient, using light as an energy source for this purpose. When plants are shaded, conversely, it is possible for ions to be released from a bound state and exit the cells into the nutrient medium.

A decrease in illumination also has a negative impact on the intake of nitrogen into plants. Moreover, light has a stronger effect on the uptake of ammonium than nitrates by plants. The latter, upon entering the plant through the roots, must first be reduced. Consequently, the stronger assimilation of ammonium nitrogen is explained by lower energy costs than the assimilation of nitrates.

When plants are shaded or illumination decreases, usually in cloudy weather, the intake and assimilation of phosphorus by them decrease sharply. This process is influenced not only by light intensity but also by the spectral composition of light.

Under the influence of short-wave (380-580 nm) radiation, these processes are activated more significantly than by long-wave (580-730 nm) radiation, which is explained by the special role of phosphorus in energy processes. Furthermore, it is precisely through phosphorus metabolism that the interconnection between carbon and mineral nutrition is realized.

Shading of plants, although to a lesser extent, still suppresses the intake of potassium by plants. Therefore, plants are particularly in need of this element in cloudy weather, which is manifested in the beneficial effect of high doses of potash fertilizers in years with increased cloud cover. In this case, the possibility is not excluded that potassium, possessing radioactivity, may participate in energy processes in plants and thereby compensate for the lack of illumination in cloudy weather.

Fertilizer application in field conditions should be carried out taking into account the intensity of illumination:

• increased doses of nitrogen and phosphorus are advisable only under conditions of high light intensity;
• increased doses of potassium are advisable when there is a lack of light.

Given the great influence of lighting conditions on plant nutrition, there is a need to develop methods for regulating them in natural settings. Proper selection of sowing methods and the formation of an optimal plant stand allow for the improvement of the light regime. The direction of the rows is also of significant importance.

Recommendations for row orientation depending on the zone:

Using west-to-east row orientation in the south-eastern zone reduces the harmful effect of excessive insolation on plants at midday. The illumination of crops can be improved by regulating not only plant density but also their growth. Creating short-stemmed cultivars is promising in this regard. Optimal leaf canopy sizes favor better plant illumination, more complete absorption of solar radiation energy, and an increase in the physiological and metabolic activity of the roots.

Soil air regime: how carbon dioxide and oxygen control nutrition

The efficiency of root nutrition depends directly on the composition of soil air and the atmosphere near the ground. The concentration of carbon dioxide around the leaves regulates how roots will absorb essential nutrients. It is important for an agronomist to consider this factor when choosing fertilizer forms, as the plant response to ammonium and nitrate forms will differ in various air environments.

• Optimal air temperature — 25–30 °C
• Soil for root growth — 1–2 °C cooler than the air
• Minimum O₂ for root growth — 10%
• Critical O₂ level (growth arrest) — 5%
• Optimal CO₂ in soil — no more than 1%
• Limit CO₂ in soil — 15%

Nutrient uptake changes depending on the saturation of air with carbon dioxide. There is a direct relationship between the CO₂ concentration in the air and the absorption of ammonium (NH₄⁺) and hydrogen ions (H⁺): the more carbon dioxide, the more active the assimilation. For nitrates (NO₃⁻) and phosphates (HPO₄²⁻), the relationship is inverse — an excess of carbon dioxide in the air blocks their uptake by the plant.

For normal development, roots require a constant influx of oxygen. In most crops, the growth of the root system is noticeably inhibited if the oxygen content in soil air drops below 10%. When the O₂ concentration decreases to 5% or less, root development stops completely. Exceptions are only rice and plants possessing specialized air-conducting tissue — aerenchyma, which are capable of developing under anaerobic conditions.

The concentration of carbon dioxide in the soil also strictly limits development. The limit of CO₂ content in soil air that plants can withstand is 15%. However, even with a smaller excess of the 1% optimal threshold, root vitality is suppressed: their growth stops, nitrogen and phosphorus uptake decreases, ammonification and nitrification processes in the soil are suppressed, and the overall synthetic activity of the plant declines.

Temperature threshold: at what cooling is nutrition blocked

Normal mineral nutrition of plants is possible only within strictly defined temperature limits. When the soil temperature deviates from the optimum, roots lose the ability to absorb nutrients, even if they are in an easily accessible form in the soil solution. During cooling, elements are blocked to varying degrees: phosphorus and nitrates stop being assimilated first, while plants continue to assimilate magnesium and ammonium for the longest time.

Assimilation of ammonium nitrogen occurs at lower temperatures than nitrate nitrogen. This is because cold more strongly inhibits the process of reducing nitrates to ammonia within the plant, while ready-made ammonium is immediately used for the synthesis of amino acids. The ranking of elements by degree of resistance to cooling (from quickly blocked to more accessible) is as follows: PO₄ > NO₃ > Cl > K > Mg > NH₄.

Physiologically, the drop in element assimilation during cooling is explained by the weakening of root respiration and a decrease in the permeability of the protoplasm. The viscosity of cell membranes increases, chemical reactions of active substance transport slow down, the formation of high-energy phosphorus bonds (ATP) worsens, and glycolysis processes are inhibited.

Extremely high temperatures are also dangerous for crops. When the soil heats up to 40–50 °C, nutrient uptake drops sharply. This happens due to the inactivation of enzyme systems and an increase in cytoplasmic permeability, which leads to the passive leakage of elements from cells back into the soil solution.

An agronomist can regulate the temperature regime of the soil using land improvement and technological methods. Good drainage promotes faster soil warming. In winter, the soil is protected from deep freezing by snow cover and plant residues. The presence of vegetation smooths out daily temperature fluctuations: on a clear day, the air above the crops is warmer than the soil, while at night it is colder (on bare ploughland, the situation is the opposite). It is also important to consider the field topography — in the northern hemisphere, southern slopes always warm up more than northern ones due to the direct angle of incidence of sunlight.

Plant nutrition is carried out in close interaction with the soil microflora, the mass of which in the tilled layer reaches up to 8 t/ha. The greatest number of microorganisms is concentrated in the rhizosphere. Soil microflora has a strong impact on the transformation of nutrients in the soil and their availability to plants. Microorganisms carry out the process of ammonification – the decomposition of nitrogen-containing compounds with the release of ammonia. The latter is partially adsorbed by the soil and consumed by plants and microorganisms as a nitrogen source, and is also released into the atmosphere, where it is oxidized to nitrites and nitrates. The resulting nitrates are used by plants as a nitrogen source, washed out of the tilled soil layer by filtration water, immobilized by microorganisms during assimilatory nitrate reduction, and reduced to molecular nitrogen as a result of denitrification. The reduction of nitrates to molecular nitrogen is a widespread process in waterlogged soils. As a result of denitrification, 270-330 million tons of molecular nitrogen are released into the atmosphere annually. In terms of scale, this process is comparable to nitrogen fixation. Denitrification is one of the reasons for the low coefficients of nitrogen utilization by plants from fertilizers and the soil.

Microorganisms decompose plant residues, dead bodies of soil fauna, and organic fertilizers present in the soil, as a result of which the nutrients contained in them become available to plants. Along with this, they contribute to the conversion of poorly soluble soil nutrient compounds into mobile forms accessible to plants, and also fix atmospheric molecular nitrogen. The latter is carried out by nodule bacteria of legumes and free-living soil microorganisms. Rhizospheric microorganisms also act as biological immobilizers of readily soluble nutrient compounds, thereby protecting them from processes of leaching from the tilled soil layer. The immobilization of nutrients by microorganisms is temporary in nature, as after their death and decomposition, these nutrients return to the soil solution and are easily assimilated by plants.

Rhizospheric microorganisms utilize plant toxins. If there were no rhizospheric microorganisms, plants would die from the accumulation of their own metabolites in the rhizosphere. In addition to all this, microorganisms release a huge amount of growth stimulants, enzymes, and vitamins during their life cycle, which are easily assimilated by plants. Microorganisms also produce antibiotics* that possess fungistatic properties. The root-zone microflora of the soil can play not only a positive but also a negative role. Thus, some microorganisms can act as antagonists in plant nutrition. These include cellulose- and hemicellulose-degrading microorganisms that consume nitrogen and phosphorus from readily available compounds, thereby impairing plant nutrition. Many microorganisms release substances that are toxic to plants. 5.3.3. Plant response to nutritional conditions during different growth periods and nutrient removal by the harvest

The quantity and ratio of consumed nutrients depend on the hereditary nature of the plant itself and the conditions of the external environment. Plants also differ in the duration of the nutritional period. This refers to the length of time during which plants absorb nutrients from the external environment. It does not always coincide with the growing season. In the initial stages of ontogenesis, the plant uses nutrients exclusively from the seed, and towards the end of the growing season, it meets its needs through the re-utilization of previously accumulated nutrients. The need for nutrients is determined by their removal with plant biomass or in terms of main and by-products. By the beginning of ripening, the accumulation of mineral nutrition elements in the plant reaches its maximum. This corresponds to the concept of "plant nutrient requirements". In the final stages of ontogenesis, nutrient losses are observed as a result of leaf fall and their outflow from the roots into the soil. In natural biocenoses, absorbed nutrients return to the soil after plant death. In agrocenoses, where agricultural crops are harvested, the nutrients absorbed from the soil are removed. The magnitude of mineral nutrient removal depends on the type and productivity of the plant, as well as on soil and climatic conditions (Table 29; Ionas V.A., Vildflush I.R., Kukresh S.P., 1998). A distinction is made between

Antibiotics (from anti… Greek bios – life) are specific chemical substances produced by microorganisms and capable of exerting a selective toxic effect on other microorganisms in small quantities.

biological and economic nutrient uptake. Biological uptake is understood as the amount of nutrients consumed by plants to create the biological mass of the harvest, i.e., grain + straw + harvest residues, as well as nutrients partially transferred from the roots to the soil. It can be conditionally divided into two parts: economic and residual. Economic uptake is the amount of nutrients removed from the field with the harvest of the main and by-products. The residual part of the uptake includes nutrients that remain in the field as part of harvest residues, fallen leaves, grain and chaff losses, as well as a certain amount of nutrients transferred from the roots to the soil.

Table 29. Nutrient uptake per 1 centner of main product and the corresponding amount of by-product, kg. Type. Crop. Product. N P2O5 K2O CaO MgO S

Nutrient uptake and the law of the minimum

To plan yield and prevent soil depletion, fertilizer rates are calculated based on the nutrient uptake by a specific crop. This work is based on the law of the minimum (or the law of limiting factors). According to it, crop yield is always determined by the nutrient element that is in the greatest deficit in the soil. Increasing the doses of other fertilizers will not be effective until you replenish the shortage of the limiting element. After eliminating this deficit, harvesting will begin to grow in proportion to the applied doses until another element becomes the relative minimum.

For accurate calculation of the nutrient balance and planning of the fertilizer system, use the average values of nutrient uptake with crop harvests as a guide:

Critical periods and nutrient uptake dynamics

The need of plants for nutrients changes during individual development (ontogenesis) from germination to senescence. Young plants assimilate nutrients ahead of schedule — the rate of their uptake is higher than the rate of dry matter accumulation. Therefore, the specific content of elements per unit of biomass at the beginning of the growing season is always higher than before harvesting. The dynamics of uptake depends on the chemical nature of the compounds:

• mobile compounds are rapidly assimilated at early stages, outpacing the accumulation of dry mass;
• immobile compounds enter tissues in proportion to dry matter synthesis or with a lag.

When building a nutrition system, it is extremely important to distinguish between two periods of crop development: the critical period and the period of maximum consumption.

The high sensitivity of seedlings to nutrient shortage in the first two weeks is due to active synthesis of organic substances with a still weak, underdeveloped root system. This same period coincides with the low activity of soil microorganisms, which is why the natural decomposition of organic matter in the soil is slowed down. To avoid a decline in yield, nutrition must be applied in advance.

The period of maximum consumption is associated with the phase of intensive growth and the greatest accumulation of dry matter. At this time, biomass increases faster than nutrients enter from the soil. A "dilution effect" occurs — a decrease in the relative content of elements in plant tissues. Nutrient shortages during this period can be effectively corrected with top dressing.

The tasks of optimizing the soil's function as a source of nutrients are not exhausted by eliminating the deficit of necessary nutrients and liquidating toxic concentrations of certain elements. No less important is the creation of favorable conditions for the uptake of necessary elements into the plant organism.

G.V. Dobrovolsky, E.D. Nikitin, 1990

Soil is an ecological guarantor of life on Earth, and this is its functional significance within the system of other natural bodies.

Soil is an independent natural-historical organo-mineral natural body that emerged on the Earth's surface as a result of the long-term influence of biotic, abiotic, and anthropogenic factors; it consists of solid mineral and organic particles, water, and air, and possesses specific genetic and morphological features and properties that create appropriate conditions for plant growth and development. There are four physical phases of soil: solid, liquid, gaseous, and living.