Factors of plant life

Chemically bound water

Chemically bound water is included in the solid phase of the soil (chemically bound). It consists of constitutional (hydrate) and crystallization (crystal hydrate) water. It is unavailable to plants, immobile in the soil, and does not dissolve mineral substances.

Solid water

Solid water — water in the form of ice. It forms when the soil freezes in the autumn-winter period (seasonal freezing) or is permanently present at a certain depth, not thawing in summer, in the frozen thickness of soil and subsoil (permafrost, long-term frozen ground). When thawing, it represents a potential reserve of liquid and vapor water. Solid water is unavailable to plants and immobile.

Vapor water

Vapor water — water vapor contained in soil air, sometimes reaching 100%. It is mobile, moving from places with higher water vapor pressure to places with lower pressure and with air flows.

It has an insignificant effect on plant moisture supply and is practically unavailable to plants. In dense crops, owing to withdrawal of vapor water by the root system from near-root voids, it plays a noticeable role. With decreasing temperature and reaching the dew point, it condenses, passing into a liquid form available to plants.

Hygroscopic (strongly bound) water

Hygroscopic (strongly bound) water — one of the forms of physically bound, or sorbed, water. It is retained by the solid phase of the soil, mainly by colloidal particles, through sorption forces. It forms a thin layer of water film 1–3 molecules thick around soil particles. Sorption forces arise because part of a water molecule (on the oxygen atom side) is negatively charged, while the other part (on the hydrogen atom side) is positively charged. Such a charge form is called a dipole. Water molecules, due to the strict orientation of dipoles, are attracted to soil particles and retained by them.

The freezing point of hygroscopic water is -4…-7 °C; it does not dissolve water-soluble substances, has a density of 1.5–1.8 g/cm³ and higher viscosity.

It is unavailable to plants.

Maximum hygroscopicity (MH) — the amount of water that soil can absorb and retain when placed in an atmosphere saturated to 96–98% with water vapor. The value of maximum hygroscopicity makes it possible to determine plant water supply. As a rule, a value equal to 1.5–2 times the maximum hygroscopicity corresponds to the moisture of permanent wilting of plants, or the so-called “dead reserve” of water in the soil. It is taken into account in calculations of irrigation norms and productive moisture reserves. To calculate the moisture of permanent wilting of plants from maximum hygroscopicity, a coefficient of 1.34 is used.

The dead water reserve is directly proportional to the content of silty (colloidal) particles and organic substances (humus) in the soil. Thus, sandy soil poor in organic matter contains only 1% of water unavailable to plants, while heavy loamy and clayey soils rich in humus contain up to 15% of such water. In peat soils, the dead reserve can reach 20–50%.

The amount of hygroscopic moisture directly depends on the granulometric (mechanical) composition of the soil and the content of organic matter. The greater the specific surface of soil particles (that is, the smaller their size), the more hygroscopic moisture can be accumulated by the soil.

Loosely bound (film) water

Loosely bound water — the second form of physically bound (sorption) water, also called film water. It is formed due to additional sorption of water molecules to colloidal particles of the soil. Unlike hygroscopic (strongly bound) water, which forms in an atmosphere of saturated water vapor, loosely bound water is sorbed by colloidal particles from the liquid phase, owing to sorption forces sufficient to retain it.

Loosely bound water is low-mobility and poorly available to plants.

Capillary water

Capillary water — water in a droplet-liquid state in soil capillaries, retained by capillary forces.

It is subdivided into capillary-suspended water, that is, water entering the soil from upper layers through precipitation or irrigation water, and capillary-supported water, rising under the action of capillary forces from groundwater. Between the layers moistened by capillary-suspended and capillary-supported water, there is a gap of dry soil called the capillary fringe.

Capillary (meniscus) forces that retain moisture and cause the rise of groundwater depend on the thickness of the capillaries. The smaller their thickness, the greater the force and, accordingly, the height of water rise. For this reason, on structureless and dense soils, the arable layer quickly dries out, especially in southern regions. Destruction of capillaries in the upper soil layer by loosening substantially reduces moisture evaporation. At a diameter of less than 8 mm, capillary forces are insignificant.

The effect created by capillary forces is also used in the opposite direction. When it is necessary to draw moisture from lower layers, for example when sowing seeds, especially in a dry period or in arid places, they resort to soil compaction by rolling.

Minimum water capacity of soil (MWC) — the amount of capillary-suspended water that is retained by the soil after excess liquid water drains. The optimal soil moisture for plant growth and development is 70–100% of minimum water capacity.

Moisture deficit — the difference between the value of minimum water capacity and actual soil moisture; it is widely used in agriculture.

Gravitational water

Gravitational water — the liquid phase of water not bound to the soil, occupying large non-capillary voids between soil aggregates. It moves freely under the force of gravity. It displaces air from soil voids, negatively affecting the air regime and creating anaerobic conditions.

Total water capacity (TWC) — the maximum possible amount of gravitational water that soil can contain when all voids are filled.

The value of total water capacity is almost equal to soil porosity (void ratio) and ranges from 20–40 to 50–60%, sometimes reaching 80%.

Gravitational water is easily available to plants. Under natural conditions, it occurs with shallow groundwater, flooding of plots, bogs, marshes, prolonged precipitation, and spring snowmelt.

Water-holding capacity

Water-holding capacity — the ability of soil to absorb and retain water in the profile, preventing it from flowing down under the action of gravity. It is quantitatively assessed by the water capacity of the soil.

It arises mainly due to sorption and capillary forces.

Water capacity

Soil water capacity — the maximum amount of water retained by the soil.

Field water capacity (FWC), or minimum water capacity (MWC), of soil — the maximum amount of soil water that can be retained by the soil for a long time without inflow and without losses from evaporation. It is characteristic of each soil and is an almost constant value; it is important in production activity, especially in irrigated agriculture when calculating irrigation norms.

At the level of minimum water capacity, the soil contains the maximum amount of moisture available to plants, while 60–80% of the soil pores are filled with water.

Clayey and humus-rich soils are characterized by high water capacity and low water permeability and a higher dead water reserve. Sandy and humus-poor soils, on the contrary, have low water capacity and high water permeability.

Water permeability

Soil water permeability — the ability of soil to absorb and pass through its profile water entering from the surface. It depends on density, granulometric composition, soil structure, and degree of moisture.

Water-lifting capacity

Water-lifting capacity — the property of soil to create a rise of water through capillary forces.

Permanent wilting moisture

Permanent wilting moisture of plants, or dead reserve, — the amount of moisture unavailable to plants. It is determined by 1.5 times the maximum hygroscopicity. As a rule, a coefficient of 1.34 is used for calculation.

The value of permanent wilting moisture depends on the content of organic matter and soil texture and varies greatly by soil type: in sandy loam soils it is 2–3%, in loamy soils 5–6%, in clayey soils 8–10%, in humus-sandy and chernozem soils 14–16%, in peaty soils up to 40–50% of the mass of absolutely dry soil.

Wilting of plants occurs due to a lack of moisture in the soil, the so-called soil drought, or because of increased evaporation of moisture due to great dryness and high air temperature (atmospheric drought).

Permafrost type

The permafrost type of water regime is characteristic of permafrost regions. The permafrost layer serves as an aquiclude, which causes waterlogging of the upper soil layer thawing during the warm season. For this reason, soil gleying occurs, and therefore tundra soils are gleyed.

Leaching type

The leaching type of water regime is characteristic of regions where precipitation prevails over evaporation throughout the year. As a result of excess moisture, a downward water flow forms. It occurs in the taiga-forest zone, Polesie, humid subtropics, and tropics. In the annual moisture cycle during wet periods, mainly in spring and autumn, through leaching of soils and parent rocks down to groundwater is observed. Such an intense water flow leads to the removal of soil formation products deep beyond the soil profile and the formation of soils of the podzolic type of soil formation.

Periodically leaching type

The periodically leaching type is characteristic of regions with approximately equal amounts of water falling as precipitation and evaporating during the year. The alternation of dry and wet years causes alternation of non-leaching and leaching types of water regime. Moreover, the latter may occur once in 10 or more years. This type of water regime contributes to the formation of gray forest soils, leached and podzolized chernozems of the forest-steppe zone.

Non-leaching type

The non-leaching type of water regime is characteristic of regions where the annual precipitation sum is less than evaporation, while water does not reach groundwater. Wetting of the soil and subsoil thickness in the chernozem steppe reaches 4 m, in brown and gray-brown soils of semi-deserts and deserts — 1 m. Between groundwater and the upper moistened layer there is a layer with moisture approximately equal to the wilting moisture value.

Exudative type

The exudative type of water regime is characteristic of areas with a non-leaching type and close occurrence of groundwater. In this case, intensive rise of moisture through capillaries from groundwater to the surface and its evaporation occur, especially in the zone of semi-deserts and deserts. With a high content of mineral substances in groundwater, saline soils are formed, primarily solonchaks and solonetzes.

Irrigation type

The irrigation type is characteristic of territories with artificial irrigation of lands. During the year, the soil water regime may change from leaching to non-leaching, and even exudative, depending on the intensity and timing of irrigation.

Methods of regulating the water regime in arid regions

Irrigation — a method used in intensive agriculture systems to regulate the soil water regime, consisting of watering. It is especially important in arid regions, where it makes it possible to provide plants with moisture primarily during critical growth phases.

Placement of crops in crop rotation with different root systems and water consumption makes it possible to use moisture from different horizons most efficiently.

Improving soil structure prevents surface water runoff and reduces its evaporation.

The use of fertilizers, by reducing the transpiration coefficient, reduces water consumption by plants.

Breeding and using drought-resistant varieties with a reduced transpiration coefficient is a good means of rational moisture use.

Drainage

Drainage — a method aimed at reducing excessive soil moisture. It is relevant in the northwestern regions of Russia.

Excess moisture in the soil leads to waterlogging of plants, reducing their yield due to the creation of anaerobic conditions and deterioration of the soil air regime. Strong swelling of clayey soils upon moistening, followed by drying, substantially compacts them, and a dense crust forms on the surface.

Drainage methods include drainage, ridge sowing, special plowing methods, organic fertilizers, including green manure, surface tillage, leveling of micro- and mesodepressions, etc.

A network of closed or open drains makes it possible to remove excess moisture, while supplying water through drains, on the contrary, makes it possible to regulate the soil water regime.

Heat absorption capacity

Soil heat absorption capacity — the property of soil to absorb solar energy by converting light energy into thermal energy. In parallel with absorption, partial reflection of light from the soil surface occurs.

The ratio of the absorbed energy share to the reflected one is characterized by the albedo indicator, determined by Kirchhoff’s law and varying depending on the darkness of the soil: the darker the soil, the more light is converted into heat. The albedo of irrigated lands is 5–11% lower than that of dry lands. The albedo of clean dry snow is 88–91%, wet snow — 70–82%. Indirect factors affecting albedo are: structure, moisture, surface evenness, and plant features (color of leaves and stems).

High-humus soils (chernozems) absorb 10–15% more light energy than low-humus soils, just as clayey soils do relative to sandy soils.

Heat capacity

Soil heat capacity — the ability of soils to accumulate thermal energy.

Gravimetric and volumetric heat capacities are distinguished.

Gravimetric heat capacity — the amount of heat required to heat 1 kg of soil by 1 °C. It is expressed in J/(kg⋅°C). Sometimes grams are used instead of kilograms.

Volumetric heat capacity of soil — the amount of heat required to heat 1 m³ of soil by 1 °C. It is expressed in J/(m³⋅°C). Sometimes cm³ or dm³ (liters) are used instead of m³.

Gravimetric C_m and volumetric C_V heat capacities are related by the relationship:

where

ρ

— soil density.

Soil heat capacity depends on mineralogical and granulometric composition, moisture, and organic matter content. For example, quartz sand has a lower gravimetric heat capacity than peat.

The gravimetric heat capacity of water is 4187 J/(kg⋅°C), sand — 821 J/(kg⋅°C), clay — 976 J/(kg⋅°C), peat — 1997 J/(kg⋅°C), air — 1.3 J/(kg⋅°C).

Moisture increases the heat capacity of soil, therefore clayey soils, characterized by high water capacity, warm slowly in spring, but also retain more heat at night and during frosts. They are also called cold soils. Light soils — sandy and sandy loam — on the contrary, warm up faster and are called warm soils. Humus-rich soils, like loose soils, are also characterized by higher heat capacity.

Water can change soil heat capacity by 10–15 times.

Thermal conductivity

Soil thermal conductivity — the ability of soil to conduct heat, or the amount of heat passing per unit time through a unit length (area, volume) at a temperature difference of 1 °C. It is expressed in W/(m⋅°C).

Soil thermal conductivity depends on chemical and granulometric composition, moisture, air content, temperature, and soil density. Dry soils, as well as humus-rich and highly aerated soils, conduct heat very poorly.

The solid phase of the soil conducts heat approximately 100 times better than air. Therefore, dense soil is more thermally conductive than loose soil. An increase in density from 1.1 to 1.6 g/cm³ leads to a 6-fold increase in thermal conductivity.

Soil moisture increases thermal conductivity: a change in moisture from 0.1 to 25–30% leads to a 5-fold increase in thermal conductivity.

To assess the rate of temperature equalization between soil horizons, thermal diffusivity is used.

Thermal diffusivity — the change in temperature in 1 cm³ of soil as a result of the input of a certain amount of heat transferred per unit time through a unit area.

Heat loss

Soil heat loss — the ability of soil to give off heat. Heat loss can occur through thermal radiation and convection. Heat loss through convection depends on the saturation of the atmosphere with water vapor, soil temperature, and the state of the surface.

Soil heat-emitting capacity — the ability of soil to give off thermal energy through thermal radiation. It depends on the state of the soil and surface, the degree of its moistening, and also on thermal conductivity. Mineral soils, due to greater thermal conductivity, have a greater heat-emitting capacity than peaty soils. Moist soils and soils with a smooth surface also have a greater heat-emitting capacity than dry soils and soils with a rough surface.

Permafrost type

The permafrost type of thermal regime is characteristic of permafrost zones. In the warm period, the soil thaws, and in the cold period it freezes to the upper boundary of the permafrost ground. The mean annual soil temperature and the temperature at a depth of 0.2 m in the coldest month are negative.

Long-seasonally freezing type

The long-seasonally freezing type of thermal regime is characterized by thawing of the soil in the warm period, followed by deep freezing in the cold period. The duration of freezing is at least 5 months to a depth of more than 1 meter. The mean annual soil temperature is positive, but the temperature at a depth of 0.2 m in the coldest month is negative.

Seasonally freezing type

The seasonally freezing type of thermal regime is characterized by thawing in the warm period and shallow freezing in the cold period. Negative temperatures penetrate to a depth of no more than 2 m. The duration of freezing ranges from several days to 5 months. The mean annual soil temperature is positive; the temperature at a depth of 0.2 m in the coldest month is negative.

Non-freezing type

The non-freezing type of thermal regime is characterized by the absence of freezing. Soil temperatures are positive; negative temperatures may occur for several days. The temperature at a depth of 0.2 m is always positive.

Agronomic methods

Agronomic methods of regulating the thermal regime include methods of soil tillage: deep loosening, ridging, rolling, leaving stubble, mulching, and applying organic fertilizers.

Large doses of organic fertilizers, owing to heat release from active microbiological processes, create an additional source of heat in the soil.

The ridged surface, by increasing the surface area, absorbs more thermal energy, accumulates more heat, and warms up faster. The temperature of ridges is 3–5 °C higher compared with a level surface, which is especially important in northern regions.

Deep plowing creates a sharp heterogeneity of the soil along the profile, changing density, moisture, and porosity. At the same time, the thermal properties of the soil change.

Soil compaction increases its thermal conductivity, as a result of which rolling is used to increase the average daily temperature by 3–5 °C in the arable layer to a depth of 10 cm.

Mulching — a method that makes it possible both to accumulate heat and to reduce its input. For accumulation, dark-colored materials are used, for example black polyethylene films, which reduce albedo (reflection of light energy) by 10–15%. Transparent films give a similar effect; almost without changing albedo, they accumulate heat through the greenhouse effect. Light-colored films, on the contrary, make it possible to reduce heat accumulation by increasing albedo.

Shading can be used to reduce the input of light energy to the soil surface.

Agro-reclamation methods

Agro-reclamation methods of regulating the soil thermal regime include drought control, afforestation, irrigation, and drainage.

Forest belts, in addition to a number of other advantages, make it possible to regulate the soil thermal regime through snow accumulation and changes in the local microclimate, and reduce wind speed by 20–40% in inter-belt areas compared with open spaces.

Irrigation can be used both for heat accumulation and for its reduction. After watering, soils reduce the share of reflected radiation by 20%, and heat-emitting capacity decreases, which increases heat accumulation. High soil moisture promotes heat transfer, which improves its warming and reduces temperature fluctuations. At the same time, an increase in soil moisture leads to a decrease in temperature as a result of large heat expenditures for warming (since soil heat capacity increases) and water evaporation.

In southern regions, the construction of reservoirs, ponds, and limans makes it possible to increase soil and air humidity, which reduces moisture evaporation and soil heating.

Agrometeorological methods

Agrometeorological methods of regulating soil thermal regimes are aimed at combating frosts, reducing the heat-emitting capacity of the soil, etc.

Frost control is carried out by creating smoke screens (“covering” crops with smoke).

Nutrient cycling

Intensity of biological cycling — the amount of chemical elements contained in the increment of a phytocenosis per unit area over a time interval.

One of the tasks of agrochemistry is to assess the direction of biogenic element cycling and the degree of intensity of anthropogenic impact on the soil-plant system in the nutrient balance in an agrocenosis, as well as to create conditions for rational nutrient cycling in agriculture and ensure their positive balance, in order to optimize the nutrition of agricultural crops through the use of a scientifically based fertilizer system in crop rotation.

In 1825, for the first time in Russia, a professor of Moscow University, M. G. Pavlov, published the scientific work “Agricultural Chemistry”, in which he substantiated the task of increasing soil fertility by increasing nutrients in the soil or at least returning what was taken by plants. The development of research on nutrient balance in agrochemistry began with the publication of the work of J. Liebig <cite>“Chemistry in Its Application to Agriculture and Physiology”</cite> (1840) and the doctrine of the complete return to the soil of mineral substances taken from it by the crop.

The nitrogen cycle in the biosphere in the natural state of the soil:

The problem of nutrient cycling in agriculture and their balance was addressed by the founder of Russian agrochemistry, D. N. Pryanishnikov. He wrote that the development of the chemical industry is one of the most important material prerequisites for regulating nutrient cycling in agriculture and their exchange between humans and nature. He noted that if soil depletion as a result of disruption of the exchange of substances between humans and the land disrupts the natural condition of constant soil fertility, then the widespread use of fertilizers based on the chemical industry is one of the powerful factors not only for maintaining it at a constant level (as J. Liebig believed), but also for further increasing soil fertility, as can be seen in the historical example of increasing yields in Western European countries with an increase in the level of chemicalization.

Human economic activity, including intensification of agricultural production and chemicalization, leads to changes in the processes of transformation of substances and energy in nature. Thus, changes occur in the nitrogen cycle in the biosphere during the transition from the natural state of the soil to a state under intensive tillage. In soils of natural biocenoses, nitrogen losses from volatilization and denitrification are balanced by its input with atmospheric precipitation and biological fixation.

When a land plot is developed for intensive agricultural production, the nitrogen cycle undergoes changes. At the same time, nitrogen losses from the system exceed its input, which leads to depletion of the soil in this element. During agricultural development of lands, the number of pathways of nitrogen loss from the system increases: along with gaseous nitrogen losses from the soil, nitrate leaching increases. Nitrogen is also removed from the system when plant residues are burned. Significant amounts are alienated when agricultural products are used for industrial and other needs, and are also absorbed by weeds.

Natural nitrogen input into the cycle occurs through biological fixation and with precipitation. Only by applying nitrogen fertilizers and manure is it possible to eliminate the deficit in the nitrogen balance and create conditions for preserving and increasing fertility. Losses of nitrogen and other nutrients cause eutrophication of water bodies, pollute groundwater, and cause a number of undesirable phenomena in the environment.

It does not matter in what form nitrogen is applied to the soil — as part of organic or mineral fertilizers, nitrate, ammonia, amide, or molecular, fixed by legumes — ultimately only the reduced ammonium form of nitrogen (NH₄⁺) participates in the synthesis of amino acids and proteins in plants. All other forms are reduced to ammonium in the process of chemical or biological transformation in the soil or in plants.

Organic and mineral fertilizers as sources of nutrients are equivalent. However, organic fertilizers are preferable because they are less concentrated. For example, in terms of nitrogen, 0.1 t of urea is equivalent to 10 t of manure. Violations in the technology of applying mineral fertilizers lead to the creation of high concentrations of nutrients in the soil, which, entering plants in excessive amounts, worsen product quality or cause ammonia poisoning of plants. Mineralization of organic fertilizers occurs slowly and does not create increased concentrations of mineral salts.

Soil nutrient balance

Soil nutrient balance — the quantitative expression of the nutrient content in the soil of a specific area, taking into account all input and expenditure items over a certain period of time.

Sources of nutrient input:

• mineral fertilizers;
• organic fertilizers;
• plant residues;
• seed material;
• biological nitrogen fixation;
• atmospheric precipitation.

The expenditure part consists of:

• removal with the main and by-products;
• removal with plant residues;
• leaching into groundwater and washout from the surface;
• losses from erosion processes;
• gaseous losses.

For research purposes, quantitative values of balance items are taken on the basis of experimental data; for practical purposes, reference data are used.

For theoretical studies taking into account all balance items, the lysimeter method is used. This method makes it possible to determine the patterns of changes in balance items and provide a scientific explanation. In these experiments, fertilizers with labeled atoms are used. Thus, according to the results of lysimetric studies conducted at the All-Russian Research Institute of Agrochemistry named after D. N. Pryanishnikov on sod-podzolic soils of the Non-Chernozem region using the stable nitrogen isotope ¹⁵N, 30–60% of the nitrogen of the applied fertilizer is used by plants, 15–30% is accumulated in the soil, 10–30% is lost as a result of volatilization into the atmosphere, and 1–5% is leached by water.

For practical purposes, data on biological, economic, and external economic balances are used.

Biological balance covers all items of input and expenditure of nutrients participating in the cycle. It is used when assessing the fertilization system of crops and specialized crop rotations.

Economic balance takes into account only the removal of nutrients with the main and by-products and the input through the application of fertilizers. Calculation of the economic balance gives a sufficiently objective agroeconomic assessment of the fertilization system.

External economic balance takes into account the alienation of nutrient elements with marketable products outside the farm and their input with mineral fertilizers. This type of balance is important for the distribution of fertilizers and is determined by the specialization of the farm. When an agricultural enterprise specializes in the production of marketable products, the balance is more deficient than at enterprises with livestock specialization, since part of the nutrients alienated from the fields in the form of feed is returned in the form of manure.

Studies of soil nutrient balance in long-term stationary experiments with fertilizers make it possible to take into account the multi-year application of nutrient elements by crop rotation cycles and their removal with harvests. The experiments are conducted under conditions close to production, therefore the data obtained are applicable for scientific and practical purposes.

The main expenditure item is the removal of nutrients with the harvest. The amount of removal in relation to specific crops, varietal characteristics, and soil-climatic conditions may differ from reference data.

The greatest losses of nutrients occur under clean fallow, less under row crops, then in decreasing order: cereals, perennial grasses, hayfields, and pastures.