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Thermal regime of soils

Thermal regime of soils is a set of processes of heat input and output by soil, its distribution and influence on plants.

It refers to the cosmic factors of plant life. The main source of heat on the Earth is the radiant energy of the Sun, which is converted into heat. Sources of heat also include: heat transferred by atmospheric air, decomposition of soil organic matter, internal heat of the planet, radioactive processes of the soil. The last two sources of heat are negligibly small. The share of heat received from atmospheric air is also insignificant, although it sometimes has some influence, for example, when warm air masses move.

As a rule, microbial heat release has no noticeable effect on the thermal regime. However, during decomposition of “concentrated” organic matter, such as manure, due to microbiological activity, the temperature may rise to 40-60 °С. The so-called “warm beds” are based on this principle.

The importance of heat in plant life

Thermal energy is a factor of physiological and biochemical processes in plants. At low temperatures some processes are strongly slowed down, and in other cases they do not start.

The need of plants for heat is different. Differences appear not only in different species, but also in the same crop at different phases of development.

Table. Requirements of field crops to heat[1]Fundamentals of agricultural production technology. Farming and crop production. Edited by V.S. Niklyaev. — Moscow: «Bylina», 2000. — 555 с.

Crop
Biological minimum temperature, °С
Frosts damaging seedlings, °C
Optimal growth temperature, °C
seed germination
arising
formation of generative organs and flowering
Mustard, canola
0...+1
+2...+3
+8...+10
-6...-8
+15...+22
Rye, wheat, barley, oats, peas
+1...+2
+4...+5
+8...+10
-7...-9
+15...+22
Flax fiber
+3...+4
+5...+6
+10...+12
-5...-7
+16...+18
Sunflower
+3...+4
+6...+8
+12...+15
-5...-6
+20...+24
Potatoes
+7...+8
+8...+10
+11...+14
-2...-3
+20...+22 (tops)
+16...+18 (tubers)
Peas
+1...+3
+4...+5
+10...+15
-7...-8
+16...+20
Maize
+8...+10
+10...+12
+12...+15
-2...-3
+20...+24
Sugar beet
+3...+4
+6...+7
+12...+15
-4...-6
+18...+22

Depending on the physiological response of plants distinguish:

  • minimum temperature – the temperature below which physiological processes do not occur;
  • optimal temperature – the temperature at which plant growth and development proceeded most rapidly;
  • maximum temperature – the temperature above which plants sharply reduce productivity, until death.

Each phase of growth and development is characterized by its minimum, optimal and maximum temperatures.

An increase in soil temperature directly affects the growth rate of plants. For example, rye seeds germinate at 4-5 °C for 4 days, at 16 °C – a day. This property should be taken into account when choosing the timing of sowing so as to avoid sowing in cold soil, in which the seeds will lie for a long time without germinating, with the likelihood of rotting.

The root system also responds to soil temperature. Its growth is more vigorous at relatively low temperatures. For example, the root system of oats at a soil temperature of 12-14 °C was 1.5 times smaller than at 6-8 °C. The largest mass of potato tubers is formed at temperatures no higher than 15-20 °C.

For good root growth, the soil temperature should be slightly lower than the air temperature of the above-ground part of the plant. For hemp, the minimum soil temperature during emergence of seedlings is 2-3 °C; for spring cereals and peas, it is 4-5 °C.

Reproductive organs are formed at the following minimum temperatures: hemp, spring cereals and peas have 10-12 °C; buckwheat, sunflower, maize, millet have 12-15 °C; rice and cotton have 13-20 °C. During fruiting, a temperature of 10-12 °C is sufficient for most crops, and 15-20 °C for rice and cotton.

The optimal temperature for most crops is 20-25 °C. Above 30 °C there is a growth retardation. Exceeding optimal temperatures leads to a sharp increase in breathing intensity and organic matter consumption, which affects the reduction of green mass growth. Temperatures above 50-52 °C lead to plant death.

Table. Minimum and optimum soil temperatures required for seed germination and seedling emergence, °C.[2]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. — Moscow: Publishing House «Kolos», 2000. — 551 с.

Crop
Seed germination
Emergence of seedlings
minimum temperatures
optimal temperatures
minimum temperatures
optimal temperatures
Clover, alfalfa, hemp
0...+1
-
+2...+3
-
Rye, wheat, barley, oats, peas, vetch, china, timothy
+1...+2
+25...+30
+4...+5
+6...+12
Beets, buckwheat, beans, flax, lupine, chickpeas
+3...+4
+25...+30
+6...+7
-
Potato, sunflower
+5...+6
+31...+37
+8...+9
-
Corn, millet, Sudan grass, soybeans, coriander
+8...+10
+37...+45
+10...+11
+15...+18
Beans, sorghum, castor beans
+10...+12
-
+12...+13
-
Cotton, rice, sesame, peanuts
+12...+14
+37...+45
+14...+15
+18...+22

High temperatures in the summer period can cause great damage to productivity, especially with a lack of water. Plant death from drought is observed not only in southern areas, but also in northern areas, for example, in clover crops or accelerated drying out (“capture”) of buckwheat from high air temperatures.

Low temperatures are best tolerated by plants in the phase of nascent seeds. Later on, cold tolerance decreases sharply. Spring frosts may severely damage seedlings.Autumn frosts are also dangerous, they damage tomato, potato, cucumber plants; millet, buckwheat, spring wheat of late sowing dates do not mature. Therefore, for proper selection of crops, you should consider the duration of the growing season, comparing it with the frost-free period, and the sum of active temperatures for a particular zone.

Each plant during the growing cycle requires a certain amount of heat, for the estimation of which in agriculture it is customary to use the sum of active temperatures.

Table. The need of agricultural plants in heat during the growing season[3]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. — Moscow: Publishing House «Kolos», 2000. — 551 с. [4]Fundamentals of agricultural production technology. Farming and crop production. Edited by V.S. Niklyaev. — Moscow: «Bylina», 2000. — 555 с.

Crop
Sum of active temperatures, °С
Spring wheat
1200-1700
(by other reports 1100-1900)
Winter wheat
1150-2000
Barley
950-1450
(by other reports 950-1700)
Oats
1000-1600
(by other reports 1000-1800)
Millet
1400-1750
Corn on grain
2100-2900
(by other reports 2000-3000)
Corn for silage
1800-2400
(by other reports 1800-2500)
Potatoes
1600
(by other reports 1200-2000)
Sugar beet
2000
Flax fiber
1500
(by other reports 1300-1700)
Peas
1000-1700
Sunflower
1600-2400
Perennial herbs
900

The sum of active temperatures is the sum of daily temperatures above 10 °C during the growing season.

For most crops it is in the range of 1200-2000 ºC. Seed germination is also activated when a certain value of the sum of active temperatures is reached.

Heat is also necessary for normal life activity of soil microorganisms and biota. As well as for plants, they are negatively affected by low temperatures, leading to slowing down processes, as well as by elevated temperatures. Optimal temperature is 15-20°C with small fluctuations, especially in high-humus textured soils.

Plants are characterized by cold-resistance and frost-resistance, and such plants are primarily plants of the temperate zone. In hot areas, plants are characterized by heat-resistance, e.g. heat-loving plants such as sorghum, castor beans, rice, and cotton.

Thermal properties of soil

Heat absorption capacity

Heat absorption capacity of soil is the property of soil to absorb solar energy by converting light energy into heat energy. In parallel with absorption, there is a partial reflection of light from the soil surface.

The ratio of the proportion of absorbed energy to reflected energy is characterized by the albedo index, which is determined by Kirchhoff’s law and varies depending on the blackness of the soil: the darker the soil, the more light is converted into heat.

The albedo of irrigated land is 5-11% lower than that of dry land. The albedo for clean dry snow is 88-91%, and for wet snow it is 70-82%.

Indirect factors influencing albedo are: soil structure, moisture content and leveling of soil surface, plant features (color of leaves and stems).

High-humus soils (black earth) absorb 10-15% more light energy than low-humus soils, as well as clay soils in relation to sandy soils.

Heat capacity

Heat capacity of soil is the ability of soils to store heat energy.

Heat capacity is divided into weight and volume heat capacities.

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

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

Weight Cm and volume CV heat capacities are related to each other by the relation:

ρCm = CV,

where ρ is the density of the soil.

The heat capacity of soil depends on the mineralogical and granulometric composition, moisture, organic matter content. For example, quartz sand has a lower weight heat capacity than peat. 

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

Moisture increases the heat capacity of soil, so clay soils with high moisture capacity warm up slowly in spring, but also retain heat more at night and during frosts. They are also called cold soils. Light soils such as sandy and sandy loam, on the contrary, warm up faster and are called warm soils. Water can change the heat capacity of the soil by 10-15 times.

Humusy soils, like loose soils, also have a higher heat capacity.

Thermal conductivity

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

Soil thermal conductivity depends on chemical and granulometric composition, moisture, air content, temperature and soil density. 

Dry soils, as well as those rich in humus and highly aerated, conduct heat very poorly.

The solid phase of soil conducts heat about 100 times better than air. Therefore, dense soil is more thermally conductive than loose soil. Increasing density from 1.1 to 1.6 g/cm3 increases thermal conductivity 6 times.

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

To estimate the rate of temperature equalization between soil horizons, thermal conductivity is used.

Thermal conductivity is the change in temperature in 1 cm3 of the soil as a result of entering it a certain amount of heat transferred per unit time through the unit area.

Heat recoil

Heat recoil is the ability of the soil to recoil heat. Heat release can occur through thermal radiation and convection. 

Soil heat emission capacity is the ability of the soil to give off heat energy through thermal radiation.

It depends on the condition of the soil and its surface, its degree of moisture, and its thermal conductivity. Mineral soils have a higher heat emissivity than peaty soils because of their higher thermal conductivity.

Moistened soils and soils with smooth surface also have greater heat recoil than dry and rough soils.

Heat recoil by convection depends on atmospheric water vapor saturation, soil temperature, and surface condition.

Regularities of the thermal regime

The amount of light energy reaching the soil surface is characterized by daily and annual periodicity. As the light energy is converted into heat energy, the change of surface temperature has a similar pattern.

In the diurnal cycle, the surface temperature rises from sunrise to 14 h, after 14 h it begins to decrease. In the annual cycle, it increases from March to July, and then decreases. Daily temperature fluctuations, as a rule, do not spread deeper than 1 m, annual – deeper than 5 m.

Temperature fluctuations play an important role for wintering crops, as deep and rapid freezing leads to reduced resistance to low temperatures.

Snow cover has a significant impact on the temperature regime of the soil. Since snow has a low thermal conductivity, the transfer of heat from the soil to the atmosphere during the winter is markedly reduced. For example, when snow was 24 cm thick, the temperature on its surface was -26.8 °С, while under the snow the temperature on the soil surface was -13.8 °С.

The water solution, which is in the soil, due to the content of large amounts of minerals, has a freezing temperature much lower than water – down to -10 °C.

The influx of light energy from the Sun depends on the latitude of the area, weather conditions, time of day, fog, atmospheric dustiness, etc.

The thermal regime depends on the terrain. The steepness and exposition of the slopes determine the difference in the amount of heat that comes from solar energy. Soils of southern, southwest and southeast slopes are heated better than those of northern, northwest and northeast, as well as leveled areas.

Types of thermal regimes

Frost penetration of soils covered with vegetation (winter crops, grasses, forests, etc.) is much less than that of uncovered soils (without vegetation, mulch, etc.). Different soil-climatic zones have different thermal regimes of soils. Depending on the value of average annual temperature and character of freezing, there are 4 types of thermal regime:

  • permafrost,
  • long-season-frozen,
  • seasonally-frozen,
  • non-freezing.

Permafrost type

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

Long-season-frozen type

The long-seasonal-frozen 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 at a depth of more than 1 meter. The average annual soil temperature is positive, but the temperature at the depth of 0.2 m in the coldest month is negative.

Seasonally-frozen type

The seasonally frozen type of thermal regime is characterized by thawing during the warm period and shallow freezing during the cold period. Negative temperatures penetrate to a depth of not more than 2 m. Duration of freezing is from several days to 5 months. The average annual soil temperature is positive, the temperature at the depth of 0.2 m in the coldest month is negative.

Non-freezing type

Non-freezing type of thermal regime is characterized by the absence of freezing. Soil temperatures are positive, negative can be several days. Temperatures at the depth of 0.2 m are always positive.

Regulation of the thermal regime of soils

Soil optimum temperature for barley seed germination is 20°C, for oats, wheat, rye – 25°C, tobacco – 28°C, maize and sorghum – 32-35°C, cucumber and pumpkin – 33-35°C. For most crops with sufficient light and water supply, the optimum air temperature is 15 to 30 ° C.

Techniques for regulating the thermal regime due to great differences in agricultural conditions can vary greatly and be opposite. For example, in the southern regions, measures are used to reduce the inflow of heat, while in the northern regions – measures for its accumulation and preservation.

In the practice of farming three types of methods of regulating the thermal regime of soils can be distinguished:

  • agrotechnical;
  • agromeliorative;
  • agrometeorological.

Agrotechnical methods

Agrotechnical methods of regulating the thermal regime include methods of tillage: deep loosening, ridging, rolling, leaving stubble, mulching, application of organic fertilizers.

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

The ridged surface, due to the increased surface area, absorbs more heat energy, accumulates more heat and warms up faster. The temperature of ridges is 3-5 °C higher compared to the levelled surface, which is especially important in northern areas.

Deep plowing creates a sharp heterogeneity in the soil profile, changing the density, moisture and porosity. This changes the thermal properties of the soil.

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

Mulching is a method that allows both accumulating heat and reducing its inflow. Dark-colored materials are used for accumulation, such as polyethylene black films, which reduce albedo (reflection of light energy) by 10-15%. Transparent films have a similar effect, almost no change in albedo, they accumulate heat due to the greenhouse effect. Lighter colored films, on the contrary, allow to reduce the accumulation of heat, increasing the albedo.

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

Agromeliorative methods

Agromeliorative methods of soil thermal regime regulation include drought control, afforestation, irrigation, and drainage.

Forest strips, in addition to a number of other advantages, allow to regulate the thermal regime due to snow accumulation and change of microclimate of area, reduce the wind speed by 20-40% in inter-strip areas compared to open spaces.

Irrigation can be used both to accumulate heat and to reduce it. Soils after irrigation reduce the fraction of reflected radiation by 20%, heat emissivity is reduced, which increases heat storage. High soil moisture promotes soil heat transfer, which improves soil warming and reduces temperature fluctuations.

At the same time, an increase in soil moisture leads to a decrease in temperature as a result of high heat input for warming (since the heat capacity of the soil increases) and evaporation of water.

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

Agrometeorological methods

Agrometeorological methods of soil thermal regime regulation are directed to frost control, reduction of soil heat emissivity, etc.

Frost control is carried out by creating smoke screens that “cover” the crops with smoke.

Sources

Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific and Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.

Plant breeding/P.P. Vavilov, V.V. Gritsenko, V.S. Kuznetsov et al; Edited by P.P. Vavilov. – M.: Agropromizdat, 1986. – 512 p.: ill. – (Textbook and textbooks for higher education institutions).