Thermal properties of the soil: state depending on temperature and types of regime

Characteristics of the soil give it certain properties that affect the process of growing cultivated plants. Consider the varieties of thermal properties of the soil: heat-absorbing capacity, heat capacity, thermal conductivity. What could be the sources of heat for her, as well as the thermal regime and its types: freezing and non-freezing.

Possible sources of heat in the soil

The main source of heat in the soil is solar radiation, which consists of direct and diffuse. The intensity of radiation depends on the latitude and height of the area, the content of carbon dioxide in the atmosphere and its transparency.

The absorbed energy is then transferred either to the atmosphere or to the lower layers. Where the heat will go depends on the temperature of the soil and air. If the soil is warmer and the air is colder, the heat will escape into the atmosphere. With a large absorption of heat, the soil heats up, and thermal energy begins to flow down. The rate of heat input is greater, the greater the temperature difference in the upper and lower layers.

The amount of solar energy that enters the soil depends on the climatic zone, weather, topography, color, its thermal and physical properties, vegetation density.

There are still sources of heat - the energy released during the decomposition of plant residues located on the surface or in the upper layer, and the energy that is transferred from the air.

Very little heat enters the soil from inside the Earth and from the radioactive decay of the elements, but it is practically irrelevant.

How to identify

How much heat is in the soil depends on many factors. Water is a heat-consuming component of the soil, so wet soil warms up longer than dry soil. But it also takes longer to cool down. Clay moist soils warm up the longest in spring, sandy soils warm up faster, but in autumn the opposite happens: clay soils are warmer due to slow cooling.

Thermal conductivity depends on the content of air in the pores. The looser the soil, the faster it warms up, and vice versa, dense soil heats up more slowly. The amount of humus also affects the thermal properties, fertile soils retain heat for a longer time, the poor lose it faster. Vegetation in the summer, snow in the winter retain heat and help keep it in the ground.

For most cultivated plants, the favorable temperature for growth is 20-25 °C. If it is more than 30 ° C, development is inhibited. An increase in acceptable temperatures leads to a strong increase in the intensity of respiration and the waste of organic matter, which leads to a reduction in the amount of green mass.Soil temperatures above 50-52 °С lead to the death of plants.

For the normal growth of plants, a certain amount of heat is needed; in agriculture, a value called the sum of active temperatures is used. These are all days of the growing season when the temperature during the day was above 10 ° C.

Soil heat is necessary not only for plants, but also for microorganisms. They are adversely affected by cold and excessive heat; both lead to the suspension of the vital activity of bacteria and biota. The optimum temperature is 15-20 °C, minor deviations are acceptable.

Thermal properties

This category of characteristics includes: soil heat absorption capacity, heat capacity and thermal conductivity.

Heat absorption capacity

This is the ability of the soil to absorb solar energy.The radiation is not completely absorbed, some part is reflected back. The heat absorption capacity is determined by the albedo value (A). It is expressed as the amount of solar radiation that was reflected by the soil surface, and is presented as a percentage of the amount of solar radiation that hit the soil.

The lower the albedo, the more the soil can absorb heat. The heat absorption capacity depends on the color of the soil, its moisture content, structure, surface topography and vegetation density. Dark soils heat up faster than light soils.

Heat capacity

This characteristic is defined as weight and volume. Weight heat capacity - the amount of heat, measured in calories, that must be spent on heating 1 g of dry soil by 1 ° C. Volumetric heat capacity - the heat that can heat 1 cubic meter. see 1°C.

The value of heat capacity varies depending on the content of moisture and air in the soil. In the wet state, its heat capacity will be higher than in the dry state. Clay earth will have a greater heat capacity than sandy earth, as it contains less air.

Thermal conductivity

This is the ability of the soil to conduct heat from the upper layers, where the temperature is higher, to the lower, colder ones. Heat transfer occurs through the solid and liquid soil phases, measured in terms of heat expressed in calories. Soil thermal conductivity is measured in terms of the amount of heat that passes through a cube. see soil for 1 s.

Air has the lowest thermal conductivity of all soil media, the highest - in the mineral part. Thus, structureless and dense soils have a high thermal conductivity, loose - less. The value also depends on other factors: the amount of organic and mineral matter (in poor soils, the indicator is lower than in fertile ones). It also depends on the degree of moisture (if the pores are filled with water, then the thermal conductivity increases, if it is filled with air, it decreases).

Thermal regime of the soil and its types

Different thermal regimes develop in different climatic zones. According to two indicators - the average annual temperature and the nature of freezing - all soils are divided into 4 types.

Cryogenic

This thermal regime in soils located in the permafrost zone. The soil thaws during the warm annual period, completely freezes in winter. Temperatures at a depth of 20 cm and average annual sub-zero temperatures.

Long-seasonal-freezing

In summer, the soil thaws, it freezes deeply in winter, to a depth of at least 1 m. The duration of freezing is at least 5 months a year. The average annual temperature of the soil is above zero, but in January at a depth of 20 cm it is below zero.

Seasonal freezing

It freezes shallowly in winter, thaws in the warm period. The duration of freezing varies greatly - from several days to 5 months. Cold can penetrate to a depth of no more than 2 m. The average annual temperature of the soil is plus, but in January at a depth of 20 cm it is minus.

Frostproof

The soils do not freeze even in winter. The temperature is always positive, both at a depth of 20 cm and the annual average.

Soil thermal regime determines the intensity and direction of soil formation processes. The duration of the growing season, the species composition and productivity of vegetation, the number of microorganisms and the intensity of their work, which affects the rate of humus formation, the volume of organic matter, and the intensity of chemical reactions, depend on the characteristics of the regime.

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