The characteristics of the soil give it certain properties that affect the process of growing cultivated plants. Let's consider the types of thermal properties of soil: heat absorption capacity, heat capacity, thermal conductivity. What could be the sources of heat for it, 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 entering the soil is solar radiation, which consists of direct and diffuse.The intensity of radiation depends on the latitude and altitude of the area, the carbon dioxide content in the atmosphere and its transparency.
The absorbed energy is then transferred either to the atmosphere or to the lower layers. Where the heat is directed depends on the soil and air temperature. If the soil is warmer and the air is colder, heat will escape into the atmosphere. With a large absorption of heat, the soil heats up and thermal energy begins to flow down. The greater the temperature difference in the upper and lower layers, the greater the rate of heat entry.
The amount of solar energy that enters the soil depends on the climate zone, weather, relief features, color, its thermal and physical properties, and vegetation density.
There are also sources of heat - energy released during the decomposition of plant residues located on the surface or in the upper layer, and energy that is transferred from the air.
A very small amount of heat enters the soil from within the Earth and from the radioactive decay of elements, but it is practically insignificant.
How to determine
How much heat is in the soil depends on many factors. Water is a heat-intensive component of soil, so wet soil takes longer to warm up than dry soil. But it also takes longer to cool. Clay wet soils take the longest to warm up in the spring, sandy soils take the longest to warm up, but in the fall the opposite happens: clay soils are warmer due to slow cooling.
Thermal conductivity depends on the air content 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 thermal properties; fertile soils retain heat longer, poor soils lose it faster. Vegetation in summer and snow in winter retain heat and help retain 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 respiration rate and waste of organic matter, which leads to a reduction in the volume of green mass. Soil temperatures above 50-52 °C lead to plant death.
For normal plant growth, a certain amount of heat is required; 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 needed not only by plants, but also by microorganisms. They are negatively affected by cold and excessive heat; both lead to the suspension of the vital activity of bacteria and biota. The optimal 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 soil's ability to absorb solar energy. The radiation is not completely absorbed; some of it is reflected back. 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 volume of solar radiation that reached the soil.
The lower the albedo, the more heat the soil can absorb. Heat absorption capacity depends on the color of the soil, its moisture content, structure, surface topography and vegetation density. Dark soils warm faster than light-colored soils.
Heat capacity
This characteristic is defined as weight and volume. Heat capacity by weight is the amount of heat, measured in calories, that must be expended to heat 1 g of dry soil by 1 °C. Volumetric heat capacity is the heat that can be used to heat 1 cubic meter. see at 1 °C.
The value of heat capacity varies depending on the moisture and air content in the soil. When wet, its heat capacity will be higher than when dry. Clay soil will have a higher heat capacity than sandy soil because it contains less air.
Thermal conductivity
This is the ability of 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 phases of the soil and is measured in the amount of heat expressed in calories. Soil thermal conductivity is measured in the amount of heat that passes through a cube. cm of soil in 1 s.
Soil thermal regime and its types
Different climate zones have different thermal regimes. Based on two indicators - average annual temperature and the nature of freezing - all soils are divided into 4 types.
Permafrost
This thermal regime occurs in soils located in the permafrost zone. The soil thaws during the warm annual period and completely freezes in winter.Temperatures at a depth of 20 cm and average annual temperatures are below zero.
Long-seasonally freezing
In summer, the soil thaws, 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 ground temperature is above zero, but in January at a depth of 20 cm it is below zero.
Seasonally freezing
It freezes shallowly in winter and thaws during warm periods. 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 ground temperature is above zero, but in January at a depth of 20 cm it is below zero.
Anti-freeze
The soils do not freeze even in winter. The temperature is always positive, both at a depth of 20 cm and the average annual temperature.
The 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.
