Soil Heat: Sources and Factors

After reading this article you will learn about the sources and factors that influence soil heat.

Sources of Soil Heat:

(i) Solar Radiation:

Radiant energy from the sun is the power source that determines the thermal regime of the soil and the growth of plants. Agriculture is the exploitation of solar energy in the presence of an adequate water supply and sufficient plant nutrients to maintain plant growth.

Much of the radiation from the sun is dissipated and distributed by components of the earth’s atmosphere as it passes downward toward the earth. The heat absorbed by the surface of the earth from the solar radiation is affected by several parameters like latitude, distribution of land and water, slope of the land etc.

The angle at which the sun’s rays meet the earth greatly influences the amount of radiation received per unit area. Radiation reaching the earth at an angle is scattered over a wider area than the same radiation striking the earth’s surface perpendicularly.

Consequently, in the former case the amount of heat received per unit area is decreased in proportion to the increase in area covered. The radiation received per unit area decreases with an increase in the angle (between the perpendicular to the surface and the direction of the received heat rays).

The angle at which the rays of the sun strike a steep south slope is entirely different from that on a steep north slope; the southern slope received more solar radiation per unit area. The temperature of the soil is always higher on southern exposures than on northern.

The presence of large amount of water in soil tends to stabilize the temperature because of the high specific heat of water, which is responsible for the absorption of large amounts of heat. So solar radiation is the greater and main source of soil heat.

(ii) Bio-Chemical Reactions:

In the soil atmosphere a variety of chemical reactions are going on and during such reaction liberation of large amount of heat in the soil environment results. Besides decomposition of organic matter and other crop residues in the soil and other microbial processes liberate large amounts of heat in the soil and thus contributes soil heat.

(iii) Conduction:

The inner atmosphere of the earth is very hot; the conduction of the heat to the soil environment is very slow. Generally, during night, the upper most surface soil becomes cooler than sub-surface soil. Thus heat flows from the regions of sub-soil to the region of surface soil (cooler soil layer).

(iv) Precipitation:

During the winter season precipitation increases soil heat because of its (precipitation) higher specific heat.

(v) Exposure:

Exposure is of little importance in the tropics because of the high elevation of the sun. It is of significance in the middle latitude where the elevation is lower. The greater the percentage of diffuse sky radiation in the global radiation, the smaller is the difference in the incoming solar energy per unit area for slopes of different exposures.

(vi) Vegetation:

Vegetation plays a significant role of soil heat because of the insulating properties of plant cover. Bare soil is unprotected from the direct rays of the sun and becomes very warm during the hottest part of the day.

The major impact of vegetation are associated with:

(1) The albedo effect (absorptivity of soil),

(2) Decreasing the depth of penetration of global radiation through the canopy,

(3) Increasing the latent heat in evapotranspiration, and

(4) Decreasing the rate of heat loss from the soil through its insulating influence.

Factors of Soil Heat:

There are various factors that influence the loss of soil heat like, radiation conduction, evaporation and precipitation etc.

(i) Radiation:

The quantity of heat is absorbed by the surface soil does not remain constant. Some portion of the absorbed soil heat is lost to the atmospheric environment by radiation.

(ii) Conduction:

The conduction process means the transmission of anything from one point to the other point. Here heat is transmitted to the subsurface horizon of the soil from the surface soil layer by the conduction process and thereby results the loss of soil heat from the surface soil.

(iii) Evaporation:

The major portion of the global radiation in humid climates is used in the process of evapotranspiration. This process consumes 580 cal/g of water that is changed from the liquid to the gaseous phase. This heat energy is lost to the soil and ultimately returned to the atmospheric air resulting loss from the soil mass.

This results a cooling effect especially at the surface. The potential evapotranspiration process may be defined as the amount of water that will be lost by evaporation and transpiration from a surface that is completely covered with vegetation if there is sufficient water in the soil at all times for the use of the vegetation.

(iv) Precipitation:

During summer months, precipitation has generally a cooling action in soil, because rain water usually has a lower temperature than soil.