Soil Temperature - An Overview

   Soil temperature is one of the important physical properties of soil and has a greater influence in Plant Growth and Nutrition.

Sources of Soil Heat:

Solar Radiation:   

  Radiant energy from sun is the power source that determines the thermal regime of the soil and growth of plants. It is the greatest and main source of soil heat.

   Parameters that influence the absorption of heat from solar radiation:

• Latitude,
• Distribution of land and water,
• Slope of land,
• Angle of the sun rays, etc.,

   Scattering of radiation per unit area decreases with the increase in angle of radiation, whereas radiation per unit area increases with its increase.

   Southern slope receives more radiation per unit area compared to the north, hence temperature of soil is always higher on the southern exposures than on northern.

Bio-chemical Reactions:

  A variety of chemical reactions are going on in the soil atmosphere which liberate a large amount heat in the soil thus contributing as one of the soil heat sources.

Conduction:

   Even though the inner atmosphere of earth is very hot, the conduction of  this heat to the soil environment is very low, but during night time as the surface soil is cooler compared to the sub-surface soil a considerably amount of heat is transferred from the sub-soil.

Precipitation: 

   During winter season, precipitation increases the heat as rain water has high specific heat.

Exposure:

   Exposure has greater  significance in the middle latitude where the elevation of sun is lower while has little importance in tropics due to high elevation. A greater percentage of diffuse radiation makes smaller difference in the amount of radiation per unit area in the slopes of different exposures (northern and southern).

Vegetation:

   Vegetation plays a significant role on soil heat due to the insulating properties of plant cover.

Impact of vegetation are associated with:

• The albedo effect,
• Depth of radiation penetration,
• Latent heat in evapo-transpiration,
• Decreasing heat loss through its insulating influence.

Loss of Soil Heat:

 Radiation:

   Some portion of the absorbed soil heat is lost into the atmosphere as radiation, hence the quantity of heat absorbed by the surface soil does not remain constant.

Conduction:

   

  Heat is transmitted to the subsurface horizon from the surface horizon by conduction process, thereby results in the loss soil heat from the surface.

Evaporation:

  A major portion of the global radiation is lost to evapo-transpiration in humid climates, which consumes 580 cal/g of water that is changed to gaseous phase. This results in the cooling effect especially at the surface.

Precipitation: 

   During summer months, precipitation has a cooling action in soil, as rain water have low temperature than soil.