Heat Flow in Soil

  Heat Flow and Storage in Soil

Three major heat transport processes in soils are :

Conduction : It is the primary mode of heat transfer in soils

Radiation : Long wave EMR (0.5 and 30.0 micrometers (μm). Emission of energy in form of electromagnetic waves

Convection : The transfer of latent heat of vaporization during convection of water vapor

Soil : Heat Storage

Soil as a sink of heat – day time; Soil as a source of heat – night time

Annually also it stored heat in summers and release heat in winters This affects soil temperature

Heat storage affects :

  • Root spread of plants - in cold, soil roots spread laterally while in warm season penetrate in deep layers
  • affects soil respiration
  • affects photosynthesis

  Basics of Soil Physics

Components of Soil

  • Soil Solids - Sand , Silt and Clay and Organic matter
  • Soil Water
  • Soil Air/Gases


Soil Phases

  Soil Density

Density in the mass per unit volume. I case of soil it can be classified into Particle Density and Bulk Density.

Particle Density 

Particle density is also termed as true density (2.6 – 2.8)

The weight per unit volume of the solid portion of soil is called particle density. Generally particle density of normal soils is 2.65 grams per cubic centimeter.

The particle density is higher if large amount of heavy minerals such as magnetite; limonite and hematite are present in the soil

With increase in organic matter of the soil the particle density decreases


Bulk Density

The oven dry weight of a unit volume of soil inclusive of pore spaces is called bulk density

The bulk density of a soil is always smaller than its particle density

The bulk density of sandy soil is about 1.6 g / cm3, whereas that of organic matter is about 0.5. Bulk density normally decreases, as mineral soils become finer in texture

The bulk density varies indirectly with the total pore space present in the soil and gives a good estimate of the porosity of the soil. Bulk density is of greater importance than particle density in understanding the physical behaviour of the soil.

Generally soils with low bulk densities have favourable physical conditions

Factors affecting bulk density

Pore space: Since bulk density relates to the combined volume of the solids and pore spaces, soils with high proportion of pore space have lower bulk densities than those that are more compact and have less pore space. Consequently, any factor that influences soil pore space will affect bulk density

Texture: Fine textured surface soils such as silt loams, clays and clay loams generally have lower bulk densities than sandy soils. This is because the fine textured soils tend to organize in porous grains especially because of adequate organic matter content. This results in high pore space and low bulk density. However, in sandy soils, organic matter content is generally low, the solid particles lie close together and the bulk density is commonly higher than in fine textured
Organic matter content: More the organic matter content in soil results in high pore space there by shows lower bulk density of soil and vice-versa


  Types of Soils


At least 70% sand and the clay separate 15% or less of the material by weight. The properties of such soils are therefore characteristically those of sand in contrast to the stickier nature of clays                                                                                 

The silt group includes soils with at least 80% silt and 12% or less clay. Naturally the properties of this group are dominated by those of silt.


35% - 40% of the clay separate. In such soils the characteristics of the clay separates are distinctly dominant                                                                            


An ideal loam may be defined as a mixture of sand, silt and clay particles that exhibits the properties of those separates in about equal proportions. Loam soils do not exhibit dominant physical properties of sand, silt or clay

Types of Soil – Texture Triangle


  Soil Parameters



  Thermal properties of soil

Heat Capacity : Ratio of heat absorbed or released to the corresponding rise and fall in temperature

Unit – Joules  K-1

Mass Specific Heat (Cs) : Amount of heat required to raise the temperature of unit mass of soil by 10K

Unit – J  Kg-1 0K-1

Volume Specific Heat (Cv) : Amount of heat required to raise the temperature of unit volume of soil by 10K

Unit – J  lt-1 0K-1

Cv = Cs. density

  Thermal Conductivity

Thermal Conductivity is the property of soil to conduct heat

Thermal conductivity is defined as the amount of heat transferred through a unit area in unit time (heat flux density) under a unit temperature gradient

Depends on bulk density and soil water content

Unit - J s-1 m-1 0k-1

Increasing soil bulk density hence the contacts between solid particles increases the thermal conductivity. The thermal conductivity also increases with increasing water content. Soil water improves the thermal contact between the soil particles, and replaces air which has 20 times lower thermal conductivity than water.

Factors affecting Thermal Conductivity

Soil texture

In clay soil total pore space (macro  = micro ) pores is more so least thermal conductivity (as air is poor conductor of heat)


Thermal conductivity increase with increase in moisture content as all the pores gets filled with water (water is a good conductor of heat)

Bulk Density - Thermal conductivity increases with increase in bulk density

Organic Matter  (OM)- High OM increase porosity thus decrease thermal  conductivity

Compactness more compactness , less pore space, more bulk density more conductivity

  Soil Heat flux

Soil heat flux -  heat flow into or out of the soil

JH = λ . dT / dz


λ = thermal conductivity

dT = change in Temperature

dz = distance

  • If heat flow towards the surface of soil JH is positive
  • If heat flow away from the soil JH is negative

Instrument – Soil heat flux plate


  Thermal Diffusivity

Thermal Diffusivity (DH)

Thermal diffusivity is the ratio of the thermal conductivity λ to the volumetric heat capacity Cv

DH = λ / Cv

Unit – m2 s-1

Fourier's law for heat transport