Soil temperature is an important growth factor not only for crops but also for the soil microbial population.
Keep on reading and try to answer an MCQ at the end of this article…!
What is soil temperature?
Temperature is actually the measure of heat.
What is heat in relation to soil temperature?
The heat means the energy obtained from random motion i.e. vibration of molecules of a substance. The speed of vibration of moleciles present in that substances is directly proportional to temperature.
Also read our various other related articles under soil science category
Source of soil heat in relation to Soil temperature
Solar radiation is the source of soil heat. The flux of heat (colories or joules) into and out
of the soil determines the soil thermal regime, which is characterized in terms of soil
How much solar radiation reaches the earth?
On an average only 50% of solar radiation reaches the earth, because of clouds and dust
particles intercept the sun rays. This energy is primarily utilized to evaporate water from soil or leaf surface or is radiated or reflected back to the sky. Only about 10% is absorbed by soil, which is of critical importance to soil processes and to plants growing on the soil.
Albedo in relation to Soil temperature
The fraction of incident radiation that is reflected by the land surface is termed the
“Albedo”, and ranges from 0.1 to 0.2 for dark coloured, rough soil surfaces to as high as 0.5 or more for smooth, light coloured surfaces.
Thermal capacity of soil
Thermal capacity also called as the “heat capacity” of a soil is defined as the ratio of heat supplied to a body to the corresponding rise in its temperature.
The heat capacity per unit mass of a body is called the “specific heat (e) and is defined
as the quantity of heat required to raise the temperature of a unit body through 1°C. The heat capacity is expressed as quantity of heat required to raise the temperature of unit volume of soil by 1°C and is known as “Volumetric heat capacity” or simply the “heat
capacity” (gram calorie) Cv = J/m 3 /°C or Cal/ cm3 /°C
Specific heat in relation to water
The specific heat is the heat capacity of a substance in relation to that of water. To check specific heat of water with other substances, lets check out the following numbers
Specific heat of water = 1.00 cal/g
Soil forming minerals = 0.2 cal/g
OM –0.46 cal/g ; Air – 2.4 cal/g
Dry soil – 0.233 cal/g; Soil with 50% moisture – 0.53 cal/g
Practically all substances have heat capacities lesser than that of water
Thermal conductivity (T.C.) is defined as the quantity of heat passing in a unit time through a unit area of soil under a unit temperature difference between the faces and is expressed as Joules / meter / second / °C. Actual heat conductance depends on thermal conductivity and on the heat gradient. T.C. of a soil depends on its water content, texture, structure, mineralogical composition, organic matter content and compaction. Thermal conductivity increases with increased water content, till about 50% of the soil saturation. T.C. is observed to decrease with a reduction in particle size. Thermal conductivity of soil varies in the order : Sand > loam > clay > peat.
Thermal diffusivity is defined as the ratio of thermal conductivity to heat capacity. It is the measure of the rate at which changes in temperature occur in the body. The reciprocal of thermal diffusivity indicates the ability of the body to retain heat. Thermal conductivity of Quartz, water and air = 333 : 23 :1
The ability of the body to retain heat is thermal retentivity. It is equal to the reciprocal of thermal diffusivity
heat transfer processes in soil
The major physical processes responsible for the transport of heat are conduction, convection and radiation.
It may be defined as “ the flow of heat through an unequally heated body from the places of high temperature to lower temperature”. It is through the momentum of molecules. The molecules of the body nearer to the source absorb heat and increase their kinetic energy and begin to vibrate about their positions faster than before. They collide with other molecules and in this process they share their energies with the adjacent molecules so that other molecules increase their K.E and behave similarly to transfer heat energy from one end to another
May be defined as the motion of the hot body itself carrying its heat with it. In this process absorption of heat causes increase in volume there by decrease in density of molecules. Such a decrease in density make the hotter molecules to go up, giving their places to the cooler molecules which also behave like that after absorbing heat energy. This process occurs only in fluids.
Radiation may be defined as the process by which heat is transmitted from one place to other with out aid of intervening medium. In this process heat is transferred through the space in the form of electromagnetic waves
Management of soil temperature
The soil temperature depends upon the heat flux in to the soil and the heat transfer processes occurring in the soil and in between the soil and atmosphere, which in turn, depend
upon the thermal characteristics of the soil solids, gases and water. The soil thermal regimes greatly modify the microclimate of the area and exercise a major influence on growth and development of plants, particularly during germination and early seedling development stages.
Soil temperature can be modified by
a) regulating energy balance on the soil surface
b) changing the soil thermal properties and
c) heating the soil through artificial means.
Temperature regulation by energy balance
The source of heat for soil is the solar radiation. The rate at which the radiant energy reaches the earth’s atmosphere from the sun is called the solar constant. The solar radiation, which reaches the soil surface, is partly reflected, partly used for heating soil, partly used as latent heat for evaporation and partly reradiated as long wave radiation. About 90% of long wave radiation is absorbed by the water vapours present in the earth’s atmosphere, which partly reradiate to the earth’s surface. It implies that the energy available for heating the soil is function of the incoming and outgoing radiation. If a greater fraction of incoming radiation is absorbed by the soil than the fraction reflected or reradiated the soil will be heated. The energy balance on the soil surface and therefore be modified through tillage and shaping of fields, mulching and vegetation; and shading and row spacing.
Tillage and shaping of fields
Since both slope and direction affect soil temperature and heat flux, soil thermal regimes can be modified by specific tillage practices such as ridges and shaping. Comparatively, high temperature is recorded on ridges as compared to furrows. Besides, the slope facing west would be comparatively warmer.
Soil temperature is highest in east-west oriented ridges and decreases for other ridge directions in the order
– North – south ridges > East – west ridges > N-S conventional > Listing
Mulching and vegetation
From the energy point of view, mulching is the application or creation of a soil cover that
constitutes a barrier to the transfer of heat or water vapour, where as the vegetation intercepts a considerable part of the incoming radiation.
Vegetation and mulching affect the soil thermal regimes by
1) interception of the incoming radiation
2) changes in albedo conditions and
3) reduction in latent heat transfer by evaporation.
The mulches commonly used include : soil, stubble, straw, weed or trash, gravel, plastic
(black, transparent, opaque etc.) paper (different textures and colors) asphalt and aluminum foils. In general, light coloured mulches will increase albedo of the surface, thereby reduces soil temperature.
Black plastic mulch – reduces out going temperature (heating)
Paper and straw mulches – increases outing temperature (cooling).
Aluminum foil mulch – sharply increases out going temperature (cooling)
Transparent plastic mulch – green house effect
Opaque mulches – thermal insulators (decreases maximum temperature and increases minimum temperature)
Shading and row spacing
Artificial shading is often practiced either to reduce the intensity of the direct radiation falling on the soil surface or to conserve the soil crop canopy heat Increased width in between rows, increases the temperature in rows and also shows lot of variation in temperature in between rows. In regard to row direction, east – west direction planted rows showed more variation in temperature between the rows.
Modification of soil thermal properties
Soil thermal characteristics can be modified by changing soil physical conditions through tillage practices, soil compaction, irrigation and drainage. Tillage will loosen soil, increase the soil porosity and decrease the soil thermal
conductivity and heat capacity.
Importance of soil temperature
Too low or too high temperatures affect the germination of seeds. Different crops have
different optimum temperature for germination.
Absorption of water and nutrients is impaired under low temperatures.
Temperature influences nutrient availability by affecting the weathering of minerals and
decomposition of organic matter.
Low soil temperature results in white succulent roots with less branching.
Low temperature enhances disease incidence by parasitic fungi
Soil microbial activity and decomposition of organic matter is restricted below 10 degree
C and ceases below 5 degree C
Biological nutrient transformations like nitrification, ammonification etc are affected by very high or low temperatures.
Further reading on soil temperature – comprehensive overview
If you have any extra points of study or MCQs related to soil temperature, do feel free to post those in comments section below.
We will include those in this article..!
Question for readers
Highest thermal conductivity is found in which of the below
Post your answer in the comment section below..