🧫 Thermal properties of soils

Soil thermal behavior controls seed germination, root activity, nutrient availability, and crop performance under field conditions.

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Influence of Soil Temperature and Air on Plant Growth

The thermal properties of soils are a component of soil physics that has

found importants uses in engineering, climatology and agriculture. These

properties influence how energy is partitioned in the soil profile. While related to

soil temperature, it is more accurately associated with the transfer of heat

throughout the soil, by radiation, conduction and convection.

Main soil thermal properties:

Volumetric heat capacity, SI units: Jm [-3] K [-1]

Thermal conductivity, SI units: W.m [-1] K [-1]

Thermal diffusivity, SI units: m2.s

Soil temperature – Soil air – Gaseous exchange

Soil Temperature

Soil temperature is an important plant growth factor like air, water and

nutrients. Soil temperature affects plant growth directly and also indirectly by

influencing moisture, aeration, structure, microbial and enzyme activities, rate of

organic matter decomposition, nutrient availability and other soil chemical

reactions. Specific crops are adapted to specific soil temperatures. Apple grows

well when the soil temperature is about 18  C, maize 25  C, potato 16 to 21  C, and

so on.

Sources of soil heat

The sources of heat for soil are solar radiation (external), heat released

during microbial decomposition of organic matter and respiration by soil

organisms including plants and the internal source of heat is the interior of the

Earth - which is negligible. The rate of solar radiation reaching the earth’s

atmosphere is called as solar constant and has a value of 2 cal cm [-2 ] min [-1] . Major

part of this energy is absorbed in the atmosphere, absorbed by plants and also

scattered. Only a small part of it reaches soil. Thermal energy is transmitted in the

form of thermal infrared radiation from the sun across the space and through the

atmosphere.

Factors affecting soil temperature

The average annual soil temperature is about 1  C higher than mean annual

air temperature. Soil temperature is influenced by climatic conditions. The factors

that affect the transfer of heat through the atmosphere from sun affect the soil

temperature also.

Environmental factors

Solar radiation: The amount of heat received from sun on Earth’s surface is 2 cal

cm [-2] min [-1] . But the amount of heat transmitted into soil is much lower. The heat

transmission into soil depends on the angle on incident radiation, latitude, season,

time of the day, steepness and direction of slope and altitude. The insulation by air,

water vapour, clouds, dust, smog, snow, plant cover, mulch etc ., reduces the

amount of heat transferred into soil.

Soil factors

a) Thermal (Heat) capacity of soil : The amount of energy required to raise the

temperature by 1  C is called heat capacity. When it is expressed per unit mass

(Calories per gram), then it is called as specific heat . The specific heat of water is

1.00 cal g [-1] where the specific heat of a dry soil is 0.2 cal g [-1] . Increasing water

content in soil increases the specific heat of the soil and hence a dry soil heats up

quickly than a moist soil.

b) Heat of vaporization: The evaporation of water from soil requires a large

amount of energy, 540 kilocalories kg [-1] soil. Soil water utilizes the energy from

solar radiation to evaporate and thereby rendering it unavailable for heating up of

soil. Also the thermal energy from soil is utilized for the evaporation of water,

thereby reducing the soil temperature. This is the reason that surface soil

temperatures will be sometimes 1 to 6  C lower than the sub-surface soil

temperature. That is why the specific heat of a wet soil is higher than dry soil.

c) Thermal conductivity and diffusivity : This refers to the movement of heat in

soils. In soil, heat is transmitted through conduction. Heat passes from soil to water

about 150 times faster than soil to air. So the movement of heat will be more in wet

soil than in dry soil where the pores will be occupied with air. Thermal

conductivity of soil forming materials is 0.005 thermal conductivity units, and that

of air is 0.00005 units, water 0.001 units. A dry and loosely packed soil will

conduct heat slower than a compact soil and wet soil.

d) Biological activity : Respiration by soil animals, microbes and plant roots

evolve heat. More the biological activity more will be the soil temperature.

e) Radiation from soil : Radiation from high temperature bodies (Sun) is in short

waves (0.3 to 2.2  ) and that from low temperature bodies (soil) is in long waves

(6.8 to 100  ) Longer wavelengths have little ability to penetrate water vapour, air

and glass and hence soil remains warm during night hours, cloudy days and in

glass houses.

f) Soil colour : Colour is produced due to reflection of radiation of specific

wavelengths. Dark coloured soils radiate less heat than bright coloured soils. The

ratio between the incoming (incident energy) and outgoing (reflected energy)

radiation is called albedo. The larger the albido, the cooler is the soil. Rough

surfaced soil absorbs more solar radiation than smooth surface soils.

Reflected energy Albido 

Incidentenergy

g) Soil structure, texture and moisture : Compact soils have higher thermal

conductivity than loose soils. Natural structures have high conductivity than

disturbed soil structures. Mineral soils have higher conductivity than organic soils.

Moist soil will have uniform temperature over depth because of its good

conductivity than dry soils.

h) Soluble salts : Indirectly affects soil temperature by influencing the biological

activities, evaporation etc .

Soil Air

Soil air is a continuation of the atmospheric air. Unlike the other

components, it is constant state of motion from the soil pores into the atmosphere

and from the atmosphere into the pore space. This constant movement or

circulation of air in the soil mass resulting in the renewal of its component gases is

known as soil aeration. .

Composition of Soil Air: The soil air contains a number of gases of which

nitrogen, oxygen, carbon dioxide and water vapour are the most important. Soil air

constantly moves from the soil pores into the atmosphere and from the atmosphere

into the pore space. Soil air and atmospheric air differ in the compositions. Soil air

contains a much greater proportion of carbon dioxide and a lesser amount of

oxygen than atmospheric air. At the same time, soil air contains a far great amount

of water vapour than atmospheric air. The amount of nitrogen in soil air is almost

the same as in the atmosphere.

Composition of soil and atmospheric air

Percentage by volume

Nitrogen Oxygen Carbon

dioxide

Soil air 79.2 20.60 0.30

Atmospheric air 79.9 20.97 0.03

Factors Affecting the Composition of Soil Air:

1. Nature and condition of soil: The quantity of oxygen in soil air is less than that in

atmospheric air.

The amount of oxygen also depends upon the soil depth.

The oxygen content of the air in lower layer is usually less than that of the surface soil. This is

possibly due to more readily diffusion of the oxygen from the atmosphere into the surface soil

than in the subsoil.

Light texture soil or sandy soil contains much higher percentage than heavy soil.

The concentration of CO

2 [is usually greater in subsoil probably due to more sluggish aeration in ]

lower layer than in the surface soil.

2. Type of crop: Plant roots require oxygen, which they take from the soil air and deplete the

concentration of oxygen in the soil air. Soils on which crops are grown contain more CO

2 [than ]

fallow lands. The amount of CO

2 [is usually much greater near the roots of plants than further ]

away. It may be due to respiration by roots.

3. Microbial activity: The microorganisms in soil require oxygen for respiration and they take it

from the soil air and thus deplete its concentration in the soil air. Decomposition of organic

matter produces CO

2 [because of increased microbial activity. Hence, soils rich in organic matter ]

contain higher percentage of CO

2 [. ]

4. Seasonal variation: The quantity of oxygen is usually higher in dry season than during the

monsoon. Because soils are normally drier during the summer months, opportunity for gaseous

exchange is greater during this period. This results in relatively high O

2 [and low CO]

2 [levels. ]

Temperature also influences the CO

2 [content in the soil air. High temperature during summer ]

season encourages microorganism activity which results in higher production of CO

Exchange of Gases between Soil and Atmosphere

2 [. ]

The exchange of gases between the soil and the atmosphere is facilitated by

two mechanisms

1. Mass flow: With every rain or irrigation, a part of the soil air moves out into the

atmosphere as it is displaced by the incoming water. As and when moisture is lost

by evaporation and transpiration, the atmospheric air enters the soil pores. The

variations in soil temperature cause changes in the temperature of soil air. As the

soil air gets heated during the day, it expands and the expanded air moves out into

the atmosphere. On the other hand, when the soil begins to cool, the soil air

contracts and the atmospheric air is drawn in.

2. Diffusion: Most of the gaseous interchange in soils occurs by diffusion.

Atmospheric and soil air contains a number of gases such as nitrogen, oxygen,

carbon dioxide etc., each of which exerts its own partial pressure in proportion to

its concentration.

The movement of each gas is regulated by the partial pressure under which it

exists. If the partial pressure on one of the gases (i.e. carbon dioxide) is greater in

the soil air than in the atmospheric air, it (CO ) moves out into the atmosphere.

Hence, the concentration of CO is more in soil air.

On the other hand, partial pressure of oxygen is low in the soil air, as oxygen

present in soil air is consumed as a result of biological activities. The oxygen

present in the atmospheric air (partial pressure of O is greater) therefore, diffuses

into the soil air till equilibrium is established. Thus, diffusion allows extensive

movement and continual change of gases between the soil air and the atmospheric

air. Oxygen and carbon dioxide are the two important gases that take in diffusion

Importance of Soil Aeration:

1. Plant and root growth: Soil aeration is an important factor in the normal

growth of plants. The supply of oxygen to roots in adequate quantities and the

removal of CO from the soil atmosphere are very essential for healthy plant

growth.

When the supply of oxygen is inadequate, the plant growth either retards or

ceases completely as the accumulated CO hampers the growth of plant roots. The

abnormal effect of insufficient aeration on root development is most noticeable on

the root crops. Abnormally shaped roots of these plants are common on the

compact and poorly aerated soils. The penetration and development of root are

poor. Such undeveloped root system cannot absorb sufficient moisture and

nutrients from the soil

2. Microorganism population and activity: The microorganisms living in the soil

also require oxygen for respiration and metabolism. Some of the important

microbial activities such as the decomposition of organic matter, nitrification,

Sulphur oxidation etc depend upon oxygen present in the soil air. The deficiency of

air (oxygen) in soil slows down the rate of microbial activity.

For example, the decomposition of organic matter is retarded and nitrification

arrested. The microorganism population is also drastically affected by poor

aeration.

3. Formation of toxic material: Poor aeration results in the development of toxin

and other injurious substances such as ferrous oxide, H S gas, CO gas etc in the 2 2

soil.

4. Water and nutrient absorption: A deficiency of oxygen has been found to

check the nutrient and water absorption by plants. The energy of respiration is

utilized in absorption of water and nutrients. Under poor aeration condition (this

condition may arise when soil is water logged), plants exhibit water and nutrient

deficiency

5. Development of plant diseases: Insufficient aeration of the soil also lead to the

development of diseases. For example, wilt of gram and dieback of citrus and

peach.

Influence of soil temperature and air on plant growth

Effect of soil temperature on plant growth

a) Soil temperature requirements of plants : The soil temperature requirements

of plants vary with the species. The temperature at which a plant thrives and

produces best growth is called optimum range ( temperature ). The entire range of

temperature under which a plant can grow including the optimum range is called

growth range . The maximum and minimum temperatures beyond which the plant

ed survival limits.	Col2	Col3
Range	Maize (C)	Wheat (C)
Optimum range	25 - 35	15 - 27
Growth range	10 - 39	5 - 35
Survival limits	0 - 43	0 - 43

b) Availability of soil water and plant nutrients : The free energy of water

increases with temperature. Up to wilting point limit, warming of soil increases

water availability beyond which it decreases. Low temperatures reduce the nutrient

availability, microbial activities and root growth and branching. The ability to

absorb nutrients and water by plants reduces at low temperatures.

Soil temperature management

Use of organic and synthetic mulches : Mulches keep soil cooler in hot summer

and warm in cool winter.

Soil water management : High moisture content in humid temperate region lowers

soil temperature.

Tillage management : Tilling soil to break the natural structure reduces the heat

conductance and heat loss. A highly compact soil looses heat faster than loose

friable soil.

Methods of measuring soil temperature : Mercury soil thermometers of different

lengths, shapes and sizes with protective cover are buried at different depths to

measure the temperature. Thermo couple and thermister based devices are also

available. Infra-red thermo meters measure the surface soil temperature. Automatic

continuous soil thermographs record the soil temperatures on a time scale. The

International Meteorological Organization recommends standard depths to measure

soil temperatures at 10, 20, 50 and 100 cm.

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Summary Cheat Sheet

Quick Recall Points

• Key thermal parameters are heat capacity, thermal conductivity, and diffusivity.
• Soil moisture strongly modifies temperature fluctuations.
• Temperature affects germination, roots, and microbial activity.

Exam Traps

• Dark soils warm faster at the surface but moisture can moderate heating.
• Thermal conductivity increases with moisture up to relevant limits.

References

2 sources • [1] [2]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

FAO Soils Portal

Website