🌡 Soil Temperature

Thermal properties of soils

• The thermal properties of soils are a component of soil physics that has found importance 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^-3K^-1
• Thermal conductivity, SI units: W m^-1K^-1
• Thermal diffusivity, SI units: m²s

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
  • 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.

    Specific hear order: Sand < Silt < Clay < Humus

  • 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.
  • 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.
  • Biological Activity: Respiration by soil animals, microbes and plant roots evolve heat. More the biological activity more will be the soil temperature.
  • Radiation from soil: Radiation from high temperature bodies (Sun) is in short waves (0.3 to 2.2 μ) and that from low temperature bodies (soil) is in long waves (6.8 to 100 μ) 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.
  • 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.

    

  • 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.
  • Soluble salts: Indirectly affects soil temperature by influencing the biological activities, evaporation etc.