Introduction

• Is the most important factor that controls the intensity of biophysical, biochemical and microbiological process in soil
• Heat is supplied to the soil by geothermal activity, atmospheric process and irradiative transfer

Thermal properties of Soil

a) Specific heat:

• Heat required to increase the temperature of 1 gm of soil by 1 ◦C.
• Specific heat of water is 1 cal/gm-◦C. The constituents of soil have much less specific heat than this. Of the soil constituents, quartz has minimum specific heat (0.1cal/gm-◦C) and humus the maximum (0.45 cal/gm-◦C).
• In general, specific heat of the different soil textures are in following orders: Silt loam>clay loam>silty clay loam>clay soil.
• Specific heat is affected by moisture content. Specific heat at field capacity is more than that at the wilting point.
• Hence the specific heat of the soil mainly depends on the specific heat of mineral constituents and moisture contents.

b) Heat capacity/Volumetric heat:

• Quantity of heat required to raise the temperature of 1 cm3 soils by 1 °C. It is denoted by Cv and expressed in cal/cm3-°C. Heat capacity of water is 1 cal/cm3-°C. Most field soils have field capacity of 0.3 to 0.6 cal/cm3-°C.
• It depends on soil moisture content, density of soil, texture, soil characteristics as organic matter content, coverage etc. Soil with greatest heat capacity will warm up slowest.

c) Thermal conductivity/Heat flux (K):

• is the quantity of heat transfer across a unit cross sectional area in a unit time under a unit temperature gradient.
• is the rate at which soil can conduct heat.
• is the ability of the soil to transfer heat from molecule to molecule
• It shows the easiness of heat transfer in the soil. It varies with porosity, moisture content, texture and organic matter of soil.
• It is expressed in Jm-1s-1K-1or W m-1K-1(watt=1J/s).
• Thermal conductivity of the water is 0.0014 cal/cm-sec-°C.
• High thermal conductivity means low heat capacity. A sandy soil
• has higher thermal capacity (0.0046 cal/cm-sec-°C; low specific heat) than loam and clay soil. A soil with high organic matter has the least thermal capacity (0.00027 cal/cm-sec-°C).
• Thermal conductivity is greatly influenced by moisture content.