Introductory Agrometeorology/Soil temperature

Objectives of the chapter

• To understand
  • diurnal and seasonal variations in soil
  • measurement of soil temperature
  • significance of soil temperature in crop production

Soil Temperature

Soil is heated through:

-Geothermal process -Atmospheric process -Radiative process

Different biophysical, biochemical and microbiological processes are controlled by soil heat/temperature. Temperature of soil depends on soil thermal properties and heat content of soil mass.

Thermal properties of soil are

a. Specific Heat

It is the heat required to raise the temperature of 1 g soil by 1 degree C. For clay, it is generally 0.8 cal / g degree C. It is affected by moisture content. Specific heat at field capacity (FC) is more than at wilting point.

b. Heat Capacity

It is the quantity of heat required to raise the temperature of 1cm3 soils by 1 degree C. Soil with high heat capacity warms up slowly. It depends upon soil moisture content, density of soil, texture, soil characteristics, organic matter content and coverage. It is denoted by C_v . Mathematically, C_v= ρC where ρ= bulk density and C=specific heat At FC, the C_v of most soils lie between 0.3 and 0.6 cal/cm₃/degree C

c. Thermal conductivity

It is the quantity of heat transfer across a unit area in unit time under a unit temperature gradient. It shows the easiness of heat transfer. It varies with porosity, moisture content, texture and organic matter of soil. The thermal conductivity of dry sandy soil is about 0.00046 cal/cm/sec/degree C, while that of soil with OM and rich in humus is about 0.00027 cal/cm-1/sec-1/degree C.

Diurnal Variation of Soil temperature

When solar radiation falls on the soil surface, it heats the soil causing a difference of temperature between surface soil and sub-soil. At day time, heat flows from surface soil to sub-soil and viceversa at night. So, the profile goes through different temperature variations called diurnal variation of soil temperature. Amplitude of diurnal wave of soil temperature decreases rapidly with depth and is smaller at higher depth. Greater diurnal variation occurs on sandy soil than in clayey soil. The temperature fluctuation is not as rapid on a cloudy day as compared to that on a clear day.

Annual Change in Soil Temperature

In January, the upper portion of the soil profile is at minimum temperature and thesoil is progressively warmer at lower depths. Therefore, the net flow of heat is upwards. In February–March, lower layer is still cool, surface soil warms up the rate star accelerate. The flow of heat is now downwards. The whole soil profile attains maximum temperature at the last of May or June. Monsoon prevails in June and the soil temperature decreases, but the net flow heat is still downwards. The variation temperature is maximum at June–September due to heavy rain and high water table condition. After the commencement of October, movement of heat takes place from subsoil to surface soil, and the cycle continues.

Measurement

Soil thermometers are used to measure the soil temperature. Soil thermometers are of two types:

a. Contact type : Mercury thermometer, Bimetallic strip, Constant pressure or volume thermometer, Electric resistant thermometer, Thermocouple.

b. Non-contact type: Optical Pyrometer, Total intensity radiometer

Soil temperature is essential in

1. seed germination
2. functional activity of roots
3. physiology and metabolic activity of plant
4. several physical, chemical biological processes occurred in the soil
5. incidence of plant diseases and effects on plant growth
6. High soil temperature increases the rate of respiration that increases the consumption of carbohydrates. This decreases the size of potato tubers.
7. Nitrogen fixation is favoured in the temperature range of 5-27 degree C. High temperature leads to de-nitrification.
8. Microbial activities are directly influenced by soil temperature. Soil bacteria become inactive

below 5 degree C and death of bacteria occurs usually above 30 degree C.