Fundamentals of Soil Science (1+1)

Soil Temperature

Soil Temperature

• Soil temperature greatly affects the physical, chemical and biological processes which occur in the soil. Since, with every 10oC rise in temerature, rate of chemical reaction get almost doubled
• Hence, it 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.

Sources of heat to soil

• Solar radiation (sun rays) is the primary source of energy to warm the soil.
• The dust particles, clouds and other suspended particles intercept the sun rays. They absorb, scatter or reflect the solar energy. Only a small part of the total radiation actually reaches earth.
• Thermal energy is transmitted in the form of thermal infrared radiation from the sun across the space and through the atmosphere.
• Other sources of heat for soil are processes like microbial decomposition of organic matter and respiration by soil organisms including plants.

Factors influencing soil temperature:
There are three main factors affecting soil temperature as described below:
1. Solar radiation:

• Amount of solar energy received by soil depends on the constituents of the atmosphere. Clouds, water vapour and dust particles reduce the solar energy reaching soil surface.
• Some energy is used for evaporation and transpiration, and some reflected back. Only 10 per cent energy is used to warm the soil.

2. Aspect and slope:

• The land in three situations viz. southern aspect, level and northern aspect receive different amounts of solar energy, and the soil will warm accordingly.
• Solar radiation reaching perpendicular to the soil surface will heat it more as these concentrate on a smaller area than when the same amount of radiation reach a slope where they get spread on a larger area. Thus, equatorial zones are warmer than temperate and arctic regions.
• In Northern hemisphere, south facing slope is warmer than the north facing slope.

3. Soil factors:

• These factors are soil colour, soil moisture, mulching, vegetative cover and organic matter content. These affect the warming of soils through solar radiation.

Measurement of soil temperature:

• The soil temperatures are less variable. They are by and large similar to the temperature of the atmosphere. However, the temperature in the sub-soil lags behind that of the surface soil. That is why the sub-soil temperature is higher in winters and lower in summers as compared to surface soil temperature.
• The soil temperature is measured using contact or non-contact thermometric methods.
• In most of the commonly used contact methods, changes in temperature are recorded using mercury thermometers and thermocouple and thermister based devices.
• The non-contact type methods include optical pyrometers, total intensity radiometers and infra red thermometers.
• The International Meteorological Organization recommends standard depths viz. 10, 20, 50 and 100 cm, to measure soil temperatures.