What is soil temperature?
Soil temperature is an important factor in plant growth and control of chemical and biological activities of soil. The decomposition of organic matter, mineralization of organic N, seed germination and plant growth increase with the increase in temperature but slow down at extreme temperatures.
Almost all biological processes operate normally in the range of 5°–50°C but often stop outside this temperature range. soil temperature is measured by Soil thermometers.
Soil temperature for seed germination
Different soil temperatures are required for the germination of seeds of different crops. Even the soil temperature required for germination of seeds of different species of the same crop varies slightly.
Here we are describing the optimum soil temperature required for the germination of seeds of some major crops, which are as follows:
|Crops||Required soil temperature|
|Wheat||20- 25° C|
|Paddy||30- 37° C|
|Pearl millet||20- 30° C|
|Chickpea||15- 35° C|
|Pea||18- 23° C|
|Tomato||16- 29° C|
|Tobacco||18- 25° C|
Apart from the above mentioned crops, 20-25 °C soil temperature is optimal for germination of seeds of most crops.
Source of Heat
- Radiation Energy- The main source of radiation energy is the sun, from which all living things get heat. Energy is obtained from the sun in autumn at 50 calorie/cm2/day2 and in summer 500-700 calorie/cm2/day2. Only 0.5% of the total energy received is available to the crops. Less than 10% or only 0.05% of the available energy is used for grains and crops.
- Transmission of heat from the Earth’s inner surface to the soil- There is enough heat in the earth’s womb which manage the soil temperature.
- Chemical and biochemical reactions- Heat is generated in the soil by various chemical and biochemical reactions.
- Heat capacity of the soil- The total amount of heat present in the soil is called the soil heat capacity or the caloric heat required to raise the temperature of 1 gram of soil by 1 degree Celsius is called the heat capacity of soil. The amount of more water and organic matter increases the heat capacity of the soil.
Factors affecting soil temperature
Factors affecting soil temperature are as follows-
- Atmospheric conditions- Almost all the short wavelength (<0.3µ) rays of the sun spectrum are absorbed by the outermost atmosphere. Only long wavelength rays and visible rays reach the surface of the earth.
- Soil Latitude and Topography- The amount of radiation received on a unit area of the soil surface is proportional to the cosine of the angle between the perpendicular to the soil surface and the direction of the incident rays. Therefore, with an increase in this angle, the amount of radiation received per unit area will be more but the amount of heat received per unit area decreases.
- Soil and water distribution- Generally island climates are more homogeneous than continental climates. The reason for this is the huge ocean water reserves around the islands which keep the temperature stable.
- Soil properties.- Soil properties like colour, specific heat, heat conductivity etc. determine the soil temperature. Deep colored soils absorb more heat during the day. In densely organized soils, heat is transmitted easily and quickly. Soils with more and larger pore spaces hold more air, which, being a poor conductor of heat, hinders the transmission of heat.
- Vegetation cover- When there is no crop, the sun’s rays fall directly on the soil, due to which the soil temperature increases. Similarly, due to the vegetation cover on the soil surface decreases the soil temperature.
- Organic matter- The excess of organic matter in the soil increases the water holding capacity, which increases the specific heat of the soil, due to which the soil take time in being warm and cool.
Importance of soil temperature in agriculture
Temperature is directly related to agriculture and crop production. All biological reactions take place smoothly under favorable temperature. The importance of soil temperature in agriculture is as follow-
Effect on germination
Different crops require different temperatures for the germination of seeds. One type of seed germinates only within a particular temperature range. The temperature at which maximum germination takes place in the shortest time is called the most suitable temperature. Seeds of some crops require cold treatment before germination, this is called springification.
Effect on vegetative growth
Normally the temperature range for plant growth is the same as for enzyme reaction. Plants grow slowly at low temperatures and rapidly at most suitable temperatures, and growth slows at high temperatures. The minimum, most suitable, and maximum temperatures for the growth of different plant varieties are different. Temperature affects plant growth as follows-
- Absorption of water and nutrients- At low temperature the velocity of water decreases, then the protoplasm becomes less permeable and the growth of roots stops. With the decrease in temperature, the absorption of essential nutrients by plants also decreases rapidly.
- Respiration- The temperature range is wide enough for respiration in plants.
- Photosynthesis- Photosynthesis takes place well within a certain range of temperature. Photosynthesis usually occurs smoothly between 20-25 °C in plants in cold regions, but is better at 25-40 °C in warm regions.
- Transfer of water and nutritive solutes- Temperature affects plant activities related to photosynthesis, respiration, absorption of water and nutrients, thereby affecting solute transfer.
- Transpiration- If other factors of crop production remain the same, the rate of transpiration increases with increase in temperature. This is due to the effect of temperature on the stomata opening, closing and vapor pressure inclination of the leaves of plants.
Effect of temperature on reproductive stage
- Flowering stage- Flowers develop rapidly when the temperature is favorable. Flower development takes time when the temperature is low or high.
- Reproductive stage- For good yield and grain filling, the reproductive stage should be long so that a large amount of food can be produced by photosynthesis. But due to higher temperature, the reproductive stage becomes shorter and the crop matures sooner.
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