Sources of soil heat
▪ The sources of heat for soil are solar radiation 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.
Effect of soil temperature on plant growth
(i) Germination of seeds. The germination of the different seeds is greatly influenced by the variation in soil temperature. If the temperature is too low, the seed fails to germinate or germinate at a slow rate. On the other hand, seeds may be injured if the temperature will be very high
Soil temperature requirements of plants
The soil temperature requirements of plants vary with the species. The temperature at which a plant thrives and produces best growth is called optimum range temperature. The entire range of temperature under which a plant can grow including the optimum range is called growth range. The maximum and minimum temperatures beyond which the plant will die are called survival limits.
ii) Physical properties of Soil. Soil structure is greatly influenced by the temperature. The temperature has a great influence on the aggregation of the soil as well as on the binding materials present in it.
(iii) Availability of soil water and plant nutrients:
The free energy of water increases with temperature. Various physico-chemical and chemical reactions are greatly influenced by soil temperature. Temperature influences the solubility reactions of different nutrients and releases larger amount of nutrient elements in the soil solution at higher temperatures. Low temperatures reduce the nutrient availability, microbial activities, root growth and plant development. The ability to absorb nutrients and water by plants reduces at low temperatures and hence the availability also gets reduced.
(iv) Microbial activity. The microbial activity is influenced by the variation in soil temperature. A certain amount of heat is necessary for the proper functioning of various types of soil micro-organisms. Various microbiological processes like mineralization of nitrogen, nitrogen fixation are influenced by the soil temperature.
The activity of micro-organisms is lowest when soil temperature is below 50C and above 540C. The optimum temperature for the activity of most of the micro-organisms is in the range of 25-35oC
(v) Decomposition of organic matter in soil. Thedecomposition of organic matter is largely influenced by the soil temperature. At low temperature, the rate of organic matter decomposition is low resulting various toxic organic substances in soil and at high temperature the rate of decomposition is very fast resulting in beneficial products of organic matter decomposition which influences the plant growth.
(vi) Plant diseases. Development of various diseases isalso related to the soil temperature. At low temperatures, the soil contain many weakly prarasitic fungi which will grow actively and very rapidly and so those will kill the seedlings.
Soil temperature management
Use of organic and synthetic mulches: Mulches keep soil cooler in hot summer and warmer in cool winter. These mulches also supply different plant nutrients after decomposition.
Soil water management: High moisture content in humid temperate region lowers soil temperature. Dark coloured soils due to organic matter application will have high water content and lowers the soil temperature.
Tillage management: Tilling soil to break the natural structure reduces the heat conductance and heat loss. A highly compact soil looses heat faster than loose friable soil.
