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WELCOME presentation by , B.Pragadeeswaran, BSF-09-020 FC&RI Mettupalayam. : 

WELCOME presentation by , B.Pragadeeswaran, BSF-09-020 FC&RI Mettupalayam.

Soil temperature : 

Soil temperature

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SOIL TEMPERATURE: Temperature of the soil gravity affects the physical , chemical processes occurring in the soil.   Sources of soil Heat: There arevarious sources of soil heat namely ,solar radiation , Biochemical reaction ,conduction,precipitation,explosure and vegetation etc.

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(1).Solar radiation: Radient energy from the sun is the power source that determines the thermal regime of the soil and growth of plants.In presence of solar energy an aqequate water supply and sufficient plant nutrients to maintain plant growth. Radiation from the sun is distributed by components of the earth’s atmosphere as it passes downward to ward the earth.the heat is absorbed by the surface of the earth from the solar radiation is affected by several parameters like latitude ,distribution of land and water ,slope of the land etc.

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The angle at which the sun’s rays meet the earth greatly influences the amout of radiation received per unit area . the radiation received per unit area decreases with an increases in the angle. The angle at which the rays of the sun strike a steep south slope is entirely different from that on a steep north slope.the southern slope received more solar radiation per unit area. The temperature of the soil is always higher on southern exposures that on northen.

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The pressure of large amount of water in soil tends to stabilize the temperature because of the high specific heat of water ,which is responsible for the absorption of large amount of heat. Solar radiation is the greater and main source of the heat. (2).Bio –Chemical Reactions: In the soil atmosphere a variety of chemical reactions are going on and during such reaction liberation are going on and during such reaction liberation of large amount of heat in the soil environment results.Decomposition of organic matter and other crop residues in the soil and other microbial processes liberate large amounts of heat in the soil and thus contributes soil heat.

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(3).Conduction: The inner atmosphere of the earth is very hot ,the conduction of heat to the soil environment is very low.generally during night ,the uppermost surface soil becomes cooler than subsurface soil.thus heat flows from the regions of sub soil to the region of surface soil.(cooler soil layer). (4).Precipitation : During the winter season precipitation increases soil heat because of its (precipitation) higher specific heat.

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(5).Exposure : Exposure is the little importance in the tropics because of the high elevation of the sun .it is of signification in the middle latitude where the elevation is lower. The greater the percentage of diffuse sky radiation ,the smaller is the difference in the incoming solar energy per unit area for slopes of different exposures. (6).Vegetation: Vegetation plays a significent role of soil heat because of the insulating of properties of plant cover.bare soil is unprotected from the direct rays of the sun and becomes very warm during the hottest part of the day.

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The major impact of vegetation are associated with ( i).The albedo effect.(absorptivity of soil ). (ii).Decreasing the depth of penetration of global radiation through The canopy. (iii).Increasing the heat in evapotranspiration. (iv).Decreasing the rate of heat loss from the soil through its insulating Influence.   Loss of heat: There are various factors that influence the loss of soil heat like radiation ,conduction,evaporation and precipitation.

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(i).Radiation: The quantity of heat is absorbed by the surface soil does not remain constant.some portion of the soil absorbed soil heat is lost to the atmospheric environment by radiation. (ii).Conduction: The conduction process means the transmission of anything from one point to the other heat is transmitted to the sub surface horizon of the soil from the surface soil layer by the conductance process and there by results the loss of soil heat from the surface soil.

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(iii).Evaporation: The major portion of the global radiation in humid climates is used in the process consume 580 cal /g of water that is changed from the liquid to the gaseous phase.this heat energy is lost to the soil and ultimately returned to the admospheric air resulting loss from the soil mass.this results a cooling effect especially at the surface. The potential evapotranspiration process may be defined as the amount of water that will be lost by evaporation and transpiration from a surface that is completely covered with vegetation if there is sufficient water in the soil at all the times for the use of the vegetation.

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(iv).Precipitation. During summer months ,precipitation has generally a cooling action in the soil ,because rainwater usually have a lower temperature than soil.  Factors influence of soil temperature: (1). Compotition of soil – soils containing much more mineral matter get heated very easily than those of soils containing higher amount of O.M. (2).Soil structure –by controlling pore spaces spheroidal type of structure warm up more quickly. (3).Soik texture –Alight textured sandy soil ,absorb heat very quickly than heavy textured clayey soils .a heavy soils carry a great quality of water and due to this reason it warms up very slowly.

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(4).Soil moisture-Controlling thermal regime.the specific heat of water is than the soil. moist soils have a higher specific heat then dry soil .consequently a moist soil has a lower temperature than dry soil.moist soil gets heated very slowly and its cooler then dry soils. (5).Soil colour-Dark colour of soil is due to presence of large amount of humus. (6).Vegetation. (7).Irrigation and drainage-Irrigation raises the humidity of the air lowers the temperature over the soil and reduces the daily soil temperature variation.Drainage decreases the heat capacity of wet soils ,which raises the soil temperature .this plays an important role in warming up the soil in the spring .

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(8).Topography-The temperature of the ridged field is higher than those that are level. (9).Compactness of the soil. (10).Climate. (11).Season. Seasonal variation in a year –temperature vary summer months in the northern hemisphere like midday,represents,the peak of the global radiation and max.temperature. this winter months daily night temperature lower.the temperature of surface soil is always higher than the air temperature.

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Thermal Conductivity and diffusivity: Once heat is gained (Solar radiation.)by the soil ,the soil temperature is governed by the thermal characteristics of the soil (i.e) Heat capacity ,thermal conductivity and thermal diffusivity.the amount of heat required to produce a given change in the soil temperature depends upon the mass and the nature of the soil. Influence of Soil temperature on plant growth: (1). Germination of seeds : if the temperature is too low ,the seed fails to germinate at a slow rate. If temperature high may be injured to seeds. (2).Physical properties of soil: soil structure influenced by soil temperature on aggregation of the soil as well as on the binding materials present in it.

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(3).Microbial activity: Micro organisms having thermophobic and thermophilic nature variation in temperature.Microbiological process like mineralization of nitrogen,nitrogen fixation ,pesticide degradation etc. are influenced by the temperature. Temperature below 5◦c-Activity of M.O’s is lowest and above 45◦c-activity of M.O’s is lowest.25-35◦c optimum temperature for activity of M.O. (4).Decomposition of O.M in the soil: Low soil temperature –decomposition low –toxic materials developed high temperature –fast decomposition –beneficial products of M.O.

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(5).Absorpton of water. (6).Availablity of nutrients. (7).Root Growth soil temperature low temperature encourages white succulent roots with little branching,while high temperature encourages a browner ,finer and much more freely branching root system. (8).Plant diseases- low soil temperature –weekly parasitic fungi will grow actively kill the seedlings.   REFERENCES: (i).Fundamentals of soil science.(shivanand tolanur). (ii).http//

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