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In this article we will discuss about:- 1. Factors Affecting Soil Temperature 2. Role of Soil Temperature 3. Management 4. Measurement.
Factors Affecting Soil Temperature:
The Soil temperature depends upon at least three factors, directly or indirectly as follows:
(i) The net amount of heat absorbed by soil.
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(ii) The heat energy required to bring a change in the soil temperature.
(iii) The heat energy required in the process like evaporation of water which is constantly occurring at or near the surface of soils.
Besides these, there are some factors affecting soil temperature as follows:
(i) Slope of the Land:
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The angle at which the rays of the sun meet the surface of land greatly affects the amount of solar radiation received by the earth per unit area. Solar radiations falling on the earth at an angle are scattered over a wider area than those falling perpendicularly. The soil temperature on the south slope is always higher than that on the North Slope.
(ii) Climate:
Climate has a great effect on soil temperature. Soils in temperate (i.e. cool) region are cooler than those in tropical (i.e. warmer) region.
(iii) Season:
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Soil temperature varies with different season of the year. In cool season, the soil temperature is low, but in dry season, the soil temperature is high due to air temperature is low and high respectively.
(iv) Nature of the Soil:
Sandy soils, in general, warm up quickly than clay or fine textured soil. Because, fine textured soils carry a large amount of water due to which they are slow to warm. Soil structure also affects soil temperature by controlling the pore space. The granular and crumb structure warms up more rapidly because there is no water logging. Dark coloured soil usually warm up more rapidly than light coloured soil as this soil absorbs more heat.
Moist soil has lower temperature than dry soil, because evaporation of water rather helps to lower the temperature of moist soil. The temperature of surface soil shows considerable fluctuation but soil temperature below certain depth remains more or less constant and is not affected by diurnal or seasonal temperature changes.
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(v) Vegetative Cover:
Bare soil warms up quickly and cools down rapidly. Soils that are covered with vegetation absorb less heat than those that are bare. Vegetation acts as an insulating agents. It does not allow the soil to become either too hot during the summer or too cold during the winter.
Role of Soil Temperature:
Soil temperature have some effects on fertility of soil and plant growth, directly and indirectly, as follows:
(i) Germination of Seed:
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Soil temperature is one of the important factors that influences the germination of seed. The seeds germinate at a particular temperature and different seeds require different temperature. Variation of temperature affects the germination of seed very profoundly. During winter, seeds of vegetables and boro paddy have to be germinated under favourable condition and then seeds are sown in the field. Wheat seeds put forth radicle 6 days after sowing at 5°C, 3 days at 10°C and with 2 days at 18°C.
(ii) Plant Growth:
Temperature is much more important factor of influencing crop production. It affects the growth and development of crops and their flowering, pollination, fruit set, fruit quality, seed production and seed storage. Most plant grows well in a temperature of 21.1°C to 26.6°C. No plant can survive at below 4.4°C and above 48.4°C. Root elongation is very much dependent on soil temperature.
During plant growth physiological processes takes place at a certain temperature. Temperature effects on vegetation is manifested through soil drying especially in dry season when the herbaceous vegetation dies off and the woody one defoliates or stops its growth in ever green species.
(iii) Absorption of Soil Water:
The absorption of soil water by plant roots is greatly influenced by soil temperature. Very low and very high soil temperature affects this physiological processes of plants. The maximum metabolic activities and maximum absorption of water by plant roots takes place generally between 20°C and 30°C soil temperature respectively.
(iv) Availability of Nutrients:
The biological processes for the transformation and availability of nutrients are controlled by soil temperature and soil water. Water transport and ion movement also depends on soil temperature. The nutrient up take by plant roots also depends on soil temperature.
(v) Soil Microorganism:
Soil microorganism needs optimum soil temperature for their proper functioning in the soil. Soil temperature controls the rate of microbiological process involved in the mineralization of nitrogen, nitrogen fixation etc. The optimum soil temperature range for the microbiological activity is between 25°C to 35°C.
(vi) Decomposition of Soil Organic Matter:
Organic matter decomposition depends on the microbial process and the activity of microorganism is closely related with soil temperature. Decomposition of organic matter takes place rapidly at higher temperature.
(vii) Soil Formation and Physical Properties of Soil:
Temperature influences the process of soil formation by weathering directly and soil structure indirectly.
(viii) Plant Diseases:
Plant disease appears at a certain environmental condition and their occurrence is related with soil temperature.
Management of Soil Temperature:
Soil temperature has some effects on plant growth. So it is necessary to control the rise or fall in temperature of surface soil.
Soil temperature under field condition is possible to alter by adopting suitable cultural practices as follows:
(i) Tillage:
Tillage makes the soil loose and friable and increases its porosity and decreases its thermal conductivity. The upper tilled surface has higher temperature than the underlying untitled soil. Tillage influences soil temperature through its influence on aeration.
(ii) Mulching:
Mulch is any material of crop residues (i.e. straw, cotton stalk, sugarcane trash, etc.), gunny bag and waste paper and plastic materials (i.e. polythene etc.) applied to the soil surface to check the evaporation loss of water. The process of applying mulch is called mulching. Addition of mulches results in additional benefits like soil conservation, moderation of temperature, reduction of soil salinity, weed control and improvement of soil structure. Mulches keep the surface of the land cool during the hot summer season and warmer in cold climate as it cuts off much of solar radiation.
(iii) Irrigation and Drainage:
Irrigation has some effects in maintaining soil temperature. In tropical areas, irrigation generally causes rapid and substantial reduction in maximum temperature in summer and increase in minimum soil temperature in winter. A portion of heat received from solar radiation is absorbed by soil water. Waterlogged soil has a low temperature and the temperature of such soil is possible to increase by removing excess water with providing suitable drainage. The removal of excess water lowers its specific heat and increases the temperature of soil.
(iv) Vegetation:
Vegetation provides shade to the young seedlings and protects them from the intense heat during summer. It also helps in lowering the soil temperature.
The factors responsible for loss of soil temperature are as follows:
(i) Radiation – The same portion of temperature absorbed by surface soil is lost by radiation to the atmospheric environment.
(ii) Conduction – The soil temperature is lost from the soil by the process of conduction which means transmission of anything from one point to the other point.
(iii) Precipitation – Precipitation, specially during summer months cools the soil by lowering its temperature.
(iv) Evaporation – The major portion of soil temperature is used for evaporation of soil moisture and transpiration of plant, i.e., evapotranspiration. The evapotranspiration is a process by which soil moisture is lost from soil by evaporation and transpiration of plant.
Measurement of Soil Temperature:
Soil temperature is important plant growth factor.
Soil temperature can be measure by different thermometer as follows:
(i) Mercury or Some other suitable liquid (i.e. alcohol and toluene) thermometer.
(ii) Biometalic thermometer – In this thermometer, the sensitive element consist of two strips of metal with different thermal expansion co-efficient firmly welded together. When the temperature changes, the element is deformed and this deformation is used to move a pointer over a temperature scale. The accuracy available by this thermometer is about 0.5°C.
(iii) Resistance thermometer – This thermometer consist of a thin wire, usually platinum or nickel, often spiralled on a cylinder. A bridge circuit is generally used for measuring the resistance. The resistance of platinum and of most similar metals increases by 0.4 to 0.5 per cent of its value for 0°C rise in temperature above ordinary temperature. This makes the instrument specially suited for accurate measurement during prolonged periods.
A special type of resistance thermometer is the ‘Thermistor’. The resistance of the thermistor decreases rapidly with increasing temperature. The temperature coefficient is negative. At ordinary temperature, a value of – 0.05°C is common.
(iv) Thermal Scanner – Bartholio et al. (1972) made use of thermal scanner to determine soil temperature.
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