Soil Temperature: Importance, Factors and Its Control

After reading this article you will learn about:- 1. Importance of Soil Temperature 2. Factors Affecting Soil Temperature 3. Heat Balance 4. Daily and Seasonal Variation 5. Control.

Importance of Soil Temperature:

Temperature begins to affect soil characteristics from the stage of weathering of its parent rock. Temperature fluctuates frequently enough to crack and exfoliate hard rocks which are finally disintegrated enough to facilitate chemical weathering to take place.

Increase in temperature accelerates chemical weathering of primary minerals in presence of adequate amount of moisture. Soil micro-organisms are able to function within a wide range of soil temperature i.e. from 25°C to 30°C. They decompose organic matter.

The higher the temperature, more rapid is the decomposition of organic matter to release the nutrient especially nitrogen contained in it in the soluble form which are either absorbed by plant roots or lost by teaching. Hence soil organic matter content depends on mean annual soil temperature.

So temperate region soils contain more organic matter than tropical region soils. The intensity of other microbial process taking place in the soil depends on soil temperature. For example nitrification takes place best at a temperature varying from about 27°C to 32°C.

The soil must be warm and moist enough for the seeds to germinate. Seeds of different crops germinate at different temperature e.g. maize begins to germinate at a temperature varying from 7° to 10°C, The soil temperature should be 10° to 29°C and 4°C to 10°C for maximum germination and growth of maize and wheat and peas respectively.

Potato tubers sprout best and potato crop grows best at 16°C to 21°C. The optimum temperature for emergence of cabbage and pinch seeds vary from 8° to 11°C, that for emergence of beets and cauliflower seeds varies from 11° to 18°C.

If the temperature of the soil is very low then seeds either do not germinate or germinate very feebly. Seeds may be injured in very hot soil.

Temperature affects the rate of entry of water in the seed. It therefore controls the germination of seeds when it affects the development of plumule and the radicle when plant grows, photosynthesis and respiration are influenced by temperature of the soil which also influences the growth of roots and absorption of nutrients by them. Most of the warm season i.e. kharif crops grow best at a temperature of about 25°C.

Cold soils tend to retard the absorption of phosphate by plant roots from it. This may be remedied by draining them. Drainage tends to warm these soils up.

Alternate freezing and thawing of soils results in the lifting up of roots of plant. This phenomenon is known as having of soils. Roots get broken by this phenomenon.

However alternate freezing and thawing of soil improves the structure of cloddy soil if it contains moderate amount of moisture but destroy the soil structure if the soil contains excessive moisture.

Anhydrous ammonia should be applied at a depth of 10 cm when the soil is at 10°C or less at 10 cm depth because ammonium ions are very slowly converted to nitrate below 10°C. So leaching loss of nitrate is less.

Factors Affecting Soil Temperature:

(i) Nature of the soil:

Inorganic vs. Organic:

As the average specific heat of dry inorganic soil is about 0.22 cal/gm. and that of humus is about 0.44 cal/gm. so dry inorganic soil would warm up quicker and also would cool down quicker than the dry organic soil.

This means that organic soils, once they have been warmed up, will remain warm for longer period, than inorganic soils. Hence organic soils remain warmer in winter and cooler in summer than the inorganic soils because the former warms up slowly in summer and cools down slowly in winter in comparison to the latter.

(ii) Soil moisture content:

As the specific heat of water is 1 cal/gm and the average specific heat of dry inorganic soil is 0.22 cal/gm., so the specific heat of the soil increases with the increase in its moisture content as shown below:

Suppose the soil contains 20 per cent moisture on weight basis.

This means that 100gms of a dry mineral soil has absorbed 20gms of water.

Heat required to raise the temperature of 100gms of dry inorganic soil by 1°C is 0.22 x 100 = 22 calories.

Heat required to raise the temperature of 20gms of water by 1°C is 20 calories.

Therefore the heat required to raise the temperature of 100 + 20 = 120gms of moist soil by 1°C is 22 + 20 = 42 calories

Therefore the heat required to raise the temperature of 1gm of moist soil will be 42/120 = 0.35 calories.

Therefore the specific heat of the inorganic soil has increased from 0.22cal/gm. to 0.35cal / gm. when the soil moisture content has increased from 0 to 20 per cent.

Hence moist soil remains cool in summer and warm in winter because it warms up slowly in summer and cools down slowly in winter.

As water evaporates especially in summer from moist soil, so it loses heat and cools down. Hence moist soil remains cooler than dry soil in summer.

(iii) Soil texture:

The specific heat of pure quartz is 0.17cal/gm. and that of pure kaolinite is 0.24gms /cal. clayey soils contain more humus and more moisture, so the specific heat of average clayey soil is more than that of the sandy soil.

Therefore sandy soils would warm up quicker in summer and cool down quicker in winter than clayey soils. In other words, clayey soils remain cooler in summer and warmer in winter than sandy soils because the former warms up more slowly in summer and cools down more slowly in winter than the latter.

(iv) Slope of the land:

The angle at which the sun rays meet the land surface, affect the amount of solar radiation reaching the unit area of the land surface. If the same solar radiation reaches the plane surface of the land, and the land surface inclined at an angle against the horizon line, then it would meet more area of the land in the latter case than Sun rays in the former case as illustrated in Fig. 4.5.

So the amount of solar radiation received by the unit area of the land surface would be more on plane land surface than on the inclined land surface. Hence the soil would warm up quicker in the former case than in the latter case.

(v) Vegetative cover of the land surface:

Bare soil absorbs heat and becomes very hot during summer very quickly and becomes very cold during the winter. This will not happen, if the surface of the land is kept covered with organic matter which would insulate the soil. So the soil will neither become too hot nor too cold.

(vi) Soil Depth:

Heat is conducted very slowly from the surface to the subsurface and back. So the temperature of the subsurface soil remains lower in summer and higher in winter than the surface soil.

(vii) Soil colour:

Suppose a black coloured soil and a white coloured soil are located side by side and both the soils are absolutely dry and do not Contain any humus. As black coloured substance absorbs heat and a white coloured soil reflects heat incident on them, so under this circumstances only, the black coloured soil would be warmer than the white coloured soil.

However usually a black/darker coloured soil contain more clay and therefore more humus and moisture than a white/light coloured soil, so the former may not be warmer than the latter.

Heat Balance of Soils:

The heat balance of a soil refers to the gains and losses of heat energy. Some portion of the incoming radiation is reflected back to the atmosphere while some portion of it is absorbed by the soil surface. A dark coloured soil may absorb 80 per cent of the incoming solar radiation while a light coloured sand my absorb 30 per cent of it. The albedo is the amount of reflected solar radiation.

Albedo of soil and water are less than 10 per cent and about 20 per cent respectively. More than half of the incoming solar radiation is reflected from the snow whose albedo is more than 50 per cent.

Some part of the solar radiation is expended for evaporating water, some other part of it is expended for heating the soil and some part of it is expended for heating the air while the remaining part of it is reradiated from the surface of the earth. A gain of heat to the soil and its losses from the soil balances each other in the long run.

Daily and Seasonal Variation in Soil Temperature:

During the day the soil temperature is maximum between noon and 2.00 p.m. and minimum during the night and morning. The average monthly temperature is usually highest in June and minimum in December and January. The variation in soil temperature decreases as the soil depth increases. Soil which remains covered with vegetation has undergone less variation is tem­perature than bare unprotected soil.

Control of Soil Temperature:

In a cold climate, the soil may be kept warmer by keeping it covered with some organic matter called mulches and draining the soil. Mulches also keep the surface of the land cool during the hot summer season.

Organic matter mulches increase the infiltration of water in the soil; so the soil remains moist and therefore takes longer time to warm up in summer and to cool down in winter. Light coloured organic matter mulch also reflects some solar radiation incident on the land surface.

The surface of the soil may be kept covered by dark coloured plastic mulches which would absorb more incoming solar radiation, prevent the soil from radiating heat and reduce the evaporation of water from the soil. The soil may thus be kept warm by keeping it covered with da: k coloured plastic mulches during the winter.