TEMPERATUR TANAH

TEMPERATUR TANAH• Just as important to plant growth as air temp. The

temp of the surface soil fluctuates greatly both during a 24 hr period and with the seasons.

• Where soil is covered by a dense growth of plants or a thick layer of mulch, temperature variations are much less severe and do not penetrate deeply

• Soil temperature has a direct effect on plant growth and also influences microbial activity.

• Freezing and thawing of the soil water also affects soil structure. Slow and occasional freezing and thawing (like under mulch) is beneficial for soil structure.

Soil temperature:-- determined by transport processes for heat to and within the soil.

• Heat transport processes:• 1. Radiation• 2. Conduction• 3. Convection• 4. Latent heat transfer

• Radiation --transfer of thermal energy from a mass by electromagnetic waves.

• Both convection and conduction of heat require molecules to transfer heat.

• Radiative heat exchange requires no molecules to transfer heat from one surface to another. Radiant energy is given off by all objects.

• Both the sun and earth emit radiation but because the sun is hotter (at a higher temperature), the radiant energy is greater for the sun's surface than for the earth's surface.

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• 1. Radiation• 2. Conduction• 3. Convection• 4. Latent heat transfer

• Conduction -- occurs by transmission of thermal energy of motion from one particle (microscopic size) to another.

• Convection -- transport of heat by some heat carrying mass.‑

• Latent Heat -- heat that can change matter without raising its temperature. (Example is: humidity, humid days snow will melt).

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FACTORS AFFECTING RATE AT WHICH SOIL WARMS OR COOLS

A. Energy input (root zone)1. Solar radiation absorbed depends on the amount of radiation ‑‑received and surface albedo.2. Convection movement of a heat carrying mass.‑‑ ‑

a. Air mass warmer than soil surface (parking lot effect).b. Rain that is warmer than the soil.c. Condensation of water vapor.

3. Conduction movement of heat within a body‑‑a. Heat conductance from warmer soil below or above.

B. Energy losses

1. Radiation to colder atmosphere (at night).2. Convection losses

a. To colder air (gaseous exchange).b. Rain or snow that is colder than the soil.c. Evaporation.

3. Conduction lossesa. Heat flow to colder soil above or below.

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Albedo for selective materials Material AlbedoSoil, dry silt loam uncultivated 0.23Soil, dry silt loam cultivated 0.15Soil, wet clay loam 0.11Soil, dry clay loam 0.18Grass 0.24-0.26Alfalfa 0.25Corn 0.16-0.17Black painted concrete 0.095White painted concrete 0.45

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CONDUCTION OF HEAT IN SOIL

• Heat flux, qh, or heat flow, Qh, is proportional to a temperature gradient, )T/)X and the area of soil, A, that the flow takes place.

• The proportionality constant is the thermal conductivity, Kt. Thus heat flux is given by

• Qh = KtA()T/)X)

• where T is temperature and X is vertical distance.

• Note: This is similar to Darcy's Law for water flow in soil.

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HEAT CAPACITY• Volumetric heat capacity (Cv) = the amount of energy required to raise a unit bulk

volume of a substance (in our case, soil) 1oC.

• The volumetric heat capacity of a soil, Cv, is summation of the heat capacity of the different soil components (Ci), weighted according to their fractional volume (fi):

• Cv = 3 Cifi = Cmfm + Cofo + Cwfw + Cafa

• The subscripts m, o, w, and a refer to the soil mineral, organic, water, and air components, respectively.

• The heat of air is 103 < that for organic and quartz (mineral soil particles), and 104 < water.

• So the contribution by air to the soil heat capacity is negligible. • Thus,• Cv = fmCm + foCo + fwCw

• The heat capacity can also be expressed on a per unit weight basis, Cg. The product, DbCg, is equal to Cv.

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THERMAL CONDUCTIVITY

• The thermal conductivity, Kt, = the amount of heat transferred across a unit area in unit time under a unit temperature gradient.

• Kt varies considerably between various soil constituents.

• Kt is also a function of the soil bulk density and water content.

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Heat Conduction

• Movement of Sensible Heat– Q - Quantity of heat– A - Cross sectional area– Kt - Thermal conductivity– T - Temperature– X - Distance

dXdTAKQ

Solid Dry Soil Wet Soil

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SOIL TEMPERATURE PROFILES

• A. The soil receives most of its energy input during the day and loses energy by radiation and convection at all times, but generally more at night.

• B. The effects of energy gains and/or losses on soil temperature are greatest at the surface and decrease with depth.

• 1. The diurnal fluctuations in soil temperature in most soils does not penetrate below 50 cm.

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SOIL TEMPERATURE PROFILES

• B. The effects of energy gains and/or losses on soil temperature are greatest at the surface and decrease with depth.

• 1. The diurnal fluctuations in soil temperature in most soils does not penetrate below 50 cm.

• 2. The annual fluctuations in soil temperature does not extend below 3 m.

• a. During spring and summer, there is a net gain in energy and the soil warms up.

• b. During fall and winter, there is a net loss of energy and the soil cools down.

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Soil Temperature Profile with SeasonFrost Free Region

3010

Summer

FallSpring

Winter

0

3

Dept

h, m

Temperature, oC

20

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EFFECT OF SOIL TEMPERATURE ON SOIL WATER MOVEMENT

1. Water moves from soil at high temperatures to soil at lower temperatures.

2. If freezing occurs, water movement can be substantial.

3. Freezing is similar to drying, and water will flow toward a freezing zone, resulting in almost solid packing of the pores with ice if the water movement to the freezing zone is fast enough.

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EFFECT OF SOIL TEMPERATURE ON PLANT GROWTH

• Seed germination and plant growth • Each species of plant has a minimum

soil temperature below which no germination or elongation (growth) occurs. Above that minimum, elongation rate increases to a maximum, with increasing temperature, then falls rapidly.

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