1. TEMPERATUR TANAH

2. 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.

3. Heat transport processes:
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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4. Convection -- transport of heat by some heat‑carrying mass.
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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5. 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 losses
a. Heat flow to colder soil above or below.
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6. Albedo for selective materials Material Albedo
Soil, dry silt loam uncultivated 0.23
Soil, dry silt loam cultivated 0.15
Soil, wet clay loam
Soil, dry clay loam
Grass
Alfalfa
Corn
Black painted concrete
White painted concrete
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7. where T is temperature and X is vertical distance.
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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8. Cv = 3 Cifi = Cmfm + Cofo + Cwfw + Cafa
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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9. Kt varies considerably between various soil constituents.
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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10. Heat Conduction Movement of Sensible Heat Q - Quantity of heat
A - Cross sectional area
Kt - Thermal conductivity
T - Temperature
X - Distance
Solid
Dry Soil
Wet Soil
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12. 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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15. 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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16. Soil Temperature Profile with Season Frost Free Region30 10
Summer
Fall
Spring
Winter 3
Depth, m
Temperature, oC 20
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17. EFFECT OF SOIL TEMPERATURE ON SOIL WATER MOVEMENT
Water moves from soil at high temperatures to soil at lower temperatures.
If freezing occurs, water movement can be substantial.
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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18. 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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