SOIL CLIMATE - heat flow and soil temperature


Sunlight - necessary for plant growth and development as well as the warming of the soil or earth surface.

Radiant energy is a form of energy that can be transferred as electromagnetic radiation.

All objects above 0oK (absolute zero) emit electromagnetic radiation.

Electromagnetic radiation is in two forms:

Shortwave radiation = high energy - x-ray, ultraviolet, visible light and near infrared radiation.

Sunís energy is only shortwave radiation (0.3 to 3 uM)

Longwave radiation - low energy - far-infrared, microwave, radar, TV and radio.

x-ray wavelength is 10-9 whereas radio waves 1M>100M

Radiation less than < 4um (4 x 10-6m) is shortwave.

Radiation originating from the earthís surface is low energy - longwave radiation (>4um)

Ĺ of the sunís energy is in the visible light range (.04 to 0.7 uM)

When radiation encounters an object it can be

Transmitted through the material

It can be scattered



Converted to electrical or chemical energy or sensible heat (warmth).

For sensible heat to be transferred, it requires a conductor.

Radiation requires no material media for transmission.

On a clear day, 75% solar radiation reaches the soil surface

On a cloudy day only about 30% reaches the soil surface

Global average for any day is about +- 50%

Factors affecting the utilization of radiation

3-5% radiation is used in photosynthesis

10-15% is used to heat the plant and soil

30-45% is reflected back to the atmosphere as albedo

(shortwave) or thermal radiation (longwave)

Affects of heat in the soil

Necessary for microorganism activity - nitrification temp. of 5oC (40F)

Plant root growth 27 - 32oC (80-90F)

C = 5/9(F-32)

Heat transfer process





Transfer of absorbed heat from one particle to another in a gradient from high to low

Thermal conductivity is the ability of materials to conduct heat

Metals copper iron = high conductivity

Wood and plastic = low conductivity, are insulators

Conductivity within the soil depends on the volume occupied by:

Solid, liquid and gaseous phases

Mineral soil thermal conductivity is 5 times water

10 times greater than organic matter

100 times greater than air

Wet soils have a much higher thermal conductivity than dry soil - air acts like and insulator in the dry soil


Movement of heat by a heated fluid

Warm air over cool soil - warms the soil

Cold air over warm soil - warms the air

Ceiling fans in tall rooms move air back to the floor

Warmed soils in spring warm the air above the soil that rises into the atmosphere

Why place orchards on the hillside rather than the bottom of the valley?


Sensible heat

All objects around us radiate energy in the form of invisible electromagnetic waves

Campfires with rock - rock radiates sensible heat

Car radiator

Emits shortwave radiation which has a high energy capacity

The earth (ave. temp. of 27oC (80F) radiates longwave radiation which is a low energy capacity

Why does the snow melt on the south side of a tree in the winter first?

Net radiation

The sun is the only significant source of energy to the earthís surface

Net radiation is the total amount of radiation reaching the earth minus the amount reflected by the earth

The net radiation during the day is positive and negative at night

Extremes in diurnal temperatures in desert exist because the desert is dry as well as the air above it so that longwave radiation is not trapped but dissipated to the atmosphere

Less diurnal temperate differences in moist tropical regions with abundant humidity or water vapor because it traps and holds or reradiates back the longwave radiation

Without the natural greenhouse effect, the earthís coverage temperature would be -18oC (-0.4F) instead of the 27oC (80F)


radiant heat used to change the state of water from liquid to vapor

the heat changes the state of water without changing the temperature

heat of vaporization is the amount of heat needed to change water from liquid to a vapor

this phase change requires a large amount of heat

condensation of water vapor release heat


heat is consumed when ice melts and released when water frozen

it takes about 1/6 as much energy to melt ice as it takes to evaporate the same amount of water

freezing water on leaves warms the leaves in the spring time that tends to prevent some frost damage

Why spray trees with water during an impending frost?

Will a dry environment freeze before a moist-humid environment?


Soil temperatures dependent on

Net radiation (shortwave) radiation absorbed

Rate of energy dissipation

Heat capacity of the soil

Net radiation used to warm the layer just above the soil surface

Called sensible heat because you can feel it

Negative sensible heat is used to heat air

Positive sensible heat mens the air is cool and extracts heat from its surroundings

There is a interaction between latent and sensible heat - wet soils require large amounts of latent heat to evaporate the water and the air stays cool. When water is unavailable, less latent heat is used and sensible heat occurs heating the air and the soil. During extremely dry periods, plants may not only suffer from drought but excessive heat due to a large sensible heat term.

Heat capacity is the amount of heat required to raise the temperature of soil by one degree. Generally measure in Joules or calories.

The greater the heat capacity of the material, the more energy it takes to change the temperature and the reverse is the same the longer it takes to cool down.


During day, shortwave radiation heats the soil and some of it is conducted into the soil

Water has a thermal conductivity 20 times greater than air, but it takes 3,000 times as much energy to warm an equal volume of water as compared to air

Thus wet soils are often called cold soils because they conduct less heat because it takes more energy to raise its temperature

Night time loss of longwave radiation from the soils results in conductivity of heat towards the surface from deeper in the soil

In order for the soil to heat any time of they year, it needs a net increase of heat into the soil during the day over the net loss at night.

Spring and summer we get a net increase

Fall we get a net decrease due to heating of the air by the soil



Plant growth

Crop yields

Planting dates

Germination rate or date

Days to maturity of the crop

Greater soil temperatures (warmer soils)

Root development is faster

Nutrient availability is greater

Water movement is usually higher

Microbial activity is greater

Seed germination and growth is enhanced

Germination of crop

Dependent on soil temperatures

Most seeds germinate at about 40 F (4.5 C)

55 to 60 F (12.5 to 15 C) for cotton sorghum, soybeans

Corn needs 50 to 55 (10 to 12.7 C)

Wheat needs 45 F (7.2 C)

Soil temperature variation - see overhead


Surface may vary as much as 40 F (22.2 C)

6 inches may vary only 10 F

24 inches little significant change in a day

Annually - temperature profiles

High temperatures reach in the upper soil late summer, while the lower depth are cooler

Winter the upper is coldest and the deeper the soil the warmer it may be

Lag time difference may be as much as two to three months

How does permafrost develop?

How does a heat exchange system work using the soil as the source of heat or cooling?

Factors affecting soil temperature


Amount of solar radiation reaching the soil


Black soil absorb 75% available heat

Brown soil absorb 50%

Light tan soil absorb 25%

Soil wetness

Reflect more sunlight

Have a greater heat capacity

Slower to change temperatures

Cold in the spring which delays planting and germination

Just enough water to provide good thermal conductivity but not so much to increase the heat capacity beyond the capability of the radiation input

Kind and amount of vegetational cover

Thick canopy results in shading - reducing radiation input

Crops residues in spring time reduce heating and reduce evaporation increasing the heat capacity

What effect will minimum or no till have on the warming of the soil?

The direction of the slope (aspect)

South facing slope verses that of the north face

Plant crops on the south side of the row - may increase germination by 3 to 5 days

Time of year - angle at which radiation hits soil surface

Midsummer radiation strikes the soil at right angles

Winter a greater angle and less heating takes place

The amount of surface the sun actually strikes the surface of the soil and the distance it has to travel

Combined effects of water and temperature

Freezing and Thawing

Repeated freezing and thawing results in soil formation

Clods eventually breakdown to a smooth seed bed

May cause a heave - pull up young seedlings - alfalfa may suffer from frost heave



Solution of plant nutrients

Warm water hastens the dissolution of minerals

Abundant rainfall and warm temperatures lend to low nutrient soils

High soil temperature and adequate soil moisture means, in most cases, more nutrients in solution

Leaching and Eluviation (see overhead)

Water movement through soil results in

Leaching of soil nutrients

Eluviation - downward movement of clay particles from topsoil to subsoil

Illuviation is the accumulation of eluviated materials downward in the soil

Swelling and Shrinking

Smectite clays swell and shrink as water is added or the soil dries

Soil Temperature Control

Snow cover

High albedo

Eearly snow cover - higher soil temperatures

Black plastic cover - absorbs heat but the air soil interface doesnít heat the soil as much as one would think

Transparent plastic - heat better - greenhouse factor

Organic mulches


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