BOTANY 3214 - SOILS
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?
All objects around us radiate energy in the form of invisible electromagnetic waves
Campfires with rock - rock radiates sensible heat
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?
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 OF FUSION IS THE HEAT GIVEN OR USED WHEN WATER IS EITHER FROZEN OR THAWED.
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.
SOIL HEAT STORAGE
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
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%
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
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
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