1. Soil Temperature
Soil temp determines palnting time, germination time, days for crop to produce

2. Significance: Affects growth rate of roots
Seed germination and survival of seedlings
Affects microbial decomposition
Soil water temperature changes solubility of minerals
Changes in soil temperature control life cycle of some soil animals

3. Soil temperature regimes measured at 50 cm
Cryic: very cold; mean annual temperature 0°C to 8°C.
Frigid: cold with warmer summer than cryic; mean annual
temp < 8°C.
Mesic: mean annual temp between 8°C and 15°C
Thermic: mean annual temp between 15°C and 22°C
Hyperthermic: warm; mean annual temp >22°C
Isohyperthermic: hot, tropical

5. Isofrigid--The mean annual soil temperature is lower than 8°C
Isofrigid--The mean annual soil temperature is lower than 8°C. Isomesic--The mean annual soil temperature is 8C or higher but lower than 15°C. Isothermic--The mean annual soil temperature is 15°C or higher but lower than 22°C. Isohyperthermic--The mean annual soil temperature is 22°C or higher. pergelic: permafrost

6. Energy balance Incoming shortwave from sun
Outgoing longwave from earth-atmosphere

8. Spring or early summer; Temperate region
Schematic representation of the radiation balance in daytime and nighttime in the spring or early summer in a temperate region. About half the solar radiation reaches the earth, either directly or indirectly, from sky radiation. Most radiation that strikes the earth in the daytime is used as energy for evapotranspiration or is radiated back to the atmosphere. Only a small portion, perhaps 10%, actually heats the soil. At night the soil loses some heat, and some evaporation and thermal radiation occur.

9. Overall balance in soil:
Daily (diurnal):
Net gain in day
Net loss at night
Annual:
Net gain in summer
Net loss in winter

10. Factors affecting amount of solar radiation reaching soil
Land Cover
Vegetation canopy
Snow
Mulch
Sources of shade; reduce evaporation
Color
Affects albedo
Dark vs light colored soil
Wet vs dry soil
Vegetation vs bare soil
Aspect of Slope
South facing vs North-facing

11. Soil heat flux
amount of thermal energy that moves through an area of soil in a unit of time.

12. Heat flux diagram Annual: Winter heat flux Summer heat flux
Duluth: mean annual air = 39.1; this depth = ~20 m high lat’s; 15 m midlatitudes; 10 m tropics
Crossover at 4 m represents penetration lag of cold and warm transmitted to depth
Spring, fall: transitional
Turnovers are important triggers to soil animals (come out of hibernation, etc.

13. heat flux : Constant temp. at depth MAST (mean annual soil temp.)
About 1°C (2F) warmer than mean annual air temp
(39.5⁰ F Duluth)
“depth of zero annual range”
high lat :20 m
midlat: 15 m
Tropics: 10 m

14. HEAT FLUX Crossover at ~ 4 m.
penetration lag of cold and warm transmitted to depth
Spring, fall: transitional
Turnovers are important triggers for soil animals

15. MAST corresponds roughly to the water temperature measured in groundwater wells 30 – 50’ deep
MAST observations at individual stations,
superimposed on well-water temperature contours.

16. Geothermal heat (not volcanic type)
Transfers heat to building from soil
In summer, soil can remove heat from building
In winter, can transfer heat from soil to building
Tubing in trenches
“ground source heat pump”

18. Soil temp cycles
Annual pattern
Diurnal pattern

19. Soil temperature cycles :
Notice:
1. Decreased amplitude and increased lag time with depth
Changes in conductivity with depth
2. Diurnal temp wave discernible to about 0.8 m
3. Annual temp wave to 14 m

20. Snow and Soil Temperature
Snow insulates and significantly dampens the diurnal range in temperature

21. Thermal properties
1.Specific heat/heat capacity 2. Thermal conductivity

22. 1. Specific heat/heat capacity
Ability to store heat
Amount of heat required to raise temperature of 1 g of substance by 1 degree C
greater the heat capacity of a substance, the more heat it can gain (or lose) per unit rise (or fall) in temperature
Soil: 0.2 cal/g
Water: 1.0 cal/g
(It takes more energy to heat a wet soil than a dry soil)
Texture determines how quickly soil will heat up in spring
Sandy soils contain less water and therefore heat more quickly than clay
Texture determines how quickly soil will heat up in spring
Sandy soils contain less water and therfore heat more quickly than clay

23. Bulk density and heat capacity
Dry, compacted soils (high BD) have higher heat capacity than dry, noncompacted (low BD) soils.
Fewer macropores in compacted
For wet soils, BD and porosity do not affect heat capacity

24. light dry soils experience greater seasonal temperature swings at a given depth than wet soils.

25. 2. Thermal conductivity Ability to conduct (transfer) heat
Conduction is molecule to molecule transfer of heat
Affected by:
moisture
texture

26. a. moisture
thermal conductivity of water is about two to three times greater than that of soil.
In saturated soils, pore spaces filled with water rather than air
Wet soils have higher conductivity than dry

27. b. texture:
thermal conductivity of air is about one hundred times less than that of soil .  
Finer soils have more particle-to-particle contact and smaller pore spaces, therefore increased conductivity.
Conductivity increases as texture becomes increasingly fine.

28. Notice that adding water makes texture have opposite effect (wet sand higher cond. than wet clay)

29. Texture determines how quickly soil will heat (in spring) or cool (in fall).
Sandy soils contain less water (lower porosity) and therefore heat more quickly than clay.

30. Ways to manage soil temperature
Ridge and furrow tillage; can create N or S facing slopes

31. Mulching! Color controls albedo
Can be used to hold moisture and control evaporation