Soil Temperature

1. Soil Temperature

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 regimesmeasured at 50 cm soil • 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. Energy balance • Incoming shortwave from sun • Outgoing longwave from earth-atmosphere

7. Spring or early summer; Temperate region

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

9. 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

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

11. Heat flux diagram • Annual: • Winter heat flux • Summer heat flux

12. 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

13. 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

14. 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.

15. 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”

17. Soil temp cycles Annual pattern Diurnal pattern

18. 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

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

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

21. 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

22. 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

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

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

25. 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

26. 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.

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

28. 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.

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

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