Soil Temperature

Heat Energy

The following discussion will define temperature, heat flow transfer by conduction, radiation, and convection. Convection is very minor in soil so only radiation and conduction will be covered in this treatment of heat flow in soils.

Soil is the Earth's reactive surface. Energy, water, and chemicals flow into, through, and out of the pedon.

Pedon: A three-dimensional body of soil that is the soil individual. A pedon with lateral dimensions large enough to permit the study of horizon shapes and relations. It is no smaller than one meter square to a depth of ‘not soil’ and is no larger than eight meters to a depth of ‘not soil’.

Irradiation and Heating

Transmitted, scattered, and reflected radiation remain radiation; only the direction is changed. But when radiant energy is absorbed, it ceases to be radiation; it is converted to electrical or chemical energy, or, most usually, sensible heat. Sensible heat—ordinary, everyday heat—can be sensed or felt and consists of an increase in the motion of molecules in the materials.

Sensible heat exists only in materials and requires a material medium, or conductor, to be transmitted. Radiant energy, by contrast, needs no media; it is sensed by people and thermometers after they have absorbed it and converted it to heat. In common usage (incorrectly), heat includes both "sensible heat" and certain kinds of radiation (e.g., we speak of 'heat lambs' and 'in the heat of the sun'), but in this lesson, heat means "sensible heat" and excludes all forms of radiation.

Daily heating and nightly cooling set up diurnal temperature fluctuations whose amplitude is greatest at the soil surface and progressively lower amplitude with depth. The extremes of soil temperature depend on (one) how much energy comes in, (two) how rapidly heat is dissipated, and (three) how much heat the soil absorbs for a given rise in temperature (a soils heat capacity).

The control and management of soil temperature are dominated by two variables; soil cover and soil water content. Soil cover moderates soil temperature extremes because it blocks incoming radiation by day and impedes outgoing radiation and direct heat transfer from moving air by night. Soil water moderates extremes and temperature because water adds to the soils heat capacity, helps heat move through the soil, and cools the soil by evaporation.

Radiant Heating and Heat Dissipation

Soils and plants are heated by the absorption of solar radiation, and they in turn heat the air. The input and dissipation of energy at the ground govern the temperature regimes in the soil and in the plant canopy. In consequence, they strongly influence other processes: air movement near the ground, evaporation, transpiration, and the transfer of carbon dioxide and oxygen.

The intensity of heating —that is, the temperatures achieved— depends on the soils heat capacity, the rate of absorption of radiation, and the rate of heat dissipation.

What is temperature?

Temperature is a measure of heat energy which is the average kinetic energy of the particles in a substance.

Temperature Notation

Three scales are used to describe temperature: the Fahrenheit (F), Centigrade (C) and Kelvin (absolute) (K) temperature scales. In the English (Fahrenheit) system, the freezing point of water is 32℉ and the boiling point is 212°F. In the metric (centigrade) system, 0℃ is defined as the temperature of freezing water and 100℃ as the temperature of boiling water (at atmospheric pressure). In physics and some other branches of science, zero is defined as the temperature at which mechanical motion of molecules should stop. This is "absolute" zero on the Kelvin (K) scale. Absolute zero is equal to -273℃ and -459℉. The freezing point of water, 0℃, is defined as 273 K.

Importance of Heat Energy

The heat energy in a soil is important because it impacts chemical and biological process within the soil. Specifically the impact is on rates of chemical and biological reactions. A generalization is that more heat energy in the soil speeds up reactions. However, if the heat energy is great it can be detrimental to biological reactions. The following discussion will define temperature, heat flow transfer by convection, radiation, and convection. Soil is the Earth's reactive surface. Energy, water, and chemicals flow into, through, and out of the pedon.

Distribution of Heat Energy

Energy in the form of solar radiation strikes the pedon surface, where it may be absorbed or reflected. All soils normally gain heat by absorbing solar energy (shortwave radiation). Heat is then dissipated (spread, redistributed) by several processes: conduction from the hot surface to the subsurface and to the air, evaporation of water, and re-radiation (long-wave infrared) to the atmosphere and space. Daily heating and nightly cooling set up diurnal temperature fluctuations whose amplitude is greatest at the soil surface and progressively less with depth. The extremes of soil temperature depend on (one) how much energy comes in, (two) how rapidly heat is dissipated, and (three) how much heat the soil absorbs for a given rise in temperature (its heat capacity).

The control and management of soil temperature are dominated by two variables; soil cover and soil water content. Soil cover moderates soil temperature extremes because it blocks incoming radiation by day and impedes outgoing radiation and heat transfer from moving air by night. Soil water moderates extremes and temperature because water adds to the soils heat capacity, helps heat move through the soil, and cools the soil by evaporation.

The driving force for heat flow is the temperature gradient. Heat (energy) flows from hot to cold.

Recall that the three ways heat moves is radiation, convection, and conduction.

Radiation

Radiation is the emission or transfer of energy in the form of electromagnetic waves.

The wavelength of radiation is inversely proportional to the temperature of the radiating body (as temperature increases, the wavelength of the emitted radiation decreases). The Sun (about 6,000 ℃) radiates short-wave radiation and the Earth (-27℃) radiates long-wave radiation.

Solar radiation (short-wave radiation) is the primary source of energy to heat the soil. Approximately 50% of the suns energy reaches the earth's surface.

Energy reaching the soil is i) reflected, ii) consumed by evapotranspiration, iii) reradiated back to atmosphere as thermal or long-wave radiation, or iv) consumed by heating the soil and air.

Conduction (Diffusive Transfer)

Conduction is when heat moves through a solid or liquid via collisions between atoms, passing heat energy from one atom to the next.

Conduction is the principal way in which sensible heat moves in soil. The conduction of heat requires a heat-conductive material: a medium.

Heat flows more rapidly through some media than others. Thermal conductivity (K) is a measure of a material's ability to transmit heat. For example, metals are excellent heat conductors, enhanced the popularity of metal cookware. Areas of poor heat conductor, hence the common use of air trapped in clothing, blankets, and household insulation to hold heat. Note that the air must be trapped and held stationary to be a good insulator and do a good job of impeding heat flow. When air water or fluids are free to mix or flow, they transmit heat rapidly, by processes (turbulent transfer and mass flow, discussed later) that are important in the atmosphere, but not in the soil, where fluids are nearly stationary.

Conductive flow also depends on the area of the conductors cross-section perpendicular to the direction of flow. Flow per unit cross-sectional area is the flux.

Finally, conductive flow depends on the driving force, the tendency for heat to flow. This driving force is the gradient of temperature. Heat tends to flow from the hotter to the cooler parts of a connected system (just as water tends to flow from wetter to drier parts of the soil). Heat flow is a process in which excited hot molecules share their excitement with the cooler neighbors. It is this excitement—the rotational and vibrational energy of molecules—that flows. The process is slow because it depends on individual interactions among molecules more or less fixed in place. Net heat flow stops when it has eliminated temperature differences throughout the system—that is, when the system has reach thermal equilibrium. Thermal equilibrium is unusual in soils.

Heat flow: The mathematical formula to describe the movement of heat.

\(\frac{\text{Q}}{\text{A}\cdot \text{t}} = \text{K}\cdot \frac{\Delta \text{T}}{\Delta \text{x}}\)

T = temperature; ΔT is the difference in temperature between two
points

x = distance; Δx is the change in distance between two points

K = the thermal conductivity (a measure of how fast heat flows through a material)

Relative K values: rock ≈ 4 -- 7; water ≈ 1.4; air ≈ 0.06 (mcal/s cm ℃)

Q = heat flow (calorie is a common unit of heat or energy; 1 Food calorie = 1000 heat calories)

A = area

t = time

Most importantly, heat flow depends on two things:

Heat flow is proportional to thermal conductivity.

as the conductivity of the soil material increases, heat moves faster through the soil

Heat flow in proportional to the temperature gradient

as the difference in temperature between two points increases, heat moves faster through the soil

Soil Temperature = ƒ (net amount of heat absorbed, soil heat capacity, evapotranspiration)

Heat Absorbed: Energy absorbed comes from sun (short-wave) and atmosphere (long-wave)

Heat Capacity: quantity of heat required to change the temperature of a material by 1° C

water = \(\frac{1\thinspace\text{cal}}{\text{cm}^3\cdot {}^\circ C }\); dry soil \(\approx \frac{0.2\thinspace\text{cal}}{\text{cm}^3\cdot {}^\circ C }\); air \(\approx \frac{0.003\thinspace\text{cal}}{\text{cm}^3\cdot {}^\circ C }\)
you can think of heat capacity as how resistant a material is to changing its temperature, water is much more resistant to temperature change than air.

Evapotranspiration - heat energy is consumed by the change of liquid water to water vapor, so less heat energy is available to heat the soil and change the soil's temperature.

Soil Temperature is influenced by:

Soil Color

Albedo = the fraction of the radiation reflected by a surface (black = low, white = high)
Slope -north versus south aspect
Vegetative cover (reflects radiation & ET) or mulch (insulation-high porosity)

Moisture content

affects heat capacity
affects thermal conductivity: heat passes from soil to water 150x faster than from soil to air, thus moist soils have a higher thermal conductivity
affects evapotranspiration (ET consumes 540 cal/g water)

Compaction

particle-particle contact increases thermal conductivity

Mineral soils have a higher thermal conductivity than organic soils.
Why?

Daily temperature fluctuations have larger daily temperature fluctuations occurring in the surface layer (upper 5 cm) and considerably deminished fluctuation occurs below a depth of about 30 cm.

Seasonal temperature fluctuations are measured at 50 cm depth as the impacts of daily fluctuations are dampened at this depth.

soil temperature is warmer than air temperature in winter
soil temperature is colder than air temperature in the summer
this is due to the high heat capacity and low thermal conductivity of soils

Regulating soil temperature:

Mulch or vegetative cover
Controlling soil water content
Avoid compaction. Keep lower ρ_b.