1. Soil temperature & modes of energy transfer.

There are three principal modes of energy transfer: (1) radiation, (2) convection, & (3) conduction. Below freezing there is extremely limited biological activity. A soil horizon as cool as 5^OC acts as a deterrent to the elongation of roots. Changes in temperature affect rock weathering, structure formation and the heaving of plant roots.

By radiation, we refer to the emission of energy in the form of electromagnetic waves from all bodies about 0⁰ K. According to the Stephen-Boltzmann law:

T_t=ET⁴

The absolute temperature also determines the wavelength distribution of the emitted energy (Wein’s law):

Where is in microns. From this it follows that the radiation emitted by the soil surface is in the realm of infrared or heat radiation. A very different spectrum is emitted by the sun which acts as a black body at an effective surface temperature of about 6000 ⁰K. The sun’s radiation includes the visible light range of 0.3-0.7µm, as well as some infrared radiation (up to about 3 µm) and some ultraviolet radiation ( <0.3 µm).

The second mode of energy transfer, convection, involves the movement of a heat-carrying mass, as in the case of ocean currents or atmospheric winds.

Conduction, the third mode of energy transfer, is the propagation of heat within a body by internal molecular motion. Temperature is an expression of the kinetic energy of a body’s molecules. Within the soil, heat transfer by radiation and convection are generally of secondary importance and the primary process of heat transport is by molecular conduction.

2. Conduction of heat in soil. Heat-related soil properties.

The first law of heat conduction, know as Fourier’s law, states that the flux of heat in a homogeneous body is in the direction of and proportional to the temperature gradient: q_h= -K_z dT/d_z.

Among the heat (or thermal) properties of the soil the most important to agronomist and ecologist are: (1) heat absorption or reflection, (2) heat capacity, (3) heat conductivity and (4) thermal conductivity.

Heat absorption is called Albedo in meteorology - Albedo shows (in % or other units) how much of the entire amount of short-wave radiation is reflected by the soil (or in a more general sense the earth’s) surface. Albedo fluctuates within 8-40% and depends upon the color and character of soil surface, which, in their turn, may depend on soil texture and wetness.

The volumetric heat capacity C_v of a soil is defined as the change in heat content of a unit bulk volume of soil pert unit change in temperature. Its units are calories per cubic centimeter per degree (Kelvin) or joules per cubic meter per degree. C_v depends on the composition of the soils solid phase, bulk density and the soil’s wetness:

Quartz	0.48 cal/cm3 oK
Other minerals (average)	0.48
OM	0.6
Water	1.0
Air	0.003

Specific heat capacity C_m of soil is defined as the change in heat content of a unit mass of soil per ^oK.