SOME PHYSICAL ASPECTS OF HEAT TRANSFER IN SOIL
Heat transfer in soils can be calculated if it is considered as a periodic phenomenon in an isotropic soil. A quantitative description of the heat fluxes and temperatures is based on solutions of the Fourier equation of heat conduction with appropriate initial and boundary conditions. In this equation two independent thermal properties appear: the thermal conductivity () and the volumetric heat capacity (C) of the soil. The ratio of these quantities (/C) is called the thermal diffusivity (a). The greater the value of a, the faster and the deeper a particular heat wave at the soil surface penetrates into the soil.
The moisture content of the soil has a strong influence on its thermal properties. The depth of the groundwater level therefore may affect for example the germination period of crops by the different soil temperatures that will occur. For a non-homogeneous soil, e. g. a layered soil, the mathematical solution is more difficult because the theory of heat conduction in a homogeneous soil has then to be applied to each layer separately. This occurs with tillage and the application of a different surface layer. With tillage, the thermal properties of the upper layer are changed and so the temperatures in the soil near the surface will differ from those in a homogeneous soil under equal conditions of irradiation. The differences are often small, but will be important when the temperature is critical, as will be the case in periods of night frost.
Since top layers act as insulating layers, changes in the surface layer are often used in warm climates in order to reduce evaporation or to keep the soil cooler, and in temperate climates to keep the soil warmer during the winter months. A brief discussion on the aspects mentioned above, based on own experiments, has been given.