A new perspective on soil thermal properties

The soil thermal properties-heat capacity (C), thermal diffusivity (alpha), and thermal conductivity (lambda)-are important in many agricultural, engineering, and meteorological applications. Soil thermal properties are largely dependent on the volume fraction of water (theta), volume fraction of solids (v(s)), and volume fraction of air (n(a)) in the soil. In many natural settings theta, v(s), and n(a) vary greatly over time and space, but data showing the effects of these variations on thermal properties are not readily available. We used a heat-pulse method to measure the thermal properties of 59 packed columns of four medium-textured soils covering large ranges of theta, v(s), and n(a). The measured data reveal the commonly overlooked but dominant influence of n(a). on soil thermal properties. Notably, the measurements show that the lambda of these soils at 20 degreesC can be accurately described as a decreasing linear function of n(a) (r(2) = 0.93). Good agreement exists between the measured data and common models for lambda and C.