Thermal Properties of Soils as affected by Density and Water Content

Thermal properties dictate the storage and movement of heat in soils and as such influence the temperature and heat flux in soils as a function of time and depth. The ability to monitor soil heat capacity is an important tool in managing the soil temperature regime to affect seed germination and crop growth. The effect of water content and bulk density on the specific heat, volumetric heat capacity, and thermal diffusivity of some sieved and repacked soils was investigated through laboratory studies. These laboratory experiments used the calorimetric method to determine specific heat of soils. The soils used were classified as sand and clay. For the type of soils studied, specific heat increased with increased moisture content. Also, volumetric heat capacity increased with increased moisture content and soil density. Volumetric heat capacity ranged from 1·48 to 3·54 MJ m−3 °C−1 for clay and from 1·09 to 3·04 MJ m−3 °C−1 for sand at moisture contents from 0 to 0·25 (kg kg−1) and densities from 1200 to 1400 kg m−3. Specific heat ranged from 1·17 to 2·25 kJ kg−1 °C−1 for clay and from 0·83 to 1·67 kJ kg−1 °C−1 for sand at moisture contents from 0·02 to 0·25 (kg kg−1) and soil density of 1300 kg m−3. The volumetric heat capacity and specific heat of soils observed in this study under varying moisture content and soil density were compared with independent estimates made using derived theoretical relations. The differences between the observed and predicted results were very small. Clay soil generally had higher specific heat and volumetric heat capacity than sandy soil for the same moisture content and soil density. The results also show that thermal diffusivity vary with moisture content and soil texture. Sandy soil exhibited a thermal diffusivity peak at a definite moisture content range. Clay soil, however, did not exhibit a sharp thermal diffusivity peak.