Soil Moisture Vs. Thermal Conductivity

Factors Influencing Thermal Conductivity in Soils

• A variety of factors influence the heat flux through soil. The primary influences are the soil's moisture content and its dry density. Other parameters that have a secondary effect are soil type or mineral composition, dry density of the soil, temperature, texture and time.

  Soil is composed of weathered rock particles, water, and void spaces. The void spaces are very important in thermal conduction. They are not really empty like a vacuum but always contain either air or water. Air is a poor conductor of heat because the gas molecules are spaced far apart and do not physically contact one another. Water is an excellent thermal conductor because it's a liquid and there's more particle contact in the void spaces between soil particles and water. That heat energy is passed along through this medium in the voids until it contacts solid soil particles which absorb and move the thermal energy.

Soil Moisture and Dry Density

• The consolidation of the soil determines its dry density. As soil becomes more compacted, the void spaces are reduced and air is squeezed out. With further consolidation, water flows into the smaller void spaces and the soil becomes more saturated as its density increases. As the soil moisture or the density of the soil increases, its thermal conductivity also increases. Saturation describes the amount of water present in the soil. An unsaturated soil still has air in the void spaces and its thermal conductivity is less than that of saturated soil. The dry density parameter refers to the mass of soil particles per unit volume and reflects a measure of the particle contact in the soil.

Moisture Content of the Soil

• As the saturation levels of a soil increase, the thermal conductivity of the soil also increases. Moisture only partially coats the soil particles at low saturation levels. With more water, the void gaps between the particles begin filling and the thermal conductivity continues to rise accordingly. As the one hundred percent saturation level of the soil is approached, the voids fill completely and the heat flux or the thermal conductivity reaches its highest flow for that soil sample. A saturated soil has a thermal conductivity level near that of pure water.

Soil Types and Composition

• There are several different soil types: gravels, sands, silts, clays, and soil containing organic materials or peat. Whether or not these soils are in a frozen or a thawed state will also affect their thermal conductivity. As the organic content of the soil increases, the thermal conductivity generally decreases because organics don't carry a heat flux very well. However, the peat is decomposing and giving off heat in that process so peat bogs can be very warm.

  Even if a soil has a high moisture content, it may not necessarily mean that the soil will warm up faster than a dry soil. Evaporation plays a role in removing much of the solar energy before the soil can warm up. So dry soils warm up faster under the sun but they cool more quickly at night. Soils with a high moisture content evaporate the water and the soil doesn't warm as fast during the day but it cools more slowly at night because of the higher moisture content.

Other Factors

• In agriculture, the soil's microclimate which nourishes seeds is determined through the thermal conductivity of the soil and its moisture content. This determines the early growth and development of a crop. The microclimate influences seed germination, seedling growth and establishment of the crop. Research has shown that increasing the percentage of organic content in the soil decreases the thermal conductivity. In hot areas, adding mulch or other organic components to the soil will help prevent seeds from baking under intense temperatures.