In soil, solids can be anything from a clay particle to a gravel piece. Every solid can be made up of any element however the majority of soil particles are silicon or derivatives of silicon. Thermal resistivity or conductivity is usually dependent on the solids in the soil.
Usually voids in soil are air but can be made up of any gas.
Most common of all liquids found in soil is water. Liquids can be any element that is in a liquid state at room temperature.
Now you probably noted I said above “usually consists of three basic constituents”. Soil can consist of just solids and water or solids and gas however theses states are rarely found naturally. As discussed in an earlier blogg air is the enemy when it comes to soil thermal resistivity so always try to remove the air with water where possible by forcing the particles closer together with compaction.