Bulk Density - Soil

Soil

The bulk density of soil depends greatly on the mineral make up of soil and the degree of compaction. The density of quartz is around 2.65g/cm³ but the (dry) bulk density of a mineral soil is normally about half that density, between 1.0 and 1.6g/cm³. Soils high in organics and some friable clay may have a bulk density well below 1g/cm³

Bulk density of soil is usually determined on a Core sample which is taken by driving a metal corer into the soil at the desired depth and horizon. This gives a soil sample of known total volume, . From this sample either the wet bulk density, the dry bulk density, or both can be determined.

For the wet bulk density (total bulk density) this sample is weighed, giving the mass . For the dry bulk density, the sample is oven dried and weighed, giving the mass of soil solids, . The relationship between these two masses is, where is the mass of substances lost on oven drying (often, mostly water). The dry and wet bulk densities are calculated as

Dry bulk density = mass of soil/ volume as a whole

Wet bulk density = mass of soil plus liquids/ volume as a whole

The dry bulk density of a soil is inversely related to the porosity of the same soil: the more pore space in a soil the lower the value for bulk density. Bulk density of a region in the interior of the earth is also related to the seismic velocity of waves travelling through it: for P-waves, this has been quantified with Gardner's relation. The higher the density, the faster the velocity.