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 but the (dry) bulk density of a mineral soil is normally about half that density, between . In contrast, soils rich in soil organic carbon and some friable clays tend to have lower bulk densities () due to a combination of the low-density of the organic materials themselves and increased
porosity. For instance, peat soils have bulk densities from . In a detailed study which has used 6,000 analysed samples in the European Union, a high resolution map (100m) of soil bulk density for the 0-20cm using regression model. Croplands have almost 1.5 times higher bulk density compared to woodlands. Bulk density of soil is usually determined from 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 the wet bulk density and the dry bulk density 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 :\rho_b = \frac{M_s}{V_t} Wet bulk density = mass of soil plus liquids/ volume as a whole :\rho_t = \frac{M_t}{V_t} 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. ==See also==