Inmaterials science,bulk density, also calledapparent density, is amaterial property defined as themass of the many particles of the material divided by thebulk volume. Bulk volume is defined as the totalvolume the particles occupy, including particle's own volume, inter-particlevoid volume, and the particles' internalpore volume.^[1]

Bulk density is useful for materials such aspowders,granules, and other "divided"solids, especially used in reference tomineral components (soil,gravel),chemical substances,pharmaceuticalingredients, foodstuff, or any other masses of corpuscular orparticulate matter (particles).

Bulk density is not the same as theparticle density, which is anintrinsic property of the solid and does not include the volume for voids between particles (see:density of non-compact materials). Bulk density is anextrinsic property of a material; it can change depending on how the material is handled. For example, a powder poured into a cylinder will have a particular bulk density; if the cylinder is disturbed, the powder particles will move and usually settle closer together, resulting in a higher bulk density. For this reason, the bulk density of powders is usually reported both as "freely settled" (or "poured" density) and "tapped" density (where the tapped density refers to the bulk density of the powder after a specified compaction process, usually involving vibration of the container.^[2])

The bulk density of soil depends greatly on the mineral make up of soil and the degree ofcompaction.^[3] The density ofquartz is around2.65 g/cm³ but the (dry) bulk density of a mineral soil is normally about half that density, between1.0 and 1.6 g/cm³. In contrast, soils rich in soil organic carbon and some friable clays tend to have lower bulk densities (<1.0 g/cm³) due to a combination of the low-density of the organic materials themselves and increasedporosity. For instance, peat soils have bulk densities from0.02 to 0.98 g/cm³.^[4] 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.^[5] Croplands have almost 1.5 times higher bulk density compared to woodlands.

Bulk density of soil is usually determined from acore sample which is taken by driving a metal corer into the soil at the desired depth and horizon.^[6] This gives a soil sample of known total volume,V_t. From this sample the wet bulk density and the dry bulk density can be determined.^[7]

For the wet bulk density (total bulk density) this sample is weighed, giving the massM_t. For the dry bulk density, the sample is oven dried and weighed, giving the mass of soil solids,M_s. The relationship between these two masses isM_t =M_s +M_l, whereM_l 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

${\displaystyle {\rho }_b=\frac{M_s}{V_t}}$

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

${\displaystyle {\rho }_t=\frac{M_t}{V_t}}$

The dry bulk density of a soil isinversely 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 theseismic velocity of waves travelling through it: forP-waves, this has been quantified withGardner's relation. The higher the density, the faster the velocity.

• Brazil nut effect
• Characterisation of pore space in soil
• Effective porosity
• Density meter
• Number density

• University of Leicester podcast 'How to measure dry bulk density'Archived 2012-04-26 at theWayback Machine
• 'Determination of bulk density'

• Mass density
• Particulates
• Soil physics

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