Volumetric Density vs. Bulk Density
- Bulk density is defined as the weight of a certain sample of soil, or any other mixture of small, loose particles, divided by the volume they occupy. This volume includes the air pockets between particles, as well as pore spaces within particles.
- In order to measure bulk density, a sample of soil is taken using a container of known volume and weight. The container, full of soil, is placed in an oven overnight to completely remove all water from the sample and then is weighed. The weight of the container is subtracted to get the weight of the soil, which is then divided by the volume to yield bulk density.
- Bulk density can change, particularly when a soil is compressed by footprints or farm machinery, for example. Compressed soils have a higher bulk density than untouched soils. This can be a problem because heavily compressed soils hold less water and make it harder for plant roots to penetrate. The density of the soil particles themselves is not changing -- only the amount of empty air between them.
- Typically, density measurements are not supposed to take into account empty air space. In soil science, particle density measures the actual density of the particles themselves and therefore does not change like bulk density does. The term "volumetric density" is not actually very specific, and sometimes when you perform actual measurements, it is difficult to avoid measuring air pockets in certain materials -- but "density" usually refers to the density of a solid material alone, without pore spaces.