Bulk density is a physical property of a granular solid, such as soil, sand, or powder. It is calculated by dividing the weight of a given volume of the material by the volume it occupies. This includes not only the volume of the particles themselves, but also the volume of space and weight of any material between the particles.
The density of a continuously solid article, such as a metal bar, is simply its volume divided by its weight. Such an item is considered incompressible, and as such, its density is essentially constant. On the other hand, there are empty spaces between the solid particles of granular materials. These materials are sometimes referred to as divided solids. Such materials can be compacted or expanded depending on the forces applied to them, and the space between the solid particles can vary.
Bulk density, therefore, is not inherent to the material itself, but varies depending on the space between the particles and the substance present in that space. It depends on a number of variables such as the measurement method, the amount of moisture in the material, how the material is handled, and others. Most typically, this property is expressed on what is known as an “oven-dry” basis thereby excluding additional weight contributed by moisture mixed into the substance.
It may also be qualified as “freely settled” or “poured” when the substance is simply poured into a vessel or as “tapped” when it is compacted. If the substance is compacted, it is typically done according to some standardized specification so that comparisons across samples are fairly consistent. Where there are multiple compaction specifications in use, a sample may be described, for example, as “tapped 150 times” or whatever number the specification indicated.
There are a number of ways to measure bulk density depending on the substance, the location where the measurement must be taken, and so on. One common example is the core method which is the method usually used to measure soil bulk density. In this method, a soil sample is taken using a coring tool that is driven to desired depth of the soil. This tool extracts a known volume of soil, which is then dried and weighed. The tool used extracts a fixed volume of soil, so only the weight needs to be determined in order to make the calculation. Other methods may also be used which involve determining both the weight and volume.
Bulk density is a particularly important variable with regard to soil quality for a number of reasons. In agricultural consideration, it can be an indicator of the ability of oxygen and water to move through soil, which is necessary for crop growth. High bulk density soil compacted together relatively tightly can hinder both root growth down into the soil and seedling emergence from the soil’s surface. In urban settings, it may also impede water from infiltrating into the ground leading to increased rainwater runoff and degraded water quality.
While bulk density is most commonly associated with soil, it may also be a relevant measure for a variety of other materials. For example, it can be an important measure for pharmaceutical products and chemicals, where precise information about the substances is critical for safety. Dehydrated foods, such as dried milk products, or cosmetics in powdered form may also be measured in terms of bulk density for manufacturing or packaging purposes.