Bulk density is the weight of a given volume of granular solid, including the space and weight between particles. It varies based on factors such as moisture, handling, and measurement method. It is important in soil quality and can hinder crop growth and water infiltration. It is also relevant for pharmaceuticals and dehydrated foods.
Bulk density is a physical property of a granular solid, such as earth, sand or dust. It is calculated by dividing the weight of a given volume of material by the volume it occupies. This includes not only the volume of the particles themselves, but also the volume of the 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 element is considered incompressible and, as such, its density is essentially constant. On the other hand, there are gaps between the solid particles of the granular materials. These materials are sometimes referred to as split solids. Such materials can be compacted or expanded depending on the forces applied to them, and the space between the solid particles can vary.
The bulk density, therefore, is not inherent in the material itself, but varies according to 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 more. More typically, this property is expressed on what is known as an “oven dry” base, thus excluding the additional weight provided by moisture mixed into the substance.
It can also be qualified as “freely deposited” or “poured” when the substance is simply poured into a container or as “capped” when it is compacted. If the substance is compacted, it is typically done to some standardized specification so that comparisons between samples are fairly consistent. Where multiple compaction specifications are in use, a sample may be described as, for example, “tapped 150 times” or any number indicated by the specification.
There are different ways to measure bulk density depending on the substance, where the measurement is to be performed, and so on. A common example is the basic method, which is the method usually used to measure the bulk density of soil. In this method, a soil sample is taken using a coring tool which is drilled to the desired depth of 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 to make the calculation. Other methods involving the determination of both weight and volume may also be used.
Bulk density is a particularly important variable in soil quality for a variety of reasons. In view of agriculture, it can be an indicator of the ability of oxygen and water to move through the soil, which is necessary for crop growth. A high bulk density soil compacted together relatively compactly can hinder both the growth of roots in the soil and the emergence of seedlings from the soil surface. In urban environments, it can also prevent water from infiltrating the ground, leading to increased stormwater runoff and deterioration of water quality.
While bulk density is most commonly associated with soil, it can also be a relevant measure for a variety of other materials. For example, it can be an important measure for pharmaceuticals and chemicals, where accurate substance information is critical to safety. Dehydrated foods, such as dry milk products or powdered cosmetics, can also be measured in terms of bulk density for manufacturing or packaging purposes.
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