Bulk density

BULK DENSITY

Bulk Density (Db)

Soil bulk density, like all density measurements, is an expression of the mass to volume relationship for a given material. Soil bulk density measures total soil volume.

Thus, bulk density takes into account solid space as well as pore space.
Soils that are loose, porous, or well-aggregated will have lower bulk densities than soils that are compacted or non aggregated.
This is because pore space weighs less than solid space (soil particles). Sandy soils have less total pore than clayey soils, so generally they have higher bulk densities.
Bulk densities of sandy soils vary between 1.2 to 1.8 Mg m-3. Fine-textured soil, such as Clays, silty clays, or clay loams, have bulk densities between 1.0 and 1.6 Mg m-3.

Factors Affecting Bulk Density

Bulk density is an indirect measure of pore space and is affected primarily by texture and structure.
As aggregation and clay content increase, bulk density decreases.
Tillage operations do not affect texture, but they do alter structure (soil particle aggregation). Primary tillage operations, such as plowing, generally decrease bulk density and increase pore space, which is beneficial. Secondary tillage (cultivation) generally increases bulk density and decreases pore space.
The compaction resulting from cultivation can be detrimental to plant growth.
Cropped soils generally have higher bulk densities than uncropped soils.
The movement of machinery over the field forces solid particles into spaces once occupied by water or air, resulting in less pore space and increased bulk density.

Clayey Loamy Sandy

Well-aggregated Moderately aggregated Non aggregated

High organic Moderate organic Low organic matter

Matter content matter content content

>-----------BULK DENSITY INCREASING-------->

. Relationship of soil bulk density to texture, organic matter content, and aggregation

The density of water is 1.0 Mg m-3 and mineral soils are usually heavier than water. However, organic soils generally have a bulk density less than water.
As the organic matter content of mineral soils increases, the bulk density decreases. Manure additions in large amounts tend to lower the surface bulk density of mineral soils because of the addition of low bulk density material and the consequent promotion of soil aggregation.
Soil bulk density increases with soil depth primarily because of less organic matter and decreased aggregation.