Definition - What does Bulk Density mean?
In horticulture, bulk density is a term that refers to the weight of dry soil within a specific volume, such as within a specific area in an in-ground garden, or within a flower pot or other grow container.
Determining the bulk density of your particular soil is done by taking a core sample, drying the sample thoroughly (often in an oven or kiln), and then weighing it. Prior to drying, the sample should be weighed as well to determine the wet bulk density in order to compare that to the dry density.
A soil or grow medium's bulk density is important to know to help ensure proper root growth and overall plant health.
MaximumYield explains Bulk Density
Whether you’re growing fruits, vegetables, or flowers, it’s important to know as much as possible about the health of the soil to ensure proper plant growth. While you should know the availability and concentration of important nutrients and minerals, you also need to know the overall soil porosity.
The greater the density of your soil, the more it will compact around the plant roots. The greater the compaction, the less root growth there will be, and the poorer the plant health. By measuring soil bulk density, you can determine whether plant root growth will be restricted, and soils that have a significant density (pounds per cubic foot) can be amended to help ensure stronger growth and better root spread.
Higher densities will also cause poor drainage of water (standing or pooling water around plant stems or stalks), and can also indicate soil compaction in the area.
Amending soils with high densities can be done by adding minerals or other materials, although organic matter is perhaps the best option, as it will deteriorate and decompose over time, adding vital nutrition to the soil while still providing porosity to encourage root growth.