The particle density of a soil measures the mass in a given volume of particles. Particle density focuses on just the soil particles themselves and not the volume they occupy in the soil. Bulk density includes both the volume of the solid (mineral and organic) portion of the soil and the spaces where air and water are found.
Density is measured as mass per unit volume (mass divided by volume). Soil particle density depends on the chemical composition and structure of the minerals in the soil. Most mineral particles in soils have a particle density ranging from 2.60 to 2.75 g/cm3. However, the density can be as high as 3.0 g/cm3 for very dense mineral particles, and as low as 0.9 g/cm3 for organic particles.
Particle density is important to determine because it allows us to understand many other properties of the soil. For example, knowing the particle density allows us to know something about the relative amount of organic matter vs. mineral particles in the soil sample. Because particle density can be compared to the density of known minerals such as quartz, feldspar, and micas, or denser minerals such as magnetite, garnet, or zircon, this measurement also helps to indicate the chemical composition and structure of the soil minerals.
If we have information on both the particle density and the bulk density of the soil, we can calculate the pore space (or porosity) that is occupied by air and water. This is useful because it helps us to understand other important soil properties such as how much water can be stored in the soil, how fast water and heat will be moved through the soil, how easily roots can move through the soil, and the potential for flooding or drought in an area.