Soil weight is most often expressed on a soil volume basis rather than on a particle basis. Bulk density is defined as the dry weight of soil per unit volume of soil. Bulk density considers both the solids and the pore space; whereas, particle density considers only the mineral solids. Figure 2.6 illustrates the difference between bulk density and particle density.
For our ideal soil, one-half of it is solids, and one-half is pore space. Using our example of a 1 cm3 volume, the ideal soil would have 0.5 cm3 of pore space and 0.5 cm3 of solids. Pore space filled with air weighs 0 g. Organic matter is a very small portion of the solids, so it is usually ignored in this calculation. The mineral solids would weigh 1.33 g when dry, and is determined by multiplying particle density by the volume of solids:
2.66 g/cm3 x 0.5 cm3 = 1.33 g
The bulk density, then, is the dry weight of soil divided by the volume of soil:
1.33 g / 1 cm3 = 1.33 g/cm3
For practice, consider a box of undisturbed soil from the field. The box has dimensions of 2.5 cm by 10 cm by 10 cm. The volume of the box can be determined by multiplying the height of the box times its width and its depth. The wet soil in the box weighed 450 g. The dry soil weighed 375 g. Now calculate the bulk density. Your answer should be 1.5 g/cm3. In this calculation, you did not have to use the particle density because the weight of soil in the box was already known.
Bulk density of the surface soil is lowest in the spring immediately after soils have thawed and before field operations have begun. Each field operation compacts soil beneath the tires. If soils are wetter than field capacity, bulk density may increase. However, if soils are dry, bulk density is not affected much. Root growth, in general, starts to be restricted when the bulk density reaches 1.55 to 1.6 g/cm3 and is prohibited at about 1.8 g/cm3. Tillage can increase bulk density if it breaks down aggregates and allows soil separates to pack more tightly. Adding organic material decreases bulk density because organic material has a lower bulk density. However, additions are typically so small in proportion to the weight of soil that they do not markedly influence bulk density except at the soil-atmosphere interface. Bulk density is also important because it tells us about the porosity of a soil.