1.9: Bulk Density, Particle Density, and Porosity
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Upon completion of this laboratory exercise students should be able to directly measure bulk density and particle density using the graduated cylinder method for coarse textured, non-aggregated soil samples. Students will also learn to calculate soil porosity.
Upon completion of this exercise you should be able to:
- directly measure bulk density and particle density using the graduated cylinder method for coarse textured, non-aggregated soil samples
- determine bulk density of a soil core, accounting for compaction during collection
- calculate soil porosity
From Brady and Weil, The Nature and Properties of Soils, 13 th Ed. Soils are composed of a combination of solids (soil particles), liquids (soil water), and gases (soil atmosphere). The liquid and gas portions are essential for plant growth and are found in the pore spaces among the soil solids. Bulk density is a measure of the mass of a soil per given volume (i.e. g/cm3 ), including solids and pore spaces. Bulk density is a commonly measured soil property by agriculturalists and engineers. High bulk density soils are soils with little pore space, so water infiltration is reduced, root penetration is inhibited, and aeration is restricted – reducing agricultural productivity. Low bulk density soils are easily compacted and may settle considerably to the detriment of roads, sidewalks, and building foundations.
Particle density is a measure of the mass of soil solids per given volume (g/cm3 ); however, pore space is not included as it is with bulk density. Particle density is similar to the specific gravity of a solid and is not impacted by land use. Particle density is approximated as 2.65 g/cm3 , although this number may vary considerably if the soil sample has a high concentration of organic matter, which would lower particle density, or high-density minerals such as magnetite, garnet, hornblende, etc.
Porosity, the percent by volume of a soil sample not occupied by solids, is directly related to bulk density and particle density. If particle density remains constant, as bulk density increases porosity decreases.
- 2 sandy soil samples – one coarse and one fine
- Top load balance
- Soil spatula
- 100 mL graduated cylinder
- 50 mL beaker (2)
- Water bottle
- Paper towels
- Mud bucket
- Add slightly more than 50 mL of the two soil samples to 50 mL beakers.
- Clean and thoroughly dry a 100 mL graduated cylinder. Weigh and record weight (A).
- Slowly add Soil Sample #1 to pre-weighed graduated cylinder to the 10 mL line. Compact the soil by dropping onto a padded surface like a book, notebook, etc. at least ten times from a height of about 2-3 inches.
- Repeat this process in ten mL intervals until you reach the 50 mL mark.
- Use a soil spatula to level the top of the sample in the graduated cylinder and add soil with the spatula until the top of the soil sample is exactly even with the 50 mL line – this is the bulk volume of compacted soil (B) (1 mL = 1 cm3 ).
- Weigh and record graduated cylinder plus compact soil weight (C).
- Return any soil sample remaining in beaker to sample storage container and dry clean beaker.
- Return 50 mL sample in graduated cylinder to 50 mL beaker. Remove all of sample within graduated cylinder.
- Now add exactly 50 mL of water to the graduated cylinder, record volume (E).
- Slowly pour approximately 25 mL of soil sample from beaker into water in the graduated cylinder. Gently stir soil/water mixture to remove any air bubbles. Add the second 25 mL of soil sample and stir again to remove air bubbles.
- Record the new volume (F)
- Repeat process for Soil Sample #2.
- Calculate bulk density, particle density, and porosity using the following formulas:
|SAMPLE 1||SAMPLE 2|
|A. Weight of 1st graduated cylinder|
|B. Bulk volume of compacted soil||50 mL||50 mL|
|C. Weight of 1st cylinder plus compacted soil|
|D. Weight of soil sample (C – A)|
|E. Volume of water in 2nd cylinder||50 mL||50 mL|
|F. Volume of soil and water in 2nd cylinder|
|G. Volume of soil (F – E)|
|H. Bulk density (g/cm3 ) (D/B)|
|I. Particle density (g/cm3 ) (D/G)|
|J. Soil porosity (%) (100 – (H/I x 100))|
|A. Weight of 1st graduated cylinder||138.4 g|
|B. Bulk volume of compacted soil||50 cm3|
|C. Weight of 1st cylinder plus compacted soil||223.90 g|
|D. Weight of soil sample (C – A)||85.43 g|
|E. Volume of water in 2nd cylinder||50 cm3|
|F. Volume of soil and water in 2nd cylinder||83 cm3|
|G. Volume of soil (F – E)||33 cm3|
|H. Bulk density (g/cm3 ) (D/B)||1.66 g/cm3|
|I. Particle density (g/cm3 ) (D/G)||2.52 g/cm3|
|J. Soil porosity (%) (100 – (H/I x 100))||34.13%|