Agriculture and Food 7–12 · Years 11–12
Soil bulk density, porosity and water content from intact cores
Plant/Animal production: Soil, nutrients and water
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The idea
Bulk density is the dry mass of soil in a known volume; compaction raises it and shrinks the pore space that roots, air and water need.
What you need
- steel sampling ring of known volume, for example 7 cm internal diameter and 10 cm high, with a sharpened lower edge
- flat timber offcut and mallet, spade, flat knife, scissors
- sealable bags and labels
- ovenproof containers, balance reading to 0.1 g
- oven at 105 °C (or a microwave oven for the quick method)
- vernier calipers or ruler
How to do it
- Choose sites that differ in traffic: a vehicle or stock track, a cultivated bed and an untrafficked fence line. Sample moist soil without gravel, three rings per site at 0–10 cm and again at 30 cm.
- Prepare a flat horizontal surface at the sampling depth, then push or gently hammer the ring in with the mallet on a timber offcut laid across it, without driving it in too far.
- Dig around the ring, lift it with the core intact, trim the soil flush with both ends, cut off plant material and tip the core into a labelled bag.
- Weigh the container, add the moist soil and weigh again; dry at 105 °C (or in a microwave) and weigh the dry soil. The fact sheet's times for this ring, 2 hours at 105 °C or 10 minutes in a microwave, are a first drying period: keep drying and reweighing until two weighings agree, which a full core may take much longer to reach.
- Measure the ring's internal diameter and height and calculate its volume.
- Calculate bulk density, porosity and water content for every core with the model, then the mean and standard deviation for each site and depth.
What you should see
The track has a higher bulk density and lower porosity than the fence line. The soilquality.org.au fact sheet gives 1.3–1.7 g/cm^3 as usual for sandy soils and 1.1–1.6 g/cm^3 for silts and clays, with values above 1.6 g/cm^3 tending to restrict root growth and organic-rich soils below 0.5 g/cm^3. A 7 cm by 10 cm ring holds 384.8 cm^3; 500.0 g of dry soil in it is 1.30 g/cm^3 and, with a particle density of 2.65 g/cm^3, a porosity of 51%. If the moist core weighed 600.0 g, gravimetric water content is 20.0% and volumetric water content 26.0%. The learner knows it worked when the three cores from one site agree and trimming was flush.
What changes
- What you change
- site (track, cultivated bed, fence line) and depth
- What you measure
- bulk density (g/cm^3), porosity (%), gravimetric and volumetric water content (%)
- What you keep the same
- ring volume
- drying temperature and time to constant mass
- soil moisture at sampling
- trimming method
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Heavy clay soils always have the highest bulk density.
- Compaction only affects the soil surface.
- A soil with more pore space always drains faster.
Safety card
Hazards
- mallet blows and the sharpened ring edge
- hot oven and containers
- lifting soil-filled buckets
Controls
- gloves, and a timber offcut between mallet and ring
- teacher manages the oven
- carry samples in small loads
Note
No chemicals. Oven heat and general rules: NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
Sources
The pages the author read to write this activity.
- www.soilquality.org.au/factsheets/bulk-density-measurement
- www.soilquality.org.au/factsheets/making-sense-of-physical-indicators-nsw
- www.nsw.gov.au/education-and-training/nesa/curriculum/tas/agriculture-stage-6-2013
- www.nsw.gov.au/sites/default/files/noindex/2025-03/agriculture-amended-st6-syl-2013.doc
- curriculum.nsw.edu.au/learning-areas/tas/agriculture-11-12-2026/outcomes
- education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/5._Volume_2_Appendices.pdf