Teacher sheet · Ages 15–16 · about 45–50 minutes
Soil physics: texture, bulk density and porosity
Lesson page: echoesof.science/school/soil/ages-15-16/ · Pupil worksheet: echoesof.science/school/soil/ages-15-16/print/
Learning aims
- Describe soil as solids plus pores of air and water.
- Classify particles by size into sand, silt and clay.
- Calculate bulk density and porosity, with units, and evaluate errors.
- Discuss uncertainty in soil formation rates and why soil is treated as non-renewable.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: a sponge and a brick — which is denser, and why does a sponge hold water? |
| 10 min | Text and worked example of ρb and f. |
| 25 min | Core sampling and weighing (dry mass from a pre-dried set if no time). |
| 5 min | Sand, silt or clay? game. |
| 5 min | Quiz; set worksheet for homework. |
Materials for the experiment
- A short metal or rigid plastic tube of known volume (or a 60 ml syringe with the end cut off, prepared by the technician)
- Soil from the school grounds and a sandy sample
- Top-pan balance (0.1 g)
- Drying oven at about 105 °C (technician) or air-drying trays for a few days
- Ruler, calculator, trays
- Gloves
Safety: Only the technician or teacher uses the drying oven; let trays cool before handling. Wear gloves when collecting soil, avoid areas used by animals, and wash hands. Do not cut tubes yourself.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Denser soil is more fertile. | Compacted soil has high bulk density and low porosity, which can restrict roots and drainage. |
| Bulk density equals particle density. | Bulk density includes the pore space; particle density refers to the solids only (about 2.65 g/cm³). |
| Soil forms at one fixed rate. | Rates differ by orders of magnitude with weathering stage and climate; the FAO figure is an average. |
Worksheet answers
- Bulk density ρb = dry mass / total volume (g/cm³); porosity f = pore volume / total volume (no unit, or %); f = 1 − ρb/ρs.
- ρb = 152/98 ≈ 1.55 g/cm³; f = 1 − 1.55/2.65 ≈ 0.42. Yes: for clayey soils values above about 1.47 g/cm³ tend to restrict roots.
- Examples: soil lost when trimming the core makes ρb too low; incomplete drying (water still in the soil) makes ρb too high; squashing the core while inserting makes ρb too high.
- Rates depend on the weathering stage, parent material and climate, so a global average can sit inside a wide range. Even fast rates are slow compared with erosion, so soil is effectively non-renewable within a lifetime and should be protected.
Quiz answers
- A dry core of 100 cm³ has a mass of 145 g. What is its bulk density? 1.45 g/cm³. ρb = 145 g / 100 cm³ = 1.45 g/cm³.[9]
- Using ρs = 2.65 g/cm³, what is the porosity of a soil with ρb = 1.06 g/cm³? 0.60. f = 1 − 1.06/2.65 = 1 − 0.40 = 0.60.[9]
- A particle 0.01 mm across is classed as… silt. Silt is 0.02–0.002 mm.[1]
- Why does a small clay fraction affect a soil so much? Clay is plate-like with a huge surface area. Its tiny, flat particles pack tightly and hold water and nutrients.[1]
- Which is NOT one of the five soil-forming factors? Wind speed on the day of sampling. The five factors are parent material, climate, topography, organisms and time.[5]
- Compaction of a soil… raises bulk density and lowers porosity. Higher bulk density means less pore space and can restrict roots.[9]
Going further
Use a soil texture triangle with your jar-test percentages to name the texture class of your school soil, and compare with your drainage and bulk-density results.
Sources used in this lesson
- [1] Soil basics (University of Illinois Extension): reliability High
- [2] Biology 2e, 31.2 The Soil (OpenStax (Rice University)): reliability High
- [3] Polluting our soils is polluting our future (Food and Agriculture Organization of the United Nations (FAO)): reliability High
- [4] Soil: Time (Encyclopaedia Britannica): reliability High
- [5] Soil facts (USDA Natural Resources Conservation Service): reliability High
- [6] Earthworms (Royal Horticultural Society (RHS)): reliability High
- [7] Microorganisms and soil processes (Ontario Ministry of Agriculture, Food and Agribusiness): reliability High
- [9] Basics of soil bulk density (Oklahoma State University Extension): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/soil/sources/
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