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Soil is a three-phase system: solids (mineral grains and organic matter) plus pores filled with air and water. A typical mineral soil is about 45% minerals, 5% organic matter, 25% air and 25% water by volume.

Soil scientists explain soil formation with five factors: parent material, climate, topography, organisms and time. Mineral soils form by the weathering of bedrock through physical, chemical and biological processes.

Texture is set by particle size: sand 2–0.02 mm, silt 0.02–0.002 mm and clay below 0.002 mm. Clay is plate-like with a large surface area, so even a small clay fraction strongly affects how a soil behaves. A loam is about 40% sand, 40% silt and 20% clay.

Experiment: Measuring bulk density and porosity

1. Calculate the volume of your tube: V = πr²h (in cm³).
2. Push the tube gently into undisturbed soil, dig it out and trim the ends flush with a flat spatula.
3. Empty the core onto a weighed tray. Dry it fully (oven by the technician, or air-dry for several days).
4. Weigh the dry soil and find m_s in g.
5. Calculate bulk density ρb = m_s / V and porosity f = 1 − ρb / 2.65.
6. Repeat for a second site (e.g. a path edge vs a flower bed) and a sandy sample. Pool class results and give a mean and range.

Site	Volume V (cm³)	Dry mass (g)	ρb (g/cm³)	Porosity f
Flower bed	Prediction
Path edge	Prediction
Sandy sample	Prediction

Questions

1. Define bulk density and porosity, with units, and write the equation that links them.

2. A core has V = 98 cm³ and a dry mass of 152 g. Calculate ρb and f (take ρs = 2.65 g/cm³). Would roots be restricted if this soil is clayey?

3. Give two sources of error in your experiment and say how each would change your value of ρb.

4. The FAO says about 1000 years per cm of topsoil; Britannica gives under 10 years to several hundred years per cm. Explain why both can be right and what this means for soil protection.

Sources for every fact on this sheet: echoesof.science/school/soil/sources/

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