Soil physics: texture, bulk density and porosity
Soil is a porous material whose behaviour depends on particle size and packing. Learn how texture is classified, calculate bulk density and porosity from measurements, and weigh up how fast soil forms compared with how fast it can be lost.
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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.[1][2]
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.[5][2][4]
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.[1]
Bulk density is the mass of dry soil divided by its total volume: ρb = ms / Vt, in g/cm³. Porosity is the fraction of the volume that is pores: f = 1 − ρb/ρs, where ρs is the particle density, often assumed to be about 2.65 g/cm³.[9]
Example: a soil with ρb = 1.33 g/cm³ has f = 1 − 1.33/2.65 ≈ 0.50, so about half its volume is pores, matching the 'typical soil' figure. Compaction raises bulk density: above about 1.47 g/cm³ in clayey soils, 1.65 in silty soils and 1.80 in sandy soils, root growth tends to be restricted.[9][1]
Life shapes soil. Fungi and bacteria decompose organic matter into humus and release nutrients; nitrogen-fixing bacteria living with peas, clover and similar plants add nitrogen; earthworms pull leaves into burrows and help form crumb-like aggregates.[7][2][6][1]
Formation is slow but its rate is uncertain. The FAO estimates about 1000 years for 1 cm of topsoil, while Britannica gives rates from under ten years per centimetre in young soils to several hundred years in strongly weathered soils. Either way, a third of the world's soils are already moderately or highly degraded, and about 95% of our food depends on soil.[3][4]
Try it: Measuring bulk density and porosity
You need
- 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.
What to do
- Calculate the volume of your tube: V = πr²h (in cm³).
- Push the tube gently into undisturbed soil, dig it out and trim the ends flush with a flat spatula.
- Empty the core onto a weighed tray. Dry it fully (oven by the technician, or air-dry for several days).
- Weigh the dry soil and find m_s in g.
- Calculate bulk density ρb = m_s / V and porosity f = 1 − ρb / 2.65.
- 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.
What happens, and why?
Bulk density compares the dry mass with the total volume including pores, so a compacted soil (path edge) has a higher value and lower porosity than a loose bed. Assuming a particle density of 2.65 g/cm³ lets you estimate porosity, but it is an assumed average, not a measured value for your soil. Losing soil while trimming the core, or not drying it fully, are the largest errors.[9]
Play: Sand, silt or clay?
Classify each particle size or description by soil texture class.
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Quiz
Other ages: Ages 6–7 · Ages 10–11