Bulk density, particle density and porosity
The three phases of soil, how bulk density, particle density and porosity are defined and measured, why compaction matters, and worked calculations.
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A handful of soil is roughly half solid and half empty space. The solids are mineral grains and organic matter; the spaces, the pores, hold the water and air that roots, microbes and soil animals need. How dense the soil is, and how its pores are divided between water and air, governs root growth, drainage, aeration and how much work a tractor must do.
Bulk density and porosity are basic measurements in soil physics, and the calculations linking them are among the most common numerical questions in soil science and agricultural engineering exams.
The three phases
A soil sample of total volume Vt contains:
- Solids (volume Vs, mass Ms): mineral particles and organic matter.
- Water (volume Vw).
- Air (volume Va).
The pore space is Vw + Va. When the soil is saturated, water fills it; as the soil drains and dries, air takes its place.
The definitions
- Particle density depends only on what the solids are made of. For most mineral soils it is close to 2.65 g/cm3, the density of quartz; soils rich in organic matter have lower values, and soils rich in iron oxides higher ones.
- Bulk density depends on how tightly the solids are packed. Typical values are about 1.0 to 1.6 g/cm3 for mineral soils: lower for well-aggregated, organic-rich topsoils and higher for sands and compacted layers. Organic soils can be well below 0.5.
- Total porosity is usually 40 to 60 per cent. Clays often have higher total porosity than sands, but their pores are small.
- Void ratio is used mainly in engineering and soil mechanics.
Pores differ in size and function:
- Macropores (larger than about 0.08 mm): drain quickly after rain, carry air and water, and let roots and earthworms move. They are formed by structure, roots and fauna.
- Micropores (smaller): hold water against gravity, much of it available to plants.
Sandy soils have mostly macropores between grains; clays mostly micropores within aggregates. Good structure gives a clay soil macropores between its aggregates as well.
Measuring bulk density
- Core method: a metal cylinder of known volume is driven into the soil, the core is trimmed flush, dried at 105 °C and weighed. Bulk density = dry mass ÷ core volume. The standard method for most soils.
- Clod method: a natural clod is coated in paraffin wax or resin and its volume found by water displacement. Used for very hard or stony soils.
- Excavation method: a hole is dug, its volume measured with sand or water in a membrane, and the excavated soil dried and weighed; useful for stony soils.
- Radiation methods: gamma density gauges, mainly in engineering.
Particle density is measured with a pycnometer (density bottle), by the volume of water displaced by a known mass of dry soil.
From a soil core to porosity and air-filled porosity
A core of volume 100 cm3 is taken from moist soil. Wet, it weighs 168.8 g; dried at 105 °C, 135.0 g. Take particle density as 2.65 g/cm3.
- Bulk density = 135.0 ⁄ 100 = 1.35 g/cm3.
- Total porosity = 1 − 1.35 ⁄ 2.65 = 1 − 0.509 = 0.49 (49 per cent).
- Gravimetric water content = (168.8 − 135.0) ⁄ 135.0 = 33.8 ⁄ 135.0 = 0.25.
- Volumetric water content = 0.25 × 1.35 = 0.338 (33.8 per cent).
- Air-filled porosity = 0.49 − 0.338 ≈ 0.15 (15 per cent).
Fifteen per cent air is adequate for most roots; below about 10 per cent, roots and soil organisms begin to run short of oxygen, a common problem in compacted or waterlogged soils.
How much does a hectare of topsoil weigh?
With a bulk density of 1.35 t/m3, the top 20 cm of one hectare weighs 10,000 m2 × 0.20 m × 1.35 t/m3 = 2,700 t. A fertiliser or contaminant added at 1 mg per kg of this soil therefore amounts to 2.7 kg per hectare. This conversion, from concentration to mass per hectare, uses bulk density every time.
Compaction
Compaction squeezes soil, reducing macropores and raising bulk density. Causes include heavy machinery and high axle loads (especially on wet soil), repeated tillage at the same depth creating plough pans, and trampling by livestock.
Effects:
- Roots grow more slowly or not at all through dense layers. As a general guide from soil surveys, root growth begins to be restricted at bulk densities around 1.6 g/cm3 in sandy soils and around 1.4 to 1.5 in clays, and is severely restricted above about 1.8 and 1.5 respectively; texture matters because it changes pore size at a given density.
- Infiltration and drainage fall, causing runoff, erosion and waterlogging.
- Aeration falls, favouring denitrification and root diseases.
- Yields fall, and tillage needs more power.
Prevention and remedies:
- Avoid traffic on wet soil; reduce axle loads; use wider or low-pressure tyres or tracks.
- Controlled traffic farming, confining wheels to permanent lanes so most of the field is never driven on.
- Vary tillage depth; deep ripping or subsoiling to break pans, followed by measures that keep them from re-forming.
- Deep-rooted crops and cover crops, organic matter and earthworm activity rebuild structure over time.
- Rotational grazing and avoiding grazing on saturated soil.
Penetrometers, which measure the resistance of soil to a pushed cone, are a quick field tool for finding compacted layers; resistance above about 2 MPa is commonly taken to restrict root growth, though it depends on soil moisture.
Key points
- Soil has three phases: solids, water and air; pores (water + air) usually make up 40–60 per cent of the volume.
- Bulk density = dry mass ÷ total volume (typically 1.0–1.6 g/cm3); particle density = dry mass ÷ solid volume (about 2.65 g/cm3).
- Porosity f = 1 − ρb ⁄ ρs; void ratio e = f ⁄ (1 − f).
- Air-filled porosity = total porosity − volumetric water content; below about 10 per cent, aeration limits roots.
- Bulk density converts concentrations to mass per hectare.
- Compaction raises bulk density, reduces macropores and restricts roots; prevent it with traffic control and timing, and repair it with deep loosening and biological activity.
What the exams ask
- School-leaving and entrance papers: definitions of bulk density, particle density and porosity, and why compaction harms crops.
- Degree and research-fellowship entrances: calculations of porosity, void ratio, water content and air-filled porosity, macropores and micropores, and measurement methods.
- Agricultural engineering examinations: soil compaction under machinery, controlled traffic, subsoiling, and converting soil test results to kg per hectare.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why is bulk density always lower than particle density?
Because bulk density divides the mass of solids by the total volume, including the pores, while particle density divides it by the volume of the solids alone. They would be equal only for a soil with no pores at all.
Do clay soils weigh less than sandy soils?
Usually yes, per unit volume. Clay soils have more total pore space, so their bulk density is often lower than that of sands, which is why sands are called "light" only in the sense of being easy to plough.
Can earthworms fix compaction?
They help a great deal over time, by burrowing and mixing organic matter, especially when tillage and traffic are reduced. Severe compaction from heavy machinery usually also needs mechanical loosening and a change in traffic.
Sources
- Brady, N. C. and Weil, R. R. The Nature and Properties of Soils, 15th edition. Pearson, 2017. Chapter 4.
- Hillel, D. Introduction to Environmental Soil Physics. Academic Press, 2004.
- Grossman, R. B. and Reinsch, T. G. "Bulk density and linear extensibility." In Dane, J. H. and Topp, G. C. (eds), Methods of Soil Analysis, Part 4: Physical Methods. Soil Science Society of America, 2002.
- USDA Natural Resources Conservation Service. Soil Quality Indicators: Bulk Density. USDA, 2008.
- Hamza, M. A. and Anderson, W. K. "Soil compaction in cropping systems: a review of the nature, causes and possible solutions." Soil and Tillage Research 82, 121–145 (2005).
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