Soils I - Part 3 - The Soil as a Three Phase System

1. Gas Phase (soil air): ca. 25%
2. Liquid Phase (soil solution): ca. 25%
3. Solid Phase (soil matrix): ca. 50%

These numbers do not account for any type of soil. The phases are not distributed equally in permanently wet groundwater soils or in desert soils. Additionally soils can be subject to short-term changes (swelling/ shrinking, freezing/ thawing, soil management).

Soil structure (also soil texture) is the arrangement of the solid soil components in relation to each other.

Pore structure is the structure of the cavities, holes, voids, gaps , interstices of the soil.

A pore is an interstice between solid substances. It es the negative matrix of the solid substance (25% air, 25% water). It is idealized as capillaries (small interconnected tubes).

Distinction by...

...size (diameter), formation and fucntion.

Depends on

• Grain size: rounded particles fit together better than angled i.e. clay platelets
• Grain shape: small grains can fit in the pores of bigger grains. Therefore the porosity is smaller in soils with mixed grain size
• Soil organic matter content
• Soil development

Bulk density and porosity are closely linked to the pore volume. They are both very important for the water and air budget of the soil. They depend on four factors: Grain size, organic matter, structure/ texture and management (→ compaction!).

• Minerals soils typically have a density of 0.8-1.8 g/cm³
• Organic soils typically have a density of 0.12-0.48 g/cm³

Coarse pores: > 10 μm

• Bioturbation, shrinkage cracks, roots
• Important for ventilation, oxygen supply of the soil
• Macroporous flow: fast transport, no water binding, easy drainage
• Restriction of the filter function of the soil for fertiliser, pesticides, pollutants
• Plant available seepage water, accessible for root hairs

Medium pores: 0.2 - 10 μm

• Plant available capillary water (easily accessible)
• Material exchange
• Accessible for bacteria and other microorganisms

Fine pores: < 0.2 μm

• Tightly bound water (not available to plants)
• Not accessible to living beings → no life

The Size of Microorganisms matters for the pores they live in. The more different pore sizes there are the more divers your soil life is. Most microorganisms live in the big and medium pore sizes.

There are primary pores (in all substrates, grain interstices, obvious with gravel and sand: interstitial or intergranular pores) and secondary pores (formed through soil development, worm and root tubes, shrinkage cracks).

Soil Porosity is important for 

• the water balance (water available to plants)
• the oxygen supply for soil organisms: big pores, good oxidated soils, good climate for microorganisms and transport
• Water and material transfers
• Soil stability
• Rooting and habitat