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

= the arrangement and cohesion of the solid components of the soil and the shape and arrangement of the cavities (pore system) between these components (→ organized juxtaposition of arrangements which form larger units. 

Plowing forms a quite lose structure on the top surface of the soil due to different agricultural and soil management practices. The structure has a major influence on growth, filtering and buffering function, yields and the overall soil development.

1. drainage (percolation, evaporation): you lose water from the soil and collapse of the soil
2. particles come closer together
3. soil surface sinks and soil matrix shrinks
4. bulk density increases and pore volume decreases.

Through shrinking processes forces build up until they exceed the forces holding the soil together and cracks form. This process is not reversible. It destroys the aggregate that was built before the shrinkage.

1. cause: precipitation, groundwater rise, rewatering, rewetting
2. particles absorb water and water shells get thicker → swelling of the particles, 
3. cracks close and the soil surface is lifted

But the original state is not restored. Additionally, the swelling strength depends on the clay content (e.g. 2:1 clay types (smectite, vermiculite)).

• same charge vs. different charge
• van der waals forces (neutral and very small particles)

Flocculation = formation of aggregates by precipitation and attraction of suspended colloids (bringing particles together, electric double-layer that cause attraction)

Peptization = destruction of aggregates by formation of suspended colloids (bringing particles apart, reduction of the electric layer)

(a) peptized particles

(b) aggregated surface-surface at neutral to high pH. Here the soil is very condensed and not well aeriated

(c) aggregated surface-edge the positive charge on the edge will become attracted to the negative charge on the sides of the clay

(d) aggregated edge to edge particles only get this one-sided charge under acidic conditions, because of more H+. This aggregate is much more porous and lose.

• induces aggregate formation
• increased microbial activity by OM addition (mucilage: biopolymer that is around the microorganism that will stick to inorganic particles like clay and sand that participates in the aggregation)
• faecal (dt. Kot/ Exkremente) aggregates of earthworms and enchytraeids
• root exudates (roots release small carbohydrate molecules), fungal hyphae, hair roots

• Fe- and Al-oxides: Give structure to loose aggregates (in the mm range); Precipitation around contact point of particles
• Silicon oxides (Silcrete, Duripan): dissolved and reprecipitated
• Ca saturation: additional stabilization of aggregates through Ca-bridges between clay and humus particles
• Al very stabilising, but reduces plant growth due to toxicity

Quartz - Organic Matter - Clay Mineral

• surface-surface
• edge-surface
• edge-edge

Single Grain Structure (non-bonded primary particles)

• Stability: meniscus forces (e.g. Sand Castle), frictional forces (e.g. Sand Dunes), gravitational forces
• Properties: well ventilated, good water permeability, well rootable, no worm and root tubes, easy trickling with steep sand slopes (→ not very good for plant growth)
• Occurrence: sandy and gravely soils, low iron-oxide soils

Coherent Structure (structureless mass, assembled particles)

• Stability: cohesion (force between molecules of the same kind = aggregation between different sizes: small particles fill the pores between big particles), not changed by dehydration (because the pores are too small)
• Properties: poor water permeability, poorly ventilated, unfavourable for plant growth
• Occurrence: lower parts of silty and clayey soils → steep sand slopes made stable

Cemented Structure or Massive Structure (bonded particles stuck together)

• Stability: cohesion, adhesion, binders (iron oxides, carbonates, organic substances around the particles contact points)
• Properties: poor water permeability, poorly ventilated, unfavourable for plant growth
• Occurrence: undergrounds of podsol, hardpan, bog iron in gley soils (similar process), calcrete in calcisol

Prismatic Structure (Pelosol/ Vertisol-Pseudogley)

• Origin: shrinkage
• Properties: distinct boundaries, sharp angles and edges, vertical, 0-30 mm/ 10-50 cm,  decay into polyhedra, moderate to very stable, poor to good ventilation (prism size), average plant available water
• Occurrence: clay-rich soils, P-Horizon (for pelosol in german classification system = Vertisol WRB, > 45% clay (= a lot of clay))

Polyhedral Structure

• Origin: shrinkage or ground frost
• Properties: distinct boundaries, sharp corners and edges, 2-50 mm, very stable aggregates, high plant available water
• Occurrence: clayey, humus-poor soils, P-Horizon, Bt-Horizon (clay accumulation), horizon >75% carbonates

Subpolyhedral Structure (further degraded polyhedral structure)

• Origin: Compression (animals, soil cultivation), Shearing (partial shrinking and swelling)
• Poperties: distinct boundaries; no sharp corners and edges (rounded, due to sheering forces), generally stable, well ventilated, high plant available water (PAW) (→ very good for plants)
• Occurrence: Bw Horizon (iron oxide formation)

Plate Structure

• Origin: Compression, Compaction (soil cultivation), Freezing and Thawing, Root pressure (wind and around roots) (→ leads to Horizontal plates and different thickness 1 to 50 mm)
• Properties: poorly ventilated, poor natural drainage, little to very stable
• Occurrence: Surface of silty soils with waterlogging (→ can lead to stagnant water or surface runoff)

Crumbs

• Origin: high biological activity needed (droppings from insects and microfauna), high OM, intensive root penetration
• Occurrence: humus-rich soils, Ah-horizon grassland
• Properties: stable, high porosity, well ventilated, high in plant available water, optimal structural shape

→ very good in terms of soil ecosystem services

Earthworm Casts

• Origin: Fine soil particles stuck together With the secretions of the intestinal flora of the worms
• Properties: htable + high porosity, well ventilated, high plant available water, optimal structural shape, high organic content
• Occurrence: humus-rich soils, Ah/Ap horizons grassland

Roll aggregates

Origins

• Physical-mechanical by rotary hoes through shearing processes: sphere as the most energy-efficient state
• Biotic: Breeding balls of the dung beetles
• Anthropogenic effects: Relocation of soil material: transport on belts

Peds (< 5cm)

• Origin: soil management (ploughing, harrowing (breaking down the bigger structures)) under good consistent conditions and moderate clay content
  • small to medium size
  • similar to subpolyhedron (larger)
  • coarse surface
  • usually, all the same size
• Occurrence: Ap-Horizons

→ Aim of soil tillage

Clods (> 5cm)

• Origin: Soil management (ploughing, harrowing) (under unfavourable consistency conditions (too wet/dry))
• Occurrence: Clayey or loamy soils (not being ploughed or harrowed), similar to prismatic structure (rough), similar to subpolyhedron (larger), surface mostly smeared, usually all the same size

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

Soil air depends on 

• pore size distribution
• pore volume
• water content
• soil development and cultivation

Keep in mind that soil air varies with time. 

The chemical composition of soil air ist not equal to that of atmospheric air: O₂, CO₂ (more in soils than in the atmosphere because microorganisms additionally generate CO₂), CH₄, N₂O, NH₃.

The chemical composition also varies throughout the year. Typically there is more oxygen during the summer months in the upper 30mm than in the upper 90mm (globally).

Water Molecule: Polarity, solvents for salts and other polar substances (water arranges itself like a magnet)

Surface tension: Tendency of a liquid to keep its surface area small (because of the polarity of the water molecules that “stick together” and build something like a film or trampoline)

Surface tension and wettability: Adsorption and capillary water → Depending on surface and liquid (wetting or non- wetting systems). Force to keep the water surface as small as possible.

Capillarity is defined as "the action by which the surface of a liquid where it is in contact with a solid (as in a capillary tube) is elevated or depressed depending on the relative attraction of the molecules of the liquid for each other and for those of the solid" (Source: https://www.merriam-webster.com/dictionary).

Rise of a liquid in a tube: Adhesion must be greater than cohesion (wetting angle < 90°)

At wetting angles > 90°: capillary depression (sphere, not a wetting system)

Capillary (from the surface tention) rise depends on...

• Capillary Diameter
• Grain Size
• Surface Tension

Water rises higher in soils with smaller pore sizes, because the area on which the water m olecules can adsorb on is bigger.

Seepage Water: Water that seeps through the soil and into the ground water.

Free Water: Ground water and stagnant water (stagnant water = when there is a clay layer that doesn't let the water pass through)

Adsorption Water: Water which is adsorbed on the solid substance. Due to adhesion forces between two layers, it is not easily removable. It can be removed by drying the soil at 105°C for 24h.