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

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.

Capillary Water: Easy to remove. Accumulates on the adsorption water due to cohesive forces. Annular deposition of water molecules due to surface tension (forms rings around the soil particles).

Fill Capacity: Is the highest amount of water that a soil can hold (differs).

1. Grain size (smaller retains more water)
2. Soil Structure (pores)
3. Type of colloids (= small particles floating in the water)
4. Cation coating of particles
5. Soil Dryness (water is sucked up faster by dryer soils)

⍦z = Gravitational potential (does not change throughout the soil)

• Indicates the work required to lift a certain amount of water from reference level to a certain height (m x g x h)
• Unit: m3 kg-1 s-2
• If the weight is used as the reference unit, then the gravitational potential appears as the local altitude h (mass x gravitational acceleration (g) removed)
• Unit of the gravitational potential is then: cm

⍦m = Matric Potential (often negligible)

• The matric potential is opposite to the gravitational potential, therefore it is given a negative sign. The smaller the pore size or the dryer the soil, the smaller the matric potential, the more tightly bound is the water.

• If the reference unit is the weight of water, the unit of gravitational potential is also: cm (centimeter)

⍦g = Gas Potential

If the air pressure in the ground is different from the reference level, this has an effect on the movement of water.

⍦o = Osmotic Potential (often negligible)

Concentration gradients of dissolved substances that have an effect on water movement.

⍦H = Hydraulic Potential = gravitational potential + matric potential

Water movements are caused by a change of potential equilibrium through precipitation/ irrigation, transportation and evaporation.

a) in a soil that is at equilibrium, the hydraulic potential is always at 0 because the matric potential and gravity cancel each other out.

b) Red line: How things are, when the soil is at equilibrium. The orange lines show what happens when circumstances change (e.g. capillary rise or infiltration).

c) if the top soil is dried, the water content in % will be lowered. Because the soil is dry, the matric potential will rise (it will at- tract more water). The gravitational potential stays the same. The hydraulic potential will go down. If the soil is dried, more water will go up towards the surface until the forces at work cancel each other out.

d) if water is added to the top, the matric potential will decrease. Gravity then takes over and the water will go down by gravity until a new equilibrium is reached.

→ If you have a very small capillary, water will go further up than in a wider one (more water in the wider one and therefore more gravity). In the smaller one there is less gravity and therefore more matric potential which is adsorption of the water on the surface.

Water tension (= suction tension) is equal to the logarithm of the negative matric potential. It describes the relationship between water content and water tension which depend on the soil type (grain size).

Field Capacity (FC)

Amount of water a soil holds after it is completely drained.

Permanent Wilting Point (PWP)

Point at which the water is so tightly bound that it cannot be accessed by the plant.

Plant Available Water

The difference between the field capacity (FC) and the permanent wilting point (PWP).

• driving potential gradient (gravitational and capillary forces)
• permeability of the soil (soil texture)

Unsaturated flow, due to more interaction (big pores empty faster than smaller pores).

Water typically flows through the largest pores (can be primary or secondary). The flow through middle sized pores is done through microorganisms or through processes of swelling and shrinking. 

• instrument to measure soil water content
• measures the suction tension (matric potential)
• needs a soil-specific calibration to calculate the water content through a pF curve

• to determine the soil water content
• TDR = Time Domain Reflectometry
• based on the property that a signal will travel through any material. Speed is based on the water content
• propagation of the signal speed will depend on the soil moisture

When one determines the soil water content directly via gravimetrie, a soil samples weight is measured before and after drying the sample. The weight difference from before to after drying the sample is the soil water content. 

= 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