The 3 steps of pedogenesis

1. Weathering	2. Enrichment	3. Horizonation
Degradation of parent materials	Addition of organic matter (required to be defined as soil)	Vertical differentiation of horizons (from top to bottom)

Factors influencing pedogenesis

• There are 5 main factors influencing the process of soil creation:

  • Climate (Cl)
  • Organisms (O)
  • Relief (R)
  • Parent material (P)
  • Time (T)

• This may be remembered as 'ClORPT' (Hans Jenny, 1941)

  $$Soil=f(Cl,O,R,P,T)$$

• The combination of these different factors gives a multitude of soils.

Climate

• T° and H₂O influence how fast rocks weather: soils develop faster in warm, moist climates and slowest in cold or arid ones.

Organisms

• Mineral particles resulting from the alteration of parental bedrocks need to be enriched in organic matter to define a soil

Organisms

• Soil organic matter is an important carbon pool (~twice bigger that the atmospheric pool)

  The fate of soil carbon can have a strong influence on climate change, depending on whether carbon is stored or mineralized

• But there are still many things to discover about the nature of soil organic carbon

Relief

• Soils are generally shallow on slopes due to erosion...

• ...whereas deeper soils form at the bottom of a hill because gravity and water move soil particles down the slope.

Parent materials

• At the Earth's surface, all rocks are subject to alteration

• Soils inherits traits from the parent material from which they formed

  • For example, soils that form from limestone are rich in calcium, with high pH, while soils evolving from granite have low pH

• As they evolve, soils can drift away from these inherited characteristics

• The two main types of parent materials are:

  • Magmatic rocks
  • Sedimentary rocks

Parent materials

Magmatic rocks
Eruptive rocks	Plutonic rocks
Rock resulting from a surface solidification of the lava	Rocks that solidified from a melt at great depth
Example: Basalt	Example: Granite

Parent materials

Sedimentary rocks
Formation at the surface of the earth or oceanic crust by : (i) Erosion, transport then sedimentation (detrital sedimentary rocks) OR (ii) Accumulations or precipitations of biological or physico-chemical origin in the oceans (limestones...)
Silicates (Mostly SiO2)	Carbonates (Mostly CaCO3)
Example: Sandstone	Example: Limestone

Time

• The process of pedogenesis is slow and the creation of a soil takes several tens to hundreds of thousands of years.

But if soils seem stable on a human scale, they are in constant evolution on a geological scale.

• The following presentation will present one example of a chronosequence

• This example will present one possible evolution of a limestone bedrock

Soil denominations

Rendosol

Clay formation

• Weathering of bedrock minerals, such as feldspar, produces clay

  As minerals, clays are phyllosilicates, characterized by alternating layers of silica (tetrahedral sheet) and aluminum (octahedral sheet)

• Si sheets are negatively charged and can thus 'store' cations (K⁺, Na⁺...) or H₂O (because of its polarity)

Clay formation

• Soils are differentiated according to their Si:Al ratio

• Due to the retention capacity of the silica sheets, the higher the ratio, the higher the storage capacity of the soil

• 2:1 clays are more likely to be expansive

Calcisol

Decarbonation

$$CaCO_{3} + CO_{2} + H_{2}O ⇄ 2HCO_{3}^{-} + Ca_{2}^{+}$$

• Rainwaters absorb atmospheric CO₂, and these rainfalls erodes limestone, which is then transformed into HCO₃^- and Ca₂⁺.

• The residual solution is leached out at depth

• So the horizons will be decarbonated from top to bottom

Brunisol

First part of the chronosequence

Luvisol

Water dynamics

• A B horizon enriched in clay is called Bt

• Bt horizon induce water accumulation, which will intereact with Fe through redox reactions (electron exchanges with oxygen)

  Pseudogley with oxidation traces: markers of temporary flooding (O₂ after flooding)
  

  Gley with reduction traces: markers of permanent flooding (no O₂)
  

Luvisol redoxic

Reductisol

Continuation of chronosequence

Soil evolutions

• Given examples represent only few possibilities of soil evolution, there are a large number of alternatives

French soil map

European soil map