Summary + Lectures second part - Natural Processes

Summary Natural Processes PT2 Book & Lectures



Chapter 18: Soils in their environment
Lecture 1: Soil Science
Residual soils: soil which is still at the place of the original parent material.
Transported soils: transported from original parent materials (by water,
glaciers, wind, etc.)

18.1 Soil formation
Two soil formation processes
1. formation of soil parent materials through weathering. This is the C
horizon material.
2. formation of the soil profile. This is when the C horizon material is
changed into A, E, and B horizons. This happens near the surface.

Vertical organisation: sequence of soil layers (horizons) with different
characteristics. Downwards:
- O: organic matter
- A: topsoil
- E: eluviated topsoil (bleached)
- B: subsoil
- C: parent material
- R: bedrock

How to determine the composition of soils
Antropogenic controls: fertilizers, heavy metals, etc.
Geogenic controls: parent soil material, climate, etc.

 Q1: Which type of control dominantly determines the composition of
agricultural soils?
A1: parent soil materials  so, geogenic

Soil components:
1. Resistant residue (quarts, etc)
2. Alteration compounds (secondary minerals)
3. Organic matter
4. Weathering solution

Controlling factors in soil formation (PClORT):
 Parent material (e.g. limestone)
- Weathered bedrock
- Freshly deposited sediments
- C horizon
- Large variety of chemical deposition (this composition determines
the speed of the weathering)

 Q2: Why is parent material important in soil formation?
A2: The parent material has a vertical influence on the topsoil.

,Summary Natural Processes PT2 Book & Lectures


 Climate
- Temperature is very important (high temp, high activity /
weathering / amount of water)
- Precipitation
o Precipitation > evaporation: leaching, peat formation
o Precipitation < evaporation: e.g salt accumulation
- Soil water balance (precipitation)
o tropical soils: intense weathering, low organic matter
(because it decomposes really quickly)
o desert: little leaching, low organic matter
o steppe: high organic matter, high leaching
o taiga (naaldwouden/boreal forest): slow decomposition, high
organic matter, high leaching
- climate determines growth rates and decomposition

 Q3: Why is climate an important factor in soil formation?
A3: Climate governs the amount of CO2, the amount of
precipitation and temperature (determining weathering). Climate is
the most important factor in soil formation.

 Biota (organisms)
- Flora
o The main source of organic material
o Plants enhance consolidation, the compaction of clay, which
influences the hydrology
- Fauna
o Organisms and plants can move/transfer soils etc and
determine structure. Moles, worms, roots, etc.
o Humans also influence soil with agriculture

 Q4: why are the biota important in soil formation?
A4: Important for the mixing of the soils, getting minerals
everywhere, mixing, etc. (?)

 Topography (relief)
- Slopes influence the water content of the soil
- Influences erosion and transport of materials

 Q5: Why is topography important for soil formation?
A5: Transfer of materials and water distribution which also
influences the decomposition etc.
 Time
- Whether or not the soil is disturbed
- Soil processes take time – ‘new’ soils (azonal) have yet to develop,
while older soils have clear horizons

Zonality:
Zonal soils: typical for certain climate
Azonal soil: younger, not yet in balance
Intrazonal soils: climate is not the determining factor (e.g. parent material, salt
etc)

, Summary Natural Processes PT2 Book & Lectures




Lecture 2: Soil formation
18.2 Processes of soil formation (pedogenic processes)

Important groups for soil formation processes:
- Additions (inputs)
o Weathering of parent material
 Unloading (remove pressure from top layer, stone expands
and breaks)
 Thermoclasty (expanding and shrinking of rocks bc of
temperature changes)
 Gelicfraction (by ice)
 Halofraction (volume changes bc of salt)
o Sedimentation
o Atmospheric depositions (wind etc)
o Anthropogenic influences
o Litterfall
- Losses (outputs)
o Erosion
o Leaching
 Leaching: continuously washing the soil with rain water.
Well leached soils have a lower pH.
 In solution (nitrate)
 In suspension (clay)
 Decalcification: removal of free calcium carbonate by
leaching. Leads to lower pH.
 Calcifiation: In arid regions, bases will be washed
downwards during rainfall but moved up by capillary
movement in dry periods. Causes the accumulation of
calcium and other bases.
 Podzolisation: pale (bleached) horizon between eluviated
(washed out) and illuviated (washed in) soil. Due to extreme
leaching of fulvic acids (litter). Temperate climates.
- Transformations (internal change)
o Chemical weathering
 Acid dissolution, chalk dissolution, etc.
o Organic material (decomposition, mineralisation)
o Redox processes (high ground water levels)
 Redox is the exchange of electrons.;
+ is oxidation
- is reduction
 Aerobic, is O2 the e- acceptor
 Anaerobic is e.g. NO3- e- acceptor
 Gleying: when a soil is filled with water. Driven by seasonal
soil water differences. Colour changes: reddish/iron,
grey/manganese etc.
o Soil maturation (change of fresh soil to older soils)
 Physical (soil dries out, etc)
 Chemical (oxidation>reduction, dehydration, etc)