SOIL FORMATION
Chapter 2
Rough Outline
Processes of soil formation
Rock is ultimate parent material of soil
Factors of soil formation
More on horizon designations
There are 4 processes of soil formation
and
There are 5 factors of soil formation
Processes form soil
Factors control processes
And these 4 processes are
Processes of Soil Formation
Transformation
Weathering / synthesis of minerals
Decomposition / synthesis of organic
matter
Processes of Soil Formation
Translocation
Movement of mineral and organic particles
in the developing profile
Processes of Soil Formation
Addition
Organic matter produced
Material deposited on top
Processes of Soil Formation
Loss
And how do losses occur?
How about an example of soil
formation?
Processes of Soil Formation
Start with uniform parent material
1 Organic matter added by plants
2 Microbes biochemically transform it
You get
A horizon
50 years
Processes of Soil Formation
3 Salts leached
translocated
lost
Minerals weather
transformed
Clay moves down
translocated
You get weak B horizon 2500 years
Processes of Soil Formation
4 With more accumulation of clay, structure
develops
You get for real B horizon 10,000 years
Transformation
Translocation
Addition
Loss
These processes are controlled by the
factors of soil formation which are
Do you think climate might be one?
grass
What about organisms, especially
type of vegetation?
Same type of soil expected
regardless of whether on hilltop,
side or bottom? Topography?
Do soils form overnight?
→B
A
→D
C
A and C are different starting materials
and
B and D are different end products
So does starting material also control
soil formation? Parent material
Five Factors of Soil Formation
Parent material
Climate
Organisms
Topography
Time
We know that
Soils either
Formed in place from rock
Formed in weathered materials
transported from elsewhere
But ultimately
Unconsolidated mineral matter at the
surface of the earth came from rock
Rock
Rocks
Mixtures of minerals
Igneous
Metamorphic
Sedimentary
Rock
Igneous
From molten magma
Contain primary minerals
Rock
Sedimentary
By deposition and cementation of
weathered products
Sandstone
Shale
sand particles
clay particles
Rock
Metamorphic
From igneous or sedimentary
High pressure and temperature
Quartzite from sandstone
Slate
shale
Rock
Weathering
Physical disintegration
Chemical decomposition
Physical decreases size
Chemical alters composition
Rock
Thermal stress
Ice expansion
Abrasion by water and wind
Rock
Chemical reactions
Hydrolysis
Hydration
Al2O3
Al2O3 · 3H2O
2KAlSi3O8 + 13H2O →
Al2O3 + 6H4SiO4 + 2K+ + 2OH-
Rock
Acid dissolution
CaCO3 + H2CO3 →
 Ca2+ + 2HCO3Oxidation
Particularly, Fe →
 Fe
Fe
2+
3+3+
Rock
What happens when you drop a little
3M HCl on some CaCO3?
By the way, what happens is a test for
carbonate in soil.
It fizzes because
CaCO3 + 2HCl → Ca2+ + 2Cl- + H2CO3
H2CO3 → H2O + CO2 ↑
Factors of Soil Formation
Parent Material
Geologic material in which a soil forms
Residual
Transported
Factors of Soil Formation, Parent Material
Transported
Colluvial
Alluvial
Marine
Lacustrine
Glacial
Eolian
These are the types of transported parent material.
Residual parent material is residual, i.e., weathered
rock. Organic soils don’t fit into this dichotomy.
Organic parent material
residual or transported?
Factors of Soil Formation, Parent Material
Residual parent material comes from
underlying rock
Soil properties tend to reflect parent
material
Factors of Soil Formation, Parent Material
For example,
Limestone
sand or clay impurities
Sandstone
shallow if SiO2 cements
deep if CaCO3
Shale
clay minerals
As the cementing agent (carbonate) dissolves, limestone residue can be any
size particulate, from sand to clay. Sandstone give sandy soils but how deep
these are depends on how fast the cementing agent dissolves. Shale gives
clayey soils.
Factors of Soil Formation, Parent Material
Colluvial is coarse material carried
downslope by gravity
Soils formed in colluvial parent material are found
in landscapes like to right.
Factors of Soil Formation, Parent Material
Alluvial from streams and rivers
Alluvial fan
Flood plain
Delta
These are the three types of alluvial deposits.
Factors of Soil Formation, Parent Material
Alluvial fan occurs at the discharge of an
upland stream into a broader valley below
These not all that common.
Factors of Soil Formation, Parent Material
Common and often important in agriculture.
Flood plain next to river
What happens during a flood?
Water with sediment spills over the banks,
and then what?
Think particle size sedimentation rate
Factors of Soil Formation, Parent Material
Coarse sediment deposited near to or
away from channel
Fine sediment where?
Deposition forms a natural levee
Deposition is greatest near the bank leading to highest elevation. Furthermore,
this area of deposition is comparatively high in sand + silt. Thus, since the coarser
particles are deposited quickly, deposition from the flood water further away is
low in sand + silt and high in clay.
Factors of Soil Formation, Parent Material
A laterally compressed cross section of the Mississippi River natural levee.
Notice any familiar names to the soils? The name comes from some place or
geographic feature nearby where the soil was first described.
Factors of Soil Formation, Parent Material
Another depiction of the Mississippi River natural levee. The course has followed
a low position in the landscape, and river sediments have been deposited in the
channel and outside it, the latter creating the levee.
Factors of Soil Formation, Parent Material
Changes in course produce complex pattern
of coarse and fine sediments
Apparent old
levee, thus
earlier course.
Factors of Soil Formation, Parent Material
A delta occurs at mouth of river
End of flood plain
Factors of Soil Formation, Parent Material
Marine sediments
Uplifted / exposed
Variable from sandy to clayey
These old materials vary in composition depending on their ancient source.
Due to uplift or sea level drop, these became exposed and have undergone
pedogensis. As with alluvial parent materials (floodplain and deltaic deposits),
there are a lot of soils in Louisiana that have formed in marine sediments.
Soils in East Baton Rouge Parish
formed in residual parent material,
right?
Obviously, no.
Near the river channel you’ve got clay
soil but in the backswamp it’s more
sandy (True / False).
Factors of Soil Formation, Parent Material
Glacial deposits
Partially melted in summer, giving sediment
deposited at leading edge or further away.
Advancing ice
sheet accumulated
unconsolidated
material
Deposited when
glacier melted
and retreated
See various topographic features
in a glaciated landscape.
Factors of Soil Formation, Parent Material
Material directly deposited called
glacial till
Streams from glacier produced
outwash plains
In low areas, lakes formed with lacustrine
deposits
None of this in La., however, there are soils in eolian parent material that
came from glacial melt. See next slide.
Factors of Soil Formation, Parent Material
This figure shows
loess only to east
of river, however,
there is loess to
the west, just not
as much. Also,
not nearly as deep
in Baton Rouge
as further north,
like Natchez.
Eolian deposits from outwash areas
Silt and some fine sand plus clay (loess)
Along Mississippi and Missouri Rivers
So, eolian deposits are more or loess silt.
Well, not necessarily because there are deposits that are more sandy.
Factors of Soil Formation, Parent Material, Misfits
Organic soils
Wet places
Plant growth faster than residue
decomposition
Deposits called peat
Factors of Soil Formation, Parent Material, Misfits
This is the general progression.
Sedimentary
(aquatic
plants)
Herbaceous
(sedges and
so forth)
Woody
(trees)
Factors of Soil Formation, Climate
Clearly, parent material is the most important of the 5 factors of soil formation.
Climate comes in next. Without precipitation (water) and warm temperature,
there will not be a lot of plant growth, mineral weathering nor translocation of
particulate matter in the profile –thus, pedogenesis is slow.
Climate
Precipitation and temperature
Affect rates of soil formation processes
Factors of Soil Formation, Climate
Example effects
High rain and low temperature
(increase / decrease) organic matter
High rain (increase / decrease) salt leaching
The reason why low temperature leads to accumulation of organic matter
is the effect it has on slowing microbial decomposition of organic matter.
Factors of Soil Formation, Climate
High rain (increase / decrease) clay translocation
High rain and high temperature
(increase / decrease) mineral weathering
represents generally cool and dry climate
Soils in Utah likely
A) Contain few weatherable minerals
B) Are leached of soluble salts
C) Both of the above
D) Neither of the above
D is the right choice. If it’s cool and dry there, there hasn’t been a lot of
weathering and leaching going on. The opposite would be true for Panama
in the next slide.
represents generally warm and wet climate
Soils in Panama likely
A) Contain few weatherable minerals
B) Are leached of soluble salts
C) Both of the above
D) Neither of the above
Factors of Soil Formation, Climate
Climate indirectly influences soil formation
by its effect on vegetation
Like (match these)
Trees
Grasses
Brush
Semiarid
Arid
Humid
The 5 factors of soil formation are interrelated to varying extent. In this example,
ample water is needed for forest vegetation and jointly due to greater rainfall and
specific effect of trees, forest soils are different from prairie soils.
Factors of Soil Formation, Organisms
Organisms
Let’s compare soils formed under forest and
prairie vegetation. Big contrast.
Factors of Soil Formation, Organisms
Thick, organic matter rich A under grass
Thinner A but E over clayey B in forest
Factors of Soil Formation, Organisms
Deciduous
versus coniferous
affects soil
development
Nutrient cycling
faster under
deciduous
Slows leaching
of base cations
Slows acidification
The preceding slide was omitted earlier because the explanation is long. It goes
like this.
Soils are open and leaky systems, at least in humid climates. Thus, soluble
substances are leached from them, including nutrient cations, like Ca2+, Mg2+
and K+. To some extent these are replaced by deposition from the atmosphere
but this is not sufficient to offset natural leaching. To make this matter worse,
there is continuous generation of H+ in the soil. It comes from CO2 released in
respiration (microbes, roots, etc.) which forms H2CO3, a weak but abundant
acid. Furthermore, the decomposition of organic matter in soil results in release
of various organic acids (R-COOH) and small amounts of the strong acids, nitric
(HNO3) and sulfuric (H2SO4). The H+ from these acids tends to replace base
cations like Ca2+, Mg2+ and K+ that are adsorbed onto the negatively charged
soil colloidal particles (clay and humus), thus acidifying them, and with the bases
in solution, they are subject to loss in water that drains through the soil. The net
effect is long-term acidification of soil. This is the natural course of things.
However, if the base cations, which are nutrients, are taken up in large quantities
by plant roots, the acidification process is slowed. Ca2+ etc. taken up by plants
is returned to the soil in litter so that these nutrients are cycled between soil
and plant. Some plants, like the deciduous trees compared with coniferous trees,
cycle nutrients quickly, thereby more effectively slow soil acidification.
Factors of Soil Formation, Organisms
Biosequence in Louisiana loess
Soil
Calhoun
Jeanerette
Cover
forest
prairie
Solum
Clay
pH
Deeper development in which?
More weathered minerals in
which?
Lower pH in which?
So how can a soils guy say that observed differences between two soils are due to
one or another of parent material, climate, organisms, topography or time? If the
focus is effect of organisms, find 2+ soils that come from the same parent material,
formed under the same climate, on the same type of landscape and are about the
same age. These 2+ soils would constitute a biosequence of soils.
Factors of Soil Formation, Organisms
Soil
Calhoun
Jeanerette
Cover
forest
prairie
Solum 175 cm
Clay
weathered
pH
4.5
125 cm
less weathered
6.5
So there you see. The effect of trees has been to cause deeper profile
development, greater mineral weathering and greater soil acidification.
Factors of Soil Formation, Topography
Topography
Modifies effects of organisms and climate
Thinner soil and less mature profile on side
slopes or top / bottom?
Why?
Erosion takes away topsoil. Furthermore, less water infiltrates so
there is less translocation of clay to form a clayey B horizon.
Factors of Soil
Formation,
Topography
Profile development slowed by erosion and
less water infiltration
Factors of Soil Formation, Topography
Topography affects soil development by
affecting depth to water table
Surface
Does the water table do this or that?
It does the this, not the that. Where there is a shallow groundwater table, it tends
to roughly parallel the soil surface, but not exactly. The effect of a shallow water table
is to impede drainage, thus minimize translocation of particulates through the profile.
Factors of Soil Formation, Topography
Water table impedes drainage
Does slow drainage speed up or
slow down profile development?
To the left, there is deeper
soil development at the top
of the hillock but in the lower
scenario there may be
deeper development on the
sides. However, erosion
probably comes into play,
complicating interpretations.
Factors of Soil Formation, Topography
More organic matter on N or S slopes?
Probably more on side away from
heat source, lower temperature so
slower rate of microbial activity
and organic matter decomposition.
The low wet spot is prime for
accumulation of organic matter
since poorer aeration slows
overall microbial activity.
More organic matter here or there?
Factors of Soil Formation, Time
Time
It takes time for soils to form
Effect of time seen in chronosequences
Factors of Soil Formation, Time
Like in Red and Mississippi River alluvia
Severn, Roxanna and Gallion on natural
levees of the Red River
Soil
Channel Age
Severn
recent / old ?
Roxanna recent / old
Gallion
recent / old
CaCO3 Depth
< 50 cm
> 50 cm
leached
Red River soils are somewhat special in that they can have carbonate (from the
West Texas, New Mexico, Oklahoma source area) in them. However, it tends to be
neutralized and leached with time. So which is oldest and on what levees is it found?
Factors of Soil
Formation, Time
Gallion on
abandoned
levees and
Severn on
current levees.
Roxanna on
both.
Factors of Soil Formation, Time
Convent, Bruin and Dundee on Mississippi
natural levees
Match soil depth with right soil
15 cm
Soil
45 cm
Belt
Convent 5
5-3
Bruin
Dundee 4 - 2
60 cm
Age
< 3,000
> 3,000
> 4,000
Solum
depth
Soils defined –dynamic natural bodies
having properties derived from the
combined effects of climate and biotic
activities, as modified by topography,
acting on parent material over time.
Horizon Designations
More on Horizon Designations
Five master horizons
Can you still name them?
Here’s a mnemonic device
Horizon Designations
O CEBA Building,
How Dear Thou Art to Me
Patrick F. Taylor Hall, alas
Horizon Designations
Some vocabulary for E and B horizons
Eluvial washed out of
E
Illuvial washed into
B
Horizon Designations
Ap Plowed
Bt Clay
The t is for translocated, of course.
Bg Gley
Bx Fragipan (dense and brittle)
Transitional horizons such as AE, EB, BE and
BC
Not clearly an A or an E but
more like an A than an E, etc.