Summary of Important Concepts
• Soil is a layer of weathered rock, minerals, and organic matter at
the earth’s surface that supports plant life.
• The main factors that determine the characteristics of a soil are
climate (mainly temperature and rainfall), topography, type of
parent rock material, organic activity, and the amount of time that
the soil has been forming.
• Soil forms from regolith - broken up rock material at the earth’s
surface. Soil forms as regolith undergoes weathering - the physical
and chemical breakdown of rock.
- Physical weathering is breakdown of rock by physical forces.
Example: rock wedged apart by freezing water or by plant roots.
- Chemical weathering is breakdown of rock by chemical
reactions. Examples: solution, oxidation, and hydrolysis reactions.
Summary of Important Concepts,
continued
• A soil profile is a set of distinct layers, or soil horizons, that
occur in a soil. These horizons have different features in different
types of soil.
• The main types of soil on earth are:
Remember : These are NOT soil
orders !!!
- Loess (pronounced “luss”): fertile soils developed on windblown glacial silt deposited during the Ice Ages.
- Laterites: red, iron oxide-rich soils of wet, hot tropical areas,
created by intense chemical weathering of parent rock material.
- Pedalfers: rich soils with brown color, high in aluminum and
iron; typical of cooler, wet temperate climates world-wide.
- Pedocals: soils typical of warm, arid regions; high in calcium
and commonly contain caliche (white deposits of calcium
carbonate)
- Tundra soils: soils forming in polar climates of permafrost
(permanently frozen ground).
What is SOIL?
It has been said that soil represents “a few inches between humanity
and starvation”. This phrase puts the importance of soil in
perspective. Soil is a layer of weathered rock, minerals, and organic
matter at the earth’s surface that supports plant life. Without soil,
human life would not be possible.
This figure illustrates
the composition of a
typical soil. Soil is
composed of mineral
matter from
weathered rock;
water, gases, and
organic matter (the
remains of plant and
animal material and
bacteria).
The water table or phreatic surface is the upper limit of abundant groundwater. In the vadose zone, above the
water table, the interstices between particles of earth are filled by air, or by air and water (with the exception of the
capillary fringe). Below it, every available space is saturated with water. The definition of the water table is surface
where the pressure head is equal to atmospheric pressure (where gauge pressure = 0).
Soil’s main
important uses for
humanity are
summarized here.
Soil Profiles
Digging down into a
soil, you would notice
that the soil zone has a
layered appearance. This
layering is called a soil
profile. Each layer of a
soil profile is called a
horizon.
Soil profiles vary
between different types
of soils, but one can
often recognize the “O”,
“A”, “B”, and “C”
horizons in many soils.
Simplified Soil Profiles
Birchwood
Castelleia
Haven
Examples of Soil Profiles
Nickerson
Creek versus
floodplain soil
profiles
horizon description of detailed soil horizons
O
consists mainly of organic matter from the vegetation, which
accumulates under conditions of free aeration.
A
eluvial (outwash) horizon consisting mainly of mineral matter
mixed with some humified (decomposed) organic matter.
E
strongly eluviated horizons having much less organic matter
and/or iron and/or clay than the horizons underneath. Usually pale
coloured and high in quartz.
B
illuvial (inwashed) horizon characterised by concentrations in
clay, iron or organic matter. Some lime may accumulate, but if the
accumulation is excessive, the horizon is named K.
K
horizon containing appreciable carbonate, usually mainly lime
or calcium carbonate.
G
gleyed horizons which form under reducing (anoxic) conditions
with impeded aeration, reflected in blueish, greenish or greyish colour.
C
weathered parent material lacking the properties of the solum
and resembling more the fresh parent material.
R
regolith, the unconsolidated bedrock or parent material.
Types of Soils
Many schemes have been proposed for classifiying soils. For our
purposes, we can recognize FIVE great soil types on earth.
• Loess
• Laterites
Some would consider this heretical !!
• Pedalfers
• Pedocals
• Tundra soils
Loess (pronounced “luss”) covers about 10% of the earth’s land
area, and is perhaps the most fertile soil on earth. Vast areas of
farmland in the U.S. and in Asia are underlain by loess.
Loess is formed on deposits of wind-blown glacial silt that were
laid down over vast areas of the continents during the Ice Ages.
As glaciers moved across the high northern latitudes they ground
up rock material to a powder. This was washed out from the
glaciers by streams, then picked up by winds and blown over
great distances.
This map shows the world-wide distribution of loess. Notice the
vast areas of the U.S. and Asia that are covered by loess. These
areas are some of the world’s most productive farmland.
Loess is tan to yellow in color, and porous and light. The small
mineral grains weather readily, producing nutrients that get taken
up by plants. The porous soil drains well, and is easily tilled to
make fields.
This photo
shows an
excavated
layer of loess
in China -home to the
largest loess
deposits in
the world.
Loess is a soil defined by its origin (deposits of wind-blown glacial
silt). The four other great soil types we will consider are defined not
by their origin, but by the climate in which they form. The two
climatic factors that are most important here are precipitation and
temperature. As shown in the figure below:
• Laterites form in hot, wet climates.
• Pedalfers form in cool, wet climates.
• Pedocals form in hot, dry climates.
• Tundra soils form in cold, dry climates.
Very generalized !
Soil Order
Formative Terms
Pronunciation
Alfisols Alf, meaningless syllable
Pedalfer
Andisols
Modified from ando
Ando
Aridisols
Latin, aridies, dry
Arid
Entisols
Ent, meaningless
Recent
Gelisols
Latin gelare, to freeze Jell
Histosols
Greek, histos, tissue
Histology
Inceptisols
Latin, incepum, beginning Inception
Mollisols
Latin, mollis, soft
Mollify
Oxisols
French oxide
Oxide
Spodosols
Greek spodos, wood ash Odd
Ultisols
Latin ultimus, last
Ultimate
Vertisols
Latin verto, turn
Invert
Puerto Rico
Rocky Mountains
Crater Lake
Saint John
Also don’t forget parent material and
Time !!!
Transported versus Residual Overburden
Residual = soil ( developed from bedrock )
Transported ; may have a soil developed on it !
Laterite
Terra rossa clay replaces Salem limestone, Bloomington,
via a set of moving alteration zones – a unique outcrop
Clay+FeOx replace calcite, terra rossa,
Bloomington: early start of a diffuse FeOx patch
Weathering’s crucial feedback, deduced by adjusting
bottom reaction on volume (since it’s a replacement)
Merino et al, Am J Sci 93
Gibbsite partly replaces Plagioclase,
left, and Quartz, below
Soil Profiles
Pit 1 - Peaty Gley
The peaty gley is found on a low
lying flat land, the vegetation cover
consists of wetland vegetation such
as sedges, marsh type plants and
rushes.
The texture of this soil is course and
sandy with lumpy particles of wood
etc. Within the structure there is
capillary action from ground water
taking place.
The peaty gley is a very dark colour
this is because their is alot of
organic matter, the surface layer is
40cm which is not mixed in through
the A horizon. Also as it is very wet
so no oxidation can take place.
The bioto found in the gley is
limited because it is too cold
and too wet. There are very few
stones in this soil profile.
The parent material is the
material deposited by melt
water at the end of the ice-age.
The ground water provides
moisture and the water table is
near the surface.
Soil Profile
Pit 2 - Cultivated Gley
The land is used for crops with grass present.
The land form is hollow and low-lying. The
texture of the soil is fine and has clay
particles. The structure of the soil profile is
wet and heavy on the top layer. The next layer
has humus which binds the fine soil together.
The C Horizon has got wet sticky clay. The
colour of this soil is grey with orange specs
on the A Horizon and on the B Horizon it is
an orange colour with bits of iron compounds.
The organic matter is a thick layer but it is
mixed in with the A Horizon and the biota
consists of worms and there are also small
stones in the soil. The soil is also poorly
drained.
Soil Profile
Pit 3 - Cultivated Podsol
The cultivated podsol is found in a gently
sloping field. The vegetation cover
consists of trees and crops. The texture of
this profile is coarse and sandy. The
structure is better than podols because of
humus, there are small rounded particles
and the structure is stable. The colour in
the A horizon is dark, then there is an iron
pan and the B horizon is dark aswell. The
soil is acidic and the humus has been
mixed through the top thirty centimetres.
There are many worms because the
weather is warmer here. This helps to
make it less acidic than it could be. Within
this soil profile there are small stones and
it’s well drained.
Soil Profiles
Pit 4 - Iron Podsol
The iron podsal is found in dry moorland areas. The type
of vegetation they like is dry heeth, and heather.
The landform is undulating hilltop.
The texture of the soil is sandy, fairly course for all
horizons.
The structure of the horizons are poor, weak structure.
There is no organic material mixed through the or Bhorizon to blend the soil together.
There are big rocks, which will be found 35cm from the
surface. The colour of the soil in the AO horizon is black.
In the A horizon its light orange and the B horizon is an
orange/ grey colour.
The organic matter is 2-3 cm its black humus and is below
the surface. Biota is the type of life found in the soil. In
the A horizon there are no worms as its too acidic.
There are a lot of stones in this soil podsal. The iron podsal
is very well drained soil.
The climate that the iron podsal most prefers is cool, humid
conditions with precipitation evaporation so that
transiocation or leaching of material is active
Parent Material
The parent material is significant in the early development
of the soil and it’s mineral content. It can vary from
bedrock to a range of deposits including alluvium and
sand.
The parent material in Podsols are weathered rock and are
acidic.
The parent material in Gley soils are impermeable clay
which results in waterlogging.
Time
Time is important in the
development of soils before
they fully mature. When they
are young soils retain the
features of the parent
material.
As time increases the young
soils gradually change their
characteristics from their
parent material to form their
own type of soil.
Conclusion
Peaty gley is useful for nothing due to it’s covering of
heather and marsh. This covering ensures that no other crops
would be able to grow there.
Cultivated gley is used for growing crops as it is low lying
so that it is warmer as it is further down. Crops can grow in
warmer conditions.
Cultivated podsol is used for farming because it has a better,
smaller stable structure. It also has remnants of rich iron
material.
Iron podsol is not used for anything as it is an acidic soil and
is covered in dry heather, long grass and conifers. It is also
very bumpy, hilly and stony.
Chapter 7
Clay structure and properties
Soil Colloids
• “Organic and inorganic matter with very small
particle size and a correspondingly large
surface area per unit mass” (“Soil bank”)
• Four categories:
–
–
–
–
Crystalline silicate clays (phyllosilicates)
Noncrystalline silicate clays
Iron and aluminum oxide clays
Organic matter (humus)
“Clay” is . . .
• A particle size class (<0.002 mm)
• A mineral type with specific properties and
characteristics (secondary mineral)
Relative Size Comparison of Soil
Particles
“Big”  smaller  really small
Sand  silt 
clay
Shape of silicon tetrahedron
and aluminum octahedron
O
OH
O
OH
O
O
Si
O
OH
Al
OH
O
O
OH
Tetrahedral sheet
Octahedral sheet
Tetrahedral sheet
Tetrahedral sheet
Octahedral sheet
Tetrahedral sheet
Ionic Radii of elements in silicate
clays – Tetrahedral & Octahedral sheets
Types of clay minerals
• Based on numbers and combinations of
structural units (tetrahedral and octahedral sheets)
• Number of cations in octahedral sheet
• Size and location of layer charge
(due to isomorphic substitution)
• Absence or presence of interlayer
cations
• Two general categories: 1:1, 2:1
Clay minerals
1:1 clays
2:1 clays
(one tetrahedral
sheet for each
octahedral sheet)
(two tetrahedral
sheets for each
Kaolinite
nacrite, dickite,
halloysite, etc.
Weird-o,
not truly
2:1
octahedral sheet)
Montmorillonite,
beidellite,
saponite, etc.
Illite,
Tri- or dimuscovite, vermiculite
biotite, etc.
Cookeite,
chamosite
ETC
1:1 Silicate Clays
• Layers composed of one tetrahedral
sheet bound to one octahedral sheet
• Kaolinite: one of the most widespread
clay minerals in soils; most abundant in
warm moist climates
• Stable at low pH, the most weathered
of the silicate clays
• Synthesized under equal concentrations
of Al3+ and Si4+
Kaolinite
•
•
•
•
A 1:1 clay
Little or no isomorphous substitution
“nutrient poor”
No shrink-swell (stable because of Hbonding between adjacent layers)
• A product of acid weathering (low pH,
common in soils of the SE USA
Structure of Kaolinite
NO ISOMORPHOUS SUBSTITUTION!!!
Sheets of silicon tetrahedra and aluminum octahedra
linked by shared oxygen atoms.
2:1 Silicate Clays
• Two silica tetrahedral sheets linked to
one aluminum octahedral sheet
• Three key groups:
– Smectites (e.g., montmorillonite)
– Vermiculites
– Micas (e.g., illite)
• And one weirdo (the chlorites)
Smectite (2:1, Montmorillonite)
• Layer charge originates from the substitution
of Mg2+ for Al3+ in the octahedral sheet
• Unstable (weathers to something else)
under low pH and high moisture
• Most swelling of all clays
• “Nutrient rich”
Structure of basic Smectite
(Montmorillonite)
Structure of montmorillonite (a smectite): it is built of two sheets of silicon
tetrahedra and one sheet of aluminum octahedra, linked by shared
oxygen atoms.
Structure of basic Smectite
(Montmorillonite)
Causes cations to
move into the
interlayer space,
where they can
be replaced by
other cations
Isomorphous
substitution in the
octahedral sheet
= Mg (this slide only)
(2:1, Fine-grained Mica: Illite)
• Al3+ substitution for Si4+ on the
tetrahedral sheet
• Strong surface charge
• “fairly nutrient poor”
• Non-swelling, only moderately plastic
• Stable under moderate to low pH, common
in midwestern US
Structure of Illite
Structure of Illite
K+
K+
1. Isomorphous
substitution is in
the tetrahedral
sheets
2. K+ comes into
the interlayer
space to satisfy the
charge and “locks
up” the structure
Chlorites (2:1:1)
•
•
•
•
Hydroxy sheet in the interlayer space
Restricted swelling
“Nutrient poor”
Common in sedimentary rocks and the
soils derived from them
• Isomorphic substitution in both
tetrahedral and octahedral sheets
Structure of Chlorite
1. Iron-rich
Mg-Al
hydroxy sheet
Mg-Al
hydroxy
sheet
= Al
= Fe
= Mg
2. “locked”
structure
3. Low
nutrient
supply
capacity
Comparison of common silicate clays
Property
Kaolinite
Smectite
Swelling
Low
High
Bonding
Net negative
Fertility(CEC)
charge
Charge
location
General class
Fine-grained
mica
Low to
none
Van der
Waal’s
(weak)
Potassium
ions (strong)
Low: 2-5
cmolc/kg
High: 80-120
cmolc/kg
Mod: 15-40
cmolc/kg
Edges only – NO
isomorphic substitution
Octahedral
sheets
Tetrahedral
sheets
1:1 (TO)
2:1 (TOT)
2:1 (TOT)
Hydrogen
(strong)
Where to find different clays
Laterite
New Caledonia Laterite
Reactions :
2Fe2+ + 3H2O = Fe2O3 + 6H+ + 2e3Fe2+ + 4H2O = Fe3O4 + 8H+ + 2e2Fe3+ + 3H2O = Fe2O3 + 6H+
Al3+ + 3H2O = Al(OH)3 + 3H+
Al(OH)4- + H+ = Al(OH)3 + H2O
Eh-pH
diagrams
Supergene Enrichment
1
0.1 ppm
Eh
Gibbsite forms
pH
Al
Fe
Eh-pH
diagrams
New Caledonia Laterite