Phyllosilicate compositions
2:1 - Illite -
Ky(Si8-yAly)(Al4)O20(OH)4
2T
y<2
O
K+ in the interlayer
2:1 - Smectite - X0.8(Si7.7Al0.3)(Al2.6Fe+30.9Mg0.5)O20(OH)4
2T
O
Individual particles are 1 layer thick; hydrated cations are
associated with both surfaces when water is present.
Phyllosilicate compositions
2:1:1 - Chlorite –
(Mg,Fe+2,Al)6(OH)12.(Si,Al)8(Mg,Fe+2,Al)6O20(OH)4
O
2T
O
1:1 - Kaolinite - Si4Al4O10(OH)8
Hydrogen bonds between the
octahedral surface of one sheet and the
tetrahedral surface of the next.
Frost Heave
The fundamentals of how:
chemical factors,
mineral surfaces,
soil texture
and
freezing temperatures
combine
to produce
frost heaving
Frost heave
An elevation of the soil surface
associated with the growth of pure
ice (ice lenses) within the soil.
Requirements for frost heave
Freezing temperatures
Water
Freezing point depression
A frost-susceptible soil
Frost Heave in a Silty Clay Soil
Freezing point depression of water
Factors:
Osmotic pressure:
dissolved salts and ions.
Physical pressure:
Capillary reasons:
Confinement in a small space:
the smaller the radius of curvature,
the greater the internal pressure, and
the lower the freezing temperature
Schematic diagram of an
ice lens, on/in soil, that
lacks a frozen fringe
Schematic diagram of an ice
lens with a frozen fringe
Schematic
diagram of a
portion of the
frozen fringe
from previous
figure
The relative
thicknesses of the
unfrozen water films,
their cation and anion
concentrations
and
the relative solution
concentrations
at points
A, B, C, and D
on the
ice/water interface,
as governed
by
the temperature
at each location
in the frozen fringe.
Frost susceptibility (FS) of a soil
Is Determined by:
Ability of pores to support a freezing point depression (FPD)
Ability to transmit water to the freezing front
(hydraulic conductivity (HC)
Ability to transmit water to the ice lens bases above the soil particles (UFC)
Sand
Clay
Silt
FPD
low
high
moderate
HC
high
low
moderate
insignificant
high
moderate
UFC
FS
low
low to moderate
high
Questions on frost heave?
Soils as ecosystems
Soils, as the ‘skin’ of the Earth, are located at the
geosphere/atmosphere boundary and are part of the
biosphere.
Their location makes them vulnerable, yet easy to ignore
Study of the soil ecology is greatly hampered by the
inability to study soils without greatly disturbing them.
NEVERTHELESS
There are more organisms per unit area under the soil
surface than above, and in many cases the total biomass
is greater below the soil surface than above.
Diversity of soil organisms
• Fauna: range from single-celled Protozoa, through
insects and worms to large vertibrates.
• Flora: range from bacteria and fungi to the roots of
herbaceous plants and trees.
• It’s a hugely diverse ‘jungle’ underground!
Roles of soil organisms
1) To build up soil organic matter: accomplished by
green plants (from algae to trees);
2) To destroy organic matter: accomplished by
bacteria, fungi, other micro-organisms and soil animals;
3) To redistribute organic matter: accomplished by soil
animals (mainly).
Soil fauna
Microfauna
Herbivores
Detritivores
Predators
Mesofauna
Protozoa springtails
mites
nematodes
Protozoa
Macrofauna
insects
vertebrates
slugs & snails
springtails woodlice earthworms
millipedes insects
nematodes
centipedes
insects
vertebrates
spiders
Soil flora
Bacteria:
- single-celled
- 1 to 4,000 million per gram of soil
- tremendous variety
- most active above pH 5.5
- It is believed that we have identified only a small proportion of the
types present – those that we can culture in the laboratory. Bulk soil
DNA studies indicate that there must be many, many more.
- some are pathogens
Two major types:
Autotrophic: obtain their energy by oxidizing mineral substances,
particularly iron and sulfur.
Heterotrophic: obtain their energy solely from organic sources. The
majority are free-living forms involved in the breakdown of organic
compounds. Some are symbiotic.
Rhizobium and other nitrogen fixers
Rhizobium bacteria are symbiotic and form nodules on the roots of
leguminous plants (clovers, alfalfa, beans, peas, soybeans, etc.) .
They are capable of taking atmospheric nitrogen and converting it to
plant-available forms (amides and amino acids). They use what they
need and pass the rest on to their host plant’ in return for the
carbohydrates (their energy source) supplied by the host plant. They
are said to ‘fix’ nitrogen.
There are several other N-fixing bacteria that form similar relationships
with other types of plants.
There are also free-living N-fixing bacteria.
These N-fixing bacteria are the major agents that make atmospheric N
available for use by living organisms. Once N is in organic forms,
these forms are repeatedly recycled.
Fungi
Fungi:
- multi-cellular
- 5,000 to 1 million per gram of soil
- greater total mass than the bacteria
- heterotrophic
- very important in the early stages of organic decomposition
- predominate at low pH, and active over wide pH range
- some act as mycorrhizae
- some are pathogens
Mycorrhizae:
These fungi either form a sheath around plant rootlets or invade
plant root cells. Both forms send hyphae out into the soil. These
hyphae facilitate the obtaining of nutrients by the plant. They are
symbiotic.
Actinomycetes
Actinomycetes:
Intermediate in complexity between bacteria and fungi.
They are able to break down some highly resistant
organic compounds.
The best known is Streptomycetes, the source of the
antibiotic ‘streptomycin’. The discovery of streptomycin
earned Sterling Waksman (whom we soil scientists claim
as a soil microbiologist), the Nobel Prize in Medicine.
Since this discovery, soil microorganisms have been a
major source of antibiotics
Some are pathogens – potato scab is an example.
Green plants
Algae:
- single celled
- photosynthetic – must live near the soil surface
- some fix nitrogen (in rice paddies)
Lichens:
- symbiotic, photosynthetic combinations of algae and fungi.
Higher plants:
- supply the greatest amount of the organic matter in soils
- protect the soil surface from erosion by wind and water
- cycle plant nutrients and minimize nutrient loss by leaching
- the nature of the vegetation influences the pedological
development of the soil.
Soils as Natural Bodies
How do soils get to be as they are?
In North America, this approach to
studying soils is called ‘Pedology’.
Soil forming factors,
horizon development
and
regional differences
Soil forming factors
The soil that develops at a site is a function of the
interaction of five soil forming factors:
Parent material
Climate
Organisms
Relief (Topography)
Time
The relative importance of these factors changes with
location.
Soil profiles and soil horizons
As soils develop under the influence of the soil forming
factors, differentiation of a series of layers occurs.
These differentiated layers are called ‘soil horizons’.
The soil horizons are designated by the letters
O, A, E, B, and C
modified by
other letter subscripts to provide
more specific information
Soil horizons – dominant features
O - a zone of organic matter accumulated on the soil surface
A - the mineral horizon enriched in organic matter
E - a mineral horizon characterized by intense leaching
B - a mineral horizon characterized by accumulations of
materials washed down from overlying horizons
C - the relatively unchanged parent material of the soil
Parent material
Originally, investigations of soil development and soil distributions were
most commonly conducted by chemists and geologists and were
conducted at local or state scales, hence, it is probably not surprising
that parent material was initially considered to be the most important
of the soil forming factors in determining the characteristics that a soil
would exhibit.
Influences
Mineralogical composition (primary and secondary)
Soil texture and the properties that texture influences
most notably
the finer the texture, the lower the permeability,
and
the finer the texture, the less advanced is the state of soil development
because of slow water movement, higher buffer capacity and more
work needing to be done
Climate
At the continental and global scales, it becomes obvious
that climate is the dominant factor determining soil
distribution.
Climate acts through the agencies of
temperature, moisture, their seasonalities and
climate’s influence on vegetation.
Climatic zones
Climatic zones can be established on the basis of the effects of
temperature [3 ‘provinces’] and precipitation [5 ‘provinces’]
Dry, cold
Wet, cold
Perpetual ice and snow (no vegetation)
Tundra (mosses, lichens, stunted trees)
Taiga (coniferous forest)
Arid
Semi-arid Subhumid
Humid
Wet
<25 cm
25-50 cm
50-100
cm
100-200
cm
>200 cm
Desert
shrubs
and
grasses
Short
grasses
Long
grasses
Forests
Rain
forests
Dry, hot
Wet, hot
Temperature
Main effects:
1) Effects on amount and type of plant growth
2) Direct effect on the soil temperature
- rates of chemical reaction (doubles with a 10ºC rise)
- amount of biological activity (optimum is about 35ºC)
3) Effect on evapotranspiration:
- 30-70% of annual rainfall in humid temperate region
- 70-100% in semi-arid region
Moisture
Main effects
1) Must be present for chemical weathering to occur.
2) The ratio between precipitation and potential
evapotranspiration iP/E) is important:
- the more by which pptn > evapotranspiration , the greater
the amount of leaching of soluble materials. The subsoil is
normally wet.
- if potential evapotranspiration > pptn, there is little leaching,
but there is redistribution of soluble materials. The subsoil is
normally dry.
Seasonality
Rainfall that occurs during season of major plant growth tends to cause
relatively little leaching of nutrients because plant nutrient uptake is
occurring and evapo-transpiration losses are at a maximum. These
combine to limit the downward movement of both water and
nutrients. Surface erosion is also limited by plant cover and by plant
uptake of water.
Rainfall that occurs during the non-growing season for plants ,in
regions where the soil does not freeze, has a greater proportion
penetrate to the water table, and nutrient loss is enhanced by both
the increased downward flow and the lesser plant uptake. Surface
erosion is also more likely to occur.
In regions where the soils freeze, very little water moves downward
during the freezing period, but surface erosion can be severe on
non-vegetated areas during the spring melt season.
P/E regional relationships
Humid regions:
soluble alkali (Na, K) compounds are lacking and Ca and Mg salts
are in lower concentration than in the parent material.
Semi-arid regions:
Ca and Mg salts are present and have commonly accumulated
somewhere within the top meter to concentrations greater than in
the parent material.
Arid regions:
Ca, Mg, K and Na salts may all be present in amounts greater than
in the parent material.
Organisms
The roles of organisms in soil development are to:
Produce organic matter and contribute it to the soil, commonly onto
the surface (tree leaves, needles and above ground parts of
herbaceous plants) and in the near surface (from root death, mainly
of herbaceous plants).
Break down organic matter to humus
A role of microorganisms and soil animals
Redistribute the organic matter within the soil to produce A horizons
A role of soil animals, particularly worms and insects
Effects of Relief
Drainage
Sloping areas are better drained than valley bottoms, so aeration and
plant communities differ with location on the slope
Large flat areas of fine textured soils tend to have poor drainage
Erosion
Erosion of slopes tends to prevent a mature soil from forming by
constantly truncating the upper surface
Erosion onto the base of the slope has the same effect by constantly
bringing new material onto the surface
Slope aspect
The sunny side is warmer and drier: enhances soil development in both
warm humid and cold regions and slows it down in dry regions
The windward side is drier, which slows development in dry regions
Time
The effect of time is a hard time to quantify, but:
It requires time to develop a ‘mature’ soil profile (one which has the
sequence of horizons that normally develops in the specific soilforming conditions that are present)
Longer periods of time may deepen the horizons and strengthen the
expression of their characteristics
Soils developed in sand may reach a ‘mature’ state in less than 100
years, but the depth of occurrence and strength of expression of
horizons may continue to change over the long term
In fact, the soil is an ever changing entity. Most of the material that now
comprises sedimentary rocks was once soil material, the mineralogy
of the sedimentary rock is influenced by production of secondary
minerals during its time as soil material, and ultimately the
sedimentary rock may be uplifted, form mountains, be weathered
and eroded and contribute to the next interval of soil formation in the
rock-soil-sediment-rock-etc cycle.
Soil development
Soil development refers to the effects of the various processes that
contribute to the differentiation of soil horizons and the development
of soil profiles
In the most general sense, the processes of soil formation can be
categorized as a series of:
Additions
Losses
Transformations
Translocations
General Model of Soil Profile Development
Additions
precipitation, with its dissolved materials
particles moved by wind and water
organic matter and materials cycled by plants
Translocations
Transformations
organic matter
organic matter to humus
clays and oxides
primary to secondary minerals
other elements
oxidation/reduction processes
Losses
water and dissolved material
erosion (from the surface)
(After Simonson)
Specific processes of soil development
1 – Additions
2 – Losses
3 – Translocations
4 –Transformations
Eluviation (3) – movement of material out of
Illuviation (3) – movement of material into
Leaching (2) - eluviation out of the solum (A + B horizons)
Enrichment (1) – addition of material to the soil
Erosion (2) – loss of particles from the surface
Cumulization (1) – addition of mineral particles to the surface
Salinization (3) – accumulation of soluble salts
Desalinization (3) – removal of soluble salts
Alkalinizarion (3) – accumulation of Na ions on CEC sites
Dealkalinization (3) – leaching of accumulated Na and Na salts
Calcification (3) – accumulation of calcium carbonate
Decalcification (3) – removal of calcium carbonate
Lessivage (3) – mechanical migration of small mineral particles from A and E to B
horizons
Pedoturbation (3) – biological /physical mixing that homogenizes the solum
After Buol et al.
Specific processes of soil development
1 – Additions
2 – Losses
3 – Translocations
4 –Transformations
Podzolization (3,4) – chemical migration of Fe, Al, &/or OM, leaving Silica behind
Desilication (4,2) – migration of Si, resulting in concentration of Fe /Al oxides/hydroxides
Decomposition (4) – breakdown of minerals and OM
Synthesis (4) – formation of secondary minerals and new organic molecules
Melanization (1,3) – darkening of soil by OM addition
Leucinization (4,2) – lightening of soil by OM transformation or removal
Littering (1) – accumulation of organic litter
Humification (4) – transformation of OM to humus
Paludization (1,4) – accumulation of deep organic deposits (muck and peat)
Ripening (4) – chemical, biological and physical of organic soil after drainage
Mineralization (4) – release of oxide solids by OM decomposition
Braunification, Rubification, Ferrugination (3,4) – release of iron by weathering to yield
brownish, reddish-brown and red colours
Gleization (3,4) – reduction of ferric iron to ferrous iron (gray) under anaerobic conditions
Loosening (4) – increase in pore space by any means
Hardening (4) – decrease in pore space by collapse, compaction of infilling
After Buol et al
Development Scenario
Humid cool temperate, coniferous forest, sand, well-drained, 10,000 yr
Additions:
Translocations
Very little mixing of OM into
mineral surface layer
Soluble OM compounds and Fe &
Al oxides translocated from O & E
to the B horizon
(eluviation) to the B horizon
where they accumulate
podzolization).
leaf litter on surface;
rain + dissolved;
recycled nutrients (few bases)
O
E
________
Bsh
________
C
The B horizon is red to black
Spodosol developed
(Podzol – Canada)
Losses:
water + dissolved
Serious loss of bases
Transformations
OM to humus
Primary to secondary
minerals
Mainly Fe & Al oxides
A bit of 2:1 clay
formed
Spodosol - Quebec
Development Scenario
Humid cool temperate, mixed forest, loam, well-drained, 10,000 yr
Additions:
leaf litter;
rain + dissolved; recycled
nutrients
Translocations
OM mixed into A
Some secondary clays
and oxides translocated
from A & E
(eluviation) to the B
horizon where they
accumulate (illuviation)
- lessivage ( cutans
developed.
Alfisol developed
(Luvisol – Canada)
O
Ah
________
___E ?__
Bt
________
C
Transformations
OM to humus
OM to humus
Primary to secondary
minerals
2:1 and iron oxides
Some primary to
secondary
Losses:
water + dissolved
Carbonates (pH increases with depth)
Alfisol (Michigan)
Development Scenario
Humid warm temperate, mixed forest, loam, well-drained, 100,000 yr
Additions:
leaf litter;
rain + dissolved;
recycled nutrients
Translocations
OM mixed into A
Some secondary clays and oxides
translocate from A & E (eluviation)
to the B horizon (illuviation)
Cutans developed in B(lessivage)
O (thin)
Ah
_______
___E___
Bt
_______
Transformations
OM to humus
OM to humus
Primary to secondary minerals
2:1 , 1:1 and iron oxides
Primary to secondary minerals
in both the B and upper C
horizons
C
Ultisol developed
(Not in Canada)
Losses:
water + dissolved
Carbonates (pH decreases with depth)
Ultisol – N. Carolina
Development Scenario
Humid tropical, tropical forest, loam, well-drained, 1,000,000 yr
Additions:
Translocations
OM mixed into A
Residual accumulation
of Fe & Al oxides as the
layer silicate clays are
broken down and the
soluble products
(including Si) are
leached
Oxisol developed
(Not in Canada)
Regular fall of leaf litter;
Abundant rain, with some
dissolved material;
Efficient plant recycling of nutrients
__A (thin__
Bo
may be
very
thick
_________
C
Transformations
OM to humus – quickly
Mineral weathering is extreme;
very resistant primary minerals
remain; 2:1 clays completely
weathered; 1:1 clays remain; Fe
and Al oxides and hydroxides
dominate (desilication)
Colour depends on whether the
parent rock was Fe-rich or Alrich
Losses:
Water + dissolved bases and silica
Extreme leaching
Oxisol - Brazil
High Fe – near
Sao Paolo
High Al – near
Manaus
Development Scenario
Semi-arid to sub-humid,(60 cm) grassland, loam, well-drained, 20,000 yr
Additions:
Translocations
rain + dissolved;
Particles brought by wind and water
erosion
Organic matter (shoots and roots)
OM mixed into A, A is fairly
deep; thins as rainfall decreases
Clays translocated from A to B
((illuviation) - lessivage (cutans).
Calcium carbonates accumulates
at about the annual depth of
percolation of rainfall (lower part
of B to upper part of C)
Transformations
Ah
________
OM to humus
Primary to secondary
minerals (2:1 clays)
Bt
prismatic
________
Some primary to
secondary minerals
C
Losses:
Mollisol developed
Water & dissolved - periodically
(Chernozem – Canada)
Mollisol – S. Dakota
Well-drained,
with a calcic horizon
With Na dominating CEC,
with a natric horizon
Development Scenario
Humid and sub-humid temperate, mixed forest, loam, poorly-drained, 10,000 yr
Additions:
OM
rain & run-on, with dissolved material and
eroded particles
Translocations
OM mixed into A
Some clay may be
transported from A to B
Transformations
Ah
_______
Btg or
Bg
_______
Cg
Losses:
Aquic suborders developed water + dissolved
(Gleysol – Canada)
carbonates
OM to humus, but oxidation
of OM retarded – high OM
content, often black
Little mineral weathering
Reduction process to
produce gray colours and
rust coloured mottles.
Mottles are in the zone
where the water table
fluctuates.
Aquic soil
Aquic Mollisol -
Aquic Alfisol Ontario