The lithosphere and the soil as power equipment and hazard
Basics of pedology
5.
1. Definition of soil
Soil is a natural body consisting of layers (soil horizons) that are primarily
composed of minerals which differ from their parent materials in their
texture, structure, consistency, color, chemical, biological and other
characteristics. It is the unconsolidated or loose covering of fine rock
particles that covers the surface of the earth.
Among numerous properties and functions of soils and the Pedosphere as the
whole, the following ones, having especially important ecological significance, are
isolated:
- functions of soils as unique habitat for a great diversity of life forms;
- function of soil cover as a link between geological and biological substance
circulation in terrestrial ecosystems and the biosphere; and
- function of the soil fertility in agriculture and biological productivity in natural
landscapes.
2. The Pedosphere
The Pedosphere is the soil cover of the Earth, which consist of specific natural
bodies (elements), i.e. of soils, which are diverse by their compositions and
properties.
As soil differ in their
properties both in the vertical
succession of overlying horizons
and spatial variations.
The pedon and the solum
(SZALAI – JAKAB 2011)
The first aspect requires the observation and study of a soil profile pit,
approximately 1m x 1m x 1m in size and being considered representative
for the soil around. This small basic entity, from which one can observe
variations in properties and extract samples for analytical investigations,
is called a pedon. It is the smallest volume that can be called a soil, but
large enough to exhibit a full set of horizons. While the combination of
various pedons with minor differences within a larger landform is called a
polypedon.
3. Genetic soil levels
The soils are differentiated from one
another by the characteristics and properties
of their profiles. A soil profile is a physical and
chemical description of the layers (called
horizons) that make up the soil, from the
surface to the depth where pedogenic (soil
forming) processes are no longer evident. If a
person digs a hole in the ground and looks at
the wall of that hole, he is looking at the soil's
profile. The horizons of the profile have
formed and differentiated from the original
parent material in place - they are not the
result of geologic processes, although some
features of a profile may be caused by geologic
events.
The horizons of a general soil profile
’A-szint’-nek nevezzük általában a talajképződés folyamán kialakult jellegzetes
réteget. Ez a talaj szerves anyagban leggazdagabb legfelső szintje. A legfelső,
döntően kevéssé lebomlott szerves anyagokból álló szintet A0-lal jelöljük.
’B-szint’-nek nevezzük az A-szint alatt fekvő felhalmozódási szintet. Itt
halmozódnak fel az A-szintből kilúgozott anyagok azokban a talajokban,
amelyekben a felszínre jutó csapadékvíz az év nagy részében a mélybe
szivárog.
A
B-szint
differenciálódása
esetén
az
agyagban,
vagy
szeszkvioxidokban gazdagabb felső szintjét B1-gyel, az alatta húzódó szintet
B2-vel jelöljük.
A ’C-szint’ elnevezést a talajképző kőzet jelölésére használjuk.
C-szintnek a talajképző kőzet azon részét tekinthetjük, amelyben a talajképző
folyamatok már hatnak, de anyagában még a kiinduló kőzettest
jellegzetességeit hordozza.
A ’D-szint’ az ágyazati kőzet jele, vagyis annak kifejezésére használjuk, hogy a
talajképző kőzet alatt valamilyen más kőzetféleség található, amely azonban
nem alapja a talajképződésnek (Stefanovits et al. 1999, Szalai – Jakab 2011).
4 Physical properties of soils
The physical properties of soils, in order of decreasing importance, are
texture, structure, density, porosity, consistency, temperature, colour and
resistivity. Most of these determine the aeration of the soil and the ability
of water to infiltrate and to be held in the soil. Soil texture is determined
by the relative proportion of the three kinds of soil particles, called soil
"separates": sand, silt, and clay.
The most characteristic soil properties are the following:
4.1. Texture
4.2. Structure
4.3. Porosity
4.4. Soil water
4.5. Soil atmosphere
4.6. Temperature
4.7. Colour
4.1. Texture
The mineral components of soil, sand, silt and clay, determine
a soil's texture (Table 5.1.). In the illustrated USDA textural
classification triangle, the only soil that does not exhibit one
of these predominately is called "loam" (Fig. 5.3.). While even
pure sand, silt or clay may be considered a soil, from the
perspective of food production a loam soil with a small
amount of organic material is considered ideal. The mineral
constituents of a loam soil might be 40% sand, 40% silt and
the balance 20% clay by weight. Soil texture affects soil
behaviour, in particular its retention capacity for nutrients
and water.
Size (mm)
<0,002
0,002 –
0,002 – 0,05
0,02
0,05 – 0,1
0,02 – 0,2
0,1 – 0,5
0,2 – 2,0
0,5 – 1,0
1,0 – 2,0
>2,0
Atterberg
classification
clay
USDA
classification
clay
mud
mud
fine grained sand
fine grained sand
medium grained
sand
coarse grained sand
coarse grained sand
very coarse grained
sand
rock fragment,
rock fragment,
gravel
gravel
Classification of the grains according to Atterberg and the USDA
Sand and silt are the products of physical and chemical weathering; clay,
on the other hand, is a product of chemical weathering but often forms as
a secondary mineral precipitated from dissolved minerals. It is the
specific surface area of soil particles and the unbalanced ionic charges
within them that determine their role in the cation exchange capacity of
soil, and hence its fertility. Sand is least active, followed by silt; clay is
the most active. Sand's greatest benefit to soil is that it resists
compaction and increases porosity. Silt is mineralogically like sand but
with its higher specific surface area it is more chemically active than
sand. But it is the clay content, with its very high specific surface area
and generally large number of negative charges, that gives a soil its high
retention capacity for water and nutrients. Clay soils also resist wind and
water erosion better than silty and sandy soils, as the particles are
bonded to each other
Types of soil textures on the base
of the USDA classification
(FÜLEKY 2011)
Soil components larger than 2.0 mm are classed as rock and
gravel and are removed before determining the percentages of the
remaining components and the texture class of the soil, but are
included in the name. For example, a sandy loam soil with 20%
gravel would be called gravelly sandy loam.
4.2. Structure
The adhesion of the soil textural components by organic substances, iron oxides,
carbonates, clays, and silica, and the breakage of those aggregates due to
expansion-contraction, freezing-thawing, and wetting-drying cycles, shape soil
into distinct geometric forms. These peds evolve into units which may have
various shapes, sizes and degrees of development. A soil clod, however, is not a
ped but rather a mass of soil that results from mechanical disturbance. The soil
structure affects aeration, water movement, conduction of heat, plant root
growth and resistance to erosion. Water has the strongest effect on soil
structure due to its solution and precipitation of minerals and its effect on
plant growth.
Types of the soil aggregates: a) microaggregates ; b) macroaggregates ; c) pores in
macroaggregates (STEFANOVITS ET AL. 1999)
Soil structure often gives clues to its texture, organic matter
content, biological activity, past soil evolution, human use, and the
chemical and mineralogical conditions under which the soil formed.
While texture is defined by the mineral component of a soil and is an
innate property of the soil that does not change with agricultural
activities, soil structure can be improved or destroyed by the choice
and timing of farming practices.
At the largest scale, the forces that shape a soil's structure result
from swelling and shrinkage that initially tend to act horizontally,
causing vertically oriented prismatic peds. Clayey soil, due to its
differential drying rate with respect to the surface, will induce
horizontal cracks, reducing columns to blocky peds. Roots, rodents,
worms, and freezing-thawing cycles further break the peds into a
spherical shape.
Cracks because of drying in clayey soil
Cracks beacause of frozen in clayey soil
Fracturing effect of roots
Biological weathering
4.3. Porosity
Pore space is that part of the bulk volume that is not occupied
by either mineral or organic matter but is open space occupied by
either air or water. Ideally, the total pore space should be 50% of
the soil volume. The air space is needed to supply oxygen to
organisms decomposing organic matter, humus, and plant roots.
Pore space also allows the movement and storage of water and
dissolved nutrients.
There are four categories of pores:
1.
Micropores: < 2 microns
2.
Mesopores: 2-20 microns
3.
Macropores: 20-200 microns
4.
Megapores: 200 microns-0.2 mm
Name of pore type
Micropores
Mesopores
Macropores
Megapores
fine pores
Medium pores
Diametre (μm)
<0,2
0,2-10
medium-coarse pores
10-50
coarse pores
very coarse pores and
cracks
50-1000
>1000
Function at soil water
pores of adherent water
pores of capillar water
pores of capillar-gravitation
water
pores of gravitation water
Classification of soil pores in size
The fine and medium-textured soils (e.g. clay loams, silty clay loams, sandy
silt loams) are favorable from an agricultural viewpoint because of their high
available retention of water and exchangeable nutrients. In fine pores the
water is strongly adsorbed in pores but not available for plants, i.e. cohesion
and adhesion water occupy the micropore space and they are retained in soil
by forces that exceed gravity. In medium-sized pores the available water
content is high, whereas in macropores water is more weakly held and
percolation is high (gravitational water). In silty soils the distribution of
macropores, medium-sized, and fine pores is optimal relating to available
water content.
4.4. Soil water
Water affects soil formation, structure, stability and erosion but is
of primary concern with respect to plant growth. Water is essential to
plants for four reasons:
1.
It constitutes 85%-95% of the plant's protoplasm.
2.
It is essential for photosynthesis.
3.
It is the solvent in which nutrients are carried to, into and
throughout the plant.
4.
It provides the turgidity by which the plant keeps itself in
proper position.
In addition, water alters the soil profile by dissolving and
redepositing minerals, often at lower levels, and possibly leaving the soil
sterile in the case of extreme rainfall and drainage. In a loam soil,
solids constitute half the volume, air one-quarter of the volume, and
water one-quarter of the volume, of which only half will be available to
most plants.
The forces with which water is held in soils determine its availability to plants.
Forces of adhesion hold water strongly to mineral and humus surfaces and less
strongly to itself by cohesive forces. A plant's root may penetrate a very small
volume of water that is adhering to soil and be initially able to draw in water
that is only lightly held by the cohesive forces.
Model of the capillarity (FÜLEKI
2011)
4.5. Soil atmospher
The atmosphere of soil is radically different from the atmosphere above. The
consumption of oxygen, by microbes and plant roots and their release of carbon
dioxide, decrease oxygen and increase carbon dioxide concentration.
Atmospheric CO2 concentration is 0.03%, but in the soil pore space it may range
from 10 to 100 times that level. At extreme levels CO2 is toxic. In addition, the
soil voids are saturated with water vapour. Adequate porosity is necessary not
just to allow the penetration of water but also to allow gases to diffuse in and
out. Movement of gases is by diffusion from high concentrations to lower.
Oxygen diffuses in and is consumed and excess levels of carbon dioxide, diffuse
out with other gases as well as water. Soil texture and structure strongly affect
soil porosity and gas diffusion. Platy and compacted soils impede gas flow, and a
deficiency of oxygen may encourage anaerobic bacteria to reduce nitrate to the
gases N2, N2O, and NO, which are then lost to the atmosphere. Aerated soil is
also a net sink of methane CH4 but a net producer of greenhouse gases when
soils are depleted of oxygen and subject to elevated temperatures
4.6. Temperature
Soil temperature regulates seed germination, root growth and the availability of
nutrients. Soil temperatures range from permafrost at a few inches below the
surface to 38°C in Hawaii on a warm day. The colour of the ground cover and its
insulating ability has a strong influence on soil temperature. Snow cover will reflect
light and heavy mulching will slow the warming of the soil, but at the same time they
will reduce the fluctuations in the surface temperature.
Below 50 cm, soil temperature seldom changes and can be approximated by adding
1.8°C to the mean annual air temperature.
Most often, soil temperatures must be accepted and agricultural activities
adapted to them to:
1.
maximize germination and growth by timing of planting
2.
optimise use of anhydrous ammonia by applying to soil below 10°C
3.
prevent heaving and thawing due to frosts from damaging shallow-rooted
crops
4.
prevent damage to desirable soil structure by freezing of saturated soils
5.
improve uptake of phosphorus by plants
Otherwise soil temperatures can be raised by drying soils or the use of clear
plastic mulches. Organic mulches slow the warming of the soil
4.7. Colour
Soil colour is often the first impression one has when viewing soil.
Striking colours and contrasting patterns are especially noticeable. The
Red River (Mississippi watershed) carries sediment eroded from
extensive reddish soils like Port Silt Loam in Oklahoma. The Yellow
River in China carries yellow sediment from eroding loess soils. Mollisols
in the Great Plains of North America are darkened and enriched by
organic matter. Podsols in boreal forests have highly contrasting layers
due to acidity and leaching.
In general, colour is determined by organic
matter content, drainage conditions, and the
degree of oxidation. Soil colour, while easily
discerned, has little use in predicting soil
characteristics. It is of use in distinguishing
boundaries within a soil profile, determining the
origin of a soil's parent material, as an indication
of wetness and waterlogged conditions, and as a
qualitative means of measuring organic, salt and
carbonate contents of soils (Fig. 5.7.). Colour is
recorded in the Munsell color system as for
instance 10YR3/4.
Possible colour of soils on the Munsell scale
5. Chemical properties
5.1. Soil salinity
5.2. Colloids
5.3. Soil pH
5.4. Cation and anion exchange
5.5. Buffering of soils
5.6. Organic matter
The chemistry of soil determines the
availability of nutrients, the health of
microbial populations, and its physical
properties. In addition, soil chemistry also
determines its corrosivity, stability, and
ability to absorb pollutants and to filter
water.
5.1. Soil salinity
Soil salinity is the salt content in the soil; the process of increasing the salt
content is known as salination. Salt is a natural element of soils and water.
Salination can be caused by natural processes such as mineral weathering or
the gradual withdrawal of an ocean. It can also be caused by artificial
processes such as irrigation. Salt is a natural element of soils and water. The
ions responsible for salination are: Na+, K+, Ca2+, Mg2+ and Cl-. As the Na+
(sodium) predominates, soils can become sodic. Sodic soils present particular
challenges because they tend to have very poor structure which limits or
prevents water infiltration and drainage. Over long periods of time, as soil
minerals weather and release salts, these salts are flushed or leached out of
the soil by drainage water in areas with sufficient precipitation. In addition to
mineral weathering, salts are also deposited via dust and precipitation. In dry
regions salts may accumulate, leading to naturally saline soils
5.2. Colloids
Colloids are the most important elements of soils, which determine the chemical
properties. They have two types, mineral and organic colloids. Due to its high
specific surface area and its unbalanced negative charges, clay is the most active
mineral component of soil. It is a colloidal and most often a crystalline material.
In soils, clay is defined in a physical sense as any mineral particle less than two
microns (2x10−6 inches) in effective diameter. Chemically, clay is a range of
minerals with certain reactive properties. Clay is also a soil textural class. Many
soil minerals, such as gypsum, carbonates, or quartz, are small enough to be
classified physically as clay but chemically do not afford the same utility as do
clay minerals.
Clay was once thought to be very small
particles of quartz, feldspar, mica,
hornblende or augite, but it is now known
to be (with the exception of mica-based
clays) a precipitate with a mineralogical
composition different from its parent
materials and is classed as a secondary
mineral. The type of clay that is formed is
a function of the parent material and the
composition of the minerals in solution.
Mica-based
clays
result
from
a
modification of the primary mica mineral
in such a way that it behaves and is
classed as a clay. Most clays are
crystalline, but some are amorphous. The
clays of soil are a mixture of the various
types of clay, but one type predominates.
Colloid model of Stern (SZALAI – JAKAB 2011)
5.3. Soil pH
At 25°C an aqueous solution that has a pH of 3.5 has 10-3.5 moles hydrogen ions
per litre of solution (and also 10-10.5 mole/litre OH-). A pH of 7, defined as
neutral, has 10−7 moles hydrogen ions per litre of solution and also 10−7 moles of
OH- per litre; since the two concentrations are equal, they are said to neutralise
each other. A pH of 9.5 is 10-9.5 moles hydrogen ions per litre of solution (and
also 10-2.5 mole per litre OH-). A pH of 3.5 has one million times more hydrogen
ions per litre than a solution with pH of 9.5 (9.5 - 3.5 = 6 or 106) and is more
acidic.
Very acidic
Acidic
pHH2O
<4,5
4,5–5,5
FILEP
characteristics
Exchangeable H+ is measurable
Dissolving of Al 3+
pHH2O
<4,0
4,0–5,5
Poorly acidic
5,5–6,8
Exchange of Ca2+, Mg2+, Na+, K+
5,5–7,0
Neutral
6,8–7,2
pH doesn’t increase while soil contains calcite
and dolomite
7,2–8,5
Maximal basic saturation. V~100%
8,5–9,0
9,0<
Rate of exchangeable Na+ is growing
Exchangeable Na+ is dominant
Categories of soil acidity (after FILEP GY.and BOHN, H. ET AL.)
Poorly
alcalic
Alcalic
Very alcalic
BOHN ET AL.
characteristics
Exchangeable H+ is measurable
Exchangeable Al3+ is measurable
Exchangeable H+ doesn’t
measurable, Al-hydroxyl polymers
are dominant
5.4. Cation and anion exchange
Cation exchange, between colloids and soil water, buffers (moderates) soil
pH, alters soil structure, and purifies percolating water by adsorbing cations
of all types, both useful and harmful. The negative charges on a colloid particle
make it able to hold cations to its surface. The charges result from four
sources.
1. Isomorphous substitution occurs in clay when lower-valence cations
substitute for higher-valence cations in the crystal structure.
Substitutions in the outermost layers are more effective than for the
innermost layers, as the charge strength drops off as the square of the
distance. The net result is a negative charge.
2. Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral
and octahedral structures are incomplete at the edges of clay.
3. Hydrogens of the clay hydroxyls may be ionised into solution, leaving an
oxygen with a negative charge.
4. Hydrogens of humus hydroxyl groups may be ionised into solution, leaving an
oxygen with a negative charge.
5.5. Buffering of soils
The resistance of soil to changes in pH and available cations from the
addition of acid or basic material is a measure of the buffering capacity
of a soil and increases as the CEC increases. Hence, pure sand has almost
no buffering ability, while soils high in colloids have high buffering
capacity. Buffering occurs by cation exchange and neutralisation.
The addition of highly basic aqueous ammonia to a soil will cause the
ammonium to displace hydrogen ions from the colloids, and the end
product is water and colloidally fixed ammonium, but no permanent
change overall in soil pH.
5.6. Organic matter
Most living things in soils, including plants, insects, bacteria and fungi, are
dependent on organic matter for nutrients and energy. Soils have varying organic
compounds in varying degrees of decomposition. Organic matter holds soils open,
allowing the infiltration of air and water, and may hold as much as twice its
weight in water. Many soils, including desert and rocky-gravel soils, have little or
no organic matter. Soils that are all organic matter, such as peat (histosols), are
infertile. In its earliest stage of decomposition, the original organic material is
often called raw organic matter. The final stage of decomposition is called
humus.
Humus refers to organic matter that has been decomposed by bacteria, fungi,
and protozoa to the final point where it is resistant to further breakdown.
Humus usually constitutes only five percent of the soil or less by volume, but it is
an essential source of nutrients and adds important textural qualities crucial to
soil health and plant growth. Humus also hold bits of undecomposed organic
matter which feed arthropods and worms which further improve the soil. Humus
has a high cation exchange capacity that on a dry weight basis is many times
greater than that of clay colloids. It also acts as a buffer, like clay, against
changes in pH and soil moisture.
Humic acids and fulvic acids, which begin as raw organic matter, are important
constituents of humus. After the death of plants and animals, microbes begin to
feed on the residues, resulting finally in the formation of humus. With
decomposition, there is a reduction of water-soluble constituents including
cellulose and hemicellulose and nutrients such as nitrogen, phosphorus, and sulfur.
As the residues break down, only complex molecules made of aromatic carbon
rings, oxygen and hydrogen remain in the form of humin, lignin and lignin
complexes as humus. While the structure of humus has few nutrients, it is able to
attract and hold cation and anion nutrients by weak bonds that can be released in
response to changes in soil pH.
6 Edaphon
Organisms affect the type of organic matter that is added to the soil, the
rate at which the organic matter is decomposed, the part of the soil to which
the organic matter is added and translocated, and the types of chemical
reactions that occur in the soil.
The soil and the organisms living on and in it comprise an ecosystem. The
active components of the soil ecosystem are the vegetation, fauna, including
microorganisms, and man
Prokaryotes
Bacteria
(5 μm)
Actinobacteria
(10 μm)
Cyanobacteria
(archeons)
Fungi
micro fungi
(50 μm)
mushrooms
(20 mm)
Virus
(0,1 μm)
Plants
micro
(10 μm)
algae
macro
micro
(100 μm)
Animals
meso
(2 mm)
seeds
Protista
rhyzomes
fonálférgek termite
bulb roots
macro
(20 mm)
Collembola insects
mite
bulbs
roots
(100 μm)
Characteristic organisms in the soil (STEFANOVITS ET AL. 1999)
Molluscs
earth worm
. Lichens on rock surface
High-quantity roots in soil
Thank you for your attention!