BIOL 4120: Principles of Ecology
Lecture 5: Terrestrial
Environment
Dafeng Hui
Office: Harned Hall 320
Phone: 963-5777
Email: dhui@tnstate.edu
Topics for this class:
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
Soil is the foundation of all life
Formation of soil
Physical properties of soil
Soil horizons
Soil water holding capacity
Soil ion exchange capacity
Soil classification
Light distribution in plant canopy
5.1 Soil is the foundation upon
which all terrestrial life depends

Before life invaded the land from the sea,
there was probably little that looked like
soil today
• Dust like Mars; Little organic matter
• A few microorganisms.


Soil is medium for plant growth; the basis
of all terrestrial life. Without soil, there
would be no plants, no soil microorganism
and no land animals
Plants obtain many of their water and
nutrients from soil and it provides an place
to attach to.
Definition of soil

Soil is hard to define because it is so
complex
• Soil is a natural product formed and synthesized
by the weathering of rocks and the action of
living organism.
• Soil is a collection of natural bodies of earth,
composed of mineral and organic matter and
capable of supporting plant growth.
• The stratum below vegetation and above hard
rock

Soil is a living system made up of a
three dimensional matrix (length,
width, and depth) and of minerals and
organic matter, and organisms:
• plants, both roots and stems
• Bacteria
• Fungi
• Algae
• Small animals
• Larger animals
5.2 Formation of soil
Starting Point: weathering of rocks and their
minerals.
 Mechanical and Chemical Weathering

Mechanical: interaction of several forces
•
•
•
•
•

Water
Wind
Temperature
Creates loose material
Sorted and moved
Chemical
• Acids produced by lichens and mosses
• Addition of organic matter (dead plants and animal
tissues)
• Oxidization
• etc
Five interrelated factors

Five factors are involved in the formation of soil
• Parent Material

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Igneous rock
Sedimentary rock
Metamorphic rock
• Climate

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
Temperature
Wind
Latitude
Rainfall
Elevation
• Biotic Factors

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Living organisms (plants, animals, bacteria, fungi).
Degradation by living organisms
• Topography

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Water runoff
Draining
Erosion
• Time

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Weathering, accumulation, decomposition and mineralization take
time
Initial differentiation can be within 30 years
Formation of true soil, 2000 to 20000 years
5.3 Soils show a great deal of variation

Color
• No direct effect on how soil
function
• Allows classification
 Red
• Possibly oxides
 Black
• Possible high organic
content

Texture
• Variation in size and shape of
soil particles
 Gravel
• >2mm
 Sand
• 0.05mm to 2mm
 Silt
• 0.002mm to 0.05mm
 Clay
• <0.002mm
Soil texture is percentage of sand, silt
and clay. (Texture chart)

Structure
• Space for roots etc
• Pore space

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Amount of water held
Rate of water movement
Aeration
Compaction
Aggregation
Depth
• Depends on

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Slope
Weathering
Parent material
Vegetation
• Grasslands are deep
• Forests are shallow
5.4 Soil has horizontal layers
Soil profile
Layers or
horizons
5.5 Moisture holding capacity is an essential
feature of soils

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Soil can become saturated if
all pores filled
All water is hold by soil
particulars, at field capacity
(FC)
Capillary water is usually
present
• Extracted by plants

Wilting point (WP)
• Plant no long extract water
Available water capacity (AWC)

All affected by soil texture
• Sand

Lower capacity
• Clays

Higher capacity
Water content at different soils
5.6 Ion exchange capacity is
important to soil fertility
Soil soluble nutrients are charged
particles, ions.
Cations: positively charged (Ca2+, Mg2+,
NH4+)
Anions: negatively charged (NO3–, PO34–)
Ions are attached to soil particles, so do
not leach out of the soil.
Ion exchange capacity: total number of
charged sites on soil particles in a
standard volume of soil.
Soils have an excess of negative
charged sites


Cationic exchange
dominant (colloids)
Cation exchange
capacity (CEC): total # of
negatively charged sites, located on
the leading edges of clay particles
and SOM.

Concentration and
affinity
Al3+ > H+ > Ca2+ > Mg2+ > K+ = NH4+
> Na+
Soils have an excess of negative
charged sites
Change in pH affects
binding capacity for
ions
• Immovable


Hydrogen (H+)
Aluminum (Al+++)
• Removable in order

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Calcium (Ca++)
Magnesium (Mg++)
Potassium (K+)
Ammonium (NH4+)
Sodium (Na+)
Process of cation exchange in soils
In soils with high Mg++ or Ca++, K+ is lacking, why?
5.7 Basic Soil Formation Processes
Produce Different Soils

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Regional differences in geology,
climate, and vegetation give rise to
characteristically different soils
The broadest level of soil
classification is soil order
There are twelve orders of soil
•
•
•
•
•
•
Entisol
Mollisol
Alfisol
Andisol
Aridisol
Inceptisol
•
•
•
•
•
•
Histosol
Oxisol
Vertisol
Spodosol
Ultisol
Gelisol
5.7 Soils vary with climate and vegetation
Ultisols

Ultisol
• Warm climate soil
• Redish or yellowish
• Low nutrient content
Laterization:
when PPT
greatly exceeds ET in warm
climates, water rapidly
percolated through soil and
into groundwater. Soluble soil
nutrients are constantly
leached out of soils, leaving
behind the less soluble ions
(Al+++ and Fe++) which
give soil color (whitish for Al
and red for Fe) and H+ make
soil acidic and nutrient poor.
Salinization (Aridisol)
Salinization:
in very dry
climates and when loss of soil
moisture due to ET exceeds
PPT, water leaves the soil
through the surface. The
minerals (NaCl) dissolved
move upward from the
groundwater and result in a
salt crust on the surface of
the soil.
Irrigation of dryland can result
salinization. This becomes a
problem in US southwest,
Australia, Northern Africa,
China, and major areas of
dryland irrigation.
5.8 Light distribution within plant
canopy

Influencing factors
• Vegetation types
• Leaf area index (LAI)
• Leaf angles

Plant cover dramatically changes the light
environment underneath it

types of plants making up the cover can have an
effect (small portion reach ground)
• Deciduous forest – 1% to 5%
• Coniferous forest – 10% to 15%
• Tropical rain forest – 0.25% to 2%
Total
LAI=315/78.5=4

Effect of leaf angle on
leaf area index and
light penetration
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Season also
affects the light
penetration
The ground
under a
deciduous forest
will undergo a
seasonal cycle
affected by leaf
loss
The ground
under a
coniferous
forest will
undergo a
seasonal cycle
unaffected by
leaf loss
The ground
under a tropical
rainforest will
neither have a
seasonal cycle
or an effect
from leaf drop
Quantifying Ecology: Beer’s Law
and the Attenuation of Light


The greater the surface area of
leaves, the less light will penetrate
the canopy and reach the ground
The attenuation (vertical reduction)
of light through a stand of plants is
estimated using Beer’s law
Beer’s Law
The End
Basic Soil Formation Processes
Produce Different Soils

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Laterization is a process common to soils
found in humid environments in the
tropical and subtropical regions  heavy
leaching of nutrients
Calcification occurs when evaporation
and water uptake by plants exceed
precipitation  deposition and buildup of
alkaline salts (CaCO3) in the subsoil
Salinization occurs in very dry climates
or coastal regions as a result of salt spray
 salt deposits near the soil surface


Podzolization occurs in cool, moist
climates where coniferous vegetation
(pine forests) dominates  acidic soil
enhances leaching of cations, iron, and
aluminum from the topsoil
Gleization occurs in areas with high
rainfall or in areas of poor drainage 
organic matter is slowly decomposed and
accumulates in the upper layers of soil
Padilla and Pngnaire (2007, Functional Ecology. Mediterranean Woody
seedling)