This document discusses the key components and factors involved in soil formation. It defines soil and regolith, and explains the five main factors that influence soil development: geology, climate, topography, biology, and time. It then provides details on each factor, describing how they impact properties such as chemical composition, structure, porosity, and nutrient content. The document also outlines the four main components of soil: inorganic materials, organic matter, air, and water, explaining their characteristics and functions within soil.

1. Title Page Photo
“A nation that destroys it's soils destroys itself.”
—Franklin D. Roosevelt (Brainyquote.com)
Vocabulary Chapter 12
A horizon (p. 367) loam (p. 364)
B horizon (p. 367) O horizon (p. 367)
calcification (p. 371) parent material (p. 355)
cations (p. 359) peds (p. 364)
C horizon (p. 367) regolith (p. 354)
colloid (p. 366) R horizon (p. 367)
E horizon (p. 367) salinization (p. 371)
eluviation (p. 362) soil (p. 353)
gleization (p. 370) soil order (p. 373)
horizon (soil horizon) (p. 367) soil profile (p. 367)
humus (p. 357) Soil Taxonomy (p. 372)
illuviation (p. 362) soil-water balance (p. 362)
laterization (p. 368) wilting point (p. 362)
leaching (p. 362)
Soil and Regolith
• Soil—zone of plant growth.
– The upper portion of
lithosphere characterized by
its ability to produce and store
plant nutrients.
• Average depth is about 15
centimeters (6 inches).
– An infinitely varying mixture of
weathered mineral particles,
decaying organic matter, living
organisms, gases, and liquid
solutions.
– Stage in a never-ending
continuum of physical–
chemical–biotic activities.
• Regolith—a layer of broken
and partly decomposed rock
particles that covers bedrock;
its upper part is soil.
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2. Soil-Forming Factors
• Five factors are involved in forming soil:
1. Geology
2. Climate
3. Topography
4. Biology
5. Time
The Geologic Factor
• Parent material—the source of the
weathered fragments of rock from
which soil is made; solid bedrock or
loose sediments that have been
transported from elsewhere by the
action of water, wind, or ice.
– Influences chemical composition of soil
and plays role in soil development.
• This influence diminishes with time, as
other factors become increasingly
important.
The Climatic Factor
• In long run, climate is generally the most
influential factor.
– Temperature and moisture are most
significant.
• High temperatures and abundant moistures
accelerate chemical and biological processes in
soil.
2

3. The Topographic Factor
• Slope and drainage are main features in this
factor.
• Change through lowering of bottom (because of
rock weathering and plant root extension) and
top of soil layer (because of erosion).
– Slope and soil drainage
• Waterlogged soils in valley bottoms
– Slope and soil depth
- Fig. 12-5 Slope is a
determinant of soil
depth.
The Biological Factor
1. Organic matter
only small fraction
of soil volume, but
of utmost
importance.
a) Gives life to soil.
b) From living and
dead plants and
animals
3

4. The Biological Factor
2. Vegetation provides passageways for drainage and
aeration.
3. Pedoturbation—mixing of soil provided by animals,
which counteracts tendency of other soil-forming
processes to accentuate the vertical differences among
soil layers.
4. Microorganisms generate estimated 75% of soil’s
metabolic activity.
a) Create humus—decomposed organic matter of utmost
important to agriculture.
b) Loosens soil structure, lessens density, and promotes root
development.
• Provides reservoirs for plant nutrients and soil water.
The Biological Factor
5. Earthworms
– Of large variety of animal life in soil, earthworm is
most important to soil formation and development.
a) Tunnels facilitate drainage and aeration, deepening
of soil profile.
b) Movement creates a crumbly soil structure, which is
favorable for plant growth.
c) Movement brings in leaf litter, which fertilizes
subsoil.
d) Digestive actions and tunnels increase porosity and
help soil impact of raindrops, which helps deter
erosion.
e) Casts, which are the inorganic material worms
excrete, increase nutrients of soil through their
physical and chemical nature.
f) Movement also brings deeper material to surface,
where it can be weathered more rapidly.
g) Movement, digestive action, and decomposition of
own bodies help promote nitrification.
The Biological Factor
6. Microorganisms in the Soil
– An estimated three quarters of a soil’s
metabolic activity is generated by
microorganisms.
– Microbes decompose organic material into
humus (a dark adhesive of minute
particles).
– This makes nutrients usable by plants.
4

5. The Chronological Factor
• Most soil develops
with geologic
slowness: changes
imperceptible within
human lifespan.
– Nonrenewable
resource:
• Can be degraded
through erosion or
depletion of nutrients in
just a few years.
Soil Components
• Four neutral components to soil:
1. Inorganics
2. Organics
3. Air
4. Water
Inorganic Materials
• Bulk of soil is mineral matter.
• Half of average soil is small,
granular mineral matter called
sand and silt.
– Mineral composition depends on
parent material.
• Quartz (silica, SiO2) most
common.
• Clay provides an important
reservoir for plant nutrients and
soil water.
– Only clay particles take part in the
intricate chemical activities that
occur in soil.
– Negatively charged, so attracts
positively charged nutrients.
• Cation—an atom or group of
atoms with a positive electrical
charge.
5

6. Organic Matter
• Varies from alive to dead, partially
decomposed to completely decomposed.
• Litter—the collection of dead plant parts
that accumulate at the surface of the soil.
• Decomposition rates depend on climate.
Soil Air
• Pore spaces make up more than half the
volume of average soil.
– Allow water and air to penetrate.
• Soil air is saturated with moisture, rich in carbon
dioxide, and poor in oxygen.
– Plants, roots, and soil organisms remove oxygen from
and respite carbon dioxide into pore spaces.
Soil Water
1. Water performs number
of important functions:
a) Dissolves essential
nutrients for plant roots;
b) Helps complete
necessary chemical
reactions;
c) Assists microorganisms
producing humus;
d) Mixes soil particles.
6

7. – Four forms of soil moisture • Gravitational Water
(Free Water)
• Capillary Water
(Water of Cohesion)
• Hygroscopic Water
(Water of Adhesion)
• Combined
- Fig. 12-12
Soil Water
2. Leaching—the process in which gravitational
water dissolves soluble materials and carries
them downward in solution to be redeposited at
lower levels.
3. Eluviation—the process by which gravitational
water picks up fine particles of soil from the
upper layers and carries them downward.
4. Illuviation—the process by which fine particles
of soil from the upper layers are deposited at a
lower level.
Soil Water
5. Soil–Water Budget—the relationship
between gain, loss, and storage of soil
water (percolation of rainfall or
snowmelt vs. evapotranspiration).
a) Field capacity—the maximum amount
of water that can be retained in the soil
after the gravitational water has drained
away.
• Most of the pore spaces are filled with
water.
b) Wilting point—the point at which plants
are no longer able to extract moisture
from the soil because the capillary water
is all used up or evaporated.
c) Soil–water budget—an accounting that
demonstrates the variation of the soil–
water balance over a period of time.
d) Four forms of soil moisture: gravitational
water, capillary water, hygroscopic
water, combined water.
• Gravitational water is mostly superfluous
to plant development, while capillary
water is the most important.
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8. Soil Properties
• Color
– Most conspicuous
property, and can
provide clues to nature
and capabilities.
– 175 gradations of
color.
Soil Properties
• Texture
– No soil is made up of particles of
uniform size.
– Texture is determined by the
relative amounts of various
separates present.
• Separates—the size groups
within the standard classification
of soil particle sizes.
• Three principal types of soil
separates:
1. Sand
2. Silt
3. Clay
• Loam—a soil texture in which
none of the three principal soil
separates— sand, silt, and
clay—dominates the other two.
Soil Properties
• Structure
– Ped—a larger mass or
clump in which individual
soil particles tend to
aggregate; determines the
structure of the soil.
• Four basic ped shapes:
– spheroidal, plate-like,
block-like, prism-like.
– These four shapes give rise
to seven generally
recognized soil types (see
Fig. 12–16, page 349).
– Structure is key in
determining soil’s porosity
and permeability.
8

9. Soil Chemistry
• Intricate series of chemical reactions
determine the presence and availability of
nutrients.
Colloids
• Colloids—organic and inorganic microscopic
particles of soil that represent the chemically
active portion of particles in the soil.
– Smaller than about 0.1 micrometer.
– Inorganic colloids consist of clay in thin, crystalline,
platelike forms.
– Organic colloids consist of decomposed organic
matter (humus).
– Major determinants of water-holding capacity of soil.
– Structure types (continued)
- Fig. 12-18
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10. Cation Exchange
• Colloidal complex—the combination of colloid
and attached cations.
– Created by colloid’s negative charges attracting
swarms of nutrient cations (positively charged).
– Too weak of bond allows nutrients to leach away; too
strong means plants won’t be able to absorb.
– Cation exchange capacity (CEC) —capability of soil
to attract and exchange cations.
– Generally, the higher the CEC, the more fertile the
soil.
– Most fertile soils tend to be those with a notable clay
and humus content; both have high-CEC activity.
Acidity/Alkalinity
• Acid—chemical compound that produces
hydrogen ions or hydronium ions when
dissolved in water.
• Base—chemical compound that produces
hydroxide ions when dissolved in water.
• Acidity—measure of dissolved acids in a
solution.
– Highly acidic solution is likely to dissolve and
leach away nutrients too rapidly for plants to
absorb them.
• Alkalinity—measure of dissolved bases in a
solution.
– Overly alkaline soil solution is inefficient in
dissolving minerals and releasing nutrients.
• Scale for measuring acidity and alkalinity
ranges from 0 to 14 pH.
– Based on relative concentration of hydrogen
ions.
– pH value of 7 is neutral, and that value is most
suitable for great majority of plants and
microorganisms.
Soil Profiles
• Four processes deepen and age soils:
– Addition, loss, translocation, and
transformation.
– Five soil-forming factors influence the rate of
these processes.
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11. Soil Profiles
• Horizon—a more or less distinctly
recognizable layer of soil, distinguished from
another by differing characteristics and forming
a vertical zonation of the soil.
– Six different horizons: O, A, E, B, C, R.
1. O (organic litter; not typical for soils to have)
2. A (topsoil; mineral and organic)
3. E (eluvial layer; concentration of sand and silt particles)
4. B (subsoil; mineral layer that contains materials removed
from E level)
5. C (unconsolidated regolith; no organic matter)
6. R (bedrock)
• Soil Horizons
– O horizon
• Organic matter
– A horizon
• Top soil/dark color
– E horizon
• Eluviation layer
– B horizon
• Illuviation layer
– C horizon
• Beyond reach of roots
– R horizon - Fig. 12-22
• bedrock
Soil Profiles
• Soil profile—a vertical cross section from
Earth’s surface down through the soil layers into
the parent material beneath.
• Solum—the true soil that includes only the top
four horizons.
• Water plays critical role in development of
profile.
• Time also important.
– Formation of B horizon normally indicates mature soil.
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12. -Fig. 12-25: Map of Major Soil Orders
 Global Distribution of Major Soils
– Figure 12-28. The general relationship among the soil
orders in terms of weathering, soil development, and
broad environmental conditions.
 Global Distribution of Major Soils
• Entisols
– “Ent”, from recent
formation
– Very little profile
development
• Thin and sandy
• Low fertility
- Fig. 12-27
12

13. • Inceptisols
– Latin Inceptum, “beginning”,
young
– Few Diagnostic Features
• Faint horizons
– Tundra, mountains, old valley
bottoms
- Fig. 12-28
• Andisols
– “Andi”, andesite (a lava
rock) named after the
Andes Mts.
– Volcanic ash soils
– Mild weathering
– Inherently fertile
- Fig. 12-29
• Gelisols
– Latin gelatio, “freezing”
– Permafrost layer
– Young soils
– Arctic and subarctic
regions
• Cryoturbation
- Fig. 12-30
13

14. • Histosols
– Greek histos, “living
tissue”
– Organic soils
– Waterlogged conditions
• Glaciated areas
• Poorly drained coastal areas
- Fig. 12-31
• Aridisols
– Latin aridus, “dry”; dry
soils
– Thin, low organic content
– High in soluble minerals
– Unproductive due to lack
of moisture
- Fig. 12-32
• Vertisols
– Latin verto, “turn”
– Swelling and
cracking clays
– Alternating wet and
dry climate
• Churning effect
inhibits soil-horizon
development
- Fig. 12-33
14

15. • Mollisols
– Latin mollis, “soft”; soft
soils
– Best agricultural soil
• Rich clay-humus content
– Central Eurasia, Pampas
of Argentina, North
American Great Plains
- Fig. 12-34
• Alfisols
– “al” for aluminum, “f” for iron
– Moderate leaching
• Subsurface clay accumulation
with high nutrient bases
– Second to Mollisols in fertility
- Fig. 12-35
• Ultisols
– Latin ultimus, “last”; last of
their nutrient bases leached
out
• Low fertility due to leaching
• Reddish color throughout
– Possible fate of Alfisols
- Fig. 12-36
15

16. • Spodisols
– Greek spodos, “wood ash”
– Light color due to heavy
leaching
• Notoriously infertile
– Acid, sandy forest soils
• Forms under coniferous forest
- Fig. 12-37
• Oxisols
– “Ox”, large amount of oxygen
containing compounds
– Highly weathered and leached
• laterization (Alt. : Latosols)
• Infertile
– Humid tropics
- Fig. 12-38
 Distribution of Soils in the United
States
- Fig. 12-39
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