chapter twelve

Title Page Photo

“A nation that destroys it's soils destroys itself.”—Franklin D. Roosevelt (Brainyquote.com)

Vocabulary Chapter 12A horizon (p. 367)

B horizon (p. 367)

calcification (p. 371)

cations (p. 359)

C horizon (p. 367)

colloid (p. 366)

E horizon (p. 367)

eluviation (p. 362)

gleization (p. 370)

horizon (soil horizon) (p. 367)

humus (p. 357)

illuviation (p. 362)

laterization (p. 368)

leaching (p. 362)

loam (p. 364)

O horizon (p. 367)

parent material (p. 355)

peds (p. 364)

regolith (p. 354)

R horizon (p. 367)

salinization (p. 371)

soil (p. 353)

soil order (p. 373)

soil profile (p. 367)

Soil Taxonomy (p. 372)

soil-water balance (p. 362)

wilting point (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.

Soil-Forming Factors

• Five factors are involved in forming soil:1. Geology2. Climate3. Topography4. Biology5. 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.

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 Factor1. 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

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 Factor5. 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.

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.

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.

– Four forms of soil moisture

- Fig. 12-12

• Gravitational Water(Free Water)

• Capillary Water(Water of Cohesion)

• Hygroscopic Water(Water of Adhesion)

• Combined

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 Water5. 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.

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. Sand2. Silt3. 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.

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

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.

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• bedrock

- Fig. 12-22

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.

-Fig. 12-25: Map of Major Soil Orders

– 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

Global Distribution of Major Soils

- Fig. 12-27

• 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

• 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

• 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

• 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