Chapter 12: Soils and Soil Development

Physical Physical GeographyGeographyNinth EditionNinth Edition

Robert E. Gabler

James. F. Petersen

L. Michael Trapasso

Dorothy Sack

Soils

• Soil: dynamic natural body capable of supporting a vegetative cover• It contains chemical solutions, gases, organic refuse, flora, and fauna

Soils

• Soil integrates:– Atmosphere– Hydrosphere– Lithologic– Biotic

• Soil is home to organisms, forming environments in which they live

12.1 Major Soil Components

• Four major components of Soil:– Inorganic materials– Soil water– Soil air– Organic matter

• Interaction and proportion of each are important factors

12.1 Major Soil Components

• Inorganic Materials– Insoluble materials– Rock fragments and

minerals that will not readily dissolve in water

– Chemical composition of soil result from:

• Weathering• Old deposits• Dissolved minerals

– Soil Fertilization

12.1 Major Soil Components

• Soil Water– Original source is precipitation– Open system– Capillary water– Hygroscopic water– Gravitational water– Eluviation– Illuviation– Hardpan– Stratification

12.1 Major Soil Components

Q: What are some examples of energy and matter that flow into and out of the soil system?

12.1 Major Soil Components

Q: How does deposition by capillary water differ from deposition (illuviation) by gravitational water?

12.1 Major Soil Components

• Soil Air– As much as 50% of soil may consist of spaces

between soil particles and clumps– Supplies microorganisms with oxygen and

carbon dioxide

• Organic Matter– Humus: decayed remains of plant and animal

material– Humus supplies nutrients and minerals to soil

12.2 Characteristics of Soil

• Readily Testable properties:– Color– Texture– Structure– Acidity– Alkalinity– Capacity to hold and

transmit water

12.2 Characteristics of Soil

• Color– Red or yellow (iron)– Black (decomposed)

• Texture– Soil texture: particle size– Clay (< 0.002 mm)– Silty (0.002 to 0.05 mm)– Sandy (0.05 to 2.0 mm)– Rocks (> 2.0 mm)

12.2 Characteristics of Soil

• Texture and proportion of particle size– Soil grade (% sand,

silt, and clay)– Loams– Soils with a higher

proportion of large particles tend to be well aerated and allow for infiltration

12.2 Characteristics of Soil

• Structure– Soil peds– Porosity– Permeability– Classified by form:

• Columns• Prisms• Angular blocks• Nutlike spheroids• Laminated plates

• Crumbs• Granules

12.2 Characteristics of Soil

• Acidity and Alkalinity– pH scale (0-14)– Lower pH (higher

acidity)– Higher pH (alkaline

conditions)– Soil acidity or alkalinity

helps determine available nutrients

12.2 Characteristics of Soil

• Acidity and Alkalinity– Optimum pH varies

by plant species– Leaching– Soils in the east tend

to be acidic– Soils in the west tend

to be alkaline

12.3 Development of Soil Horizons

• Parent material

• Soil profiles show:– Color– Structure– Composition– Other characteristics

with depth

12.3 Development of Soil Horizons

• Soil Horizons: – Distinct layers

distinguished by their physical and chemical properties

– Designated by set of letters that refer to:

• Composition• Dominant process• Position in the soil profile

12.3 Development of Soil Horizons

• Soil Horizons: – O horizon– A horizon– E horizon– B horizon– C horizon– R horizon

12.4 Factors Affecting Soil Formation

• Weathering

• Chemical reactions

• Physical Processes

• Soil development is a function of:– Climate (Cl)– Organic matter (O)– Relief (R)– Parent material (P)– Time (T)

12.4 Factors Affecting Soil Formation

• Parent Material– Residual parent material

(e.g. physical or chemical breakdown)

– Transported parent material (e.g. carried by water, wind, etc.)

– Sandstone– Chemicals & nutrients

reflect composition

12.4 Factors Affecting Soil Formation

• Organic Material– Plant die and

decompose– Leaves fall to the

ground– Most fertile soil are

typically grasslands– Microorganisms (e.g.

bacteria)– Earthworms, ants…

12.4 Factors Affecting Soil Formation

• Climate– Global vs. Local scale– Equatorial:

• Higher temps increase soil microorganisms, preclude thick accumulations of humus

– Middle Latitudes:• Cooler temps slow decay and produce rich humus

– Polar Latitudes• Cold temperatures and limited plant growth result in

thin humus

12.4 Factors Affecting Soil Formation

• Climate, Temperature, and organic materialQ: What range of mean annual temperature is

most favorable for the accumulation of humans?

12.4 Factors Affecting Soil Formation

• Climate– Moisture Conditions

• Ample precipitation supports plant growth which increases organic content

• Too much precipitation will cause leaching

– Evaporation rate

12.4 Factors Affecting Soil Formation

• Land Surface Configuration– Slope– Aspect

(direction of its faces)

• Time– Young &

mature soils– Alluvium

12.5 Soil-Forming Regimes

• Self-forming regimes: vary mainly due to climate and vegetation

• Climate differences produce 3 primary soil-forming regimes:– Laterization– Podzolization– Calcification

12.5 Soil-Forming Regimes

• Laterization– Humid and subtropical

climates– Result of high temp

and abundant precip.– Laterite: soil type

(brick-like)– No O horizon,

absence of organic acids

– Topsoil reddish

12.5 Soil-Forming Regimes

• Podzolization– High middle latitudes– Moist with short, cool

summers and severe winters

– Podzol– East Coast of U.S.

12.5 Soil-Forming Regimes

• Calcification– Evapotranspiration

exceeds precipitation– Often thick calcium

carbonate (alkali dusts)

– Deserts of American west

12.5 Soil-Forming Regimes

• Regimes of Local Importance– Salinization:

concentration of salts– Occurs most often:

• in dry areas• Intensive irrigation

– Gleization: poorly drained soils in cold, wet climates

12.6 Soil Classification

• Soil Taxonomy– Soil classification system that is based on their

characteristics– Natural Resources Conservation Service

(NRCS)– Mapped by their spatial distribution

• Soil Surveys– Books that outline and describe soils in a region– Useful for factors such as fertility, irrigation, &

drainage

12.6 Soil Classification

• NRCS Soil Classification System– Based on development and composition– Soil order: largest division– Subdivisions– Horizons below the surface:

• Subsurface horizon• Epipedons

12.6 Soil Classification

• Common Soil Horizons (NRCS Soil System)

12.6 Soil Classification

• NRCS Soil Orders: based on a variety of characteristics and processes

12.6 Soil Classification

• Map of dominant soil orders in U.S.

12.6 Soil Classification

• World Map of dominant soil orders (NRCS system)

12.6 Soil Classification

• World Map of dominant soil orders (NRCS system)

12.6 Soil Classification

– Entisols

– Inceptisols

– Histosols

– Andisols

– Gelisols

– Aridisols

12.6 Soil Classification

– Vertisols

– Mollisols

– Alfisols

– Spodosols

– Ultisols

– Oxisols

12.7 Soil as a Critical Natural Resource

• Soil Fertility

Q: What could have been done to prevent the kind of soil loss shown in this example?

Q: What other soil conservation practices are often used to preserve the soil resource?

Physical Geography

End of Chapter 12: Soils and Soil Development