Soil: Supporting All Life

Why Soils?

• Food and water resources

• Disposing of waste

• Support for human

structures

• Growing timber and fibers

• Interact with forestry, wildlife,

and water resources

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Why Soils?

Characteristics of Soil

5.2 Soil

� Soil is part of the regolith that supports

the growth of plants.

• Regolith is the layer of rock and mineral

fragments that covers most of Earth’s land

surface.

5.2 Definition of Soil

• Depends on field or intended use

• Consists of minerals, organics, water, air

• Supports plants or shows evidence of pedogenesis

• Consists of naturally formed bodies

• Limited by rock/ice and deep water horizontally

• Upper limit air/shallow water• Lower limit where biological

activity stops/rock

Pedogenesis is the

science and study of the

processes that lead to the

formation of soil (soil

evolution)

Characteristics of Soil

5.2 Soil

� Soil Composition

• Soil has four major components:

1. Mineral matter: or broken-down rock

2. Humus: which is the decayed remains of organisms

3. Water

4. Air.

Composition by Volume of

Good-Quality Soil

Characteristics of Soil

5.2 Soil

� Soil Texture

• Loam (a mixture of all three sizes) is best suited

for plant life.

• Texture refers to the proportions of different

particle sizes.

- Sand (large size)

- Silt

- Clay (small size)

Soil Texture

Characteristics of Soil

5.2 Soil

� Soil Structure

• Soil particles clump together to give a soil its

structure.

Characteristics of Soil

5.2 Soil

� Soil Texture

• Soil particles clump together to give a soil its

structure.

• Sand- will not hold a ball in hand• Loamy sand- will hold a ball in hand, won’t ribbon• Sandy loam- ribbons, gritty texture• Silt loam- ribbons well, smooth texture, silt is floury when dry• Fine sandy loam- ribbons, easily mistaken for silt loam, but

less soapy/slippery in hand• Gravel- >2 mm, “pebbles”

• Note that sand particles can be seen with the naked eye, silt cannot

Soil Formation – Five Factors

5.2 Soil Forming Processes - 5

� The most important factors in soil

formation are parent material, time,

climate, organisms, and slope.

1. Parent material

• Transported soil—parent material has been

carried from elsewhere and deposited

• Residual soil—parent material is the bedrock

5.2 Soil Forming Processes

• Till: Mixed unsorted (unstratified) particle sizes laid down below ice

• Outwash: stratified sands and gravels carried by glacial water, flat areas

• Loess: wind-blown silt-sized materials deposited over till and outwash

• Peat and Muck: organic, poorly drained soils

Receding glacier in Iceland, NASA

Parent Material and Soils

Soil Formation

5.2 Soil

• RI soils are young/weakly

developed

• Wisconsonian glacier

retreated in RI ~15,000

years ago

• Mature soils have

maximum horizonation and

are stable/ in equilibrium

with environment

2. Time

• The longer a soil has been forming, the thicker it becomes.

• Important in all geologic processes

Soil Formation

5.2 Soil

Climate

• Precipitation, temperature, humidity, seasonal variability

• Direct effects: temperature and precipitation weathering

• Indirect effects: vegetation

3. Climate

• Greatest effect on soil

formation

Soil Formation

5.2 Soil

• Vegetation most important, depends on climate, parent material, topography, drainage, etc.

• Microorganisms-decompose OM to provide nutrients, aggregation, water holding capacity

4. Organisms

• Furnish organic matter to

soil

• Organisms influence

the soil's physical and

chemical properties.

Soil Formation

5.2 Soil

5. Slope• Orientation, or direction the slope is facing,

influences soil formation.

- Soil temperature

- Moisture

The Soil Profile

5.2 Soil

� Soil varies in composition, texture, structure,

and color at different depths. Soil horizonsare zones or layers of soil. A soil profile is a vertical section through all the soil horizons.

• The A horizon is commonly know as topsoil.

• The B horizon is subsoil and contains clay

particles washed out from the A horizon.

• The C horizon is between B horizon and

unaltered parent material.

Soil Profile

Soil Profile

• Vertical cross-section

of soil layers

(horizons)

• Horizons differ in

color, texture,

structure, etc

• Common sequence in

RI:

– O, A (E), B, C

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Soil Horizons

• O: formed from organic matter (plants and animals), commonly seen in forested areas but often absent in developed/cultivated areas

Oa- highly decomposed

Oi- undecomposed/fibric

• A: “topsoil,” mineral with lots of organics (making color darker), biologically active (roots, fungi, bacteria, mammals, bugs, worms, etc), most affected by acid rain

Ap horizon- plow/disturbed layer, mixed O, A, E, B horizons. Thicker than A horizon (>2 inches)

Soil Horizons

• E: Eluvial horizon, zone of leaching, light in color, rare in RI due to cultivation

• B: “Subsoil,” yellowish or reddish brown in well drained soils, accumulation zone (illuviation)

• C: Under the solum (A and B), less biological activity, weathering, structure, but still more weathered than bedrock

• R: Bedrock- RI soils are not formed in weathered bedrock as in many other areas (we will get to glaciation later)

A Soil Profile Showing

Different Horizons

5.2 Soil Reaction

• pH: 0-14, 7 neutral

• In RI soils acidic due to:

– Granite parent material

– Leaching of bases

– Plants give off H+

• RI soils “very strongly

acid” pH 4.5-5.0

• pH often increased for

agriculture by liming

(CaCO3)

Soil Types

5.2 Soil

� Three common types of soil are pedalfer,

pedocal, and laterite.

1. Pedalfer

• Best developed under forest vegetation

• Accumulation of iron oxides and aluminum-rich

clays in the B horizon

Soil Types

5.2 Soil

2. Pedocal

• Accumulates calcium carbonate

• Associated with drier grasslands

3. Laterite

• Hot, wet, tropical climates

• Intense chemical weathering

Soil Erosion

5.2 Soil

• Human activities that remove natural vegetation,

such as farming, logging, and construction, have

greatly accelerated erosion.

� Water erodes soil.

� Rates of Erosion

� Sediment Deposition

• Reservoirs fill with sediment.

• Sediments are contaminated by pesticides

and fertilizers.

Soil Erosion

5.2 Soil

• Planting rows of trees called windbreaks

� Controlling Erosion

• Terracing hillsides

• Plowing along the contours of hills

• Rotating crops