Soils

How does soil form?

1. Animals, plants, microbes, and fungi interact with sediment, absorbing nutrients and leaving behind waste and remains. 2. Rainwater enters sediments and percolates downward. Leaches dissolves ions and picks up very fine clay, then accumulation = new minerals precipitate out of water, and water leaves behind load of fine clay. 3. Burrowing organisms like ants, worms, and gophers churn the soil so organic material from ground surface gets mixed in.

What is soil composed of?

1. Fragments of bedrock 2. Clay minerals formed by alteration of bedrock minerals 3. Organic matter produced by the organisms that live in it

Factors in Soil Composition

1. Substrate composition 2. Substrate resistance 3. Climate 4. Slope steepness 5. Duration of soil formation 6. Vegetation type

A1-Horizon

2nd layer of soil. Included in Topsoil and Zone of Leaching. Consists of humus that has decayed further and mixed with mineral grains (clay, silt, sand).

A2-Horizon

3rd layer of soil. Included in Transition soil and Zone of Leaching. Lighter-colored Contains less organic material than topsoil.

B-Horizon

4th layer of soil. Included in Subsoil and Zone of Accumulation. Where ions precipitate to form new minerals (ex. iron oxides) and clay collects. Can be red-colored.

C-Horizon

5th layer of soil - at base of soil profile. Zone of weathering. Consists of material derived from the substrate (base) that's been chemically weathered and broken apart.

Spodosol

Acidic Low in plant nutrients Ashy Has accumulations of iron and aluminum Forms in humid forests

Vertisol

Clay-rich soils capable of swelling when wet, and shrinking and cracking when dry

Positive-feedback process

Describes soil production: The product of the process works to advance the process itself.

How does vegetation type affect soil?

Different kinds of plants extract or add different nutrients and quantities of organic matter to a soil. Some plants have deeper root systems than others, and more effectively prevent soil from washing away.

How does substrate composition affect soil?

Different substrates consist of different materials, so soils formed on them have different chemical compositions. Soil develops faster on unconsolidated material (ash/sediment) than on bedrock.

O-Horizon

First layer of soil. Included in Topsoil and Zone of Leaching. Consists almost entirely of humus.

Andisol soil

Forms in volcanic ash

Alfisol soil

Gray/brown Subsurface clay accumulation Abundant plant nutrients Forms in humid forests

Entisol

Has no horizons Formed recently

What is humus?

Humus is partially decayed organic matter.

How does climate affect soil?

Large amounts of rainfall accelerates chemical weathering and leach soluble minerals, creating more new soil.

How does substrate resistance affect soil?

Less resistant underlying rocks weather more deeply thus forming thicker soil.

Aridisol soil

Low in organic matter Has carbonate horizons Forms in arid environments

Inceptisol

Moist Poorly developed horizons Formed recently

Topsoil

O-Horizon and A1-Horizon Nutrient-rich Usually dark in color Contains diverse life

What is regolith?

Regolith is any kind of unconsolidated debris that covers bedrock. Soil is a type of regolith.

What is residual soil?

Residual soil forms directly from underlying bedrock.

Mollisol

Soft Black Rich in nutrients Forms in subhumid to subarid grasslands

What are soils?

Soil consists of rock and sediment that have been modified by physical and chemical interaction with organic material and rainwater, over time, to produce a substrate that can support the growth of plants.

How is soil a positive-feedback process?

Soil retains rainwater & plays host to variety of vegetation, bacteria, and other organisms. These create an acidic environment that, in combination with moisture, promotes chemical weathering, which alters/dissolves minerals. Plant roots and organisms tunneling through soil aid physical weathering by helping create fractures.

How does slope steepness affect soil?

Soil thickness increases as slope angle decreases.

What is transported soil?

Transported soil forms from sediment that has been carried in from elsewhere (deposits left by rivers, glaciers, wind).

What is bedrock?

Unaltered rock of any kind.

Gelisol Soil

Underlaid with permanently frozen ground

Ultisol

Very mature strongly weathered soils Low in plant nutrients

Histosol

Very rich in organic debris Forms in swamps and marshes

Oxisol

Very weathered Rich in aluminum & iron oxide Low in plant nutrients Forms in tropical regions

How does duration of soil formation affect soil?

Young soils tend to be thinner and less evolved.