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What is “Soil”?

Chapter 1pages 1 – 30

Soil is NOTDirt:• A four-letter word• Worthless material• Something swept under

the rug• Something wished upon

your neighbor

Soil is:A three-dimensional natural and dynamic body covering the Earth’s surface in a thin layer, synthesized in profile form from weathered and weathering mineral matter and decomposed and decomposing organic matter, which when adequately supplied with air and water, provides mechanical support, and in part, sustenance for plants.

Soils are:Dynamic natural bodies comprising the uppermost layer of the Earth, exhibiting distinct organization of their mineral and organic components, including water and air, which formed in response to atmospheric and biospheric forces acting on various parent materials under diverse topographic conditions over a period of time.

Other soil adjectives:

• Alive• Biologically diverse • Chemically complex• Chemically reactive• Nature’s water purification system

Soil functions:• Sustain plant and animal life above

and below the surface• Filtering, buffering, degrading,

immobilizing, and detoxifying• Storing and cycling nutrients• Providing support for structures• Regulating water flow

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Soil characteristics depend on:

• Parent material• Biota• Topography• Climate• Time

Soils form by the weatheringof parent material

• Parent material: bedrock or unconsolidated debris that are deposited over bedrock

• Weathering: physical and chemical processes that break-up, dissolve, and reform new minerals

Soils consist of:• Solids: mineral, biota (plants, animals,

& microbes), and organic matter (products and remains of life)

• Solution: soil water and dissolved substances

• Atmosphere: soil air consisting of N2, CO2, O2, and trace gases

Soils consist of:

Porespace

Soilsolids

Mineral45 %

Organic5 %

Air20-30 %

Water20-30 %

Soils Solids• Mineral (inorganic)

– Primary: formed as liquid magma cools– Secondary: a product of weathering

• Organic– Debris: source recognizable– Biota: plants, animals, & microbes– Humus: source unrecognizable

• Non-humic: biochemical organic compounds

• Humic: recalcitrant byproducts of microbial activity

Soil has:

• Texture, consisting of– Sand particles: 2.0 to 0.05 mm– Silt particles: 0.05 to 0.002 mm– Clay particles: < 0.002 mm

• The distribution of sand, silt, and clay in a soil is the textural class

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Textural classes Soil has:• Structure: the individual particles are

arranged together into aggregates• Aggregate: an agglomeration of soil

particles that are cemented together by organic matter and secondary mineral precipitates

• Soil structure can range from no structure (single-grained or massive) to well-structured

Prismatic

Platy

Granular

Columnar

Blocky

Soil has:

• Pores, both large (macropores) and small (micropores), that hold air and water [soil breathes through pores]

• Pores are responsible for soil air and water transport and water retention

• Porosity: the amount of pore space in a soil; a function of soil texture and structure

Soil hasColor: indicator of soil processes and properties• Black: soil contains high organic matter• Gray: soil contains reduced iron (Fe2+) and is

poorly drained & water saturated (low O2)• Red, browns, yellows, & tans: soil contains

oxidized iron (Fe3+) and is well-drained with adequate O2

• White: soil contains quartz (silica) or salts (calcite, gypsum)

Soil Color• Described using the Munsell

color system• Three components of color:

– Hue, the spectral color of visible light

– Value, the lightness or darkness of the hue

– Chroma, intensity or brightness

• Format: hue value/chroma

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Soil Color

• Most soils have a hue that is a combination of reds, yellows, & grays

• 10YR is a hue that is a 10:1 mixture of yellow-to-red

• 10R is a hue that is 100 % red

Soil Color

• The value ranges from 0 (absolute black) to 10 (pure white)

• Soil is lighter as the value increases• The chroma ranges from 1 (high gray,

low purity) to 8 (low gray, high purity)• Chroma < 2 indicates a gleyed

condition (poor drainage)

10YR 5/4yellowish brown

Soil AirA mixture of gases consisting of• N2 dominates, as in atmospheric air• Water vapor, ~100% relative humidity• CO2, 100- to 1000-times more

concentrated than atmospheric air• O2, less than soil CO2 and

atmospheric O2

Soil Water• A solution consisting of water (the

solvent) plus dissolved substances (the solutes)

• Dissolved substances are– Salts (electrolytes): organic and

inorganic ions (cations and anions)– Molecules (non-electrolytes): organic

substances that are not, or do not ionize– Hydrogen (H+) and hydroxide (OH–)

{pH; a master soil chemical property}

Soil Water• Plant nutrients are in their most

available forms when dissolved in the soil solution

• The mineral nutrients are described as macronutrients or micronutrients

• C (CO2), H (H2O), and O (O2 and H2O) are macronutrients obtained from soil air and water

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Soil Water

org-S, SO42–S:K+K:

Mg2+Mg:org-P, H2PO4–, HPO4

2–P:Ca2+Ca:org-N, NO3

–, NH4+N:

Macronutrients; obtained primarily from soil solids

ElementSpecies

Soil Water

MoO42–Mo:H3BO3

0B:Ni2+Ni:Zn2+Zn:

Cu2+Cu:

Co2+Co:Mn2+Mn:Cl–Cl:Fe2+ & Fe3+Fe:

Micronutrients; obtained from soil solids

Soil is Highly Reactive

• Clay-sized materials, e.g., clay minerals and organic matter, are colloidal

• Colloidal materials adsorb substances from solution

• Adsorbed substances resist leaching, but may be extracted by plant roots

Soils are Three-Dimensional

• Only two dimensions of a soil can be viewed on a landscape; the third dimension is depth

• The smallest 3-dimensional body that contains all the characteristics of a soil is a pedon

• A pedon is the soil individual; typically1 m × 1 m × 1.5 m deep, or to lithic contact

Soils are Three-Dimensional• Many proximate pedons = polypedon = soil

individual• Many soil individuals is a soil series• Soil formation results in soil horizons• Soil horizons are layers that have

contrasting physical (and chemical) features

• A vertical (two-dimensional) view of the soil horizons is called the soil profile

Soils Horizons • Soil horizons are identified according

to properties and characteristics• The master horizons of the Regolith:

O, A, E, B, and C; O, A, E, and B comprise the Solum– O horizons: soil surface layers that are

dominated by organic matter– A horizons: soil surface layers that are

dominated by mineral matter, but darkened by organic matter

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Soils Horizons The master horizons of the Solum• E horizons: soil subsurface layers that

represent the zone of eluviation; organic matter, clay minerals, Fe and Al have been leached from this horizon

• B horizons: soil subsurface layers that represent the zone of illuviation; organic matter, clay minerals, Fe and Al have accumulated in this horizon

Soils Horizons

The remaining soil horizons (parent material)

• C horizons: unconsolidated materials that are little affected by soil genesis

• R horizons: hard bedrock, like granite, basalt, sandstone, and limestone

O

A

E

B

C

Soil Profile

So

lum

Re

go

lith

Soils are classified: Soil Taxonomy

Order (12)Suborder (64)

Great Group (~ 300)Subgroup (> 2,400)

FamilySeries (~ 23,000)

Entisols (Recent): soils with little or no development. Occur in areas of recent deposition (sand dunes & flood plains), and in a wide variety of environments. Occupy ~ 16 % of the Earth’s ice-free surface.

Inceptisols (Latin, inceptum, beginning, inception): only weakly developed. Have a wide range of characteristics and occur in a wide variety of environments. Occupy ~ 17 % of the Earth’s ice-free surface.

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Aridisols (Latin, aridies, dry arid): too dry for soil development and for the growth of most plants. Often contain subsurface layers of accumulated salts or clay minerals. Occupy ~ 12 % of the Earth’s ice-free surface.

Vertisols (Latin, verto, vertical cracking): clayey soils with high shrink/swell capacity. They develop cracks that open and close periodically. Occupy ~ 2 % of the Earth’s ice-free surface.

Mollisols (Latin, mollis, soft, mollify): grassland and prairie soils that contain abundant organic matter and have a dark-colored surface. Quite fertile. Occupy ~ 7 % of the Earth’s ice-free surface.

Alfisols (alf from a combination of aluminum and ferrous): between Mollisols and Ultisols (moderately leached), with moderate fertility. Form primarily under forest or mixed vegetative cover. Occupy ~ 10 % of the Earth’s ice-free surface.

Spodosols (Greek spodos, wood ash): acid forest soils; surface layer is stripped of OM, Al, and Fe, and subsoil is enriched with these substances. Occupy ~ 4 % of the Earth’s ice-free surface.

Ultisols (Latin, ultimus, last, ultimate): strongly leached soils in humid environments. Generally acidic and nutrient-poor, requiring additions of lime and fertilizers. Occupy ~ 8 % of the Earth’s ice-free surface.

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Oxisols (French, oxide): intensely weathered (old) soils of tropical and subtropical environments. Occur on land surfaces that have been stable for a long time. Occupy ~ 8 % of the Earth’s ice-free surface.

Histosols (Greek histos, tissue): primarily composed of organic matter; bogs, moors, peats, and mucks. They are always wet. Occupy ~ 1 % of the Earth’s ice-free surface.

Andisols (Japanese ando, dark): formed on volcanic ash. Common in cool areas with moderate to high precipitation. Occupy ~ 1 % of the Earth’s ice-free surface.

Gelisols (Latin gelare, to freeze): permafrost near the soil surface. Common in the higher latitudes (e.g., Alaska & Siberia) and at high elevations. Occupy ~ 9 % of the Earth’s ice-free surface.