1 What is “Soil”? Chapter 1 pages 1 – 30 Soil is NOT Dirt: • 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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1
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
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.