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Introduction to Soils
Laboratory Exercise #1 By: Carlin Conk
Definition of Soils
Soil is the collection of natural bodies on earth’s surface containing living matter and supporting, or capable of supporting plants. Its upper limit is the atmosphere (air) or water, and at its lateral margins it grades to deep water or barren areas of rock and ice. Its lower limit is normally considered to be the lower limit of the common rooting zone (root zone) of the native perennial plants, a boundary that is shallow in the deserts and tundra and deep in the humid tropics.
A soil profile is the vertical display of soil horizons.
O A E
Pedon is a 3-D unit that represents the entire soil body and is the smallest volume of soil that shows all of the characteristic properties of a particular soil. Typically represents 10100 ft2 of surface area. A group of pedons is calles polypedons.
A soil series includes soils that have developed from similar materials by similar processes resulting in similar appearances and properties. The characteristic properties of a soil series are unique. There are more than 20,000 soil series in the United States.
Soil Series Example
Soil color can suggest soil properties that may influence plants. Dark soil near the surface may indicate a high organic matter content, which may lead to easier cultivation and higher nutrient content.
Soil Color in Young & Old Soils
A young, unweathered soil may have a soil of uniform color. In older, weathered soils, yellow and red colors may dominate well below the surface of the soil. In North Carolina, we typically have bright red subsoils, would this indicate young or old soils? Weathered or unweathered?
Soil Forming Factors
CL - Climate OR - Organisms P - Parent Material T - Time T – Topography
=CLORPTT (an easier way to remember the soil forming factors)
Climate refers to rainfall and temperature. The main effects are: weathering, the production of organic matter , and the decomposition of organic matter.
Climate of North Carolina
In North Carolina, rainfall is fairly similar across the Piedmont and Coastal Plain. However, the rainfall is more variable in the mountains of North Carolina. Mountain
Organisms or Biotic Activity
Vegetation is the main biotic factor. The type of vegetation affects the soil color and organic matter content, especially in the A (or top) horizon.
Grasslands vs. Forests
Trees contribute less organic matter to the soil each year when compared to grasslands. The top horizons, or A horizons, in forests are usually thinner than A horizons in grasslands.
OM content http://www.soils.umn.edu/academics/classes/soil2125/doc/s10chap1.htm
Parent materials are defined as the materials underlying the soil and from which the soil developed. There are many different parent materials, including: Residual minerals and rocks (bedrock) Glacial deposits Loess deposits Alluvial and marine deposits Organic deposits
Residual Rocks and Minerals
Residual minerals weather in place to form soils.
Glacial deposits formed as massive ice sheets, moving across North America, approximately one million years ago. As the glaciers expanded, they "bulldozed" rocks, minerals and soil in front of them. As the ice sheets melted, the exposed parent material began to weather and soil was formed. http://www.soils.umn.edu/academics/classes/soil2125/img/1usglac.jpg
Boulders and Rocks
Loess deposits formed as high speed winds picked up predominantly silt-sized particles and carried them across open areas. As the wind speed slowed the particles fell to the ground covering the native soil and parent material.
Loess Deposits Example
Alluvial deposits refer to sediments carried by and deposited in fresh water. Flooding events carry rocks and minerals, which are deposited in bands.
Marine sediments refer to sediments carried by fresh water but deposited in salt water. Marine sediments can build up over long periods of time until eventually they are quite deep. http://www.mo15.nrcs.usda.gov/ features/gallery/bonneau.jpg
Organic deposits originate from plants that have died or shed their leaves. Organic deposits are common to areas such as swamps and marshes since the chemical and biological process that decompose the organic matter are greatly limited by the saturated conditions
Organic Soils of North Carolina
http://www.soil.ncsu.edu/publication s/Soilfacts/AG-439-26/image2.gif http://soils.usda.gov/technical/class ification/orders/images/histosol.jpg
Time as a Soil Forming Factor
We may regard time as continuous yet still recognize a "time zero" for a given soil. Time zero is the point in time at which a catastrophic event (flood or earthquake) is completed and a new cycle of soil development is initiated. Time is important in soil formation because it determines the degree to which the other soil forming factors express themselves.
"Old" soils are those that have experienced intense weathering of parent material in the presence of biotic factors. These soils will have well developed profiles containing A, E, and B horizons. http://soils.ag.uidaho.edu/soilorders/spodosols_02.htm
Young soils have weakly developed horizons and may often lack E and B horizons. This soil profile lacks any horizon development.
If the absolute age of a soil is of interest, a soil scientist may measure the activity of radioactive carbon. Knowing the activity of the radioactive carbon the scientist can estimate the age of the parent material that has been subjected to weathering.
Topography consists of three parts: elevation, slope, and aspect. Slope is the tilt or inclination of the land. Elevation is the height above mean sea level. Aspect is the direction the slope is facing.
Soils on a Steep Slope
As slopes increase, soils become shallower and have thinner and fewer horizons. This is because the steeper the slope the greater the runoff and the greater the erosion.
Review of Soil Forming Factors
Climate Organisms Parent Material Time Topography
As a soil develops on the landscape, distinct layers or bands parallel to the earth's surface may form. These layers or bands are called soil horizons. Soil horizons, are soil layers that differ from the overlying and underlying layers in some property, such as color, clay content, abundance of cracks, etc. Color is one property that is commonly used to separate different soil horizons.
Soil Horizon & Layer Designation
In the United States, soil horizons are designated by a code of letters and numbers developed by soil scientists of the National Cooperative Soil Survey. Master horizons are major layers designated by capital letter such as 0, A, E, B, C and R.
Master Horizons – O Horizon
0 Horizon: Organic horizons are dominated by organic material. The 0 horizons contain organic litter from plants and animals. 0 horizons are usually present on the soil surface except in the case of peats and mucks where the 0 horizon extends almost to the bottom of the soil.
Master Horizon – A Horizon
A Horizon : Mineral horizons that have formed at the soil surface or just below the 0 horizon. The A horizon may contain some organic material mixed with mineral material. Properties of the A horizon may reflect plowing, pasturing or similar activities. http://soils.ag.uidaho.edu/soilorders/spodosols_02.htm
Master Horizon – E Horizon
E Horizon : Mineral horizon in which the major characteristic is loss of clay, iron and aluminum oxides by eluviation or leaching. An increase in concentration of sand and silt size particles of resistant minerals occurs as clay is leached to lower depths. Color is lighter than the overlying A horizon. http://soils.ag.uidaho.edu/soilorders/spodosols_02.htm
Master Horizon – B Horizon
B Horizon: Mineral horizon that includes layers in which illuviation or accumulation of materials has taken place. Clay, iron and aluminum oxides from the overlying E horizon have accumulated here. http://soils.ag.uidaho.edu/soilorders/ultisols_04.htm
Master Horizon – C & R Horizons
C Horizon: Mineral horizon consisting of unconsolidated, partially weathered material that is neither soil or rock. The horizon is below the zone of most biological activity. The upper layer of the C horizon may become part of the B horizon as weathering continues. R Layer: Underlying consolidated bedrock http://soils.ag.uidaho.edu/soilorders/spodosols_01.htm
Transitional horizons are layers of soil between two master horizons.
Subordinate distinctions are specific features within master horizons that are designated by lowercase letters. There are many subordinate distinctions such as: p – plow layer t – accumulation of clay b – buried layer http://soils.ag.uidaho.edu/soilorders/ultisols_07.htm