HOW SOIL ORIGINATES
Clip Art: Soil Quality Institute

You need to know :   

1. How the five soil forming factors--climate, organic matter, relief, parent material, and time (CLORPT) influence the type of soil 
2. Common parent materials from which soils derive
3. The definition and characteristics of  major soil horizons
4.  The definition and characteristics of  diagnostic horizons

Misconceptions

1. Weathering and erosion are synonymous

1. Soil forming factors

Introduction:

Soils are not rocks.  Unlike rocks, a soil forms at normal earth surface temperatures and ___________________________. 

Soil genesis includes reducing the size of parent material particles (physical weathering), rearranging the mineral particles, adding organic matter, changing the kinds of                             and                            minerals (including clay formation) through chemical weathering, and the formation of soil                                               .  Soil genesis is a continuous but slow process.

A soil horizon is a layer of soil or soil material approximately parallel to the land surface and differing from adjacent horizons in physical, chemical, and biological properties.  (place diagram)

A soil profile is a vertical section (cross section) , usually 6 ft. deep, exposing major  horizons. 

Five soil forming factors--climate (cl), organic matter (o), relief, (r)  parent material (p), and time (t) (CLORPT) influence the type of soil.

The points listed below are interrelated and represent only SOME of the ways CLORPT influences the  soil type.  For a  overview of CLORPT from San Diego State University click here (requires Macromedia Flash viewer)

A) climate (CL) helps determine

--soil horizon                                       through the rate of weathering.                                                                                            (Image:  Jim Turenne 2004)

--soil formation rate through weathering

--rates of soil                                            

--precipitation and temperature determine the kind and amount of vegetation which in turn helps determine rates of nutrient cycling and availability. 

--clay type

Weathering results in a decrease in particle size, a release of soluble constituents, but also results in the                                 of clays. Weathering is important because it makes many nutrients available to plants AND helps create clays that can retain these nutrients.  Water is the main weathering agent that converts minerals from their original form into new forms.  

For the next class, bring either a Baby Ruth or Snickers bar.  Come hungry. 

Apple Exercise: How much arable soil exists on the earth?  Dissect an apple to gain an appreciation of how little decent soil actually exists on earth. (Based on J. Cassidy's Earthsearch) 

Difference between weathering and erosion:

physical weathering: breakdown without change in chemical composition

examples:

Field Trip: discovering evidence of thermal expansion and pressure release on within 50 m of Brooks. 

chemical weathering: breakdown in size and chemical composition.  Chemical reactions generally ___________________for every 10 degree C increase in temperature.

B) organic matter (O) helps determine

--                                                             through bioturbation and the creation of organic acids
--rates of nutrient cycling and availability

C) relief (R) helps determine

--soil profile thickness: thickness of the A horizon varies with slope position
--soil fertility:                                                           
Compare the crest, slope face, and slope base for soil fertility.  Where would you expect soils to be most fertile? 

--rates of                                                                  

d) parent material (P)  helps determine

e) time (T) helps determine

--soil horizon                                                   

2. Common parent materials

 Why is it important to know common soil parent materials?  The mineralogy of the parent material contributes to soil fertility, color, texture, and the kinds and amount of minerals in the soil.

n A soil formed from quartz sand (SiO₂) usually does not contain                             because the necessary Al and associated cations are not present.

n Some soils are considered to be                                  because these soils form  in-place without first being moved by wind or water.                          

n Most soils have weathered from transported parent materials. The transporting agents include wind, liquid water, ice, and gravity.

1. Soils derived from wind transported material: 
2. Soils derived from river transported material: alluvium or fluvial:
3. Soils derived from gravity transported material: 
4. Soils derived from glacially transported material: glacial

3. Major soil horizons and diagnostic horizons

Soil horizon: a soil layer parallel to the earth's surface.  Horizons are exposed in soil profiles.  Horizons are distinguished on the basis of their physical, chemical, and biological properties.  Examples of distinguishing properties:

As indicated in Table 1, one capital letter is used to designate master horizons (A, B, C) and two are used for transitional horizons (AB, BC) (Table 2).  Look at Table 3 and see that lowercase subscripts are used to subdivide master and transitional horizons (Bt, Ap) and to designate important horizon properties.  Master horizons with two or more subordinate horizons keep the identical letter identification and are numbered consecutively; for example A1, A2, Bt1, Bt2.

Horizon forming processes

a. eluviation- leaching, percolation, downward movement (out).
b. illuviation: deposition of                                                  (in).

Typical horizon characteristics: 

O horizon

A horizon: maximum biological activity occurs within the A horizon and results in the rapid weathering of                           and the eluviation of soluble products like calcium carbonate.

E horizon: The E horizon is found below an A or O horizon. It is a mineral horizon of strong eluviation, formed when organic acids from the A or O horizon combine with leaching waters to weather and mobilize silicate clays, iron and aluminum, leaving a bleached layer of resistant minerals such as quartz.

B horizon: Eluviated clays, iron and aluminum, CaCO_3, humus, silicates, and salts, alone or in various combinations, accumulate or become illuviated in B horizons.  The kind of B horizon formed depends on the soil-forming factors. As water percolates through a soil, it may become saturated, at which time materials will come out of solution or precipitate.

Bt horizons are found in moist climates.  A layer of illuviated clays.  Implications on permeability? 

C horizon: a layer little affected by CLORPT.

R horizon: hard, consolidated bedrock showing no signs of soil formation. 

Descriptive Soil Profile Symbols

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