1. Outline and describe the general weathering processes in soil formation.
2. List the 5 soil-forming factors and explain the importance of each factor.
3. Relate specific factors of soil formation and the process of soil genesis to observable soil properties.
4. Describe a soil profile including: a. Soil order b. Landscape position and aspect c. Master horizons and depth d. Soil texture of each horizon e. Structure of each horizon
The properties that a soil exhibits are determined by the type of parent material from which the soil developed, as well as the influences of climate , relief , and organisms that act over time to transform or weather the parent material into a soil. The wide variety of soil characteristics observed in nature is due to the combined influences of these 5 soil forming factors!
Inorganic parent material may include igneous, metamorphic, or sedimentary rocks. Parent materials can be weathered to soils in situ or transported prior to their formation. The rate of weathering is determined primarily by the temperature and moisture regime of the environment (the climate). The amount of water present during soil formation regulates most soil reactions. The rate of most chemical reactions and biological processes increase with temperature, therefore, the quantity of water available for reactions and the temperature of the soil are important soil forming factors.
The interaction of temperature and moisture causes different soils to form from the same parent material.
An example of this is the soil formation from basalt parent material in two different climates. If the climate is hot and dry most of the summer with only small amounts of rainfall occurring during the cool winter, the rate of soil formation would be slow and the profile would be shallow. In a climate with uniform rainfall throughout the year, and warm temperatures in the summer and cool in the winter, the rate of soil formation would be more rapid, indicated by deeper soil.
Each of the world’s soils are assigned to one of 12 soil orders , largely on the basis of soil properties that reflect the major course of soil development (i.e. diagnostic horizons). From the soil profile characteristics or the diagnostic horizons, soil scientists can determine the relative degree of soil development. Soil development ranges from essentially no profile development (Entisols) to deep, highly
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weathered soils of the humid tropics (Oxisols and Ultisols). The soil orders are named for the predominant forming element and have a common ending – sols, from the Latin word solum which means soil. Understanding the soil forming factors and their influence on soil development is essential to selecting the correct soil order.
The location of the soil in relationship to its position in the landscape can hasten or delay soil formation.
The steepness of the slope will affect the rate of erosion potential as well as the water intake of a soil.
The position on the slope will affect whether erosion or deposition is predominant. The slope is described as the percent of elevation loss.
The landscape position is described as the summit , shoulder , backslope , toeslope , or floodplain .
From and erosional perspective, the summit position is the most stable, the shoulder is a zone of loss, and the backslope, toeslope, and floodplain are zones of deposition. The aspect, or compass direction the slope faces, strongly influences the microclimate which in turn influences the rate of soil development and overlying vegetation. North aspects will have cooler temperatures and more available moisture that the corresponding south aspects.
Summit
Shoulder
Backslope
Toeslope
Floodplain
Soil is an open, dynamic system. The components of the soil are continually undergoing physical, chemical, and biological changes. The great diversity and variation exhibited by soils throughout the world may be attributed to the differences in the intensity and length of time the processes of horizon development have continued. The layers resulting from the soil formation and horizon development are grouped into master horizons designated by the capital letters O, A, E, B, and C.
O – Organic horizon: Typically forms above the mineral soil due to the deposition of litter derived from dead plant and animal matter. These horizons are commonly observed in wetlands and forested areas – and are generally absent from grasslands and cropland soils. This horizon must contain >20% organic material by weight.
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A – Surface horizon: Typically the uppermost mineral horizon containing an accumulation of organic matter that imparts a dark brown/black color to the soil.
E – Eluvial horizon: This is the horizon designation for a horizon with a great deal of leaching or eluviation of clay, primary minerals (Fe and Al oxides), and humus. This horizon is typically lighter due to the uncoated mineral grains that make up this horizon.
C – Unconsolidated horizon: This is the designation for the horizon that contains unconsolidated mineral materials that are similar to the parent material. This zone exhibits little biological activity and little effects of the factors of soil formation.
Transition horizons: Used when a horizon exhibits properties on one master horizon while also having properties of another master horizon. Two capital letters are used to describe these zones. The letter of the dominant horizon is placed first.
Subordinate distinctions: These lowercase letters further describe particular master horizons. They call attention to unique horizon features, such as and accumulation of clay (Bt), a zone of increased soil density (Bx), an accumulation in CaCO 3 (Bk)…
Structure
See previous lab (Soil Profile Description) for more details.
Structure Type Abbrev. Aggregate Description Common Location
Granular
Platy
Angular blocky
Sub-angular blocky
Columnar
Prismatic
Single grained
(structure less)
Massive
(structure less)
Spring 2006 gr pl abk sbk
Small, spherical peds
Thin, flat, and plate like. Cleave horizontally
Block-like peds with sharply angled edges.
Block-like peds with rounded edges
A
E and compacted horizons, or inherited from parent material
Common in B , may occur in A
Common in B , may occur in A co pr sg
Column-like peds with rounded caps
Column-like peds with flat, angular, un-rounded caps
Loose soil ma Large structure less chunks
B horizons in arid regions
B horizons
Sandy soil
Sandy soil
3

Notes:
Name
Location
Aspect/Slope
Landscape Position
Parent Material
Horizon Depth (cm) Texture Structure
Date
Soil Order
Vegetation
Climate
Color (Munsell notation)
Observations
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Study Questions
1) Why do you think texture, structure, or color change with profile depth (choose one)?
2) How does and E horizon form?
3) How does a Bt horizon form?
4) How do temperatures and moisture influence soil development?
5) Given the same parent material, would you expect the soil profile to be deeper in a tropical climate or a desert climate?
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