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Soil Classification

1. Factors of Soil Formation

1.1. Parent material  
1.2. Climate  
1.3. Biota  
1.4.Topography  
1.5. Time  
Interactions of Soil Formation Factors

Soil forms as a result of  five soil formation factors.  Differences in soil type within and between regions are a result of the interactions between these factors. 

1) Parent material: unconsolidated material in which soil development occurs
2) Climate: particularly precipitation and temperature  
3) Biota: living organisms including vegetation, microbes, soil animals, and human beings
4) Topography: slope, aspect, and elevation  
5) Time: period that parent materials are subjected to soil formation  

 

1.1. Parent material

Parent material is the material from which a soil forms. It consists  of unconsolidated and more or less chemically weathered mineral or organic material.  

Slide Show Images

Glacio-marine sediments, with subfossil shells. James Ross Island, Antarctica. Photo: IIngólfsson, 1987.

Glaciotectonically folded marine sediments, below till. Severnaya Zemlya, Siberia. Photo: Ólafur Ingólfsson 2002.

Source: provided by CanSIS http://sis.agr.gc.ca/cansis/

Numerous landform features were formed following glaciation and deposition of morainal (till) material. The animation below illustrates formation of outwash deposits formed by meltwater streams that issued from a glacier.

The nature of the parent material influences both texture and the mineral composition of the soil. Soil parent material consists of rocks, which can be classified as:

1) Residual or sedentary: sediments developed in place (in situ) from underlying rock over a long period of intense weathering.

2) Cumulose: organic deposits developed in place from plant residues that have been preserved by a high water table, or some other factor that inhibits decomposition. Examples are peat (undecomposed or slightly decomposed organic matter) and muck (highly decomposed organic matter)

3) Transported: loose sediments or surficial material that were transported and deposited by gravity, water, ice, or wind. The following table lists transported parent materials and their modes of deposition.  

Mode of deposition

Resulting parent material

Water

 

  • Alluvium (deposited from flowing water)

  • Lacustrine (sediments in still water, especially lakes)

  • Marine (deposited in oceans or reworked by oceans)  

Water and ice

 

Ice

  • Till (or morainal material) 

Wind

Gravity

1.2. Climate

Climate determines the nature (physical, chemical or biological) and rate of weathering (that acts on parent material to form soil). The most important elements of climate for soil formation are precipitation and temperature. For example, the amount of precipitation determines the extent of leaching through a soil profile and seasonal temperature fluctuations influence the number and rate of chemical reactions and overall biological activity.

Slide Show Images - Source: provided by M. David Bennett

 

1.3. Biota

Living organisms, including plants, microbes, soil animals, and humans, are collectively referred to as biota. Soil development is affected by both the type and number of organisms that live in and on the soil. Plants influence the amount of organic matter buildup in the soil. For example, soil developed under grassland vegetation has organic matter incorporated into the rooting zone, while in forest soils organic matter accumulates on the surface.

Slide Show Images - Source: Sask Interactive
© www.micrographia.com

Human activity also influences soil formation. Destruction of natural vegetation by changing the frequency and extent of natural fires or by soil tillage abruptly modifies the soil forming factors. These changes have influenced the relative distribution of forests and grasslands in many areas of British Columbia .  

Example of Human Impacts on Soil Formation: The image shows a dust bowl approaching Stratford, Texas (1935)

Source: Photo from NOAA George E. Marsh Album http://www.photolib.noaa.gov/index.html

 

1.4.Topography

There is a strong interaction between topography and vegetation and their influence on soil formation.  Depressions in dry grassland areas are usually occupied by trees due to greater soil water content. As a result, these depressions have different soil types than adjacent areas.

Example of the Influence of Topography & Vegetation on Soil Formation: The image of the Nicola Valley illustrates how within grasslands, trees occupy slight depressions where soil moisture is higher than in the surrounding parts of the landscape.

Source: Photo from Nicola Valley in southern BC: Maja Krzic

Slope influences 

1) the relative rate of water infiltration into the soil, 
2) surface runoff and its associated soil erosion, and 
3) distribution of vegetation.  

Example of the Influence of Slope on Soil Formation: The image from the Sumas Prairie shows flat topography as influenced by slope.

Source: Natural Resources Canada
http://www.nrcan-rncan.gc.ca/com/index-eng

Aspect influences the angle at which the sun’s rays strike the earth’s surface. Slope and aspect together influence soil temperature, soil water content and vegetation, which in turn affects soil formation. For example, in the Northern Hemisphere cooler north-facing slopes are usually forested with slower soil development than on warmer south-facing grasslands . Consequently, a north-facing slope has a thinner soil than a south-facing slope on the same ridge.

Elevation influences vegetation and soil type. In the interior of British Columbia climate becomes cooler and more wet as elevation increases from valley bottoms to mountain tops. Such climate gradients are usually reflected in a shift from grassland to forest and alpine plant communities.

There is a strong interaction between topography and vegetation and their influence on soil formation. The figure below illustrates how in a grassland-forest transition zone, trees occupy slight depressions where soil moisture accumulates.  These depressions have different soil types than adjacent upland areas.  This photo is from the Nicola Valley in interior British Columbia.  

1.5. Time

Soil formation is a slow process that takes hundreds or even thousands of years. A younger soil will reflect characteristics of the parent material better than an older soil, since insufficient time has elapsed to permit significant development. Canadian soils, including British Columbia’s, are relatively young when compared to soils of the southern United States. They have been developing since the recession of the last ice age 10,000 years ago.  

“The Oldest Life on Land”. Fossilized remnants of a microbial mat provide evidence that life existed on land as early as 2.6 to 2.7 billion years ago. The findings suggest that an oxygen atmosphere and a protective ozone layer were in place around Earth by that time. Review website for details.

Examples of Time as a Factor in Soil Formation: Canadian soils have been developing since the recession of the last ice age 10,000 years ago as illustrated in the glaciers of St Elias Mountains, Kluane National Park, Yukon (top) and Glaciers at  Piedmont Glacier, Byolot Island, Nunavot (bottom). 

Source: Natural Resources Canada
http://www.nrcan-rncan.gc.ca/com/index-eng

 

Interactions of Soil Formation Factors

The formation of soil is a complex process, and the five soil formation factors are active simultaneously and interdependently. Individual factors are of interest because they help us simplify and explain soil formation. They are particularly useful for soil survey and mapping, since soils are stratified on the basis of individual formation factors.

Faculty of Land and Food Systems
THE UNIVERSITY OF BRITISH COLUMBIA