Factors of Soil Formation

Arif M Sikder

Center for Environmental Studies (CES)

Virginia Commonwealth University (VCU)

What is soil?

Soil is the unconsolidated cover on the surface of the earth.

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Soil is made up of

mineral particles,

organic particles,

air, and

water.

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Soil is capable of supporting plant growth.

SOIL

Geologic definition:

Loose surface of the earth as distinguished from solid bedrock.

Traditional definition:

Material which nourishes and supports growing plants; includes rocks, water, snow, air.

Component definition:

Mixture of mineral matter, organic matter, water, and air.

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Soil Components�The 4 parts of soil

SOIL

Soil Taxonomy definition:

Collection of natural bodies of the earth’s surface, in places modified or even made by man or earthy materials, containing living matter and supporting or capable of supporting plants out of doors.

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Its upper limit is air or shallow water and its lower limit is

the depth to which soil weathering has been effective.

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SOIL

As a portion of the landscape:

Collection of natural bodies occupying portions of the earth’s surface that support plants and that have properties due to the integrated effect of climate and living matter, acting upon parent material, as conditioned by relief, over periods of time.

United States Department of Agriculture

Soils are very different, depending on how they form

History of Modern Soil Science

• Aristotle (384 -322 B.C.)
• Considered the relationship between plant and soil
• Justus von Liebig (1803-1873)
• Chemistry in its Application to Agriculture and Physiology
• V.V. Dokuchaev (1846-1903)
• Soil is an independent natural evolutionary body formed under the influence of five factors
• C.F. Marbut (1863-1935)
• Introduced the concepts of soil genesis and classification to the U.S. soil survey
• Hans Jenny (1907-1981)
• Factors of Soil Formation

Soil Formation

Soil forms when weathered parent material interacts with environment.

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Soil environment includes:

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• Climate and weather
• Animals
• Microbes
• Human use
• Hazards ( natural and unnatural )
• Topographical relief

Soil Formation

Five Factors of Soil Formation

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• Parent Material

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• Climate

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• Topography

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• Biological Factors

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• Time

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Soil = f (Cl, O, R, P, T…)

Climate

Organisms

Relief

Parent material

Time

V. Dokuchaev, founder of Soil science defined soil as a result of interaction of soil formation factors: rocks, climate, plants, organisms, relief and time.

H Jenny refined the ideas of Russian scientist VV Dokuchaev

into the following equation:

Factors of Soil Formation

Which factor is most important ?

Climate

Organisms

Parent material

Topography

Soil Forming Factors�

• Climate
• Organisms
• Parental Material
• Topography
• Time

Decomposition and Humification

Capillary action

Translocation

Weathering

Leaching

Soils are complex and dynamic systems, in which many processes are taking place.

SOIL PROCESSES

Factors of Soil Formation

Time

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• Development and destruction of soil horizons

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• Reaction rates are slow, the longer a rock unit has been exposed, the more likely it is to be weathered.

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Rates of Soil Development

U.S. Department of Agriculture estimates that it takes 500 years to form an inch of topsoil.

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That’s less than 0.01 mm yr^-1

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Modern rates of soil loss are 100 to 1000 times rates of soil formation (typically mm yr^-1 to cm yr ^-1 in agricultural settings).

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Sets up a fundamental problem due to the erosion of natural capital!

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Several thousand years are needed for formation of a soil profile.

Profiles of different types of soil

Rates of soil formation

Slow--------------------------------------Fast

~ 1 cm/1000 yr ~ 30cm/50yr

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Factors of Soil Formation

Climate

• Temperature and precipitation
• Indirect controls (e.g., types of plants)
• Weathering rates

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The greater the rainfall amount, the more rapid the rate of erosion and leaching.

Climate

• Physical weathering – results from mechanical agents, such as heating & cooling, freezing & thawing, &/or abrasion.
• Materials are still the same at the molecular level.
• Materials are classified by the way they have been moved or scattered

Climate

• Chemical weathering – Results from the action of water, oxygen, & carbon dioxide on the rocks.
• Carbon dioxide mixed with rain water forms a weak acid, speeding up the weathering of parent material.
• Materials that have been chemically weathered are no longer the same at the molecular level.
• Example – Clay has been synthesized into something physically and chemically different from the parent rock.

Components of Climate

• 1. Temperature
• -for every 10°C , biochemical reaction rates 2X

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• 2. Effective precipitation
• (water that moves through entire soil column, including regolith)

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• -depth of water = depth of weathering
• -water moves soluble & suspended materials

Seasonal distribution of precipitation:

Every month

6 rainy months only

Location A

600 mm/yr

Location B

600 mm/yr

50mm

100mm

Temperature and evaporation:

Location A

hot

Location B

cool

600 mm

600 mm

High

evapotranspiration

Low

evapotranspiration

Higher effective ppt

Lower effective ppt

Factors of Soil Formation

Topography:

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• Ground slope
• Elevation
• Aspect (e.g., north facing vs. south facing slopes)

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Topography

• Topography refers to the “lay of the land” or the angle & length of the slope.
• It affects soil formation because it influences runoff, drainage, & erosion.
• It also may affect the type & amount of plant growth (biggest factor in determining type & amount of organic matter)

Topography

• Steep slopes
• High runoff rates
• Less water infiltration
• Fewer plant forms
• Less organic matter
• Increased erosion
• Thinner surface soil
• Slower formation/development

Topography

• Level to Gentle slope
• Little runoff
• Increased water infiltration
• Faster plant growth leading to more organic matter
• Less erosion
• Deeper surface soil
• Faster formation/development
• Excessive water can slow development

Topography

Topography:

level

slope

concave or

bottom of slope

(receiving)

Topography:

Factors of Soil Formation

Downslope transport of soil is a function of slope:

• Erosion rate = f(S)

The steeper the surface slope, the more likely any eroded material is to be transported out of the system.

Parent Material

• Alluvium – finely pulverized rock laid down by running water. Mostly silt and clay particles with some sand.
• Marine Deposits – finely pulverized rock laid down under sea. This material has been thrust up by shifts in the earth’s crust.
• Loess – silty, floury material laid down by winds. Mostly silt and clay with little or no sand.

Factors of Soil Formation

Parental Material:

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• Chemistry

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• Mineralogy

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• Grain size

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Factors of Soil Formation

Organisms

• Types of native vegetation
• Weathering is dependent of plant growth

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• Plant and animal activity produces humic acids that are powerful erosion agents.

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• Plants can physically erode as well as chemically erode.

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• Plants stabilize soil profiles, Animals (including humans) tend to increase erosion.

Organisms�plant and animal

(Living plants and animals on and in soil)

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-type of vegetation influences soil type

-base pumping

-sources of organic matter

-nutrient recycling

-vegetation prevents erosion

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Biota

• Biota is made up of living organisms such as plants, animals, insects, bacteria, & fungi.
• Plants have the most important affect because they determine the type & amount of organic matter.
• Animals & insects mix and aerate the soil as well as add organic matter.
• In the US, organic matter in the soils increases as you move north &/or east. Why?

Type of vegetation influences soil type

Type of vegetation influences soil type

Least

weathered

Most

Weathered

Soil horizons

Master Soil Horizons

RR Weil

Processes

Factors

Physical

Chemical

Biological

Agricultural

Industrial

Urbanization

• decline in texture;
• compaction
• crust formation;
• erosion;
• break in
• moisture, air and temperature regimes

• leaching;
• exhaustion
• salinization
• laterization
• contamination

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decline in total biomass; decline in total humus content

decline biodiversity and activity of soil fauna and flora

• deforestation
• over ploughing;
• cultivation too much vegetables and monocrop;
• over fertilization;
• over pasturing

• buildings;
• acid precipita-tion;
• man-caused degradation

excluding soils from agricultural use due to urbanization

Factors and processes of soil degradation

Soil degradation

Two Basic Types of Soil

• Organic Soil 
• Mineral Soil

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