Environmental Science 102
Soil and Soil Erosion


Natural surface layer containing organic and inorganic matter.
Contains matter in solid, liquid and gaseous form
Organic matter:
Inorganic matter:
Soil without organic material is called:
Soil can be described using seven characteristics


Indicates composition of the soil


Proportion of particle sizes present in soil
Sand, silt and clay
Size classification
Determines the water retention capacity

Soil Colloids

Particles less than 1/100,000 mm (0.00001)
Very fine clay mineral particles and organic matter
Tend to attract nutrient to their surfaces

Soil Acidity/Alkalinity

Dependent on presence or absence of H+ ions
Acidic soil is caused by many factors including climate, pollution precipitation, and bedrock.

Soil Structure

Refers to the way the soil grains are grouped together
These groupings are called peds
Bound together by colloids

Soil Minerals

Minerals found in the soil.
Primary minerals:
- quartz, feldspar, mica, hornblende, pyroxene, olivine, calcite
Secondary minerals:
- clay minerals, mineral oxides

Soil Moisture

Amount of water retained in the soil after excess water has drained
Excess water is removed by evaporation, transpiration, groundwater transport

Soil Horizons

- distinctive horizontal layers that differ in physical, chemical and organic composition
- develop as a result of climate, living organisms, and configuration of land surface
- distinguished by color
- collection of horizons at a given location is called the soil profile

Soil Development

Soil Horizons:
Develop as a result of climate, living organisms, configuration of land surface
Distinguished by color
Soil Profile:

Soil Profile

The following is a typical sequence in a mid-latitude forest.
Not all soil profiles will contain all of the soil horizons listed below.

Soil horizons diagram

Organic Horizon - O

Accumulated organic material.
- Oi - identifiable organic material - undecomposed material.
- Oa - amorphous organic horizon - decomposed beyond the point of recognition - humus.

Solum Horizon

Mineral horizon which includes the A1, A2, and B Horizons
A1 Horizon - eluvial horizon - leaching of material from the soil.
A2 Horizon - horizon of maximum eluviation - typically has a light color, washed out look.
B Horizon - illuvial horizon - material from A1 and A2 is washed into this layer.

C Horizon

"calcareous" horizon - unconsolidated parent material - not affected much by organic processes.

R Horizon

regolith - loose bedrock material.


Solid rock - not considered to be part of the soil profile but underlies all soil profiles

Soil Forming Processes

Soil Enrichment - process of adding new material to the soil from outside sources.

Soil Removal or Depletion - removal of material from the soil - erosion and leaching

Translocation - movement of materials within the soil profile. Includes:
- Eluviation:
- Illuviation:
- Decalcification:
- Calcification:
- Salinization:

Transformation - changing of the material within the soil - primary mineral weather to become secondary - organic matter decomposes to humus (humification)

Soil Temperature

Controls soil development by regulating the rate of chemical and biologic activity
Biologic activity
- below 0°C there is little or no activity
- below 10°C activity is slow
- decomposition and chemical activity is slowed by low temperature

Slope Configuration

Degree of the slope
- steep slopes:
- gentle slopes:

Orientation of the slope
- how is the slope oriented relative to insolation and wind.
- orographic precipitation, microclimates and insolation will influence soil development

Soil Classification

Soil Profiles are distinguished based on the presence or absence of soil horizons.
Diagnostic horizons are layers within a profile that are unique or easily identified.
Three groups of soil orders.

Soil Group I

Well developed horizons.
Develop in areas where soil has had a long period of adjustment.
Generally have high base status
- high base status - rich in nutrients
- low bas status - poor in nutrients
7 orders - Oxisols, Ultisols, Vertisols, Alfisols, Spodosols, Mollisols, Aridisols

Soil Group II

Soils with a thick, organically rich upper layer.
1 order - Histosols
Commonly called peat or muck.
Forms in lakes/ponds that fill up with organic debris.

Soil Group III

Soils with poorly developed horizons or no horizons.
Recent accumulations of material.
Poorly developed horizons indicates that the soil formation process has only recently begun.
3 orders - Entisols, Inceptisols, Andesols

Soil Erosion in the US

Soil erosion is a natural process of soil development

Splash Erosion

Surface Runoff

Only detrimental when the rate of erosion is faster than the rate of development.
Most human activity tends to increase soil erosion

Soil Erosion Factors


Rates of Erosion

In the US, the average rate of soil formation is one inch per century
Annual loss of top soil from agricultural land:

Total Annual Loss - 5 billion tons of soil
4 billion tons removed by water
1 billion tons blows away in the wind

Where Does the Soil Go?

1 billion tons is lost at sea
3 billion tons settles in lakes, reservoirs and floodplains

Sediment Problems

Sediment clogs river channels
Reservoir Problems due to sediment fill: