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Soil
What is soil?

Mixture of inorganic material (rocks) and organic
material (SOM) as well as organisms (microbes).

Soil formation involves:
 Parent Material (glacial deposits? Sediment? Lava?)
 Biotic Factors

Plant roots break up soil, add nutrients and organic material
 Climate influences

Temp, precipitation, wind: leaching: movement solute thru soil
 Topography

More water flow? Steep slopes?
 Time
Characteristics of Soil

Color
 Dark: humus, other colors: minerals present

Texture
 Gravel, sand, silt, clay

Structure
 moisture, air capacity and ion exchange ability

Moisture
 Wet soils vs dry soils support different forms of life

Depth
What role does soil serve in the
ecosystem?

Part of both food webs
 Provides nutrients for plants

Plants are a sink for toxic metals, organic toxins, carbon

Plants are a food source

Plants prevent erosion
 Detritovores: recycle nutrients so they can be used by
other organisms




Turn Nitrogen into nitrates
Recycle the nitrogen and carbon out of dead things
(mineralization)
Eat up pollutants
Holds moisture/ filters water
Structure of Soil: Inorganic

Silicon is to geologists what
carbon is to biologists...

Rocks and soils basic
structure:
 Silica: SiO2
 Aluminosilicate:
Al3+ can be substituted for Si4+
1 in 4 will give a -1 charge
2 in 4 gives a -2 charge


AlSi3O8- or AlSi3O8Negative charge allows soil to
hold on to important mineral
cations: Ca2+, K+, Na+, Mg2+,
NH4+,
Soil Organic Matter

Non-living organic components present in soil
resulting from decomposition of once living
creatures

Holds onto nutrients to exchange with plants

Improves soil structure
 Increase air

More oxygen,

Easier for roots to grow
 Increased moisture

Heat capacity / smaller temperature deviations
 Reduces soil erosion
Soil Organic Matter

The organic material from bacterial breakdown of
plants and other organisms
 Nature's way of recycling important nutrients:

Sugars, amino acids, proteins, polysaccharides
 Humic acids (Hummus)



Still don't really know what it is
Stable organic material that isn't broken down any more by
organisms.
Organisms will only “eat” what will give them a net energy
gain.
Soil Ecosystem: Detrital Food Chain
Soil Organic Matter: Possible
Structure

Polar parts
 Hold water
 Hold inorganic nutrients (ions: nitrate, phosphate)
Importance of Soil Organic Matter

SOM gives the soil better “Structure”
 More moisture, more oxygen can diffuse, more pockets
for microorganisms to live

Better soil structure supports more microorganisms
 Microorganisms mineralize nutrients


Amino acids → Nitrates → Natural fertilizer for plants
Using compost is a way to increase soil organic
matter into bad soil
Pollutants

Pesticides

Excess Fertilizer

Organic Pollutants
 VOCs, Semi-VOCs,
PCBs, PAHs, petroleum,
solvents, organotin
compounds


Sources:
 Agrichemicals
 Household cleaners
 Gasoline, oil
 Dry cleaning
 Paints
 Partition in SOM (humic
substances)
 Sludge
 Adsorb to surface of
inorganic soil
 Etc (see pg 458 in IB bk)
Heavy metals
 Landfills
Soil Degradation  Salinization
 Water used for irrigation leaves
behind salts
 Too much salt and plant life cannot
survive
 Water tables rise (due to
deforestation), more evaporation
occurs since surface is warmer so
salinization occurs

Acid Rain and Storm water
runoff
 Leaching of important nutrients for
plants
Sources

Green J., Damji S. Chemistry 3rd Ed. IBID Press, 2007.

http://www.science.org.au/nova/032/032box01.htm

Smith, T. M., and R. L. Smith. 2009. Elements of Ecology, 7th
edition. Benjamin Cummings.