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: AlSi3O8

- or AlSi3O8-

Negative charge allows soil to hold on to important mineral cations: Ca2+, K+, Na+, Mg2+, NH4

+,

Al3+ can be substituted for Si4+

1 in 4 will give a -1 charge

2 in 4 gives a -2 charge

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

Partition in SOM (humic substances)

Adsorb to surface of inorganic soil

Heavy metals

Sources: Agrichemicals Household cleaners Gasoline, oil Dry cleaning Paints Sludge Landfills Etc (see pg 458 in IB bk)

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.