and
Nancy A. Van Wagoner
Acadia University

 Soil is a Critical Resource
– World’s farmers must feed an additional 90 million people every single year
 At the current rate of population growth
– Limiting constraint = availability of fertile land
– i.e.. good quality soil, and proper soil management

 Soil is a Nonrenewable resource on the human time scale
– How long does it take to produce a 10 cm thick layer of soil from bedrock?
 100 years to 10,000 years!!!
– What are current rates of soil loss
 India
– 4.3 billion tons per year
 USA
– 3.9 billion tons per year
– *1987 National Resources
Inventory, USDA SCS

 Processes that contribute to the loss
– contamination
– removal of surface vegetation and residue
 agricultural cultivation
 forest harvesting
 rangeland grazing
 surface mining
 urbanization (hwy, building construction)
– degradation

 Crop residue
– foliage, stubble, straw
– left on soil by crops
– before and after harvest
 Decreases surface runoff
– absorbs energy of
 wind
 rain

 Summary
– global food security requires
 understanding of soils
 proper soil management
– WWW resource on Soil Erosion
 http://soils.ecn.purdue.edu/~wepphtml/wepp/wepptu t/jhtml/intro.html

 What is weathering?
– The decomposition and disintegration of rocks and minerals at the
Earth’s surface by mechanical and chemical processes
 converts rock to gravel, sand, clay and soil

combine

 What is erosion
– The removal of weathered rocks and minerals from the place where they formed
 water
 wind
 glaciers
 gravity

 Types of weathering
– mechanical weathering
 The physical disintegration of rock into smaller pieces each retaining their original characteristics
– Example
– chemical weathering
 The decomposition of rocks and minerals as a result of chemical reactions (removal and/or addition of elements
– Example

 increases surface area
 exposes more surfaces to chemical attack

Mechanical Weathering
 Major Mechanisms
– frost wedging
– salt cracking
– abrasion
– biological activity
– thermal expansion and contraction
– pressure release fracturing

Mechanical Weathering
 Frost Wedging (fig. 10.4)
– When water freezes it expands
 Example
 Volume increases by about 9%
– Water migrates into cracks in rocks
– Ice crystal growth puts tremendous pressure on surrounding rock
 Enough to break rock
– Most effective in mountainous areas where daily freeze/thaw
 Talus slopes
– Dangers to hikers

Mechanical Weathering
 Salt Cracking (fig. 10.5)
– salts crystallize in cracks in rocks
 puts pressure on surrounding rock
– important in
 dry climates (arid regions)
– ground water is salty, salts precipitate out of solution
 coastal areas
– salt spray blows into cracks in rocks

Mechanical Weathering
 Abrasion
– breakup of rock by friction and impact
 glaciers (fig. 10-8)
 wind (fig. 10-7)
 running water (fig. 10-6)
 waves

Mechanical Weathering
 Biological Activity (fig. 10-9)
– plants growing in cracks in rocks
– burrowing animals
– humans blasting for roads, development, exploration, etc..

Mechanical Weathering
 Pressure release fracturing (fig. 10-10)
– buried rocks are under confining pressure
– when exposed they expand due to release of confining pressure erosion surface exfoliation joints cracks dev. parallel to erosion surface
– problem for miners (underground)--causes rock bursts

 WATER = main agent of chemical weathering
– pure water by itself is relatively inactive, but
 pH = 7
– with small amounts of dissolved substances it becomes highly reactive
 many of these substances are found in the atmosphere
 and soil

 78% Nitrogen
 21% Oxygen
 1% other
– inert gases = 0.93%
– carbon dioxide CO2
– methane CH4
– Hydrogen
– oxides of Nitrogen
– carbon monoxide
– ozone O3

 reactions with oxygen
 common, ~21% of atm. = oxygen
 example, Iron bearing minerals oxidize to form rust
 4FeSiO3 + 2H
Fe-pyroxene rain
2
O + O
2 oxygen
> 4FeO(OH) + 4SiO4 dissolved limonite silica from atm.
hydrated Fe-oxide

 solution of soluble substances, such as
 salt in water

 CO
2 dissolved in water, rain or snow, produces
– Carbonic Acid
 Remember, pure water is neutral (not acid or base)
– If we increase the number of H + ions in water, it becomes an acid, pH < 7
– If increase the number of Hydroxyl ions (OH ) it becomes a base
 Acids and bases are more corrosive than pure water


All natural rain water is “acid rain”
 Why
– as rain drops fall through the atmosphere and through soil
– react with carbon dioxide in the air, and produced by decaying organisms in soil
– to form carbonic acid
 H2O + CO2 H2CO3 H + +HCO3

 carbonic acid reacts with limestone to dissolve it
 draw equation

 result is dissolved Ca++ and HCO3effect on neutralizing acid

 Certain minerals react with acid solutions to neutralize them
 Examples are:
– Calcite (limestone)
– minerals of mafic igneous rocks
 Ca-rich feldspar
 Olivine

Carbonic
 idealized by the reaction with the mineral orthoclase, a common mineral found in granite
–
EQUATION (draw on board)
 What has happened
– The feldspar is weathered to clay.
– Ions are released to be soil nutrients.
– Silica goes into solution.
– H + replaces K in the crystal structure as OH ions = hydrolysis
 this disrupts and expands the crystal structure
– Al is retained

 Only one hydrogen ion is neutralized for each mole of feldspar consumed
 because clay minerals are by-products of weathering
– form at surface conditions
– very stable at surface conditions
– comprise a high percentage of the inorganic component of soil

 The emissions are
– SO2 and
– gases of nitrogen (NO2, N2O)
 draw reactions on board

 Canadian Shield
 Appalachians
 New England
 Nova Scotia
 Therefore lakes in these geographic settings have a poor buffer against the effects of acid rain.
 Soils in these settings also have a poor natural buffer and farmers must add lime (CaCO3) to the soil.
 figure 23.2

 constituent mineral growth, increase in mineral volume
 puts pressure on the framework of the rock resulting in:
– gruss
– spheroidal weathering

 form where hydrated minerals fall off and collect at the base of a weathering rock

 sequence of events:
– pressure release forms orthogonal joints
– water percolates through cracks
– -hydrated minerals disrupt the framework of the rock
– put pressure outward
– weathering first reacts more intensely at corners, producing a rounded shape
– finally, onion-skin pieces of rock flake off
– end up with what looks like giant pile of marbles

 Particle Size
 Porosity and Permeability
 Climate
– optimum environment for chem. weathering
– optimum environment for mech. weathering
 Mineral Stability