Download Soils As A Resource Weathering Mechanical Weathering Frost

Soils As A Resource
Weathering
• Mechanical – physical breakup of rocks
– Tree roots
– Frost wedging
– Abrasion
• Chemical – transformation of minerals in rock
– Solution
– Hydrolysis
– Oxidation
Mechanical Weathering
Frost Wedging and Talus Slope
Abrasion
Tree Roots
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Hydrolysis
• Example
• Weathering of feldspar to kaolinite
2KALSi3O8 + 2H+ +H2O >>>>>
Al2Si2O5(OH)4 + 2K+ + 4SiO2
Oxidation
4FeO + 2H2O + O2 >>>>> 4FeOOH
SOILS AND SOIL EROSION
Five factors of soil formation:
Climate
Topography (slope/exposure)
Parent material (geology)
Biota
Time
Soil horizons = Soil development =
weathering, accumulation,
translocation, and soil structure (soil
vs. raw dirt)
topsoil vs. subsoil
(“tilth” and productivity decline with
depth)
Dissolution
CO2 +H2O >>>> H2CO3 >>> H+ + HCO3CACO3 + H+ >>>>> Ca2+ + HCO3-
Idealized Soil
Profile
Horizons
O (organic)
A (dark, humus)
E (eluviation)
B (illuviation)
C (weathered rock)
R (rock)
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Parent Material
• Some rocks and minerals are more
resistant – example quartz takes longer to
break down than feldspar
Time
• Greater time = greater soil development
Climate
• This is the most important factor
– Humid tropical climates result in intense chemical weathering.
• End result is soil depleted of most minerals except iron oxides and
aluminum oxides
– Humid mid-latitudes
• Dissolved species recombine to form stable clay minerals
– Hot Arid Climates
• Little water for chemical weathering
• Little water for leaching
• Clays formed may be removed by deflation
– Cold climates
• Very dry
• Less biological activity
• Mechanical weathering is more dominant
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Topography
• Slope and exposure
• Greater slope = less moisture retention
• Northern exposure in northern hemisphere
= less intense summer sun
Photo By Werner Schellmann, 1982
Biota
Soil Characteristics
• Plant roots – open avenues for water, air
and other chemicals to reach deeper in
solid
• Earthworms – stir soil, increase water
infiltration
• Rodents, stir soil
• Deer, cattle, etc. – add nutrients
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Texture
Color
Structure
Porosity
Chemistry
Moisture content
Soil Texture Triangle
Types of Soil Structure
Figure 15.3
Figure 15.4
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SOIL CLASSIFICATION (a Linnean system):
SIX CATEGORIES
Soil Color
• Determined by composition and degree of
soil formation
NUMBER OF SOILS IN EACH CATEGORY
order
suborder
great group
subgroup
family
series
12 worldwide
47
230
1,200
6,000
15,000 in the USA
SOIL ORDERS:
• Provides evidence about how the soil
formed
– Organic rich, poorly drained – dark
– Gray – wet most of the time
– Yellow, brown, tan – good drainage
Alfisols:
Andisols:
Aridisols:
Entisols:
Gelisols:
Histosols:
Inceptisols:
Mollisols:
Oxisols:
Spodosols:
Ultisols:
Vertisols:
world breadbasket, temperate forest soils
volcanic soils
arid soils
new soils
frozen soils with permafrost
organic soils
young, weakly developed soils
grassland soils, most naturally fertile on earth
tropical soils, extremely leached
Northern conifer forest soils
highly leached forest soils, low natural fertility
heavy clay soils, high shrink/swell
world breadbasket
temperate forest soils
grassland soils
most naturally fertile on earth
highly leached forest soils
low natural fertility
heavy clay soils
high shrink/swell
new soils
No humus/salinization
tropical soils
extremely leached
coniferous forest soils
Aridisols
Figures 15.12, 15.13
Spodosols
Figure 15.20
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Expandable clay soils
(peat/swamp)
Vertisols
Figures 15.24, 15.25
Histosols
Figure 15.26
Where Does it Come From?
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Farming
Off road vehicles
Human trails
Horse trails
Construction
Deforestation
Measurement of Soil Erosion
• Measure directly on each slope
• Not realistic
• Too many variables
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Estimate depletion of reservoir capacity
Sediment yield/yr to reservoir
Size of basin
Can calculate m3/acre or tons/km2
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Natural Resource Conservation
Service (NRCS)
• Use an empirical equation to estimate
• Universal Soil Loss Equation
water
sediment
Example: Initial Volume of Reservoir = 100,000 m3
Volume of Reservoir after 1 year = 99,000 m3
– A=RxKxLxSxCxP
– Where:
A = average annual soil loss
R = long term rainfall-runoff factor
K = soil erodibility index
L = hill slope length
S = hill slope gradient
C = soil cover factor
P = erosion control practices
Size of Basin = 5,000 km2
Yield = 0.2 m3/km2/yr
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