Transcript Soil Chemistry (continued)

The final exam
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Mon, 8p WGND 115
Comprehensive
“Essentials of CSS 305” on BB
Don’t forget calculator!
Review Exams 1 and 2 and labs and
homework assignments (especially
practice problems)
Erosion:
• A process that transforms soil into sediment
– Natural = geologic erosion: weathering means soil
formation usually > soil loss
– Human-induced
• over-grazing – 1/3 of all land degradation,
• forest harvest – in rain forests, bad practices
responsible for 0.5 b ha of land degradation…so far
• Tied with damage to plant communities
(increasing susceptibility to erosion)
• 85% of degradation of soils is due to the
destructive action of wind & water (2/3 of that is
by the action of water)
Estimated cost in US $9-$44 billion annually!!!
Geologic erosion greatest in semi-arid environments
Soil degradation
•Physical properties:
compaction, crusting
•Chemical properties:
acidification, salt
accumulation
•Erosion (loss): wind &
water
Downward spiral of land degradation
Degraded land g poor
crops g human poverty
g reduced protection of
soil resources g
increased erosion g
desperate people clear,
cultivate, & degrade more
land
Easter Island Volcanic PM, equatorial climate…
1st settlers – 400ad – rich in plant and animal resources
Building of moai (statues)
by 1400 – the two native trees (one palm one woody) were extinct
Rapid and catastrophic loss of forest nutrients and soil erosion
Consumed the birds & fisheries till they ran out
Ate rats to survive – cannibalism emerges – societal breakdown
Abandon villages – moved to caves
The most total environmental and social breakdown in recorded history
Factors affecting rates of erosion
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•
Factoid:
Approximately 53% of all wood
Topography
consumed globally is used for home
Land use practices heating and cooking.
Vegetation type
Rainfall amount, frequency, and
intensity
• Soil chemical properties (high CEC =
more plant cover = less erosion)
(Bowyer et. al., Forest
Products and Wood Science: An Introduction. Iowa State Press 2003.
p.ix)
Effect of agriculture
Fig 17.5, Brady & Weil
health
hazard
too!
Erosion mechanics
Factoid: large
raindrops fall at
30 km/hr !
1. Detachment
2. Transport
3. Deposition
Fig. 13.8, p. 333
Most erosion is initiated by the
impact of raindrops, NOT by
the flow of running water
USLE - Universal Soil Loss Equation
A = RKLSCP
• R= rainfall erosivity
• K= soil erodibility
• L = slope length
Soil-related
factors
• S = slope steepness
Land
• C = cover and management
management
• P = erosion-control practices
factors
In 1990s Revised (RUSLE) computer-based, more data,
improved more sophisticated modeling
See Table 17.3 of Brady & Weil for details
K: soil erodibility factor
• Structural
stability
(next slide)
Rain intensity
(cm/hr)
• Infiltration
capacity
Infiltration rate
A = RKLSCP
Excess water
(runoff)
Ksat
Time
K: soil erodibility factor
A = RKLSCP
• Infiltration capacity (previous slide)
• Structural stability
– Particle cohesion
• frictional resistance (block vs. sphere)
• cementation by OM, clays
– Particle mass (2.0 g/cm3 for bauxite, 5.3 for
hematite, 7.6 for galena)
Soil properties resulting in
low K values (less erosion)
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•
•
•
High organic matter content
Non-expansive clays
Strong granular structure
“stoniness” & macropores
Practices that minimize erosion
A = RKLSCP
L = slope length
S = slope steepness
C = cover and management
1. Mulching - C
2. Contour cultivation – C, L & S
3. Grass contour hedges - L
1. Mulch as a means to reduce erosion
A = RKLSCP
C =cover and management
Effects of mulching, contour
cultivation, and grass hedges
Practice
Mulching
Contour
cultivation
Grass contour
hedges
Reduction in
soil erosion
(%)
88
Increase in
crop yield (%)
46
36
55
56
98
A few practices to reduce soil
loss caused by timber production
• Tree removal: cable not skidder
• Scheduling: when dry or frozen. snow
great.
• Road design: 99% of soil loss avoided
by gravel, planting grasses on road cuts
• Buffer strips: 1.5 times the height of the
tallest trees
Wind erosion
Surface creep, saltation, suspension.
• 40% of eroded soil is transported by
wind erosion in USA
• In six of the Great Plains states, wind
erosion exceeds water erosion
• Fine particles (aeolian dust) can even
be transported to other continents
(Saharan sands found in Hawaii)
Wind erosion mechanics
1. Detachment
2. Transport
3. Deposition
Dust Bowl – 1930’s United States, great plains
Factors affecting wind erosion
• Wind velocity
> 25 km/h to get started – After which soil movement
is proportional to the cube of the wind velocity
• Wind turbulence – though it’s the airborne
particles that do most of the abrasion/erosion
• Surface roughness - decreases wind erosion
• Vegetation - decreases wind erosion
• Soil properties – Aggregate & surface crust
stability, bulk density
Naturally, there is
a wind erosion
prediction
equation . . .
E = ICKLV
I – soil erodibility
C – climate
K - soil roughness
L – field width
V – veg cover
Controls of wind erosion
• Add moisture: often not practical
• Conservation tillage
– stubble mulch
– strip-cropping perpendicular to the wind
– if tilling, do so when soils moist, not dry
• Windbreaks: trees best, but even cereal
crops can help!
Coming full circle . . .
Soil is:
A natural, evolutionary body
that is the product of the
environment under which it
develops…
The real voyage of discovery
consists not in seeking new landscapes
but in having new eyes.
~ Marcel Proust