Soil is composed of mineral grains, organic matter, water, and air. It develops in layers called horizons from the surface down. The top O and A horizons contain organic matter and form topsoil. The lower B horizon contains less organic matter. Soil type is determined by the percentages of sand, silt, and clay particles. Soil erosion degrades soil quality and harms waterways by removing fertile topsoil. Conservation efforts since 1985 have reduced U.S. soil erosion by 40%.

1. 10.4 Soil Components
• Components of
soil – mineral
grains (clay,
silt, sand, rock
fragments),
organic matter,
water, gas.
air
25%
water
25%
mineral matter
45%
organic matter
5%

2. Soil Profile
• Horizons – soils develop a layered structure, the layers are
called horizons; (from top)
o O horizon – mostly litter and humus
• Litter – organic debris on the ground’s surface
• Humus – decomposed litter, increases water-holding capacity
o A horizon – mixture of humus, sand, silt, clay
• O + A = topsoil
o B horizon – subsoil, low organics
o C horizon – partially weathered rock
o Water moves through soil and leaches ions
• Water, ions, clay from “A” end up in “B”

3. Fig. 10.15a, p.241

4. Fig. 10.15b, p.241

5. Fig. 10.16, p.244

6. 10.4 Factors affecting soil
development
• Rates of plant growth & decay – affects humus and
chemistry
• Slope aspect & steepness
o Aspect – the direction a slope faces
• Time – chemical weathering is usually slow
• Soil transport – streams and winds move sediments

7. Soil Texture
• Determined by soil particles
o Sand – largest
o Silt – middle
o Clay – smallest
• Percentage of each particle within the volume give the
soil type
• Use the soil triangle to determine soil type

8. 1) Find
Percentage
Of each
particle
Type
2)Follow the
lines for each
percentage
Until they
intersect
At a point.
This give the
soil
Type.

9. 1) Find
Percentage
Of each
particle
Type
2)Follow the
lines for each
percentage
Until they
intersect
At a point.
This give the
soil
Type.

10. Permeability and Porosity• Permeability
o The ability to transmit (or release) fluids (water)
• Porosity
o The volume of spaces between the soil particles
Water Water
High permeability Low permeability
Fig. 10.17, p. 224

11. Porosity vs Permeability• The larger the particle size the larger the spaces between
particles BUT there are fewer spaces
o Sand has a high permeability because of large pore SIZE
• Gives sand the ability to DRAIN a lot of water
o Clay has a high porosity because of the NUMBER of pores
• Gives clay the ability to HOLD onto a lot of water
https://www.youtube.com/watch?v=39FfOa1gTX4&list=PLXKxye83Oxiy0qHna6eBa
6HMIcQZuJjph&index=6

12. SOIL EROSION AND
DEGRADATION
• Soil erosion is the
movement of soil
components, especially
surface litter and topsoil,
by wind or water.
►Soil erosion increases through activities such
as farming, logging, construction,
overgrazing, and off-road vehicles.
Figure 13-9

13. Soil Degradation
• Degradation – to degrade or lower
the ability of the soil
oMechanisms of Degradation
• Erosion
• Desertification
• Salinization
• Waterlogging

14. Soil Degradation
• Erosion
oMechanisms for erosion
• Sliding downhill
• Moving water
• Wind

15. SOIL EROSION AND
DEGRADATION
• Soil erosion lowers soil fertility and can overload nearby
bodies of water with eroded sediment.
o Sheet erosion: surface water or wind peel off thin layers of soil.
o Rill erosion: fast-flowing little rivulets of surface water make small channels.
o Gully erosion: fast-flowing water join together to cut wider and deeper ditches
or gullies.

16. Global Outlook: Soil
Erosion
• Soil is eroding faster than it is forming on more than one-third
of the world’s cropland.
Figure 13-10

17. Desertification: Degrading
Drylands
• About one-third of the world’s land has lost some of its
productivity because of drought and human activities that
reduce or degrade topsoil.
Figure 13-12

18. Soil Degradation
• Causes of
Desertification
 Overgrazing
 Agricultural
overuse
 Deforestation
 Urban
development
Desertification – the complete loss of topsoil
Preventing the remaining soil from retaining
water and nutrients creating infertile substanc
Moderate Severe Very severe

19. Soil Degradation
• Salinization
o Increase in the salt
content of the soil due
to irrigation
• Waterlogging
o Impermeable clay layer
below soil traps water
innundating root
systems

20. Waterlogging of Soils: A
Downside of Irrigation
• Example of high
evaporation, poor
drainage, and severe
salinization.
• White alkaline salts
have displaced cops.
Figure 13-14

21. Fig. 13-15, p. 281
CleanupPrevention
Soil Salinization
Solutions
Reduce irrigation
Switch to salt-
tolerant crops
(such as barley,
cotton,
sugarbeet)
Flush soil
(expensive and
wastes water)
Stop growing crops
for 2–5 years
Install underground
drainage systems
(expensive)

22. Case Study: Soil Erosion in
the U.S. – Some Hopeful
Signs
• Soil erodes faster than it forms on most U.S. cropland, but
since 1985, has been cut by about 40%.
o 1985 Food Security Act (Farm Act): farmers receive a subsidy for taking highly
erodible land out of production and replanting it with soil saving plants for 10-15
years.

23. Summary
• Weathering
o Two types – Mechanical & Chemical
o Factors that affect rate of weathering
• Erosion – movement of particles
• Porosity vs Permeability
• Soil Formation
o Soil horizons & types
• Problems with soil erosion