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%

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”

Fig. 10.15a, p.241

Fig. 10.15b, p.241

Fig. 10.16, p.244

10.4 Factors affecting soil

development• Rates of plant growth & decay – affects humus and

chemistry

• Slope aspect & steepnesso Aspect – the direction a slope faces

• Time – chemical weathering is usually slow

• Soil transport – streams and winds move sediments

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

1) Find PercentageOf each particleType

2)Follow the lines for each percentageUntil they intersectAt a point.This give the soilType.

1) Find PercentageOf each particleType

2)Follow the lines for each percentageUntil they intersectAt a point.This give the soilType.

Permeability and Porosity• Permeability

o The ability to transmit (or release) fluids (water)

• Porosityo The volume of spaces between the soil particles

Water Water

High permeability Low permeability

Fig. 10.17, p. 224

Porosity vs Permeability• The larger the particle size the larger the spaces between

particles BUT there are fewer spaceso 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=PLXKxye83Oxiy0qHna6eBa6HMIcQZuJjph&index=6

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

Soil Degradation• Degradation – to degrade or lower

the ability of the soiloMechanisms of Degradation

• Erosion

• Desertification

• Salinization

• Waterlogging

Soil Degradation

• Erosion

oMechanisms for erosion

• Sliding downhill

•Moving water

•Wind

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.

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

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

Soil Degradation

• Causes of

Desertification

Overgrazing

Agricultural

overuse

Deforestation

Urban

development

Desertification – the complete loss of topsoilPreventing the remaining soil from retaining water and nutrients creating infertile substance

Moderate Severe Very severe

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

Salinization and 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

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)

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.

Summary• Weathering

o Two types – Mechanical & Chemical

o Factors that affect rate of weathering

• Erosion – movement of particles

• Porosity vs Permeability

• Soil Formationo Soil horizons & types

• Problems with soil erosion