1. Unit 4.3:
Soil and Agriculture

2. Rock Cycle

3. Weathering
The breaking down and changing of rocks at or near the Earth’s surface
2 Types:
Mechanical
Chemical

4. Mechanical Weathering
Occurs when physical forces break rock into smaller and smaller pieces without changing the rock’s mineral composition
Mechanical weathering increases surface area

5. Three processes that cause mechanical weathering
Frost Wedging
caused by the expansion of freezing water in cracks and crevices.
wedged loose rocks may tumble into large piles called talus, found at the bases of steep, rocky cliffs

6. Three processes that cause mechanical weathering
2. Unloading/Exfoliation
Reduced pressure on igneous rock causes it to expand and allows slabs of outer rock to break off in layers in a process called exfoliation

7. Three processes that cause mechanical weathering
3. Biological Activity
The activity of organisms, including plants, burrowing animals, and humans

8. Chemical Weathering
Is the transformation of rock into one or more new compounds

9. Processes of chemical weathering
Water (dissolved gases)
oxygen (Fe + O2 = rust!)
CO2 (carbonic acid)
sulfur/nitrogen oxides (acid rain)

10. Erosion
Erosion – major external process: loosened material is dissolved, or worn away from one part of the Earth's surface and deposited in another
Streams – the most important agent of erosion
produce ordinary valleys and canyons; may form deltas when they flow into lakes and oceans
Wind Erosion – produces dunes or loess
Human activities can accelerate erosion: activities that destroy vegetation

11. Soil

12. Soil
Regolith – layer of rock and mineral fragments produced by weathering
Soil – part of regolith that supports plant growth

13. Soil Composition
Mineral matter, humus (decayed, organic material), air, water

14. Soil Texture Texture refers to % of particle sizes Sand (large size)
Silt
Clay (small size)
Loam (a mixture of all three sizes) is best suited for plant life

15. Soil Texture

16. Soil Forming Factors “what makes soils look different”
Parent material
Topography
Time
processing
Soil Properties
Biology
Climate

17. Soil Formation: Topography
steep – more erosion = thin/nonexistent soil
flat – little erosion = thick soils

18. Soil Formation: Time Time (years) millions 1 2 3 4
millions
In this diagram you can see how the soil forming factors have affected the weathering of bedrock to form a soil.
Time- It takes millions of years t forma soil.
Parent material ( bedrock) is broken down by rain and ice..
Vegetation starts to grow on the soils. The roots break up the soil. Biological life arrives later and acts on the soil further braking up organic matter and soil particles.

19. Soil Formation: Organisms
plants – main source of organic matter, acids from decay increase weathering
animals – burrow and mix materials (earthworms)

20. Soil Formation: Climate
Greatest affect on soil formation
hot/wet – thick/chemically weathered soil
cold/dry – thin/mechanically weathered soil

21. Soil Profile O horizon - organic matter A Horizon – (topsoil)
accumulation and decomposition
A Horizon – (topsoil)
mixture of mineral and organic matter, insects, fungi, microorganisms
 B Horizon – (subsoil)
fine clay particles leached out of A horizon
 C Horizon -
partially weathered parent rock

23. Soil scientists categorize soils in to 12 soil orders
Soil scientists categorize soils in to 12 soil orders. There are over 50,000 soils in the US.

24. Soil Types
Pedalfer – (temperate) – eastern U.S.- rich in iron oxide and aluminum clays (forest floor)
Pedocal – (dry) – southwest U.S. – less clay, rich in calcium carbonate (grasslands)
Laterite - Hot, wet, tropical climates, intense chemical weathering

25. Soil Erosion/Fertility/Food Production

26. Soil Erosion Losing topsoil
makes a soil less fertile and less able to hold water
The sediment clogs ditches, boat channels, reservoirs, and lakes
Soil can be renewed: in tropical and temperate areas: 200 – 1000 years depending on climate for 1 inch of new topsoil to form

27. Soil Erosion Global Soil Erosion World is losing 7-21%
of its topsoil from actual
or potential cropland
each decade.
Soil Erosion in the U.S.
About 1/3 of nation's original topsoil has been washed or blown into streams, lakes and oceans – mostly as the result of overcultivation, overgrazing and deforestation.
Soil on cultivated land is eroding 16 times faster than it can form.

28. How Do Excess Salts and Water Degrade Soils?
Salinization – thin layer of dissolved salts in the topsoil as a result of irrigation
stunts crop growth , lowers yields
Cures
take land out of production for 2-5 years
install underground network of perforated drainage pipe
flush soil with large amount of low-salt water
Waterlogging – water used to flush topsoil accumulates underground, raising the water table.
Saline water lowers root productivity, eventually killing them

30. Reducing Soil Erosion
Terracing – reduces erosion on steep slopes (converted into a series of broad nearly-level terraces that run across the land contour

31. Reducing Soil Erosion
Contour Farming – on gently sloping land plowing and planting crops in rows across rather than up and down the sloped contour of the land.

32. Reducing Soil Erosion
Strip Cropping A row crop (corn) alternates with another crop (a grass or grass-legume mixture).
catch and reduce water runoff
help prevent the spread of pests and plant diseases.
Soybeans and alfalfa help restore soil fertility.

33. Reducing Soil Erosion
Alley Cropping (agroforestry) - several crops are planted together in strips or alleys between trees or shrubs that provide fruit or fuelwood
The trees provide shade (helps to retain moisture).
Trimmings from the trees and shrubs provide mulch (green manure) for the crops

34. Reducing Soil Erosion Windbreaks/Shelterbelts reduce wind erosion
long rows of trees

35. Soil Fertility
Fertilizers – partially restore plant nutrients lost by erosion, crop harvesting and leaching.
organic fertilizer – from plant and animal materials (animal manure, green manure, compost)
commercial inorganic fertilizer -produced from various minerals (nitrogen, potash, phosphorus)

36. Soil Fertility Will Inorganic Fertilizers Save the Soil?
Advantages: Easy to transport, store, apply
Disadvantages :
Do not add humus to the soil - will become compacted.
Decreased soil porosity leads to reduced oxygen content and fertilizer from being taken up efficiently
Usually supply only two or three of the needed 20 or so nutrients
Cause water pollution (eutrophication)

37. Soil Fertility Crop Rotation Plant corn, tobacco or cotton one year
following year plant legumes to add nitrogen to the soil (soybeans, oats, rye, barley, sorghum)
Also helps reduce crop losses to insects by presenting them with a changing target