8/12/2019 GEOL 1003 Ch12.ppt

1/48

Soil as a Resource

Chapter 12

Erosion has resulted in deep

gullying of this exposed soil

embankment. Photo from State of

California, Department of

Transportation

8/12/2019 GEOL 1003 Ch12.ppt

2/48

8/12/2019 GEOL 1003 Ch12.ppt

3/48

Soil Formation

Soilseveral ways to define

Unconsolidated material overlying bedrock

Material capable of supporting plant growth

Regolithencompasses all unconsolidated

material at the surface, fertile or not

Soil: is produced by weathering

Involves chemical, physical, biological

processes to breakdown rocks Climate, topography, source material

composition, and time are factors

8/12/2019 GEOL 1003 Ch12.ppt

4/48

Soil-Forming Processes

Weathering

Climate and water a major factor Mechanical Weathering:physical

breakdown of minerals by mechanical

action. No changes chemically. Freezing water expansion

Frost wedging

Break up of rocks and minerals without

changing the rocks composition Salt crystallization can wedge cracks

Chemical Weathering:breakdown of

minerals by chemical reaction

8/12/2019 GEOL 1003 Ch12.ppt

5/48

8/12/2019 GEOL 1003 Ch12.ppt

6/48

Soil-Forming Processes

Chemical Weathering

Calcium Carbonate dissolves in water

Some silicates dissolveclays and

oxides tend to form

Organic acids breakdown minerals in

rocks from the infiltrating water

Biological activity or roots or burrowersaid chemical weathering

Airborne chemicals such as acids or

sulfate wash into soil

8/12/2019 GEOL 1003 Ch12.ppt

7/48

8/12/2019 GEOL 1003 Ch12.ppt

8/48

8/12/2019 GEOL 1003 Ch12.ppt

9/48

Soil Profiles and Horizons

A cross section of the soil blanket betweenbedrock and atmosphere usually reveals aseries of zones of different colors, chemicalcompositions, and physical properties

A Horizon Rock material is exposed to heavy leaching

B Horizon Zone of accumulation (zone of deposition)

Zone of leaching

C Horizon Very coarsely broken-up bedrock

Below this is R horizon: bedrock or parent rockmaterial

8/12/2019 GEOL 1003 Ch12.ppt

10/48

8/12/2019 GEOL 1003 Ch12.ppt

11/48

Chemical and Physical

Properties of Soils

Color: dark or light Dark soils tend to be rich in organic matter

Light soils generally lack organic matter

Texture: size of fragments Sand-sized (2-0.05 mm)

Silt-sized (0.05-0.002 mm)

Clay-sized (less than 0.002 mm)

Structure: tendency to form peds

Ped forming soils resist erosion

Finer soils may become loess

8/12/2019 GEOL 1003 Ch12.ppt

12/48

8/12/2019 GEOL 1003 Ch12.ppt

13/48

8/12/2019 GEOL 1003 Ch12.ppt

14/48

Soil ClassificationBroad Chemical-based Classification

The effects of chemical weathering, andthus the climatic factor

Pedalfer:soil of a humid climate

More extensive leaching

Less soluble material remains after leaching

Iron and Aluminum oxides, hydroxides and clays

Pedocal:soil of a dry climate Calcium Carbonate generally present

Soil Scientists use a more sophisticated

classification

8/12/2019 GEOL 1003 Ch12.ppt

15/48

Fig. 11.07

8/12/2019 GEOL 1003 Ch12.ppt

16/48

8/12/2019 GEOL 1003 Ch12.ppt

17/48

Soils and Human Activities

Lateritesoil

Extreme version of pedalfer

Forms in tropical climates

Contains few soluble nutrients

Forests in tropical areas hold the nutrients,not the soil

Slash and burn agriculture quickly depletesthe nutrients over time

In areas where climates are monsoonal, soilmay form brick hard surfaces

Lateritic soils are difficult to farm or work forpeople to grow food with

8/12/2019 GEOL 1003 Ch12.ppt

18/48

8/12/2019 GEOL 1003 Ch12.ppt

19/48

8/12/2019 GEOL 1003 Ch12.ppt

20/48

Wetland Soils

Tend to be rich in accumulated organic

matter, are reduced chemically becausethey accumulate organic matter easily

which will decay and consumes oxygen

Provide vital habitats for birds and otherorganisms

Retain flood waters easily and often trap

sediments Also serve as pollution traps

Complex soils and land system

8/12/2019 GEOL 1003 Ch12.ppt

21/48

Soil Erosion

Weathering is the breakdown of rock ormineral material

Erosion is the physical removal of thematerial that has been weathered

Rain strikesbreaks up and softens soil Surface run and wind off picks up soil

particles

Faster moving water or wind will carry offlarge size particles and a greater load

Soil erosion is not beneficial to humans

8/12/2019 GEOL 1003 Ch12.ppt

22/48

8/12/2019 GEOL 1003 Ch12.ppt

23/48

Soil Erosion versus Soil Formation

Soil losses in U.S. amount to billions of

tons per yearabout 0.04 cm per year

Human activities, including farming,

accelerate the loss of soil

In general, soil formation is slower than

soil erosion

Important factors:

Climate and time

Nature of the source rock that weathering can

work on

8/12/2019 GEOL 1003 Ch12.ppt

24/48

8/12/2019 GEOL 1003 Ch12.ppt

25/48

8/12/2019 GEOL 1003 Ch12.ppt

26/48

8/12/2019 GEOL 1003 Ch12.ppt

27/48

8/12/2019 GEOL 1003 Ch12.ppt

28/48

8/12/2019 GEOL 1003 Ch12.ppt

29/48

8/12/2019 GEOL 1003 Ch12.ppt

30/48

Strategies for Reducing Erosion

Protect the soil from fast moving wind Plant wind breaks perpendicular to dominate

wind direction

Protect the soil from fast moving water

Reduce the slope so runoff is slowed

Plow fields parallel to contours

Terrace fields

Encourage the growth of plants withextensive root systems to hold the soil inplace

8/12/2019 GEOL 1003 Ch12.ppt

31/48

8/12/2019 GEOL 1003 Ch12.ppt

32/48

8/12/2019 GEOL 1003 Ch12.ppt

33/48

8/12/2019 GEOL 1003 Ch12.ppt

34/48

8/12/2019 GEOL 1003 Ch12.ppt

35/48

8/12/2019 GEOL 1003 Ch12.ppt

36/48

8/12/2019 GEOL 1003 Ch12.ppt

37/48

8/12/2019 GEOL 1003 Ch12.ppt

38/48

8/12/2019 GEOL 1003 Ch12.ppt

39/48

8/12/2019 GEOL 1003 Ch12.ppt

40/48

8/12/2019 GEOL 1003 Ch12.ppt

41/48

Irrigation and Soil Chemistry

Leaching modifies soil chemistry Application of fertilizers, herbicides and

pesticides alters soil chemistry

Human activity modifies soil chemistry Runoff water from irrigated fields carries

off toxic chemicals

These chemical can collect in lowland areasor wetlands

8/12/2019 GEOL 1003 Ch12.ppt

42/48

8/12/2019 GEOL 1003 Ch12.ppt

43/48

Th S il R

8/12/2019 GEOL 1003 Ch12.ppt

44/48

The Soil ResourceThe Global View

Soil degradation is a global issue Destructive processes exist such as:

Desertification

Erosion Deterioration of lateritic soil

Contamination from pollution

Chemical modification to soil by humans

These processes combine to the loss ofsoil, loss of soil quality, and degradedacreage left to grow enough food for a

hungry world. Land area is finite.

8/12/2019 GEOL 1003 Ch12.ppt

45/48

8/12/2019 GEOL 1003 Ch12.ppt

46/48

8/12/2019 GEOL 1003 Ch12.ppt

47/48

8/12/2019 GEOL 1003 Ch12.ppt

48/48