Soil as a resource

Regolith is the layer of unconsolidated material at the Earth's surface.

Soil is the layer of unconsolidated material at the Earth's surface which can support plant growth.

Because of its ability to sustain plant growth, soil constitutes an extremely valuable resource.

The nature of the soil influences the type of agriculture, and the agricultural productivity of a region.

Loss of soil represents potential loss of food productivity.

WeatheringSoil forms by weathering of pre-existing rock or unconsolidated deposits (e.g., sediments, glacial till, etc.), and from the breakdown of organic material at or near the surface.

Mechanical weathering is the physical breaking-up of bedrock or coarse unconsolidated material into smaller-sized fragments.

This size reduction facilitates chemical weathering which is the transformation of original minerals in the bedrock or unconsolidated deposit into other minerals, through chemical reactions, and changes in composition due to removal or addition of certain chemical components.

Mechanical weathering

Frost wedging ( تجمد results from(الماءexpansion of freezing water in fractures in the rock. It is a very effective mode of mechanical weathering in temperate to cold climate.

Plants affect mechanical weathering through root action on the substrate on which they grow (e.g., root wedging).

Mechanical weathering is facilitated by pre-existing weaknesses in the bedrock, such as joints.

surface area to volume ratio increases

(volume remains constant at 1 m3 )

What happens during mechanical weathering?Rock breaks down into smaller pieces

for a cube that is 1 m on each side…mechanical weatheringbreaks it down into smaller pieces, exposing more surfaces

more resistant sandstone cap rock

less resistant shale

Mechanical weathering

Chemical weatheringChemical reactions, often involving water, lead to formation of new minerals (clays, oxides, hydroxides), and removal or addition of chemical components.

Chemical weathering is enhanced by presence of dissolved species that increase the acidity of infiltrating water.

Products of chemical weathering

Silicate minerals typically break down to form other minerals (clays, oxides, hydroxides).

Other minerals, such as carbonates, completely dissolve in water leaving new pore space.

marble (calcium carbonate) dissolution at Lincoln Memorial

Chemical weathering

Acid precipitation resulted in dissolution of calcite in tombstone over time.

Intense chemical weathering of granite facilitated by exfoliation joints. Note the flaky, brittle appearance of the rock due to feldspar breaking down to clay minerals

Weathering and soil

Soil constituents:

•weathering products

•living organisms

•organic decay products

Effect of climate on weathering

Climate controls the rates of mechanical and chemical weathering.

In cool, humid climates, mechanical weathering, in particular frost wedging, is important.

Chemical weathering is favored by:

1. high humidity (more water as weathering agent) ;

2. high temperature (increases the rates at which chemical reactions proceed).

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Influences on soil formationالتجوية عمليات و التربة تكوين في تتحكم التي العوامل

•Rock type الصخور نوعية

• topography التضاريس

• climate المناخ

• vegetation النباتي الغطاء

• time الوقت

Factors controlling weathering

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differences in soils among Indonesian islands reflect parent materials

parent material: bedrock from which soil develops

minerals in parent material determine:

• nutrient richness of soil• amount of soil produced

Java: parent materials are volcanic ashes--nutrient rich, thick soils

…population density is 460/square km

Borneo: parent materials are granites, gabbros, and andesites--soils depleted of nutrients (lack of fresh ash)

…population density is 2/square km

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climate: controls precipitation/chemical weathering

moderately wet climates:

• more chemical weathering and thicker soils

• significant clay-rich layers -- may be solid enough to form hardpan

arid climates:

• less chemical weathering and thinner soils

• subsurface evaporation leads to build-up of salts

• calcite-rich accumulation zones may form -- solid enough for hardpan

extremely wet climates (tropical rain forests):

• highly leached and unproductive soils -- laterites

• most nutrients come from thick O/A horizons

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water flows quickly down steep slopes -- little soil formation

topography: differences in elevation

• relief (elevation change--valley bottoms to hill tops)

• steepness of slopes

think about whatwater does…

water accumulates inlow-lying areas --high soil formation

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vegetation: source of organic matter in soil

• produces oxygen and carbon dioxide -- chemical weathering

• yields H+ for ion exchange in feldspars/clays that givesplants Ca, Na, K

time: dependent on other factors per se• warm, moist climate -- soils develop quickly

• arid, dry climate -- soils form very slowly

thicker soil is not necessarily older

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soil - a layer of weathered, unconsolidated material on top of bedrock

contains:

• clay minerals

• quartz

• water 25%

• organic material 5%- Air 25%

Soil profileA (+O)

B

C

bedrock

E

Soils typically show a zonation from the surface to the unweathered basement including some or all of: a layer consisting primarily of organic matter (O horizon), a zone of leaching rich in organic matter (A horizon), a zone of leaching poor in organic matter (E horizon), and zone of accumulation (B horizon), and a zone of partially weathered basement (C-horizon).

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idealized soil profile

not allhorizonswill bepresenteverywhere

downward motionof water

“leached” from above

The nature of a soil is characterized by a number of properties (color, particle size and shape, mineral, organic, and chemical composition, structure, etc.).

These properties depend on the starting materials and on the relative importance of the different weathering processes.

Pedocal بيدوكال (Alkaline soil, rich

in calcium-carbonates; typical

of dry climate)

Pedalfer بيدالفير(Acidic soil, rich in Al and Fe, and relatively poor in calcium; typical of humid,

temperate climate)

Types of soil التربة انواع

Types of soil: laterites الالتيريت

In hot, humid regions, intense chemical weathering leads to leaching of all but the most insoluble components in the soil (aluminum and iron). (Tropical areas are characterized by high temperature and high rainfall.)

This produces thick, clay-rich soils rich in Al and Fe oxide and hydroxide minerals, but generally poor in other elements (calcium, potassium, magnesium, etc.) necessary for plant growth. The soils are called laterites.Tropical rainforest soils are typically laterites. So how is the vegetable luxuriance possible??

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•Caliche soil: Mainly is composed of ca carbonate,Arid environment, most of desert lands are caliche soils.

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Soil classification based on the climate

•Tropical soil: High rainfall, high chemical weathering, High leaching, not productive

• Desert soil: Dry and deserts, mainly is composedOf calcium carbonate

• Forest Soil: Wet climate, includes zones C,B,A

•Arctic soil: Thin soil, frozen during all the time except short time, not productive.

Strategies for reducing cropland erosion

• Terracing ( المصاطب reduces the slope of the land, thus reducing water(عملvelocity and the water’s erosive potential.

• Contour plowing ( الكنتورية .similarly slows water flow downslope(الحراثة• Erection of windbreaks slows the wind and decreases its potential to cause

erosion. • Planting of cover crops between cash crops ( مناسب بغطاء التربة (تزويد• minimum-tillage agriculture ( العميقة غير with stubble left in place(الحراثة

between growing seasons, both lead to increased soil retention; the plants slow wind and water flow, and their roots provide additional anchoring for soil.

• Strip cropping: Planting crops such as corn, beans, or wheat in strips. In between the strips of crops, farmers sowed plants with dense roots, such as grass or alfalfa

• Strip cropping involves planting row crops in strips across the slope, with alternate strips of grain and/or forage crops (crops and alfafa)

Strip-Cropping

11-19 Source: Photograph courtesy of USDA Soil Conservation Service.

Contour Plowing

Figure 11.19A

11-20 Source: Photograph courtesy of USDA Soil Conservation Service.

Terracing to Create Steps

Figure 11.19B

11-21 Source: Photograph courtesy of USDA Soil Conservation Service.

Concern for Soil Degradation Worldwide

11-31 Source: Data from Global Resource Information Database of U.N. Environment Programme.

Causes of Soil Degradation Worldwide

11-32 Source: Data from World Resources Institute.