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Soil and Its UsesChapter 14

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Geologic Processes

• Crust - Extremely thin, less-dense solid.• Mantle - Makes up majority of earth, and

surrounds a small core of iron.– Outermost portion is solid.

Crust and solid outer mantle collectively known as the lithosphere.

Asthenosphere is a thin layer below the outer mantle capable of plastic flow.

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Structure of the Earth

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Geologic Processes

• Plate Tectonics - Large plates of crust and outer mantle slowly moving over surface of the liquid outer mantle.– Heat from earth core causes movement.

Plates are pulling apart in some areas, and colliding in others.

Building processes counteracted by processes tending to tear down land.

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Tectonic Plates

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Geological Processes

• Weathering - Factors bringing about fragmentation or chemical change of parent material.– Mechanical - Results from physical forces

reducing size of rock particles without changing the chemical nature of the rock.

Freezing and Thawing CyclesErosion from Wind and Moving Water

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Physical Fragmentation

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Weathering

• Chemical - Involves the chemical alteration of rock in such a manner that it is more likely to fragment or be dissolved.– Rock fragments exposed to atmosphere

may oxidize, or otherwise chemically change.

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Soil and Land

• Land - Portion of world not covered by water.• Soil - Mixture of minerals, organic material,

living organisms, air, and water that together support growth of plant life.– Good Agricultural Soil:

45% Mineral25% Air25% Water5% Organic Matter

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Soil Formation

• Soil formation begins with fragmentation of parent material.– Ancient layers of rock, or more recent

deposits from lava flows or glacial activity.• First organisms to gain a foothold in

weathered parent material also contribute to soil formation.– Lichens form pioneer community.

Decomposition of lichens further alters underlying rock.

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Soil Formation

• Humus - Decaying organic material.– Mixes with top layers of rock particles, and

supplies needed nutrients to plants.– Creates crumbly soil which allows

adequate water absorption and drainage.• Burrowing animals bring nutrients up from

deeper soil layers, improving soil fertility.

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Other Factors Influencing Soil Formation:

• Plant Roots• Bacteria and Fungi (Decomposers)• Position on Slope• Climate• Time• Rainfall• Soil pH

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Soil Properties

• Texture - Determined by the size of mineral particles within the soil.– Too many large particles leads to

extreme leaching.– Too many small particles leads to poor

drainage.Gravel >2.0 mm Sand 0.05 - 2.0 mmSilt 0.002-0.05 mmClay < 0.002 mm

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Soil Texture

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Soil Properties

• Structure - Refers to the way various soil particles clump together.– In good soils 2/3 of intra-soil spaces

contain air after excess water has drained.Friable - Crumbles easily.

– Protozoa, nematodes, earthworms, insects, algae, bacteria, and fungi are typical inhabitants of soil.

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Soil Profile

• Soil Profile - A series of horizontal layers of different chemical composition, physical properties, particle size, and amount of organic matter.– Each recognizable layer of the profile is

known as a horizon.

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Soil Horizons

• O Horizon - Litter - Un-decomposed or partially decomposed organic material.

• A Horizon - (Topsoil) Uppermost layer - contains most of the soil nutrients and organic matter.

• E Horizon - Formed from leaching darker materials.– Not formed in all soils.– Usually very nutrient poor.

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Soil Horizons

• B Horizon - (Subsoil) Underneath topsoil. Contains less organic matter and fewer organisms, but accumulates nutrients leached from topsoil. Poorly developed in dry areas.

• C Horizon - Weathered parent material, very little organic material.

• R Horizon - Bedrock

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Soil Profiles

• Over 15,000 separate soil types have been classified in North America. However, most cultivated land can be classified as either grassland or forest soil.– Grassland Soils - Usually have a deep A

Horizon - low rainfall limits topsoil leaching.A Horizon supports most root growth.

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Soil Profiles

• Forest Soils - Topsoil layer is relatively thin, but topsoil leachate forms a subsoil that supports substantial root growth. (High rainfall areas)– Tropical Rainforests

Two features of great influence: High Temperatures

Rapid decomposition, little litter. High Rainfall

Excessive leaching of nutrients.

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Major Soil Types

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Soil Erosion

• Erosion - Wearing away and transportation of soil by wind, water, or ice.– Worldwide removes 25.4 billion tons/yr.

Made worse by deforestation and desertification.

Poor agricultural practices increase erosion and lead to the transport of associated fertilizers and pesticides.

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Soil Erosion

• Most current agricultural areas lose topsoil faster than it can be replenished.– Wind erosion may not be as evident as

water erosion, but is still common.Most common in dry, treeless areas.

Great Plains of North America have had four serious bouts of wind erosion since European Settlement in the 1800s.

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Worldwide Soil Erosion

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Soil Conservation Practices

• When topsoil is lost, fertility is reduced or destroyed, thus fertilizers must be used to restore fertility.– Raises food costs, and increases sediment

load in waterways.– Over 20% of U.S. land is suitable for

agriculture, but only 2% does not require some form of soil conservation practice.

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Soil Conservation Practices

• Agricultural Potential– Worldwide:

11% land surface is suitable for crops.24% in permanent pasture.

– United States:20% land surface suitable for crops.25% in permanent pasture.

– African Continent:6% land surface suitable for crops.29% can be used for pasture.

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Soil Conservation Practices

• Contour Farming - Tilling at right angles to the slope of the land. Each ridge acts as a small dam.– Useful on gentle slopes.

• Strip Farming - Alternating strips of closely sown crops to slow water flow, and increase water absorption.

• Terracing - Level areas constructed at right angles to the slope to retain water.– Good for very steep land.

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Soil Conservation Practices

• Waterways - Depressions in sloping land where water collects and flows off the land.– Channels movement of water.

• Windbreaks - Planting of trees or other plants that protect bare soil from full force of the wind.– Reduces wind velocity decreasing the

amount of soil that can be carried.

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Conventional vs. Conservation Tillage

• Plowing has multiple desirable effects:– Weeds and weed seeds are buried or

destroyed.– Crop residue is turned under.

Decays faster and builds soil structure.– Leached nutrients brought to surface.– Cooler, darker soil brought to top and

warmed.

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Problem:

• Each trip over the field is an added expense to the farmer, and at the same time increases the amount of time the soil is open to erosion via wind or water.– Reduced Tillage - Uses less cultivation to

control weeds and prepare soil, but generally leaves 15-30% of soil surface covered with crop residue after planting.

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Problem:

• Conservation Tillage - Further reduce amount of disturbance and leaves 30% or more of soil surface covered with crop residue.– Mulch Tillage - Tilling entire surface just

prior to planting.– Strip Tillage - Tilling narrow strips.– Ridge Tillage - Leave ridge.– No Till - Place seeds in slits.

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Positive Effects of Reduced Tillage

• Wildlife gain food and cover.• Less runoff - reduced siltation of waterways.• Row-crops can be planted in sloped areas.• Fewer trips means lower fuel consumption.• Double - cropping• Fewer trips means less soil compaction.

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Drawbacks of Conservation Tillage

• Plant residue may delay soil warming.• Crop residue reduces evaporation and

upward movement of water through the soil.• Accumulation of plant residue can harbor

plant pests and diseases requiring more insecticides and fungicides.

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Protecting Soil on Non-Farm Land

• By using appropriate soil conservation practices, much of the land not usable for crops can be used for grazing, wood production, wildlife production, or scenic and recreational purposes.

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Fig. 14.1

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Fig. 14.2

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Fig. 14.4

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Fig. 14.5

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Fig. 14.7

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Fig. 14.8

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Fig. 14.10

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Fig. 14.11

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Fig. 14.12

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Fig. 14.15

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Fig. 14.16

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Fig. 14.17

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Fig. 14.18

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Fig. 14.19

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Fig. 14.20

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Fig. 14.21

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Fig. 14.22

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Fig. 14.23

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Fig. p.324

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Fig. p.326

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Table 14.2

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