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Environmental Science: Toward a Sustainable Future

Richard T. Wright

Chapter 8 Soil: Foundation for Land Ecosystems


Vocabulary ­ DAY 1 Define in your notebook Horizons, soil profile, O horizon, humus, A horizon, topsoil, E horizon, eluviation, B horizon


Vocabulary ­ DAY 2 Define in your notebook subsoil, C horizon, sand, silt, clay, soil texture, loam, workability, soil fertility


Vocabulary ­ DAY 3 Define in your notebook weathering, leaching, nutrient-holding or ion-exchange capacity, organic fertilizer, inorganic fertilizer, infiltrate, water-holding capacity, soil aeration, compaction


Vocabulary ­ DAY 4 Define in your notebook composting, castings, soil structure, mycorrhizae, mineralization, splash erosion, sheet erosion, gully erosion, desert pavement, desertification

Vocabulary ­ DAY 5 Define in your notebook overcultivation, no-till agriculture, contour strip cropping, shelterbelts, sediments, Salinization 43 total vocab ­ I'll check!

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Global Trend: Where Did All the Farms Go?

Poor farming practices = loss of soils and farmland

Erosion Salinization

Development in United States = loss of 1.4 million acres of farmland per year

Over 22% of land has been degraded



Why a Study of Soil Is Important

90% of the world's food comes from landbased agriculture. Maintenance of soil is the cornerstone of sustainable civilizations. [Easter Island] Simply stated, it is the "foundation" of terrestrial life.

Soil: Foundation for Land Ecosystems

Soil and plants ­ 8.1 Soil degradation ­ 8.2 Conserving the soil ­ 8.3



Soil and Plants ­ 8.1

Soil characteristics Soil and plant growth The soil community

Topsoil Formation




Soil Profile

Soil Texture

Soil texture refers to the percentage of each type of particle found in the soil.

Loam soil is approximately 40% sand, 40% silt, and 20% clay. [KNOW THIS]



Soil Texture

Soil Texture

Why is this important? Next slide . . .

Sand Silt Clay

Large Small Smaller

Animation for soil particle size

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Soil Texture and Properties (see Table 8-2)

Soil Classes

Mollisols: fertile soils with deep A horizon; best agriculture soils [temperate grasslands] Oxisols: iron and aluminum oxides in B horizon; little O horizon; poor agriculture soils [tropical rainforests]


Water Infiltration Good Medium Poor Medium

Waterholding Capacity Poor Medium Good Medium

Nutrientholding Capacity Poor Medium Good Medium


Sand Silt Clay Loam

Good Medium Poor Medium




Soil Classes

Alfisols: well-developed O, A, E, and B horizons; suitable for agriculture if supplemented [forest soils] Aridisols: little vertical structure; thin and unsuitable for sustainable agriculture [drylands and deserts]

Water Transport by Transpiration

Powered by the sun



Plant­Soil­Water Relationships

Productive Soil

Good supply of nutrients and nutrientholding capacity Infiltration, good water-holding capacity, resists evaporative water loss Porous structure for aeration Near-neutral pH Low salt content



The Soil Community


Partly decomposed organic matter High capacity for holding water and nutrients Typically found in O horizon




Formation of Humus

Humus and Development of Soil Structure

Left: loam without humus


Right: loam with humus


Soil Degradation ­ 8.2

Erosion Drylands and desertification Irrigation and salinization

The Results of Removal of Topsoil: Sand and Gravel

Play soil video [14 minutes]



The Importance of Humus to Topsoil

Erosion: Wind or Water

Splash erosion: impact of falling raindrops breaks up the clumpy structure of topsoil Sheet erosion: running water carries off the fine particles on the soil surface Gully erosion: water volume and velocity carries away large quantities of soil, causing gullies (see Fig. 8-14)




Dryland Areas

Cover one-third of Earth's land area Defined by precipitation, not temperature United Nations Convention to Combat Desertification (UNCCD)

Fund projects to reverse land degradation In 2003, $500 million available in grants to fund projects


Formation and expansion of degraded areas of soil and vegetation cover in arid, semiarid, and seasonally dry areas, caused by climatic variations and human activities.



Dry lands and Desertification: Formation of Desert Pavement

Causes of Soil Degradation



Contour Farming and Shelterbelts

A Global View of Soil Degradation





Flood irrigation (see Fig. 8-21) Center-pivot irrigation (see Fig. 7-16)

Can extract as much as 10,000 gallons/minute

Salinization: What It Looks Like

Irrigated lands

67 million acres or one-fifth of all cultivated cropland in the United States 667 million acres worldwide, a 35% increase over the past 30 years

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A process of distilling out dissolved salts in irrigated water and leaving it on the land A form of desertification, since land is rendered useless Worldwide an estimated 3.7 million acres of agricultural land is lost annually to salinization and waterlogging

Conserving the Soil ­ 8.3

Cover the soil Minimal or zero tillage Mulch for nutrients Maximize biomass production Maximize biodiversity What do plants do for soil?

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Sustainable Agriculture

1. 2. 3.

End of Chapter 8 Review questions ­ write answers in notebook which will be graded on test day Review questions (p. 230): 1, 2, 6, 7, 9, 12, 13, 14 Thinking environmentally (p. 231): 3


Maintain productive topsoil Keep food safe and wholesome Reduce chemical fertilizers and pesticides Keep farms economically viable




Chapter 8 Exam Study Guide [can use 3x5 card during test]

Definitions: leaching, transpiration, infiltration, humus, composting, desertification, topsoil relative sizes of soil particles Improvements of adding humus Harm of removing plants from soil Objectives of sustainable agriculture Nutrient availability in tropical rainforests Cause of salinization Causes of construction site erosion Soil characteristics impact nutrient cycles Methods for maintaining soil productivity Original source of soil nutrients Know the impact of soil texture upon infiltration, aeration, nutrient availability, etc. Understand where biomass/nutrients stored in tropical rainforests Definitions: soil fertility, fertilizer, weathering, erosion, clay, sand, loam, mineralization Best textures for agricultural soil Benefits of grass/mulch on top of soil What comprises soil texture Methods for controlling erosion Causes for decrease in productive farmland Sources of sediments Soil pH Soil horizons Results of heavy irrigation of poorly drained soils Impact of overwatering on oxygen levels Release of nutrients from detritus Problems of sediment added to surface waters EXTRA CREDIT ESSAY: understand how water reaches top of tall trees (in detail)



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