Chapter 8Soil: Foundation for Land Ecosystems

Copyright © 2008 Pearson Prentice Hall, Inc.

8.1 - 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.

Why a Study of Soil Is Important• 90% of the world’s food comes from

land-based agriculture.• Maintenance of soil is the cornerstone

of sustainable civilizations.• Simply stated, it is the “foundation” of

terrestrial life.

Soil Ecosystem: Distinguishing Characteristics – Part 1• Slow rate of nutrient and energy

transfer • Few months: tropical rainforest• Few years: temperate forests

• Different textures demand different adaptations, e.g., worms and pocket gophers

• Near total reliance on decomposers

Soil Ecosystem: Distinguishing Characteristics – Part 2• Exclusive use of detritus for energy and

nutrients• Extreme susceptibility to disturbance

and slow recovery times

Topsoil Formation

Soil Formation and Nutrient Uptake

Soil Formation

Soil Formation(click to view animation)

Soil Profile

Soil Particles

• Sand - largest, can see with naked eye, 2 mm to 0.02 mm

• Silt - medium, 0.02 mm to 0.002 mm• Clay - very fine, smaller than 0.002 mm

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.

Soil Texture Triangle

Soil Formation and Nutrient Uptake

Nutrient Availability

Nutrient Availability(click to view animation)

Water Transport by Transpiration

Plant-soil-water Relationships

Productive Soil

• Good supply of nutrients and nutrient-holding capacity (limits leaching)

• Infiltration, good water-holding capacity, resists evaporative water loss

• Porous structure for aeration (avoid compaction)

• Near-neutral pH• Low salt content

The Soil Community: The 5% Organic Component

Humus• Partly decomposed organic matter• High capacity for holding water and

nutrients• Typically found in O horizon

Humus and Development of Soil Structure

8.2 - The Importance of Humus to Topsoil

GLADSOD

• Global assessment of soil degradation map (1987-1990)

• No global data on soils existed at the time

• Informal surveys (not very accurate)• Very few actual samples collected• GLADSOD estimated that

desertification had occurred on 20% of the land worldwide

• Current estimates closer to 10%

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 (next slide)

Desertification

• 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.

Formation of Desert Pavement

Formation of Desert Pavement

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

Desertification

Three Major Practices That Expose Soil to Erosion

• Overcultivation - planting crops/farming• Overgrazing - from livestock• Deforestation - cleared for farming or

construction

Causes of Soil Degradation

Alternative Farming Practices That Conserve Soil

• No-till planting - kill weeds chemically, then plant and add fertilizer

• Contour farming - right angle to slope• Shelter belts - trees around fields• Dry land farming – minimize irrigation• Crop Rotation – especially legume vs.

non-legume

Contour Farming

Shelterbelts

Irrigation

• Flood irrigation (next slide)• Center-pivot irrigation (Chapter 7)• Can extract as much as 10,000

gallons/minute

• Irrigated lands• 67 million acres or one-fifth of all cultivated

crop land in the United States• 667 million acres worldwide, a 35%

increase over the past 30 years

Flood Irrigation

Salinization

• 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 are lost annually to salinization and waterlogging

8.3 - Public Policy and Soils• Soil and Water Conservation Act of

1977 - aid landowners and users; evaluate U.S. soils, water, and related resources

• Food Security Act (1985) - “Swampbuster” - discouraged conversion of wetlands

• Subsidies• Numerous Farm Bills

Conserving the Soil

• Cover the soil - cover crop• Minimal or zero tillage• Mulch for nutrients• Maximize biomass production• Maximize biodiversity