Chapter 6

© 2009 Delmar, Cengage Learning

Chapter 6

Land and Soil


Life Requirements

• Life requires the presence of certain resources– water, sunlight, and soil

• Soil is a valuable resource requiring careful management


Land, Soil, and Life

• Soil makes up surface of Earth’s crust– unique blend of organic and inorganic material– allows for exchange of gases, water, minerals– necessary for life


Soil Formation

• Forms extremely slowly– natural forces act on mineral and

rock portions of Earth’s surface– atmosphere, sunlight, water, and

living things• Soils vary in composition

and character– temperature, organic matter, and

amount of air and water


Soil Types

• Types of soils differ with specific sites– shaped by forces of climate, living organisms, parent

soil material, topography, and time


Origins of Soil

• Parent material– materials underlying the soil and from which

the soil was formed


Origins of Soil

• General categories of soil parent materials– minerals and rocks– glacial deposits– loess deposits– alluvial and marine deposits– organic deposits


Minerals and Rocks

• Minerals– solid, inorganic,

chemically uniform, and naturally occurring substances


Minerals and Rocks

• Rocks– aggregates of minerals– three major types

• igneous: cooled molten material

• sedimentary: solidification of sediment

• metamorphic: other rock reformed by heat and pressure

http://geology.com/rocks/conglomerate.shtml


Which is which?


Glacial Deposits

• Massive rivers of slowly moving ice

• Scrape across surface of Earth– scoop up surface material

• When glaciers recede or melt, they leave material behind– great deposits of rocks, parent

materials, and already formed soil materials


Loess Deposits

• Collections of wind-blown silt

• Create soil formations that look like drifting snow

• Often create rich soil deposits– eastern Mississippi Valley– palouse soils of the Pacific

Northwest


Alluvial and Marine Deposits

• Soil deposits of waterborne sediments

• Alluvial deposits– left by moving fresh

water• moving water picks up

sediment• as water slows,

sediment deposited• creates alluvial fans


Alluvial and Marine Deposits

• Marine deposits– Soil formed on

ancient ocean floors• Soil in water drifts to

bottom and collects


Organic Deposits

• Lush plant life exists in swamps and marshes– Plants die and

decay– Partially decayed

material collects• forms muck or

peat soils


Which is it?


Which is it?


Which is it?


Weathering

• Minerals and rocks exposed to the weather break into smaller and smaller pieces


Weathering

• Major weathering forces– temperature changes– water action– plant roots– ice expansion– mechanical grinding


Soil’s Organic Content

• Decaying plant and animal parts– important part of formation and productivity of soil



• Organic matter may be in two basic forms– Original tissue

• portion of organic matter that remains recognizable• twigs and leaves covering a forest floor

– Humus• decomposed and constituents unrecognizable• gives topsoil its brown color



• Serves many important functions– stabilizes soil structure as cementing agent– returns plant nutrients to the soil– helps store soil moisture– makes soil more tillable for farming



– provides food (energy) for soil microorganisms– makes soil porous– provides storehouse for nutrients– minimizes leaching


Soil Profile

• Cross-sectional view of the different layers or horizons in soil

• Undisturbed soil will have four or more horizons in its profile– O horizon: on surface and

composed of organic matter and small amount of mineral matter


Soil Profile

– A horizon: topsoil; located near surface and combination of mineral matter and organic matter

– B horizon: subsoil; similar to A horizon but particle size and proportions differ; less organic material

– C horizon: parent material, large soil particles

– How many layers do you see in this picture?


Properties of Soil

• A variety of physical properties of soil exist– slope: angle of soil surface– texture: proportions of sand, silt, and clay– drainage: how well water flows through soil– flood hazard: likelihood area will flood– erosion: degree soil is damaged by erosion– topsoil and subsoil thickness: layer depths


Soil and Plants

• Soil provides individual plants with four basic needs– anchorage– water– oxygen– nutrients


Soil Uses

• Human societies depend on soil for– cropland– grazing land– urbanization– building– waste disposal– forests– recreation


Human Uses of Soil

• Cropland– land where crops are planted, cared for, and harvested– Two basic types of crops: annual and perennial


Human Uses of Soil

• Annual crops– planted and harvested within one growing season– requires yearly preparation of soil– prone to erosion– corn, soybeans, cotton, vegetables

• Perennial crops– remain in ground several years– alfalfa, fruits, nuts, and nursery stock


Human Uses of Soil

• Grazing land– used to feed grazing animals such as cattle and sheep– eastern United States uses pastures planted with

perennial forage– western United States uses rangeland of native grasses


Human Uses of Soil

• Urbanization– land used to build cities, towns, factories, and roads– continues to grow at expense of other land uses


Human Uses of Soil

• Buildings– land used to prepare foundations for construction

• Several properties of soil are considered, including– shrink-swell potential– load-bearing capacity


Human Uses of Soil

• Shrink-swell potential– how much soil swells when wet and shrinks when

dry• Load-bearing capacity

– ability of soil to support great weight


Human Uses of Soil

• Waste disposal– using soil to filter, contain, or confine waste– waste disposal takes a variety of forms

• sanitary landfills• hazardous waste landfills• personal home septic systems


Human Uses of Soil

• Forests– soil often left undisturbed– harvesting of trees can damage soil

• tear up vegetation• compact soil

– may result in increased erosion or limited growth potential


Human Uses of Soil

• Recreation– also takes a variety of forms

• golf courses, playing fields, parks, campgrounds– may require special construction or management


A Living Environment

• Soil is living medium containing great variety of organisms– amphibians, mammals, insects, reptiles, worms,

microbes, etc.• Most diverse ecosystem on Earth• Many organisms in soil are decomposers


A Living Environment

• Decomposers– break down material that was once living– decomposers play irreplaceable role


Erosion

• Natural process forming channels and canyons– However, human activity is accelerating the process


Erosion

• Major environmental issue– Each year, almost 2 billion tons of soil lost

• mostly result of water erosion– Approximately 21 percent of U.S. cropland suffers

excessive soil losses– We must conserve soil resources by reducing or

eliminating erosion losses


Erosion by Water

• Water erosion occurs in three steps– detachment– transport– deposition


Erosion by Water

• Factors affecting rate of erosion– soil texture and structure– organic matter content– slope– soil cover– plant cover


Erosion by Wind

• Accounts for 40 percent of soil loss in United States

• Dry areas with high winds are most prone to wind erosion

• Bare soil is at greatest risk


Erosion by Wind

• Effects of wind erosion– Removes topsoil– reduces nutrient- and water-holding capacities– “sandblasts” young plants– fills in road or drainage ditches– affects respiratory health of humans and other animals– wears down paint and other surfaces


Preventing and Controlling Erosion

• Two key practices in controlling erosion– protecting soil surfaces– slowing movement of water and wind


Preventing and Controlling Erosion

• Soil conservation must be practiced to preserve our soil resources– Implementation of best management practices must

become a priority– A wide variety of management practices exist to help

control erosion

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Chapter 6