Soils, Chapter 10Soils, Chapter 10

© Brooks/Cole Publishing Company / ITP

Chapter Overview Questions

What is soil? How is soil formed? What are the characteristics of soil? What is the importance of soil as a

resource?

SOIL: A RENEWABLE RESOURCE

Soil is a slowly renewed resource that provides most of the nutrients needed for plant growth and also helps purify water.– Soil formation begins when bedrock is

broken down by physical, chemical and biological processes called weathering.

Mature soils, or soils that have developed over a long time are arranged in a series of horizontal layers called soil horizons.

Weathering- when rocks are exposed to air, water, certain chemicals or biological agents that degrade the rock.

Physical weathering- the mechanical breakdown of rocks and minerals.

Weathering

Water gets into cracks in rock, where it expands when it freezes. Growing plant roots can pry rock sections apart.

Chemical weathering- the breakdown of rocks and minerals by chemical reactions.

Rain water contains carbonic acid, which wears away limestone

Weathering

Erosion- the physical removal of rock fragments from a landscape or ecosystem. Wind, water, ice transport and living organisms can erode materials.

Deposition- the accumulation or depositing of eroded material such as sediment, rock fragments or soil.

Erosion

Soft rocks, such as sedimentary rocks, erode more easily than metamorphic and igneous rocks.

Soil Resources

Soil- weathered rock mixed with organic material (humus), mineral nutrients, microorganisms, water and air

Soil is important because itIs a medium for plant growthServes as a filter for waterIs a habitat for living organismsServes as a filter for pollutants

Ecosystem Services of Soil

Factors That Determine the Formation of SoilParent material- what the soil is made

from influences soil formationClimate- what type of climate influences

soil formationTopography- the surface and slope can

influence soil formationOrganisms- plants and animals can have

an effect on soil formationTime- the amount of time a soil has spent

developing can determine soil properties.

Fig. 3-23, p. 68

Fern

Mature soil

Honey fungus

Root system

Oak tree

Bacteria

Lords and ladies

Fungus

Actinomycetes

Nematode

Pseudoscorpion

Mite

RegolithYoung soil

Immature soil

Bedrock

Rockfragments

Moss and lichen

Organic debrisbuilds upGrasses and

small shrubs

Mole

Dog violet

Woodsorrel

EarthwormMillipede

O horizonLeaf litter

A horizon

Topsoil

B horizonSubsoil

C horizon

Parent material

Springtail

Red Earth Mite

Soil Horizons O horizon – surface litter layer. Contains organic

material in various stages of decomposition (leaves, twigs, etc.)

A horizon – top soil. Contains humus and mineral particles. Where overlying organic material mixes with underlying inorganic material

B horizon – subsoil. Zone of accumulation of metals and nutrients

C horizon – least weathered portion of profile, same composition as parent material

Bedrock or parent material – the rock material from which soil is derived.

As soils form, they develop characteristics layers.

Soil Horizons

Layers in Mature Soils Infiltration- the downward movement of

water through soil. Leaching- dissolving of minerals and

organic matter in upper layers carrying them to lower layers.

The soil type determines the degree of infiltration and leaching.

Animation: Soil Profile

PLAYANIMATION

Soil Profiles of the Principal

Terrestrial Soil Types

Figure 3-24Figure 3-24

Texture- the percentage of sand, silt and clay the soil contains.

Soils vary in the size of the particles they contain, the amount of space between these particles, and how rapidly water flows through them.

Physical Properties of Soil

Soil Triangle

Soil texture can be determined using the soil triangle

Physical Properties of Soil

Soil texture determines porosity, permeability and structure of soil– Porosity- measure of volume of pores per

volume of soil– Permeability- rate at which water and air

move from upper to lower soil layers– Structure- way in which soil particles are

organized and clumped together

Physical Properties of Soil

Porosity- the ratio of the volume of pore space in a unit of material to the total volume of material.

Which has the highest porosity?

Permeability- how quickly the soil drains (which depends on its texture)

Physical Properties of Soil

Sand has large, loosely-packed particles and drains quickly. Clay, with its platy particles, drains much more slowly.

Chemical Properties of Soil

• pH- measure of how acidic or basic a substance is. It determines plants’ ability to take up nutrients from soil

• Cation exchange capacity- the ability of a soil to adsorb and release cations (positively charged mineral ions)

• Soil bases- calcium, magnesium, potassium and sodium

• Soil Acids- aluminum and hydrogen

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Biological Properties of Soil

Many organisms are found in the soil including fungi, bacteria, protozoans, rodents and earthworms.

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Soil type affects vegetation: trees are growing on

soil formed from sandstone that

holds water

Differences in rocks, soil and microclimates determine the plants that live on two sides of the same mountain.

Soil Degradation

Soil degradation- loss of some or all of soil’s ability to support plant growth

Causes:– Soil erosion– Soil compaction (machines, foot traffic or

overgrazing)– Salinization- excess salts added to soil through

incorrect irrigation practices– Desertification- transformation of arable land into

desert or unproductive land

Review Questions How do soils form? What are the roles of soils in

ecosystems? How do a soil’s physical and chemical

properties influence its role as a medium for plant growth?