chapter 13

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Enger & Smith

Environmental ScienceA Study of Interrelationships

Twelfth Edition

Chapter 13

Soil and Its Uses

Soil and Its Uses

Outline

Geologic Processes Soil and Land Soil Formation Soil Properties Soil Profile Soil Erosion Soil Conservation Practices Conventional Versus Conservation Tillage Protecting Soil on Nonfarm Land

Geologic Processes

Structure of the Earth

Layered Earth

Geologic Processes

The crust of the Earth is an extremely thin, less-dense solid covering over the mantle.

The mantle makes up the majority of the Earth, and surrounds a small core of iron.

The outermost portion of the mantle is solid. The crust and solid outer mantle are collectively

known as the lithosphere. The asthenosphere is a thin layer below the outer

mantle capable of plastic flow.

Plate Tectonics

Fig. 2-32

Geologic Processes

Tectonic plates

Geologic Processes

Tectonic plates

Plate tectonics

Fig. 2.5

Geologic Processes

Plate tectonics is the concept that the outer surface of the Earth is made of large plates of crust and outer mantle that are slowly moving over the surface of the liquid outer mantle.• Heat from the Earth causes the slow movement.• Plates are pulling apart in some areas, and colliding in

others.• These building processes are counteracted by

processes tending to make elevated surfaces lower.

Geological Processes

Weathering processes are important in reducing the size of particles that can then be dislodged by moving water and air.

Mechanical weathering results from physical forces that reduce the size of rock particles without changing the chemical nature of the rock.

– Freezing and thawing cycles– Actions of plants and animals

Geologic Processes

Physical fragmentation by freezing and thawing

Geologic Processes

Chemical weathering involves the chemical alteration of rock in such a manner that it is more likely to fragment or be dissolved.• Rock fragments exposed to the atmosphere may oxidize

or hydrolyze. The process of loosening and redistributing

particles is called erosion.

Geologic Processes

An eroded landscape

Soil and Land

Land is the portion of world not covered by water.

Soil is a mixture of minerals, organic material, living organisms, air, and water that together support growth of plant life.• Good agricultural soil:

– 45% Mineral– 25% Air– 25% Water– 5% Organic Matter

Soil and Land

The components of soil

Soil Formation

Soil formation begins with fragmentation of parent material.• Parent material consists of ancient layers of rock, or

more recent deposits from lava flows or glacial activity.

The first organisms to gain a foothold in modified parent material also contribute to soil formation.• Lichens form pioneer communities.• Decomposition of dead lichens further alters

underlying rock.

Soil Formation

Humus is the organic material resulting from the decay of plant and animal remains.• It mixes with top layers of mineral particles, and

supplies needed nutrients to plants.• It creates a crumbly soil that allows adequate water

absorption and drainage. Burrowing animals such as earthworms bring

nutrients up from deeper soil layers, improving soil fertility.

Soil Formation

Other factors influencing soil formation include:• Plant roots• Bacteria and fungi (decomposers)• Position on slope• Climate• Time• Rainfall• Soil pH

Soil Properties

Soil texture is determined by the size of mineral particles within the soil.• Too many large particles (sand, gravel) lead to extreme

leaching.• Too many small particles (clay) lead to poor drainage.

Soil Properties

Soil texture

Soil Properties

Soil structure refers to the way various soil particles clump together.• An ideal soil for agricultural use is loam, which

combines the good aeration and drainage properties of large particles with the nutrient retention and water-holding ability of clay particles.

• In good soils, one-half to two-thirds of spaces contain air after excess water has drained.

• A good soil is friable, which means that it crumbles easily.

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

Soil Profile

The soil profile is 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.

Soil Profile

O horizon is made of litter, undecomposed or partially decomposed organic material.

A horizon is the topsoil, or the uppermost layer. It contains most of the soil nutrients and living organisms.

E horizon is formed from leaching darker materials.• Usually very nutrient poor.

Soil Profile

B horizon is the subsoil. It contains less organic matter and fewer organisms, but accumulates nutrients leached from topsoil. It is poorly developed in dry areas.

C horizon is weathered parent material. R horizon is bedrock.

Soil Profile

Soil Profile

Over 15,000 separate soil types have been classified in North America.

Most cultivated land can be classified as either grassland or forest soil.• Grassland soils usually have a deep topsoil layer. A

lack of leaching results in a thin layer of subsoil. • In forest soils, which are typically high rainfall areas,

the topsoil layer is relatively thin, but topsoil leachate forms a subsoil that supports substantial root growth.

Soil Profile

Two features of tropical rainforests have great influence over the nature of the soil:• High temperatures lead to rapid decomposition of organic

matter, with little litter.• High rainfall leads to excessive leaching of nutrients.

Soil Profile

Major soil types

Soil Erosion

Erosion is the wearing away and transportation of soil by wind, water, or ice.

Worldwide, erosion removes 25.4 billion metric tons of soil per year.• Made worse by deforestation and desertification.• Poor agricultural practices increase erosion and lead

to the transport of associated fertilizers and pesticides.

Soil Erosion

Most current agricultural practices lose soil faster than it can be replenished.

Wind erosion may not be as evident as water erosion, but is still serious.• It is 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.

Soil Erosion

Worldwide soil erosion

Soil Conservation Practices

When topsoil is lost, fertility is reduced or destroyed, thus fertilizers must be used to restore fertility.• This practice 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.

Soil Conservation Practices

Agricultural Potential• Worldwide:

– 11% of land surface is suitable for crops.– An additional 24% is 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.

Soil Conservation Practices

Soil Quality Management Components:• Enhance organic matter.• Avoid excessive tillage.• Manage pests and nutrients efficiently.• Prevent soil compaction.• Keep the ground covered.• Diversify cropping systems.

Soil Conservation Practices

Contour farming is tilling at right angles to the slope of the land. Each ridge acts as a small dam.• Useful on gentle slopes.• One of the simplest methods for preventing soil

erosion. Strip farming is the practice of alternating strips

of closely sown crops to slow water flow, and increase water absorption.

Soil Conservation Practices

Contour farming

Soil Conservation Practices

Strip farming

Soil Conservation Practices

Terracing is the practice of constructing level areas at right angles to the slope to retain water.• Good for very

steep land.

Terraces

Soil Conservation Practices

Waterways are depressions in sloping land where water collects and flows off the land.• Channels movement of water.

Windbreaks are plantings of trees or other plants that protect bare soil from full force of the wind.• Windbreaks reduce wind velocity, decreasing the

amount of soil that can be carried.

Soil Conservation Practices

A well-maintained, uncultivated waterway

Soil Conservation Practices

Windbreaks

Conventional vs. Conservation Tillage

Plowing has multiple desirable effects:• Weeds and weed seeds are buried.• Crop residue is turned under, where it will contribute to

soil structure.• Leached nutrients brought to surface.• Cooler, darker soil brought to top and warmed.

Conventional vs. Conservation Tillage

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 is a practice that uses less cultivation to control weeds and prepare soil, but generally leaves 15-30% of soil surface covered with crop residue after planting.

Conventional vs. Conservation Tillage

Conservation tillage further reduces amount of soil 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 that will receive

seeds.• Ridge tillage: Leaves ridges; the crop is planted on

the ridge with residue left between ridges.• No till farming: Involves special planters that place

seeds in slits cut in the soil.

Conventional vs. Conservation Tillage

Positive Effects of Reduced Tillage:• Wildlife gain winter food and cover.• Less runoff results in reduced siltation of waterways.• Row crops can be planted in sloped areas.• Fewer trips over the field means lower fuel

consumption.• Two crops may be grown on a field in areas that had

been restricted to a single crop. • Fewer trips over the soil means less soil compaction.

Conventional vs. Conservation Tillage

Drawbacks of Conservation Tillage• Plant residue may delay soil warming.• Crop residue reduces evaporation and upward

movement of water through the soil, which may retard the growth of plants.

• Accumulation of plant residue can harbor plant pests and diseases, requiring more insecticides and fungicides.

Protecting Soil on Nonfarm 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.

Summary

The surface of the Earth is in constant flux. The movement of tectonic plates results in the

formation of new land as old land is worn down by erosive activity.

Soil is an organized mixture of minerals, organic material, living organisms, air, and water.

Organisms affect soil building by burrowing into and mixing the soil, releasing nutrients, and decomposing.

Summary

The ability of soil to grow crops is determined by the inorganic matter, organic matter, water, and air spaces in the soil.

A soil profile typically consists of the: • O horizon of litter• A horizon, which is rich in organic matter• E horizon, from which materials have been leached• B horizon, which accumulates materials leached from

above• C horizon, which consists of slightly altered parent

material.

Summary

Soil erosion is the removal and transportation of soil by water or wind.

Proper use of conservation practices such as contour farming, strip farming, terracing, waterways, windbreaks, and conservation tillage can reduce soil erosion.

Land unsuitable for crops may be used for grazing, lumber, wildlife habitats, or recreation.