Workbook pages 92 thru 97. Be sure to read before you highlight. Also, complete the assignment on page 97.

●2 The Nature of Soil

Before You Read

Think of the plants that grow where you live. Do you think

your area would be a good place to grow vegetables? Explain

why or why not on the lines below.

Formation of Soil

Soil science is called pedology. What is soil? Where does

it come from? As you read in the last section, weathering

slowly breaks rocks into smaller and smaller fragments.

A layer of weathered rock and mineral fragments covers

Earth’s surface. But these fragments don’t become good

quality soil until plants and animals live in them. Plants and

animals add organic matter, the remains of once living

organisms. Organic matter can include leaves, twigs, roots,

and dead worms and insects. Soil is a mixture of weathered

rock, decayed organic matter, mineral fragments, water,

and air.

What factors affect soil formation?

Soil can take thousands of years to form. In some places

soil is 60 m thick, but in other places it is only a few

centimeters thick. Five factors—climate, slope of the land,

types of rock, types of plants, and the amount of time that

rock has been weathering—affect soil formation. For

example, different types of soil develop in tropical areas

than in polar regions. Soils that form on steep slopes are

different from soils that develop on flat land.

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92 Weathering and Soil

How does soil form?

Over time, soil can form from rock. Natural acids in

rainwater begin to weather the surface of rock. Water seeps

into cracks in the rock and freezes, causing the rock to

break apart. Then, plants start to root in the cracks. As the

roots of the plants grow, they continue breaking down the

rock. A thin layer of soil begins to form. Organisms like

grubs and worms live among the plant roots, adding organic

matter to the soil. As organic matter increases, the layer of

soil gets thicker. Over time, a rich layer of soil forms that

can support trees and plants with larger roots.

Composition of Soil

Soil is made up of rock and mineral fragments, organic

matter, air, and water. The rock and mineral fragments

come from rocks that have been weathered. Most of the

pieces of rock and mineral are small particles of sediment

such as sand, silt, and clay.

Where does organic matter come from?

Most of the organic matter in soil comes from plants.

Animals add organic matter to soil when they die. After

plant and animal material gets into the soil, bacteria and

fungi help it decay. The decayed organic matter turns into a

rich, dark-colored material in soil called humus

(HYEW mus). Humus is a source of nutrients for plants.

Animals are important in making good soil. As worms,

insects, and rodents, like mice, burrow in the soil, they mix

the humus with the fragments of rock. Good soil is made up

of equal amounts of humus and weathered rock material.

There are many small spaces between the sediment and

humus particles in soil that are filled with water or air. The

spaces in moist soil hold water that plants need to grow.

During a drought, the spaces in the soil are filled with air.

When water soaks into the ground, it moves into the pores.

Soil Profile

If you’ve ever seen a deep hole, you’ve probably seen

different layers of soil. Most plant roots grow in the top

layer of soil. The top layer is usually darker than the layers

below it. The different layers of soil are called horizons. All

the horizons of a soil form a soil profile. Most soils have

three horizons that are referred to as A, B, and C horizons.

What is found in the A horizon?

The top layer of soil is called the A horizon. In a forest, the

A horizon might be covered with organic plant litter. Litter

consists of twigs, leaves, and other organic matter that will

become humus. Litter traps water and keeps the A horizon

moist. Litter also prevents erosion. When litter decays and

turns into humus, it provides nutrients for plant growth.

Another name for the A horizon is topsoil. Topsoil has more

humus and fewer rock and mineral fragments than other layers.

The A horizon is usually dark-colored and rich, or fertile.

What is found in the B horizon?

The layer below the A horizon is the B horizon, also

known as subsoil. The B horizon is lighter in color than the

A horizon because it contains less humus. As a result, the

B horizon is not as fertile. Sometimes material from the

A horizon moves down into the B horizon through a

process called leaching.

Leaching is the carrying away of minerals that have been

dissolved in water. Water seeps through the A horizon. The

water reacts with humus and carbon dioxide to form acid.

This acid dissolves some of the minerals in the A horizon.

The acid seeps deeper into the soil, carrying the minerals

into the B horizon. The figure below shows how leaching

occurring in forest soil.

94 Weathering and Soil

What is found in the C horizon?

The C horizon is the bottom horizon in a soil profile. It is

often the thickest soil horizon. The C horizon contains partly

weathered rock and little organic matter from plants and

animals. Leaching generally does not affect the C horizon.

At the bottom of the C horizon lies rock—the rock that

formed the soil above it. This rock is called the parent

material of the soil. Because the C horizon is nearest to the

parent material, it is most like the parent material.

What is soil structure?

Soil structures can be granular, platy, blocky, or prismatic.

Granular structures are common in surface soils with high

organic content that glues minerals together. Platy structures

are often found in subsurface soils that have been leached or

compacted by animals or machinery. Blocky structures are

common in soils with high clay content. Prismatic structures,

found in B horizons, are dense and difficult for plant roots

to penetrate.

Soil Types

If you travel around the country and look at soils, you

will notice that they are not all the same. Some soils are

thick and red. Others soils are brown and rocky. The figure

below shows some of the different soil types in various areas

in the United States.

How does climate affect soil?

Different regions of Earth have different climates.

Different climates produce different types of soil. Deserts

are dry and have few plants. Because desert soil contains

little organic matter, desert soil is thin and light. On the

grasslands of the prairie, the soil contains plenty of organic

matter from grasses. Lots of organic matter gives prairie

soils thick, dark A horizons. Other regions, such as

temperate forests and tropical areas, have their own

particular types of soil.

What other factors affect soil type?

Parent material affects the kind of soil that develops

from it. Clay soils often form on parent rocks like basalt.

The minerals in basalt weather to form clay. If the parent

material is sandstone, it weathers into sand, producing

sandy soil.

Plants Rock type also affects the types of plants that grow

in an area. Different rocks provide different nutrients that

plants need to grow. Soil pH controls many chemical and

biological activities in soil.

Time

Soil development is affected by time. If rock has been

weathering for a short time, the sediment will be similar to

the parent material. If rock has been weathering for a long

time, the soil is less like the parent material.

Slope

The slope of the land is a factor in soil profiles. On

steep slopes, soils are poorly developed because sediment

does not have time to build up before it is carried downhill.

In bottomlands, sediment and water are plentiful. As shown

in the figure below, the soils of bottomlands and valleys are

usually thick, dark, and full of organic material.

96 Weathering