homework encyclopedia

48 Earth

Soil is one of Earth’s most precious resources. It provides the support and

nourishment that plants need in order to grow. In turn, plants provide food

for animals and people. As well as rock fragments,. SOIL LAYERS contain

air and water, and plant and animal remains.

HOW DO LIVING THINGS HELP THE SOIL?

Living things play an important role in helping to

recycle nutrients (nourishing minerals) that enrich the

soil. When plants and animals die, their remains are

broken down by scavenging creatures, such as beetles,

microscopic bacteria, and fungi. This releases minerals

into the soil. The minerals fertilize plants so they can

grow, and so the cycle of life begins again.

HOW MANY SOIL LAYERS ARE THERE?

A layer of dark, fertile humus made of rotting plants

lies at the soil’s surface. Underneath, the topsoil

contains plant roots, and plant and animal remains

that bacteria and fungi are helping to rot down. The

subsoil contains fewer plant and animal remains but

has plenty of minerals washed down from the layers

above. Below are rock fragments, then solid bedrock.

HOW IS SOIL FORMED?

Soil is formed as rock is broken up by ice, frost, wind,

and water. Plants take root among the rock fragments

and bind them together. When plants die, they fertilize

the soil. Soil takes many years to form but it can be

destroyed very quickly by bad farming methods, such

as deforestation (clearing the land of trees).

ARE THERE DIFFERENT TYPES OF SOIL?

There are three main types of soil – clay,

sandy, and loamy. There are also other types

of soil, depending on the type of underlying

rock, and the climate and vegetation. Loams

are a mixture of clay, sand, and silt, and are

more fertile than other soils.

Soil scientists divide the soil into layers

from the surface down to the underlying

bedrock. This is called a soil profile.

The layers in a soil profile are known

as horizons. The depth of each horizon

varies among different types of soil.

4 BUSY BURROWERS

Earthworms and other burrowing animals

such as moles, mice, and rabbits make

their homes underground, in the topsoil.

Their tunnels allow air and water to enter

the soil, which helps to enrich it. Up to

one million earthworms live in just 1 sq

km (0.4 sq miles) of soil.

4 LIFE IN THE SOIL

A small patch of soil just 1 sq m

(1 sq yd) holds up to a billion

living things. These include insects,

spiders, worms, centipedes, mites,

fungi, and tens of thousands of

bacteria (shown magnified).

Soil

SOIL LAYERS

1 CLAY

Clay soils are usually

sticky and waterlogged.

4 BORDER ZONE

Soil forms a vital zone above rock

in which living things can grow.

1 PEAT

Acidic peaty soils are made

up of rotting plants.

4 SAND

Sandy soils are

loose and dry.

Topsoil

(A horizon)

often rich in

humus and

minerals

4 SOIL PROFILE

This slice of soil

from surface to

bedrock shows

its five layers.

Weathered

rock fragments

(C horizon)

little or no plant

or animal life

1 CHALK

Chalky soils are thin

and dry.

Subsoil

(B horizon)

poor in humus,

rich in minerals

Humus Decomposing

leaf

Wildflowers

Bedrock

(D horizon)

Grass Roots

Humus

FIND OUT MORE. Erosion 55 • Micro-organisms 85 • Plants 88–89 • Rocks 46–47

soil