as rock. It has many small spaces, called
pores, that hold water and air. In some
places on Earth, a thin layer of soil only
6 inches (15 centimeters) thick lies on
top of rock. In other places, though, soil
may be hundreds of feet deep.
The Importance of Soil
Without soil, most life on Earth could
not survive. Soil provides a place for
plants to grow. It holds water in place
for their roots. It contains nutrients, or
food substances, needed for their
growth. Soil also provides a home for
many animals and other living things.
Soil forms slowly. A layer of soil that is 1
inch (2.5 centimeters) thick may take
500 to 1,000 years to form. For this
reason it is important to protect soil
from erosion—being blown or washed
away.
How Soil Forms
The mineral part of soil forms from
rocks. Such forces as wind, water, and
Dark layers of soil lie near the ground’s
surface in the U.S. state of Montana.
Beneath them is glacial till—a mixture of
clay, sand, and rocks that a glacier left
behind when it melted away.
A player pitches the ball during
a women’s softball game at the
Summer Olympics. Unlike in
baseball, softball pitchers throw
the ball with an underhand
motion.
124 Soil BRITANNICA STUDENT ENCYCLOPEDIA
temperature changes break rocks down
into tiny particles, or bits. The smallest
particles are called clay. Medium-sized
particles are called silt. The largest particles
are called sand. Different types of
soil contain different mixtures of clay,
silt, and sand.
The organic part of soil forms when
plants and animals die. Their bodies
decay, or break down, and mix with the
rock particles. The organic material,
called humus, helps to form the pores in
the soil. It keeps the soil soft and loose.
Humus also provides the nutrients used
by plants.
Layers of Soil
Soil is made up of layers. The layers rest
on solid rock, called bedrock. A layer of
broken rock rests on the bedrock. Some
of this rock may have gone into forming
the soil above. The soil above the broken
rock is called subsoil. Subsoil contains
mostly minerals and a small amount of
humus. Only the deepest plant roots
reach the subsoil. The top layer is called
topsoil. Topsoil contains a lot of humus.
It is the layer where plants grow.
#More to explore
Clay • Erosion • Mineral • Plant • Rock
• Sand
Solar Energy
Solar energy is light, heat, and other
forms of energy given off by the sun.
Solar energy can be collected and used
to heat buildings and to make electricity.
Solar Heating
Most solar heating systems capture solar
energy with a device called a flat-plate
collector. The collector is a large plate
of black metal covered with a sheet of
glass. It is usually placed on the roof of
a building. The plate absorbs sunlight
and uses it to heat air or water that
flows through pipes behind it. The air
or water then goes through the
building’s heating or plumbing system.
It can also be held in a storage tank for
later use.
Making Electricity
Devices called solar furnaces and solar
cells can turn solar energy into electricity.
A solar furnace uses the sun’s heat to
make electricity. It has mirrors that focus
large amounts of solar energy into a
small area. A solar furnace can produce
temperatures of up to 3,630° F (2,000°
C). This heat can be used to make
steam. The steam can be used to make
electricity in a power plant.
Solar cells use the sun’s light rather than
its heat. When the sun shines on a solar
The roof of a house has flat-plate collectors
that capture solar energy to heat air or water.
BRITANNICA STUDENT ENCYCLOPEDIA Solar Energy 125
cell, the cell turns the light energy into
electricity. A single solar cell makes only
a little electricity. However, groups of
solar cells can provide electricity for
whole buildings. Solar cells are also used
in a variety of products, including calculators,
watches, electronic toys, and portable
radios.
Benefits and Costs
Solar energy has two big benefits over
fossil fuels (coal, oil, and natural gas).
First, though fossil fuels can be used up,
there is an endless supply of sunlight.
Second, solar energy does not cause pollution,
like burning fossil fuels does.
However, the equipment needed for
collecting and using solar energy is
expensive. The high cost of solar energy
has limited its use.
#More to explore
Electricity • Energy • Heat • Sun
Solar System
The solar system consists of the sun and
everything that orbits, or travels around,
the sun. This includes the eight planets
and their moons, dwarf planets, and
countless comets, asteroids, and other
small, icy objects. However, even with
all of these things, most of the solar system
is empty space.
The solar system consists of the planets that orbit the sun as well as smaller bodies such as
dwarf planets, comets, and asteroids.
126 Solar System BRITANNICA STUDENT ENCYCLOPEDIA
The solar system itself is only a small
part of a huge system of stars and other
objects called the MilkyWay galaxy. The
MilkyWay galaxy is just one of billions
of galaxies that in turn make up the universe.
The Sun
At the center of the solar system is a star
called the sun. It is the largest object in
the solar system. The sun is a very hot
