Name: ______________________________________________ Period #: ______ Date: __________________
Notes: Soil Formation and Composition
Soil Formation
SOIL Horizons
Many organisms rely on soil for water, nutrients, shelter, or support. Soil
is the loose material that covers the ground in most places. Most soil is
made up of a combination of sediment, water, air, and the decayed
remains of organisms. However, soils are not all the same. They can
have different compositions and can form in different ways. A soil
profile is broken down into horizons. The thicknesses of these horizons
can vary from place to place.
The horizon just below the Earth’s surface is called the topsoil or the
“A” Horizon. The topsoil is usually a fairly thick layer of dark, finegrained, soft soil. Topsoil contains plant roots and many living
organisms. The topsoil also contains a component of soft, spongy, darkcolored material called humus. Humus is the decayed remains of
organisms. Humus contains most of the nutrients in the topsoil. It also
helps the topsoil hold water. This is the most fertile layer of the soil. A
fertile soil contains the nutrients which help plants grow.
The horizon below the topsoil is called the subsoil or the “B”
Horizon. Subsoil layers are very thick and are composed of coarsergrains. These layers are described as nutrient-poor and can be a light
brown, gray, or reddish color. Very few organisms live in these layers.
If you were to dig down below the subsoil, you would find partially
weathered bedrock or the “C” Horizon and eventually solid bedrock.
This bedrock is the parent material from which the soil forms.
Soil Texture
The texture of a soil is the relative amounts of gravel, sand, silt, and clay.
Different soils have different ratios of these sediment types. Some soils
contain mostly clay. Others contain mostly sand. A soil that contains
about equal amounts of clay, silt, and sand is called a loam. The texture
of a soil determines its porosity and permeability. The porosity of a soil
is a measure of the amount of space between the solid particles in the soil.
The permeability of a soil is a measure of how easily and quickly water
moves through it. Soils made up mostly of clay have a high porosity and
low permeability. Therefore, these soils hold a lot of water. Soils made
up mostly of sand have a low porosity and hold very little water. In
other words, gravely and sandy soils drain better than soils that contain
a lot of clay.
Permeability & porosity
The Formation of Different Soil Types and Their Characteristics
Soil Type
Tundra Soils
{Tundra Biome}
Northern Forest Soils
{Temperate Forest Biome}
*Example: Pennsylvania
Prairie Soils
{Temperate Grasslands Biome}
{Example: Midwestern U.S.}
Mountain Soils
Southern Forest Soils
{Taiga Biome}
Desert Soils
{Desert Biome}
Tropical Soils
{Tropical Rain Forest Biome}
Soil Formation and Characteristics
Tundra biomes tend to have relatively thin soils. Because the tundra is so
cold, most of the soil remains frozen most of the year. In the tundra, rocks
weather very slowly, so little humus accumulates. For these reasons, tundra
soils are fairly thin, rocky and infertile.
*NOT SUITABLE FOR AGRICULTURAL
Temperate forests tend to have fairly thick, fertile soil because the trees and
other organisms produce a great deal of humus.
*IDEAL FOR AGRICULTURE
Temperate grasslands tend to have very thick, fertile soils because the
grasses and organisms produce a great deal of humus.
*IDEAL FOR AGRICULTURE
Topsoil is often thin because cold temperatures slow chemical weathering
and erosion causes soil loss.
*NOT SUITABLE FOR AGRICULTURAL
Soils in taiga biomes are more fertile than tundra soils because the taiga has
a warmer climate. However, most of the plants that live in the taiga are
evergreen trees. The needles of these trees form much of the humus in taiga
soils. These needles are very acidic, so the soils in the taiga also tend to be
acidic.
*NOT SUITABLE FOR AGRICULTURE
Desert biomes tend to have thin, sandy, less fertile soil. Because deserts are
so hot and dry, few organisms live there and it is difficult for plants to grow.
For these reasons, little humus forms in desert soils making them less fertile.
*NOT SUITABLE FOR AGRICULTURE
Some of the thickest soils are found in tropical rain forests. Because it is so
warm and wet there, rocks weather very rapidly, and soils can be several
meters thick. Large numbers of plants and animals live in these forests as
well, so a great deal of humus forms. However, because so many plants
grow in the rain forest, most of the nutrients in the humus are used up as
soon as they become available. In addition, large amounts of rainfall wash
many nutrients out of the soil. Therefore, soils in tropical rain forests tend to
be very thick but not very fertile.
*SOMEWHAT SUITABLE FOR AGRICULTURE
Additional Information:
Soils Containing Loess
*Not Found on Map
Wind can carry and deposit large amounts of fine-grained sediment in certain
areas. These windblown sediments are called loess. In some places, loess is
the main sediment that forms soil. These soils can be very thick and fertile.
*IDEAL FOR AGRICULTURE