Regal Dirt
By Trista L. Pollard
Have you ever wondered where soil and dirt come
from? Maybe you thought it just appeared under your feet
or in your parents' garden magically everyday. Just as
weathering breaks apart rock, it also gives birth to our soil.
Without it, many of the plants and animals (including us)
on our planet would not survive.
1
Soil has very distinctive characteristics that are
influenced by its parent rock. How this parent rock is weathered helps to
determine the characteristics of that soil. Before we get into the "dirt" about soil,
there are some layers that you should know about. Regolith is the layer of rock
fragments that is produced from weathering. These fragments cover most of our
land surface. Bedrock lies underneath regolith, and it is solid unweathered rock.
Soil is located in the uppermost region of regolith as very fine particles that were
weathered from rock fragments. The lower regions of regolith tend to weather
more slowly because they are protected by the upper layers of regolith. Soil
contains a complex mixture of materials like minerals, water, gases, and remains of
dead organisms.
2
The weathering of the parent rock can produce two types of soil. Once residual
soil is formed, it remains directly over its parent rock. Keep in mind that wind,
water, and glaciers may carry this soil away from its parent rock. The deposition of
this material produces transported soil. Transported soil, since it traveled from
another location, may have different characteristics than the bedrock beneath it.
Clay soils are usually produced by parent rocks made of feldspar or aluminum
based minerals. Sandy soils are produced by quartz bearing parent rocks that go
through weathering. The minerals and other compounds found in the parent rock
make up the materials or composition of the soil. These same materials also
produce different soil colors. Organic materials that enter the soil usually produce
black soils. Parent rocks that contain large amounts of iron form red soils. The
amount of water within the soil also affects its color.
3
Another characteristic of soil is its texture. There are three main types of soil
particles. The diameter of clay particles measures less than 0.004 millimeters
(mm). The diameter of silt particles measures from 0.004 mm to 0.06 mm. Silt
tends to be gritty. The last type of soil particle is sand. Its diameter ranges from
0.06mm to 2 mm. The composition of the parent rock determines the proportion of
the clay, sand, and silt in the soil.
4
Geologists, when studying soil, try to develop soil profiles. Soil profiles are
vertical cross sections of soils that show the layers or horizons. Soil contains three
main horizons. The A horizon, also known as topsoil, contains organic material
and small rock particles. Soil-dwelling organisms live in this horizon. Once these
organisms die, their remains form humus, a dark organic material. Water from the
surface leaches minerals from this horizon. These leached minerals from the
topsoil, clay, and humus of the A horizon make their way to the B horizon or
subsoil. B horizon soils that are in dry climates tend to have minerals that were
produced by water evaporation. Weathered bedrock is found in the C horizon.
This horizon is where the first stages of chemical and mechanical changes occur.
5
The climate of an area has a huge impact on the composition of soil. Soils that
are found in humid tropical climates tend to be thick. These soils develop rapidly
because of the amount of rainfall and high temperatures. It is the chemical
weathering that produces these thick soils. Laterites are infertile because they
have iron and aluminum minerals that are resistant to dissolving in water. The
heavy rainfall causes a large amount of topsoil leaching and the collection of
minerals in the A horizon. As a result, the A horizon is thin. The humid, warm
tropical weather produces dense vegetation which adds more organic material to
the soil continuously. The B horizon is covered in this area by thin layers of
humus.
6
The soils in temperate climates tend to be thick and fertile with each horizon
measuring several meters thick. Pedalfer and pedocal soils are found in these
climates. Temperate climates tend to have temperatures that range from warm to
cool. Different from tropical climates, the rainfall is not excessive in these areas.
Pedalfer soils are produced in temperate climates that receive more than 65
centimeters (cm) of rainfall each year. They have clay, quartz, and iron compounds
as part of their composition. These soils are also more acidic than pedocal soils.
You will usually find pedalfer soils in the Gulf Coast states and states located east
of the Mississippi River in the United States. Pedocal soils are found in the
southwest United States and states located west of the Mississippi River. Areas that
have pedocal soils receive less than 65 cm of rain each year. This soil is less acidic
and very fertile due to the high amount of calcium carbonate.
7
Quick Speck: What can you infer about the rainfall amounts per year for the
southwestern United States and states located west of the Mississippi River?
8
9
Land topography has an effect on how soil is formed. Along a slope, the soil is
usually thin because running rainwater washes away the topsoil. Geologists in
Canada studied soil composition on flat and sloping land. They found that the A
horizon of soil located on 10 degree slopes was twice as thin as the A horizons
found in flat land. Along the slope, the topsoil tends to be thin and does not
adequately support plants and other vegetation. Take away the vegetation and you
have soil that does not contain humus and is of poor quality. The soils that are
located along the sides of mountains contain very few nutrients because the soil is
thin and rocky. Without vegetation to protect the soil, a large amount is washed
away. This is different from low-lying areas that hold on to water. These areas
have thick soils and an abundance of organic material. It's the organic matter that
produces the humus. The areas that provide the best soils are flat areas that have
good drainage. These areas have residual soils that are thick and fertile. So, that
brown stuff in your backyard beneath the grass contains more information than you
first thought.
Copyright © 2013 edHelper
Name _____________________________
Date ___________________
Regal Dirt
1. How is transported soil different
from residual soil?
3. The first changes of mechanical and
chemical weathering happen in the
______.
A horizon
B horizon
C horizon
A and B horizon
5. Why is the soil on the slope of a hill
thinner than the soil at the top or
bottom of a hill?
7. What are laterites?
2. Why do lower regions of regolith
tend to weather more slowly than
upper regions of regolith?
4. Based on its composition, what can
you conclude about pedalfer soils?
6. Describe the characteristics of each
horizon found in residual soil.
8. True or False: Clay soils are usually
produced by parent rocks that
contain feldspar or aluminum based
minerals.
False
True