BACKGROUND INFORMATION
Getting the Dirt on Soil
The lessons in this unit will help you explore soil quality in the Bronx River Watershed
with your students. They are an excellent follow up to the WHAT IS A WATERSHED? lessons,
as they further illustrate the importance of working soils for the water cycle and the overall
urban ecosystem.
Soil’s ability to hold nutrients and filter water contributes to its quality. Students should
consider how soil compaction, lack of biodiversity, and human development including
paving, shore bulkheads, and dumping, can impede infiltration, increase runoff, and affect
soil quality overall.
An overview follows to assist you in understanding the science and soil study procedures
behind the lessons.
TERMS OF INTEREST:
Biodiversity, soil compaction,
infiltration, soil profile, soil horizons,
contamination, anthropogen.
BACKGROUND INFORMATION:
What is soil?
Soil is a mixture of mineral material
(sand, silt, and clay), organic matter,
water, air, and other gases. The
proportions of each of these
components vary from soil to soil and
can change over time. Climate, macro
and microorganisms,
location/topography, and time affect
these individual characteristics. In other words, the type of soil you find in any given area is
affected by everything you see, what kind and shape of rock is deep down below, and how long
these factors have been in play.
When most people think of soil, they think of the mineral material that forms the majority of
soil solids. There are three main types of mineral material. All of these are composed of
different sizes of weathered rock. The largest of these particles is sand. Sand feels gritty, and is
relatively inert. The second largest soil particle is silt. Silt feels smooth when wet and has the
consistency of baking flour when dry. Silt is also relatively inert. The smallest and most
chemically active is clay. Clay feels sticky. Its very fine particles have a high surface area and
also hold an electrical charge. The relative amounts of each of these particles give the soil its
texture. The texture of the soil affects how the soil behaves, what kind of plants can grow in it,
and even the way that buildings are built.
Soil can also have larger pieces or coarse fragments mixed in with the sand, silt, and clay.
These are commonly referred to as gravel, cobbles, stones, or boulders. These coarse fragments
can influence drainage and affect root growth, as well as the storage of water and nutrients.
Coarse fragments in the Bronx are usually gneiss, granite or schist; the same type of rock that
lies beneath the Bronx soil.
There are also organic components of the soil, which contribute and cycle nutrients, help to
aggregate or loosen soil, and hold water. This vibrant underground community is usually
more diverse than the above-ground community! Some common soil organism types are
bacteria, fungi, protozoa, nematodes, and arthropods. In this cycle, these organisms
decompose plant and animal matter and the resulting nutrients are made available to living
plants and animals. The larger organisms, like earthworms, also help by aerating the soil.
Most of these organisms require air to live in the soil. 100 million to 1 billion organisms can
be found per teaspoon of soil.
Air and water are both very important to soil. In fact, about 50% of the volume of a healthy
soil is composed of pore space filled with air and water. All of this relatively empty space
allows for the movement of the plants and animals that live in the soil, as well as root growth
and drainage. Having enough space between the soil particles also affects the way that water
moves through the soil.
Soil Horizons
All of these distinctly different components make
up soil in varying proportions; soil is not a
uniform mixture. If you dig down a few feet into
an undisturbed section of soil, you should be able
to see soil horizons.
The O horizon is the topmost layer, consisting of
organic material such as decaying leaves, grass,
and plants. This layer is usually very dark in color
because the decaying matter forms humus. Many
times, especially if the soil that is being examined
is in a disturbed area, this layer is absent or has
been mixed with the next horizon.
The A horizon is the next horizon, and it is what
most people normally refer to as topsoil. The A
horizon is usually dark in color, because it has
been enriched from the O horizon above. The A
is not as dark as the O horizon. In most soils,
except those that are saturated by water, the A
horizon is where much of the aerobic activity
takes place in the soil.
Soil forms horizons through natural processes.
These horizons are composed of different
combinations of the elements of soil.
The B horizon is called the subsoil. It is usually lighter in color with very little organic
material.
The C horizon is made of partially weathered or broken down
parent material. In some cases, soft bedrock will also show up
in the C horizon.
Under these horizons is the bedrock, which is sometimes
referred to as the R horizon. The bedrock can be located far
below the surface or just a few inches under the surface. Not all
soils have an R horizon!
Humans and Soil
Soil plays a very important part in each of our lives. It provides
a growth medium for the plants that we eat and with which we
build shelter; it supports the buildings that we build. Soils soak
up rainwater and then filter and store it. They also provide and
store nutrients like nitrogen, carbon, and phosphorus.
The Charlton Series, formed in
glacial till, is one of the most
important natural soils in the
Bronx.
Many urban soils have also received fill during construction activities or gardening. Common
fill materials are bricks or other construction debris, topsoil, coal ash, municipal solid waste, or
dredged material from waterways. These can be found alone or
in combination. The addition of these or other materials
changes the way that soils drain, their chemical properties, and
even what can grow in them.
Different geographic areas face different issues when it comes to
soil health. These issues can be anthropogenic or natural. The
three major issues with soils in the Bronx are compaction,
contamination, and erosion.
There are many items that can
become mixed into the soil
due to human activity, such as
the solid waste in this soil
profile.
Compaction occurs when the soil particles are squeezed
together, reducing the pore size in the soil. This affects the way
that water moves through the soil and can lead to pooling of
water rather than infiltration or soaking of water. Places that
you can see compaction are along unpaved footpaths or in park
fields where many people walk or play.
Erosion is the act of water or wind moving or carrying away soil
particles. Exposed soil can be washed or blown away by rainfall
or the wind. This takes away the nutrient rich parts of the soil, exposing the less nutritious
horizons. Erosion by water can also carry silt into
the Bronx River and other waterways in the area,
creating issues in waterbody management and
causing difficulties for the organisms living in the
water.
Contamination, or the addition of harmful
chemicals or compounds, is another issue that is
faced in the Bronx. Many times contamination
occurs as an accident, like with oil spills or
Compaction causes the pore size and number in
affected soils to be decreased.
gasoline leaks. Sometimes, chemicals are buried on purpose. In many cases, it is possible to
remove or remediate the soil, although it involves a great deal of time and money. In fact, one
contaminated site at Starlight Park is being remediated so that the land can become an
improved New York City Park and part of the Bronx River Greenway. Remediation will be
completed by early 2008.
Starlight Park from 174th Street Bridge.
Written by Karlee Yurek.
Edited by Jill Weiss and Rich Shaw.