Dr Charles E. Kellogg, a long-time head of Soil Survey at
the United States Department of Agriculture (USDA) was quoted as saying:
“Essentially,
all life depends upon the soil ... There can be no life without soil and no
soil without life; they have evolved together.” --- Charles E. Kellogg,USDA Yearbook of Agriculture, 1938
Image courtesy of USDA
National Agricultural Library
When you think about the basic needs for most forms of life (e.g.
oxygen, nutrition/food, shelter) it can all be traced back to soil. Soil helps
the trees and plants that produce our oxygen to grow and provide us with food.
It is the link connecting many ecosystems and natural cycles.
Soil is extremely diverse, and this is evident in the amount
of science research that is dedicated to it.
Soil VS Dirt
Soil may be defined as the topmost layer of the Earth’s
crust, consisting of:
- loosely arranged products of rock weathering & erosion
- products of organic decay and
- living bacteria and fungi
Dirt is soil that has ended up in the wrong place; like on
our skin, our clothes or shoes. It contributes to household dust along with a
lot of dead skin cells that we shed every day!
For a PALMS activity looking at soil, dirt and dust click here for teacher notes and here for a student worksheet.
Soil Profiles and Components
If we were to dig a deep hole in an area of soil, the colour
and composition of the soil will change as we go down into the Earth’s crust
until we eventually reach an area of solid rock known as bedrock*. The diagram
below shows a simple soil profile. Note – the depth measurements (in inches) are
just an example and not all layers are present everywhere.
Image
courtesy of USDA
The soil layers shown in the diagram are:
- O – Topsoil – composed of mulch, humus and broken bits of rock containing minerals. This is the most fertile part of the soil. Humus is mostly dead and decaying plant and animal material which has been decomposed by bacteria. It’s an important part of making soil fertile. The broken bits of rock are formed through the process of weathering and erosion.
- E – Eluviated – composed of weathered rock and minerals but less humus.
- B – Subsoil – composed of minerals that moved down (leached) from the upper layers with water soaking into the soil. Less fertile than O and E layers.
- C -Bedrock – composed of rock such as granite, basalt, quartzite, limestone or sandstone that is weathered to form the mineral particles of soil.
*A common question we get asked during incursions on the
topic of soil relates to bedrock and the popular game Minecraft. Our
understanding is that in Minecraft you can’t break bedrock. To address this increasingly common misconception, this is not true in real life. Bedrock is the rock at your location and could
be a range of different rock types depending on where in the world you are (e.g.
limestone is commonly found close to the coast in WA). Most of these rock types can be
broken by mechanical means or by the use of explosives. And no, you will not
fall to the centre of the Earth if you break through the bedrock. That’s a
whole other misconception to be addressed at another time!
To find out more about how weathering can occur, take a look
at these PALMS activities:
S
The process of soil formation can take tens of thousands of
years however it’s surprisingly simple to separate out the components of topsoil.
Check out this video on just how you can do this using
equipment you’ll find around your home.
You’ll end up with something that looks a little like an
alien landscape.
Why not try this process with some soils from different
places? You could compare some garden soil with some beach sand and think about
why there’s not many big trees and other plants growing on the beach.
Some other resources about soil can be found at this website.