In this video Paul Andersen explains how soils are formed and classified. Weathering of rock creates particles which are mixed with water, air, and organic material. Soils are classified according to particle size, chemical makeup, and horizon distribution. A brief discussion of soil loss through erosion, compaction, and salinization is included.
Transcript provided by YouTube:
Hi. It’s Mr. Andersen and this is environmental science video 6. It is on soils. Soils are
incredibly important. It is where we grow our food. But they take along time to form.
We start with regular rock and then over time what we have is weathering. We have physical
weathering where we break the rock down into smaller particles. We have chemical reactions
or chemical weathering. We have biological weathering as well. And so it takes a long
time for us to go from rock to soil, soil that we can grow our crops in. And the sad
thing is that it can all be lost over night. And so rocks remember are made of minerals
which are recycled on our planet using the rock cycle. They can undergo weathering both
physical and chemical. And that produces the particles in the soil. It makes up about half
of the soil. We also have the biosphere contributing life. We have the atmosphere and the hydrosphere.
And also a lot of time. And so all of these contribute to soil. We are going to have lots
of different types of soil on our planet. One way to look at classified soil is to look
at the different horizons or the layers in the soil. We could also look at the particle
size in the soil. Going from large to small it goes from sand to silt to clay. Now that
contributes to the porosity of the soil. How easy is it for the water to get down and bring
water and nutrients to the roots of the plant? And then we also have the chemistry of the
soil. Where did the parent rock come from? And one of the big things that is important
in the chemistry is the CEC or the cation exchange capacity. How easily does that soil
deliver important ions to the roots themselves. Conservation is incredibly important with
soils. We have soil erosion where we are physically removing the soil. And then we also have salinization
or salting of the soil which is contributing to soil loss. So on our planet remember we
have a rock cycle where we can move from igneous rock, which are crystallized magma to sedimentary
rock. So we have this weathering, erosion moves it and then we have this compaction
that forms this sedimentary rocks like sandstone. And then we can have metamorphism where we
are actually putting heat and pressure on that rock to convert it into a metamorphic
rock. But it is weathering that contributes to our soils. First type of weathering is
going to be physical weathering. This is an example of a rock that has been weathered
physically. You can see it is just broken apart. So that is water. It could be ice wedging.
It could be the roots of a plant. But anything that increases the surface area of the rock,
in other words anything that physically breaks it down into smaller bits, that is going to
be physical weathering. And that is only half of it. We also have chemical weathering. So
on this rock here you can see there is rust or oxidation going on on the outside of that
rock. And so that chemical reaction breaks down a rock into particles of the soil as
well. So if we look at this granite, the felspar here can react with naturally forming acids
and form something called clay. And we will talk about the importance of clay in a little
bit. Now we can talk about soil being three phases coming together. So we have solid,
liquid and gas. If we look at the solid phase we are going to have the minerals of the soil
itself. So that was the particles that came from the parent rock. We also have the organic
materials. So the living the material. So roots would be an example or dead material.
We also have the hydrosphere so the water coming together. And then we are going to
have the air. And so this is the breakdown of how much contributes to the soil. It is
going to differ on what soil we have. But if you thing about it what is soil? It is
the coming together of the lithosphere or the earth on our planet. It is also the air
or the atmosphere and the hydrosphere. And then it is where we are headed next in this
course. It is the biosphere, the living material. And so that is why soil is important. It is
at this interface between all these different spheres on our planet. And if we look at how
it is formed, bedrock is broken down, weathered over time, and we eventually get what are
called the horizons of the soil. We could classify some of the major soil horizons.
At the top we are going to have what is called the O horizon. That is going to be the organic
horizon. That is going to be where we have a mix of a lot of dead or dying material.
Below that we are going to have the A horizon. That is going to be our topsoil. That is going
to be a nice mix of minerals and also all the organics from the horizon above. As we
go below that we are going to have the B horizon which is the subsoil. Not a lot of organics
found in here. We are still going to have minerals and nutrients that are pushed down
from the soil layers above. And then finally we get down to the C horizon. And that is
going to be where we have parent rock. Now in certain soil horizons we will also have
an E horizon. And so that if eluviation taking place. In other words we have the movement
of water down. It is pulling those minerals out. And we are just left with sand and silt,
kind of this dry layer. Particle size contributes to what type of a soil we are talking about.
So this is a loam right here on the right side. So if we look at that soil, it is going
to have varying sizes. So we could go from very big, like boulder to gravel, but eventually
when we get to the level of the soil we have sand. Sand is going to be relatively large
in the soil. We then have silt. And then finally we have clay. Clay is going to be particles
that are smaller than 2000th of a millimeter. So really, really fine particles. Now what
do those particles contribute to? It is the type of the soil and the porosity of that
soil. So let’s say we take those three particles and fill up a container with sand, silt and
clay. And then we fill it up with water? Well you can imagine what is going to happen. In
the container that has sand it is going to drain out. Or we are going to have high porosity.
And that is going to take hours. In the silt it is going to take days. And in the clay
it is going to take years. And so having a lot of clay can really stifle the movement
of water into the pores where those roots need it. Now we can classify soil based on
which of these particles we have. And so this chart takes a second to get used to. So this
would be the clay on the left side, from 0 percent to 100 percent. And this would be
the silt on the right side and then the sand down below. Remember is particle size sand
is biggest. Then silt and then clay. And so you can see on this chart that anything that
has 50 percent or higher clay, we just call that soil clay. And we are not going to have
good drainage. And this is not going to be a great place to grow crops. What is the perfect
soil? Well if we have about 20% clay and we have about 40% of sand and silt we have what
is called a loam. And that is going to be a nice balance of all of those particle sizes.
Because clay is important. If we look at the cation exchange capacity, what is that? That
is the ability of a soil to deliver important ions, important nutrients to the root itself.
And the more clay we have, so this is going to be at the microscopic level, and the more
organics we have, they are going to be attracting those cations and they are going to deliver
it to the root itself. Another important property of soil that goes along with the CEC is going
to be the base saturation. So how can these minerals buffer the acidity of the soil as
it comes in as well? Because that acidity can damage the plants. Soil conservation you
can see is incredibly important. It takes hundreds of years to form these soils. We
can turn that around over night. So this is going to be soil erosion, where we are rinsing
that top soil off. And so as a farmer you would want to mediate that. We can also have
soil compaction. That is when if the soil is wet and we are driving on it with heavy
machinery what we can is we can compact those pores. And so that is going to destroy that
soil as well. And then we can have salinization, of increase is salt. So if we have plants
growing on the soil then they are going to draw the water out. It is going to leave these
natural salts behind. Normally not a problem because we are going to have rain water. Rain
water is fresh water. And as we have that rain water it pushes those salts out. And
so it is not going to damage the crops. Now what is the problem? If we start to irrigate.
So now we are going to use irrigation. And we are going to spray water on the crops.
Where is that water coming from? It is not coming from the sky. It is coming from the
soil itself. We are probably pumping it up. And so that means that those little droplets
in the irrigation are going to have salts inside it. And so as that lands on the field
we are going to increase the salt levels to the point where we cannot grow crops there
anymore. And so how can we solve this problem? We could flush it out with freshwater. We
could change the type of crops we have. Maybe get more salt tolerant crops. Or we could
use crops that have bigger roots so that they can push that salt farther down. But you can
see what I am getting at. It is this idea that soil is a non-renewable resource. It
takes a long time to form and it is really hard to balance the proper chemistry and particle
size in the soil. And this map shows us on areas on our planet where soil loss is vulnerable.
And so it is important that we conserve our soil. So did you learn all of this about soils?
Can you pause the video here and fill in the empty boxes? Let me show you what goes there.
So the rocks and minerals are weathered, both physically and chemically to produce soil.
Along with the bio, atmosphere and hydrosphere, this takes a lot of time. Types of soils could
be characterized by the horizons or the levels. So we have at the top O, A, sometimes E and
then B and C. We have particle size, sand, silt and clay is going to be the smallest.
And that leads to the porosity of the soil. And then the chemistry is important. CEC or
the cation exchange capacity is incredibly important. And I hope that was helpful.
This post was previously published on YouTube.