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.