APES Soil formation

Professor: Hey guys, let's have another,
uh, quick video on, uh, Earth systems,

we're going to talk specifically in this case
about soil formation, how do we get from

the rocks in the rock cycle that obviously
come from the, uh, crust and the tectonic

plates, how do we get from all that to the
soil that obviously is going to have a

pretty much direct impact on human and
really all biological life.

So, essentially what soil is, is soil is a
mix of geologic and organic components, so

you've got parts of the biosphere here,
um, that as they die and decay they get

deposited, uh, and of course their
manure and stuff like that, and then

you've got part of the Geosphere which
are the rocks being broken down.

You need both of these things together,
um, to actually make soil, so like if you

just have sand, things don't grow well in
sand, that's just a rock, um, and so for

it to actually become soil and to grow
you've gotta mix in organic

matter with that.

That's why if you've got a really sandy
soil, that's its usually necessary either

mix in some fertilizer or some manure
or some compost, something that gives it

some organic component, so that's really
what compost is, is compost is organic

components that are going to decay down
and then if you mix those in with the

rock components, some dirt, some clay,
stuff like that, then it can

become actual soil.

So, two parts here, you've got the break-
down of the rocks, you've got deposition

of organic matter, those two things
together are going to create soil.

So, we talked about the rocks and the
rock cycle, uh, the video we did before

this, um, the deposition of organic matter
again is, um, as things die and are broken

down by bacteria, uh, stirred up by
earthworms stuff like that, um manure

being deposited, all those things are
going to go into making

proper soil as well; and there are
basically, five factors that are going to

effect what the soil is like and we all
know that different kinds of soil are

going to be used it--er are going to be--
appear in different areas and going

to be used to grow different things.

So, the first thing is, is the parent
material, so what kind of rock, um, is

actually being broken down, and so, it's
a little hard to see in the video here,

but this topic example, this is some, uh,
granite, uh, and if that's broken down,

you tend to not really get super rich
soils because granite has a lot of

silica components in it, which is,
think about sand, um, and so that's not

going to be very nutrient rich just
from the rocks component, now of course

if you add an organic component to it
that's what makes it soil and that's

what helps out.

But on the other hand, if something
like say, limestone breaks down,

limestone is essentially calcium, um, and
so the calcium that's in that, um, is

going to actually be one of the active
nutrient components in the soil and so the

parent material is, what kind of rock
breaks down to actually form the rock

component, um, it's pretty important to
what the soil is.

Um, climate obviously has a pretty big
effect on this, um, the temperature

and precipitation, particularly are going
to affect how quick the soil forms, um,

soils not going to form very well
typically in really, uh, cold climates,

um, but in warmer climates, in
particularly climates where, um, you get

a lot of precipitation, therefore a lot
of weathering, a lot of erosion, um, a lot

of deposition, um from p-precipitation
then the soil is going to form much

more quickly and that's why in, uh,
tropical areas you tend to see much

richer types of soils, in the Tempa
regions obviously, as compared to

arctic regions, you are going to get much
better soil out of that.

Um, the third thing that is going to
affect the rate--er affect the soil

formation is topography, in other words,
how steep is the land, um, it tends to be

just generally speaking, that the steeper
the slope that the more quickly it's going

to erode because the higher up it is, the
steeper it is, um, the faster the water

is going to flow down it, the more it's
going to be subjected to wind, all those

things are going to weather and break down
the rock and then

lead to quicker soil formation.

Uh, what kind of organisms are in the
area, and so again this works against

arctic regions because there's not quite
the vast variety of organisms.

Um, and so plants and animals are going to
take nutrients out of the soil, uh,

bacteria and stuff are going to break
stuff down, earthworms are going to

help to mix up the soil and so all of
those things together, um,

are the organisms, the biological
part that's going to help to start to

make up the soil.

And the firth component, um, is time,
okay?

So, the longer the time goes, in general,
the more of what we call mature the soil

now, the reason that we call them
mature soil is because A, the particles

of the rock are usually broken down much
finer, and then secondly, there's going

to tend to be a lot more organic material
there.

So, like for instance, if we were talking
about like a volcanic island that just

So, if we were talking about like a
volcanic island that has just formed, um,

some of the rock might break down, but
until animals start to come there, and you

might get birds and stuff like that
obviously going by, but not you're not

going to have a lot of organic matter so
that's going to be a very immature soil.

As time goes on and that volcanic isle
ages a little bit, the rocks break down a

little bit more, more plant life starts to
come in, seeds get drawn by birds,

stuff starts to grow, break down the soil
more, it dies off and forms more organic

material and as time goes on you get
a much more mature soil out of that.

Okay, so, um, let's talk a little bit
about the layers in soil, um, layers in

soil are called Soil Horizons, don't let
that term like sort of throw you off,

Horizon means layer for us, um, and so
we've got some basic layers here.

So, we've got an organic, um, layer at the
top which basically just means at the

very top layer of the soil is essentially
just the plant life typically, um,

especially in a forest or grassland,
that's what's going on at the very top,

is you've basically got
a little bit of dirt mixed in

with mostly organic material, um,

in the form of actual living organisms.

Right under that is
the really important area for us,

and you see here in the diagram
from about 2 inches to 10 inches down.

Of course, that varies very drastically

depending on where you are,

what type of climate,
what type of soil you have,

but that's the topsoil layer.

Um, and that's where everything
is really well mixed together.

You've got a really nice blend
of the mineral components,

the rock components, and of the organic
components to supply the nutrients.

And so you've got a firm base
to hold everything there

and you've got a lot of nutrients there
that can be pulled up.

Um, in areas
where the topsoil isn't there,

that's why we have to add that in.

Um, if there's not a good mix
of organic materials

or if, or if maybe if the uh,
the actual soil itself

maybe can't, isn't very porous
and doesn't let a lot of water in,

then we might have to add
some soil to the top

to be able to properly
grow things there, okay.

So that's that A layer,
that's the topsoil layer.

Underneath that, we've got
a couple of subsoil layers.

Now you see here in the B subsoil layer,

we've still got some
root formations going through.

Um, but there's not really
a whole lot of organic material.

Usually those root systems
that are going to go deep down,

usually what they're pulling up
is more water.

Um, there is some nutrients there,
but not a whole lot.

It sort of depends on, again, what,
what area you're in,

um, and and what they're trying
to pull out of there.

And as we get down to level C,
you'll see that

there's not really any plant action
going on here.

Um, you're almost to the bedrock area.

Um, there's just not, the rocks
aren't broken up enough

to even allow for any real mixing

and there's no plant life
or animal life going on there

to be able to actually mix everything up

and make the soil rich enough
to help to grow things.

Um, one thing
that I didn't put here in the notes

is that right in between um,
the A and the B layer

is sometimes what's called an E layer.

Um, if you're in a very acid soil,
you usually get an acid layer right there.

Um, it is usually very distinguishable.

Um, and typically speaking,
we don't really want acidic soils.

So, the physical properties of the soil.

In other words, this essentially means
what are the size of the rock particles.

So we've, I've got three main categories.

We've got sand and silt and clay.

Sand particles tend to be
really big, okay,

and, and you'll see here, this is
a sand particle and so is this.

So um, sand has a really wide variance.

You can have kind of big particles
and then much smaller particles.

Anything bigger than about two millimeters though
isn't really sand anymore.

It's more like gravel, um,
than it is sand.

And you still have some of those particles
around occasionally,

but it doesn't really make
for very good soil most of the time.

Um, smaller particles
than sand are called silt.

Um, the way I always think about this
is if you like

step in the bottom of a river usually

and you get sort of that uh,
muddy sort of slick feeling soil,

a lot of times that's silt.

And of course, in Tennessee,
what that is actually

probably more likely to be is clay

and that's sort of the vast bulk
of our soil around here.

Um, and that essentially means that
everything is ground down really fine.

Now one of the reasons why
we have so much clay here

is that the mountain system
that surrounds us

um, is a very old mountain system

and so it's been eroded a lot

and so the rock particles
have had time to break down

into these really really small
and fine particles

that form the clay that is pretty much
in every river system

and makes up all the soil around us.

You know, if you, it's not
just the U2 logo.

If you dig down in the soil in Tennessee,

pretty much does bleed orange.
It's pretty much clay all the way um, through.

Um, this triangle is called
a soil triangle

and essentially what this allows you to do

is it allows you to determine
the type of soil,

not just that it's clay, silt, or sand,

but knowing the percent
of clay, silt, and sand

allows you to actually say
this is this kind of soil,

so this is a loamy soil

or this is a sand or a silt
or it's silty clay loam

or sandy clay
or whatever those things are.

It all depends on what the percentage is.

So for instance, you would basically
read this thing um, in multiple directions.

So if I had, say, 40% clay
and 10% silt, okay,

and then um, that would obviously
leave another 50% in that sand

so I'd come up in the 50 in sand,

I hit the 40 from the clay,

and then I'd hit the 10 from the silt,

and so I'd be right here in this area

and so that would be a sandy clay.

Um, we're going to do some actual labs

where you're going to determine
what type of soil is in

a specific area that we're going
to take a sample of.

Now of course, we're in Tennessee,

chances are, probably pretty likely to be

somewhere up here in the top part,
in the clay region,

siltier sandy clay or some clay loam
or something like that.

Um, couple other characteristics
about soil

that are important for us is the porosity.

The porosity depends essentially
on what type of particles you have.

Um, and what porosity means
is how quickly does the,

does, uh, water um,

or really any liquid, but water
in particular, drain through the soil.

And the way it works is that
since sand has much bigger particles,

then there are bigger spaces
between the particles

because the particles
can't fit as tightly together.

Think about if you had some,
like, basketballs

and you were trying to you know,
put them together.

You'd have these big gaps between them.

But if you took, like, golf balls
and put them together,

you'd still have gaps

but they'd be much smaller gaps

because the circumference
and the radius of the,

uh, particle overall would be
much smaller.

And that's exactly what's going on here
in the sand and the silt and the clay,

is the smaller the particles,
the less porous it is.

So for instance,
when we did our filtering thing,

if you pour-- when you pour water in,

it essentially went right through the sand

if you poured it directly in.

You had to sort of move it around
or it would just go right through

and not filter very well at all.

Silt, it's going to take
a little bit longer to get through.

And then clay,
if you've got a pack of you know,

especially densely packed clay, um,

really it, it pretty much
blocks water altogether.

Water can, I mean,
it might take it 100 years or so

to actually get through that clay,

because the particles
are so tightly packed together.

This is why a lot of times
when they want to uh,

sort of contain some hazardous waste,

they'll put some clay areas over it
and then some plastic over it,

because even if some liquid
gets through the plastic,

then it actually has long time,
it's really difficult to get through the clay

because it's not very porous.

Um, another important characteristic
of soils is uh,

the cation exchange capacity,
and we'll talk more about

the actual implications of this
when we do agriculture later in the year.

Um, but essentially this just means how,

how well does the soil grab onto ions.

Um, and, and by ions,
we mean like you know, metal cations,

so things like calcium
and potassium, magnesium.

These are all important nutrients
that the soil needs

um, to supply to the biomass.

And so, if it has
a high cation exchange capacity,

that's good because
that means that it can absorb--

I'm sorry, adsorb um,
a lot of cations to it.

Um, the word adsorb means that
it's actually going to hold it on the outside.

Absorb with a B means that it would
pull it into it much like a sponge would.

Um, and so, in general
this is really good.

Now, this can be bad
because if you notice down here,

sodium is one of our cations too.

And most soils don't do very well
with sodium at all.

Um, plants don't tend
to grow very well in sodium,

except for a few, um,
things that are called halophiles,

um, and actually do grow
relatively well in salt, um, areas.

And they're actually being used
to remediate some really rough patches,

uh, like in Australia and stuff like that.

Um, our next thing is the base saturation.

Uh, remember that base
essentially is the opposite of acid

and so essentially
what base saturation means

is how many bases are in the soil
compared to the acids.

In general, this is a broad
generalization, is we want bases

and not acids in most of the soil
because it tends to be most of

our nutrients are bases, most of the
acids tend to be heavy metals and

things that are going to lower the
PH and make it so the organisms

can't leave, uh, live there, so we
really want more base saturation,

so in general, if you've got a high
cation change capacity, and a

high base saturation, that's usually going
to mean that your soil's a lot healthier,

it's going to be much more conducive
to growing living organisms.

Then of course we've got the biological
properties of the soil, so, um, you know,

how much, um, living bio mass is in there,
so there's bio mass that is dead bio mass

that's been decomposed, but living
organisms are important too.

Perhaps the most important thing for
us on this planet is that we need that

nitrogen fixing bacteria, okay, we could
have some way to pull the nitrogen out

of the air, remember the nitrogen cycle,
we have some way to pull that nitrogen

out of the air and get it in to the
soil in a form like nitrates that the

plants can actually absorb and use as
nutrients to make their amino acids

and proteins.

Um, those organisms that are in, the
biological organisms in the soil

are also gonna act to mix up the
soil, and then of course, they're

gonna, um, you got detritivores, um,
that are gonna help to break down that

organic matter as well, and again return
those nutrients back to the soil.

Um, and finally, there are a lot of
different ways that our soil can get

degraded.