Hey everybody, welcome back! Today we're
going to be doing a lecture on soils and
agriculture. The connection between soils
and agriculture and with a strong
emphasis on soil structure and its role
in plant growth. I think you're gonna
really enjoy it. Let's just dive right in.
Right here at the very beginning is a
nice introduction to the whole concept.
We have these nice taproot plants, these
are carrots, living right in the soil.
We're gonna be talking about the
relationship that exists between the
root and the soil itself. The
structure and what conditions make this
soil acceptable for, say a plant like a
carrot. Some other plants would not be
able to survive in this type of soil. So
we're going to talk about the different
conditions that might arise as a result
of soil structure and plant growth. So in
order to talk about this stuff we've got
to get a little vocabulary under our
belts, and so we're gonna start right off
with a definition of: "What is agriculture?"
So agriculture is the practice of
raising crops and livestock for human
use and consumption. So here we see an
agricultural field...this is one in Brazil
that's growing soybeans...it's entirely
grown for human purposes. Right. Humans
have cultivated this field. They
have taken care of the field. They have
harvested this field. This is all
soybeans. And it's for human use. OK. So
that is agriculture. Cropland is land
used to raise plants for human use. So
this would be cropland, in this case, that
was used , in this case, for
agriculture. Rangeland or pasture are two
extremely important and interchangeable
terms in a lot of ways. Land used for
grazing livestock. So cattle, sheep, goats...things like this. And in terms of how we
use the land, we need to be aware of the
fact it's a large amount of land
we're talking about here. The amount of
land is devoted to agriculture on earth
covers about 38% of Earth's land
currently. Now we have to remember the
earth is a water planet.
It's 71% ocean. So 29% of the
land surface, er the Earth's surface,
is land.
And of that 29%, 38% of it is used for
human purposes of Agriculture. And what
is it that they're actually doing out
there? Is it the plants? Is it the animals?
Well it's actually the soil which is
driving people there. So soil is a
complex plant-supporting system. Consists
of disintegrated rock, organic
matter, water, gases, nutrients, and
microorganisms. And it's a renewable
resource that can be depleted if it's
abused. So when you think of soil and you
think of what's beneath your feet,
sometimes we just kind of fall into the
fallacy of thinking it's just rock or
dirt or there's really nothing exciting
down there. It turns out there's a lot
going on beneath your feet, and we're
gonna spend some time digging through
that. And one last term that we need to
bring up before we move on here is
"sustainable agriculture." So sustainable
agriculture is agriculture we can
practice in the same way in same place
far into the future. Right, so it's a type
of Agriculture. And one of the things
we're gonna learn by the end of, I would
say the next lecture or two, we're gonna
learn that while this is wonderful, lots
of soybeans are coming out of this field
in Brazil, it's not particularly
sustainable. OK. This is something we
can do for a while but it is having
effects, and so we're going to talk about
that going forward. Alright, let's jump
into it. So the reason why we're gonna
focus so strongly on soil when we get
into our agricultural portions or
agricultural lectures going forward, is
because soil is the base. It's the...it's
what supports all agriculture. So
agriculture and plants require healthy
soil to 1) provide nutrients and 2)
have a structure that allows roots to
penetrate deeply, and 3) to retain
water. So those are three important
features. Now I'm going to direct your
attention to this photograph
over here to the side where we have a
cutaway of soil with a plant. And we can
see that this plant has about half of
its mass is probably underground, maybe
even more. Right. Because we're only
seeing this in two dimensions. So this
plant is growing into the soil, it almost
has like a spider web type structure
moving through here, and then it turns
out about half of this is sticking up
above...
and there's the leaves, the leaves are
undergoing photosynthesis. So here we see
the energy accumulation and chemical
storage through glucose, and then down
here we see a totally different thing
happening. So what we see is that we have
this black soil, that it's built up
around the root structures here. And it
turns out that black or brown color is
usually organic matter. So organic matter
provides nutrients and helps with
structure and water retention. It turns
out the organic matter prevents the
water from flushing down through the
soil too easily, but it also keeps it
from water logging or flooding the soil
at the same time. So it's a very good
mediator of water. And it turns out it's
a big deal. Soils that retain water make
the right amount accessible to plants.
You don't want to drown the
plant and of course you don't want the
plant to dry out. So you have to have
enough constantly in reserve, and
soils are actually really good at doing
that. Now nutrients, such as nitrogen and
phosphorus, enable plant growth. And those
are also derived from the soil.
Nutrients, in particular nitrogen, is derived
through the nitrogen cycle which
comes from the atmosphere through a
complicated series of nitrogen-fixing
reactions that wind up allowing the soil
roots to be able to take it up.
Whereas phosphorus is generally consumed
from dead organisms that have died and
are being decomposed and it also could
be derived from the rock cycle from
which the soil itself is derived from.
We're going to get into the origins of
soil here and a couple of slides so hang
on for that. So soil structure and
texture influence root penetration and
growth. If you have solid soils that are
too hard the roots won't penetrate and
the plant won't grow, if it's too soft
the roots won't have anything to hold on
to then the plant will be removed or
damaged very easily by wind or some type
of surface feature. OK. So it has to
have the right texture for the roots to
be able to hold that particular plant in
place. So you need to have the right
roots, you have to...er, the roots have to
match the soil, the roots have to match
the amount of water. The
nutrients have to be right. The amount of
organic matter needs to be right. You can
start to see pretty quickly that there's
certain plants that live in certain
kinds of soils around the world. So
that's what we're seeing here in this
image. Livestock depend on healthy soil
because they depend on the plants that
grow there. That's kind of common sense,
it's the next step. Right. If you want to
have healthy livestock you need to have
healthy soils to grow the grasses that
they survive off of. And then of course
soils have sustained agriculture for
thousands of years, and it's our plan to
create a sustainable system that's going
to continue to do this forever.
Nature actually makes very sustainable
soils automatically. Human beings, through
their interventions in soils and
with our machines and so on and so forth,
tend to degrade the soil. So our next
lecture...it won't be in this one...but our
next lecture will actually deal with
that very issue. So let's talk about
where agriculture came from. Turns out
different cultures independently
invented agriculture. If we look at a map
of the earth, right here we notice that
there's orange zones and yellow zones.
The orange zones are centers of origin
of food production. So this is where
human beings came together and developed
agriculture based upon the plants that
reside in that area. One of the primary
places this happened, or actually I should
say the first place this happened, is
probably the Fertile Crescent or
Mesopotamia. This is based upon
archaeology. About ten thousand years ago,
we have very good evidence that human
beings were coming in and they were
doing agriculture, which is planting
crops. OK. Shortly thereafter we have
very good evidence that this was
happening in China, Guinea, Ethiopia, West
Africa, the Andes Mountains, and
Mesoamerica, as well as in the eastern
United States. So these are places that
independently developed the origins of
food production. Now the yellow zones
show us where food is grown today and
they don't necessarily always coincide...
for example, in South America the great
food growing region of Argentina and
Brazil does not match where we see it up
here in the northern portion of the
Amazonian rainforest, OK.
Or up in the Andes. The same thing is true,
of course, in Africa and where we see
tremendous amounts of production of a
lot of things that were from the Middle
East as well as from Africa grown in
Europe. Same thing in India. China and the
eastern United States are two exceptions
to this where we actually see the food
being grown still heavily in the very
areas where it was developed. Here and
here, alright. So it turns out that
raising crops produces a positive
feedback cycle. So let's go over that
cycle. So you're harvesting the crops
which required people to be sedentary. In
other words, if you're growing a whole
field full of something you can't be
moving around. You're gonna be there you
could be taking care of it and making
sure things don't get into it. So you're
gonna be tending that field. So that
makes you sedentary. That means you're not
moving around. You're gonna you got time
to do other things perhaps. Being
sedentary, of course, encouraged the
planting of more crops and production of
more food. So if you're gonna be there
anyway you might as well just plant
larger fields, even more than what you
would consume. So it's an overproduction
that might happen. And these more, er
more crops allowed larger populations
and then larger populations required
time to more crops. And there's your
positive feedback cycle right there.
OK. And very shortly thereafter that
you start seeing the development of
cities. You know, cities are areas where
people are generally sedentary. They're
not moving around. They're not traveling
hundreds of miles following Buffalo
or deer or something else that might be
hunted. In this case they're staying
somewhere nearby and they're engaging in
something that would develop into an
economy. Something called a market
economy in fact. We'll get to that here
in a moment.
So it turns out that agriculture, from
the very beginnings, has evolved a lot.
But most of that evolution is pretty recent.
Traditional agriculture is a very basic
system. Most of us would recognize it.
some of us even have great grandparents
that described having to work in some of
these environments of what we call
traditional agriculture. And this is
where we are talking about
biologically-powered forms of
agriculture that use human and animal
muscle power to make everything happen.
So that means you're using an ox or a
horse to maybe pull a plow. You might be
using hand tools like a hoe or a rake.
Simple machines might have been
implemented at the time. And in fact we
see in very ancient art...this is actually
from a tomb in Egypt from the 15th
century BC...we see things that we
recognize that farmers would be doing as
recently as 200 years ago. Right. So here
we see...this would be the harvesting or
threshing of wheat, the storing of wheat...
so this is a grain store that we see
here, we see digging and tree-cutting
that's happening over here. We clearly
see a domesticated animal that's doing
some plowing, and these are things that
happened right up...like I said...right up
into the 1850s, 1860s. And a lot of that
was replaced by tractors and machinery
later on, but we recognize that this is
something that was pretty standard
worldwide everywhere you go.
This is, in here, we're actually seeing it
on a piece of art in a tomb in 15th
century BC Egypt. Now what was going on
back then is something called
"subsistence agriculture" which is a form
of Agriculture in which families proves
only enough food for themselves. So they
would have a local farm and they would
grow what they needed. And on that farm
would be numerous...different...numerous
crops. Some of different amounts
depending on the demand for that family
of a crop. And so they would develop
something called a polyculture which
is different crops that are planted in
one field. So you might have an area
where you're growing...I don't know...
pumpkins and corn, you know. Whatever it
is. Watermelon. You can think of whatever
it is that you're growing or what your
desire is and you would grow that in
your field and then you would live off
that. So that's your subsistence
agriculture. You don't need any money, all
you need to do is be able to go into the
field and pick what you need. And you
need to be careful with how you stored
that material to make sure that you
got through the winter or whatever it is
that you were planting those crops for.
Now market economies came along a little
bit later and it
change things considerably. So a market
economies allowed...my pen's not
working here, here we got it....market
economies allowed farmers to sell their
product. So, in other words, they could
sell their corn and get coin in response.
Right. So you can't eat a coin but you
can trade it for future needs. Future...future goods and services. So you didn't
have to use it all at that moment. So
what that allowed people to do was to
overproduce and to keep the difference
in a...you know...in the form of coin. So
this also, in order to do this, of course
that increased the use of irrigation and
fertilizer and we start to see a strong
desire to have a large amount of
Agriculture because you can produce a
large amount of income by doing this. And
this happened for a very long time. Since
about the 1850s right up into especially
into the 1920s, we see a very rapid
transformation where we go from
traditional agriculture into
industrialized agriculture. And that's
what dominates to this very day.
So industrialized agriculture is a form
of Agriculture that uses large-scale
mechanization and fossil fuels to boost
yields. So here we see a farmer...this is
on an old corn picker and a trailer...this
is actually pretty outdated stuff today.
Most large companies that are out
harvesting corn would use a very
different type of corn-picking device.
But anyways, you can see that even with
this older equipment he's much better
off than the old traditional subsistence
farmers were who would come out with
their hands and pick the corn. This
machine is doing it for him.
What would happen here is he would put
fuel into his tractor, so it's gonna use
fossil fuels to make this happen, but he
and that tractor can do the work of many
many, many other workers. And so that what
that does is that liberalizes a lot of
people so they don't have to go into the
farm and work...there's a lot of other
things that they could do with their
labor. And so they went on and did other
things..and made it that so that fewer
people can produce just as much or
sometimes even more food than it was
happening
before. So that's one of the advantages of
industrialized agriculture. But in order
to make this field happen you need a
large amount of pesticides and
irrigation, fertilizers, and usually it's
done using monoculture. So monoculture is
uniform planting of a single crop. So in
this image here where the farmers
working, this is all corn as far as the
eye can see. He's probably using some
type of herbicide to keep weeds out of
here and the only thing that's growing
out here is effectively corn. And why
does he want to do this? Well because
it's really easy to harvest, right. If you
invest in machinery that only harvests
corn, that's the only thing you want out
in your field. So there's a strong
financial incentive to make that happen.
Now a lot of this really took off after
about World War II, under...during a period of
time called the "Green Revolution." So
during the Green Revolution new tech, new
technology, crop varieties, and farming
practices were introduced to developing
countries. And what this did was it
increased yields and decreased
starvation. And you would say that's
perfectly great. Right. We don't want
people starving but it does come at a
price....this style. We're gonna talk
about what the Green Revolution brought
about in a little bit, but it degraded
the integrity of the soil
over time and it is still having that
effect. And we're gonna talk about how
that happens over the next couple
of videos. Alright, so the basis for all
of this is the soil, right. So you can't
grow corn without the soil. You can't
grow wheat without the soil.
You can't even grow a polyculture
without the soil. So we need to talk
about what the soil is and get into what...
this how the system that is soil works.
So soil is 50 percent mineral matter and
it's up to 5 percent organic matter. It
consists of dead and living
microorganisms, decaying material from
plants and animals, and it's all kind of
in a single, coherent block working
together. The remainder of
that space, which is about 45 percent, is
pore space taken up by air and water. We
don't necessarily always think about how
much open space is beneath our feet
but those openings beneath our feet
allow insect penetration, root
penetration, water penetration, gas
penetration. It's all
beneath you. And if you're able to get
those things cycling around, those are...
remember... those are the four biospheres
here. Minerals are part of the geosphere,
the biosphere, the hydrosphere, and the
atmosphere are all coming together in
that soil. And so you're gonna have a
vibrant ecosystem down there. And so it
turns out that soil is teeming with
bacteria, algae, fungi, protists, and
provides habitat for earthworms, insects,
mammals, reptiles, and amphibians.
It's incredible what we'll find down
there. Here we see in this block
diagram here earthworms, bacteria, spring
tail, and a nematode, protozoa, and of
course as insects slide through the soil
they have little biofilms on them that
makes them feel slippery, but it helps
them work their way through this soil
matter. OK. Now since soil is composed
of interacting living and nonliving
matter it is considered an ecosystem.
Alright, so in order to really
understand soils we have to understand
where it comes from. So one of the ways
that we're gonna argue about the
structure of soil and how it should
operate properly with plants is to know
where it comes from, what its origins are,
and it turns out that it's  pretty
straightforward. It makes sense. But you
kind of follow it through carefully so
let's go ahead and build this case on
where it comes from. First off, it comes
through time. So it forms slowly and it's
a combination of time and parent
material. So parent material is the base
geologic material of soil. It could be
lava, could be old volcanic ash, it could
be rock of any kind. It could be sand
dunes, for example. And it also could be
derived from bedrock, which is the solid
rock comprising the Earth's crust. This
is stuff down deep below. Its crystalline,
it's hard, it's involved in tectonics, and
that bedrock is very much playing a role
in the kinds of soils that are gonna
form on top of it. So we take parent
material, combine that with time, and we
have to add one more, er actually two
more elements...the next element is
weathering. So weathering is a process
that happens at the Earth's surface. And
this process occurs in several ways. It's
processes that break down large rock
particles down into smaller
ones. So when we think of a soil we think
of that mineral matter, we know that it
came from something that was cohesive
and large at some point in the past, but
it's all broken down now. OK. And it can
happen in three different ways. There can
be a physical way of weathering things
down, something called physical or
mechanical. This is usually wind or rain
beating up on the material over a very
long period of time. Turns out water is
extremely effective at breaking down
material physically. OK. Another way is
chemical weathering. So chemical
weathering is pretty impressive and what
it can do. It turns out that rainwater
that falls out of the sky has a pH of
about five and a half. The reason why it has
a pH of about five and a half is that there's a little
bit of carbon dioxide in the atmosphere
at all times and it combines with water
and it forms carbonic acid. So you get a
little bit of carbonic acid blended in
with rainwater that moves the pH to five
and a half. So you're basically dousing
all the landscape and slightly...well
not just slightly...it's acidic rain water
for millions of years. And of course if
you do that long enough,
even on rocks, even something like a
granite, you're going to see pretty
dramatic chemical changes to that rock.
And one of the results is you start to
form...er, you start to break down this
rock and it gives the material that you
need to form soil. Another way that we
could form or cause weathering is
biological. Simply having organisms that
are physically running through the soil...er, running through the rock...little
roots from trees coming in and breaking
the rock material up. So that's what I
mean by biological. All of those
processes together will take large rocks,
over time, and smash them into smaller
and smaller pieces...smaller than sand...
it'll get down to something called silt
or even clay in some cases. Another
process or another thing that is formed
right alongside the the weathering cycle
is humus. So humus is spongy material
formed by partial decomposition of
organic matter and it holds moisture. So
here we see a bunch of that humus
right here. Here's a plant growing out of it,
but that black material...that organic-rich,
wonderful material that holds all the
good stuff
we need for plant growth is going
to be in that humus. And so the
decomposition mixed with them weathering
and good time and a good parent material
will give you a set of soils in which
certain plants can grow. Alright so let's
talk about some of these key processes
because it turns out when you go to one
location the soils are very different
than they are at a different location.
And it's because of the combination of
those things. But it's not just simply
the combination of those things, it turns
out there's a couple of other
influencing factors. So we got weathering
and accumulation and transformation of
organic matter...we just talked about that
in the previous slide...they're
influenced by the following factors: so
climate is a big deal here. So soils form
faster in warm, wet climates. So here we
see a desert. This is an African desert
and in African deserts they tend to not
be so wet, and because they're not so wet
they don't get the type of climate where
you find very rapid formation of soils.
So when we look out here, we don't see
the traditional soils where we got
plant-supporting material. While there's
plants out here, you don't have huge
accumulations of it. Whereas in a
tropical rainforest, where it tends to be
warm and wet at the same time, you get
trees all over the place very, very
vibrant ecosystems that are set up. And
here there's an ecosystem but it's very,
very thin and very, very fragile. OK.
The organisms play a role. Plants and
decomposers add organic matter. If the
decomposers aren't there you don't get
the humus, and if you don't get the
humus you don't get the soil
formation that's plant-supporting.
Topography plays a big role. This is the
way that the hills roll up and down. So
hills and valleys affect exposure to the
sun, wind, and water which, of course, the
sun gives you the energy, the wind is
going to give you that weathering, and so
is that water is going to give you both
the physical and the chemical weathering
components. The parent material also is
going to affect it. You're gonna get a
different kind of soil if it's from
limestone versus if it's from granite or
sandstone or something like this. It all
is coming to pass. It's kind of a
really complicated story behind almost
every single soil. And of course time.
Soil can take decades to millennia to
form. It's an extremely
interesting field to study. It's
complicated, the number of soils that
exists on earth are in the
tens of thousands, and we are constantly
classifying new soils all the time. And
it's all based upon all these different
criteria. OK.
So, I just want to
let you know that this is a huge field
and in no way shape or form are we going
to cover any [note: meant to say "all"] of it. What we're gonna do
is we're gonna tackle this from a very
simplistic viewpoint, right. This is kind
of a prototypical or typical soil...it
depends on where you live...but it could
be pretty typical of soil formation in
most places on earth. That's what we're
going to focus on. So when we look at
soil we usually imagine just the stuff
up here at the top. So here we actually
have a cutaway of an imaginary earth
where we got a tree with the roots
coming in, and we usually think about
that black or brown material up at the
very surface. OK. But it turns out when
we go below it there's layers down below.
And those layers aren't called layers
that when we're talking about sediment...er, I'm sorry...when we're talking about
soils...in sedimentology we talk about
layers but when were talking about soils
we talk about things called "horizons." So
a horizon is each layer of soil. So here we
have a layer, here we have a layer, here,
here, and then this is actually the
unweathered original parent material
bedrock...we wouldn't call that a layer
but we frequently call it the R-horizon, R standing for "rock." OK. So
soils can have up to six horizons. It's
pretty impressive how much difference
exists between each one of these
horizons. And they're all part of the
same system. So where does that come from?
OK. Well we're gonna get into that here
in a moment, we gotta go over some
processes and a little bit of definition
right now. So a soil profile is the
cross-section of the soil as a whole. So
here we see a profile of a soil. OK.
With the material...the brown
material up on top, and it's gonna change
color pretty dramatically as we go down
through that profile down to the rock.
OK. And this is a result
of the degree of weathering and amount
of organic matter that decrease in lower
horizons. So we got a lot of organic
material up here on top, not so much down
here. And that's going to have a chemical
difference...er, chemical effect that's
going to result in these differences.
Another process is leaching. So this is a
process whereby dissolved particles move
down through the horizons and may end up
in drinking water. So constantly, it's
raining up here, you might be watering
this field that...imagine this is an apple
tree or an apple field or something, so
you're out here irrigating...that water
is constantly running through that soil,
from the surface down to the bedrock.
OK. And as it moves it has the ability
to remove particles, dissolve those
things, and to move them from the top
down lower in the soil. OK. So some
materials, it turns out in drinking water,
are hazardous. And this is pretty common
in countries like Bangladesh where
there's a type of mineral that exists
called arsenopyrite
that...arsenopyrite, if it's interacting
with oxygen in the atmosphere through
complicated groundwater processes, that
arsenopyrite can release arsenic into
the water, and that can get leached out
of the rock and out of the minerals and
into the groundwater and that can cause
some pretty serious health hazards.
Another thing we want, need to bring up is
this concept of topsoil. The topsoil is
the really crucial resource for plant
growth that we're really going to focus
on in this lecture. So topsoil is the
inorganic and organic material most
nutritive for plants. And it's vital for
agriculture. And so when we think of
topsoil we're really talking about the A-horizon up here. So we haven't got into
what these are here, but we're gonna get
into them here in a moment, but we have
the A-horizon, the B-horizon, and the C-horizon, and occasionally we have a
couple of extra horizons in there that
might show up under special conditions.
If we have good plant growth and a lot
of leaf litter and dead things that are
on the surface, we might create an O-horizon way up here on the top. The O
standing for "organic." OK. So this is the
loose and partly decayed organic matter
and then right below that is the A-horizon which is where we have the
mineral matter mixed with some humus.
OK. And then down below we have the
B-horizon. This is where we see some pretty
interesting effects that actually
usually stain this an orange or a pink...in
some cases...relative to the A-horizon
even though it's made out of largely the
same stuff. We're gonna get into where
these come from over the next couple of
slides. First thing I want to focus on is
the O-horizon. So in the O-horizon
there's no mineral matter. It's only made
up of leaf litter and organic material.
So here we have a soil core that has
has been removed, and it shows the O-horizon
of a type of soil called a "spodosol."
Don't worry about what a spodosol is,
just focus on the fact that when we look
at this here we see no mineral material
in here. We've got leaves, decaying twigs,
probably some dead bugs, things like this
in there. Now decaying organic matter
creates humic and fulvic acid in that
layer, which are very important for a
plant growth it turns out. But remember,
we're already raining acid down through
rainwater...which is a pH of five and a
half...it interacts with that dead and
decaying material at the surface, and
then we're creating fulvic and humic
acid, which...while important for plant
growth...also is going to have important
chemical effects going down through the
soil. The next layer down is the
A-horizon. So the A-horizon is a layer of
mineral soil...so this is where we get
minerals...remember the O had no minerals,
so now we have minerals...with most
organic matter accumulation and soil
life happening in this layer. So this is
where the action is, right. This is the
really rich, good stuff that we use to
grow plants in. Additionally, due to
weathering oxides...mainly iron oxide. Now
if you think of what iron oxide is, iron
oxide is basically rust, right. It's a
form of rust, depending on the level of
oxidation and so on and so forth and
it's kind of a complicated little
process. But these oxides are usually an
orange or
pink or a red color and so these things
tend to form and they could be
accumulated or they could actually be
leached out, right. They can be passed
down. And it usually has a pronounced
soil structure. So let's go ahead and
look at an image here. This is a cutaway
of a profile showing the A, B, and C
horizon. So here's the A-horizon. The
A-horizon's got a mixture of that nice,
beautiful organic material that's coming
in from that very thin O-horizon up
here at the top...you can see where the
where the roots are and things are dying
up here, this seems to be somebody's yard
or field or something like this...so the
O-horizon is quite thin but here's the
A right where this arrow is. And here
we've got mineral material, we've got
open spaces, we've got little divots in
here which is probably where earthworms
and roots were in the past, so it's a
very active and vibrant system. OK.
Sometimes when we get a tremendous
amount of...I should say...dissolution of
certain materials, you can...and clay minerals...you can move these things, you
can dissolve them and move them downward.
So here we see the surface...here's the
O-horizon, and this is in a different
soil, here's the A...and what happens is
you can get intense leaching that will
happen, right. So, let's go ahead and read
this: and some soils clay minerals, iron,
aluminum, organic compounds, and other
constituents are soluble and move
downwards. So water comes in, it's
leaching this stuff out, it's moving it
downwards when this
eluviation...this is that leaching and
transport is what that means, OK...when
that happens, a lighter colored E-
subsurface soil horizon is apparent at
the base of the A-horizon. So here we see
it, right. It's highly leached. It's a nice,
beautiful, white band right here in the
middle between these two black sections
right here. OK, so what we see here is a
nice E-horizon and it exists as part of the
A, it's the base of the A-horizon, okay. Or
near the base of the A.
So here we have the A-horizon and here
we have the E-horizon, okay. E stands for
for "eluviated." This is a zone of
transport away, right. So materials are
leaving that zone of the base of the E, er...of the base of the A-horizon which
forms the E. OK. So the next layer is
going to be, as we move down, we've
got O, A, E now we're gonna head into B. OK. So this is gonna take us back to
our previous picture. It's commonly
referred to as subsoil, the B-horizon
can also accumulate minerals and organic
matter that are migrating downward from
the A and E horizons. If so this layer is
known as the illuviated or illuvial horizon.
OK. So this is a zone of accumulation.
Well, what is it accumulating? It's
accumulating a lot that clay, it's
accumulating a lot of the iron oxides,
it's accumulating a lot of the aluminum
oxides. When that happens
we get a zone that tends to be kind of
pink, because you got the iron. The iron
tends to stain the soil. So here we don't
have a pronounced E but we have a clear
A up on top. So we've got A and
here's our B, and then down below we have
a little different texture, it's a
different type of material. It turns out
that is our next horizon, the C horizon.
In this case it looks like it's derived
from sand dunes, but the C horizon is
usually stuff that is very similar to
the original parent material, okay. It can
contain material as large as boulders
and cobbles in it. This looks like it's
sand dunes or something like this. I
actually I'm not familiar with what the
material is here in the C-horizon on
this particular image, but here you can
clearly see all three: A, B, and then down
here is C. OK. This is the original
parent material with the soil forming on
top. Alright, so we've gotta talk about R-horizons. If we've talked about C you
might as well go all the way to R. So
R-horizons denote the layer of
partially weathered or unweathered
bedrock at the base of the soil profile.
So here we have a soil profile here
cut right into this cliff face. Unlike
the above layers, R-horizons
largely comprise continuous masses of
hard rock that cannot be excavated by
hand. So here we see that hard rock down
here. It's strong. This is something that
is not soil at this point. It can
if it's exposed to weathering long
enough and broken up, it can become soil.
On top of that though, if we look
carefully, we can see that there's
gravels and cobbles in here. And this is
that C-horizon right here. Where we see
it's broken up and then presumably...if we
got up here onto the top...we'd actually
start to see the B, possibly an E, and an
A and an O horizon right up to the top
of that cliff face. But this is a clear
R-horizon right here. OK. Alright, to
figure out another important parameter
of soils...er, to classify soils is to
understand its color and its texture and
what those things mean. And as well as
the structure in the pH. So those are
four important things. So let's talk
about soil a little bit. So soil color
indicates its composition and its
fertility. So if it's black or dark brown
soil, it usually means it's pretty rich
organic matter. It means it's probably
gonna be pretty easy to grow stuff in.
If its pale gray or white soil, it usually
indicates it's pretty leached or pretty
organic poor. Alright. So usually you
would take...if you just happen to have
white soils in your yard and you're trying
to grow things and things aren't growing
well try using something but with a
little humus in it to kind of pump up
the the fertility of that soil. Soil
texture is determined by the size of the
particles. So it turns out there's four...
er, sorry three main particle sizes that
we're going to be concerned about for soils:
the clays, the silts, and the sands. So the
smallest is clay, the middle is silt, and
the largest is sand, and there's kind of
an easy way to figure out the difference
between the three of them if you're ever
making an estimate in your backyard, there's
actually easier way...there's more
direct ways...I shouldn't say necessarily
easier, but there's direct ways of doing
this...but clay you can always detect as
different from silt and sand by their
texture and by their look. So sand you
can feel and see. You can see the
individual grains.
Take a little bit of sand put it in the
palm of your hand and you can actually
play with individual grains. And you
could see them. Silt on the other hand is
so fine that the human eye has very, very
strong...has a lot of difficulty
identifying a single grain but you can
feel it with your finger. So it's kind of
this weird zone where you can feel it
but you can't see it.
That's silt. And then clay is where you
can't see it and you can't feel it. It's
just too tiny but you can feel its
texture. It's usually sticky. It might
make your skin feel a little dry, that's
clay that's making that happen. OK. So
clay you can't see and you can't feel an
individual grain, silt you can feel the
grain but you can't see it, sand you can
see it and you can feel it. OK. So those
are the three different things. And it
turns out that the stuff that really
grows well or really kind of has the
stuff for planting or for plants to grow
well in, are loams. So loam is a soil with
an even mixture of the three. So that
means you've got large sand which allows
good drainage but it also has clay in it
which allows some adhesive or some
protection against the water just
dropping right out of it. So you want a
blend of these things. Of course it
affects, also, how easily air and water
travel through the soil. If you have a
large amount of clay it turns out water
and air get through it very slowly, it's
very difficult for these
components to get through the soil. If
it's got too much sand, it has difficulty
holding water, right. But the water will
go through it very easily. So how that
happens is pretty complicated, and so
that can have an effect on the kinds of
plants that you can grow in that kind of
soil. OK.
Influences how easy soil is to
cultivate, of course, as well. Now let's
use an example. Thirty percent sand, fifty
percent silt, and twenty percent clay. If
we go out and we do a test and we find
out that that's how much...er, what the
percentages are in a soil sample, we can
classify that. We want to know if it's a
if it's a loam or if it's a clay or a
sand or whatever, and the way that we do
that is we can plot it on this chart. So
this is a ternary plot over here.
A ternary plot is unlike your regular X-Y
axis. It actually has three different
ends. And so each one of these corners of
this triangle is referred to as an apex.
So here's an apex for sand. So this is
the sand percentage as we go this way. A
hundred percent sand is at this apex
and we call that sand. If we go to this
apex,
you know this increases in numbers and
here we get to clay, so that's a hundred
percent clay up here. And then silt is a
hundred percent over here at this apex.
You can see we call it silt, clay, and
sand for that very reason. So let's
imagine we come back to our hypothetical
sample: 30 percent sand, 50 percent silt,
and twenty percent clay. Well the first
thing we could plot of course is the
sand. So the sand is right here, and you
see that I just drew the line here, so we
come up to here...here's 30 percent...and
we draw that line and its 30 percent away
from sand. OK. Then we put on the next
line. The next line is fifty percent silt.
So here's our silt line...here's 50...and we
just draw our line down from fifty to
fifty right here.
Of course our last one you can almost
kind of see right away is gonna be...
here's our silt apex over here...so here's
the twenty percent on silt...I'm sorry, on
clay...sorry I misspoke...here's our clay
apex over here, twenty percent...so this is
gonna be this line right here and so
let's go ahead and draw that. You'll see
that it all comes through and it plots
at this point right here. What's really
cool about this, actually, is that you
don't really need all three. You can see
where the two cross where the sand and
the silt crossed is also at the same
place where the clay came through. But we
have a pretty good
way to classify this in a soil that has
that composition would either be a loam
or a silt loam...it's right on the border
it turns out between the two. But
that's actually pretty good growing soil.
Alright, another thing that we need to
be concerned about is soil
structure, which is to say it's measure
of a soil's "clumpiness," which is to say how
its cohesive, how it holds together. So a
medium amount of clumpiness is best for
plants. Repeated tilling, that turns out to
compact the soil. It's actually not
good to make...to constantly till the
ground. It hardens, especially down about 18
inches down, because you're running a
tractor over in it all the time. So
you've got to be very careful. And of
course if you compact it, that decreases
its water-absorbing capabilities. We need
to talk about soil pH too. So we got
clumpiness and the last one is soil pH. So
soil pH affects the soil's ability to
support plant growth.
So soils that are too acidic or basic
can kill plants. We're going to get into
why that is here in a moment. OK. But pH,
it turns out, influences the ability of
the availability of nutrients for plants.
Let's talk about why. The main reason why
is through...is because of a process
called cation exchange. So cation
exchange is how plants go and get the
nutrients that it needs for survival. So
imagine we have a soil, right. So a
negatively charged soil is going to be
holding cations, right. So cations, if
we remember from our energy and matter
lecture, positive...er, cations are
positively charged. So if you have a
negatively charged soil, it'll hold those
cations. What kind of cations might the plant need? Calcium, magnesium,
and potassium are pretty common, as well
as possible iron and things like this. So
it needs these cations. They're already
in the soil, the soil is going to hold on
to them because the soils got a slight
negative charge. Now what the roots do is
something very interesting as they
donate hydrogen to soil in exchange for
these nutrients. So in the plant world, in
interacting with soil, hydrogen is almost
like currency and the roots are able to
come in and trade...they push in the
hydrogen. Remember, hydrogen is basically
a proton. It's a positive charge. And what
it does is it forces a proton in and
what that does is cause the soil to
regurgitate back a cation which is also
positively charged. So it's a trade
that's happening here. And in order for
that to happen you have to have the
ability to accept those hydrogens from
the roots. So that leads us to an
important concept called the cation
exchange
capacity. This is a soil's ability to hold
cations. So cations that don't leach
are more available to plants if they're
not washed out all the time. So that's
actually where a lot of the humic and
fulvic acid comes in and it helps
prevent a lot of these things from
moving out and it holds it within the
soil structure. It's also a useful
measure of soil fertility.
So it's greatest in fine-textured or
richly organic soils, but the cation
exchange capacity decreases with lower
pH. And the reason why that happens is, if
you remember, the lower the pH that means
the more protons the more hydrogen is
already present in that material. So if a
plant is already coming into the soil
and the soil has got a very low pH which
means it has a large number of
hydrogens, it doesn't necessarily want
to do that exchange because it's already
saturated or has plenty of hydrogen
already. So it's not going to allow that
to happen. So very acidic soils, it turns
out, will have very low fertility because
they have a very low cation exchange
capacity. I want to quickly mention some
soil structures...alternative soil
structure because it's a really big deal
when we start looking, especially, at
tropical rainforests. So soil
characteristics tend to change no matter
where you are. The rainforest is a big
deal. So in rainforests, the nutrients are
in plants not the soil. There's very, very
little in the in the profile for an A-horizon. The A-horizons are usually very
thin and sometimes they're non-existent.
All you have is an O sitting on top of a
B-horizon with a C. And let's look at
this hypothetical soil profile that we
have. Here we have a soil profile
where the O-horizon is like two inches,
maybe an additional eight inches of A-horizon in this hypothetical one, and
then we go down into the B and then we
go into C. By the time you get to the B
bar about 30 inches down, which isn't far,
so we can look at this cutaway that we
see here of a tropical rainforest and
we'll notice that pretty much everything
is happening right along that edge. All
the trees, everything, all that life, is
happening right there at the top; right
there in the O-horizon.
There's almost no A-horizon at all. There
might not be one at all. And then here we
see that orange very, very eluviated, er...
I'm sorry...sorry illuviated B-horizon.
Right. This is a zone of accumulation of
those iron oxides over very long periods
of time. It's turned it into a red, a red
soil that is actually not very fertile.
OK. So rain will leach the minerals and
nutrients reducing the accessibility to
roots and rapid decomposition of
leaf litter results in a thin topsoil
layer with little humus. So this is
where the whole action happens. And so if
you do any damage to the upper couple of
inches you can damage this whole thing
for a very long period of time. And so
this is what we see for a soil profile
from us from a typical tropical
rainforest in the Amazon for example.
Alright, so we're about finished with this
lecture, but what I want you to do is
look into the comments section down
below and you'll notice that I have a
link to an amazing video put together by
Dr. Friedland.
You're gonna like it. It's where he and a
graduate student go out and actually
look at a soil profile. It's amazing, it's a
beautiful spodosol and I highly
recommend that you do it as soon as you
finish this video. Alright, so with that
said we'll see you in the very next
video where we talk about soil
conservation and techniques for
basically dealing with the loss of the
topsoil and the abuse that is suffering
across the world. Alright, catch you in
that video.
Take care!
