Soil Basics: Soil Profiles


[MUSIC] My name is John Graveel and I’m a professor in the agronomy
department at Purdue University. And what we’re going to do today is talk
about soil profiles or soil monoliths. Here in the agronomy department we have
over 400 soil monoliths that we use in teaching a wide variety of
different soil science classes. So, as you’re driving down the road, one
thing you notice about soils is you notice that most of them look fairly dark on top,
we call that horizontal variability. But what’s really interesting and
cool about soils is once you dig into the soils and you start to
look at the vertical variability, that’s when soils in my
mind really come to life. So, for example, on this monolith right
here, it looks sorta dark on top. But when we dig down into it, we find a soil horizon that looks something like
this, very light in appearance, all right? And then we get a darker color area here,
and then we get a redder color area here. So we call that vertical variability. So what you notice about
soils by just looking at them is the wide variety of
different colors that we have. We have grays and browns, we have tans and reds, we have white colors,
we have black colors. So we have this wide range
of different colors, and we can use colors to actually
help us define soil horizons. And so what I’m gonna do over the next
couple of minutes is define for you the six master horizons that
we can find in the soil. So I’m gonna start off
talking about the top soil. The top soil is the A horizon. And the A horizon is at the very top,
all right, it’s usually dominated by organic matter
as you can see in these profiles. So organic matter imparts are very
dark color, either black or dark brown. That is typical of an A horizon. A B horizon is called the subsoil. In the subsoil is where
everything has accumulated. So, as soils weather, what happens is that things sort of move out of the A horizon
and they accumulate down in the B horizon. So over here we have nice B horizons, and essentially the B horizon,
it has a higher clay content. So for example, it would be like a silty
clay loam texture in the B horizon, compared to the top horizon,
which would be probably a silt loam. So the B horizon is
the horizon of accumulation. It is also the horizon which is most
colorful in a soil, is the B horizon. The C horizon is the parent material, and that’s the material from
which the soil formed. So at the bottom of this profile here,
and that profile, and the profile on the far left-hand side. Those soils over there, all right, which you’re actually looking at
is called the parent material. And again that’s the material
from which the soil was formed. So those are the three
of the master horizons, we have three others that we use. One of them is called the O, and
the O horizon stands for organic. Usually the O horizon is just used,
typically with organic soils or
in virgin forested sites, sites that have not been clear cut for
many years. So essentially that’s possibly where
we could use the O horizon, so this would be an O horizon,
an organic horizon. The other master horizon that
we have is called the E horizon. The E horizon is a leached,
bleached horizon, and what that means is that everything’s been leached out it,
all the clay and the iron and so forth. And so what’s left behind is a very white sort of appearance to that soil horizon,
that’s called the E horizon. At the bottom of this particular
profile here, we have a lot of rocks. And that’s the R horizon, or the regolith. So we have, essentially,
the A horizon at the very top. We have the B horizon, and
we have the C horizon and the R horizon, and then,
in some soil profiles we have unique features like the O and
the E horizon. [MUSIC]

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