Beyond
the Horizons—Eating Your Soil
Do
you really understand what is under your feet… and what it took to get it there?
When I was young, I didn’t ponder on the purpose of
that dark stuff under the grass. It was
something that got stuck under my fingernails and was fun to dig up and find
bugs in. It was everywhere—even in the
house, if you didn’t wipe your feet.
So when I opened my textbook on my first day of Soil
Science 101, I was nervous. I knew that
there was more to soil than a block of dark matter that held roots and
water. I barely knew anything about
farming, much less the intricate details of the teeming ecosystem underneath
the crops I was learning about.
But you know what?
The book actually had some familiar words! Soil really isn’t that different from you or
I in some regards, believe it or not.
In my first blog, we established that soil is NOT
dirt, and that dirt is soil where you don’t want it.
But
what is soil?
Soil is the natural
medium for the growth of land plants.
Therefore, not all of earth’s land surface is
covered with soil. If it can’t grow
plants, it isn’t soil. What isn’t
soil? Polar ice, high elevation areas,
hardened lava, salt flats, and bare rock mountain slopes. If an area has anything that would make a
plant recoil its little roots and say “Nope, not going there”—that is not
soil.
Sounds simple, right? Well, enter NASA and explorers asking about
mineral substrates on other planets.
Have you read or seen “The Martian”?
Is that soil? It isn’t on
earth….is oxygen required to make something soil? I’m not going down that path—that’s for
another day and another argument.
Where
did soil come from?
Well, where did YOU come from? Your parents, right? Soil has a parent, too—the substance from
which the soil was made is called its parent material. Most soils came from minerals and rocks. Parent materials go through chemical and
physical processes called weathering over many, many years to create
loosened material. In this case,
weathering doesn’t mean a soil is checking the radar or the forecasted high
temperature. It means a rock is being
worn down into something useful to a plant.
So your mom isn’t the only one who is worn down. And mom?
Next time you’re frazzled, you can say you are being weathered! Ha!
Why
aren’t all soils the same?
You may have noticed that not all soils look
alike. Some are deep, almost black,
crumbly, and grow just about anything.
Others are light tan, grainy, and seem to blow around too much. Some soils are red, some salty, some deep,
some barely there. How a soil forms
depends on these factors:
·
Parent
Material: You aren’t the only one who inherited things from
your parents! Properties such as rates
of weathering, nutrient composition, and particle size depend on the substances
that the soil came from.
·
Climate:
Precipitation
and temperature are the main things in a climate that affect how a soil
develops.
·
Biota:
This
is a fancy word for living things and their waste, including dead parts and
poop of plants and animals of all sizes—even the ones you can’t see.
·
Topography:
This
is how the earth’s surface is shaped. A
soil will be affected by how level or sloped the land is, elevation, and even
what direction the soil is facing on the slope (does it see lots of sunshine,
or is it hidden from the light and heat?).
·
Time:
A
soil can take 200 to thousands of years to develop.
What
does a plant need from soil?
So if soil is a medium or substance that can grow
plants, what is needed in order to call it soil? Soil provides the following growth factors
to plants:
·
Support:
No, not positive affirmations, or tight pantyhose. Plants need help standing up, and soil provides
support as the roots grow into the substrate.
·
Oxygen:
It’s
often misunderstood that a plant only gives off oxygen. It is true that oxygen is a product of
photosynthesis, but a plant also needs to breathe, and oxygen is required for
this. Roots need oxygen in the
soil. Little tiny spaces between the
loosened parent material hold air that contains oxygen.
·
Water:
Part
of those tiny spaces between soil particles hold water, too, and the roots
absorb the water as they need it.
·
Proper
temperatures: The roots can’t freeze or fry. Temperature requirements depend on the
species.
·
Nutrients:
Plants
require 16 nutrients, 13 of which are supplied by the soil, either by sticking
to the soil particles themselves or being dissolved in the water in the pores.
A
soil has layers?
Nope. And
yes. But read on.
First, you have the weathering, or creation of small
particles. Then, you have soil
development. These two put together are
called soil formation. It doesn’t
all happen at once. If you dig a deep
hole, from the top all the way down to the parent material at the bottom, you
would see what looks like layers of soil.
But they aren’t layers.
Layers
are for cakes.
Don’t
eat it. It’s not a cake.
These areas of variation are called horizons. The horizon with the least amount of
formation will be at the bottom, and the most developed horizon that will grow
the best plants will be at the top.
What kind of horizons are present depends on where
you are. Most soils in my area, the midwestern
United States, have an A horizon (the topsoil, where you would find the roots
and the most critters crawling around and where a farmer would till and plant
crops), a B horizon (a little less developed than the A horizon), and a C
horizon (resembles the parent material, but has started to break down). There are many versions, amendments,
subgroups, and additions to this basic idea.
I
can friend a soil on Facebook?
Ha. Caught
you there. You know when you see someone
that is particularly good looking from the side, and think, “nice profile”? Well, a soil has a profile, too. Does that mean it can fill out information on
Instagram and post silly photos and quips?
“The earthworms are tickling me particularly
vigorously this morning”
“This rain is really bringing me to
field capacity”
I heard you groan.
We were just talking about the soil’s profile
earlier and you didn’t even know it.
Remember that hole you dug from the top of the soil all the way down to
the parent material so you could look at the horizons? The collection of horizons is the soil’s
profile. A soil profile is a
vertical section of the soil through all its horizons and extending into the
parent material.
Now that you’ve become a little more familiar with
soil, I hope it makes you feel more grounded.
(You’re supposed to laugh here).
And I hope since you learned that soil has a parent
that it has worn down just like you, you’ll feel a little closer to it. So the next time you see its profile exposed,
you can tell it, “Gee, I like you’re A horizon”. (Ahem….laugh
again, please).
Thanks
for reading!
November
8, 2019
Julie S. Paschold
Reference:
Soils in Our Environment by Raymond W. Miller & Roy L. Donahue.
1995. Prentice-Hall, Inc. Englewood Cliffs, NJ