Let me make something super clear.
If you take nothing else away, remember this: Soil is NOT dirt.
Soil is productive, it’s useful.
It’s fundamental to life as we know.
It is an essential natural resource, a major component of most ecosystems, and has been
celebrated in art and song for millenia.
Dirt is just soil in the wrong place.
Soil is the thin layer of inorganic and organic material wrapping the earth like a cozy blanket.
It is where the abiotic lithosphere (that is, the upper mantle and crust of the Earth,
the airless, unmoving underground stuff) meets all the living things in the biosphere.
Here’s the nitty gritty.
A soil’s make-up looks something like this: tiny bits of rocks (like sand, silt, and clay),
usually a sprinkling of water and air mixed in, then tons of living stuff, and also formerly
living stuff that is decomposing, adding organic material into the soil.
All of these components of soil are different sizes, from the comparatively massive earthworm
to the large particles of sand, down to the teensy pieces of clay and the miniscule bacteria.
And, maybe obviously, not all soil is the same.
You might live somewhere with really sandy soil, or a place with distinct layers, or
lots of rocks.
But regardless - all these different soils with funny names are still soil.
To get a better understanding of the diversity of all these different types of soil, we’re
going to look at the 5 main components that drive soil formation.
One, the parent material, the type of rock that over a really really really really long
time broke down.
Two, how long that breaking down process took.
Three, the types of organisms living in the soil.
Four, the climate of the place the soil is.
And Five, the topography of the area.
Let’s dig into that list a bit shall we?
Soil doesn’t just appear out of nowhere, so let’s imagine for a moment that we’re
hanging out, just minding our own business, and we see a volcano erupt.
We’re safe, but we see a little landslide of lava.
It completely burns and covers whatever was there before.
So there isn’t any soil now, but if we hit fast forward, the soil will come back.
It starts with the parent material, in this case volcanic rock.
Over time that rock will slowly break up into loads of tiny pieces through a process called
weathering.
This could be physical like wind or water smashing into the rock and breaking off bits,
or it could be chemical, the rock particles interacting with materials in the air or water,
like acid rain, or - pioneer species.
Pioneer species like lichen can hop on our lava rock and secrete an acid that breaks
down the parent material.
Pioneer species, lichen or bacteria, are all r-selected species meaning they have a high
reproductive rate and will crank out lots of little offspring and babies and spores
and seeds, knowing that the majority of them will not make it in this harsh habitat but
the ones that do, will grow and thrive.
Eventually these pioneer species are going to die and when they die they're going to
decompose and their decomposed tissues are going to go into the weathered pieces of rock
and now this mixture is soil.
The broken down parent material is the base of the soil.
And kinda like a baby inherits characteristics from their parents, soil inherits characteristics
from its parent material.
For example, limestone is a rock that's rich in calcium carbonate.
So the soil that results from limestone rock typically has a lot of calcium in it.
Or, in our lava field, the soil contains a bunch of different nutrients because it is
made up of materials pulled from deep underground.
This obviously takes a long time, rocks are pretty hard, so breaking them down is a whole
process, which is why time really matters in classifying soil types.
As soil goes from young and immature to mature and robust, the living community on top of
the soil gets bigger and more complex along with the soil.
Over time a mature soil with distinct layers forms.
And like magic, and over a few hundred years, our barren lava field will transform to a
forest.
In addition to the parent material and age of the soil, we’ve gotta look at what organisms
are hanging out in and on it.
There are a lot of living things in soil.
Like a LOT.
More than a million microorganisms live in a SINGLE TEASPOON of fertile soil.
There are microorganisms and worms and moles and gophers and plants and fungi.
Those plants are taking nutrients out of the soil, but when they die and decompose, they'll
bring it right back in.
And all those animals and plants squirming around aerates and mixes the soil.
This combination of living and dying organisms all mixed up with the parent material’s
minerals and nutrients is what makes soil such a key component of the biogeochemical
cycles of our planet.
Which means, soil plays a major part in how nutrients and minerals flow through the biosphere.
So let's take a look at one part of one of those processes.
This is a rough cartoon of the nitrogen cycle.
Nitrogen is a pretty important for plants, because they need it for photosynthesis, BUT,
the nitrogen gas in the air the plants can’t use, it is in the wrong shape.
Enter: rhizobium, this little bacterium, well, all bacteria are little, but this one, has
the incredible ability to turn atmospheric nitrogen into ammonia.
Ammonia, is the form of nitrogen that plants love.
Rhizobium basically gives the plant its own microscopic fertilizer factory, converting
unusable nitrogen into a form usable by the plant, all in the soil beneath our feet.
And rhizobium isn’t alone, the nitrogen cycle, the carbon cycle, basically all nutrient
cycling between the abiotic and biotic parts of our planet is driven by these soil microbes.
Climate’s influence on soil formation is pretty varied depending where you are on Earth,
but temperature and humidity are some of the big factors.
Super cold temperatures or really dry air will slow down decomposition.
Conversely, warm and wet conditions will speed up decomposition and soil formation.
Think a frozen tundra versus a rainforest.
Or how when you’re cooking in the kitchen, you can start with the same ingredients and
get a different meal depending on how you cook them.
And finally, topography will also influence soil formation.
Topography is the land forms and landscapes of an area - is it hilly, or flat, or full
of canyons?
Something like a steep slope might result in high erosion rates, and a loose, dusty
soil, whereas thicker soils might form down in the valley between two mountains.
These are the five factors that create the diversity of soil: parent material, time,
organisms, climate and topography.
But so what?
Why do we care that there's these different types of soil?
Well, because each type does different things and we, and other species, use them differently.
And that all comes down to the soil’s properties.
We’ll start with the physical properties: soil texture, porosity, and permeability.
Soil texture is determined by the percentages of sand, silt, and clay, those weathered down
tiny pieces of rock, that make-up the soil.
Scientists use the ‘soil triangle’ as a way to represent soil texture.
Up here at the top of the triangle - that’s 100% clay - great for making pottery, over
here 100% sand, great for making castles.
But most soils are a combination of all three: sand, silt, clay.
So we can triangulate the kind of soil we have.
21% clay would be along this line, 66% sand would be right here, and then 13% silt would
be here.
So this particular type of soil, with those percentages is called sandy clay loam.
And the word ‘loam’ is important.
Loamy soils are almost like little bear’s porridge for goldilocks, not too hot or too
cold, it's just right.
Loamy soil is just the right mix of sand, silt and clay to be beneficial for agricultural
uses.
And here’s where we point out what might be really obvious to you.
The vast majority of our caloric intake comes from things that are grown in the soil.
Just like we learned in the water lesson that preserving ecosystems and watersheds is a
cheaper and more effective way to clean up drinking water, we rely on soil as an ecosystem
service, because it’s a fundamental component of the agricultural system we use every single
day.
The next physical property: porosity.
This is simply the amount of space between the soil particles that determines how much
water and air can move through the soil.
Which is related to the soil’s permeability - how quickly will water go through the soil.
Sand is more permeable than clay.
So imagine if you had a handful of sand and you poured some water into it, most of the
water would just pass right through as opposed to clay, which will actually hold the water
in there.
That’s why pottery works.
So if we combine what we know about soil texture, porosity, and permeability, we can start to
get an idea of what different types of soils can do and be useful for.
When farmers are planting their crops and taking care of their crops, identifying the
soil’s physical properties is a big part of making sure you irrigate, or water, your
plants (and soil) appropriately.
Farmers also need to consider the chemical properties of their soil, like soil fertility.
As we’ve talked about, plants need certain nutrients to grow, and ideally these are going
to come from a well-balanced soil.
This balance of the micronutrients like, nitrogen, but also phosphorus and potassium, is the
soil’s fertility.
And those micronutrients and other organic matter the plant’s crave largely comes from
the chemical properties of the parent material of the soil as well as the organisms, like
our rhizobium living in the soil.
When well managed, soils with high fertility can support abundant, healthy plant life.
But, I don’t want to just talk about the benefits of healthy soil for our food system.
The ecosystem services that soils provide are so much bigger.
Remember, soil is everywhere, it is the top layer of earth, and we rely on it for a lot
of things underground.
Especially groundwater.
Soil is a fantastic filter.
After a rainstorm, water which could of picked who knows what in the atmosphere or some gross
stuff on the gound, slowly makes its way downward heading toward an aquifer - an underground
body of water.
But, to get to that aquifer, all along the way, like a bajillion little coffee filters,
soil pulls out the stuff that isn’t good to drink, and in some places, we barely have
to treat our groundwater to make it drinkable because soil does it for us.
Soil provides all these incredible benefits for people - filtering water, helping us grow
food – but we don’t really return the favor, in fact, human activity is messing
up a lot of our great soil out there.
Unsustainable agricultural practices can lead to a loss of soil fertility.
So things like, planting the same thing over and over and over again in the same soil without
proper fertilizers and soil management can result in soil that can no longer support
life.
Overgrazing, when there are too many cattle for an area and they overeat the grasses and
compact the soil with their hooves.
And once the grasses are all gone, the soil no longer has their roots to hold it in place...so
erosion increases and we lose soil.
This is a bit heartbreaking, because remember soil takes so long to form from a parent material,
erosion erases millenia of hard work in a matter of days or weeks.
Similarly, deforestation does a real number on soils, because by getting rid of these
trees with massive root systems that hold a lot of soil in place, suddenly that good,
healthy soil supporting an entire forest will just wash away when it rains.
This also happens when soil is tilled.
Tilling is the process a lot of folks who grow food use to overturn the soil and get
rid of old crops in preparation for the next planting.
But this process often leads to loads of erosion.
Roots, as I’ve said, are important for soil, even the old roots from last year’s crops
matter.
And when those rich structures are disturbed and all tossed around during tillage, the
wind can easily blow away the soils - because they aren’t being held together anymore.
Along the way - that soil is also at risk of compaction, all those big tractor tires,
squishing along on the soil can mess up the permeability and porosity.
Unsustainable agriculture, overgrazing, deforestation, over-tilling, and compaction are all ways
human activity is degrading our soils - reducing agricultural productivity, negatively impacting
ecosystems, and increasing erosion.
But there’s one more big reason to protect our soils.
Soil is where the air meets the ground making it a major player in the global climate system.
Soil, is a carbon sink.
Which means it can take carbon dioxide from the atmosphere and store it underground - something
that is super needed right now because we’re putting a lot of carbon dioxide into the atmosphere
and warming up the global climate.
This is the famous Keeling curve, which reports continuous measurements of carbon dioxide
in the air from 1958 to now, and we can see that over time carbon dioxide is increasing.
Why is soil a carbon sink?
Well, if I’m a plant and I’m doing some photosynthesizing, I’m drawing carbon dioxide
out of the air and transforming it into organic compounds, which I store in my little plant
body as leaves or kernels of corn or a beautiful flower or roots.
If I get eaten, whatever eats me then turns that carbon into muscle or fur or bone.
Eventually, plant me, and whatever I got turned into, is going to die, and the carbon in the
plant or animal tissue will re-enter the soil as the decomposers do their work.
We call this ‘soil organic matter.’
Through this process carbon moves from the air, to the tissues of plants and animals,
and then into the soils where it stays trapped for a long time, assuming we don’t go digging
it up and letting all that carbon dioxide back out again.
This is a reminder that when it comes to climate change, not only is reducing emissions important,
we also need to preserve Earth’s carbon storage systems: in this case, complex soil
ecosystems.
The earth is really good at cleaning up carbon, so we should make sure it can keep doing its
job.