Deep, Down and Dirty: The Science of Soil Page #3

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the foundation of soil development.

Rock fragments permeate the soil

from the bedrock

all the way to the surface.

It's mainly this stuff that

was left behind

when I burned the plant matter

away from the topsoil.

But, though these

particles are from lifeless rock,

that doesn't mean

they have no purpose.

In fact, they are fundamental

to how soil works.

Soil particles are divided into

three different categories

depending on the size

of the particle.

The largest being sand.

There you can see them

just coming into focus,

wonderful, rounded particles.

The next size down, well, it's silt.

And there you can start to see

the individual silt particles.

And the very smallest are the clays.

Search for the clay. There they are,

much smaller.

Relatively speaking, if the sand was

the size of a beach ball

then the clay particles would be

the size of a pin head.

Incredibly small and

flat in their profile.

What's curious about the particles

is that the relative

proportions of them in any

soil fundamentally affect

how that soil behaves, and, more

importantly, how it supports life.

'To see exactly how, I've come to

the James Hutton Institute

'in Aberdeen.

'I'm here to meet soil

scientist Dr Jason Owen.'

Jason, what will this

experiment demonstrate?

What we have here are three

cylinders. One with a sand, one

with a silt-dominated soil

and one with a clayed soil.

When we pour water in the top

what we'll see is the water

percolating through the soil

profile.

With the sand it'll go very quickly.

With a clay it'll go very slowly.

And the silt will be

somewhere in between.

To me, this is familiar stuff,

as it will be to any gardener.

It's the age-old question

of drainage. How well water

moves through different

types of soil.

With the sand, large particles,

there's quite large gaps,

comparatively speaking,

and water can

go down through the profile.

With the clay, very small particles,

and as a result the gaps

where water can penetrate

are exceptionally small.

The silt is somewhere in between

the two extremes.

But to really see what's

going on inside the soil

we have to look at it in far

greater detail.

Here, they're using cutting edge

technology to examine soil

on an incredibly small scale.

We're joined by Evelyne Delbos,

operator of the Scanning Electron

Microscope at the Hutton Institute.

She's looking at soil

magnified 400 times.

I have the three main

parts of the soil.

The sand grains here.

On the right is the silt

and the clay at the bottom.

Well, you can sort of see with

the clay, for example,

it's stacked so tightly together

that you can actually not see

discernible gaps between them.

Whereas here we've got these very

large sand particles

and even through they're

right on top of each other

you can still see the far larger

gaps.

That allows air,

for aeration of the soil,

and it also allows water movement

through the soil.

But there's more going on here

than just how the particles

are packed together.

Let's imagine this is

a grain of sand.

And the surface area of that

grain of sand is that surface,

that surface, that surface,

and that's it.

It we take, by comparison,

the same volume of clay

then you have that surface plus that

surface plus that surface, so you

can imagine already that the surface

area is much, much, much larger.

So what does the surface area

do to the water?

What's the relationship

between those two things?

What's interesting about many clays,

it has an electric charge

associated with its surfaces.

Many nutrients that are dissolved

within the water can be

attracted to these clay sites, to

this large surface area,

and then held,

basically for root systems

then to uptake for plant growth.

So clay particles have an electrical

charge that can bind nutrients

and water to them.

This allows soil to

act as both larder

and reservoir for plants

and animals.

Sounds ideal, but there's a catch.

Too much clay and the soil can act

like a sponge

and can quickly become waterlogged.

At the other end of the scale,

too much sand

and the water can run through

too quickly,

washing the nutrients out and

leaving behind soil that's dry.

Have we got an image of what

a good soil should look like?

Here you can see some grains

of sand, they are different sizes.

It's a mixture and you can also have

there and there the clay

and the silt all mixed up.

So this is demonstrating the ideal,

in terms of soil. It would

be free draining,

retain sufficient moisture,

sufficient nutrients,

what about microbial activity?

This is a very,

very complicated 3D structure

which gives all of the microbiota

within the soil effectively a niche,

a home to live, and as a result

the ecosystems that exist in the soil

are exceptionally complicated.

This is a classic example where

you've got the mix between the

large particles, the clay particles

and silt all working together.

So the elements that make up soil

come from two very different places.

The chaos of life,

and the inert world of rock.

Together, they create an intricate

substance that can naturally

feed and water all

plant life on earth.

And it makes me wonder just how did

this strange

alliance between rock

and life begin?

'How did the very first soil

come to exist?'

To find out, we need to go back

to a time

and place before the first soil

appeared on the planet.

That's not quite as difficult as it

might sound.

This is Malham Cove, an inland cliff

deep in the Yorkshire Dales.

It's a striking landscape,

built from limestone

and sculpted by the awesome

power of ice.

This place offers a wonderful

window into the Earth

billions of years ago,

before there was soil.

That's because at the end

of the last Ice Age,

as temperatures rose and the ice

retreated, it left this

naked rock. Any soil that had been

here had been scoured away

and deposited somewhere in that

direction.

And as a consequence any soil

you see here is relatively new,

in fact, it's still forming.

Making this one of the best

places in the country to discover

how we get from this naked rock,

to this. Soil that supports life.

I'm joined by Professor Steven

Nortcliff from Reading University.

Landscape is fascinating in terms of

the soil.

First, I want to know what could

possibly start to break up

something as seemingly

permanent as rock.

We've got to break it down.

And we've got evidence here in this

landscape

of those early stages of breakdown.

We have ice forming in the fissures

in the rock and as the ice expands

it forces the rock apart. And that's

the first form of disintegration.

When water freezes, it expands.

If that expansion

happens within a crack,

it can exert a force strong enough

to break rock apart.

And you can witness this in your own

freezer at home.

You fill the ice tray and when it

freezes there's expansion.

But it seems remarkable that

that expansion is powerful enough

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