Deep, Down and Dirty: The Science of Soil Page #5

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and, below, the various layers or

horizons of soil,

each with a different characteristic

in terms of colours and textures.

The topsoils, going down into

the subsoils with the roots

penetrating,

this is what we saw in the forest.

But, as we go further down, the dark

organic plant material disappears.

We seem to have left

the soil behind.

These deeper layers are mainly

made up of fragments

of the underlying rock.

And then further down

we're into bedrock.

Collectively, these layers form

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.

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