Deep, Down and Dirty: The Science of Soil Page #2

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hundreds of others to thrive.

One such animal is so important it's

been called an ecosystem engineer.

In this field, there might well be

over two million of them.

There are no prizes for guessing

which animal I'm seeking out here.

It's one that's inspired

generations of horticulturists

and agriculturists.

It is possibly the greatest

gardener on earth.

And it's this, the humble earthworm.

As a gardener,

I've long known that worms play

an important role in soil.

The great Charles Darwin devoted

over 40 years of study

to them, culminating in the

publication of his seminal work,

The Formation Of Vegetable Mould

Through The Actions Of Worms

With Observations On Their Habits.

You may not have heard of it,

but it sold faster

than On The Origin Of Species.

Darwin's studies, lesser known

than his work on evolution,

revealed an organism that was

essential for the life of the soil.

He became obsessed by them.

He fed them different diets,

tested their intelligence

and even tested their senses

by playing a bassoon to them.

What is about the earthworms

that beguiled Darwin?

Just why are they so important?

Well, first of all the sheer

scale of the worm operation.

As they tunnel into the ground

in their millions,

their burrows permeate the earth

like a vast ventilation system,

providing essential supplies of air

to everything else that

lives in the soil.

But that's not the earthworms'

only talent.

They also continue

what the fungi began.

They eat and digest dead leaves

underground,

unlocking their trapped nutrients.

The way they do this reveals

one of the most fundamental

secrets of soil.

But it's hard to see.

'So I've come to meet

Mark Hodson, Professor

'of Environmental Science

at York University.'

I find they're very fun creatures,

you see them a lot.

If you walk around after the rain

you see them crawling around.

'He's spent years studying what

and how worms eat.'

They go up and down.

During the day, they stay in

the bottom of their burrows.

At night they come out onto

the surface, they look round,

sort of, sometimes they keep their

tails anchored in their burrows.

They sort of stretch out

and eat or grab organic material,

they pull it down into their

burrows to eat later on.

And the undigested material gets

squirted out of the back end and that

helps make all of this black, browny

stuff which is the soil.

Nothing is quicker at breaking down

dead leaves than an earthworm.

It's thought that in the average

field the worms get through

a staggering one and a half

tonnes of plant matter every year.

They're like leaf-processing

factories,

operating on an industrial scale.

Yet they look nothing more than

a simple, fleshy tube.

So what's going on inside?

To help answer that, Mark has been

doing a rather unsavoury experiment.

This Petri dish contains

a sample of plain soil.

And this one was made using earth

that has passed through

an earthworm.

In other words, worm poo.

Mark's been comparing the two

and he's uncovered

evidence of a hidden army of secret

agents at work within the worm.

Bacteria.

So each of these spots is

a bacterial colony. You can see

there are far more growing here

from the material that's just

come out of the earthworm gut.

So the earthworm ingests the soil,

there are bacteria in there already,

and the earthworm gut environment

is good for bacteria.

It's moist, its got the right pH,

the earthworm is secreting mucous

full of polysaccharide sugars,

which the bacteria love to eat.

So it's bacteria

that finish the job of breaking down

dead plant matter.

There are billions of them

naturally present in the ground,

like workers on a production line

turning dead plants into new soil.

But inside the earthworm

this activity is magnified to levels

that are truly mind-blowing.

If you do counts on the soil

in earthworm guts

you can have 1,000 times more active

bacteria in that soil

than the bulk soil

surrounding the earthworm.

What it's proving is

the earthworms have ramped up

the bacterial activity in the soil.

And it's this army of bacteria,

hidden in the guts of earthworms,

that completes the vital cycle.

Unlocking all

the nutrients from dead leaves

and releasing them

back into the soil.

We very often think of soil as being

brown, solid, inert stuff.

But there's more life within in it

than flies, swims or walks above it.

And, far from being a haphazard

array of organisms,

this is a complex

range of interconnected structures

that support the life above.

As we've seen, it takes

a combination of plants, fungi,

animals and bacteria all working

together to keep nutrients

flowing from the dead to the living.

In the process, new soil is created

which in turn supports

even more life, making a cycle that

keeps the soil fertile.

Yet so far we've only scratched

the surface of the soil.

Everything we've seen happens within

just the topmost layers.

'Look deeper and there's

far more to soil than this.

'To reveal just how much,

I first need a bit of heat.'

What I have here is dried topsoil.

I want to find out

how much of this is

derived from plants by setting

fire to it.

If it's 100% plant material,

there should be nothing left.

So I'm starting with 100g.

'Let's see how much remains.'

As this is burning away, the soil is

completely transforming colour.

It's going from a soft brown

to almost a carbon colour.

Very similar to the

embers in a barbecue.

The soil particles are fracturing,

breaking apart. The organic matter

binding them together is burning

away and the soil particles are

just falling to pieces.

'The plant matter is turning

into gases like carbon dioxide

'that are lost into the air.

'After about 15 minutes of intense

heat, I'm going to weigh it again.'

See how much we've lost?

We started off with about 100,

it's now down to 70.

So about 30% of this original soil

was plant based.

It's burnt away.

Clearly, there's more to soil than

just plant material.

To see what that is, we need to get

beneath the topsoil

and look deeper down.

'This is Scolly's Cross

in Aberdeenshire, where

'a landslide has exposed the layers

of soil beneath the pine forest.

'It's something we rarely

get to see,

'as all this is usually hidden

underground.'

In a landslip situation like this

we get to examine perfectly

the soil profile, the horizons or

layers of various materials.

At the top we've got the vegetation

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

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