THE DWELLERS IN THE SOIL
Apparatus required.
_Garden soil. Six bottles and corks [1]. Twelve Erlenmeyer flasks, 50
c.c. capacity [2]. Cotton wool. Milk (about half a pint). Leaf
gelatine. Soil baked in an oven. Six saucers [3]. The apparatus in
Fig. 28 (two lots). Wash bottle containing lime water (Fig. 27, also
p. 19)._
In digging a garden a number of little animals are found, such as
earthworms, beetles, ants, centipedes, millipedes and others. There
are also some very curious forms of vegetable life. By carefully
looking about it is not difficult to find patches of soil covered with
a greenish slimy growth; they are found best under bushes where the
soil is not disturbed, or else where the soil has been pressed down by
a footmark and not touched since. Any good soil left undisturbed for a
time shows this growth.
Put some fresh moist garden soil into a bottle and cork it up tightly
so that it keeps moist. Write the date on the bottle and then leave it
in the light where you can easily see it. After a time--sometimes a
long, sometimes a shorter time--the soil becomes covered with a slimy
growth, greenish in colour, mingled here and there with reddish brown.
The longer the {54} soil is left the better. Often after several
months something further happens; little ferns begin to grow and they
live a very long time indeed. There is at Rothamsted a bottle of soil
that was put up just like this as far back as 1874. For a number of
years past a beautiful fern has been growing inside the bottle, and
even now it is very healthy and vigorous. If, instead of being kept
moist, the rich garden soil is left in a dry shed during the whole of
the winter so that it gradually loses its moisture, it will generally
show quite a lot of white fluffy growth.
All of these living things are very wonderful, and some, especially
earthworms, are very useful to gardeners and farmers.
After a shower of rain look carefully in the garden or else on a lawn,
common, or pasture field where the grass is closely grazed by cattle or
does not naturally grow long, and you will find numbers of tiny heaps
of soil scattered about. Carefully brush away a heap and a little hole
is seen, now hit the ground near it a few times with a stick or stamp
on it with your foot and the worm, if he is near the top, comes up.
When he is safely out of the way dig carefully down with a knife or
trowel so as to examine the hole or "burrow." At the top you generally
find it lined with pieces of grass or leaves that the worm has pulled
in; lower down the lining comes to an end, but the colour of the burrow
is redder than that of the rest of the soil wherever the soil has a
greenish tinge. These holes are useful because they let air and water
down into the soil.
[Illustration: Fig. 24. Soil in which earthworms have been living and
making burrows]
The following experiment shows what earthworms can do. Fill a pot with
soil from which all the worms have been carefully picked out and
another {55} with soil to which earthworms have been added, one worm to
every pound of soil. Leave them out of doors where the rain can fall
on to them. You can soon see the burrows and the heaps of soil or
"casts" thrown up by the worms: these casts wash or blow over the
surface of the soil, continually covering it with a thin layer of
material brought up from below. Consequently the soil containing
earthworms always has {56} a fresh clean look. After some time the
other soil becomes very compact and is covered with a greenish slimy
growth. When this happens carefully turn the pots upside down, knock
them so as to detach the soil and lift them off. The soil where the
earthworms had lived is full of burrows and looks almost like a sponge.
Fig. 24 shows what happened in an experiment lasting from June to
October. The other soil where there were no earthworms shows no such
burrows and is rather more compact than when it was put in.
Earthworms therefore do three things:--
(1) They make burrows in the ground and so let in air and water.
(2) They drag leaves into the soil and thus help to make the mixture of
soil and leaf mould.
(3) They keep on bringing fresh soil up to the surface, and they
disturb the surface so much that it is always clean and free from the
slimy growth.
All these things are very useful and so a gardener should never want to
kill worms. The great naturalist, Darwin, spent a long time in
studying earthworms at his home in Kent and wrote a very interesting
book about them, called _Earthworms and Vegetable Mould_. He shows
that each year worms bring up about 1/50th of an inch of soil, so that
if you laid a penny on the soil now and no one took it, in 50 years it
might be covered with an inch of soil. Pavements that were on the
surface when the Romans occupied Britain are now covered with a thick
layer of soil.
[Illustration: Fig. 25. Fresh soil turns milk bad, but baked soil does
not]
But besides these there are some living things too small to see, that
have only been found by careful experiments, but you can easily repeat
some of these {57} experiments yourselves. Divide a little rich garden
soil into two parts and bake one in the kitchen oven on a patty tin.
Pour a little milk into each of two small flasks, stop up with cotton
wool (see Fig. 25) and boil for a few minutes very carefully so that
the milk does not boil over, then allow to cool. Next carefully take
out the stopper from one of the flasks and drop in a little of the
baked soil, label the flask "baked soil" and put back the stopper.
Into the other flask drop a little of the untouched soil and label it;
leave both flasks in a warm place till the next day. Carefully open
the stoppers and smell the milk: the baked soil has done nothing and
the milk smells perfectly sweet; the unbaked soil, on the other hand,
has made the milk bad and it smells like cheese. If you have a good
microscope you can go further: look at a drop of the liquid from each
flask and you find in each case the {58} round fat globules of the
milk, but the bad milk contains in addition some tiny creatures,
looking like very short pins, darting in and out among the fat
globules. These living things must have come from the unbaked soil or
they would have been present in both flasks: they must also have been
killed by baking in the oven.
[Illustration: Fig. 26. Soils contain tiny things that grow on
gelatine]
Another experiment is easy but takes a little longer to show. Mix two
sheets of leaf gelatine with a quarter {59} of a pint of boiling water,
pour into each of three saucers, and cover over with plates. Then stir
up some baked soil in a cup half full of cold boiled water, and quickly
put a teaspoonful of the liquid into a second cup, also half full of
cold boiled water. Stir quickly and put a spoonful on to the jelly,
tilting it about so that it covers the whole surface and label the
saucer "baked soil." Do the same with the "unbaked soil," labelling
the saucer; leave the third jelly alone and label it "untouched."
Cover all three with plates and leave in a warm place. After a day or
so little specks begin to appear on the jelly containing the unbaked
soil, but not on the others (Fig. 26); they grow larger, and before
long they change the jelly to a liquid. The other jellies {60} show
very few specks and are little altered. These creatures making the
specks came from the soil because so few are found on the jelly alone;
they were killed in the baking and so do not occur on the baked soil
jelly.
[Illustration: Fig. 27. Bottle containing lime water, used to show
that breath makes lime water milky]
You can also show that breathing is going on in the soil even after you
have picked out every living thing that you can see. First of all you
must do a little experiment with your own breathing so that you may
know how to start. Shake up a little fresh lime with water and leave
it to stand for 24 hours. Pour a little of the clear liquid into a
flask or bottle fitted with a cork and two tubes, one long and one
short like that shown in Fig. 27. Then breathe in through the tube _A_
so that the air you take in comes through the lime water: notice that
no change occurs. Next breathe out through the tube _B_ so that your
breath passes through the lime water; this time the lime water turns
very milky. You therefore alter in some way the air that you breathe:
you know also that you need fresh air.
Now we can get on with our soil experiments. Take two small flasks of
equal size fitted with corks and joined by a glass tube bent like a U
with the ends curled over. Put some lime water into each flask and a
little water in the U-tube. Now make a small muslin bag like a
sausage: fill it with moist fresh garden soil, tie it up with a silk
thread and hang it in one of the flasks by holding the end of the
thread outside and pushing in the cork till it is held firmly (see Fig.
28). Fix on the other flask, and after about five minutes mark the
level of the liquid with a piece of stamp paper; leave in a warm place
but out of the sun. {61} In one or two days you will see that the
water in the U-tube has moved towards the soil flask, showing that some
air has been used up by the soil; further, the lime water has turned
milky. But in the other flask, where there is no soil, the lime water
remains quite clear.
This proves, then, that some of the tiny creatures want air just as
much as we do. The air readies them through passages in the soil,
through the burrows of earthworms and other animals, or by man's
efforts in digging and ploughing.
[Illustration: Fig. 28. A bag of soil is fixed into a flask containing
lime water. In a few days some of the air has been used up, and the
lime water has turned milky]
Now try the experiment with very dry garden soil: little or no change
takes place. As soon as you add water, however, breathing begins
again, air is absorbed and the lime water turns milky just as before.
Water is therefore wanted just as much as air.
If you had very magnifying eyes and could see things so enlarged that
these little creatures seemed to {62} you to be an inch long, and if
you looked down into the soil, it would seem to you to be an
extraordinarily wonderful place. The little grains of soil would look
like great rocks and on them you would see creatures of all shapes and
sizes moving about, and feeding on whatever was suitable to them, some
being destroyed by others very much larger than themselves, some
apparently dead or asleep, yet waking up whenever it becomes warmer or
there was a little more moisture. You would see them changing useless
dead roots and leaves into very valuable plant food; indeed it is they
that bring about the changes observed in the experiments of Chap. VI.
Occasionally you would see a very strange sight indeed--a great
snake-like creature, over three miles long and nearly half a mile
round, would rush along devouring everything before it and leave behind
it a great tunnel down which a mighty river would suddenly pour, and
what do you think it would be? What you now call an earthworm and
think is four inches long, going through the soil leaving its burrow
along which a drop of water trickles! That shows you how tiny these
little soil creatures are.
These busy little creatures are called micro-organisms because of their
small size. But they are not all useful. Some can turn milk bad as we
have already seen, and therefore all jugs and dishes must be kept clean
lest any of them should be present. Others can cause disease. It has
happened that a child who has cut its finger and has got some soil into
the cut, and not washed it out at once, has been made very ill. You
may sometimes notice sheep limping about in the fields, especially in
damp fields; an organism gets into the foot and causes trouble.
{63}
SUMMARY. The soil is full of living things, some large like earth
worms, others very small. Earthworms are very useful: they make
burrows in the soil, thus allowing air and water to get in: they drag
in leaves and they keep on covering the surface with soil from below.
Besides these and the other large creatures, there are micro-organisms
so small that they cannot be seen without a very good microscope: they
live and breathe and require air, water and food. Some are very useful
and change dead parts of plants or animals into valuable plant food.
Almost anything that can be consumed by fire can be consumed by them.
Others are harmful.
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