THE PART THAT BURNS AWAY
Apparatus required.
_Leaf mould. Mould from a tree. Peat. About 1 lb. soil from a wood,
a well-manured garden and a field; also some subsoil. Six crucibles or
tin lids. Six tripods, pipe-clay triangles, and bunsen burners or
spirit lamps. Six beakers and egg-cups [1]._
In the autumn leaves fall off the trees and form a thick layer in the
woods. They do not last very long; if they did they would in a few
years almost bury the wood. You can, in the springtime or early summer
find out what has happened to them if you go into a wood or carefully
search under a big hedge in a lane where the leaves were not swept
away. Here and there you come across skeleton leaves where only the
veins are left, all the rest having disappeared. But generally where
the leaves have kept moist they have changed to a dark brown mass which
still shows some of the structure of a leaf. This is called leaf
mould. The top layer of soil in the wood is soft, dark in colour, and
is evidently leaf mould mixed with sand or soil.
Leaf mould is highly prized by gardeners, indeed gardeners will often
make a big heap of leaves in autumn and let them "rot down" and change
into mould. If you can in autumn collect enough leaves to make a heap
you {34} should do so and leave it somewhere where the rain can fall on
it, but cover it with a few small branches of trees to prevent the wind
blowing the leaves away. The heap shrinks a great deal during the
first few months, and in the end it gives a supply of mould that will
be very useful if you want to grow any plants in pots.
Some of the little hollows in the bank under a hedge, especially on
chalky soils, are filled with leaf mould which has sometimes changed to
a black powder not looking at all like leaves.
You can also find mould in holes in decayed trees; here it has formed
from the wood of the tree.
It appears, then, that dead leaves, etc., slowly change into a black or
brown substance, shrinking very much as they do so. For this reason
they do not go on piling up year after year till finally they fill the
wood; instead they decay or "rot down" to form leaf mould: the big pile
of the autumn has changed by the next summer to a thin layer which
mixes with the soil.
We want now to see what happens on a common or a piece of waste ground
that is not cultivated. Grass and wild plants grow up in summer and
die during winter; their stems and roots are not taken away, but
clearly they do not remain where they are, because next year new plants
grow up. We may suppose that the dead roots and stems decay like the
leaves did, and change to a brown or black mould. It looks as if we
are right, because on digging a hole or examining the side of a freshly
cut ditch we shall find that the top layer of soil, just so far as the
living roots go, is darker in colour than the layer below.
We must, however, try and get some more proof, and to do this we must
study some of our specimens a little {35} more closely. We will take
some leaf mould, some black mould from a hollow in the bank, some from
a tree, soils from a wood, a well-manured garden, a field and some
subsoil. All except the subsoil have a dark colour, but the wood and
garden soils are probably darker than the field soil. Now weigh out 2
grains of each of these and heat in a dish as you did the soil on p. 4;
notice that all except the subsoil go black and then begin to smoulder,
but the moulds smoulder more than the soils. Then weigh again and
calculate how much has burnt away in each case. Here are some results
that have been obtained at Harpenden:--
Amount Percentage
Colour before of loss on Colour of
burning smouldering burning residue
Leaf mould dark brown much 78.3 light grey
Mould from
dead tree black much 60.6 light grey
Soil from
wood dark brown less 43.4[1] white
Soil from
garden almost black less 10.1 red
Soil from
field brownish still less 5.4 red
Subsoil red none 2.0 red
The mould nearly all burns away and its dark colour entirely goes, so
also does the dark colour of the soil.
Our supposition explains why, in the case of soils, the less the
blackness, the less the loss on burning. If the {36} brown or black
combustible part is really mould formed by the decay of plant roots,
etc., then we should expect that as the percentage of mould in the soil
increased, so its blackness would increase and its loss on burning
would become greater. This actually happens.
This, then, is our idea. We suppose that the plants that have lived in
past years have decayed to form a black material like leaf mould which
stops in the soil, giving it a darkish colour. The more mould there
is, the darker the colour of the soil. We know that along with this
decay there is a great deal of shrinkage. As the black material is
formed from the plant, it only extends as far into the soil as the
plant roots go, so that there is a sharp change in colour about 6
inches below the surface (see also p. 2). Like the plant the black
material all burns away when the soil is heated sufficiently.
Thus we can explain all the facts we have observed, and in what seems a
very likely way. This does not show that our supposition is correct,
but only that it is useful. When you come to study science subjects
you will find such suppositions, or hypotheses as they are called, are
frequently used so long as they are found to be helpful. In our
present case we could only get absolute proof that the black
combustible part of the soil really arose from the decay of plants by
watching the process of soil formation. We shall turn later to this
subject.
The black material is known as humus. Farmers and gardeners like a
black soil containing a good deal of humus because they find it very
rich, and we shall see later on why this is so. Vast areas of such
soils occurring in Manitoba, in Russia, and in Hungary are used for
{37} wheat growing, while there are also areas in the Fen districts of
England.
There is something known as peat that looks rather like mould, but is
really so different that you must be careful not to confuse the two.
Peat is not good for plants, and does not make the soil fertile, but
quite the reverse. You can see it being formed on a moor or bog, and
you should at the first opportunity go and examine it. There was a
peat bog near Wye that was examined with the following results. The
peat was very fibrous and had evidently been formed from plants. It
made a layer about 2 feet thick and underneath it was a bed of clay;
this was discovered by examining the ditches, some of which cut right
through the peat into the clay below. A sample of the clay put into a
funnel, as on p. 14, did not allow water to pass through; this was also
evident from the very wet nature of the ground. The peat bed was below
the level of the surrounding land and was in a sort of basin; the water
draining from the higher land could all collect there but could not run
away, indeed it might very well have been a shallow lake. It was quite
clear that the plants as they died would _decay in very wet soil_, and
so the conditions are very different from those we have just been
studying where the plants _decay in soil that is only moist_. This
difference at once shows itself in the fact that peat generally forms a
thick layer, while mould only rarely does so. In the north of England
the moors lie high, but here again the peat bed is like a saucer or
basin, and there is soil or rock below that does not let the rain water
pass through. For a great part of the year the beds are very wet.
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Look at a piece of peat and notice how very fibrous it is, quite unlike
leaf mould. When it is dry peat easily burns and is much used as fuel
in parts of Scotland, Wales and Ireland. It is cut in blocks during
the spring, left to dry in heaps during summer, and then carried away
in autumn. Fig. 19 shows a peat bog with cutting going on. Peat does
not easily catch light and the fires are generally kept burning all
night; there is no great flame such as you get with a coal fire, but
still there is quite a nice heat.
Peat has a remarkable power of absorbing water. Fill an egg-cup with
peat, packing it as tightly as you possibly can, and then put it under
water and leave for some days. The peat becomes very wet and swells
considerably, overflowing the cup just like the clay did on p. 12.
After long and heavy rains peat in bogs swells up so much that it may
become dangerous; if the bog is on the side of a hill, the peat may
overflow and run down the hill like a river, carrying everything before
it. Such overflows sometimes occur in Ireland and they used to occur
in the north of England; you can read about one on Pendle Hill in
Harrison Ainsworth's _Lancashire Witches_. But they do not take place
when ditches are cut in the bog so that the water can flow away instead
of soaking in; this has been done in England.
This great power of absorbing water and other liquids, so terrible when
it leads to overflows, enables peat to be put to various uses, and a
good deal of it is sold as peat-moss, for use in stables.
[Illustration: Fig. 19. Cutting and carrying peat for fuel, Hoy,
Orkney]
In the ditches of a peat bog red slimy masses can often be found. They
look just like rusty iron, and in fact they do contain a good deal of
iron, but there are also a number of tiny little living things present.
The {40} stones and grit just under the peat are usually white, all the
red material from them having been washed out by the water which has
soaked through the peat. Then at the ditch these tiny living things
take up the red material because it is useful to them. Peat or
"moorland" water can also dissolve lead from lead pipes and may
therefore be dangerous for drinking purposes unless it is specially
purified. When you study chemistry you will be able to show that both
peat itself and moorland waters are "acid" while good mould is not.
That is why peat is not good for cultivated plants (see also p. 96).
Other things besides peat are formed when plants decay under water. If
you stir up the bottom of a stagnant pond with a stick bubbles of gas
rise to the surface and will burn if a lighted match is put to them.
This gas is called marsh gas. Very unpleasant and unwholesome gases
are also formed.
[1] The top two inches of soil only were collected here, and there were
many leaves, twigs, etc. mixed in. Soils from different woods vary
considerably. If the sample is taken to a greater depth the loss on
burning is much less, and may be only 5 or 6 per cent.
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