HOW SOIL HAS BEEN MADE
Apparatus required.
_The apparatus in Fig. 54. The under surface, of the lips of the
beakers should be vaselined to prevent the water trickling down the
sides._
It is not uncommon to find cliffs or crags in inland places, but they
usually show one very striking difference from seaside cliffs. The
seaside cliffs may be nearly vertical, but the inland cliffs are not,
excepting for a little way at the top; lower down a heap of stones and
soil lies piled against the face of the cliff and makes a slope up
which you can climb. If you look at the cliff you can find loose
fragments of it split off either by the action of freezing water (p.
83) or by other causes ready to roll down if sufficiently disturbed.
So long has this been going on that a pile has by now accumulated, and
has been covered with plants growing on the soil of the heap. Our
interest centres in this soil; no one has carried it there; it must
have been made from the rock fragments. When you get an opportunity of
studying such a heap, do so carefully; you can then see how, starting
from a solid rock, soil has been formed. This breaking down of the
rock is called weathering.
[Illustration: Fig. 52. Cliffs at the seaside, Manorbier,
Pembrokeshire]
The same change has gone on at the top of the cliff. Fragments have
split off and the rock has broken {118} down into soil which stops
where it is unless the rain can wash it away. If there are no cliffs
where you live you can see the same kind of action in the banks of the
lanes, in a disused quarry, gravel pit or clay pit. Wherever a
vertical cutting has been made this downward rolling begins and a heap
quickly forms, making the vertical cut into a slope. Plants soon begin
to grow, and before long it is clear that soil has been made out of the
fragments that have rolled down. This process is known as soil
formation, but there is another always going on that we must now study.
The heap does not invariably lie at the foot of the cliff. If there is
a stream, river, or sea at the foot the fragments may be carried away
as fast as they roll down: the differences shown in Figs. 52 and 53
between a cliff at the seaside and a cliff inland arise simply in this
way. In inland districts great valleys are in course of time carved
out, and at the seaside large areas of land have been washed away.
What becomes of the fragments thus carried away by the water? The best
way of answering the question would be to explore one of these mountain
streams and follow it to the sea, but we can learn a good deal by a few
experiments that can be made in the classroom. We want to make a model
stream and see what happens to little fragments of soil that fall into
it.
[Illustration: Fig. 53. Inland cliff. Salisbury Crags, Arthur's Seat,
Edinburgh]
Fix up the apparatus shown in Fig. 54. The small beaker A is to
represent the narrow mountain stream, the larger one _B_ stands for the
wide river, and the glass jar _C_ for the mouth of the river or the
sea. Run water through them; notice that it runs quickly through _A_,
slowly through _B_, and still more slowly through _C_: we want it to do
this, because the stream flows quickly and the river slowly.
{120}
Now put some soil into _A_. At once the soil is stirred up, the water
becomes muddy, and the muddy liquid flows into _B_. But very soon a
change sets in, the liquid in _A_ becomes clear, and only the grit and
stones are left in the bottom: all the mud--the clay and the silt--is
washed into _B_. There it stops for a long time, and some of it will
never wash out. The liquid flowing into _C_ is clearer than that
flowing into _B_. If you keep on putting fresh portions of soil into
_A_ you can keep _B_ always muddy, although _A_ is usually clear. At
the end of the experiment look at the sediment in each beaker: in _A_
it is clear and gritty, in _B_ it is muddy. If you can get hold of
some sea water put some of the liquid from _C_ into it: very soon this
liquid clears and a deposit falls to the bottom, the sea water thus
acting like the lime water on p. 20.
[Illustration: Fig. 54. Model of a stream. In _A_, where the stream
flows quickly, the water is clear and the sediment free from mud. In
_B_, where it flows slowly, the water is turbid and the sediment muddy]
{121}
The experiment shows us that the fine material washed away by a quickly
flowing stream is partly deposited when the river becomes wider and the
current slower, and a good deal more is deposited by the action of the
salt water when the river flows into the sea. The rock that crumbles
away inland is spread out on the bed of the river or at its mouth.
[Illustration: Fig. 56. The two sides of the river at the bend]
The river Stour at Wye showed all these things so clearly that I will
describe it; you must then compare it with a river that you know, and
see how far the same features occur. At the bridge the stream was
shallow and flowed quickly: the bottom was gritty and pebbly, free from
mud, and formed a safe place for paddling. Before the bridge was built
there had been {122} a ford here. But further away, either up or down,
the stream was deeper and wider, flowed more slowly, had a muddy
bottom, and so was not good for paddling. At one place about a mile
away some one had widened out the river to form a lake, but this made
the stream flow so slowly (as it was now so much wider) that the silt
and clay deposited and the lake became silted up, i.e. it became so
shallow that it was little more than a lake of mud. The same facts
were brought out at the bend of the river. On its convex side, Fig.
55, the water has rather further to go in getting round the bend than
on its concave side _B_, it therefore flows more quickly, and carries
away the soil of the bank and mud from the bottom. But on its concave
aide where it flows more slowly it deposits material. There is at the
bend a marked difference in depth at the two sides. On its convex side
the stream is rapid and deep, and scours away the bank; on its concave
side it is slower, shallower, and tends to become silted up. Thus the
bend becomes more and more pronounced unless the bank round _A_ is
protected (the other bank of course needs no protection) and the whole
river winds about just as you see in Fig. 56, and is perpetually
changing its course, carrying away material from one place, mixing it
up with material washed from somewhere else, and then deposits it at a
bend or in a pool where it first becomes a mud flat and then dry land.
Some, however, is carried out to sea. We need not follow the Stour to
the sea; reference to an atlas will show what happens to other rivers.
Some of the clay and silt they carry down is deposited at their mouths,
and becomes a bar, gives rise to shoals and banks, or forms a delta.
The rest is carried away and deposited on the floor of the sea. {124}
Material washed away by the sea from the coast is either deposited on
other parts of the coast, or is carried out and laid on the floor of
the sea. Thus a thick deposit is accumulating, and if the sea were to
become dry this deposit would be soil. This has actually happened in
past ages. The land we live on, now dry land, has had a most wonderful
history; it has more than once lain at the bottom of the sea and has
been covered with a thick layer of sediment carried from other places.
Then the sea became dry land and the sediment became pressed into rock,
which formed new soil, but it at once began to get washed away by
streams and rivers into new seas, and gave rise to new sediments on the
floor of these seas. And so the rock particles have for untold ages
been going this perpetual round: they become soil; they are carried
away by the rivers, in time they reach the sea; they lie at the bottom
of the sea while the sediment gradually piles up: then the sea becomes
dry land and the sediments are pressed into rocks again. The eating
away of the land by water is still going on: it is estimated that the
whole of the Thames valley is being lowered at the rate of about one
inch in eight hundred years. This seems very slow, but eight hundred
years is only a short time in geology, the science that deals with
these changes.
[Illustration: Fig. 56. The winding river Stour. The river winds from
the right to the left of the picture, then back again, and then once
more to the left, passing under the white bridge and in front of the
barn.]
Water does more than merely push the rock particles along. It
dissolves some of them, and in this way helps to break up the rock.
Spring water always contains dissolved matter, derived from the rocks,
some of which comes out as "fur" in the kettles when the water is
boiled.
Rocks are also broken up by other agents. There is nearly always some
lichen living on the rock, and if you {125} peel it off you can see
that it has eaten away some of the rock. When the lichen dies it may
change into food for other plants.
We have learnt these things about soil formation. First of all the
rocks break up into fragments through the splitting action of freezing
water, the dissolving action of liquid water, and other causes. This
process goes on till the fragments are very small like soil particles.
Then plants begin to grow, and as they die and decay they give rise to
the black humus that we have seen is so valuable a part of the soil (p.
51). This is how very many of our soils have been made. But the
action of water does not stop at breaking the rock up into soil; it
goes further and carries the particles away to the lower parts of the
river bed, or to the estuary, to form a delta, and mud flats that may
be reclaimed, like Romney Marsh in England and many parts of Holland
have been. Many of our present soils have been formed in this way.
Finally the particles may be carried right away to sea and spread out
on the bottom to lie there for many ages, but they may become dry land
again and once more be soil.
One thing more we learnt from the river Stour. Why did it flow quickly
at the bridge and slowly elsewhere? We knew that the soil round the
bridge was gravelly, whilst up and down the stream it was clayey. The
river had not been able to make so wide or so deep a bed through the
gravel as it had through the clay, and it could therefore be forded
here. We knew also that there was a gravel pit at the next village on
the river, where also there was a bridge and had been a ford, and so we
were able to make a rough map like Fig. 57, showing that fords had
occurred at the gravel {126} patches, but not at the clay places. Now
it was obvious that an inn, a blacksmith's forge, and a few shops and
cottages would soon spring up round the ford, especially as the gravel
patch was better to live on than the clay round about, and so we
readily understood why our village had been built where it was and not
a mile up or down the stream. Almost any river will show the same
things: on the Lea near Harpenden we found the river flowed quickly at
the ford (Fig. 58), where there was a hard, stony bottom and no mud:
whilst above and below the ford the bottom was muddy and the stream
flowed more slowly. At the ford there is as usual a small village.
The Thames furnishes other examples: below Oxford there are numerous
rocky or gravelly patches where fords were possible, and where villages
therefore grew up. Above Oxford, however, the possibilities of fording
were fewer, because the soil is clay and there is less rock; the roads
and therefore the villages grew up away from the river.
[Illustration: Fig 57. Sketch map showing why Godmersham and Wye arose
where they did on the Stour. At _A_, the gravel patch, the river has a
hard bed and can be forded. A village therefore grew up here. At _B_,
the clay part, the river has a soft bed and cannot be forded. The land
is wet in winter, and the banks of the stream may be washed away. It
is therefore not a good site for a village]
[Illustration: Fig. 58. Ford and Coldharbour, near Harpenden]
{128}
APPENDIX
The teacher is advised to procure, both for his own information and in
order to read passages to the scholars:
Gilbert White, _Natural History of Selborne_.
Charles Darwin, _Earthworms and Vegetable Mould_ (Murray).
A. D. Hall, _The Soil_ (Murray).
Mr Hugh Richardson has supplied me with the following list of
questions, through many of which his scholars at Bootham School, York,
have worked. They are inserted here to afford hints to other teachers
and to show how the lessons may be varied. They should also prove
useful for revising and testing the scholars' knowledge.
1. Collect samples of the different soils in your neighbourhood--garden
soil, soil from a ploughed field, from a mole-hill in a pasture field,
leaf mould from a wood, etc. Collect also samples of the sub-soils,
sand, gravel, clay, peat.
2. Supplement your collection by purchasing from a gardener's shop some
mixed potting soil and also the separate ingredients used to form such
a mixture--silver sand, leaf mould, peat.
3. How many different sorts of peat can you get samples of? Peat
mould, peat moss litter, sphagnum moss, turf for burning, dry moor peat?
4. Find for what different purposes sand is in use, such as mortar
making, iron founding, scouring, bird cages, and obtain samples of each
kind.
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Analysis of Garden Soil. About a handful of soil will be required by
each pupil.
5. Describe the appearance of the soil. Is it fine or in lumps? Does
it seem damp or dry? Can you see the separate particles of mineral
matter? How large are these? Is there any evidence of vegetable
matter in the soil?
6. Put some of the soil in an evaporating basin and over this place a
dry filtering funnel. Warm the basin gently. Is any moisture given
off?
7. Dry some of the soil at a temperature not greater than that of
boiling water, e.g. by spreading it out on a biscuit tin lid, and
laying this on a radiator. How have the appearance and properties of
the soil been changed by drying?
8. Crumble some of the dried soil as finely as you can with your
fingers. Then sift it through a sheet of clean wire gauze. What
fraction of the soil is fine enough to go through the gauze? Describe
the portion which will not pass through the gauze. Count the number of
wires per linear inch in the gauze.
9. Mix some of the soil with water in a flask. Let it stand. How long
does it take before the water becomes quite clear again?
10. Mix some more soil with water. Let it settle for 30 seconds only.
Pour off the muddy water into a tall glass cylinder. Add more water to
the remaining soil, and pour off a second portion of muddy water,
adding it to the first, and so on until all the fine mud is removed
from the soil. Allow this muddy water ample time to settle.
11. When the fine mud has settled pour off the bulk of the water; stir
up the mud with the rest of the water; transfer it to an evaporating
basin, and evaporate to dryness.
12. Does this dried mud consist of very tiny grains of sand or of some
material different from sand? Can you find out with a microscope?
13. If the mud consists of real clay and not of sand it should be
possible to burn it into brick. Moisten the dried mud again. Roll it
if you can into a round clay marble. Leave this to dry slowly for a
day. Then bake it either in a chemical laboratory furnace or in an
ordinary fire.
14. Return to the soil used in Question 10, from which only the fine
mud has been washed away. Pour more water on to it, shake it {130}
well, and pour off all the suspended matter without allowing it more
than 5 seconds to settle. Repeat the process. Collect and dry the
poured off material as before. What is the material this time, sand or
clay?
15. Wash the remaining portion of the soil in Question 14 clean from
all matter which does not settle promptly. Are there any pebbles left?
If so, how large are they, and of what kind of stone?
16. Take a fresh sample of the soil. Mix it with distilled water in a
flask. Boil the mixture. Allow it to settle. Filter. Divide the
filtrate into two portions. Evaporate both, the larger portion in an
evaporating basin over wire gauze, the smaller portion in a watch glass
heated by steam. Is any residue left after heating to dryness?
17. Take a fresh sample of soil. Spread it on a clean sand bath and
heat strongly with a Bunsen flame. Does any portion of the soil burn?
Is there any change in its appearance after heating?
18. To a fresh sample of soil add some hydrochloric acid. Is there any
effervescence? If so, what conclusions do you draw?
19. Make a solution of soil in distilled water, and filter as before.
Is this solution acid, alkaline or neutral? Are you quite certain of
your result? Did you test the distilled water with litmus paper? And
are you sure that your litmus does not contain excess of free acid or
free alkali?
Peat.
20. Examine different varieties of peat collected (see Question 2) and
describe the appearance of each.
21. Burn a fragment of each kind of peat on wire gauze. What do you
notice?
22. Boil some peat with distilled water and filter the solution. What
colour is it? Can you tell whether it is acid, neutral or alkaline?
Evaporate some of the solution to dryness.
Out-of-doors.
23. Describe the appearance of the soil in the flower beds (_a_) during
hard frost, (_b_) in the thaw which follows a hard frost, (_c_) after
an April shower, (_d_) in drought at the end of summer, (_e_) in damp
October weather when the leaves are beginning to fall.
24. Is the soil equally friable at different times of the year?
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25. In what way do dead leaves get carried into the soil?
26. Can you find the worm holes in a garden lawn? in a garden path?
27. Take a flower bed or grass plot of small but known area (say 3
yards by 2 yards) and a watering can of known capacity (say 3 gallons).
Find how much water must be added to the soil before some of the water
will remain on the surface. What has been the capacity of the soil in
gallons per square yard?
28. Take two thermometers. Lay one on the soil, the other with its
bulb 3 inches deep in the soil. Compare their temperatures at morning,
noon and night.
29. Find from the 25-inch Ordnance map the reference numbers of the
fields near your school. Make a list of the fields, showing for what
crop or purpose each field is being used.
{132}
INDEX
Acid waters, 40
Air in soil, 16, 70, 95
Bars in estuaries, 122
Black soils, 36
Blowing sands, 22
Bricks, 10, 16-18
Chalk, 26, 96
Chalk soils, 110-112
Clay, 6, 9-21
Clay soils, 75, 100-102
Cliffs, 116-119
Darwin's experiments, 11, 56
Deltas, 122
Drainage, 19, 96
Dwellers in the soil, 53-63
Earthworms, 54-56
Error of experiment, 48
Fallow, 14, 95
Fens, 112
Flora, 114
Fords, 126
Frost, action of, on soil, 83
Grassland, 75
Grit, 6
Hales's experiment, 73
Heaths, 104
Heavy soils, 100
Hoeing, 86-93
Humus, 36, 51, 93, 125
Hypotheses, 36
Land slips, 12
Leaf mould, 33
Light soils, 104
Lime, action of, on clay and soil, 19-21, 96-98
Lime water, 19
Loams, 2, 65, 108
Marsh gas, 40
Micro-organisms, 56-62
Moorland, 80
Mulch, 87, 90
Peat, 37-40, 130
Peat bogs, overflow of, 38
Perspiration of plants, 74
Plant food, 41-52, 62
Plant requirements, 64
Ploughing, 82
Pot experiments, 41-52, 54, 69, 71
Roads, 30-32, 101-112
Rolling the soil, 84
Sand, 6, 22-32, 41
Sand dunes, 22
Sandy soils, 68-72, 102-108
Shrinkage of clay, 10
Silt, 7
Soil sampler, 82, 88
Sowing seed, 84
Springs, 24-31, 111
Subsoil, 2, 4, 42, 48-51
Swelling of clay, 11
Swelling of peat, 38
Temperature of soil, 86-93
Tilth, 86
Van Helmont's experiment, 46
Villages, situation of, 24, 30, 126
Wastes, 101, 104
Water content of soil, 88-93
Water, movement of in soils, 65-68
Water supply and plant growth, 69-74
Weathering, 116
Weeds, 94, 97
Woodland, 80, 101