Chapter 11

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.

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.

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.

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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.

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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.

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.

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.

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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.

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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