Lessons on Soil

The columns are given in Fig. 3.

CHAPTER II

MORE ABOUT THE CLAY

Apparatus required.

Clay, about 6 lbs.; a little dried, powdered clay; sand, about 6 lbs. Six glass jars or cylinders [2]. Six beakers [1]. Six egg-cups [1]. Six funnels and stands [2]. Six perforated glass or tin disks [2]. Six glass tubes [2]. Two tubulated bottles fitted with corks. Some seeds. Six small jars about 2 in. x 1 in. [2]. Bricks. The apparatus in Fig. 9. Pestle and mortar.

We have seen in the last chapter that clay will float in water and only slowly settles down. Is this because clay is lighter than water? Probably not, because a lump of clay seems very heavy. Further, if we put a small ball of clay into water it at once sinks to the bottom. Only when we rub the clay between our fingers or work it with a stick—in other words, when we break the ball into very tiny pieces—can we get it to float again. We therefore conclude that the clay floated in our jars (p. 6) for so long not because it was lighter than water, but because the pieces were so small.

Clay is exceedingly useful because of its stickiness. Dig up some clay, if there is any in your garden, or procure some from a brick works. You can mould it into any shape you like, and the purer the clay the better it acts. Enormous quantities of clay are used for making bricks. Make some model bricks about an inch long and half an inch in width and depth, also make a small basin of about the same size, then set them aside for a week in a warm, dry place. They still keep their shape; even if a crack has appeared the pieces stick together and do not crumble to a powder.

If you now measure with a ruler any of the bricks that have not cracked, you will find that they have shrunk a little and are no longer quite an inch long. This fact is well known to brickmakers; the moulds in which they make the bricks are larger than the brick is wanted to be. But what would happen if instead of a piece of clay one inch long you had a whole field of clay? Would that shrink also, and, if so, what would the field look like? We can answer this question in two ways; we may make a model of a field and let it dry, and we can pay a visit to a clay meadow after some hot, dry weather in summer. The model can be made by kneading clay up under water and then rolling it out on some cardboard or wood as if it were a piece of pastry. Cut it into a square and draw lines on the cardboard right at the edges of the clay. Then put it into a dry warm place and leave for some days. Fig. 4 is a picture of such a model after a week's drying. The clay has shrunk away from the marks, but it has also shrunk all over and has cracked. If you get an opportunity of walking over a clay field during a dry summer, you will find similar but much larger cracks, some of which may be two or three inches wide, or even more. Sometimes the cracking is so bad that the roots of plants or of trees are torn by it, and even buildings, in some instances, have suffered through their foundations shrinking away. We can now understand why some of our model bricks cracked. The cracks were caused by the shrinkage just as happens with our model field. As soon as the clay becomes wet it swells again. A very pretty experiment can be made to show this. Fill a glass tube or an egg-cup with dry powdered clay, scrape the surface level with a ruler, and then stand in a glass jar full of rain water so that the whole is completely covered. After a short time the clay begins to swell and forces its way out of the egg-cup as shown in Fig. 5, falling over the side and making quite a little shower. In exactly the same way the ground swells after heavy rain and rises a little, then it falls again and cracks when it becomes dry. Darwin records some careful measurements in a book called Earthworms and Vegetable Mould—"a large flat stone laid on the surface of a field sank 3.33 millimetres[1] whilst the weather was dry between May 9th and June 13th, and rose 1.91 millimetres between September 7th and 19th of the same year, much rain having fallen during the latter part of this time. During frosts and thaws the movements were twice as great."

Another thing that you will have noticed is that anything made of clay holds water. A simple way of testing this is to put a round piece of tin perforated with holes into a funnel, press some clay on to it and on to the sides of the funnel (Fig. 7), and then pour on rain water. The water does not run through. Pools of water may lie like this on a clay field for a very long time in winter before they disappear, as you will know very well if you live in a clay country. So when a lake or a reservoir is being made it sometimes happens that the sides are lined with clay to keep the water in.

If water cannot get through can air? This is very easily discovered: plug a glass tube with clay and see if you can draw or blow air through. You cannot. Clay can be used like putty to stop up holes or cracks, and so long as it keeps moist it will neither let air nor water through. Take two bottles like those in Fig. 8, stop up the bottom tubes, and fill with water. Then put a funnel through each cork and fit the cork in tightly, covering with clay if there is any sign of a leak. Put a perforated tin disk into each funnel, cover one well with clay and the other with sand. Open the bottom tubes. No water runs out from the first bottle because no air can leak in through the clay, but it runs out very quickly from the second because the sand lets air through. These properties of clay and sand are very important for plants. Sow some seeds in a little jar full of clay kept moist to prevent it cracking, and at the same time sow a few in some moist sand. The seeds soon germinate in the sand but not in the clay. It is known that seeds will not germinate unless they have air and water and are warm enough. They had water in both jars, and they were in both cases warm, but they got no air through the clay and therefore could not sprout. Pure clay would not be good for plants to grow in. Air came through the sand, however, and gave the seeds all they wanted for germination.

Let us now compare a piece of clay with a brick. The differences are so great that you would hardly think the brick could have been made from clay. The brick is neither soft nor sticky, and it has not the smooth surface of a piece of clay, but is full of little holes or pores, which look as if they were formed in letting the steam out. A brick lets air through; some air gets into our houses through the bricks even when the windows are shut. Water will get through bricks more easily than it does through clay. After heavy rain you can often find that water has soaked through a brick wall and made the wall paper quite damp. A pretty experiment can be made with the piece of apparatus shown in Fig. 9: bore in a brick a hole about an inch deep and a quarter of an inch wide, put into the hole the piece of bent glass tubing, and fix it in with some clay or putty, then pour some water blackened with ink into the tube, marking its position with a label. Stand the brick in a vessel so full of water that the brick is entirely covered. Water soaks into the brick and presses the air out: the air tries to escape through the tube and forces up the black liquid.

One more experiment may be tried. Can a brick be changed back into clay? Grind up the brick and it forms a gritty powder. Moisten it, work it with your fingers how you please, but it still remains a gritty powder and never takes on the greasy, sticky feeling of pure clay. Indeed no one has ever succeeded in making clay out of bricks. All these experiments show that clay is completely altered when it is burnt. We also found that soil is completely altered by burning, and if you look back at your notes you will see that the changes are very much alike, so much so that we can safely put down some of the changes in the burnt soil—the red colour, the hard grittiness, and the absence of stickiness—to the clay. Let us now examine a piece of dry, but unburnt, clay. It is very hard and does not crumble, it is neither sticky nor slippery. Directly, however, we add some water it changes back to what it was before. Drying therefore alters clay only for the time being, whilst baking changes it permanently.

[1] A little more than one-eighth of an inch.

CHAPTER III

WHAT LIME DOES TO CLAY

Apparatus required.

Clay, about 6 lbs. Some of the clay from Chapter II may, if necessary, be used over again. Lime, about 1/2 lb. Six funnels, stands and disks [2]. Twelve glass jars [2]. Lime water[1].

If you are in a clay country in autumn or early winter you will find some of the fields dotted with white heaps of chalk or lime, and you will be told that these things "improve" the soil. We will make a few experiments to find out what lime does to clay. Put some clay on to a perforated tin disk in a funnel just as you did on p. 14, press it down so that no water can pass through. Then sprinkle on to the clay some powdered lime and add rain water. Soon the water begins to leak through, though it could not do so before; the addition of the lime, therefore, has altered the clay. If you added lime to a garden or a field on which water lay about for a long time in winter you would expect the water to drain away, especially if you made drains or cut some trenches along which the water could pass. There are large areas in England where this has been done with very great advantage.

The muddy liquid obtained by shaking clay with water clears quickly if a little lime is stirred in. Fill two jars A and B (Fig. 10) with rain water, rub clay into each and stir up so as to make a muddy liquid, then add some lime water to B and stir well. Leave for a short time. Flocks quickly appear in B, then sink, leaving the liquid clear, but A remains cloudy for a long time. But why should the liquid clear? We decided in our earlier experiments that the clay floated in the water because it was in very tiny pieces; when we took a larger lump the clay sank. The lime has for some reason or other, which we do not understand, made the small clay particles stick together to form the large flocks, and these can no longer float, but sink. If we look at the limed clay in our funnel experiment we shall see that the same change has gone on there; the clay has become rather loose and fluffy, and can therefore no longer hold water back.

Lime also makes clay less sticky. Knead up one piece of clay with rain water alone and another piece with rain water and about 1/20 its weight of lime. The limed clay breaks easily and works quite differently from the pure clay.

SUMMARY. This, then, is what we have learnt about clay. Clay is made up of very, very, tiny pieces, so small that they float in water. They stick together when they are wetted and then pressed, and they remain together; a piece of clay moulded into any pattern will keep its shape even after it is dried and baked. Clay is therefore made into bricks, earthenware, pottery, etc., whilst white clay, which is found in some places, is made into china. Wet clay shrinks and cracks as it dries; these cracks can easily be seen in the fields during dry weather. This shrinkage interferes with the foundations of houses and other buildings, causing them to settle. Dry clay is different from wet clay, it is hard, not sticky and not slippery, but it at once becomes like ordinary clay when water is added. After baking, however, clay permanently alters and cannot again be changed back to what it was before. Clay will not let water pass through; a clay field is therefore nearly always wet in winter and spring. Nor can air pass through until the clay dries or cracks.

Lime has a remarkable action on clay. It makes the little, tiny pieces stick together to form feathery flocks which sink in water; lime therefore causes muddy clay water to become clear. The flocks cannot hold water back, and hence limed clay allows water to pass through. Limed clay is also less sticky than pure clay. A clay field or garden is improved by adding lime because the soil does not remain wet so long as it did before; it is also less sticky and therefore more easily cultivated.

[1] Lime water is made by shaking up lime and water. It should be kept in a well-corked bottle.

CHAPTER IV

SOME EXPERIMENTS WITH THE SAND

Apparatus required.

Sand, about 6 lbs.; clay, about 6 lbs. Six funnels, stands and disks [1]. Six glass jars [2]. One box with glass front shown in Fig. 13 filled with clay and sand, as indicated. Quarry chalk (about 5 lbs.). Six beakers [1]. Six egg-cups [1].

If there is a sand pit near you, or a field of sandy soil, you should get a supply for these experiments; if not, some builder's sand can be used. When the sand is dry you will see that the grains are large and hard. Further, they are all separate and do not stick together; if you make a hole in a heap of the sand, the sides fall in, there is nothing solid about it, and you can easily see the mistake of the foolish man who built his house upon the sand. When the sand is wet it sticks better and can be made into a good many things; at the seaside you can make a really fine castle with wet sand. But as soon as the sand dries it again becomes loose and begins to fall to pieces.

Strong winds will blow these fragments of dry sand about and pile them up into the sand hills or dunes common in many seaside districts (Fig. 11). Blowing sands can also be found in inland districts; in the northern part of Surrey, in parts of Norfolk and many other places are fields where so much of the soil is blown away by strong winds that the crops may suffer injury. In Central Asia sand storms do very much harm and have in the course of years buried entire cities. Fig. 12 shows the Penhale sands in Cornwall gradually covering up some meadows and ruining them.

At the foot of a hill formed of sand you often find a spring, especially if clay or solid rock lies below. It is easy to make a model that will show why the spring forms at this particular place. Fill the lower part of the box shown in Fig. 13 with wet clay, smoothing it out so that it touches all three sides and the glass front; then on top of the clay put enough sand to fill the box. Bore four holes in the side as shown in the picture, one at the bottom, one at the top, one just above the junction of the sand and clay, the fourth half way up the sand, and fix in glass tubes with clay or putty. Pour water on to the sand out of a watering can fitted with the rose so as to imitate the rain. At first nothing seems to happen, but if you look closely you will notice that the water soaks through and does not lie on the surface; it runs right down to the clay; then it comes out at the tube there (c in the picture). None goes through the clay, nor does enough stay in the sand to flow out through either the top or the second tube; of the four tubes only one is discharging any water. The discharge does not stop when the supply of water stops. The rain need only fall at intervals, but the water will flow all the time.

Just the same thing happens out of doors in a sandy or chalky country; the rain water soaks through the sand or chalk until it comes to clay or solid rock that it cannot pass, then it stops. If it can find a way out it does so and makes a spring, or sometimes a whole line of springs or wet ground. Rushes, which flourish in such wet places, will often be found growing along this line, and may, indeed, in summer time be all you can see, the water having drained away. But after much rain the line again becomes very wet. Fig. 14 shows the foot of a chalk hill near Harpenden, where a spring breaks out just under the bush at the right-hand side of the gate. In Fig. 15 the bush itself is seen, with the little pool of water made by the spring. Here the water flows gently, but elsewhere it sometimes happens, as in Fig. 16, that the spring breaks out with great force.

Now stop up the glass tubes so that the water cannot get out. Soon the sand becomes flooded and is no better than clay would be. A second model will show this very well. Make a large saucer of clay and fill with sand: pour water on. The water stays in the sand, because it cannot pass through the clay. A sandy field saturated like this will therefore not be dry, but wet, and will not make a good position for a house. We must therefore distinguish the two cases illustrated in Fig. 17. A shows sand on a hill, dry because the water runs through until it comes to clay or rock, when it stops and breaks out as a spring, a tiny stream, or pond; this is a good building site and you may expect to find large houses there. B shows the sand in a basin of clay, where the water cannot get away: here the cellars and downstairs rooms are liable to be wet, and in a village the wells give impure water. Matters could be improved if a way out were cut for the water, but then the foundations of the buildings might move a little.

On the other hand large tracts of clay which remain wet and sticky during a good part of the year are not very attractive to live in, and even near London they were the last to be populated: Hither Green in the south-cast and the clay districts of the north-west have only of late years been built on; while the sands and gravels of Highgate, Chiswick, Brentford and other places had long been occupied. Elsewhere, villages on the clay do not grow quickly unless there is much manufacturing or mining; the parishes are large, the roads even now are not good while they used to be very bad indeed. Macaulay tells us that at the end of the seventeenth century in some parts of Kent and Sussex "none but the strongest horses could in winter get through the bog, in which at every step they sank deep. The markets were often inaccessible during several months. . . The wheeled carriages were, in this district, generally pulled by oxen. When Prince George of Denmark visited the stately mansion of Petworth in wet weather, he was six hours in going nine miles; and it was necessary that a body of sturdy hinds should be on each side of his coach to prop it up. Of the carriages which conveyed his retinue several were upset and injured. A letter from one of the party has been preserved in which the unfortunate courier complains that, during fourteen hours, he never once alighted, except when his coach was overturned or stuck fast in the mud." The Romans knew how to make roads anywhere, and so they made them run in a straight line between the two places they wished to connect, but the art was lost in later years, and the country roads made in England since their time usually had to follow the sand or the chalk, avoiding the clay as much as possible. These roads we still use. Fig. 18 shows the roads round Wye; you should in your rambles study your own roads and see what soil they are on.

There are several other ways in which sand differs from clay. It does not shrink on drying nor does it swell on wetting, and you will find nothing happens when you try with sand the experiment with the model field (p. 11) or the egg-cup (p. 12).

CHAPTER V

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 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 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:—

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 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 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.

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.