Read Ebook: Roundabout to Moscow: An Epicurean Journey by Bouton John Bell

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SOME EXPERIMENTS WITH THE SAND

Apparatus required.

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 . 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.

Sand particles, being large, do not float in water. If we shake up sand in water the sand sinks, leaving the water entirely clear. So running water does not carry sand with it unless it is running very quickly: the sand lies at the bottom.

Unlike clay, sand does not hold water. Pour some water on to sand placed on the tin disk in a funnel ; it nearly all runs through at once. We should therefore expect a sandy field or a sandy road to dry up very quickly after rain and not to remain wet like a clay field. So much is this the case that people prefer to live on a sandy soil rather than on a clay. The most desirable residential districts round London, Hampstead on the north, and the stretch running from Haslemere on the south-west to Maidstone on the south-east, and other favoured regions, are all high up on the sand.

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 . 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.

The experiment should now be tried with some chalk from a quarry; it gives the same results and shows that chalk, like sand, allows water readily to pass.

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.

It often happens that villages are situated at the junction of sand and clay, or chalk and clay, because the springs furnish forth a good water supply.

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 or the egg-cup .

THE PART THAT BURNS AWAY

Apparatus required.

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:--

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 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 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 . 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.

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.

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.

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 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 .

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.

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.

THE PLANT FOOD IN THE SOIL

Apparatus required.

It is a rare sight in England to see land in a natural uncultivated state devoid of vegetation. The hills are covered with grasses and bushes, the moors with ling and heather, commons with grass, bracken and gorse, a garden tends to become smothered in weeds, and even a gravel path will not long remain free from grass. It is clear that soil is well suited for the growth of plants. We will make a few experiments to see what we can find out about this property of soil.

We have seen that a good deal of the soil is sand or grit, and we shall want to know whether this, like soil, can support plant life. We have also found that the subsoil is unlike the top soil in several ways, and so we shall want to see how it behaves towards plants. Fill a pot with soil taken from the top nine inches of an arable field or untrenched part of the garden; another with subsoil taken from the lower depth, 9 to 18 inches, and a third with clean builder's sand or washed sea-sand. Sow with rye or mustard, and thin out when the seeds are up. Keep the pots together and equally well supplied with water; the plants then have as good a chance of growth in one pot as in any other.

Figs. 20 and 21 are photographs of sets of plants grown in this way; the weights in grains were:--

Green weight After drying

Rye Mustard Rye Mustard

Plants grown in top soil 14.5 17.7 5.6 2.6

" " " subsoil 2.9 5.1 1.6 1.1

" " " sand 2.0 4.6 0.8 1.0

The plants in the soil remained green and made steady growth. Those in the sand never showed any signs of getting on, their leaves turned yellow and fell off; in spite of the care they received, and the water, warmth and air given them, they looked starved, and that, in fact, is what they really were. Nor did those in the subsoil fare much better. The experiment shows that the top soil gives the plant something that it wants for growth and that it cannot get either from sand or from the subsoil; this something we will call "plant food."

Further proof is easily obtained. At a clay or gravel pit little or no vegetation is to be seen on the sloping sides or on the level at the bottom, although the surface soil is carrying plants that shed innumerable seeds. A heap of subsoil thrown up from a newly made well, or the excavations of a house, lies bare for a long time. The practical man has long since discovered these facts. A gardener is most particular to keep the top soil on the top, and not to bury it, when he is trenching. In levelling a piece of ground for a cricket pitch or tennis court, it is not enough to lift the turf and make a level surface; the work has to be done so that at every point there is sufficient depth of top soil in which the grass roots may grow.

How much plant food is there in the top soil? To answer this question we must compare soil that has been cropped with soil that has been kept fallow, i.e. moist but uncropped. Tip out some of the soil that has been cropped with rye, and examine it. Remove the rye roots, then replace the soil in the pot and sow with mustard; sow also a fallow pot with mustard. Keep both pots properly watered. The soil that has carried a crop is soon seen to be much the poorer of the two. Fig. 22 shows the plants, while their weights in grams were:--

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