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 (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.

CHAPTER VI

THE PLANT FOOD IN THE SOIL

Apparatus required.

The pot experiments (p. xiii).

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:—

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:—

The rye has taken most of the plant food that was in Pot 1 leaving very little for the second crop. Our soil therefore contained only a little plant food, not more, in fact, than will properly feed one crop. But yet it did not seem to have altered in any way, even in weight, in consequence of the plant food being taken out. In our experiment the soil was dried and weighed before and after the mustard was grown; the results were:—

The experiment is not good enough to tell us exactly how much plant food was present at the beginning. But we can say that the amount of plant food in the soil is too small to be detected by such weighing as we can do.

Here is an account of a similar experiment made 300 years ago by van Helmont in Brussels, and it is interesting because it is one of the first scientific experiments on plant growth:—

"I took an earthen vessel in which I put 200 pounds of soil dried in an oven, then I moistened with rain water and pressed hard into it a shoot of willow weighing 5 pounds. After exactly five years the tree that had grown up weighed 169 pounds and about 3 ounces. But the vessel had never received anything but rain water or distilled water to moisten the soil (when this was necessary), and it remained full of soil which was still tightly packed, and lest any dust from outside should have got into the soil it was covered with a sheet of iron coated with tin but perforated with many holes. I did not take the weight of the leaves that fell in the autumn. In the end I dried the soil once more, and got the same 200 pounds that I started with, less about two ounces. Therefore the 164 pounds of wood, bark and root arose from the water alone." The experiment is wonderfully good and shows how very little plant food there is in the soil. The conclusion is not quite right, however, although it was for many years accepted as proof of an ancient belief, which you will find mentioned in Kingsley's Westward Ho!, that all things arose from water. It is now known that the last sentence should read, "Therefore the 164 pounds of wood, bark and root arose chiefly from the water and air, but a small part came from the soil also."

But to return to our experiment with Pots 1 and 2. They had been kept moist before the mustard was sown. Did this moisture have any effect on the soil? Take two of the pots that have been kept dry and uncropped, and two that have been kept moist and uncropped, also one of dry uncropped subsoil and one of moist uncropped subsoil. Sow rye or mustard in each pot and keep them all equally supplied with water.

It is soon evident that the top soil is richer in plant food than the subsoil, and the soil stored moist is rather richer than that stored dry, although the difference here is less marked. In an experiment in which the soils were put up early in July and sown at the end of September the weights of crops in grams obtained were:—

The crops on Pots 10 and 11 ought of course to weigh the same, and so should the crops on Pots 8 and 9. The differences arise from the error of the experiment. In all experimental work, however carefully carried out or however skilful the operator, there is some error.

There is clearly an increase in crop as a result of storing the surface soil in a moist condition, showing that additional plant food has been made, since these pots were put up. On the other hand it does not appear that much plant food has been made in the subsoil during this time. Further evidence on this point is given by an experiment similar to that in Fig. 22, but where mustard is grown in subsoil kept moist, but uncropped for some time, and in subsoil previously cropped with rye. The results in grams were:—

These should be compared with the figures on p. 45. Although the subsoil lay fallow for a long time it produced no plant food but is just as poor as the subsoil that has been previously cropped. These observations give us a clue that must be followed up in answering our next question.

What has the plant food been made from? Clearly it is not made from the sand, the clay or the chalk since all these occur in the subsoil. We have seen (Chap. I.) that the top soil differs from the subsoil in containing a quantity of material that will burn away and is in part at any rate made up of plant remains. It will be easy to find out whether these remains furnish any appreciable quantity of plant food.

Fill one pot with surface soil and another with the same weight of surface soil well mixed up with 30 grams of plant remains—pieces of grass, or stems and leaves of other plants cut up into fragments about half an inch long. At the same time put up two pots of subsoil, one of which, as before, is mixed with 30 grains of plant remains, and also put up two pots of sand, one containing 30 grams of plant remains and the other none. Sow all six pots with mustard and keep watered and well tended. The result of one experiment is shown in Fig. 23 and the weights of the crop in grams were:—

Now let us look at these results carefully. The experiment with surface soil shows that the pieces of stem and leaf have furnished a good deal of food to the mustard and have caused a gain of 24.3 grams in the crop. If we knew what the pieces were made of we could push the experiment still further and find out more about plant food, but this involves chemical problems and must be left alone for the present. We can, however, say that plant remains are an important source of plant food, and since we suppose the black material of the soil to be made of plant remains (see p. 36), it will be quite fair to say also that this black material, the humus, is a source of plant food. We have therefore answered the question we set, and we can explain some at any rate of the differences between the surface soil and the subsoil. The surface soil contains a great deal of the black material, which forms plant food, while the subsoil does not. Thus plants grow well on the surface soil and starve on the subsoil. We can also explain why gardeners and farmers speak of black soils as rich soils; they contain more than other soils of this black material that makes plant food. Still further, we can explain why the farmer often sows plants like mustard, tares or clover, and then ploughs them into the ground. They are not wasted, but they make food for the next crop that goes in.

Now let us turn to the results of the subsoil experiments. The leaves and stems have increased the crop, but only by 5.4 grams: they have not been nearly so effective as in the surface soil. It is evident that the mustard did not feed directly on the leaves and stems put in; if it had there should have been an equal gain in both cases. The leaves and stems clearly have to undergo some change before they are made into plant food and the soil has something to do with this change. After the crops are cut the soils should be tipped out and examined. More of the original pieces of leaf and stem are found in the subsoil than in the surface soil. That is to say, there has been more change in Pot 6 containing surface soil than in Pot 7 containing subsoil. The "something," whatever it may be, that changes plant remains like leaves, stems, pieces of grass, roots, etc. into plant food therefore acts better in the surface soil than in the subsoil. Here then we have another difference between surface and subsoils.

SUMMARY. The experimental results obtained in this chapter may now be summed up as follows:—

(1) Plant food is present in the top soil only and not to any extent in the subsoil.

(2) There is not much present, so little indeed that we could not detect it by weighing.

(3) It is, however, always being made in the top soil, if water is present. Only little is made from the subsoil.

(4) The remains of leaves, stems, roots, etc. furnish an important source of plant food.

(5) But they have first to undergo some change, and the agent producing this change is more active in the top soil than in the subsoil.

(6) The top soil is much the most useful part of the soil and should never be buried during digging or trenching, but always carefully kept on top.

CHAPTER VII

THE DWELLERS IN THE SOIL

Apparatus required.

Garden soil. Six bottles and corks [1]. Twelve Erlenmeyer flasks, 50 c.c. capacity [2]. Cotton wool. Milk (about half a pint). Leaf gelatine. Soil baked in an oven. Six saucers [3]. The apparatus in Fig. 28 (two lots). Wash bottle containing lime water (Fig. 27, also p. 19).

In digging a garden a number of little animals are found, such as earthworms, beetles, ants, centipedes, millipedes and others. There are also some very curious forms of vegetable life. By carefully looking about it is not difficult to find patches of soil covered with a greenish slimy growth; they are found best under bushes where the soil is not disturbed, or else where the soil has been pressed down by a footmark and not touched since. Any good soil left undisturbed for a time shows this growth.

Put some fresh moist garden soil into a bottle and cork it up tightly so that it keeps moist. Write the date on the bottle and then leave it in the light where you can easily see it. After a time—sometimes a long, sometimes a shorter time—the soil becomes covered with a slimy growth, greenish in colour, mingled here and there with reddish brown. The longer the soil is left the better. Often after several months something further happens; little ferns begin to grow and they live a very long time indeed. There is at Rothamsted a bottle of soil that was put up just like this as far back as 1874. For a number of years past a beautiful fern has been growing inside the bottle, and even now it is very healthy and vigorous. If, instead of being kept moist, the rich garden soil is left in a dry shed during the whole of the winter so that it gradually loses its moisture, it will generally show quite a lot of white fluffy growth.

All of these living things are very wonderful, and some, especially earthworms, are very useful to gardeners and farmers.

After a shower of rain look carefully in the garden or else on a lawn, common, or pasture field where the grass is closely grazed by cattle or does not naturally grow long, and you will find numbers of tiny heaps of soil scattered about. Carefully brush away a heap and a little hole is seen, now hit the ground near it a few times with a stick or stamp on it with your foot and the worm, if he is near the top, comes up. When he is safely out of the way dig carefully down with a knife or trowel so as to examine the hole or "burrow." At the top you generally find it lined with pieces of grass or leaves that the worm has pulled in; lower down the lining comes to an end, but the colour of the burrow is redder than that of the rest of the soil wherever the soil has a greenish tinge. These holes are useful because they let air and water down into the soil.

The following experiment shows what earthworms can do. Fill a pot with soil from which all the worms have been carefully picked out and another with soil to which earthworms have been added, one worm to every pound of soil. Leave them out of doors where the rain can fall on to them. You can soon see the burrows and the heaps of soil or "casts" thrown up by the worms: these casts wash or blow over the surface of the soil, continually covering it with a thin layer of material brought up from below. Consequently the soil containing earthworms always has a fresh clean look. After some time the other soil becomes very compact and is covered with a greenish slimy growth. When this happens carefully turn the pots upside down, knock them so as to detach the soil and lift them off. The soil where the earthworms had lived is full of burrows and looks almost like a sponge. Fig. 24 shows what happened in an experiment lasting from June to October. The other soil where there were no earthworms shows no such burrows and is rather more compact than when it was put in.

Earthworms therefore do three things:—

(1) They make burrows in the ground and so let in air and water.

(2) They drag leaves into the soil and thus help to make the mixture of soil and leaf mould.

(3) They keep on bringing fresh soil up to the surface, and they disturb the surface so much that it is always clean and free from the slimy growth.

All these things are very useful and so a gardener should never want to kill worms. The great naturalist, Darwin, spent a long time in studying earthworms at his home in Kent and wrote a very interesting book about them, called Earthworms and Vegetable Mould. He shows that each year worms bring up about 1/50th of an inch of soil, so that if you laid a penny on the soil now and no one took it, in 50 years it might be covered with an inch of soil. Pavements that were on the surface when the Romans occupied Britain are now covered with a thick layer of soil.

But besides these there are some living things too small to see, that have only been found by careful experiments, but you can easily repeat some of these experiments yourselves. Divide a little rich garden soil into two parts and bake one in the kitchen oven on a patty tin. Pour a little milk into each of two small flasks, stop up with cotton wool (see Fig. 25) and boil for a few minutes very carefully so that the milk does not boil over, then allow to cool. Next carefully take out the stopper from one of the flasks and drop in a little of the baked soil, label the flask "baked soil" and put back the stopper. Into the other flask drop a little of the untouched soil and label it; leave both flasks in a warm place till the next day. Carefully open the stoppers and smell the milk: the baked soil has done nothing and the milk smells perfectly sweet; the unbaked soil, on the other hand, has made the milk bad and it smells like cheese. If you have a good microscope you can go further: look at a drop of the liquid from each flask and you find in each case the round fat globules of the milk, but the bad milk contains in addition some tiny creatures, looking like very short pins, darting in and out among the fat globules. These living things must have come from the unbaked soil or they would have been present in both flasks: they must also have been killed by baking in the oven.

Another experiment is easy but takes a little longer to show. Mix two sheets of leaf gelatine with a quarter of a pint of boiling water, pour into each of three saucers, and cover over with plates. Then stir up some baked soil in a cup half full of cold boiled water, and quickly put a teaspoonful of the liquid into a second cup, also half full of cold boiled water. Stir quickly and put a spoonful on to the jelly, tilting it about so that it covers the whole surface and label the saucer "baked soil." Do the same with the "unbaked soil," labelling the saucer; leave the third jelly alone and label it "untouched." Cover all three with plates and leave in a warm place. After a day or so little specks begin to appear on the jelly containing the unbaked soil, but not on the others (Fig. 26); they grow larger, and before long they change the jelly to a liquid. The other jellies show very few specks and are little altered. These creatures making the specks came from the soil because so few are found on the jelly alone; they were killed in the baking and so do not occur on the baked soil jelly.

Now we can get on with our soil experiments. Take two small flasks of equal size fitted with corks and joined by a glass tube bent like a U with the ends curled over. Put some lime water into each flask and a little water in the U-tube. Now make a small muslin bag like a sausage: fill it with moist fresh garden soil, tie it up with a silk thread and hang it in one of the flasks by holding the end of the thread outside and pushing in the cork till it is held firmly (see Fig. 28). Fix on the other flask, and after about five minutes mark the level of the liquid with a piece of stamp paper; leave in a warm place but out of the sun. In one or two days you will see that the water in the U-tube has moved towards the soil flask, showing that some air has been used up by the soil; further, the lime water has turned milky. But in the other flask, where there is no soil, the lime water remains quite clear.

This proves, then, that some of the tiny creatures want air just as much as we do. The air readies them through passages in the soil, through the burrows of earthworms and other animals, or by man's efforts in digging and ploughing.

If you had very magnifying eyes and could see things so enlarged that these little creatures seemed to you to be an inch long, and if you looked down into the soil, it would seem to you to be an extraordinarily wonderful place. The little grains of soil would look like great rocks and on them you would see creatures of all shapes and sizes moving about, and feeding on whatever was suitable to them, some being destroyed by others very much larger than themselves, some apparently dead or asleep, yet waking up whenever it becomes warmer or there was a little more moisture. You would see them changing useless dead roots and leaves into very valuable plant food; indeed it is they that bring about the changes observed in the experiments of Chap. VI. Occasionally you would see a very strange sight indeed—a great snake-like creature, over three miles long and nearly half a mile round, would rush along devouring everything before it and leave behind it a great tunnel down which a mighty river would suddenly pour, and what do you think it would be? What you now call an earthworm and think is four inches long, going through the soil leaving its burrow along which a drop of water trickles! That shows you how tiny these little soil creatures are.

These busy little creatures are called micro-organisms because of their small size. But they are not all useful. Some can turn milk bad as we have already seen, and therefore all jugs and dishes must be kept clean lest any of them should be present. Others can cause disease. It has happened that a child who has cut its finger and has got some soil into the cut, and not washed it out at once, has been made very ill. You may sometimes notice sheep limping about in the fields, especially in damp fields; an organism gets into the foot and causes trouble.

SUMMARY. The soil is full of living things, some large like earth worms, others very small. Earthworms are very useful: they make burrows in the soil, thus allowing air and water to get in: they drag in leaves and they keep on covering the surface with soil from below. Besides these and the other large creatures, there are micro-organisms so small that they cannot be seen without a very good microscope: they live and breathe and require air, water and food. Some are very useful and change dead parts of plants or animals into valuable plant food. Almost anything that can be consumed by fire can be consumed by them. Others are harmful.

CHAPTER VIII

THE SOIL AND THE PLANT

Apparatus required.

Dry powdered soil, sand, clay, leaf mould, seeds. Six funnels, disks, stands and glass jars [3]. Six glass tubes about 1/2 in. diameter and 18 in. long [2]. Muslin, string, three beakers. Six lamp chimneys standing in tin lids [3]. Pot experiments (p. xiii), growing plant. Two test tubes fitted with split corks (Fig. 35).

If you have ever tried to grow a plant in a pot you must have discovered that it needs much attention if it is to be kept alive. It wants water or it withers; it must be kept warm enough or it is killed by cold; it has to be fed or it gets yellow and starved; also it needs fresh air and light. These five things are necessary for the plant:

Water,
Warmth,
Food,
Fresh air,
Light.

We may add a sixth: there must be no harmful substance present in the soil.

Wild plants growing in their native haunts get no attention and yet their wants are supplied. We will try and find out how this is done.

Water comes from the rain, but the rain does not fall every day. How do the plants manage to get water on dry days? A simple experiment will show you one way. Put about four tablespoonsful of dry soil on to the funnel shown in Fig. 29 and then pour on two tablespoonsful of water. Measure what runs through. You will find it very little; most of the water sticks to the soil. Even after several days the soil was still rather moist. Soil has the power of keeping a certain amount of water in reserve for the plant, it only allows a small part of the rain to run through. Do the experiment also with sand, powdered clay, and leaf mould. Some water always remains behind, but less in the case of sand than in the others. In one experiment 30 cubic centimetres of water were poured on to 50 grains of soil but only 10 cubic centimetres passed through, but when an equal amount was poured on to 50 grains of sand no less than 20 cubic centimetres passed through. Very sandy soils, therefore, possess less power of storing water than do soils with more clay or mould in them, such as loams, clays or black soils.

Further, water has a wonderful power of passing from wet places to dry places in the soil. Tie a piece of muslin over the end of a tube and fill with dry soil, tapping it down as much as you can, then stand the tube in water as in Fig. 30. Fill another with sand and place in water. Notice that the water at once begins to rise in both tubes and will go on for a long time, always passing from the wet to the dry places. It rises higher in the soil than it does in the sand. Enough water may pass up the tube in this way to supply the needs of a growing plant. Fill a glass lamp chimney with dry soil, packing it down tightly, put into water and then sow with wheat. The plants grow very well. A longer tube may be made from two chimneys fastened together by means of a tin collar stuck on with Canada balsam or sealing wax (Fig. 31). Our plants grew well in this also, but on a sandier soil, where the water could not rise so high, it might happen that they would not.

You must find out in two ways. Visit a soil that you know is dry—a sandy, gravelly or chalky soil in a high situation—and look carefully at the plants there, then go to some moister, lower ground and see what the plants show. You cannot be quite certain, however, that anything you see is simply due to water supply, because there may be other differences in the soil as well. So you must try the second method, and that is to find out by experiments what is the effect of varying quantities of water on the plant growth. Both methods must be used, but it may be more convenient to start the experiments first, and while they are going on to collect observations in your rambles.

Fill four glazed pots with dry soil: keep one dry; one only just moist; the third is to be very moist and should be watered more frequently than the second; and the fourth is to be kept flooded with water, any way out being stopped up. Sow wheat or mustard in all four and keep out of the rain. The result of one experiment with mustard is shown in Fig. 32. Where no water was supplied there was no growth and the seeds remained unaltered. Where only little water was supplied (Pot 16) the plants made some growth, but not very much: the leaves were small and showed no great vigour; where sufficient water was given (Pot 3) the plants grew very well and had thick stems and large leaves; where too much water was given (Pot 15) the plants were very sickly and small.

The weights were:—