 |
LESSONS ON SOIL
BY
E. J. RUSSELL, D.Sc. (Lond.)
GOLDSMITH COMPANY'S SOIL CHEMIST, ROTHAMSTED EXPERIMENTAL STATION
Cambridge:
at the University Press
1911
{v}
PREFACE
The Syndics of the Cambridge University Press propose to issue a Nature Study Series of which this is the first volume.
We count ourselves fortunate in securing Dr E. J. Russell as author and Soil as subject. The subject is fundamental, for, just as the soil lies beneath the plant and animal life we see, so is a knowledge of the soil necessary for all understanding of flora and fauna. The real complexity of the apparently simple element "Earth," and the variety of methods required for exploring it, are typical of the problems which the tout ensemble of the outdoor world presents to the naturalist.
Dr E. J. Russell has not only acquired a first-rate and first-hand knowledge of his subject at Wye and at Rothamsted; his own researches have recently extended our knowledge of the micro-organisms in the soil and their influence on fertility. Further, what is very much to our purpose, he has himself had practical experience in teaching at an elementary school in Wye and at a secondary school in Harpenden.
Just at the present moment, County Councils are trying to push rural education and to awaken the intelligence of country children by interesting them in their surroundings. It is, therefore, a favourable opportunity to offer these pages as a concrete suggestion in model lessons and object lessons, showing exactly what can be done under existing conditions.
{vi}
The book is intended to help children to study nature; there is no attempt to substitute book study for nature study. Hence, whilst there are passages of continuous reading, it is not a mere "reader." Many teachers, myself among them, have felt the difficulty of organising practical work for large classes. Dr Russell has written so that, whilst nominally showing the pupil how to learn, he is secretly scattering hints for the teacher who is learning how to teach.
Abundant and varied practical exercises have been suggested, and careful instructions have been given so that the book shall seem intelligible even in the absence of a teacher. The proposed practical work is not only what might be done by eager boys and girls on half-holidays, but what can be done by every scholar in the course of ordinary school work. The pictorial illustrations are intended as aids to observation, not as substitutes. Drawing is one form of practical exercise, and the preparation of corresponding illustrations in the scholars' notebooks from the apparatus used in the classroom and the fields around the school may afford exercises in artistic work with pen, brush or camera.
Sufficient directions are given for the supply of necessary materials and apparatus. The apparatus proposed is of the simplest character.
It is suggested that the book will be found most useful in the higher standards of elementary schools, in preparatory schools and in the lower forms of secondary schools, that is, where the ages of scholars average from 12 to 14.
HUGH RICHARDSON
YORK, 7 January 1911
{vi}
CONTENTS
CHAP. PAGE
I. WHAT IS THE SOIL MADE OF? . . . . . . . . . . . . . 1 II. MORE ABOUT THE CLAY . . . . . . . . . . . . . . . . 9 III. WHAT LIME DOES TO CLAY . . . . . . . . . . . . . . . 19 IV. SOME EXPERIMENTS WITH THE SAND . . . . . . . . . . . 22 V. THE PART THAT BURNS AWAY . . . . . . . . . . . . . . 33 VI. THE PLANT FOOD IN THE SOIL . . . . . . . . . . . . . 41 VII. THE DWELLERS IN THE SOIL . . . . . . . . . . . . . . 53 VIII. THE SOIL AND THE PLANT . . . . . . . . . . . . . . . 64 IX. CULTIVATION AND TILLAGE . . . . . . . . . . . . . . 82 X. THE SOIL AND THE COUNTRYSIDE . . . . . . . . . . . . 100 XI. HOW SOIL HAS BEEN MADE . . . . . . . . . . . . . . . 116 APPENDIX . . . . . . . . . . . . . . . . . . . . . . 128 INDEX . . . . . . . . . . . . . . . . . . . . . . . 132
[Transcriber's note: The page numbers below are those in the original book. However, in this e-book, to avoid the splitting of paragraphs, the illustrations may have been moved to the page preceding or following.]
LIST OF ILLUSTRATIONS
FIGURE PAGE
1. Soil and subsoil in St George's School garden . . . . 2
2. Columns showing what 100 parts of soil and subsoil were made of . . . . . . . . . . . . . . . . . . . . 4
3. Columns showing what 100 parts of dried soil and subsoil were made of . . . . . . . . . . . . . . . . 8
4. Clay shrinks when it dries . . . . . . . . . . . . . . 11
5. Clay swells up when it is placed in water . . . . . . 12
6. Landslip in the Isle of Wight. Phot. Valentine & Son . . . . . . . . . . . . . . 13
7. Clay does not let water run through . . . . . . . . . 14
8. Sand allows air to pass through but clay does not . . 15
9. A brick allows both air and water to pass through it 17
10. Lime added to turbid clay water soon makes the clay settle . . . . . . . . . . . . . . . . . . . . . . . 20
11. Sand dunes, Penhale, Cornwall. Phot. Geological Survey . . . . . . . . . . . . . 23
12. Blowing sand covering up meadows and ruining them. Phot. Geological Survey . . . . . . . . . . . . . 25
13. Model of a spring . . . . . . . . . . . . . . . . . . 26
14. Foot of chalk hill at Harpenden where a spring breaks out. Phot. Lionel Armstrong . . . . . . . . . . . 27
15. The little pool and the spring. Phot. Lionel Armstrong . . . . . . . . . . . . . . 28
16. Water spouting up from a bore hole, Old Cateriag Quarry, Dunbar. Phot. Geological Survey . . . . . 29
17. Sandy soils in wet and in dry positions . . . . . . . 31
18. Map of the roads round Wye . . . . . . . . . . . . . . 32
19. Peat bog in Hoy, Orkney: peat is being cut for fuel. Phot. Valentine & Son . . . . . . . . . . . . . . 39
20. Rye growing in surface soil, subsoil, and sand . . . . 42
21. Mustard growing in surface soil, subsoil, and sand . . 43
22. Mustard growing in soil previously cropped with rye, and in soil previously uncropped . . . . . . . . . . 45
23. Pieces of grass, leaves, etc. change to plant food in the surface soil lint not in the subsoil . . . . . 50
24. Soil in which earthworms have been living and making burrows . . . . . . . . . . . . . . . . . . . . . . 55
25. Fresh soil turns milk bad, but baked soil does not . . 57
26. Soil contains tiny living things that grow on gelatine 58
27. Our breath makes lime water turn milky . . . . . . . . 59
28. Something in the soil uses up air and makes lime water turn milky . . . . . . . . . . . . . . . . . . . . . 61
29. Soils are able to stick to water: clay or loam soils do this better than sands . . . . . . . . . . . . . . . 65
30. Water can pass from wet to dry places in the soil, it can even travel upwards . . . . . . . . . . . . . . 66
31. Plants growing in soils supplied from below with water. All the water the plants get has to travel upwards 67
32. Mustard growing in soils supplied with varying quantities of water . . . . . . . . . . . . . . . . 69
33. Wheat growing in moist and in dry soils . . . . . . . 71
34 a and b. Plants found on a dry soil had narrow leaves, those on a moist soil had wider leaves. Phot. S. T. Parkinson . . . . . . . . . 72, 73
35. Plants give out water through their leaves . . . . . . 74
36. Stephen Hales's experiment in 1727 . . . . . . . . . . 75
37. Hill slope near Harpenden showing woodland at top and arable land lower down. Phot. Lionel Armstrong 77
38. View further along the valley; woodland and arable above, rough grassland near the river. Phot. Lionel Armstrong . . . . . . . . . . . . . . 79
39. Rough grass pasture near the river. Higher up is arable land. Phot. Lionel Armstrong . . . . . . . 81
40. After harvest the farmer breaks up his land with a plough and then leaves it alone until seed time. Phot. Lionel Armstrong . . . . . . . . . . . . . . 83
41. Rolling in mangold seed on the farm. Phot. H. B. Hutchinson . . . . . . . . . . . . . . 85
42. Soil sampler . . . . . . . . . . . . . . . . . . . . . 88
43. Cultivation and mulching reduce the loss of water from soils . . . . . . . . . . . . . . . . . . . . . 90
44 a and b. Maize cannot compete successfully with weeds . . . . . . . . . . . . . . . . . . . . 94, 95
45. A plot of wheat left untouched since 1882 at Rothamsted has now become a dense thicket. Phot. Lionel Armstrong . . . . . . . . . . . . . . 97
46. A badly drained wheat field . . . . . . . . . . . . . 99
47. Highly cultivated sandy soil in Kent . . . . . . . . . 103
48. A Surrey heath . . . . . . . . . . . . . . . . . . . . 105
49. Woodland and heather on high sandy land, Wimbledon Common. Phot. R. H. Carter . . . . . . . . . . . 107
50. Poor sandy soil in Surrey, partly cultivated but mainly wood and waste . . . . . . . . . . . . . . . . . . . 109
51. Open chalk cultivated country, Thanet . . . . . . . . 113
52. Cliffs at the seaside, Manorbier. Phot. Geological Survey . . . . . . . . . . . . . 117
53. Cliffs in inland district, Arthur's Seat, Edinburgh. Phot. Geological Survey . . . . . . . . . . . . . 119
54. Model of a stream . . . . . . . . . . . . . . . . . . 120
55. The bend of a river . . . . . . . . . . . . . . . . . 121
56. The winding river—the Stour at Wye. Phot. R. H. Carter . . . . . . . . . . . . . . . . 123
57. Sketch map showing why Godmersham and Wye arose where they did on the Stour . . . . . . . . . . . . 126
58. Ford at Coldharbour near Harpenden. Phot. Lionel Armstrong . . . . . . . . . . . . . . 127
The photographs of the pot experiments are by Mr Lionel Armstrong.
{xi}
INTRODUCTION
The following pages contain the substance of lessons given at the village school at Wye to the 4th, 5th, 6th and 7th standards (mixed) and at St George's School, Harpenden, to the 3rd form. There is, however, an important difference between the actual lessons and the book. The lessons had reference to the soils round about the village, and dealt mainly with local phenomena, general conclusions being only sparingly drawn; while in the book it has been necessary to throw the course into a more generalised form. The teacher in using the book will have to reverse the process, he must find local illustrations and make liberal use of them during the course; it is hoped that the information given will help him over any difficulties he may experience.
This necessity for finding local illustrations constitutes one of the fundamental differences between Nature Study subjects and other subjects of the school curriculum. The textbooks in some of the others may be necessary and sufficient; in Nature Study it is at most only subsidiary, serving simply as a guide to the thing that is to be studied; unless the thing itself be before the class it is no better than a guide to a cathedral would be without the cathedral. And just as the guide is successful only when he directs the attention of the stranger to the important features of the place, and fails directly he becomes garrulous and distracts attention, so a Nature Study book succeeds {xii} only in as far as it helps in the study of the actual thing, and fails if it is used passively and is substituted for an active study. No description or illustration can take the place of direct observation; the simplest thing in Nature is infinitely more wonderful than our best word pictures can ever paint it.
The author recommends the teacher to look through the chapter before it has to be taken in class and then to make a few expeditions in search of local illustrations. It is not strictly necessary that the chapters should be taken in the order given. The local phenomena must be dealt with as they arise and as weather permits, or the opportunity may pass not to return again during the course. In almost any lane, field, or garden a sufficient number of illustrations may be obtained for our purpose; if a stream and a hill are accessible the material is practically complete, especially if the children can be induced to pursue their studies during their summer holiday rambles. Of course this entails a good deal of work for the teacher, but the results are worth it. Children enjoy experimental and observation lessons in which they take an active part and are not merely passive learners. The value of such lessons in developing their latent powers and in stimulating them to seek for knowledge in the great book of Nature is a sufficient recompense to the enthusiastic teacher for the extra trouble involved.
It is not desirable to work through a chapter in one lesson. Children unaccustomed to make experiments or to see experiments done, will probably require three or four lessons for getting through each of the first few chapters, and two or three lessons for each of the others.
{xiii}
The pot experiments of Chaps. VI., VII. and VIII. should be started as early in the course as possible. Twenty flower pots are wanted for the set; they should be of the same size, about eight inches being a convenient diameter, and should be kept together in a warm place. Three are filled with sand, seven with subsoil, and the remaining ten with surface soil. Three of the subsoil pots are uncropped, two being stored moist and one dry. Four pots of the surface soil are uncropped and moist, a fifth and sixth are uncropped and dry, one of these contains earthworms (p. 54). Four glazed pots, e.g. large jam or marmalade jars, are also wanted (p. 69). Mustard, buckwheat, or rye make good crops, but many others will do. Leguminous crops, however, show certain abnormal characters, while turnips and cabbages are apt to fail: none of these should be used. It is highly desirable that the pots should be duplicated.
The plots also should be started in the school garden as early as convenient. Eight are required for the set: their treatment is described in Chap. IX. Plots two yards square suffice.
A supply of sand, of clay, and of lime will be wanted, but it is not necessary to have fresh material for each lesson. The sand may be obtained from a builder, a sand pit, the sea shore or from a dealer in chemical apparatus. The clay may be obtained from a brick yard; it gives most satisfactory results after it has been ground ready for brick making. Modelling clay is equally satisfactory. A supply of rain water is desirable.
For a class of twelve children working in pairs at the experiments the following apparatus is wanted for the whole course:—
{xiv}
Six tripods and bunsen burners or spirit lamps [2] twelve pipe-clay triangles [4] twelve crucibles or tin lids [3] sixteen gas jars [4] twelve beakers 250 c.c. capacity [4] two beakers 500 c.c. two beakers 100 c.c. six egg-cups [2] twelve funnels [3] six funnel stands [1] six perforated glass disks [3] two tubulated bottles 500 c.c., four corks to fit cork borers 4 lbs. assorted glass tubing pestle and mortar twelve Erlenmeyer flasks 50 c.c. [3] six saucers twelve flatbottomed flasks 100 c.c., six fitted with India rubber stoppers bored with one hole [3], and six with ordinary corks [3] box as in Fig. 13 six glass tubes 1/2" diameter, 18" long [2] six lamp chimneys [3] six test tubes, corks to fit three thermometers soil sampler (p. 88) balance and weights two retort stands with rings and clamp.
The figures given in square brackets are the quantities that suffice when the teacher alone does the experiments, it not being convenient for the scholars to do much.
{xv}
In conclusion the author desires to tender his best thanks to the Rev. Cecil Grant of St George's School, and to Mr W. J. Ashby of the Wye School, for having allowed him the use of their schools and appliances during the progress of these lessons. Especially are his thanks due to Mr Lionel Armstrong for much help ungrudgingly rendered in collecting material, taking photographs, and supervising the experiments.
E. J. R.
HARPENDEN, February, 1911.
{1}
CHAPTER I
WHAT IS THE SOIL MADE OF?
Apparatus required.
Soil and subsoil from a hole dug in the garden. Clay. Six tripods and bunsen burners or spirit lamps [2]. Six crucibles or tin lids and pipe-clay triangles [2]. Twelve glass jars or gas cylinders [4]. Six beakers [2] [1].
If we talk to a farmer or a gardener about soils he will say that there are several kinds of soil; clay soils, gravel soils, peat soils, chalk soils, and so on, and we may discover this for ourselves if we make some rambles in the country and take careful notice of the ground about us, particularly if we can leave the road and walk on the footpaths across the fields. When we find the ground very hard in dry weather and very sticky in wet weather we may be sure we are on a clay soil, and may expect to find brick yards or tile works somewhere near, where the clay is used. If the soil is loose, drying quickly after rain, and if it can be scattered about by the hand like sand on the sea shore, we know we are on a sandy soil and can look for pits where builder's sand is dug. But it may very likely happen that the soil is something in between, and that neither sand pits nor {2} clay pits can be found; if we ask what sort of soil this is we are told it is a loam. A gravel soil will be known at once by its gravel pits, and a chalk soil by the white chalk quarries and old lime kilns, while a peat soil is black, sometimes marshy and nearly always spongey to tread on.
We want to learn something of the soil round about us, and we will begin by digging a hole about three feet deep to see what we can discover. At Harpenden this is what the scholars saw:—the top eight inches of soil was dark in colour and easy to dig; the soil below was reddish brown in colour and very hard to dig; one changed into the other so quickly that it was easy to see where the top soil ended and the bottom soil began; no further change could, however, be seen below the eight inch line. A drawing was made to show these things, and is given in Fig. 1. You may find something quite different: sand, chalk, or solid rock may occur below the soil, but you should enter whatever you see into your notebooks and make a drawing, like Fig. 1, to be kept for future use. Before filling in the hole some of the dark coloured top soil, and some of the lighter coloured soil lying below (which is called the subsoil), {3} should be taken for further examination; the two samples should be kept separate and not mixed.
First look carefully at the top soil and rub some of it between your fingers. We found that our sample was wet and therefore contained water; it was very sticky like clay and therefore contained clay; there were a few stones and some grit present and also some tiny pieces of dead plants—roots, stems or leaves, but some so decayed that we could not quite tell what they were. A few pieces of a soft white stone were found that marked on the blackboard like chalk. Lastly, there were a few fragments of coal and cinders, but as these were not a real part of the soil we supposed they had got in by accident. The subsoil was also wet and even more sticky than the top soil, it contained stones and grit, but seemed almost free from plant remains and from the white chalky fragments.
A few experiments will show how much of some of these things are present. The amount of water may be discovered by weighing out ten grams of soil, leaving it to dry in a warm place near the fire or in the sun, and then weighing it again. In one experiment the results were:—
Weight of top soil before drying ... 10 grams = 100 decigrams " " " after " ... 8.3 " = 83 " —— —- Water lost ... 1.7 " = 17 "
A column 100 millimetres long was drawn to represent the 100 decigrams of soil, and a mark was drawn 17 millimetres up to show the amount of water (see Fig. 2).
Weight of bottom soil before drying ... 10 grams = 100 decigrams " " " after " ... 8.7 " = 87 "
Water lost ... 1.3 " = 13 "
{4} Another column should be drawn for the subsoil. On drying the soil it becomes lighter in colour and loses its stickiness, but it has not permanently changed because as soon as water is added it comes back to what it was before.
The dried lumps of soil are now to be broken up finely with a piece of wood, but nothing must be lost. It is easy to see shrivelled pieces of plant, but not easy to pick them out; the simplest plan is to burn them away. The soil must be carefully tipped on to a tin lid, or into a crucible, heated over a flame and stirred {5} with a long clean nail. First of all it chars, then there is a little sparkling, but not much, finally the soil turns red and does not change any further no matter how much it is heated. The shade of red will at once be recognised as brick red or terra cotta, indeed "terra cotta" means "baked earth." When the soil is cold it should be examined again; it has become very hard and the plant remains have either disappeared or have changed to ash and crumble away directly they are touched. On weighing a further loss is discovered, which was in our experiment:—
Weight of top soil after drying but before burning ... 83 decigrams " " " " " " after " ... 76 " — The part that burnt away weighed ... 7 "
Weight of subsoil after drying but before burning ... 87 decigrams " " " " " after " ... 84 " — The part that burnt away weighed ... 3 "
These results are entered on the column in Fig. 2.
The surface soil is seen to contain more material that will burn away than the subsoil does. When the burnt soil is moistened it does not become dark and sticky like it did before, it has completely changed and cannot be made into soil again. It is more like brick dust than soil.
For further experiments we shall want a fresh portion of the original soil.
On a wet afternoon something was noticed that enabled us to get a little further with our studies. The rain water ran down a sloping piece of ground in a tiny channel it had made; the streamlet was very muddy, and at first it was thought that all the soil was washed away. But we soon saw that the channel was lined {6} with grit, some of which was moving slowly down and some not at all. Grit can therefore be separated from the rest of the soil by water.
This separation can be shown very well by the following experiment. Rub ten grains of finely powdered soil with a little water (rain water is better than tap water), and carefully pour the muddy liquid into a large glass jar. Add more water to the rest of the soil, shake, and again pour the liquid into the jar; go on doing this till the jar is full. Then get some more jars and still keep on till the liquid is no longer muddy but nearly clear. The part of the soil that remains behind and will not float over into the jars is at once seen to be made up of small stones, grit, and sand. Set the jars aside and look at them after a day or so. The liquid remains muddy for some time, but then it clears and a thick black sediment gathers at the bottom. If now you very carefully pour the liquid off you can collect the sediments: they are soft and sticky, and can be moulded into patterns like clay. In order to see if they really contain clay we must do the experiment again, but use pure clay from a brick yard, or modelling clay, instead of soil. The muddy liquid is obtained as before, it takes a long time to settle, but in the end it gives a sediment so much like that from the soil, except in colour, that we shall be safe in saying that the sediments in the jars contain the clay from the soil. And thus we have been able to separate the sticky part of the soil—the clay—from the gritty or sandy part which is not at all sticky. We may even be able to find out something more. If we leave the soil sediment and the clay sediment on separate tin lids to dry, and then examine them carefully we may find that the {7} soil sediment is really a little more gritty than the clay. Although it contains the clay it also contains something else.
When the experiment is made very carefully in a proper way this material can be separated from the pure clay. It is called silt, but really there are a number of silts, some almost like clay and some almost like sand; they shade one into the other.
If there is enough grit it should be weighed: we obtained 14 decigrams of grit from 10 grams of our top soil and 17 decigrams from 10 grams of bottom soil. We cannot separate the clay from the silt, but when this is done in careful experiments it is found that the subsoil contains more clay than the top soil. We should of course expect this because we have found that the subsoil is more sticky than the top soil. These results are put into the columns as before so that we can now see at once how much of our soil is water, how much can burn away, how much is grit, and how much is clay and other things.
What would have happened if the sample had been dug out during wetter or drier weather? The quantity of water would have been different, but in other respects the soil would have remained the same. It is therefore best to avoid the changes in the amount of water by working always with 10 grams of dried soil. The results we obtained were:—
Top soil Subsoil Weight of dry soil before burning ... 100 100 decigrams " " " after " ... 92 97 " —- —- The part that burned away weighed ... 8 3 Weight of grit from 10 grams of dried soil 17 19 "
The columns are given in Fig. 3.
{8}
Summary. The experiments made so far have taught us these facts:—
1. Soil contains water, grit or sand, silt, clay, a part that burns away, and some white chalky specks.
2. The top layer of soil to a depth of about eight inches is different from the soil lying below, which is called the subsoil. It is less sticky, easier to dig, and darker in colour. It contains more of the material that burns away, but less clay than the subsoil.
3. When soil is dried it is not sticky but hard or crumbly; as soon as it is moistened it changes back to what it was before. But when soil is burnt it completely alters and can no longer be changed back again.
[1] See p. xiv for explanation of the figures in square brackets.
{9}
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 {10} 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. {11} 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, {12} 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."
You must have found out by now how very slippery clay becomes as soon as it is wet enough. It is not easy to walk over a clay field in wet weather, and if the clay forms part of the slope of a hill it may be so slippery that it becomes dangerous. Sometimes after very heavy rains soil resting on clay on the side of a hill has begun to slide downwards and moves some distance before it stops. Fortunately these land slips as they are called, are not common in England, but they do occur. Fig. 6 shows one in the Isle of Wight, and another is described by Gilbert White in The Natural History of Selborne.
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 {14} 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 {15} 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 {16} 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.
This also explains something else that you may have noticed. If you tried baking one of your model bricks in the fire you probably found that the brick exploded and shattered to pieces: the water still left in the brick changed to steam when it was heated, but the steam could not escape through the clay, and so it burst the clay. In a brick works the heat is very gradually applied and the steam only slowly forms, so that it has time to leak away, then when it has all gone the brick can be heated strongly. You should try this with one of your model bricks; leave it in a hot place near the stove or on the radiator for a week or more and then see if you can bake it without mishap.
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 {17} 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 {18} 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.
{19}
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.
{20}
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 {21} 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.
{22}
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 {24} 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 (Fig. 8); 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 {26} 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.
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 {28} 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: {30} 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.
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 {31} 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 {32} 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).
{33}
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 {34} 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 {35} 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[1] 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 {36} 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 {37} 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.
{38}
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.
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 {40} 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.
{41}
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.
{42}
{43}
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 (Pot 3) 14.5 17.7 5.6 2.6
" " " subsoil (Pot 4) 2.9 5.1 1.6 1.1
" " " sand (Pot 5) 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 {44} 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:—
{45}
Green weight After drying
Mustard growing in soil previously cropped with rye, Pot 1 17.8 62.3
Mustard growing in soil previously uncropped, Pot 2 3.3 8.6
{46}
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:—
Pot 2 Pot 2a
lbs. oz. lbs. oz.
Weight of dried soil before the experiment 6 6 6 7
" " " after " " 6 6 6 6 ——— ——— Difference 0 0 0 1
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 {47} 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:—
{48}
Green weight After drying
Plants grown in top soil stored in 16.9 2.6 moist condition (Pots 10 & 11) 18.9 2.8
Plants grown in top soil stored in 12.1 1.8 dry condition (Pots 8 & 9) 14.4 1.9
Plants grown in subsoil stored in moist condition (Pot 13) 5.5 0.9
Plants grown in subsoil stored in dry condition (Pot 12) 5.6 0.8
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:—
Green weight After drying
Mustard growing in subsoil previously cropped with rye 12.6 2.27
Mustard growing in subsoil previously uncropped 12.9 2.26
{49}
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:—
Green weight After drying
Top soil and pieces of plants (Pot 6) 42.0 5.0
Top soil alone (Pot 3) 17.7 2.6
Difference in top soil 24.3 2.4
{50}
Green weight After drying
Subsoil and pieces of plants (Pot 7) 10.5 1.9
Subsoil alone (Pot 4) 5.1 1.1
Difference in subsoil 5.4 0.8
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 {51} 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 {52} 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.
{53}
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 {54} 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 {55} 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 {56} 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 {57} 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 {58} 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 {59} 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 {60} 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.
You can also show that breathing is going on in the soil even after you have picked out every living thing that you can see. First of all you must do a little experiment with your own breathing so that you may know how to start. Shake up a little fresh lime with water and leave it to stand for 24 hours. Pour a little of the clear liquid into a flask or bottle fitted with a cork and two tubes, one long and one short like that shown in Fig. 27. Then breathe in through the tube A so that the air you take in comes through the lime water: notice that no change occurs. Next breathe out through the tube B so that your breath passes through the lime water; this time the lime water turns very milky. You therefore alter in some way the air that you breathe: you know also that you need fresh air.
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. {61} 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.
Now try the experiment with very dry garden soil: little or no change takes place. As soon as you add water, however, breathing begins again, air is absorbed and the lime water turns milky just as before. Water is therefore wanted just as much as air.
If you had very magnifying eyes and could see things so enlarged that these little creatures seemed to {62} 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.
{63}
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.
{64}
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.
{65}
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 {66} 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 {68} 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. |
|
