LESSONS ON SOIL

BY

E. J. RUSSELL, D. Sc. (Lond. )

GOLDSMITH COMPANY'S SOIL CHEMIST, ROTHAMSTED EXPERIMENTAL STATION

Cambridge:

at the University Press

1911

[Transcriber's note: Page numbers in this book are indicated by numbersenclosed in curly braces, e. G. {99}. They have been located where pagebreaks occurred in the original book, in accordance with ProjectGutenberg's FAQ-V-99. In the HTML version of this book, page numbersare placed in the left margin. ]

{v}

PREFACE

The Syndics of the Cambridge University Press propose to issue a NatureStudy Series of which this is the first volume. 

We count ourselves fortunate in securing Dr E. J. Russell as author andSoil as subject. The subject is fundamental, for, just as the soillies beneath the plant and animal life we see, so is a knowledge of thesoil necessary for all understanding of flora and fauna. The realcomplexity of the apparently simple element "Earth, " and the variety ofmethods required for exploring it, are typical of the problems whichthe _tout ensemble_ of the outdoor world presents to the naturalist. 

Dr E. J. Russell has not only acquired a first-rate and first-handknowledge of his subject at Wye and at Rothamsted; his own researcheshave recently extended our knowledge of the micro-organisms in the soiland their influence on fertility. Further, what is very much to ourpurpose, he has himself had practical experience in teaching at anelementary school in Wye and at a secondary school in Harpenden. 

Just at the present moment, County Councils are trying to push ruraleducation and to awaken the intelligence of country children byinteresting them in their surroundings. It is, therefore, a favourableopportunity to offer these pages as a concrete suggestion in modellessons and object lessons, showing exactly what can be done underexisting conditions. 

{vi}

The book is intended to help children to study nature; there is noattempt to substitute book study for nature study. Hence, whilst thereare passages of continuous reading, it is not a mere "reader. " Manyteachers, myself among them, have felt the difficulty of organisingpractical work for large classes. Dr Russell has written so that, whilst nominally showing the pupil how to learn, he is secretlyscattering hints for the teacher who is learning how to teach. 

Abundant and varied practical exercises have been suggested, andcareful instructions have been given so that the book shall seemintelligible even in the absence of a teacher. The proposed practicalwork is not only what might be done by eager boys and girls onhalf-holidays, but what can be done by every scholar in the course ofordinary school work. The pictorial illustrations are intended as aidsto observation, not as substitutes. Drawing is one form of practicalexercise, and the preparation of corresponding illustrations in thescholars' notebooks from the apparatus used in the classroom and thefields around the school may afford exercises in artistic work withpen, brush or camera. 

Sufficient directions are given for the supply of necessary materialsand apparatus. The apparatus proposed is of the simplest character. 

It is suggested that the book will be found most useful in the higherstandards of elementary schools, in preparatory schools and in thelower forms of secondary schools, that is, where the ages of scholarsaverage 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 originalbook. However, in this e-book, to avoid the splitting of paragraphs, the illustrations may have been moved to the page preceding orfollowing. ]

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 thevillage 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 thebook. The lessons had reference to the soils round about the village, and dealt mainly with local phenomena, general conclusions being onlysparingly drawn; while in the book it has been necessary to throw thecourse into a more generalised form. The teacher in using the bookwill have to reverse the process, he must find local illustrations andmake liberal use of them during the course; it is hoped that theinformation given will help him over any difficulties he may experience. 

This necessity for finding local illustrations constitutes one of thefundamental differences between Nature Study subjects and othersubjects of the school curriculum. The textbooks in some of the othersmay be necessary and sufficient; in Nature Study it is at most onlysubsidiary, serving simply as a guide to the thing that is to bestudied; unless the thing itself be before the class it is no betterthan a guide to a cathedral would be without the cathedral. And justas the guide is successful only when he directs the attention of thestranger to the important features of the place, and fails directly hebecomes garrulous and distracts attention, so a Nature Study booksucceeds {xii} only in as far as it helps in the study of the actualthing, and fails if it is used passively and is substituted for anactive study. No description or illustration can take the place ofdirect observation; the simplest thing in Nature is infinitely morewonderful than our best word pictures can ever paint it. 

The author recommends the teacher to look through the chapter before ithas to be taken in class and then to make a few expeditions in searchof local illustrations. It is not strictly necessary that the chaptersshould be taken in the order given. The local phenomena must be dealtwith as they arise and as weather permits, or the opportunity may passnot to return again during the course. In almost any lane, field, orgarden a sufficient number of illustrations may be obtained for ourpurpose; if a stream and a hill are accessible the material ispractically complete, especially if the children can be induced topursue their studies during their summer holiday rambles. Of coursethis entails a good deal of work for the teacher, but the results areworth it. Children enjoy experimental and observation lessons in whichthey take an active part and are not merely passive learners. Thevalue of such lessons in developing their latent powers and instimulating them to seek for knowledge in the great book of Nature is asufficient recompense to the enthusiastic teacher for the extra troubleinvolved. 

It is not desirable to work through a chapter in one lesson. Childrenunaccustomed to make experiments or to see experiments done, willprobably require three or four lessons for getting through each of thefirst 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 asearly in the course as possible. Twenty flower pots are wanted for theset; they should be of the same size, about eight inches being aconvenient diameter, and should be kept together in a warm place. Three are filled with sand, seven with subsoil, and the remaining tenwith surface soil. Three of the subsoil pots are uncropped, two beingstored moist and one dry. Four pots of the surface soil are uncroppedand moist, a fifth and sixth are uncropped and dry, one of thesecontains earthworms (p. 54). Four glazed pots, e. G. Large jam ormarmalade jars, are also wanted (p. 69). Mustard, buckwheat, or ryemake good crops, but many others will do. Leguminous crops, however, show certain abnormal characters, while turnips and cabbages are apt tofail: none of these should be used. It is highly desirable that thepots should be duplicated. 

The plots also should be started in the school garden as early asconvenient. Eight are required for the set: their treatment isdescribed in Chap. IX. Plots two yards square suffice. 

A supply of sand, of clay, and of lime will be wanted, but it is notnecessary to have fresh material for each lesson. The sand may beobtained from a builder, a sand pit, the sea shore or from a dealer inchemical apparatus. The clay may be obtained from a brick yard; itgives most satisfactory results after it has been ground ready forbrick making. Modelling clay is equally satisfactory. A supply ofrain water is desirable. 

For a class of twelve children working in pairs at the experiments thefollowing 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 sufficewhen the teacher alone does the experiments, it not being convenientfor 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 WyeSchool, for having allowed him the use of their schools and appliancesduring the progress of these lessons. Especially are his thanks due toMr Lionel Armstrong for much help ungrudgingly rendered in collectingmaterial, 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 tripodsand bunsen burners or spirit lamps [2]. Six crucibles or tin lids andpipe-clay triangles [2]. Twelve glass jars or gas cylinders [4]. Sixbeakers [2] [1]. _

If we talk to a farmer or a gardener about soils he will say that thereare several kinds of soil; clay soils, gravel soils, peat soils, chalksoils, and so on, and we may discover this for ourselves if we makesome rambles in the country and take careful notice of the ground aboutus, particularly if we can leave the road and walk on the footpathsacross the fields. When we find the ground very hard in dry weatherand 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, wherethe clay is used. If the soil is loose, drying quickly after rain, andif it can be scattered about by the hand like sand on the sea shore, weknow we are on a sandy soil and can look for pits where builder's sandis dug. But it may very likely happen that the soil is something inbetween, and that neither sand pits nor {2} clay pits can be found; ifwe ask what sort of soil this is we are told it is a loam. A gravelsoil will be known at once by its gravel pits, and a chalk soil by thewhite chalk quarries and old lime kilns, while a peat soil is black, sometimes marshy and nearly always spongey to tread on. 

[Illustration: Fig. 1. Soil and subsoil in St George's school garden]

We want to learn something of the soil round about us, and we willbegin by digging a hole about three feet deep to see what we candiscover. At Harpenden this is what the scholars saw:--the top eightinches of soil was dark in colour and easy to dig; the soil below wasreddish brown in colour and very hard to dig; one changed into theother so quickly that it was easy to see where the top soil ended andthe bottom soil began; no further change could, however, be seen belowthe eight inch line. A drawing was made to show these things, and isgiven in Fig. 1. You may find something quite different: sand, chalk, or solid rock may occur below the soil, but you should enter whateveryou see into your notebooks and make a drawing, like Fig. 1, to be keptfor future use. Before filling in the hole some of the dark colouredtop soil, and some of the lighter coloured soil lying below (which iscalled the subsoil), {3} should be taken for further examination; thetwo samples should be kept separate and not mixed. 

First look carefully at the top soil and rub some of it between yourfingers. We found that our sample was wet and therefore containedwater; it was very sticky like clay and therefore contained clay; therewere a few stones and some grit present and also some tiny pieces ofdead plants--roots, stems or leaves, but some so decayed that we couldnot quite tell what they were. A few pieces of a soft white stone werefound that marked on the blackboard like chalk. Lastly, there were afew fragments of coal and cinders, but as these were not a real part ofthe soil we supposed they had got in by accident. The subsoil was alsowet and even more sticky than the top soil, it contained stones andgrit, but seemed almost free from plant remains and from the whitechalky fragments. 

A few experiments will show how much of some of these things arepresent. The amount of water may be discovered by weighing out tengrams of soil, leaving it to dry in a warm place near the fire or inthe sun, and then weighing it again. In one experiment the resultswere:--

 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 decigramsof soil, and a mark was drawn 17 millimetres up to show the amount ofwater (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 soilit becomes lighter in colour and loses its stickiness, but it has notpermanently changed because as soon as water is added it comes back towhat it was before. 

[Illustration: Fig. 2. Columns showing what 100 parts of soil andsubsoil were made of]

The dried lumps of soil are now to be broken up finely with a piece ofwood, but nothing must be lost. It is easy to see shrivelled pieces ofplant, but not easy to pick them out; the simplest plan is to burn themaway. The soil must be carefully tipped on to a tin lid, or into acrucible, 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 matterhow much it is heated. The shade of red will at once be recognised asbrick red or terra cotta, indeed "terra cotta" means "baked earth. "When the soil is cold it should be examined again; it has become veryhard and the plant remains have either disappeared or have changed toash and crumble away directly they are touched. On weighing a furtherloss 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 awaythan the subsoil does. When the burnt soil is moistened it does notbecome dark and sticky like it did before, it has completely changedand cannot be made into soil again. It is more like brick dust thansoil. 

For further experiments we shall want a fresh portion of the originalsoil. 

On a wet afternoon something was noticed that enabled us to get alittle further with our studies. The rain water ran down a slopingpiece of ground in a tiny channel it had made; the streamlet was verymuddy, 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 whichwas moving slowly down and some not at all. Grit can therefore beseparated 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 wateris better than tap water), and carefully pour the muddy liquid into alarge glass jar. Add more water to the rest of the soil, shake, andagain pour the liquid into the jar; go on doing this till the jar isfull. Then get some more jars and still keep on till the liquid is nolonger muddy but nearly clear. The part of the soil that remainsbehind and will not float over into the jars is at once seen to be madeup of small stones, grit, and sand. Set the jars aside and look atthem after a day or so. The liquid remains muddy for some time, butthen it clears and a thick black sediment gathers at the bottom. Ifnow you very carefully pour the liquid off you can collect thesediments: they are soft and sticky, and can be moulded into patternslike clay. In order to see if they really contain clay we must do theexperiment again, but use pure clay from a brick yard, or modellingclay, instead of soil. The muddy liquid is obtained as before, ittakes a long time to settle, but in the end it gives a sediment so muchlike that from the soil, except in colour, that we shall be safe insaying 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--theclay--from the gritty or sandy part which is not at all sticky. We mayeven be able to find out something more. If we leave the soil sedimentand the clay sediment on separate tin lids to dry, and then examinethem carefully we may find that the {7} soil sediment is really alittle more gritty than the clay. Although it contains the clay italso contains something else. 

When the experiment is made very carefully in a proper way thismaterial can be separated from the pure clay. It is called silt, butreally there are a number of silts, some almost like clay and somealmost like sand; they shade one into the other. 

If there is enough grit it should be weighed: we obtained 14 decigramsof grit from 10 grams of our top soil and 17 decigrams from 10 grams ofbottom soil. We cannot separate the clay from the silt, but when thisis done in careful experiments it is found that the subsoil containsmore clay than the top soil. We should of course expect this becausewe have found that the subsoil is more sticky than the top soil. Theseresults are put into the columns as before so that we can now see atonce how much of our soil is water, how much can burn away, how much isgrit, and how much is clay and other things. 

What would have happened if the sample had been dug out during wetteror drier weather? The quantity of water would have been different, butin other respects the soil would have remained the same. It istherefore best to avoid the changes in the amount of water by workingalways 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}

[Illustration: Fig. 3. Columns showing what 100 parts of dried soiland subsoil were made of]

Summary. The experiments made so far have taught us these facts:--

1. Soil contains water, grit or sand, silt, clay, a part that burnsaway, and some white chalky specks. 

2. The top layer of soil to a depth of about eight inches is differentfrom the soil lying below, which is called the subsoil. It is lesssticky, easier to dig, and darker in colour. It contains more of thematerial that burns away, but less clay than the subsoil. 

3. When soil is dried it is not sticky but hard or crumbly; as soon asit is moistened it changes back to what it was before. But when soilis 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. Someseeds. Six small jars about 2 in. X 1 in. [2]. Bricks. The apparatusin Fig. 9. Pestle and mortar. _

We have seen in the last chapter that clay will float in water and onlyslowly settles down. Is this because clay is lighter than water?Probably not, because a lump of clay seems very heavy. Further, if weput 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 astick--in other words, when we break the ball into very tinypieces--can we get it to float again. We therefore conclude that theclay floated in our jars (p. 6) for so long not because it was lighterthan water, but because the pieces were so small. 

Clay is exceedingly useful because of its stickiness. Dig up someclay, if there is any in your garden, or procure some from a brickworks. You can mould it into any shape you like, and the purer theclay the {10} better it acts. Enormous quantities of clay are used formaking bricks. Make some model bricks about an inch long and half aninch in width and depth, also make a small basin of about the samesize, then set them aside for a week in a warm, dry place. They stillkeep their shape; even if a crack has appeared the pieces sticktogether and do not crumble to a powder. 

If you now measure with a ruler any of the bricks that have notcracked, you will find that they have shrunk a little and are no longerquite an inch long. This fact is well known to brickmakers; the mouldsin which they make the bricks are larger than the brick is wanted tobe. But what would happen if instead of a piece of clay one inch longyou 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 twoways; we may make a model of a field and let it dry, and we can pay avisit to a clay meadow after some hot, dry weather in summer. Themodel can be made by kneading clay up under water and then rolling itout on some cardboard or wood as if it were a piece of pastry. Cut itinto a square and draw lines on the cardboard right at the edges of theclay. 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 hasshrunk away from the marks, but it has also shrunk all over and hascracked. If you get an opportunity of walking over a clay field duringa dry summer, you will find similar but much larger cracks, some ofwhich may be two or three inches wide, or even more. Sometimes thecracking is so bad that the roots of plants or of trees are torn by it, and even buildings, in some instances, have suffered through theirfoundations shrinking away. {11} We can now understand why some of ourmodel bricks cracked. The cracks were caused by the shrinkage just ashappens with our model field. As soon as the clay becomes wet itswells again. A very pretty experiment can be made to show this. Filla glass tube or an egg-cup with dry powdered clay, scrape the surfacelevel with a ruler, and then stand in a glass jar full of rain water sothat the whole is completely covered. After a short time the claybegins 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 exactlythe same way the ground swells after heavy rain and rises a little, then it falls again and cracks when it becomes dry. Darwin recordssome careful measurements in a book called _Earthworms and VegetableMould_--"a large flat stone laid on the surface of a field sank 3. 33millimetres[1] whilst the weather was dry between May 9th and June13th, {12} and rose 1. 91 millimetres between September 7th and 19th ofthe same year, much rain having fallen during the latter part of thistime. During frosts and thaws the movements were twice as great. "

[Illustration: Fig. 4. Clay was plastered over a square piece of boardand completely covered it. After drying for a week the clay had shrunkand cracked]

You must have found out by now how very slippery clay becomes as soonas it is wet enough. It is not easy to walk over a clay field in wetweather, and if the clay forms part of the slope of a hill it may be soslippery that it becomes dangerous. Sometimes after very heavy rainssoil resting on clay on the side of a hill has begun to slide downwardsand moves some distance before it stops. Fortunately these land slipsas 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 GilbertWhite in _The Natural History of Selborne_. 

[Illustration: Fig. 5. Clay swelling up when placed in water andoverflowing from the egg-cup into which it was put]

[Illustration: Fig. 6. Landslip in the Isle of Wight]

Another thing that you will have noticed is that anything made of clayholds water. A simple way of testing this is to put a round piece oftin perforated {14} with holes into a funnel, press some clay on to itand on to the sides of the funnel (Fig. 7), and then pour on rainwater. The water does not run through. Pools of water may lie likethis on a clay field for a very long time in winter before theydisappear, 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 thatthe sides are lined with clay to keep the water in. 

[Illustration: Fig. 7. A thin layer of clay _a_ entirely prevents thewater running through]

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 airthrough. You cannot. Clay can be used like putty to stop up holes orcracks, 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 thebottom tubes, and fill with water. Then put a funnel through each corkand fit the cork in tightly, covering with clay if there is any sign ofa leak. Put a perforated tin disk into each funnel, cover one wellwith clay and the other with sand. Open the bottom tubes. No waterruns out from the first bottle because no air can leak in through theclay, but it runs out very quickly from the second because the sandlets air through. These properties of clay and sand are very importantfor plants. Sow some seeds in a little jar {16} full of clay keptmoist to prevent it cracking, and at the same time sow a few in somemoist 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 andwater and are warm enough. They had water in both jars, and they werein both cases warm, but they got no air through the clay and thereforecould 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 wantedfor germination. 

[Illustration: Fig. 8. Sand allows air to pass through it, and sowater runs out of the bottle. Clay does not let air pass, and thewater is therefore kept in, even though the tube is open. ]

This also explains something else that you may have noticed. If youtried baking one of your model bricks in the fire you probably foundthat the brick exploded and shattered to pieces: the water still leftin the brick changed to steam when it was heated, but the steam couldnot escape through the clay, and so it burst the clay. In a brickworks the heat is very gradually applied and the steam only slowlyforms, so that it has time to leak away, then when it has all gone thebrick can be heated strongly. You should try this with one of yourmodel bricks; leave it in a hot place near the stove or on the radiatorfor 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 areso great that you would hardly think the brick could have been madefrom clay. The brick is neither soft nor sticky, and it has not thesmooth surface of a piece of clay, but is full of little holes orpores, which look as if they were formed in letting the steam out. Abrick lets air through; some air gets into our houses through thebricks even when the windows are shut. Water will get through bricksmore easily than it does through clay. After heavy rain you {17} canoften find that water has soaked through a brick wall and made the wallpaper quite damp. A pretty experiment can be made with the piece ofapparatus shown in Fig. 9: bore in a brick a hole about an inch deepand a quarter of an inch wide, put into the hole the piece of bentglass tubing, and fix it in with some clay or putty, then pour somewater blackened with ink into the tube, marking its position with alabel. Stand the brick in a vessel so full of water that the brick isentirely 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. 

[Illustration: Fig. 9. A brick standing in water. The air in thebrick is driven inwards by the water and forces the liquid up the tubein order to escape]

One more experiment may be tried. Can a brick be changed back intoclay? 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 grittypowder and never takes on the greasy, sticky feeling of {18} pure clay. Indeed no one has ever succeeded in making clay out of bricks. Allthese experiments show that clay is completely altered when it isburnt. We also found that soil is completely altered by burning, andif you look back at your notes you will see that the changes are verymuch alike, so much so that we can safely put down some of the changesin the burnt soil--the red colour, the hard grittiness, and the absenceof stickiness--to the clay. Let us now examine a piece of dry, butunburnt, clay. It is very hard and does not crumble, it is neithersticky nor slippery. Directly, however, we add some water it changesback to what it was before. Drying therefore alters clay only for thetime 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, ifnecessary, 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 findsome of the fields dotted with white heaps of chalk or lime, and youwill be told that these things "improve" the soil. We will make a fewexperiments to find out what lime does to clay. Put some clay on to aperforated tin disk in a funnel just as you did on p. 14, press it downso that no water can pass through. Then sprinkle on to the clay somepowdered lime and add rain water. Soon the water begins to leakthrough, 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 afield on which water lay about for a long time in winter you wouldexpect the water to drain away, especially if you made drains or cutsome trenches along which the water could pass. There are large areasin England where this has been done with very great advantage. 

{20}

The muddy liquid obtained by shaking clay with water clears quickly ifa little lime is stirred in. Fill two jars A and B (Fig. 10) with rainwater, 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, butA remains cloudy for a long time. But why should the liquid clear? Wedecided in our earlier experiments that the clay floated in the waterbecause it was in very tiny pieces; when we took a larger lump the claysank. The lime has for some reason or other, which we do notunderstand, made the small clay particles stick together to form thelarge flocks, and these can no longer float, but sink. If we look atthe limed clay in our funnel experiment we shall see that the samechange has gone on there; the clay has become rather loose and fluffy, and can therefore no longer hold water back. 

[Illustration: Fig. 10. Addition of lime to turbid clay water nowmakes the clay settle and leaves the water quite clear]

Lime also makes clay less sticky. Knead up one piece of clay with rainwater alone and another piece {21} with rain water and about 1/20 itsweight of lime. The limed clay breaks easily and works quitedifferently from the pure clay. 

SUMMARY. This, then, is what we have learnt about clay. Clay is madeup of very, very, tiny pieces, so small that they float in water. Theystick together when they are wetted and then pressed, and they remaintogether; a piece of clay moulded into any pattern will keep its shapeeven after it is dried and baked. Clay is therefore made into bricks, earthenware, pottery, etc. , whilst white clay, which is found in someplaces, is made into china. Wet clay shrinks and cracks as it dries;these cracks can easily be seen in the fields during dry weather. Thisshrinkage interferes with the foundations of houses and otherbuildings, causing them to settle. Dry clay is different from wetclay, it is hard, not sticky and not slippery, but it at once becomeslike ordinary clay when water is added. After baking, however, claypermanently alters and cannot again be changed back to what it wasbefore. Clay will not let water pass through; a clay field istherefore nearly always wet in winter and spring. Nor can air passthrough until the clay dries or cracks. 

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

[1] Lime water is made by shaking up lime and water. It should be keptin 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. 13filled 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 shouldget a supply for these experiments; if not, some builder's sand can beused. When the sand is dry you will see that the grains are large andhard. Further, they are all separate and do not stick together; if youmake a hole in a heap of the sand, the sides fall in, there is nothingsolid about it, and you can easily see the mistake of the foolish manwho built his house upon the sand. When the sand is wet it sticksbetter and can be made into a good many things; at the seaside you canmake a really fine castle with wet sand. But as soon as the sand driesit again becomes loose and begins to fall to pieces. 

[Illustration: Fig. 11. Sand dunes, Penhale sands, Cornwall]

Strong winds will blow these fragments of dry sand about and pile themup into the sand hills or dunes common in many seaside districts (Fig. 11). Blowing sands can also be found in inland districts; in thenorthern part of Surrey, in parts of Norfolk and many {24} other placesare fields where so much of the soil is blown away by strong winds thatthe crops may suffer injury. In Central Asia sand storms do very muchharm and have in the course of years buried entire cities. Fig. 12shows the Penhale sands in Cornwall gradually covering up some meadowsand ruining them. 

[Illustration: Fig. 12. Sand from Penhale sand dunes blowing on to andcovering up meadows]

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

Unlike clay, sand does not hold water. Pour some water on to sandplaced on the tin disk in a funnel (Fig. 8); it nearly all runs throughat once. We should therefore expect a sandy field or a sandy road todry 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 soilrather than on a clay. The most desirable residential districts roundLondon, Hampstead on the north, and the stretch running from Haslemereon the south-west to Maidstone on the south-east, and other favouredregions, 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 amodel 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 glassfront; then on top of the clay put enough sand to fill the box. Borefour holes in the side as shown in the picture, one at the bottom, oneat the top, one just above the junction of the sand and clay, thefourth 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 therose so as to imitate the rain. At first nothing seems to happen, butif you look closely you will notice that the water soaks through anddoes not lie on the surface; it runs right down to the clay; then itcomes out at the tube there (_c_ in the picture). None goes throughthe clay, nor does enough stay in the sand to flow out through eitherthe top or the second tube; of the four tubes only one is dischargingany 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 thetime. 

[Illustration: Fig. 13. Model spring. A box with glass front containsa layer of clay and one of sand. Water that falls on the sand runsright down to the clay but can get no further, and therefore flows outthrough the tube _c_ at the junction of the clay and the sand. Thesame result is obtained when chalk takes the place of sand]

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

[Illustration: Fig. 14. Foot of a chalk hill at Harpenden where aspring breaks out just under the bush at the right-hand side of thegate]

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 clayor solid rock that it cannot pass, then it stops. If it can find a wayout it {28} does so and makes a spring, or sometimes a whole line ofsprings or wet ground. Rushes, which flourish in such wet places, willoften be found growing along this line, and may, indeed, in summer timebe all you can see, the water having drained away. But after much rainthe line again becomes very wet. Fig. 14 shows the foot of a chalkhill near Harpenden, where a spring breaks out just under the bush atthe 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 flowsgently, but elsewhere it sometimes happens, as in Fig. 16, that thespring breaks out with great force. 

[Illustration: Fig. 15. "The little pool below the tree"]

Now stop up the glass tubes so that the water cannot get out. Soon thesand becomes flooded and is no better than clay would be. A secondmodel will show this very well. Make a large saucer of clay and fillwith sand: {30} pour water on. The water stays in the sand, because itcannot pass through the clay. A sandy field saturated like this willtherefore not be dry, but wet, and will not make a good position for ahouse. We must therefore distinguish the two cases illustrated in Fig. 17. _A_ shows sand on a hill, dry because the water runs through untilit comes to clay or rock, when it stops and breaks out as a spring, atiny stream, or pond; this is a good building site and you may expectto find large houses there. _B_ shows the sand in a basin of clay, where the water cannot get away: here the cellars and downstairs roomsare 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 thenthe foundations of the buildings might move a little. 

[Illustration: Fig. 16. Water bursting out from an underground spring, Old Cateriag Quarry, Dunbar]

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

On the other hand large tracts of clay which remain wet and stickyduring a good part of the year are not very attractive to live in, andeven near London they were the last to be populated: Hither Green inthe south-cast and the clay districts of the north-west have only oflate 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 ismuch manufacturing or mining; the parishes are large, the roads evennow are not good while they used to be very bad indeed. Macaulay tellsus that at the end of the seventeenth century in some parts of Kent andSussex "none but the strongest horses could in winter get through thebog, in which at every step they sank {31} deep. The markets wereoften inaccessible during several months. . . The wheeled carriageswere, in this district, generally pulled by oxen. When Prince Georgeof Denmark visited the stately mansion of Petworth in wet weather, hewas six hours in going nine miles; and it was necessary that a body ofsturdy hinds should be on each side of his coach to prop it up. Of thecarriages which conveyed his retinue several were upset and injured. Aletter from one of the party has been preserved in which theunfortunate courier complains that, during fourteen hours, he neveronce alighted, except when his coach was overturned or stuck fast inthe mud. " The Romans knew how to make roads anywhere, and so they madethem run in a straight line between the two places they wished toconnect, but the art was lost in later years, and the country roadsmade in England since their time usually had to follow the sand or thechalk, avoiding the clay as much as possible. These roads we stilluse. Fig. 18 shows the roads round Wye; you should in your ramblesstudy your own roads and see what soil they are on. 

[Illustration: Fig. 17. Two positions of sand. _A_ is dry because thewater can drain away and break out as a spring at _c_. _B_ is wetbecause the water cannot drain away]

There are several other ways in which sand differs from clay. It doesnot shrink on drying nor does it {32} swell on wetting, and you willfind nothing happens when you try with sand the experiment with themodel field (p. 11) or the egg-cup (p. 12). 

[Illustration: Fig. 18. The roads round Wye. As far as possible theykeep off the clay (the plain part of the map) and keep on the chalk orthe sand (the dotted part of the map)]

{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 ortin lids. Six tripods, pipe-clay triangles, and bunsen burners orspirit lamps. Six beakers and egg-cups [1]. _

In the autumn leaves fall off the trees and form a thick layer in thewoods. They do not last very long; if they did they would in a fewyears almost bury the wood. You can, in the springtime or early summerfind out what has happened to them if you go into a wood or carefullysearch under a big hedge in a lane where the leaves were not sweptaway. Here and there you come across skeleton leaves where only theveins are left, all the rest having disappeared. But generally wherethe leaves have kept moist they have changed to a dark brown mass whichstill shows some of the structure of a leaf. This is called leafmould. The top layer of soil in the wood is soft, dark in colour, andis evidently leaf mould mixed with sand or soil. 

Leaf mould is highly prized by gardeners, indeed gardeners will oftenmake a big heap of leaves in autumn and let them "rot down" and changeinto mould. If you can in autumn collect enough leaves to make a heapyou {34} should do so and leave it somewhere where the rain can fall onit, but cover it with a few small branches of trees to prevent the windblowing the leaves away. The heap shrinks a great deal during thefirst few months, and in the end it gives a supply of mould that willbe very useful if you want to grow any plants in pots. 

Some of the little hollows in the bank under a hedge, especially onchalky soils, are filled with leaf mould which has sometimes changed toa black powder not looking at all like leaves. 

You can also find mould in holes in decayed trees; here it has formedfrom the wood of the tree. 

It appears, then, that dead leaves, etc. , slowly change into a black orbrown substance, shrinking very much as they do so. For this reasonthey do not go on piling up year after year till finally they fill thewood; instead they decay or "rot down" to form leaf mould: the big pileof the autumn has changed by the next summer to a thin layer whichmixes with the soil. 

We want now to see what happens on a common or a piece of waste groundthat is not cultivated. Grass and wild plants grow up in summer anddie during winter; their stems and roots are not taken away, butclearly they do not remain where they are, because next year new plantsgrow up. We may suppose that the dead roots and stems decay like theleaves did, and change to a brown or black mould. It looks as if weare right, because on digging a hole or examining the side of a freshlycut ditch we shall find that the top layer of soil, just so far as theliving roots go, is darker in colour than the layer below. 

We must, however, try and get some more proof, and to do this we muststudy some of our specimens a little {35} more closely. We will takesome leaf mould, some black mould from a hollow in the bank, some froma tree, soils from a wood, a well-manured garden, a field and somesubsoil. All except the subsoil have a dark colour, but the wood andgarden soils are probably darker than the field soil. Now weigh out 2grains 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 andcalculate how much has burnt away in each case. Here are some resultsthat 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, soalso does the dark colour of the soil. 

Our supposition explains why, in the case of soils, the less theblackness, the less the loss on burning. If the {36} brown or blackcombustible part is really mould formed by the decay of plant roots, etc. , then we should expect that as the percentage of mould in the soilincreased, so its blackness would increase and its loss on burningwould become greater. This actually happens. 

This, then, is our idea. We suppose that the plants that have lived inpast years have decayed to form a black material like leaf mould whichstops in the soil, giving it a darkish colour. The more mould thereis, the darker the colour of the soil. We know that along with thisdecay there is a great deal of shrinkage. As the black material isformed from the plant, it only extends as far into the soil as theplant roots go, so that there is a sharp change in colour about 6inches below the surface (see also p. 2). Like the plant the blackmaterial all burns away when the soil is heated sufficiently. 

Thus we can explain all the facts we have observed, and in what seems avery likely way. This does not show that our supposition is correct, but only that it is useful. When you come to study science subjectsyou will find such suppositions, or hypotheses as they are called, arefrequently used so long as they are found to be helpful. In ourpresent case we could only get absolute proof that the blackcombustible part of the soil really arose from the decay of plants bywatching the process of soil formation. We shall turn later to thissubject. 

The black material is known as humus. Farmers and gardeners like ablack soil containing a good deal of humus because they find it veryrich, and we shall see later on why this is so. Vast areas of suchsoils occurring in Manitoba, in Russia, and in Hungary are used for{37} wheat growing, while there are also areas in the Fen districts ofEngland. 

There is something known as peat that looks rather like mould, but isreally 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, butquite the reverse. You can see it being formed on a moor or bog, andyou should at the first opportunity go and examine it. There was apeat bog near Wye that was examined with the following results. Thepeat was very fibrous and had evidently been formed from plants. Itmade 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 rightthrough the peat into the clay below. A sample of the clay put into afunnel, as on p. 14, did not allow water to pass through; this was alsoevident from the very wet nature of the ground. The peat bed was belowthe level of the surrounding land and was in a sort of basin; the waterdraining from the higher land could all collect there but could not runaway, indeed it might very well have been a shallow lake. It was quiteclear that the plants as they died would _decay in very wet soil_, andso the conditions are very different from those we have just beenstudying where the plants _decay in soil that is only moist_. Thisdifference at once shows itself in the fact that peat generally forms athick layer, while mould only rarely does so. In the north of Englandthe moors lie high, but here again the peat bed is like a saucer orbasin, and there is soil or rock below that does not let the rain waterpass 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 unlikeleaf mould. When it is dry peat easily burns and is much used as fuelin parts of Scotland, Wales and Ireland. It is cut in blocks duringthe spring, left to dry in heaps during summer, and then carried awayin autumn. Fig. 19 shows a peat bog with cutting going on. Peat doesnot easily catch light and the fires are generally kept burning allnight; there is no great flame such as you get with a coal fire, butstill there is quite a nice heat. 

Peat has a remarkable power of absorbing water. Fill an egg-cup withpeat, packing it as tightly as you possibly can, and then put it underwater and leave for some days. The peat becomes very wet and swellsconsiderably, 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 maybecome dangerous; if the bog is on the side of a hill, the peat mayoverflow and run down the hill like a river, carrying everything beforeit. Such overflows sometimes occur in Ireland and they used to occurin the north of England; you can read about one on Pendle Hill inHarrison Ainsworth's _Lancashire Witches_. But they do not take placewhen ditches are cut in the bog so that the water can flow away insteadof soaking in; this has been done in England. 

This great power of absorbing water and other liquids, so terrible whenit leads to overflows, enables peat to be put to various uses, and agood deal of it is sold as peat-moss, for use in stables. 

[Illustration: Fig. 19. Cutting and carrying peat for fuel, Hoy, Orkney]

In the ditches of a peat bog red slimy masses can often be found. Theylook just like rusty iron, and in fact they do contain a good deal ofiron, 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 thered material from them having been washed out by the water which hassoaked through the peat. Then at the ditch these tiny living thingstake up the red material because it is useful to them. Peat or"moorland" water can also dissolve lead from lead pipes and maytherefore be dangerous for drinking purposes unless it is speciallypurified. When you study chemistry you will be able to show that bothpeat 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. Ifyou stir up the bottom of a stagnant pond with a stick bubbles of gasrise to the surface and will burn if a lighted match is put to them. This gas is called marsh gas. Very unpleasant and unwholesome gasesare also formed. 

[1] The top two inches of soil only were collected here, and there weremany leaves, twigs, etc. Mixed in. Soils from different woods varyconsiderably. If the sample is taken to a greater depth the loss onburning 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 uncultivatedstate devoid of vegetation. The hills are covered with grasses andbushes, the moors with ling and heather, commons with grass, brackenand gorse, a garden tends to become smothered in weeds, and even agravel path will not long remain free from grass. It is clear thatsoil is well suited for the growth of plants. We will make a fewexperiments 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 shallwant to know whether this, like soil, can support plant life. We havealso found that the subsoil is unlike the top soil in several ways, andso we shall want to see how it behaves towards plants. Fill a pot withsoil taken from the top nine inches of an arable field or untrenchedpart of the garden; another with subsoil taken from the lower depth, 9to 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 thepots together and equally well supplied with water; the plants thenhave as good a chance of growth in one pot as in any other. 

{42}

[Illustration: Fig. 20. Rye growing in surface soil (Pot 3), subsoil(Pot 4), and sand (Pot 5)]

{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

[Illustration: Fig. 21. Mustard growing in surface soil (Pot 3), subsoil (Pot 4), sand (Pot 5)]

The plants in the soil remained green and made steady growth. Those inthe sand never showed any signs of getting on, their leaves turnedyellow and {44} fell off; in spite of the care they received, and thewater, warmth and air given them, they looked starved, and that, infact, is what they really were. Nor did those in the subsoil fare muchbetter. The experiment shows that the top soil gives the plantsomething that it wants for growth and that it cannot get either fromsand or from the subsoil; this something we will call "plant food. "

Further proof is easily obtained. At a clay or gravel pit little or novegetation is to be seen on the sloping sides or on the level at thebottom, although the surface soil is carrying plants that shedinnumerable seeds. A heap of subsoil thrown up from a newly made well, or the excavations of a house, lies bare for a long time. Thepractical man has long since discovered these facts. A gardener ismost 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 cricketpitch or tennis court, it is not enough to lift the turf and make alevel surface; the work has to be done so that at every point there issufficient depth of top soil in which the grass roots may grow. 

How much plant food is there in the top soil? To answer this questionwe must compare soil that has been cropped with soil that has been keptfallow, i. E. Moist but uncropped. Tip out some of the soil that hasbeen cropped with rye, and examine it. Remove the rye roots, thenreplace the soil in the pot and sow with mustard; sow also a fallow potwith mustard. Keep both pots properly watered. The soil that hascarried a crop is soon seen to be much the poorer of the two. Fig. 22shows 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

[Illustration: Fig. 22. Mustard growing in surface soil previouslycropped with rye (Pot 1) and in surface soil previously uncropped (Pot2)]

{46}

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

 Pot 2 Pot 2_a_

 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 plantfood was present at the beginning. But we can say that the amount ofplant food in the soil is too small to be detected by such weighing aswe can do. 

Here is an account of a similar experiment made 300 years ago by vanHelmont in Brussels, and it is interesting because it is one of thefirst scientific experiments on plant growth:--

"I took an earthen vessel in which I put 200 pounds of soil dried in anoven, then I moistened with rain water and pressed hard into it a shootof willow weighing 5 pounds. After exactly five years the tree thathad grown up weighed 169 pounds and about 3 ounces. But the vessel hadnever received anything but rain water or distilled water to moistenthe soil (when this was necessary), and it remained full of soil whichwas still tightly packed, and lest any dust from outside should havegot into the soil it was covered with a sheet {47} of iron coated withtin but perforated with many holes. I did not take the weight of theleaves 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 wateralone. " The experiment is wonderfully good and shows how very littleplant food there is in the soil. The conclusion is not quite right, however, although it was for many years accepted as proof of an ancientbelief, which you will find mentioned in Kingsley's _Westward Ho!_, that all things arose from water. It is now known that the lastsentence should read, "Therefore the 164 pounds of wood, bark and rootarose chiefly from the water _and air_, but a small part came from thesoil also. "

But to return to our experiment with Pots 1 and 2. They had been keptmoist before the mustard was sown. Did this moisture have any effecton the soil? Take two of the pots that have been kept dry anduncropped, and two that have been kept moist and uncropped, also one ofdry uncropped subsoil and one of moist uncropped subsoil. Sow rye ormustard 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 thesubsoil, and the soil stored moist is rather richer than that storeddry, although the difference here is less marked. In an experiment inwhich the soils were put up early in July and sown at the end ofSeptember 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 soshould the crops on Pots 8 and 9. The differences arise from the errorof the experiment. In all experimental work, however carefully carriedout or however skilful the operator, there is some error. 

There is clearly an increase in crop as a result of storing the surfacesoil in a moist condition, showing that additional plant food has been_made_, since these pots were put up. On the other hand it does notappear that much plant food has been made in the subsoil during thistime. Further evidence on this point is given by an experiment similarto that in Fig. 22, but where mustard is grown in _subsoil_ kept moist, but uncropped for some time, and in _subsoil_ previously cropped withrye. 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 thesubsoil lay fallow for a long time it produced no plant food but isjust as poor as the subsoil that has been previously cropped. Theseobservations give us a clue that must be followed up in answering ournext question. 

What has the plant food been made from? Clearly it is not made fromthe 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 incontaining a quantity of material that will burn away and is in part atany rate made up of plant remains. It will be easy to find out whetherthese remains furnish any appreciable quantity of plant food. 

Fill one pot with surface soil and another with the same weight ofsurface soil well mixed up with 30 grams of plant remains--pieces ofgrass, or stems and leaves of other plants cut up into fragments abouthalf an inch long. At the same time put up two pots of subsoil, one ofwhich, as before, is mixed with 30 grains of plant remains, and alsoput up two pots of sand, one containing 30 grams of plant remains andthe other none. Sow all six pots with mustard and keep watered andwell tended. The result of one experiment is shown in Fig. 23 and theweights 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

[Illustration: Fig. 23. Pieces of grass, leaves, etc. Change intoplant food in the surface but not to any great extent in the subsoil. Mustard is growing in surface soil (Pot 3), in surface soil and piecesof grass (Pot 6), in subsoil (Pot 4), and in subsoil and grass (Pot 7)]

Now let us look at these results carefully. The experiment withsurface soil shows that the pieces of stem and leaf have furnished agood deal of food to the mustard and have caused a gain of 24. 3 gramsin the crop. If we knew what the pieces were made of we {51} couldpush the experiment still further and find out more about plant food, but this involves chemical problems and must be left alone for thepresent. We can, however, say that plant remains are an importantsource of plant food, and since we suppose the black material of thesoil to be made of plant remains (see p. 36), it will be quite fair tosay also that this black material, the humus, is a source of plantfood. We have therefore answered the question we set, and we canexplain some at any rate of the differences between the surface soiland the subsoil. The surface soil contains a great deal of the blackmaterial, which forms plant food, while the subsoil does not. Thusplants grow well on the surface soil and starve on the subsoil. We canalso explain why gardeners and farmers speak of black soils as richsoils; they contain more than other soils of this black material thatmakes plant food. Still further, we can explain why the farmer oftensows plants like mustard, tares or clover, and then ploughs them intothe ground. They are not wasted, but they make food for the next cropthat goes in. 

Now let us turn to the results of the subsoil experiments. The leavesand stems have increased the crop, but only by 5. 4 grams: they have notbeen nearly so effective as in the surface soil. It is evident thatthe mustard did not feed directly on the leaves and stems put in; if ithad there should have been an equal gain in both cases. The leaves andstems clearly have to undergo some change before they are made intoplant food and the soil has something to do with this change. Afterthe crops are cut the soils should be tipped out and examined. More ofthe original pieces of leaf and stem are found in the subsoil than inthe surface {52} soil. That is to say, there has been more change inPot 6 containing surface soil than in Pot 7 containing subsoil. The"something, " whatever it may be, that changes plant remains likeleaves, stems, pieces of grass, roots, etc. Into plant food thereforeacts better in the surface soil than in the subsoil. Here then we haveanother difference between surface and subsoils. 

SUMMARY. The experimental results obtained in this chapter may now besummed up as follows:--

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

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

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

(4) The remains of leaves, stems, roots, etc. Furnish an importantsource of plant food. 

(5) But they have first to undergo some change, and the agent producingthis 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 shouldnever be buried during digging or trenching, but always carefully kepton top. 

{53}

CHAPTER VII

THE DWELLERS IN THE SOIL

Apparatus required. 

_Garden soil. Six bottles and corks [1]. Twelve Erlenmeyer flasks, 50c. C. Capacity [2]. Cotton wool. Milk (about half a pint). Leafgelatine. Soil baked in an oven. Six saucers [3]. The apparatus inFig. 28 (two lots). Wash bottle containing lime water (Fig. 27, alsop. 19). _

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

Put some fresh moist garden soil into a bottle and cork it up tightlyso that it keeps moist. Write the date on the bottle and then leave itin the light where you can easily see it. After a time--sometimes along, sometimes a shorter time--the soil becomes covered with a slimygrowth, greenish in colour, mingled here and there with reddish brown. The longer the {54} soil is left the better. Often after severalmonths something further happens; little ferns begin to grow and theylive a very long time indeed. There is at Rothamsted a bottle of soilthat was put up just like this as far back as 1874. For a number ofyears past a beautiful fern has been growing inside the bottle, andeven now it is very healthy and vigorous. If, instead of being keptmoist, the rich garden soil is left in a dry shed during the whole ofthe winter so that it gradually loses its moisture, it will generallyshow quite a lot of white fluffy growth. 

All of these living things are very wonderful, and some, especiallyearthworms, 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 ordoes not naturally grow long, and you will find numbers of tiny heapsof soil scattered about. Carefully brush away a heap and a little holeis seen, now hit the ground near it a few times with a stick or stampon 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 ortrowel so as to examine the hole or "burrow. " At the top you generallyfind it lined with pieces of grass or leaves that the worm has pulledin; lower down the lining comes to an end, but the colour of the burrowis redder than that of the rest of the soil wherever the soil has agreenish tinge. These holes are useful because they let air and waterdown into the soil. 

[Illustration: Fig. 24. Soil in which earthworms have been living andmaking burrows]

The following experiment shows what earthworms can do. Fill a pot withsoil from which all the worms have been carefully picked out andanother {55} with soil to which earthworms have been added, one worm toevery pound of soil. Leave them out of doors where the rain can fallon 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 thesurface of the soil, continually covering it with a thin layer ofmaterial brought up from below. Consequently the soil containingearthworms always has {56} a fresh clean look. After some time theother soil becomes very compact and is covered with a greenish slimygrowth. When this happens carefully turn the pots upside down, knockthem so as to detach the soil and lift them off. The soil where theearthworms had lived is full of burrows and looks almost like a sponge. Fig. 24 shows what happened in an experiment lasting from June toOctober. The other soil where there were no earthworms shows no suchburrows 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 ofsoil and leaf mould. 

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

All these things are very useful and so a gardener should never want tokill worms. The great naturalist, Darwin, spent a long time instudying earthworms at his home in Kent and wrote a very interestingbook about them, called _Earthworms and Vegetable Mould_. He showsthat each year worms bring up about 1/50th of an inch of soil, so thatif you laid a penny on the soil now and no one took it, in 50 years itmight be covered with an inch of soil. Pavements that were on thesurface when the Romans occupied Britain are now covered with a thicklayer of soil. 

[Illustration: Fig. 25. Fresh soil turns milk bad, but baked soil doesnot]

But besides these there are some living things too small to see, thathave only been found by careful experiments, but you can easily repeatsome of these {57} experiments yourselves. Divide a little rich gardensoil 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 cottonwool (see Fig. 25) and boil for a few minutes very carefully so thatthe milk does not boil over, then allow to cool. Next carefully takeout the stopper from one of the flasks and drop in a little of thebaked 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 openthe stoppers and smell the milk: the baked soil has done nothing andthe 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 goodmicroscope you can go further: look at a drop of the liquid from eachflask and you find in each case the {58} round fat globules of themilk, but the bad milk contains in addition some tiny creatures, looking like very short pins, darting in and out among the fatglobules. These living things must have come from the unbaked soil orthey would have been present in both flasks: they must also have beenkilled by baking in the oven. 

[Illustration: Fig. 26. Soils contain tiny things that grow ongelatine]

Another experiment is easy but takes a little longer to show. Mix twosheets 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 stirup some baked soil in a cup half full of cold boiled water, and quicklyput a teaspoonful of the liquid into a second cup, also half full ofcold boiled water. Stir quickly and put a spoonful on to the jelly, tilting it about so that it covers the whole surface and label thesaucer "baked soil. " Do the same with the "unbaked soil, " labellingthe saucer; leave the third jelly alone and label it "untouched. "Cover all three with plates and leave in a warm place. After a day orso little specks begin to appear on the jelly containing the unbakedsoil, but not on the others (Fig. 26); they grow larger, and beforelong they change the jelly to a liquid. The other jellies {60} showvery few specks and are little altered. These creatures making thespecks 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 soiljelly. 

[Illustration: Fig. 27. Bottle containing lime water, used to showthat breath makes lime water milky]

You can also show that breathing is going on in the soil even after youhave picked out every living thing that you can see. First of all youmust do a little experiment with your own breathing so that you mayknow how to start. Shake up a little fresh lime with water and leaveit to stand for 24 hours. Pour a little of the clear liquid into aflask or bottle fitted with a cork and two tubes, one long and oneshort 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 thatno change occurs. Next breathe out through the tube _B_ so that yourbreath passes through the lime water; this time the lime water turnsvery 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 ofequal size fitted with corks and joined by a glass tube bent like a Uwith the ends curled over. Put some lime water into each flask and alittle water in the U-tube. Now make a small muslin bag like asausage: fill it with moist fresh garden soil, tie it up with a silkthread and hang it in one of the flasks by holding the end of thethread 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 thelevel of the liquid with a piece of stamp paper; leave in a warm placebut out of the sun. {61} In one or two days you will see that thewater in the U-tube has moved towards the soil flask, showing that someair has been used up by the soil; further, the lime water has turnedmilky. But in the other flask, where there is no soil, the lime waterremains quite clear. 

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

[Illustration: Fig. 28. A bag of soil is fixed into a flask containinglime water. In a few days some of the air has been used up, and thelime water has turned milky]

Now try the experiment with very dry garden soil: little or no changetakes place. As soon as you add water, however, breathing beginsagain, 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 thatthese little creatures seemed to {62} you to be an inch long, and ifyou looked down into the soil, it would seem to you to be anextraordinarily wonderful place. The little grains of soil would looklike great rocks and on them you would see creatures of all shapes andsizes moving about, and feeding on whatever was suitable to them, somebeing destroyed by others very much larger than themselves, someapparently dead or asleep, yet waking up whenever it becomes warmer orthere was a little more moisture. You would see them changing uselessdead roots and leaves into very valuable plant food; indeed it is theythat bring about the changes observed in the experiments of Chap. VI. Occasionally you would see a very strange sight indeed--a greatsnake-like creature, over three miles long and nearly half a mileround, would rush along devouring everything before it and leave behindit a great tunnel down which a mighty river would suddenly pour, andwhat do you think it would be? What you now call an earthworm andthink is four inches long, going through the soil leaving its burrowalong which a drop of water trickles! That shows you how tiny theselittle soil creatures are. 

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

{63}

SUMMARY. The soil is full of living things, some large like earthworms, others very small. Earthworms are very useful: they makeburrows in the soil, thus allowing air and water to get in: they dragin leaves and they keep on covering the surface with soil from below. Besides these and the other large creatures, there are micro-organismsso small that they cannot be seen without a very good microscope: theylive and breathe and require air, water and food. Some are very usefuland 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 and18 in. Long [2]. Muslin, string, three beakers. Six lamp chimneysstanding 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 havediscovered that it needs much attention if it is to be kept alive. Itwants water or it withers; it must be kept warm enough or it is killedby cold; it has to be fed or it gets yellow and starved; also it needsfresh 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 thesoil. 

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

{65}

[Illustration: Fig. 29. Loam and sand both retain water, but loambetter than sand]

Water comes from the rain, but the rain does not fall every day. Howdo the plants manage to get water on dry days? A simple experimentwill show you one way. Put about four tablespoonsful of dry soil on tothe funnel shown in Fig. 29 and then pour on two tablespoonsful ofwater. Measure what runs through. You will find it very little; mostof the water sticks to the soil. Even after several days the soil wasstill rather moist. Soil has the power of keeping a certain amount ofwater in reserve for the plant, it only allows a small part of the rainto run through. Do the experiment also with sand, powdered clay, andleaf mould. Some water always remains behind, but less in the case ofsand than in the others. In one {66} experiment 30 cubic centimetresof water were poured on to 50 grains of soil but only 10 cubiccentimetres passed through, but when an equal amount was poured on to50 grains of sand no less than 20 cubic centimetres passed through. Very sandy soils, therefore, possess less power of storing water thando soils with more clay or mould in them, such as loams, clays or blacksoils. 

[Illustration: Fig. 30. Water can rise upwards in soil. It can, infact, travel in any direction, from wet to dry places]

Further, water has a wonderful power of passing from wet places to dryplaces in the soil. Tie a piece of muslin over the end of a tube andfill with dry soil, tapping it down as much as you can, then stand thetube in water as in Fig. 30. Fill another with sand {68} and place inwater. Notice that the water at once begins to rise in both tubes andwill go on for a long time, always passing from the wet to the dryplaces. It rises higher in the soil than it does in the sand. Enoughwater may pass up the tube in this way to supply the needs of a growingplant. Fill a glass lamp chimney with dry soil, packing it downtightly, put into water and then sow with wheat. The plants grow verywell. A longer tube may be made from two chimneys fastened together bymeans 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, wherethe water could not rise so high, it might happen that they would not. 

[Illustration: Fig. 31. Wheat growing in soils supplied from belowwith water. All the water the plant gets has to travel upwards]

Thus we shall expect great differences in the moisture of varioussoils. In some districts there is much more rain than in others, andtherefore the soils get a larger supply of water. Sandy soils allowwater to run through while a loam holds it like a sponge, in a loamalso the water readily moves from wet to dry places. Further, waterruns down hills and collects in low-lying hollows or valleys; here, therefore, the soil is moister than it is somewhat higher up. Whatwill be the effect of these moisture differences on plants?

You must find out in two ways. Visit a soil that you know is dry--asandy, gravelly or chalky soil in a high situation--and look carefullyat the plants there, then go to some moister, lower ground and see whatthe plants show. You cannot be quite certain, however, that anythingyou see is simply due to water supply, because there may be otherdifferences 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{69} quantities of water on the plant growth. Both methods must beused, but it may be more convenient to start the experiments first, andwhile they are going on to collect observations in your rambles. 

[Illustration: Fig. 32. Mustard growing in soils supplied with varyingquantities of water. 16 very little water, 3 a nice amount of water, 15 too much water]

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 frequentlythan the second; and the fourth is to be kept flooded with water, anyway out being stopped up. Sow wheat or mustard in all four and keepout of the rain. The result of one experiment with mustard is shown inFig. 32. Where no water was supplied there was no growth and the seedsremained unaltered. Where only little water was supplied (Pot 16) theplants made some growth, but not very much: the leaves were small andshowed no great vigour; {70} where sufficient water was given (Pot 3)the plants grew very well and had thick stems and large leaves; wheretoo much water was given (Pot 15) the plants were very sickly and small. 

The weights were:--

 Green weight After drying

 Plants with too much water 3. 9 0. 5

 " " too little water 5. 3 0. 9

 " " a nice quantity of water 17. 7 2. 6

Fig. 33 shows two pots of wheat, one kept only just sufficiently moistfor growth, the other kept very moist but not too wet. You can seewhat a difference there is; in the drier pot the leaves are rathernarrow and the plants are small, in the moister pot the leaves are wideand the plants big. But there was also another difference that thephotograph does not bring out very well--the plants in the rather drysoil were, as you can see, in full ear, ripe and yellow, while those inthe very moist soil were still green and growing. We see then

(1) that on moist soils there is greater growth than on dry soils, butthe plants do not ripen so quickly;

(2) in very wet soils mustard--and many other plants also--will notgrow. 

Water is not itself harmful. It is easy to grow many plants in watercontaining the proper food, but _air must be blown through the water atfrequent intervals_. In the water-logged soil of Pot 15 the troublearose not from too much water but from too little air. Air is wantedbecause plants are living and {71} breathing in every part, in theroots as well as in the leaves. 

[Illustration: Fig. 33. This wheat growing on very moist soil wasstill green and growing vigorously, whilst this wheat growing on ratherdry soil was yellow and ripe]

Now turn to what you have seen in your walks. You would probablynotice that on the drier, sandy or gravel ground there was nothing likeas great a growth of grass or of other plants as on the moister soil. This is so much like what we found in the pot experiments that we shallnot be wrong in supposing that the difference in water supply largelyaccounted for the difference in growth. But you may also have noticedsomething else. Plants in the drier soil have generally {72} narrowleaves and the grasses are rolled up and fine, whilst those on the dampsoil, including the grasses, have usually broad leaves. Thus in thedry sandy soil you may find broom, spurrey, sheep's fescue, pine trees, all with narrow leaves; whilst on the moister soil you may findburdock, primroses, cocksfoot and other broad-leaved plants. Figs. 34_a_ and _b_ show some plants we found on a dry, gravelly patch onHarpenden common, and on a moist loam in the river valley below. 

[Illustration: Fig. 34 _a_. Plants collected on dry sandy soil. Broom, sheep's fescue, crested dogstail and gorse, all with narrowleaves]

{73}

Before we can account for this observation, we must ascertain a littlemore closely what becomes of the water the plant takes up. Itcertainly does not all stay in the plant, and the only way out seems tobe through the leaves. Put a test tube on the leaf of a growing plantand fix a split cork round the stem: leave in sunlight for a few hoursand notice that water begins to collect in the test tube (Fig. 35). The experiment shows that water passes out of the plant through theleaves. 

[Illustration: Fig. 34 _b_. Plants collected on moist loam. All havewide leaves]

This experiment was first made by Stephen Hales, and described by himthus in 1727: "Having by many {74} evident proofs in the foregoingexperiments seen the great quantities of liquor that were imbibed andperspired by trees, I was desirous to try if I could get any of thisperspiring matter; and in order to do it, I took several glass chymicalretorts, _b a p_ [Fig. 36] and put the boughs of several sorts oftrees, as they were growing with their leaves on, into the retorts, stopping up the mouth _p_ of the retorts with bladder. By this means Igot several ounces of the perspiring matter of vines, figtrees"--andother trees, which "matter" Hales found to be almost pure water. Thetest tube experiment should now be made with a narrow-leaved grass likesheep's fescue and with a wide-leaved grass like cocksfoot. You willfind that wide-leaved plants pass out more water than those with narrowleaves, and hence wide-leaved plants occur in damp situations or ondamp soils like loams and clays, while narrow-leaved plants can grow ondry, sandy soils. 

[Illustration: Fig. 35. Plants give out water through their leaves]

Another thing you will notice is that fields lying at the side of ariver and liable to be flooded, and fields {75} high up in wet hilldistricts, are covered with grass. In a clay country there is also agreat deal of grass land and not much ploughed land; if you live wherethere is much clay you can easily discover the reason. Clay becomesvery wet and sticky when rain falls, and very hard in dry weather: itis, therefore, difficult to cultivate. Farmers cannot afford to spendtoo much money on cultivation, and so they prefer grass, because onceit is established it goes on indefinitely and does not want ploughingup and re-sowing. And besides, farmers have learned by experience thatgrass can tolerate more water and less warmth than most other Englishcrops. There is much more grass land in those parts of England wherethe rainfall is high and the temperature rather {76} low--e. G. Thenorthern parts of England--than in the eastern counties where therainfall is low. 

[Illustration: Fig. 36. Stephen Hales's Experiment (from _VegetableStaticks_, Vol. I. 1727)]

The difference in water supply, therefore, leads us to expect thefollowing differences between sandy soils and clays or loams:--

On sandy soils (the water content being small) the wild plants andtrees usually have small leaves. Cultivated plants do not give veryheavy crops, but they ripen early. 

On clay soils (the water content being good) wild plants and treesusually have larger leaves. Cultivated plants give good crops, butthey ripen rather late. If the water content is too good or the clayis too sticky the land is generally put into grass. 

[Illustration: Fig. 37. Hill slope near Harpenden. Woodland at thetop, arable land lower down. In the valley there is grass land butthis is hidden by the cottages]

Plants require to be sufficiently warm. Some like tropical heat andcan only be grown in hot houses; others can withstand a certain amountof cold and will grow up on the mountains. Our common cultivated cropscome in between and will not grow in too cold or exposed a situation;thus you find very little cultivated land 800 ft. Above sea level, andnot usually much above 500 ft. At this height it is left as grassland, and higher up as woodland, moor, or waste land. Grass requiresless warmth and can therefore grow at greater heights than many othercrops. If you start at the top of a hill in Derbyshire, and walk down, you will see that the top is moorland, lower down comes grass land, still lower you may find arable land, and if the valley is damp youwill find more grass at the bottom. Figs. 37 and 38 show typical viewsof the hill slopes further south: they are taken near Harpenden. Thetop of the hill in each case is over 400 ft. Above sea level, and hasnever been thought worth cultivating, but has always been left as {78}wood because it is too exposed for farm crops. On the lower slopes thearable fields are seen, while at the bottom bordering the river isrough grass land, shown in Fig. 39. The top is too cold and windy, andthe bottom too wet, to be worth cultivating. 

[Illustration: Fig. 38. View further along the valley, woodland andarable above rough grass land near the river]

As the plant root is alive it wants air. The effect of keeping air outcan be seen by sowing some barley or onion seeds in the ground and thenpouring a lot of water on and plastering the soil down with a spade. Sow another row in nicely crumbled soil, not too wet, press the seedswell in, but do not plaster the soil. This second lot will generallydo much better than the first. If the ground round a plant isfrequently trodden so that it becomes very hard the plant makes muchless growth than if the soil were kept nice and loose. A good gardenertakes very great pains in preparing his ground before he sows hisseeds, and he is careful that no one should walk on his beds lest hisplants should suffer. 

SUMMARY. We may now collect together the various things we have learntin this chapter. Plants require water, air, warmth, food, and light, and they will not grow if harmful substances are present. Therain-water that falls remains for some time in the soil, and does notat once run away or dry off: water can also move from wet to dry placesin the soil. Therefore the plant does not need rain every day, but candraw on the stock in the soil during dry weather. A sandy soil isusually drier than a loam or a clay, especially if it lies rather high:plants growing on a sandy soil make less growth and have narrower andsmaller leaves than those on a moister soil. 

Situations more than five or six hundred feet above sea level are, inEngland, as a rule, too bleak and {80} exposed for the ordinarycultivated crops. Such land is, therefore, either grass land, moorland, downland or woodland. 

The roots of plants are living and require air. The soil must not betrodden too hard round them or air cannot get in, nor can it if toomuch water is present. 

Grass can put up with more water and less warmth than most cultivatedcrops. 

[Illustration: Fig. 39. Rough grass pasture near the river, above thatis arable land and still higher is woodland]

Instances of these facts may be found in going down any hill 500 ft. Ormore in height: the top is usually wood or waste, being too cold forcrops, below this may come grass land, lower still arable land. It isboth warmer and moister in the valley (since water runs down hill), andso we can account for the proverbial fertility of valleys. But justnear the river, if there is one, the ground may be too wet for crops, and therefore grass is grown. Clay land that is rather too wet toplough is usually left in grass. 

{82}

CHAPTER IX

CULTIVATION AND TILLAGE

Apparatus required. 

_Plot experiments, hoeing and mulching. Thermometer. Soil sampler(Fig. 42, p. 88). This tool consists of a steel tube 2 in. In diameterand 9 in. Long, with a slit cut along its length and all the edgessharpened. The tube is fixed on to a vertical steel rod, bent at theend to a ring 2 in. In diameter, through which a stout wooden handlepasses. It is readily made by a blacksmith. _

Farmers and gardeners throughout the spring, summer and autumn, arebusy ploughing or digging, hoeing or in other ways cultivating thesoil. Unless all this is well done the soil fails to produce much; thesluggard's garden has always been a by-word and a reproach. In tryingto understand why they do it we must remember that plant roots needwater, warmth and air; if the soil is too compact or if there is toomuch water the plant suffers, as we have seen. 

[Illustration: Fig. 40. After harvest the farmer breaks up his landwith a plough and then leaves it alone until seed time]

One great object of cultivation is, therefore, to prevent the soilbeing too compact and too wet. After the harvest the farmer breaks uphis ground with a plough and then leaves it alone till seed time (Fig. 40). A gardener does the same thing with a fork in his kitchengarden--he cannot very well elsewhere, or the plant roots might {83}become too cold. If there is frost during the winter both farmer andgardener are pleased because they say the frost "mellows" the ground;you can see what they mean if you walk on a frosty morning over aploughed field. The large clods of earth are no longer sticky, theyalready show signs of breaking up, and if they are not frozen too hardcan easily be shattered by a kick. The change has been brought aboutin exactly the same way as the bursting of water-pipes by frost. Whenwater freezes it expands with enormous force and bursts open anythingthat confines it; water freezing in the pores of the soil forces thelittle fragments apart. This action is so important that furtherillustrations should be looked for. A piece of wet chalk left out on afrosty night often crumbles to pieces. It is dangerous {84} to climbcliffs in the early spring because pieces of rock that have been splitoff during the winter frosts by the expanding water may easily giveway. Frost plays havoc with walls built of flints and with old bricksthat are beginning to wear. If there are several frosts, with falls ofrain or snow and thaws coining in between, the soil is moved about agood deal by the freezing and melting water. Bulbs and cuttings aresometimes forced out of the ground, whilst grass and young wheat may beso loosened that they have to be rolled in again as soon as the weatherpermits. When the ground has been dug in autumn and left in a veryrough state all this loosening work of the frost is very much helped, because so much of the soil may become frozen. If in spring you dig apiece of land that has already been dug in autumn, and then try digginga piece that has not, you will find the first much easier work than thesecond in all but very sandy soils. 

A little before the seeds are sown, the soil has to be dug orcultivated again so that it may become more level and broken intosmaller pieces. The farmer then harrows and the gardener rakes it, andit becomes still finer. Very great care is bestowed on the preparationof the seed bed, and it will take you longer to learn this than anyother part of outdoor gardening. The soil has to be made fine and dry, and no pains must be spared in getting it so. 

When at last the soil is fine enough the seed is put in. But it is notenough simply to let the seed tumble into the ground. It has to bepressed in gently with a spade or a roller, not too hard or the soilbecomes too sticky. Fig. 41 shows this operation being carried out onthe farm. Then the soil should be left alone. 

[Illustration: Fig. 41. Rolling in mangold seeds on the farm]

{86} If you watch an allotment holder who grows onions really wellworking away at his seed-bed you will see what a beautifully fine tilthhe gets. If you try to do the same you will probably fail; his seedswill be up before yours and will grow into healthier plants. Onlyafter long practice will you succeed, and then you will have masteredone of the great mysteries of gardening. 

As soon as the plants are up they have to be hoed, and the more oftenthis is done the better. Hoeing has several useful effects on thesoil; during summer time some experiments may be made to find out whatthese are. A piece of ground is wanted that has got no crop on it. Set out three strips each six feet wide and six feet long, leave oneentirely alone, hoe the second once a week, and the third three times aweek; put labels on so that no mistake can arise. The surface of theuntouched plot becomes very compact and glazed in appearance; the othersoils look nice and crumbly. Take the temperature of the soils byplacing a thermometer into it at various depths--half inch, threeinches, and six inches--also take the temperature of the air; enter upthe results as in the table, which shows what happened at Harpenden. 

 Air Date temperature Soil temperature

 Hoed Hoed Untouched once three times weekly weekly

 1910 June 20th 30 1/2 inch 35 31. 5 31. 5 3 inches 30. 5 29. 8 28. 8 6 inches 27 26. 5 24

 June 27th 18 1/2 inch 17. 5 17 17 3 inches 16. 7 16. 3 16. 2 6 inches 15. 8 15. 5 15. 6

{87}

The thermometer readings are in degrees centigrade. 

Remarks. June 20th: Hot sunny day, there had been no rain since June11th. 

June 27th: Cold, cloudy day, several cold, wet days during the pastweek. 

On the cold day there was very little difference between the plots, buton the hot day the hoed plots were cooler than the others. Now onlythe top inch is touched by the hoe, and so it appears that the layerthus loosened shields the rest of the soil from the sun's heat. Ifthis is the case we ought to find that any other loose material wouldact in just the same way. We must, therefore, set out a fourth plotalongside the others, cover it with straw or cut grass (a cover likethis is called a mulch), and take the temperature there. Some of theresults were as follows:--

 Air Date temperature Soil temperature

 Hoed plot Mulched plot

 1910 Sept. 24th 15 1/2 inch 17. 5 12. 25 3 inches 12. 5 11. 75 6 inches 12. 25 11. 5

 Oct. 5th 17 1/2 inch 17 15. 5 3 inches 16. 7 15 6 inches 15. 5 14. 5

Remarks. Sept. 24th: Warm day after a rather cold spell. Oct. 5th:After a long spell of dry, warm weather. 

The untouched plot had become smothered in weeds and could no longer beused for this experiment. The mulched soil is, however, cooler eventhan the hoed soil, and our expectation that mulching would keep thesoil cool has turned out to be correct. 

{88}

[Illustration: Fig. 42. Soil sampler. (See p. 82 for description)]

It may be expected that the hotter soil--the unhoed plot--will also bedrier than the others, and this can be found out by a simpleexperiment. Take a sample by making a hole six indies deep withstraight and not with sloping sides: this is best done by driving atube two inches wide into the soil (Fig. 42): if you have not got sucha tool you may use a trowel, but you will have to be very quick andvery careful. Weigh the soil--or a part of it if you have got a greatdeal--then set it to dry in a warm place for three or four days. Weighagain when it is dry: the difference gives the loss of water: find whatit would be in a hundred parts. Our results were:--

{89}

 Date Percentage of water in the

 Untouched soil Soil hoed once Soil hoed three weekly times weekly

 1910 June 4th 21. 1 19. 5 17. 9

 June 20th 14. 7 16. 0 16. 0

 June 27th 19. 3 18. 4 20. 5

Remarks. June 4th; The weather is still cold and the summer has notyet begun. 

June 20th: Hot day following on some hot, dry weather. 

June 27th: Rain had recently fallen. 

When hoeing is done in the early part of the summer it dries the soil, and the more frequent the hoeing the drier the soil (see June 4thresults). But later on, when the hot weather begins, the hoed soilloses much less moisture than the untouched plot; the latter lost 6. 4per cent. In 16 days in the top six inches, whilst the soil hoed onceweekly lost 3. 1 per cent. , and the one hoed three times weekly lostonly 1. 4; the two hoed soils are now equal, and are both moister thanthe untouched soil. When more rain comes they get just as wet as theothers: hoeing does not prevent water from sinking in, but it doesprevent water from getting lost. 

Our experiment has, therefore, shown us that hoeing makes a loose layerof soil which shields the rest of the soil from the sun's heat, andprevents it getting too hot or too dry. A hoed soil is cooler andmoister, and therefore better suited for the growth of plant roots thanan unhoed soil. 

{90}

The mulch of straw or dried grass was found to have the same effect inconserving the water as the loose layer of soil obtained by hoeing. Some results were:--

 Percentage of moisture in

 Date Hoed soil Mulched soil

 1910 Sept. 24th 19. 6 20. 7

[Illustration: Fig. 43. Cultivation and mulching reduce the loss ofwater from soils]

These results are so important that some indoor experiments should bemade to furnish more proof. Fix up three inverted bell jars with corksand bent tubes as shown in Fig. 43, fill all with dry soil well presseddown, then add water carefully till it appears in the glass tubes. Next day mark with stamp paper the level of liquid in each tube andthen leave one jar {91} untouched, carefully cultivate with a penknifeevery two or three days the top quarter of an inch of the second, andcover the third with a layer of grass. After a week notice again thelevels of the liquid and mark with paper; you find that the water hasfallen most in the untouched jar, showing that more has been lost fromthis than from the jars covered with a mulch either of soil or of drygrass. 

A slate or flat stone acts like a mulch; if you leave one on the soilfor a few days in hot weather and then lift it up on a hot day you willsee that the soil underneath is quite moist; you may also find severalslugs or other animals that have gone there for the sake of thecoolness and the moisture. Plants and trees also keep off the sun'sheat and so make the soil cold and moist. Grass land is in summer andautumn, and even in early winter, cooler near the surface than bareland. At Harpenden we found:--

 Soil temperature Date Grass land Bare land

 1910 Sept. 24th 1/2 inch 13 17. 5 3 inches 12. 5 12. 5 6 inches 12. 5 12. 25

 Oct. 5th 1/2 inch 17. 5 17 3 inches 15 16. 7 6 inches 14. 5 15. 5

Even if the ground is not covered a certain amount of protection isstill possible. Trees are often planted round ponds to preventevaporation of the water. The wind helps to dry the soil very much, and a hedge {92} that shields from the wind not only protects the cropbut also keeps the soil moist: a road with high hedges at each sideremains wet for a long time after more exposed parts have dried. Theeffect on the temperature can be well seen on a day when a N. E. Wind isblowing. Fix up on a piece of the experimental ground a little hedgemade of small pea-stakes or brushwood, and take the soil temperature atone inch depth, both on the windward and on the leeward side. Tworesults were:--

 Temperature at 1 inch depth--sheltered side 15. 5 " " " " windward side 14

We have already seen that on the hot day, June 20th, the top half-inchof soil was hotter than the air: the mercury in the thermometer rosedirectly it was put into the soil. There is nothing very unusual aboutthis; if you touch a piece of iron lying on the soil you find it hotterthan the air. Lower down the soil had the same temperature as the air, and still lower it was cooler[1]. The sun's heat travels so slowlyinto the soil in summer that months pass before it gets far down, butthen, as it takes so long to get in, it also takes a long time to getout, and it takes still longer to get either in or out if there is amulch or if grass is growing. 

During the early winter you may notice that the first fall of snow soonmelts on the arable land but remains longer on the grass; towards theend of the winter, however, the reverse happens and the snow meltsfirst on the grass. There is no difficulty in explaining this. Thearable land is, as we have seen, warmer in autumn and early winter thangrass land, {93} and so it melts the snow more rapidly. But duringwinter the grass land loses its heat more slowly, and therefore it iswarmer at the end of the winter than the arable land, hence the snowmelts more quickly. 

In Chap. V. It was pointed out that dark coloured soils rich in humusare greatly favoured by gardeners and farmers. The value of humus caneasily be shown: take a sample of soil from a garden that has for along time been well manured and another from a field close by--next toit if you can--and find the amounts of moisture present. Two soils atRothamsted gave the following results:--

 Date April 6th May 6th July 6th Oct. 28th

 Moisture in dark soil rich in humus 20. 0 18. 0 20. 7 23. 3

 Moisture in lighter soil poor in humus 13. 1 11. 9 12. 0 17. 5

Humus, therefore, keeps the water in the soil and saves it from beinglost. 

Another beneficial effect of hoeing is to keep down weeds. Weedsovercrowd the plant, shut out light, take food and water, and occupyspace. Few plants can compete against weeds, some fail very badly inthe struggle. Sow two rows of maize two yards apart; keep one wellhoed for a yard on each side and leave the other alone to struggle withthe weeds that will grow. Fig. 44 shows the result of this experimentat St George's School. At Rothamsted a piece of wheat was leftunharvested in 1882, and the plot has not been touched since; the wheatwas allowed to shed its seed {94} and to grow up without any attention. Weeds flourished, but the wheat did not; the next year there was butlittle wheat, and by 1886 only a few plants could be seen, so stuntedthat one would hardly recognise them. The ground still remainsuntouched, and is now the dense thicket seen in Fig. 45. Most of ourland would become like this if it were neglected for a few years. 

[Illustration: Fig. 44 _a_. The hoed plot, no weeds. Maize cannotcompete successfully against weeds]

{95}

Farmers occasionally leave their ground without a crop for a whole yearand cultivate it as often as they can to kill the weeds. This practiceis called "fallowing, " and is very ancient; it is much less common nowthat crops like mangolds and swedes are grown, which can, if necessary, be hoed all the summer. 

[Illustration: Fig. 44 _b_. Untouched plot, many weeds]

We have already seen (p. 69) that ordinary cultivated plants will notlive in a water-logged soil. {96} Wherever there is an excess of waterit must be removed before satisfactory results can be obtained. Fig. 46 shows a field of wheat in May where the crop is all but killed andonly certain weeds survive on a patch of undrained land that lay wetall the winter. Draining land is difficult and somewhat expensive;trenches are first cut to a proper depth, and drain pipes are laid onthe bottom, taking care that there is a gentle slope all the way to theditch. The rain soaks into the soil and gets into the pipes, for theyare not joined together like gas or water pipes, but left with littlespaces in between; it then runs out into the ditch. Usually only claysoils need drainage, but occasionally sandy soils do also (see pp. 30, 106). A great deal of drainage was carried out in England between 1840and 1860, and it led to a marked improvement in agriculture and incountry life generally. There is, however, a great deal that wantsdoing now. 

[Illustration: Fig. 45. A plot of wheat left untouched since 1882 atRothamsted has now become a dense thicket]

The addition of chalk or lime to soil was found in Chap. III. Toimprove it very much by making it less sticky and less impervious toair and water. Chalk or lime does more than this. It puts out ofaction certain injurious substances or acids that may be formed, andthus makes the conditions more favourable for plants and for the usefulmicro-organisms; farmers and gardeners express this by saying that it"sweetens the soil. " A United States proverb runs: "A lime country isa rich country. " Very many soils in England are improved by addinglime or chalk. There are considerable areas in the south-eastern andeastern counties where the soil is very chalky; here you find awonderfully rich assortment of flowers and shrubs. Where there is toomuch chalk the soil is not fertile, because it lets water {98} throughtoo easily, as was shown on p. 26: but for this very reason it isadmirable for residential purposes. 

There are some exceptions to the rule that plants need lime. Someplants will not tolerate it at all; such are rhododendrons, azaleas, foxgloves, spurrey, and broom; wherever you see these growing you maybe sure that lime is absent. 

Lime really differs from chalk, but changes into it so quickly in thesoil that the action of both is almost, though not quite, the same. 

[Illustration: Fig. 46. A wheat field in May. The large patch in thecentre where the crop is doing badly lay under water for much of thewinter because of the bad drainage]

SUMMARY. The various things we have learnt in this Chapter are:--

Autumn and winter cultivation are needed to loosen the soil so thatrain can soak in and not lie about in pools, and also to facilitateworking in spring. 

The soil has to be broken down very finely and made rather dry for aseed bed. The seed has to be rolled in and then left entirely alone. 

As soon as the little plants are up the soil must be hoed, and the moreoften this is done the better. Hoeing keeps the soil cool and moist inhot weather, the loose layer acting like a mulch of straw. Anythingelse that shields the soil from the sun or the wind has the same actionbut is not so effective as the mulch. Further, hoeing keeps downweeds, which successfully compete against almost any cultivated plants. Humus also prevents the loss of moisture from soils. 

Drainage may be necessary to remove excess of water. 

Liming or chalking the soil is beneficial, not only because of theimprovements mentioned in Chap. III. , but also because certaininjurious substances are thereby removed. There are, however, someplants that will not tolerate lime. 

[1] At great depths below the surface the temperature rises again fromquite another cause. 

{100}

CHAPTER X

THE SOIL AND THE COUNTRYSIDE

In this chapter we want to put together much of what we have learnedabout the different kinds of soil, so that as we go about the countrywe may know what to look for on a clay soil, a sandy soil, and so on. 

We have seen that clay holds water and is very wet and sticky inwinter, while in summer it becomes hard and dry, and is liable to crackbadly. "It greets a' winter and girns a' summer, " as one of Dr JohnBrown's characters said of his soil. Clay soils are therefore hard todig and expensive to cultivate: the farmer calls them heavy and usuallyprefers to put them into grass because once the grass is up it lasts aslong as it is wanted and never needs to be resown. But in the dayswhen we grew our own wheat, before we imported it from the UnitedStates and other countries, this clay land was widely cultivated forwheat and beans. So long as wheat was 60/- to 100/- a quarter it was avery profitable crop, but, when some forty years ago it fell to 40/-and then lower still, the land either went out of cultivation like the"derelict" farms of Essex, or it was changed to grass land and used forcattle grazing. Great was the distress that followed; some districtsindeed were years in recovering. But new methods came in: the landnear London was used for dairy {101} farming, and elsewhere it wasimproved for grazing, and the clay districts, although completelychanged, are now more prosperous again. Many of the fields still showthe ridges or "lands" in which, when they grew wheat, they were laid upto let the water run away, and many of them keep their old names, butthese are the only relics of the old days. The land is not, and neverwas, very valuable. The roads are wide, and on either side have widewaste strips cut up roughly by horse tracks, cart ruts and ant hills. Bracken, gorse, rushes, thistles and brambles grow there, and you mayfind many fine blackberries in September. The coarse Aira grass isfound with its leaves as rough as files. The villages are often builtround greens which serve as the village playground, where the boys andyoung men now play cricket and football, and their forefatherspractised archery, played quoits and other games. On a few villagegreens the Maypole can still be seen, whilst the stocks in whichoffenders were placed are also left in some places. 

The hedges are often high and straggling, and there are numerous woodsand plantations containing much oak. Some of the woods are veryancient and probably form part of the primeval forests that oncelargely covered England. Epping Forest in Essex, the Forest of Bleanand the King's Wood in Kent, have probably never been cultivated land. In the days when ships were made of oak these woods and hedges werevery valuable, but now they are of little use as sources of timber. Instead they are valued for quite another reason: they afford shelterfor foxes and for game birds. The clay districts are and always havebeen famous for fox hunting; the Pytchley, Quorn, Belvoir, {102} andother celebrated packs have their homes in the broad, clay, grassyvales of the Midlands. The vale of Blackmoor and other clay regionsare equally famous. The plantations and hedgerows are fine places forprimroses and foxgloves, while in the pastures, and especially the poorpastures, are found the ox-eyed daisy and quaking grass, that make suchfine nosegays, as well as that sure sign of poverty, the yellow rattle. But many of these poor pastures have been improved by draining, liming, and the use of suitable manures. Although the roads are better thanthey were (see p. 30) they are still often bad and lie wet for weekstogether in winter, especially where the hedges are high. Numerousbrick and tile yards may be found and iron ore is not uncommon; in someplaces it is worked now, in others it is no longer worked and nothingremains of the lost industry save only a few names of fields, of ponds, or of cottages. 

[Illustration: Fig. 47. Highly cultivated sand in Kent. Gooseberriesare growing in the foreground, vegetables behind, and hops inbackground]

A sandy soil is in so many ways the opposite of a clay soil that weshall expect to find corresponding differences in the look of thecountry. A sandy soil does not hold water: it may get water up fromthe subsoil to supply the plant (see p. 66), or, if it happens to liein a basin of clay, it may even be very wet: otherwise it is likely tobe too dry for ordinary plants. We may therefore look out for twosorts of sand country, the one cultivated because there is enough waterfor the crops, and the other not cultivated because the water islacking. These can readily be found. 

We will study the cultivated sands first. As sand is not good plantfood (p. 43) these soils want a lot of manure, and so are not good forordinary farmers. But they are very easy to cultivate--for whichreason they {104} are called light soils--and can be dug at any time;seeds can be sown early, and early crops can be got. Consequentlythese soils are very useful for men doing special work like fatteningwinter and spring sheep, or producing special crops like fruit orpotatoes, and for market gardeners who grow all sorts of vegetables, carrots, parsnips, potatoes, peas, and so on. Fig. 47 is a view of ahighly cultivated sandy region in Kent showing gooseberries in theforeground, vegetables behind, and a hop garden behind that again. 

[Illustration: Fig. 48. A Surrey heath]

The uncultivated sands are sometimes not really so very different, andsome of them, perhaps many of them, might be improved or reclaimed andmade to grow these special crops if it were worth while. But theyalways require special treatment and therefore they have been leftalone. In days of old our ancestors disliked them very much;"villanous, rascally heaths" Cobbett always called them. There werepractically no villages and few cottages, because the land was toobarren to produce enough food; the few dwellers on the heath, or the"heathen, " were so ignorant and benighted that the name came to standgenerally for all such people and has remained in our language longafter its original meaning was lost. As there were so few inhabitantsthe heaths used to be great places for robbers, highwaymen, andevil-doers generally; Gad's Hill on the Watling St. Between Rochesterand Gravesend, Finchley Common, Hounslow Heath and others equallydreaded by travellers of the seventeenth and eighteenth centuries, werebarren sandy tracts. But in our time we no longer need to dread them;we can enjoy the infinite charm of the breezy, open country with itsbrown vegetation, the pink blossom of the bell-shaped heath and thelilac blossom of the {106} heather, the splashes of yellow from theragwort or the gorse and the dark pine and larch plantations. In thespring the young shoots of bracken lend a beautiful light green colourto the scene, while in the autumn the faded growth covers it all with arich brown. People now like to live amid such surroundings, and sothese heaths, that have been untouched for so long and are part of theoriginal primeval England as it was in the days of the Britons, arebecoming dotted with red bricked and red tiled villas, and are fastlosing their ancient character. The heaths are not everywhere dry;there are numerous clay basins where the sand lies wet, where peatforms (see p. 37), and where marsh plants like the bog asphodel, sundew, or cotton grass can be found. In walking over a heath you soonlearn to find these wet places by the colour of the grass and theabsence of heather. In some places there is a good deal of wood, especially pines, larches, and silver birches: all these are verycommon on the Surrey sands, willows also grow in the damp places. Fig. 48 shows a Surrey heath--Blackheath--with heather, gorse and bracken;with pine-woods in the distance and everywhere some bare patches ofsand. Much of the New Forest is on the sand, as also is Bournemouth, famous for its fine pine woods. Fig. 49 is a view of such woods onWimbledon common. But elsewhere there is no wood: the peasants burnthe turf, and so you find their cottages have huge fireplaces: insteadof fences round their gardens or round the plantations there are wallsmade of turf. Such are the Dorchester heaths so finely described byHardy in _The Return of the Native_ and other novels. Other sands, however, are covered with grass and not with heather, and many of thesehave a special value {108} for golf links, especially some of the dry, invigorating sands by the seaside. The famous links at St Andrews, andat Littlestone, are examples. 

[Illustration: Fig. 49. Woodland and heather on light sandy soil, Wimbledon Common]

In between the fertile and the barren sands come a number that arecultivated without being very good. They are much like the others, carrying a vegetation that is usually of the narrow leaved type (p. 72), and not very dense. On the road sides you see broom, heather, heath, harebells, along with gorse and bracken with milkwort nestlingunderneath: crested dog's tail and sheep's fescue are common grasses, while spurrey, knotwood, corn marigold, are a few of the numerous weedsin the arable fields. Gardens are easily dug, but it is best to putinto them only those plants that, like the native vegetation, canwithstand drought: vegetable gardens must be well manured and welllimed. Fig. 50 shows some of this kind of country in Surrey, thebarley field is surrounded by wood and very poor grass on the higherslopes. 

[Illustration: Fig. 50. Poor sandy soil in Surrey, partly cultivatedbut mainly wood and waste]

It is easy to travel in a sand country because the roads dry veryquickly after rain, although they may be dusty in summer. Sometimesthe lanes are sunk rather deeply in the soft sand, forming very prettybanks on either side. 

Loams, as we have seen (p. 2), lie in between sands and clays: they areneither very wet nor very dry: not too heavy nor yet too light: theyare very well suited to our ordinary farm crops, and they form by farthe best soils for general farming; wheat, oats, barley, sheep, cattle, milk, fruit and vegetables can all be produced: indeed the farmer on agood loam is in the fortunate position of being able to produce almostanything he finds most profitable. In a loam district that does not{110} lie too high the land is generally all taken up, even the roadsare narrow and there are few commons. The hedges are straight and cutshort, the farm houses and buildings are well kept, and there is ageneral air of prosperity all round. Good elms grow and almost anytree that is planted will succeed. Loams shade off on one side intosand; the very fertile sands already described might quite truly becalled sandy loams. On the other side they shade off into clays; theheavy loams used to be splendid wheat soils, but are now, like clays, often of little value. But they form pleasant, undulating country, nicely wooded, and dotted over with thatched cottages; the fields areless wet and the roads are rather better than on the clays. Whenproperly managed they make excellent grass land. 

Chalky soils stand out quite sharply from all others: their whitecolour, their lime kilns now often disused, their noble beech trees, and, above all, the great variety of flowering plants enable thetraveller at once to know that he is on the chalk. Many plants likechalk and these may be found in abundance, but some, such as foxgloves, heather, broom or rhododendrons cannot tolerate it at all, and so theywill not grow. 

Chalk, like sand, is dry, and the roads can be traversed very soonafter rain. They are not very good, however; often they are onlymended with flints, which occur in the chalk and are therefore easilyobtainable, and the sharp fragments play sad havoc with bicycle tyres. The bye roads and lanes are often narrow, winding, and worn deepespecially at the foot of the hills, so that the banks get a fairamount of moisture and carry a dense vegetation. Among the profusionof flowers you can find scabious, the bedstraws, vetches, ragwort, {111} figwort, and many a plant rare in other places, like the wildorchids; while the cornfields are often yellow with charlock. In thehedgerows are hazels, guelder roses, maples, dogwood, all intwined withlong trails of bryony and traveller's joy. In the autumn thetraveller's joy produces the long, hairy tufts that have earned for itthe name of old man's beard, while the guelder roses bear clusters ofred berries. The great variety of flowers attracts a correspondingvariety of butterflies, moths and other insects; there are also numbersof birds and rabbits--indeed a chalk country teems with life in spiteof the bare look of the Downs. The roads running at the foot of thechalk Downs and connecting the villages, and farmhouses built there forthe good water supply, are particularly rich in plants because theysometimes cut into the chalk and sometimes into the neighbouring clay, sand or rock. Now and then a spring bursts out and a little streamtakes its rise: if you follow it you will generally find watercresscultivated somewhere. 

Besides the beech trees you also find ash, sycamore, maples, and, inthe church yards, some venerable yews. Usually the chalk districtswere inhabited very early: they are dry and healthy, the land can becultivated and the heights command extensive views over the country, sothat approaching enemies could easily be seen. On the chalk downs andplains are found many remains of tribes that lived there in the remoteages of the past, whose very names are now lost. Strange weapons andornaments are sometimes dug up in the camps where they lived andworked; the barrows can be seen in which they were buried, and thetemples in which they worshipped; Stonehenge itself, the best known ofall these, lies on the chalk. {112} Several of the camps still keepthe name the ancient Britons gave them--the _Mai-dun_, the encampmenton the hill, changed in the course of years to Maiden, as in MaidenHill, near Dorchester, in Dorset, Maiden Bower, near Dunstable, and soon. Some of their roads are still in use to this day, the Icknield Way(the way of the Iceni, a Belgic tribe), the Pilgrim's Way of thesouthern counties and others. 

Even the present villages go back to very ancient times, and thechurches are often seven or eight hundred years old. 

In places the land is too steep or too elevated to be cultivated, andso it is left as pasture for the sheep or "sheep walk"; wherecultivation is possible the fields are large and without hedges, likethose shown in Fig. 51; during autumn, winter and spring there are manysheep about, penned or "folded" on the arable land, eating the crops ofswedes, turnips, rape, vetches or mustard grown for them, or grazing onthe aftermath of sainfoin or grass and clover. So important are sheepin chalk districts that the whole scheme of farming is often based ontheir requirements, but corn is also a valuable crop, and, especiallyin dry districts, barley, so that chalk soils are often spoken of as"sheep and barley" soils. Although the pastures are very healthy thereis not generally much food or "keep" for the animals during the summerbecause of the dryness. 

[Illustration: Fig. 51. Open chalk cultivated country, Isle of Thanet]

The black soil of the fen districts and elsewhere is widely differentfrom any of the preceding. It contains, as its colour shows, a largequantity of combustible material (Chap. V. ), which has a great power ofholding water. These fens are therefore very wet; until they weredrained they were desolate wastes: you may {114} read in Kingsley's_Hereward the Wake_ what they used to be like in old days, and even aslate as 1662 Dugdale writes that here "no element is good. The aircloudy, gross and full of rotten harrs[1]; water putrid and muddy, yea, full of loathsome vermin; the earth spongy and boggy; and the firenoisome by the stink of smoking hassocks[2]. " But during the Stuartperiod wide ditches or drains were dug, into which the water could flowand be pumped into rivers. This reclamation has been continued to thepresent time, and the black soils as well as the others in the Fendistricts can be made very productive. 

We have seen that a change in the soil produces a change in the plantsthat grow on it. The flora (i. E. The collection of plants) of a claysoil is quite different from that of a sandy soil, and both aredifferent from that of a chalk or of a fen soil. In like mannerdraining a meadow or manuring it alters its flora: some of the plantsdisappear and new ones come in. Even an operation like mowing a lawn, if carried on sufficiently regularly, causes a change. In all thesecases the plants favoured by the new conditions are enabled to growrather better than those that are less favoured; thus in the regularlymown lawn the short growing grasses have an advantage over those likebrome that grow taller, and so crowd them out. When land is drainedthose plants that like a great quantity of water no longer do quite sowell as before, while those that cannot put up with much water now havea better chance. In the natural state there is a great deal ofcompetition among {115} plants, and only those survive that are adaptedto their surroundings. You should remember this on your rambles andwhen you see a plant growing wild you should think of it as one thathas succeeded in the competition and try to find out why it has beenenabled to do so. 

[1] Harr is an old word meaning sea-fog. 

[2] Hassock is the name given to coarse grass which forms part of theturf burnt in the cottages. 

{116}

CHAPTER XI

HOW SOIL HAS BEEN MADE

Apparatus required. 

_The apparatus in Fig. 54. The under surface, of the lips of thebeakers should be vaselined to prevent the water trickling down thesides. _

It is not uncommon to find cliffs or crags in inland places, but theyusually show one very striking difference from seaside cliffs. Theseaside cliffs may be nearly vertical, but the inland cliffs are not, excepting for a little way at the top; lower down a heap of stones andsoil lies piled against the face of the cliff and makes a slope upwhich you can climb. If you look at the cliff you can find loosefragments of it split off either by the action of freezing water (p. 83) or by other causes ready to roll down if sufficiently disturbed. So long has this been going on that a pile has by now accumulated, andhas been covered with plants growing on the soil of the heap. Ourinterest centres in this soil; no one has carried it there; it musthave been made from the rock fragments. When you get an opportunity ofstudying such a heap, do so carefully; you can then see how, startingfrom a solid rock, soil has been formed. This breaking down of therock is called weathering. 

[Illustration: Fig. 52. Cliffs at the seaside, Manorbier, Pembrokeshire]

The same change has gone on at the top of the cliff. Fragments havesplit off and the rock has broken {118} down into soil which stopswhere it is unless the rain can wash it away. If there are no cliffswhere you live you can see the same kind of action in the banks of thelanes, in a disused quarry, gravel pit or clay pit. Wherever avertical cutting has been made this downward rolling begins and a heapquickly forms, making the vertical cut into a slope. Plants soon beginto grow, and before long it is clear that soil has been made out of thefragments that have rolled down. This process is known as soilformation, but there is another always going on that we must now study. The heap does not invariably lie at the foot of the cliff. If there isa stream, river, or sea at the foot the fragments may be carried awayas fast as they roll down: the differences shown in Figs. 52 and 53between a cliff at the seaside and a cliff inland arise simply in thisway. In inland districts great valleys are in course of time carvedout, and at the seaside large areas of land have been washed away. 

What becomes of the fragments thus carried away by the water? The bestway of answering the question would be to explore one of these mountainstreams and follow it to the sea, but we can learn a good deal by a fewexperiments that can be made in the classroom. We want to make a modelstream and see what happens to little fragments of soil that fall intoit. 

[Illustration: Fig. 53. Inland cliff. Salisbury Crags, Arthur's Seat, Edinburgh]

Fix up the apparatus shown in Fig. 54. The small beaker A is torepresent the narrow mountain stream, the larger one _B_ stands for thewide river, and the glass jar _C_ for the mouth of the river or thesea. Run water through them; notice that it runs quickly through _A_, slowly through _B_, and still more slowly through _C_: we want it to dothis, because the stream flows quickly and the river slowly. 

{120}

Now put some soil into _A_. At once the soil is stirred up, the waterbecomes muddy, and the muddy liquid flows into _B_. But very soon achange sets in, the liquid in _A_ becomes clear, and only the grit andstones are left in the bottom: all the mud--the clay and the silt--iswashed into _B_. There it stops for a long time, and some of it willnever wash out. The liquid flowing into _C_ is clearer than thatflowing into _B_. If you keep on putting fresh portions of soil into_A_ you can keep _B_ always muddy, although _A_ is usually clear. Atthe end of the experiment look at the sediment in each beaker: in _A_it is clear and gritty, in _B_ it is muddy. If you can get hold ofsome sea water put some of the liquid from _C_ into it: very soon thisliquid clears and a deposit falls to the bottom, the sea water thusacting like the lime water on p. 20. 

[Illustration: Fig. 54. Model of a stream. In _A_, where the streamflows quickly, the water is clear and the sediment free from mud. In_B_, where it flows slowly, the water is turbid and the sediment muddy]

{121}

The experiment shows us that the fine material washed away by a quicklyflowing stream is partly deposited when the river becomes wider and thecurrent slower, and a good deal more is deposited by the action of thesalt water when the river flows into the sea. The rock that crumblesaway inland is spread out on the bed of the river or at its mouth. 

[Illustration: Fig. 56. The two sides of the river at the bend]

The river Stour at Wye showed all these things so clearly that I willdescribe it; you must then compare it with a river that you know, andsee how far the same features occur. At the bridge the stream wasshallow and flowed quickly: the bottom was gritty and pebbly, free frommud, and formed a safe place for paddling. Before the bridge was builtthere had been {122} a ford here. But further away, either up or down, the stream was deeper and wider, flowed more slowly, had a muddybottom, and so was not good for paddling. At one place about a mileaway some one had widened out the river to form a lake, but this madethe stream flow so slowly (as it was now so much wider) that the siltand clay deposited and the lake became silted up, i. E. It became soshallow that it was little more than a lake of mud. The same factswere brought out at the bend of the river. On its convex side, Fig. 55, the water has rather further to go in getting round the bend thanon its concave side _B_, it therefore flows more quickly, and carriesaway the soil of the bank and mud from the bottom. But on its concaveaide where it flows more slowly it deposits material. There is at thebend a marked difference in depth at the two sides. On its convex sidethe stream is rapid and deep, and scours away the bank; on its concaveside it is slower, shallower, and tends to become silted up. Thus thebend becomes more and more pronounced unless the bank round _A_ isprotected (the other bank of course needs no protection) and the wholeriver winds about just as you see in Fig. 56, and is perpetuallychanging its course, carrying away material from one place, mixing itup with material washed from somewhere else, and then deposits it at abend or in a pool where it first becomes a mud flat and then dry land. Some, however, is carried out to sea. We need not follow the Stour tothe sea; reference to an atlas will show what happens to other rivers. Some of the clay and silt they carry down is deposited at their mouths, and becomes a bar, gives rise to shoals and banks, or forms a delta. The rest is carried away and deposited on the floor of the sea. {124}Material washed away by the sea from the coast is either deposited onother parts of the coast, or is carried out and laid on the floor ofthe sea. Thus a thick deposit is accumulating, and if the sea were tobecome dry this deposit would be soil. This has actually happened inpast ages. The land we live on, now dry land, has had a most wonderfulhistory; it has more than once lain at the bottom of the sea and hasbeen covered with a thick layer of sediment carried from other places. Then the sea became dry land and the sediment became pressed into rock, which formed new soil, but it at once began to get washed away bystreams and rivers into new seas, and gave rise to new sediments on thefloor of these seas. And so the rock particles have for untold agesbeen going this perpetual round: they become soil; they are carriedaway by the rivers, in time they reach the sea; they lie at the bottomof the sea while the sediment gradually piles up: then the sea becomesdry land and the sediments are pressed into rocks again. The eatingaway of the land by water is still going on: it is estimated that thewhole of the Thames valley is being lowered at the rate of about oneinch in eight hundred years. This seems very slow, but eight hundredyears is only a short time in geology, the science that deals withthese changes. 

[Illustration: Fig. 56. The winding river Stour. The river winds fromthe right to the left of the picture, then back again, and then oncemore to the left, passing under the white bridge and in front of thebarn. ]

Water does more than merely push the rock particles along. Itdissolves some of them, and in this way helps to break up the rock. Spring water always contains dissolved matter, derived from the rocks, some of which comes out as "fur" in the kettles when the water isboiled. 

Rocks are also broken up by other agents. There is nearly always somelichen living on the rock, and if you {125} peel it off you can seethat it has eaten away some of the rock. When the lichen dies it maychange into food for other plants. 

We have learnt these things about soil formation. First of all therocks break up into fragments through the splitting action of freezingwater, the dissolving action of liquid water, and other causes. Thisprocess goes on till the fragments are very small like soil particles. Then plants begin to grow, and as they die and decay they give rise tothe black humus that we have seen is so valuable a part of the soil (p. 51). This is how very many of our soils have been made. But theaction of water does not stop at breaking the rock up into soil; itgoes further and carries the particles away to the lower parts of theriver bed, or to the estuary, to form a delta, and mud flats that maybe reclaimed, like Romney Marsh in England and many parts of Hollandhave been. Many of our present soils have been formed in this way. Finally the particles may be carried right away to sea and spread outon the bottom to lie there for many ages, but they may become dry landagain and once more be soil. 

One thing more we learnt from the river Stour. Why did it flow quicklyat the bridge and slowly elsewhere? We knew that the soil round thebridge was gravelly, whilst up and down the stream it was clayey. Theriver had not been able to make so wide or so deep a bed through thegravel as it had through the clay, and it could therefore be fordedhere. We knew also that there was a gravel pit at the next village onthe river, where also there was a bridge and had been a ford, and so wewere able to make a rough map like Fig. 57, showing that fords hadoccurred at the gravel {126} patches, but not at the clay places. Nowit was obvious that an inn, a blacksmith's forge, and a few shops andcottages would soon spring up round the ford, especially as the gravelpatch was better to live on than the clay round about, and so wereadily understood why our village had been built where it was and nota mile up or down the stream. Almost any river will show the samethings: on the Lea near Harpenden we found the river flowed quickly atthe ford (Fig. 58), where there was a hard, stony bottom and no mud:whilst above and below the ford the bottom was muddy and the streamflowed more slowly. At the ford there is as usual a small village. The Thames furnishes other examples: below Oxford there are numerousrocky or gravelly patches where fords were possible, and where villagestherefore grew up. Above Oxford, however, the possibilities of fordingwere fewer, because the soil is clay and there is less rock; the roadsand therefore the villages grew up away from the river. 

[Illustration: Fig 57. Sketch map showing why Godmersham and Wye arosewhere they did on the Stour. At _A_, the gravel patch, the river has ahard bed and can be forded. A village therefore grew up here. At _B_, the clay part, the river has a soft bed and cannot be forded. The landis wet in winter, and the banks of the stream may be washed away. Itis therefore not a good site for a village]

[Illustration: Fig. 58. Ford and Coldharbour, near Harpenden]

{128}

APPENDIX

The teacher is advised to procure, both for his own information and inorder to read passages to the scholars:

 Gilbert White, _Natural History of Selborne_. Charles Darwin, _Earthworms and Vegetable Mould_ (Murray). A. D. Hall, _The Soil_ (Murray). 

Mr Hugh Richardson has supplied me with the following list ofquestions, through many of which his scholars at Bootham School, York, have worked. They are inserted here to afford hints to other teachersand to show how the lessons may be varied. They should also proveuseful for revising and testing the scholars' knowledge. 

1. Collect samples of the different soils in your neighbourhood--gardensoil, soil from a ploughed field, from a mole-hill in a pasture field, leaf mould from a wood, etc. Collect also samples of the sub-soils, sand, gravel, clay, peat. 

2. Supplement your collection by purchasing from a gardener's shop somemixed potting soil and also the separate ingredients used to form sucha mixture--silver sand, leaf mould, peat. 

3. How many different sorts of peat can you get samples of? Peatmould, peat moss litter, sphagnum moss, turf for burning, dry moor peat?

4. Find for what different purposes sand is in use, such as mortarmaking, iron founding, scouring, bird cages, and obtain samples of eachkind. 

{129}

Analysis of Garden Soil. About a handful of soil will be required byeach pupil. 

5. Describe the appearance of the soil. Is it fine or in lumps? Doesit seem damp or dry? Can you see the separate particles of mineralmatter? How large are these? Is there any evidence of vegetablematter in the soil?

6. Put some of the soil in an evaporating basin and over this place adry filtering funnel. Warm the basin gently. Is any moisture givenoff?

7. Dry some of the soil at a temperature not greater than that ofboiling water, e. G. By spreading it out on a biscuit tin lid, andlaying this on a radiator. How have the appearance and properties ofthe soil been changed by drying?

8. Crumble some of the dried soil as finely as you can with yourfingers. Then sift it through a sheet of clean wire gauze. Whatfraction of the soil is fine enough to go through the gauze? Describethe portion which will not pass through the gauze. Count the number ofwires per linear inch in the gauze. 

9. Mix some of the soil with water in a flask. Let it stand. How longdoes it take before the water becomes quite clear again?

10. Mix some more soil with water. Let it settle for 30 seconds only. Pour off the muddy water into a tall glass cylinder. Add more water tothe remaining soil, and pour off a second portion of muddy water, adding it to the first, and so on until all the fine mud is removedfrom the soil. Allow this muddy water ample time to settle. 

11. When the fine mud has settled pour off the bulk of the water; stirup the mud with the rest of the water; transfer it to an evaporatingbasin, and evaporate to dryness. 

12. Does this dried mud consist of very tiny grains of sand or of somematerial different from sand? Can you find out with a microscope?

13. If the mud consists of real clay and not of sand it should bepossible to burn it into brick. Moisten the dried mud again. Roll itif you can into a round clay marble. Leave this to dry slowly for aday. Then bake it either in a chemical laboratory furnace or in anordinary fire. 

14. Return to the soil used in Question 10, from which only the finemud has been washed away. Pour more water on to it, shake it {130}well, and pour off all the suspended matter without allowing it morethan 5 seconds to settle. Repeat the process. Collect and dry thepoured off material as before. What is the material this time, sand orclay?

15. Wash the remaining portion of the soil in Question 14 clean fromall matter which does not settle promptly. Are there any pebbles left?If so, how large are they, and of what kind of stone?

16. Take a fresh sample of the soil. Mix it with distilled water in aflask. Boil the mixture. Allow it to settle. Filter. Divide thefiltrate into two portions. Evaporate both, the larger portion in anevaporating basin over wire gauze, the smaller portion in a watch glassheated by steam. Is any residue left after heating to dryness?

17. Take a fresh sample of soil. Spread it on a clean sand bath andheat strongly with a Bunsen flame. Does any portion of the soil burn?Is there any change in its appearance after heating?

18. To a fresh sample of soil add some hydrochloric acid. Is there anyeffervescence? If so, what conclusions do you draw?

19. Make a solution of soil in distilled water, and filter as before. Is this solution acid, alkaline or neutral? Are you quite certain ofyour result? Did you test the distilled water with litmus paper? Andare you sure that your litmus does not contain excess of free acid orfree alkali?

Peat. 

20. Examine different varieties of peat collected (see Question 2) anddescribe the appearance of each. 

21. Burn a fragment of each kind of peat on wire gauze. What do younotice?

22. Boil some peat with distilled water and filter the solution. Whatcolour is it? Can you tell whether it is acid, neutral or alkaline?Evaporate some of the solution to dryness. 

Out-of-doors. 

23. Describe the appearance of the soil in the flower beds (_a_) duringhard frost, (_b_) in the thaw which follows a hard frost, (_c_) afteran April shower, (_d_) in drought at the end of summer, (_e_) in dampOctober weather when the leaves are beginning to fall. 

24. Is the soil equally friable at different times of the year?

{131}

25. In what way do dead leaves get carried into the soil?

26. Can you find the worm holes in a garden lawn? in a garden path?

27. Take a flower bed or grass plot of small but known area (say 3yards by 2 yards) and a watering can of known capacity (say 3 gallons). Find how much water must be added to the soil before some of the waterwill remain on the surface. What has been the capacity of the soil ingallons per square yard?

28. Take two thermometers. Lay one on the soil, the other with itsbulb 3 inches deep in the soil. Compare their temperatures at morning, noon and night. 

29. Find from the 25-inch Ordnance map the reference numbers of thefields near your school. Make a list of the fields, showing for whatcrop or purpose each field is being used. 

{132}

 INDEX

 Acid waters, 40 Air in soil, 16, 70, 95

 Bars in estuaries, 122 Black soils, 36 Blowing sands, 22 Bricks, 10, 16-18

 Chalk, 26, 96 Chalk soils, 110-112 Clay, 6, 9-21 Clay soils, 75, 100-102 Cliffs, 116-119

 Darwin's experiments, 11, 56 Deltas, 122 Drainage, 19, 96 Dwellers in the soil, 53-63

 Earthworms, 54-56 Error of experiment, 48

 Fallow, 14, 95 Fens, 112 Flora, 114 Fords, 126 Frost, action of, on soil, 83

 Grassland, 75 Grit, 6

 Hales's experiment, 73 Heaths, 104 Heavy soils, 100 Hoeing, 86-93 Humus, 36, 51, 93, 125 Hypotheses, 36

 Land slips, 12 Leaf mould, 33 Light soils, 104 Lime, action of, on clay and soil, 19-21, 96-98 Lime water, 19 Loams, 2, 65, 108

 Marsh gas, 40 Micro-organisms, 56-62 Moorland, 80 Mulch, 87, 90

 Peat, 37-40, 130 Peat bogs, overflow of, 38 Perspiration of plants, 74 Plant food, 41-52, 62 Plant requirements, 64 Ploughing, 82 Pot experiments, 41-52, 54, 69, 71

 Roads, 30-32, 101-112 Rolling the soil, 84

 Sand, 6, 22-32, 41 Sand dunes, 22 Sandy soils, 68-72, 102-108 Shrinkage of clay, 10 Silt, 7 Soil sampler, 82, 88 Sowing seed, 84 Springs, 24-31, 111 Subsoil, 2, 4, 42, 48-51 Swelling of clay, 11 Swelling of peat, 38

 Temperature of soil, 86-93 Tilth, 86

 Van Helmont's experiment, 46 Villages, situation of, 24, 30, 126

 Wastes, 101, 104 Water content of soil, 88-93 Water, movement of in soils, 65-68 Water supply and plant growth, 69-74 Weathering, 116 Weeds, 94, 97 Woodland, 80, 101