Wiley & Putnam’s New Publications.

LECTURES
ON
AGRICULTURAL
CHEMISTRY AND GEOLOGY;

TO WHICH ARE ADDED,

SUGGESTIONS FOR EXPERIMENTS
IN PRACTICAL AGRICULTURE.

BY
JAS. F. W. JOHNSTON, M.A., F.R.SS. L. & E.

Fellow of the Geological Society, Honorary Member of the Royal
Agricultural Society, &c. &c.; Reader in Chemistry and
Mineralogy in the University of Durham, &c.

These Lectures will be divided into four Parts, of which the First is now ready; the others are in course of publication, and the whole will be completed in two volumes.

Outline of Part I.—“On the Organic Constituents of Plants.”—Lecture I. Elementary substances of which plants subsist. II. and III. Compound substances which minister to the growth of plants. IV. Sources from which plants immediately derive their elementary constituents. V. How the food enters into the circulation of plants—general structure of plants. VI. Into what substances the food is changed in the interior of plants—substances of which plants chiefly consist. VII. Chemical changes by which the substances of which plants chiefly consist are formed from those on which they live. VIII. How the supply of food for plants is kept up in the general vegetation of the globe.

Outline of Part II.—“On the Inorganic Constituents of Plants—the Origin, Classification, and Chemical Constitution of Soils—General and Special Relations of Geology to Agriculture—Origin, Constitution, Analyses, and Methods of Improving Soils in different Districts and under unlike conditions.—Lecture IX. Kind and proportion of inorganic matter contained in plants. X. Properties of the inorganic compounds which exist in vegetable substances, or which promote their growth. XI. Of the nature, origin, and classification of soils—Structure of the earth’s crust—Classification and general characters of the stratified rocks—Agricultural capabilities of the soils derived from them. XII. Granite and trap rocks, and the soils derived from them—Superficial accumulations. XIII. On the exact chemical constitution, the analysis, and the physical properties of soils.

Part III.—Methods of improving the soil by mechanical and by chemical means—Manures, their nature, composition, and mode of action—theory of their application in different localities.

Part IV.—The results of vegetation—the nature, constitution, and nutritive properties of different kinds of produce, and by different modes of cultivation—the feeding of cattle, the making of cheese, &c. &c. The constitution and differences of various kinds of wood, and the circumstances which favour their growth.

CRITICAL NOTICES.

“A valuable and interesting course of lectures.”—Quarterly Review.

“But it is unnecessary to make large extracts from a book which we hope and trust will soon be in the hands of nearly all our readers. Considering it as unquestionably the most important contribution that has recently been made to popular science, and as destined to exert an extensively beneficial influence in this country, we shall not fail to notice the forthcoming portions as soon as they appear from the press.”—Silliman’s American Journal of Science. Notice of Part I of the American reprint.

“We think it no compliment to Professor Johnston to say, that among our own writers of the present day who have recently been endeavouring to improve our agriculture by the aid of science, there is probably no other who has been more eminently successful, or whose efforts have been more highly appreciated.”—County Herald.

“Prof. Johnston is one who has himself done so much already for English agriculture, that to behold him still in hot pursuit of the inquiry into what can be done, supplies of itself a stimulus to further exertion on the part of others.”—Berwick Warder.

ELEMENTS
OF
AGRICULTURAL
CHEMISTRY AND GEOLOGY.

BY
JAS. F. W. JOHNSTON, M.A., F.R.S.,

HONORARY MEMBER OF THE ROYAL ENGLISH AGRICULTURAL
SOCIETY, AND AUTHOR OF “LECTURES ON AGRICULTURAL
CHEMISTRY AND GEOLOGY.”

NEW-YORK:
WILEY AND PUTNAM.
MDCCCXLII.

J. P. Wright, Printer,
18 New Street, N. Y.

INTRODUCTION.

The scientific principles upon which the art of culture depends, have not hitherto been sufficiently understood or appreciated by practical men. Into the causes of this I shall not here inquire. I may remark, however, that if Agriculture is ever to be brought to that comparative state of perfection to which many other arts have already attained, it will only be by availing itself, as they have done, of the many aids which Science offers to it; and that, if the practical man is ever to realize upon his farm all the advantages which Science is capable of placing within his reach, it will only be when he has become so far acquainted with the connection that exists between the art by which he lives and the sciences, especially of Chemistry and Geology, as to be prepared to listen with candour to the suggestions they are ready to make to him, and to attach their proper value to the explanations of his various processes which they are capable of affording.

The following little Treatise is intended to present a familiar outline of the subjects of Agricultural Chemistry and Geology, as treated of more at large in my Lectures, of which the first Part is now before the public. What in this work has necessarily been taken for granted, or briefly noticed, is in the Lectures examined, discussed, or more fully detailed.

Durham, 8th April, 1842.

CONTENTS.

ELEMENTS
OF
AGRICULTURAL CHEMISTRY, &c.

CHAPTER I.

Distinction between Organic and Inorganic Substances.—The Ash of Plants.—Constitution of the Organic Parts of Plants.—Preparation and Properties of Carbon, Hydrogen, and Nitrogen.—Meaning of Chemical Combination.

The object of the practical farmer is to raise from a given extent of land the largest quantity of the most valuable produce at the least cost, and with the least permanent injury to the soil. The sciences either of chemistry or geology throw light on every step he takes or ought to take, in order to effect this main object.

SECTION I.—OF THE VEGETABLE AND EARTHY
OR THE ORGANIC AND INORGANIC
PARTS OF PLANTS.

In the prosecution of his art, two distinct classes of substances engage his attention—the living crops he raises, and the dead earth from which they are gathered. If he examine any fragment of an animal or vegetable, either living or dead, he will observe that it exhibits pores of various kinds arranged in a certain order—that it has a species of internal structure—that it has various parts or organs—in short, that it is what physiologists term organized. If he examine, in like manner, a lump of earth or rock, he will perceive no such structure. To mark this distinction, the parts of animals and vegetables, either living or dead—whether entire or in a state of decay, are called organic bodies, while earthy and stony substances are called inorganic bodies.

Organic substances are also more or less readily burned and dissipated by heat in the open air; inorganic substances are generally fixed and permanent in the fire.

But the crops which grow upon it, and the soil in which they are rooted, contain a portion of both of these classes of substances. In all fertile soils, there exists from 3 to 10 per cent. of vegetable or other matter of organic origin; while, on the other hand, all vegetables, as they are collected for food, leave, when burned, from one-half to twenty per cent. of inorganic ash.

If we heat a portion of soil to redness in the open air, the organic matter will burn away, and, in general, the soil, if previously dry, will not be materially diminished in bulk. But if a handful of wheat, or of wheat straw, or of hay, be burned in the same manner, the proportion that disappears is so great, that in most cases a comparatively minute quantity only remains behind. Every one is familiar with this fact who has seen the small bulk of ash that is left when weeds, or thorns, or trees, are burned in the field, or when a hay or corn-stack is accidentally consumed. Yet the ash thus left is a very appreciable quantity, and the study of its true nature throws much light, as we shall hereafter see, on the practical management of the land on which any given crop is to be made to grow.

Thus the quantity of ash left by a ton of wheat straw is sometimes as much as 360 lbs.; by a ton of oat straw as much as 200 lbs.; while a ton of the grain of wheat leaves only about 40 lbs.; of the grain of oats about 90 lbs.; and of oak wood only 4 or 5 lbs. The quantities of inorganic matter, therefore, though comparatively small, yet, in some cases, amount to a considerable weight in an entire crop. The nature, source and uses of this earthy matter will be explained in a subsequent chapter.

SECTION II.—CONSTITUTION OF THE ORGANIC
PART OF PLANTS AND ANIMALS.

The organic part of plants, when in a perfectly dry state, constitutes therefore from 85 to 99 per cent. of their whole weight. Of those parts of plants which are cultivated for food, it is only hay and straw, and a very few others, that contain as much as 10 per cent. of inorganic matter.

This organic part consists of four substances, known to chemists by the names of carbon, hydrogen, oxygen, and nitrogen. The first of these, carbon, is a solid substance, the other three are gases or peculiar kinds of air.

1. Carbon. When wood is burned in a covered heap, as is done by the charcoal burners, or is distilled in iron retorts, as in making wood-vinegar, it is charred and converted into common wood charcoal. This charcoal is the most usual and best known variety of carbon. It is black, soils the fingers, and is more or less porous according to the kind of wood from which it has been formed. Coke obtained by charring or distilling coal is another variety. It is generally denser or heavier than the former, though less pure. Black lead is a third variety, still heavier and more impure. The diamond is the only form in which carbon occurs in nature in a state of perfect purity.

This latter fact, that the diamond is pure carbon—that it is essentially the same substance with the finest and purest lamp-black—is very remarkable; but it is only one of many striking circumstances that every now and then present themselves before the inquiring chemist.

Charcoal, the diamond, lamp-black, and all the other forms of carbon, burn away more or less slowly when heated in the air, and are converted into a kind of gas known by the name of carbonic acid. The impure varieties leave behind them a greater or less proportion of ash.

2. Hydrogen.—If oil of vitriol (sulphuric acid) be mixed with twice its bulk of water, and then poured upon iron filings, the mixture will speedily begin to boil up, and bubbles of gas will rise to the surface of the liquid in great abundance. These are bubbles of hydrogen gas.

If the experiment be performed in a bottle, the hydrogen which is produced will gradually drive out the atmospheric air it contained, and will itself take its place. If a bit of wax taper be tied to the end of a wire, and when lighted be introduced into the bottle, it will be instantly extinguished; while the hydrogen will take fire, and burn at the mouth of the bottle with a pale yellow flame. If the taper be inserted before the common air is all expelled, the mixture of hydrogen and common air will burn with an explosion more or less violent, and may even shatter the bottle and produce serious accidents. This experiment, therefore, ought to be made with care. It may be safely made in an open tumbler, covered by a plate or a piece of paper, till a sufficient quantity of hydrogen is collected, when, on the introduction of the taper, the light will be extinguished, and the hydrogen will burn with a less violent explosion.

This gas is also an exceedingly light substance, rising through common air as wood does through water. Hence, when confined in a bag made of silk, or other light tissue, it is capable of sustaining heavy substances in the air, and even of transporting them to great heights. For this reason it is employed for filling and elevating balloons.

Hydrogen gas is not known to occur anywhere in nature in any sensible quantity. It is very abundant, as we shall hereafter see, in what by chemists is called a state of combination.

3. Oxygen.—When strong oil of vitriol is poured upon black oxide of manganese, and heated in a glass retort: or when red oxide of mercury, or chlorate of potash, is so heated alone; or when saltpetre, or the same oxide of manganese, is heated alone in an iron bottle;—in all these cases a kind of air is given off, which, when collected and examined by plunging a taper into it, is found to be neither common air nor hydrogen gas. The taper, when introduced, burns with great rapidity, and with exceeding brilliancy, and continues to burn till either the whole of the gas disappears, or the taper is entirely consumed. If a living animal is introduced, its circulation and its breathing become quicker—it is speedily thrown into a fever—it lives as fast as the taper burned—and, after a few hours, dies from excitement and exhaustion. This gas is not light like hydrogen, but is about one-ninth part heavier than common air.

In the atmosphere, oxygen exists in the state of gas. It forms about one-fifth of the bulk of the air we breathe, and is the substance which, in the air, supports all animal life and the combustion of all burning bodies. Were it by any cause suddenly removed from the atmosphere of our globe, every living thing would perish, and all combustion would become impossible.

4. Nitrogen.—If a saucer be half filled with milk of lime, formed by mixing slaked quicklime with water, a very small tea-cup containing a little burning sulphur then placed in the middle, and a common large tumbler inverted over the whole, the sulphur will burn for a while, and will then gradually die out. On allowing the whole to remain for some time, the fumes of the sulphur will be absorbed by the milk of lime, which will rise a certain way into the tumbler. When the absorption has ceased, a quantity of air will remain in the upper part of the tumbler. This air is nitrogen gas.

If the whole be now introduced into a large basin of water, the tumbler being held in the left hand, the cup and saucer may be removed from beneath. The saucer may then be inverted and introduced with its under side into the mouth of the tumbler, which may thus be lifted out of the water and restored to its upright position, the saucer serving the purpose of a cover. By carefully removing this cover with the one hand, a lighted taper may be introduced by the other. It will then be seen that the taper is extinguished by this air, and that no other effect follows. Or if a living animal be introduced into it, breathing will instantly cease, and it will drop without signs of life.

This gas possesses no other remarkable property. It is a very little lighter than common air, and is known to exist in large quantity in the atmosphere only. Of the air we breathe it forms nearly four-fifths of the entire bulk.

These three gases are incapable of being distinguished from common air, or from each other, by the ordinary senses; but by the aid of the taper they are readily recognised. Hydrogen extinguishes the taper, but itself takes fire; nitrogen simply extinguishes it; while in oxygen the taper burns with extraordinary brilliancy and rapidity.

Of this one solid substance, carbon, and these three gases, hydrogen, oxygen, and nitrogen, all the organic part of vegetable and animal substances is made up.

Into these substances, however, they enter in very different proportions. Nearly one-half the weight of all vegetable productions which are gathered as food for man or beast—in their dry state—consists of carbon; the oxygen amounts to rather more than one-third, the hydrogen to little more than five per cent., while the nitrogen rarely exceeds two and a half or three per cent. of their weight.

This will appear from the following table, which exhibits the actual constitution by analysis of some varieties of the more common crops when perfectly dry.

These numbers represent the weights of each element in pounds, contained in 1000 lbs. of the dry hay, potatoes, &c.; but in drying by a gentle heat, 1000 lbs. of hay from the stack, lost 158 lbs. of water, of potatoes wiped dry externally 722 lbs.,[1] wheat straw 260 lbs., and oats 151 lbs.

SECTION III.—OF THE MEANING OF
CHEMICAL COMBINATION.

If the three kinds of air above spoken of be mixed together in a bottle, no change will take place, and if charcoal in fine powder be added to them, still no new substance will be produced. If we take the ash left by a known weight of hay or wheat straw, and mix it with the proper quantities of the four elementary substances, carbon, hydrogen, &c., as shewn in the above table, we shall be unable by this means to form either hay or wheat straw. The elements of which vegetable substances consist, therefore, are not merely mixed together—they are united in some closer and more intimate manner. To this more intimate state of union, the term chemical combination is applied—the elements are said to be chemically combined.

Thus, when charcoal is burned in the air, it slowly disappears, and forms, as already stated, a kind of air known by the name of carbonic acid gas, which rises into the atmosphere and disappears. Now, this carbonic acid is formed by the union of the carbon (charcoal), while burning, with the oxygen of the atmosphere, and in this new air the two elements, carbon and oxygen, are chemically combined.

Again, if a piece of wood or a bit of straw, in which the elements are already chemically combined, be burned in the air, these elements are separated and made to assume new states of combination, in which new states they escape into the air and become invisible. When a substance is thus changed by the action of heat, it is said to be decomposed, or if it gradually decay and perish by exposure to the air and moisture, it undergoes slow decomposition.

When, therefore, two or more substances unite together, so as to form a third possessing properties different from both, they enter into chemical union—they form a chemical combination or chemical compound. When, on the other hand, one compound body is so changed as to be converted into two or more substances different from itself, it is decomposed. Carbon, hydrogen, &c., are chemically combined in the interior of the plant during the formation of wood: wood, again, is decomposed when by the vinegar-maker it is converted among other substances into charcoal and wood-vinegar, and the flour of grain when the brewer or distiller converts it into ardent spirits.

CHAPTER II.

Form in which these different substances enter into Plants. Properties of the Carbonic, Humic, and Ulmic Acids—of Water, of Ammonia, and of Nitric Acid. Constitution of the Atmosphere.

SECTION I.—FORM IN WHICH THE CARBON, ETC.
ENTER INTO PLANTS.

It is from their food that plants derive the carbon, hydrogen, oxygen, and nitrogen, of which their organic part consists. This food enters partly by the minute pores of their roots, and partly by those which exist in the green part of the leaf and of the young twig. The roots bring up food from the soil, the leaves take it in directly from the air.

Now, as the pores in the roots and leaves are very minute, carbon (charcoal) cannot enter into either in a solid state; and as it does not dissolve in water, it cannot, in the state of simple carbon, be any part of the food of plants. Again, hydrogen gas neither exists in the air nor usually in the soil—so that, although hydrogen is always found in the substance of plants, it does not enter them in the state of the gas above described. Oxygen exists in the air, and is directly absorbed both by the leaves and by the roots of plants; while nitrogen, though it forms a large part of the atmosphere, is not supposed to enter directly into plants in any considerable quantity.

The whole of the carbon and hydrogen, and the greater part of the oxygen and nitrogen also, enter into plants in a state of chemical combination with other substances; the carbon chiefly in the state of carbonic acid, and of certain other soluble compounds which exist in the soil; the hydrogen and oxygen in the form of water: and the nitrogen in those of ammonia or nitric acid. It will be necessary therefore briefly to describe these several compounds.

SECTION II.—OF THE CARBONIC, HUMIC,
AND ULMIC ACIDS.

1. Carbonic Acid.—If a few pieces of chalk or limestone be put into the bottom of a tumbler, and a little spirit of salt (muriatic acid) be poured upon them, a boiling up or effervescence will take place, and a gas will be given off, which will gradually collect and fill the tumbler; and when produced very rapidly, may even be seen to run over its edges. This gas is carbonic acid. It cannot be distinguished from common air by the eye; but if a taper be plunged into it, the flame will immediately be extinguished, while the gas remains unchanged. This kind of air is so heavy, that it may be poured from one vessel into another, and its presence recognised by the taper. It has also a peculiar odour, and is exceedingly suffocating, so that if a living animal be introduced into it, life immediately ceases. It is absorbed by water, a pint of water absorbing or dissolving a pint of the gas.

Carbonic acid exists in the atmosphere; it is given off from the lungs of all living animals while they breathe; it is also produced largely during the burning of wood, coal, and all other combustible bodies, so that an unceasing supply of this gas is poured into the air. Decaying animal and vegetable substances also give off this gas, and hence it is always present in greater or less abundance in the soil, and especially in such soils as are rich in vegetable matter. During the fermentation of malt liquors, or of the expressed juices of different fruits,—the apple, the pear, the grape, the gooseberry—it is produced, and the briskness of such fermented liquors is due to the escape of this gas. From the dung and compost heap it is also given off; and when put into the ground in a fermenting state, farm-yard manure affords a rich supply of carbonic acid to the young plant.

Carbonic acid consists of carbon and oxygen only, combined together in the proportion of 28 of the former to 72 of the latter, or 100 lbs. of carbonic acid contain 28 lbs. of carbon and 72 lbs. of oxygen.

2. Humic and Ulmic Acids.—The soil always contains a portion of vegetable matter (called humus by some writers), and such matter is always added to it when it is manured from the farm-yard or the compost heap. During the decay of this vegetable matter, carbonic acid, as above stated, is given off in large quantity, but other substances are also formed at the same time. Among these are the two to which the names of humic and ulmic acids are respectively given. They both contain much carbon, are both capable of entering the roots of plants, and both, no doubt, in favourable circumstances, help to feed the plant.

If the common soda of the shops be dissolved in water, and a portion of a rich vegetable soil, or a bit of peat, be put into this solution, and the whole boiled, a brown liquid is obtained. If to this brown liquid, spirit of salt (muriatic acid) be added till it is sour to the taste, a brown flocky powder falls to the bottom. This brown substance is humic acid. But if in this process we use spirit of hartshorn (liquid ammonia), instead of the soda, ulmic acid is obtained.

These acids exist along with other substances in the rich brown liquor of the farm-yard, which is so often allowed to run to waste; they are also produced in greater or less quantity during the decay of the manure after it is mixed with the soil, and no doubt yield to the plant a portion of that supply of food which it must necessarily receive from the soil.

SECTION III.—OF WATER, AMMONIA,
AND NITRIC ACID.

1. Water.—If hydrogen be prepared in a bottle, in the way already described, and a gas-burner be fixed into its mouth, the hydrogen may be lighted, and will burn as it escapes into the air. Held over this flame a cold tumbler will become covered with dew, or with little drops of water. This water is produced during the burning of the hydrogen; and as it takes place in pure oxygen gas as well as in the open air, this water must contain the hydrogen and oxygen which disappear, or must consist of hydrogen and oxygen only.

This is a very interesting fact; and were it not that chemists are now familiar with many such, it could not fail to appear truly wonderful that the two gases, oxygen and hydrogen, by their union, should form so very different a substance as water is from either. It consists of 1 of hydrogen to 8 of oxygen, or every 9 lbs. of water contain 8 lbs. of oxygen and 1 lb. of hydrogen.

Water is so familiar a substance, that it is unnecessary to dwell upon its properties. When pure, it has neither colour, taste, nor smell. At 32° of Fahrenheit’s[2] scale (the freezing point), it solidifies into ice, and at 212° it boils, and is converted into steam. There are two others of its properties which are especially interesting in connection with the growth of plants.

1st, If sugar or salt be put into water, they disappear or are dissolved. Water has the power of thus dissolving numerous other substances in greater or less quantity. Hence, when the rain falls and sinks into the soil, it dissolves some of the soluble substances it meets in its way, and rarely reaches the roots of plants in a pure state. So waters that rise up in springs are rarely pure. They always contain earthy and saline substances in solution, and these they carry with them, when they are sucked in by the roots of plants.

It has been above stated, that water absorbs (dissolves) its own bulk of carbonic acid; it dissolves also smaller quantities of the oxygen and nitrogen of the atmosphere; and hence, when it meets any of these gases in the soil, it becomes impregnated with them, and conveys them into the plant, there to serve as a portion of its food.

2d, Water is composed of oxygen and hydrogen; by certain chemical processes it can readily be resolved or decomposed artificially into these two gases. The same thing takes place naturally in the interior of the living plant. The roots absorb the water, but if in any part of the plant hydrogen be required, to make up the substance which it is the function of that part to produce, a portion of the water is decomposed and worked up, while the oxygen is set free, or converted to some other use. So, also, in any case where oxygen is required water is decomposed, the oxygen made use of, and the hydrogen liberated. Water, therefore, which abounds in the vessels of all growing plants, if not directly converted into the substance of the plant, is yet a ready and ample source from which a supply of either of the elements of which it consists may at any time be obtained.

It is a beautiful adaptation of the properties of this all-pervading compound (water), that its elements should be so fixedly bound together as rarely to separate in external nature, and yet to be at the command and easy disposal of the vital powers of the humblest order of living plants.

2. Ammonia.—If the sal ammoniac of the shops be mixed with quicklime, a powerful odour is immediately perceived, and an invisible gas is given off which strongly affects the eyes. This gas is ammonia. Water dissolves or absorbs it in very large quantity, and this solution forms the common hartshorn of the shops. The white solid smelling-salts of the shops are a compound of ammonia with carbonic acid,—a solid formed by the union of two gases.

The gaseous ammonia consists of nitrogen and hydrogen only, in the proportion of 14 of the former to 3 of the latter, or 17 lbs. of ammonia contain 3 lbs. of hydrogen.

The chief natural source of this compound is, in the decay of animal substances. During the putrefaction of dead animal bodies ammonia is invariably given off. From the animal substances of the farm-yard it is evolved, and from all solid and liquid manures of animal origin. It is also formed in lesser quantity during the decay of vegetable substances in the soil; and in volcanic countries, it escapes from many of the hot lavas, and from the crevices in the heated rocks.

It is produced artificially by the distillation of animal substances (hoofs, horns, &c.), or of coal. Thousands of tons of the ammonia present in the ammoniacal liquors of the gas-works, which might be beneficially applied as a manure, are annually carried down by the rivers, and lost in the sea.

The ammonia which is given off during the putrefaction of animal substances rises partially into the air, and floats in the atmosphere, till it is either decomposed by natural causes, or is washed down by the rains. In our climate, cultivated plants derive a considerable portion of their nitrogen from ammonia. It is supposed to be one of the most valuable fertilizing substances contained in farm-yard manure; and as it is present in greater proportion by far in the liquid than in the solid contents of the farm-yard, there can be no doubt that much real wealth is lost, and the means of raising increased crops thrown away in the quantities of liquid manure which are almost everywhere permitted to run to waste.

3. Nitric Acid—is a powerfully corrosive liquid known in the shops by the familiar name of aquafortis. It is prepared by pouring oil of vitriol (sulphuric acid) upon saltpetre, and distilling the mixture. The aquafortis of the shops is a mixture of the pure acid with water.

Pure nitric acid consists of nitrogen and oxygen only; the union of these two gases, so harmless in the air, producing the burning and corrosive compound which this is known to be.

It never reaches the roots of plants in this free and corrosive state. It exists in many soils, and is naturally formed in compost heaps, and in most situations where vegetable matter is undergoing decay in contact with the air; but it is always in a state of chemical combination in these cases. With potash, it forms nitrate of potash (saltpetre); with soda, nitrate of soda; and with lime, nitrate of lime; and it is generally in one or other of these states of combination that it reaches the roots of plants.

Nitric acid is also naturally formed, and in some countries probably in large quantities, by the passage of electricity through the atmosphere. The air, as has been already stated, contains much oxygen and nitrogen mixed together, but when an electric spark is passed through a quantity of air, a certain quantity of the two unite together chemically, so that every spark that passes forms a small portion of nitric acid. A flash of lightning is only a large electric spark; and hence every flash that crosses the air produces along its path a quantity of this acid. Where thunder-storms are frequent, much nitric acid must be produced in this way in the air. It is washed down by the rains, in which it has frequently been detected, and thus reaches the soil, where it produces one or other of the nitrates above mentioned.

It has been long observed that those parts of India are the most fertile in which saltpetre exists in the soil in the greatest abundance. Nitrate of soda, also, in this country, has been found wonderfully to promote vegetation in many localities; and it is a matter of frequent remark, that vegetation seems to be refreshed and invigorated by the fall of a thunder-shower. There is, therefore, no reason to doubt that nitric acid is really beneficial to the general vegetation of the globe. And since vegetation is most luxuriant in those parts of the globe where thunder or lightning are most abundant, it would appear as if the natural production of this compound body in the air, to be afterwards brought to the earth by the rains, were a wise and beneficent contrivance by which the health and vigour of universal vegetation is intended to be promoted.

It is from this nitric acid, thus universally produced and existing, that plants appear to derive a large—probably, taking vegetation in general, the largest—portion of their nitrogen. In all climates they also derive a portion of this element from ammonia; but less from this source in tropical than in temperate climates.[3]

SECTION IV.—OF THE CONSTITUTION OF
THE ATMOSPHERE.

The air we breathe, and from which plants also derive a portion of their nourishment, consists of a mixture of oxygen and nitrogen gases, with a minute quantity of carbonic acid, and a variable proportion of watery vapour. Every hundred gallons of dry air contain about 21 gallons of oxygen and 79 of nitrogen. The carbonic acid amounts only to one gallon in 2500, while the watery vapour in the atmosphere varies from 1 to 2½ gallons (of steam) in 100 gallons of common air.

The oxygen in the air is necessary to the respiration of animals, and to the support of combustion (burning of bodies). The nitrogen serves principally to dilute the strength, so to speak, of the pure oxygen, in which gas, if unmixed, animals would live and combustibles burn with too great rapidity. The small quantity of carbonic acid affords an important part of their food to plants, and the watery vapour in the air aids in keeping the surfaces of animals and plants in a moist and pliant state; while, in due season, it descends also in refreshing showers, or studs the evening leaf with sparkling dew.

There is a beautiful adjustment in the constitution of the atmosphere to the nature and necessities of living beings. The energy of the pure oxygen is tempered, yet not too much weakened, by the admixture of nitrogen. The carbonic acid, which alone is noxious to life, is mixed in so minute a proportion as to be harmless to animals, while it is still beneficial to plants; and when the air is overloaded with watery vapour, it is provided that it shall descend in rain. These rains at the same time serve another purpose. From the surface of the earth there are continually ascending vapours and exhalations of a more or less noxious kind; these the rains wash out from the air, and bring back to the soil, at once purifying the atmosphere through which they descend, and refreshing and fertilizing the land on which they fall.

CHAPTER III.

Structure of plants—Mode in which their nourishment is obtained—Growth and substance of plants— Production of their substance from the food they imbibe—Mutual transformations of starch, sugar, and woody fibre.

From the compound substances, described in the preceding chapter, plants derive the greater portion of the carbon, hydrogen, oxygen, and nitrogen, of which their organic part consists. The living plant possesses the power of absorbing these compound bodies, of decomposing them in the interior of its several vessels, and of recompounding their elements in a different way, so as to produce new substances,—the ordinary products of vegetable life. Let us briefly consider the general structure of plants, and their mode of growth.

SECTION I.—OF THE STRUCTURE OF PLANTS,
AND THE MODE IN WHICH THEIR
NOURISHMENT IS OBTAINED.

A perfect plant consists of three several parts,—a root which throws out arms and fibres in every direction into the soil,—a trunk which branches into the air on every side,—and leaves which, from the ends of the branches and twigs, spread out a more or less extended surface into the surrounding air. Each of these parts has a peculiar structure and a special function assigned to it.

The stem of any of our common trees consists of three parts,—the pith in the centre, the wood surrounding the pith, and the bark which covers the whole. The pith consists of bundles of minute hollow tubes, laid horizontally one over the other; the wood and inner bark, of long tubes bound together in a vertical position, so as to be capable of carrying liquids up and down between the roots and the leaves. When a piece of wood is sawn across, the ends of these tubes may be distinctly seen. The branch is only a prolongation of the stem, and has a similar structure.

The root, immediately on leaving the trunk or stem, has also a similar structure; but as the root tapers away, the pith gradually disappears, the bark also thins out, the wood softens, till the white tendrils, of which its extremities are composed, consist only of a colourless spongy mass, full of pores, but in which no distinction of parts can be perceived. In this spongy mass the vessels or tubes which descend through the stem and root lose themselves, and by them these spongy extremities are connected with the leaves.

The leaf is an expansion of the twig. The fibres which are seen to branch out from the base over the inner surface of the leaf are prolongations of the vessels of the wood. The green exterior portion of the leaf is, in like manner, a continuation of the bark in a very thin and porous form. The green of the leaf, though full of pores, especially on the under part, yet also consists of, or contains, a collection of tubes or vessels, which stretch along its surface, and communicate with those of the bark.

Most of these vessels in the living plant are full of sap, and this sap is in almost continual motion. In spring and autumn the motion is more rapid, and in winter it is sometimes scarcely perceptible; yet the sap is supposed to be rarely quite stationary in every part of the tree.

From the spongy part of the root the sap ascends through the vessels of the wood, till it is diffused over the inner surface of the leaf by the fibres which the wood contains. Hence, by the vessels in the green of the leaf, it is returned to the bark, and through the vessels of the inner bark it descends to the root.

Every one understands why the roots send out fibres in every direction through the soil,—it is in search of water and of liquid food, which the spongy fibres suck in and send forward with the sap to the upper parts of the tree. It is to aid these roots in procuring food that, in the art of culture, such substances are mixed with the soil where these roots are, as are supposed to be necessary, or at least favourable, to the growth of the plant.

It is not so obvious that the leaves spread out their broad surfaces into the air for the same purpose precisely as that for which the roots diffuse their fibres through the soil. The only difference is, that while the roots suck in chiefly liquid, the leaves inhale almost solely gaseous food. In the sunshine, the leaves are continually absorbing carbonic acid from the air and giving off oxygen gas. That is to say, they are continually appropriating carbon from the air.[4] When night comes, this process ceases, and they begin to absorb oxygen and to give off carbonic acid. But this latter process does not go on so rapidly as the former, so that, on the whole, plants when growing gain a large portion of carbon from the air. The actual quantity, however, varies with the season, with the climate, and with the kind of tree. The proportion of the whole carbon contained by a plant, which has been derived from the air, is greatly modified also by the quality of the soil in which it grows, and by the comparative abundance of liquid food which happens to be within reach of its roots. It has been ascertained, however, that in our climate, on an average, not less than from one-third to three-fourths of the entire quantity of carbon contained in the crops we reap from land of average fertility, is really obtained from the air.

We see then why, in arctic climates, where the sun once risen never sets again during the entire summer, vegetation should almost rush up from the frozen soil—the green leaf is ever gaining from the air and never losing, ever taking in and never giving off carbonic acid, since no darkness ever interrupts or suspends its labours.

How beautiful, too, does the contrivance of the expanded leaf appear! The air contains only one gallon of carbonic acid in 2500, and this proportion has been adjusted to the health and comfort of animals to whom this gas is hurtful. But to catch this minute quantity, the tree hangs out thousands of square feet of leaf in perpetual motion, through an ever-moving air; and thus, by the conjoined labours of millions of pores, the substance of whole forests of solid wood is slowly extracted from the fleeting winds. The green stem of the young shoot, and the green stalks of the grasses, also absorb carbonic acid as the green of the leaf does, and thus a larger supply is afforded when the growth is most rapid, or when the short life of the annual plant demands much nourishment within a limited time.

SECTION II.—OF THE GROWTH AND
SUBSTANCE OF PLANTS.

In this way the perfect plant derives its food from the soil and from the air; but perfect plants arise from seeds; and the study of the entire life—the career, so to speak—of a plant, presents many interesting and instructive subjects of consideration.

When a portion of flour is made into dough, and this dough is kneaded with the hand under a stream of water upon a fine sieve, as long as the water passes through milky, there will remain on the sieve a glutinous sticky substance resembling birdlime, while the milky water will gradually deposit a pure white powder. This powder is starch, that which remains on the sieve is gluten. Both of these substances exist, therefore, in the flour; they both also exist in the grain. The starch consists of carbon, hydrogen, and oxygen only; the gluten, in addition to these, contains also nitrogen.

When ground into flour, these substances serve for food to man; in the unbruised grain they are intended to feed the future plant in its earliest infancy.

When a seed is committed to the earth, if the warmth and moisture are favourable, it begins to sprout. It pushes a shoot upwards, it thrusts a root downwards, but, until the leaf expand, and the root has fairly entered the soil, the young plant derives no nourishment other than water, either from the earth or from the air. It lives on the starch and gluten contained in the seed. But these substances, though capable of being separated from each other by means of water, as above stated, yet are neither of them soluble in water. Hence, they cannot, without undergoing a previous change, be taken up by the sap, and conveyed along the pores of the young shoot they are destined to feed. But it is so arranged that, when the seed first shoots, there is produced at the base of the germ, from a portion of the gluten, a small quantity of a substance (diastase) which has so powerful an effect upon the starch as immediately to render it soluble in the sap, which is thus enabled to take it up and convey it by degrees, just as it is wanted, to the shoot or to the root.[5] As the sap ascends, it becomes sweet,—the starch thus dissolved changes into sugar. When the shoot first becomes tipped with green, the sugar is again changed into the woody fibre, of which the stem of perfect plants chiefly consists. By the time that the food contained in the seed is exhausted,—often, as in the potato, long before,—the plant is able to live by its own exertions, at the expense of the air and the soil.

This change of the sugar of the sap into woody fibre is observable more or less in all plants. When they are shooting fastest the sugar is most abundant; not, however, in those parts which are growing, but in those which convey the sap to the growing parts. Thus the sugar of the ascending sap of the maple and the alder disappears in the leaf and in the extremities of the twig; thus the sugar-cane sweetens only a certain distance above the ground, up to where the new growth is proceeding; and thus also the young beet and turnip abound most in sugar, while in all these plants the sweet principle diminishes as the year’s growth draws nearer to a close.

In the ripening of the ear also, the sweet taste, at first so perceptible, gradually diminishes and finally disappears; the sugar of the sap is here changed into the starch of the grain, which, as above described, is afterwards destined, when the grain begins to sprout, to be reconverted into sugar for the nourishment of the rising germ.

In the ripening of fruits a different series of changes presents itself. The fruit is first tasteless, then becomes sour, and at last sweet. In this case the acid of the unripe is changed into the sugar of the ripened fruit.

The substance of plants,—their solid parts that is—consist chiefly of woody fibre, the name given to the fibrous substance, of which wood evidently consists. It is interesting to inquire how this substance can be formed from the compounds, carbonic acid and water, of which the food of plants in great measure consists. Nor is it difficult to find an answer.

It will be recollected that the leaf drinks in carbonic acid from the air, and delivers back its oxygen, retaining only its carbon. It is also known that water abounds in the sap. Hence carbon and water are thus abundantly present in the pores or vessels of the green leaf. Now, woody fibre consists only of carbon and water chemically combined together,—100 lbs. of dry woody fibre consisting of 50 lbs. of carbon and 50 lbs. of water. It is easy, therefore, to see how, when the carbon and water meet in the leaf, woody fibre may be produced by their mutual combination.

If, again, we inquire how this important principle of plants may be formed from the other substances, which enter by their roots, from the ulmic acid, for example, the answer is equally ready. This acid also consists of carbon and water only, 50 lbs. of carbon with 37½ of water forming ulmic acid, so that when it is introduced into the sap of the plant, all the materials are present from which the woody fibre may be produced.

Nor is it more difficult to see how starch may be converted into sugar, and this again into woody fibre; or how, again, sugar may be converted into starch in the ear of corn, or woody fibre into sugar during the ripening of the winter pear after its removal from the tree. Any one of these substances may be represented by carbon and water only.

Thus,—

50 lbs. of carbon with	50	of water, make	100		of woody fibre.
50 lbs.	37½		87½		of ulmic acid.
					of cane sugar,
50 lbs.	72½		122½		of starch, or
					of gum.
50lbs.	56		106		of vinegar.

In the interior of the plant, therefore, it is obvious that, whichever of these substances be present in the sap, the elements are at hand out of which any of the others may be produced. In what way they really are produced, the one from the other, and by what circumstances these transformations are favoured, it would lead into too great detail to attempt here to explain.[6]

We cannot help admiring to what varied purposes in nature the same elements are applied, and from how few and simple materials, substances, the most varied in their properties, are in the living vegetable daily produced.

CHAPTER IV.

Of the Inorganic Constitution of Plants—Their immediate Source—Their Nature—Quantity of each in certain common Crops.

SECTION I.—SOURCE OF THE EARTHY MATTER
OF PLANTS—SUBSTANCES OF
WHICH IT CONSISTS.

When plants are burned, they always leave more or less of ash behind. This ash varies in quantity in different plants, in different parts of the same plant, and sometimes in different specimens of the same kind of plant, especially if grown upon different soils; yet it is never wholly absent. It seems as necessary to their existence in a state of perfect health as any of the elements which constitute the organic or combustible part of their substance. They must obtain it therefore along with the food on which they live: it is in fact a part of their natural food, since without it they become unhealthy. We shall speak of it therefore as the inorganic food of plants.

We have seen that all the elements which are necessary to the production of the woody fibre, and of the other organic parts of the plant, may be derived either from the air, from the carbonic acid and watery vapour taken in by the leaves, or from the soil, through the medium of the roots. In the air, however, only rare particles of inorganic or earthy matter are known to float, and these in a solid form, so as to be unable to enter by the leaves; the earthy matter which constitutes the ash, therefore, must be all derived from the soil.

The earthy part of the soil, therefore, serves a double use. It is not merely, as some have supposed, a substratum in which the plant may so fix and root itself, as to be able to maintain its upright position against the force of winds and tempests; but it is a storehouse of food also, from which the roots of the plant may select such earthy substances as are necessary to, or are fitted to promote, its growth.

The ash of plants consists of a mixture of several, sometimes of as many as eleven, different earthy substances. These substances are the following:—

1. Potash.—The common pearl-ash of the shops is a compound of potash with carbonic acid; it is a carbonate of potash. By dissolving the pearl-ash in water, and boiling it with quicklime, the carbonic acid is separated, and potash alone, or caustic potash, as it is often called, is obtained.

2. Soda.—The common soda of the shops is a carbonate of soda, and by boiling it with quicklime, the carbonic acid is separated, as in the case of pearl-ash.

3. Lime.—This is familiar to every one as the lime-shells, or unslaked lime of the limekilns. The unburned limestone is a carbonate of lime; the carbonic acid in this case being separated by the roasting in the kiln.

4. Magnesia.—This is the calcined magnesia of the shops. The uncalcined is a carbonate of magnesia, from which heat drives off the carbonic acid.

5. Silica.—This is the name given by chemists to the substance of flint, quartz, and of siliceous sands and sandstones.

6. Alumina is the pure earth of alum, obtained by dissolving alum in water, and adding liquid ammonia (hartshorn) to the solution. It forms about two-fifths of the weight of porcelain and pipe-clays, and of some other very stiff kinds of clay.

7. Oxide of Iron.—The most familiar form of this substance is the rust that forms on metallic iron in damp places. It is a compound of iron with oxygen, hence the name oxide.

8. Oxide of Manganese is a brown powder, which consists of oxygen in combination with a metal resembling iron, to which the name of manganese is given. It exists in plants, and in soils only in very small quantity.

9. Sulphur.—This substance is well known. It generally exists in the ash in the state of sulphuric acid (oil of vitriol), which is a compound of sulphur with oxygen. It does not always exist in living plants, however, in this state.

Sulphuric acid forms with potash a sulphate of potash,—with soda, sulphate of soda (or Glauber’s salts),—with lime, sulphate of lime (gypsum),—with magnesia, sulphate of magnesia (Epsom salts),—with alumina, sulphate of alumina,—and with oxide of iron, sulphate of iron or green vitriol. When the sulphate of potash is combined with sulphate of alumina, it forms common alum.

10. Phosphorus is a soft pale yellow substance which readily takes fire in the air, and gives off, while burning, a dense white smoke. The white fumes which form this smoke are a compound of phosphorus with oxygen obtained from the air, and are called phosphoric acid. In the ash of plants the phosphorus is found in the state of phosphoric acid, though it probably does not all exist in the living plant in that state.

Phosphoric acid forms phosphates with potash, soda, lime, and magnesia. When bones are burned, a large quantity of a white earth remains (bone-earth), which is a phosphate of lime, consisting of lime and phosphoric acid. Phosphate of lime is generally present in the ash of plants; phosphate of magnesia is contained most abundantly in the ash of wheat and other varieties of grain.

11. Chlorine.—This is a very suffocating gas, which gives its peculiar smell to chloride of lime, and is used for bleaching and disinfecting. It is readily obtained by pouring muriatic acid (spirit of salt) on the black oxide of manganese of the shops. In combination with the metallic bases of potash, soda, lime, and magnesia, it forms the chlorides of potassium, sodium (common salt), calcium and magnesium,[7] and in one or other of these states it generally enters into the roots of plants, and exists in their ash.

Such are the inorganic substances usually found mixed or combined together in the ash of plants. It has already been observed, that the quantity of ash left by a given weight of vegetable matter varies with a great many conditions. This fact deserves a more attentive consideration.

SECTION II.—OF THE DIFFERENCE IN
THE QUANTITY OF ASH.

1. The quantity of ash yielded by different plants is unlike. Thus 1000 lbs. of

So that the quantity of inorganic food required by different vegetables is greater or less according to their nature; and if a soil be of such a kind that it can yield only a small quantity of this inorganic food, then only those plants will grow well upon it which require the least. Hence, trees may often grow where arable crops fail to thrive, because many of them require and contain very little inorganic matter. Thus while 1000 lbs. of elm wood leave 19 lbs. and of poplar 20 lbs. of ash, the same weight of the willow leaves only 4½ lbs., of the beech 4 lbs., of the birch 3½ lbs., of different pines less than 3 lbs., and of the oak only 2 lbs. of ash when burned.

2. The quantity of inorganic matter varies in different parts of the same plant. Thus while 1000 lbs. of the turnip root sliced and dried in the air leave 70 lbs. of ash, the dried leaves give 130 lbs.; and while the grain of wheat yields only 12 lbs., wheat straw will yield 60 lbs. of earthy matter. So, though the willow and other woods leave little ash, as above stated, yet the willow leaf leaves 82 lbs., the beech leaf 42 lbs., the birch 50 lbs., the different pine leaves 20 lbs. to 30 lbs., and the leaves of the elm as much as 120 lbs. of incombustible matter when burned in the air.

Most of the inorganic matter, therefore, which is withdrawn from the soil in a crop of corn is returned to it again, by the skilful husbandman, in the fermented straw,—in the same way as nature, in causing the trees periodically to shed their leaves, returns with them to the soil a very large portion of the soluble inorganic substances which had been drawn from it by the roots during the season of growth.

Thus an annual top-dressing is given to the land where forests grow; and that which the roots from spring to autumn are continually sucking up, and carefully collecting from considerable depths, winter strews again on the surface, so as, in the lapse of time, to form a soil which cannot fail to prove fertile,—because it is made up of those very materials of which the inorganic substance of former races of vegetables has been entirely composed.

2. The quantity of inorganic matter often differs in different specimens of the same plant. Thus, 1000 lbs. of wheat straw, grown at different places, gave to four different experimenters 43, 44, 35, and 155 lbs. of ash respectively. Wheat straw, therefore, does not always leave the same quantity of ash.

To what is this difference owing? Is it to the nature of the soil, or does it depend upon the variety of wheat experimented upon? It seems to depend partly upon both. Thus, on the same field, in Ravensworth dale, Yorkshire, on a rich clay soil abounding in lime, the Golden Kent and Flanders Red wheats were sown in the spring of 1841. The former gave an excellent crop, while the latter was a total failure, the ear containing 20 or 30 grains only of poor wheat. The straw of the former left 165 lbs. of ash from 1000 lbs., that of the latter only 120 lbs. Something, therefore, depends upon the variety. But as from the straw of a good wheat crop grown near Durham this last summer on a clay loam I obtained only 66 lbs. of ash, I am persuaded that the very wide variations in the quantity of ash left, by different wheat straws, must be dependent in some considerable degree upon the soil.

The truth, so far as it can as yet be made out, seems to be this—that every plant must have a certain quantity of inorganic matter to make it grow in the most healthy manner;—that it is capable of living, growing, and even ripening seed with very much less than this quantity;—but that those soils will produce the most perfect plants which can best supply all their wants,—and that the best seed will be raised in those districts where the soil, without being too rich or rank, yet can yield both organic and inorganic food in such proportions as to maintain the corn plants in their most healthy condition.

SECTION III.—OF THE QUALITY OF
THE ASH OF PLANTS.

But much also depends upon the quality as well as upon the quantity of the ash. Plants may leave the same weight of ash when burned, and yet the nature of the two specimens of ash, the kind of matter of which they respectively consist, may be very different. The ash of one may contain much lime, of another much potash, of a third much soda, while in a fourth much silica may be present. Thus 100 lbs. of the ash of bean straw contain 53½ lbs. of potash, while that of barley straw contains only 3½ lbs. in the hundred; and, on the other hand, the same weight of the ash of the latter contains 73½ lbs. of silica, while in that of the former there are only 7½ lbs.

The quality of the ash seems to vary with the same conditions by which its quantity is affected. Thus—

1. It varies with the kind of plant. 100 lbs. of the ash of wheat, barley, and oats, for example, contain, respectively,

A comparison of the several numbers opposite to each other in these three columns, shews how unlike the quantities of the different substances are, which are contained in an equal weight of the ash of these three varieties of grain. The ash of wheat contains 19 lbs. of potash in the 100 lbs., while that of oats contains only 6 lbs. In wheat are 20½ per cent. of soda, in oats only 5 per cent. Wheat also contains more sulphuric acid than either of the other grains, while barley contains a still greater predominance of phosphoric acid.

It is thus evident that a crop of wheat will carry off from the soil—even suppose the whole quantity of ash left by each the same in weight—very different quantities of potash, soda, &c. from a crop of oats. It will take more of these, of sulphuric acid, and of certain other substances, from the soil. It will, therefore, exhaust the soil more of these substances—as barley and oats will of others—hence one reason why a piece of land may suit one of these crops and not suit the others. That which cannot grow wheat may yet grow oats. Hence, also, two successive crops of different kinds of grain may grow where it would greatly injure the soil to take two in succession of the same kind, especially of either wheat or barley; and hence we likewise deduce one natural reason for a rotation of crops. The surface soil may be so far exhausted of one inorganic substance, that it cannot afford it in sufficient quantity during the present season to bring a given crop to healthy maturity, and yet may, by natural processes, be so far supplied again, during the intermediate growth of certain other crops, as to be prepared in a future season fully to supply all the wants of the same crop, and to yield a plentiful harvest.

2. The kind of inorganic matter varies with the part of the plant. Thus the grain and the straw of the corn plants contain very unlike quantities of the several inorganic constituents, as will appear by comparing the following with the preceding table:—

		Wheat Straw.	Barley Straw.	Oat Straw.
Potash,		½	3½	15
Soda,		¾	1	trace.
Lime,		7	10½	2¾
Magnesia,		1	1½	½
Alumina,		2¾	3	trace.
Oxide of Iron,		0	½	trace.
Oxide of manganese,
Silica,		81	73½	80
Sulphuric acid,		1	2	1½
Phosphoric acid,		5	3	¼
Chlorine,		1	1½	trace.
		100	100	100

Not only are the quantities of the several inorganic substances kinds of straw very unlike—especially the proportions of potash, lime, and phosphoric acid in each—but these quantities are also very different from those exhibited by the numbers in the preceding table as contained in the three varieties of grain. In this difference we see, further, one reason why the same soil which may be favourable to the growth of straw may not be equally propitious to the growth of the ear. Wheat straw contains little either of potash or of soda; the ash of the grain contains a large proportion; while the ash of the oat-straw, on the other hand, contains a much larger proportion of potash than that of its own ear does. It is clear, therefore, that the roots may, in certain plants and in certain soils, succeed in fully nourishing the straw while they cannot fully ripen the ear; or contrariwise, where they feed but a scanty straw, may yet be able to give ample sustenance to the filling ear.[8]

3. The quality of the ash varies also with the soil in which it grows. This will be understood from what is stated above. Where the soil is favourable, the roots can send up into the straw every thing which the healthy plant requires; when it is poorly supplied with some of those inorganic constituents which the plant desires, life may be prolonged, a stunted or unhealthy crop may be raised, in which the kind, and perhaps the quantity, of ash left in burning will necessarily be different from that left by the same species of plant grown under more favouring circumstances. Of this fact there can be no doubt, though the extent to which such variations may take place without absolutely killing the plant, has not yet been by any means made out.

4. It varies also with the period of a plant’s growth, or the season at which it is reaped. Thus, in the young leaf of the turnip and potato, a greater proportion of the inorganic matter they contain consists of potash than in the old leaf. The same is true of the stalk of wheat; and similar differences prevail in almost every kind of plant at different stages of its growth.

The enlightened agriculturist will perceive that all the facts above stated have a perceptible connection with the ordinary processes of practical agriculture, and tend to throw considerable light on some of the principles by which they ought to be regulated. One illustration of this is exhibited in the following section.

SECTION IV.—QUANTITY OF INORGANIC MATTER
CONTAINED IN AN ORDINARY CROP
OR SERIES OF CROPS.

The importance of the inorganic matter contained in living vegetables, or in vegetable substances when reaped and dry, will appear more distinctly if we consider the actual quantity carried off from the soil in a series of crops.

In a four-years’ course of cropping, in which the crops gathered amount per acre to—

1st year, Turnips, 25 tons of bulbs, and 7 tons of tops.

2d year, Barley, 38 bushels of 63 lbs. each, and 1 ton of straw.

3d year, Clover and Rye Grass, 1 ton of each in hay.

4th year, Wheat, 25 bushels of 60 lbs., and 1¾ tons of straw.

The quantity of inorganic matter carried off in the four crops, supposing none of them to be eaten on the land, amounts to—

or, in all, about 11 cwt.—of which gross weight the different substances form very unlike proportions.

A still clearer idea of these quantities will be obtained by a consideration of the fact, that if we carry off the entire produce, and return none of it again in the shape of manure, we must or ought in its stead, if the land is to be restored to its original condition, add to each acre every four years:—

Several observations suggest themselves from a consideration of the above statements: first, that if this inorganic matter be really necessary to the plant, the gradual and constant removal of it from the land ought by-and-by to impoverish the soil of this inorganic food; second, that the more of what grows upon the land we can again return to it in manure, the less will this deterioration be perceptible; third, that as many of these inorganic substances are readily soluble in water, the liquid manure of the farm-yard, so often allowed to run to waste, carries with it to the rivers much of the saline matter that ought to be returned to the land; and, lastly, that the utility and often indispensable necessity of certain artificial manures is owing, it may be, in some districts, to the natural poverty of the land in certain inorganic substances,—but more frequently to a want of acquaintance with the facts above stated, among practical men, and to the long continued neglect and waste which has been the natural consequence.

In certain districts, the soil and subsoil contain within themselves an almost unfailing supply of some of these inorganic substances, so that the waste is long in being felt; in others they become sooner exhausted, and hence call for more care, and, when exhausted, for a more expensive cultivation, in order to replace them.

One thing is of essential importance to be remembered by the practical farmer—that the deterioration of land is often an exceedingly slow process. In the hands of successive generations a field may so imperceptibly become less valuable, that a century even may elapse before the change prove such as to make a sensible diminution in the valued rental. Such slow changes, however, have been seldom recorded; and hence the practical man is occasionally led to despise the clearest theoretical principles, because he has not happened to see them verified in his own limited experience, and to neglect therefore the suggestions and the wise precautions which these principles lay before him.

The agricultural history of tracts of land of different qualities, shewing how they had been cropped and tilled, and the average produce in grain, hay, straw, and other crops, every five years, during an entire century, would be invaluable materials both to theoretical and to practical agriculture.

CHAPTER V.

Of Soils—their Organic and Inorganic Portions—Saline Matter in Soils—Examination and Classification of Soils—Diversities of Soils and Subsoils.

Soils consist of two parts,—of an organic part, which can readily be burnt away when the soil is heated to redness; and of an inorganic part, which is fixed in the fire, and which consists entirely of earthy and saline substances.

SECTION I.—OF THE ORGANIC PART OF SOILS.

The organic part of soils is derived chiefly from the remains of vegetables and animals which have lived and died in or upon the soil, which have been spread over it by rivers and rains, or which have been added by the hand of man for the purpose of increasing its natural fertility.

This organic part varies very much in quantity in different soils. In some, as in peaty soils, it forms from 50 to 70 per cent. of their whole weight, and even in some rich long cultivated lands it has been found, in a few rare cases, to amount to as much as 25 per cent. In general, however, it is present in much smaller proportion, even in our best arable lands. Oats and rye will grow upon a soil containing only 1½ per cent., barley when 2 to 3 are present, while good wheat soils generally contain from 4 to 8 per cent. In stiff and very clayey soils 10 to 12 per cent. may occasionally be detected. In very old pasture lands and in gardens, vegetable matter occasionally accumulates, so as to overload the upper soil.

To this organic matter in the soil the name of humus has been given by some writers. It contains or yields to the plant the ulmic and humic acids described in a previous chapter. It supplies also, by its decay, in contact with the air which penetrates the soil, much carbonic acid, which is supposed to enter the roots and minister to the growth of living vegetables. During the same decay ammonia is likewise produced,—and in larger quantity, if animal matter be present in considerable abundance,—which ammonia is found to promote vegetation in a remarkable manner. Other substances, more or less nutritious, are also formed from it in the soil. These enter by the roots, and contribute to nourish the growing plant, though the extent to which it is fed from this source is dependent, both upon the abundance with which these substances are supplied, and upon the nature of the plant itself, and of the climate in which it grows.

Another influence of this organic portion of the soil, whether naturally formed in it, or added to it as manure, is not to be neglected. It contains,—as we have seen that all vegetable substances do,—a considerable quantity of inorganic, that is, of saline and earthy matter, which is liberated as the organic part decays. Thus living plants derive from the remains of former races buried beneath the surface, a portion of that inorganic food which can only be obtained in the soil,—and which, if not thus directly supplied, must be sought for by the slow extension of their roots through a greater depth and breadth of the earth in which they grow. The addition of manure to the soil, therefore, places within the easy reach of the roots not only organic but inorganic food also.

SECTION II.—OF THE INORGANIC PART OF SOILS.

The inorganic part of soils,—that which remains behind, when every thing combustible is burned away by heating it to redness in the open air,—consists of two portions, one of which is soluble in water, the other insoluble. The soluble consists of saline substances, the insoluble of earthy substances.

1. The saline or soluble portion.—In this country the surface soil of our fields, in general, contains very little soluble matter. If a quantity of soil be dried in an oven, a pound weight of it taken, and a pint and a half of pure boiling rain-water poured over it, the whole well stirred and allowed to settle,—the clear liquid, when poured off and boiled to dryness, may leave from 2 to 20 grains of saline matter. This saline matter will consist of common salt, gypsum, sulphate of soda (Glauber’s salts), sulphate of magnesia (Epsom salts), with traces of the chlorides of calcium, magnesium, and potassium, and of the nitrates of potash, soda, and lime.[9] It is from these soluble substances that the plants derive the greater portion of the saline ingredients contained in the ash they leave when burned.

Nor must the quantity thus obtained from a soil be considered too small to yield the whole supply which a crop requires. A single grain of saline matter in every pound of a soil a foot deep, is equal to 500 lbs. in an acre, which is more than is carried off from the soil in 10 rotations (40 years), where only the wheat and barley are sent to market, and the straw and green crops are regularly returned to the land in the manure.[10]

In some countries, indeed in some districts of our own country, the quantity of saline matter in the soil is so great, as in hot seasons to form a distinct incrustation on the surface. This may often be seen in the neighbourhood of Durham; and is more especially to be looked for in districts where the subsoil is sandy and porous, and more or less full of water. In hot weather the evaporation on the surface causes the water to ascend from the porous subsoil: and as this water always brings with it a quantity of saline matter,—which it leaves behind when it rises in vapour,—it is evident that the longer the dry weather and the consequent evaporation from the surface continue, the thicker the incrustations will be, or the greater the accumulation of saline matter on the surface. Hence, where such a moist and porous subsoil exists in countries rarely visited by rain, as in the plains of Peru, of Egypt, or of India, the country is whitened over in the dry season with an unbroken covering of the different saline substances above mentioned.

When rain falls, the saline matter is dissolved, and descends again to the subsoil,—in dry weather it reascends. Thus the surface soil of any field will contain a larger proportion of soluble inorganic matter in the middle of a hot season than in one of even ordinary rain; and hence the fine dry weather which, in early summer, hastens the growth of corn, and later in the season favours its ripening, does so, among its other modes of action, by bringing up to the roots from beneath a more ready supply of those saline compounds which the crop requires for its healthful growth.

2. The earthy or insoluble portion.—The earthy or insoluble portion of soils rarely constitutes less than 95 lbs. in a hundred of their whole weight. It consists chiefly of silica in the form of sand, of alumina in the form of clay, and of lime in the form of carbonate of lime. It is rarely free, however, from one or two per cent. of oxide of iron; and where the soil is of a red colour, this oxide is present in a still larger quantity. A trace of magnesia also may be almost always detected, and a minute quantity of phosphate of lime. The principal ingredients, however, of the earthy part of all soils are sand, clay, and lime; and soils are named or classified according to the quantities of each of these three they may happen to contain.

If an ounce of soil be boiled in a pint of water till it is perfectly softened and diffused through it, and, after shaking, the heavy parts be allowed to settle for a few minutes, the sand will subside, while the clay—which is in finer particles, and is less heavy—will still remain floating. If the water and clay be now poured into another vessel, and be allowed to stand till the water has become clear, the sandy part of the soil will be on the bottom of the one vessel, the clayey part on that of the other, and they may be dried and weighed separately.

If 100 grains of dry soil leave no more than 10 of clay, it is called a sandy soil; if from 10 to 40, a sandy loam; if from 40 to 70, a loamy soil; if from 70 to 85, a clay loam; from 85 to 95, a strong clay soil; and when no sand is separated at all by this process, it is a pure agricultural clay.

The strong clay soils are such as are used for making tiles and bricks; the pure agricultural clay is such as is commonly employed for the manufacture of pipes (pipe-clay).

Soils consist of these three substances mixed together. The pure clay is a chemical compound of silica and alumina, in the proportion of about 60 of the former to 40 of the latter. Pure clay soils rarely occur—it being well known to all practical men, that the strong clays (tile clays) which contain from 5 to 15 per cent. of sand, are brought into arable cultivation with the greatest possible difficulty. It will rarely happen, therefore, that arable land will contain more than 30 to 35 of alumina.

If a soil contain more than 5 per cent. of carbonate of lime, it is called a marl; if more than 20 per cent., it is a calcareous soil. Peaty soils, of course, are those in which the vegetable matter predominates very much.

To estimate the lime, a quantity of the soil should be burned in the air, and a weighed portion, 100 or 200 grains, diffused through half a pint of cold water mixed with half a wine glassful of spirit of salt (muriatic acid), and allowed to stand for a couple of hours, with occasional stirring. The water is then poured off, the soil dried, heated to redness as before, and weighed: the loss is nearly all lime.[11]

The quantity of vegetable or other organic matter is determined by drying the soil well upon paper in an oven, and then burning a weighed quantity in the air: the loss is nearly all organic matter. In stiff clays this loss will comprise a portion of water, which is not wholly driven off from such soils by drying upon paper in the way described.

SECTION III.—OF THE DIVERSITIES OF
SOILS AND SUBSOILS.

Though the substances of which soils chiefly consist are so few in number, yet every practical man knows how very diversified they are in character—how very different in agricultural value. Thus, in some of our southern counties, we have a white soil, consisting apparently of nothing else but chalk; in the centre of England a wide plain of dark red land; in the border counties of Wales, and on many of our coal-fields, tracts of country almost perfectly black; while yellow, white, and brown sands give the prevailing character to the soils of other districts. Such differences as these arise from the different proportions in which the sand, lime, clay, and the oxide of iron which colours the soils, have been mixed together.

But how have they been so mixed—differently in different parts of the country. By what natural agency?—for what end?

Again, the soil on the surface rests on what is usually denominated the subsoil. This, also, is very various in its character and quality. Sometimes it is a porous sand or gravel, through which water readily ascends from beneath or sinks in from above; sometimes it is light and loamy like the soil that rests upon it; sometimes stiff and impervious to water.

The most ignorant farmer knows how much the value of a piece of land depends upon the characters of the surface soil,—the intelligent improver understands best the importance of a favourable subsoil. “When I came to look at this farm,” said an excellent agriculturist to me, “it was spring, and damp growing weather: the grass was beautifully green, the clover shooting up strong and healthy, and the whole farm had the appearance of being very good land. Had I come in June, when the heat had drunk up nearly all the moisture which the sandy subsoil had left in the surface, I should not have offered so much rent for it by ten shillings an acre.” He might have said also, “Had I taken a spade, and dug down 18 inches in various parts of the farm, I should have known what to expect in seasons of drought.”

But how come subsoils thus to differ—one from the other—and from the surface soil that rests upon them? Are there any principles by which such differences can be accounted for—by which they can be foreseen—by the aid of which we can tell what kind of soil may be expected in this or that district—even without visiting the spot—and on what kind of subsoil it is likely to rest?

Geology explains the cause of all such differences, and supplies us with principles by which we can predict the general quality of the soil and subsoil in the several parts of entire kingdoms;—and where the soil is of inferior quality and yet susceptible of improvement, the same principles indicate whether the means of improving it are likely, in any given locality, to be attainable at a reasonable cost.

It will be proper shortly to illustrate these direct relations of geology to agriculture.

CHAPTER VI.

Direct relations of Geology to Agriculture—Origin of Soils—Causes of their Diversity—Relation to the Rocks on which they rest—Constancy in the relative Position and Character of the Stratified Rocks—Relation of this fact to Practical Agriculture—General Character of the Soils upon these Rocks.

Geology is that branch of knowledge which embodies all ascertained facts in regard to the nature and internal structure, both physical and chemical, of the solid parts of our globe. This science has many close relations with practical agriculture, and especially throws much light on the nature and origin of soils,—on the cause of their diversity,—on the kind of materials by the admixture of which they may be permanently improved,—and on the sources from which these materials may be derived.

SECTION I.—OF THE ORIGIN OF SOILS.

If we dig down through the soil and subsoil to a sufficient depth, we always come sooner or later to the solid rock. In many places the rock actually reaches the surface, or rises in cliffs, hills, or ridges, far above it. The surface (or crust) of our globe, therefore, consists everywhere of a solid mass of rock, overlaid with a covering, generally thin, of loose materials. The upper or outer part of these loose materials forms the soil.

The geologist has travelled over great part of the earth’s surface, has examined the nature of the rocks, which everywhere repose beneath the soil, and has found them to be very unlike in character, in composition, and in hardness—in different countries and districts. In some places he has met with a sandstone, in other places a limestone, in others a slate or hardened rock of clay. But a careful comparison of all the kinds of rock he has observed, has led him to the general conclusion, that they are all either sandstones, limestones, or clays of different degrees of hardness, or a mixture in different proportions of two or more of these kinds of matter.

When the loose covering of earth is removed from the surface of any of these rocks, and it is left exposed, summer and winter, to the action of the winds and rains and frosts, it may be seen gradually to crumble away. Such is the case even with many of those which, on account of their greater hardness, are employed as building-stones, and are kept generally dry; how much more with such as are less hard, and, beneath a covering of moist earth, are continually exposed to the action of water. The natural crumbling of a naked rock thus gradually covers it with loose materials, in which seeds fix themselves and vegetate, and which eventually forms a soil. The soil thus produced partakes necessarily of the character of the rock on which it rests, and to the crumbling of which it owes its origin. If the rock be a sandstone the soil is sandy; if a claystone, it is a more or less stiff clay; if a limestone, it is more or less calcareous; and if the rock consist of any peculiar mixture of those three substances, a similar mixture is observed in the earthy matter into which it has crumbled.

Led by this observation, the geologist, after comparing the rocks of different countries with one another, compared next the soils of various districts with the rocks on which they immediately rest. The general result of this comparison has been, that in almost every country the soils have as close a resemblance to the rocks beneath them—as the loose earth derived from the crumbling of a rock before our eyes, bears to the rock of which it lately formed a part. The conclusion therefore is irresistible, that soils, generally speaking, have been formed by the crumbling or decay of the solid rocks,—that there was a time when these rocks were uncovered by any loose materials,—and that the accumulation of soil has been the slow result of the natural degradation (wearing away) of the solid crust of the globe.

SECTION II.—CAUSE OF THE DIVERSITY OF SOILS.

The cause of the diversity of soils in different districts, therefore, is no longer obscure. If the subjacent rocks in two localities differ, the soils met with there must differ also, and in an equal degree.

But why, it may be asked, do we find the soil in some countries uniform, in mineral[12] character and general fertility, over hundreds or thousands of square miles, while in others it varies from field to field,—the same farm often presenting many well marked differences both in mineral character and in agricultural value? The cause of this is to be found in the mode in which the different rocks are observed to lie, one upon or by the side of the other.

Geologists distinguish rocks into two classes, the stratified and the unstratified. The former are found lying over each other in separate beds or strata, like the leaves of a book, when laid on its side, or like the layers of stones in the wall of a building; the latter form hills, mountains, or sometimes ridges of mountains, consisting of one more or less solid mass of the same material, in which no layers or strata are any where distinctly perceptible. Thus, in the following diagram, ([No. 1]), A and B represent unstratified masses, in connection with a series of stratified deposits, 1, 2, 3, lying over each other in a horizontal position. On A one kind of soil will be formed, on C another, on B a third, and on D a fourth,—the rocks being all different from each other.

No. 1.

If from A to D be a wide valley of many miles in extent, the undulating plain at the bottom of the valley, resting in great part on the same rock (2), will be covered by a similar soil. On B the soil will be different for a short space; and again at C, and on the first ascent to A, where the rock (3) rises to the surface. In this case the stratified rocks lie horizontally; and it is the undulating nature of the country which, bringing different kinds of rock to the surface, causes a necessary diversity of soil.

But the degree of inclination, which the beds possess, is a more frequent cause of variation in the characters of the soil in the same district, and even at shorter distances. This is shewn in the annexed diagram ([No. 2]), where A, B, C, D, E, represent the mode in which the stratified rocks of a district of country not unfrequently occur in connection with each other.

No. 2.

Proceeding from E in the plain, the soil would change when we came upon the rock D, but would then continue uniform till we reached the layer C. Each of these layers may stretch over a comparatively level tract of perhaps hundreds of miles in extent. Again, on climbing the hill-side, another soil would present itself, which would not change till we arrived at B. Then, however, we begin to walk over the edges of the beds, and the soil may vary with every new stratum (or bed) we pass over, till we gain the ascent to A, where the beds are much thinner, and where, therefore, still more frequent variations may present themselves.

Everywhere over the British islands valleys are hollowed out, as in the former of these diagrams ([No. 1]), by which the rocks beneath are exposed, and differences of soil produced,—or the beds are more or less inclined, as in the latter diagram ([No. 2]), causing still more frequent variations of the land to appear. By a reference to these facts, nearly all the great diversities which the soils of the country present may be satisfactorily accounted for.

SECTION III.—OF THE CONSTANCY IN THE CHARACTER
AND ORDER OF SUCCESSION OF
THE STRATIFIED ROCKS.

Another fact alike important to agriculture and to geology, is the natural order or mode of arrangement in which the stratified rocks are observed to occur in the crust of the globe. Thus, if 1, 2, 3, in [diagram No. 1] represent three different kinds of rock, a limestone, for example, a sandstone, and a hard clay rock (a shale or slate), lying over each other, in the order here represented; then, in whatever part of the country nay, in whatever part of the world, these same rocks are met with, they will always be found in the same relative position. The bed 2 or 3 will never be observed to lie over the bed 1.

This fact is important to geology, because it enables this science to arrange all the stratified rocks in a certain invariable order,—which order indicates their relative age or antiquity,—since that which is lowest, like the lowest layer of stones in the wall of a building, must generally have been the first deposited, or must be the oldest. It also enables the geologist, on observing the kind of rock which forms the surface in any country, to predict at once, whether certain other rocks are likely to be met with in that country or not. Thus at C ([diagram, No. 1]), where the rock (3) comes to the surface, he knows it would be in vain, either by sinking or otherwise, to seek for the rock (1), the natural place of which is far above it; while at D he knows that by sinking he is likely to find either 2 or 3, if it be worth his while to seek for them.

To the agriculturist this fact is important, among other reasons,—

1. Because it enables him to predict whether certain kinds of rock, which might be used with advantage in improving his soil, are likely to be met with within a reasonable distance or at an accessible depth. Thus if the bed D ([diagram No. 2]) be a limestone, the instructed farmer at E knows that it is not to be found by sinking into his own land, and, therefore, brings it from D; while, to the farmer upon C, it may be less expensive to dig down to the bed D in one of his own fields, than to cart it from a distant spot where it occurs on the surface. Or if the farmer requires clay, or marl, or sand, to ameliorate his soil, this knowledge of the constant relative position of beds enables him to say where these materials are to be got, or where they are to be looked for, and whether the advantage to be derived is likely to repay the cost of procuring them.

2. It is observed, that when the soil on the surface of each of a series of rocks, such as C, or D, or E, in the same diagram, is uniformly bad, it is almost invariably of better quality at the point where the two rocks meet. Thus C may be dry, sandy, and barren; D may be cold, unproductive clay; and E a more or less unfruitful limestone soil: yet at either extremity of the tract D, where the soil is made up of an admixture of the decayed portions of the two adjacent rocks, the land may be of average fertility—the sand of C may adapt the adjacent clay to the growth of turnips, while the lime of E may cause it to yield large returns of wheat.[13] Thus, to the tenant in looking out for a farm, or to the capitalist in seeking an eligible investment, a knowledge of the mutual relations of geology and agriculture will often prove of the greatest assistance. Yet how little is such really useful knowledge diffused among either class of men—how little are either tenants or proprietors guided by it in their choice of the localities in which they desire to live!

And yet here and there the agricultural practice of more or less extended districts, if not really founded upon or directed by, is yet to be explained only by principles such as those I have above illustrated. I shall mention only one example. The chalk in Yorkshire, in Suffolk, and in other southern counties, consists of a vast number of beds, which, taken all together, form a deposit of very great thickness. Now, the upper beds of the chalk form poor, thin, dry soils, producing a scanty herbage, and only under the most skilful culture yielding profitable crops of corn. The lower beds, on the contrary, are marly; produce a more stiff, tenacious, and even fertile soil; and are found in a remarkable degree to enrich the soils of the upper chalk, when laid on as a top-dressing in autumn, and allowed to crumble under the action of the winter’s frost. Hence in Yorkshire, Wiltshire, Hampshire, and Kent, where the lower chalk covers the surface, or is found at no great depth beneath it, it is dug out of the sides of the hills, or pits are sunk for it, and it is immediately laid upon the land with great benefit to the soil. But in parts of Suffolk, where the soil equally rests upon the upper chalk, there is no other chalk in the neighbourhood, or to be met with at any reasonable depth, which will materially improve the land. The farmers find it, from long experience, to be more economical to bring chalk by sea from Kent to lay on their lands in Suffolk, than to cover them with any portion of the same material from their own farms. The following imaginary section will fully explain the fact here mentioned:—

No. 3.

Suffolk.Mouth of the Thames. Kent.

In this diagram 1 represents the London clay; 2, the plastic clay which is below it; 3, the upper chalk with flints, rising to the surface in Suffolk; and 4, the lower chalk, without flints, which is too deep to be reached in Suffolk, but which rises to the surface in Kent,—where it is abundant, is easily accessible, and whence it is transmitted across the estuary of the Thames into Suffolk.

3. The further fact that the several stratified rocks are remarkably constant in their mineral character, renders this knowledge of the order of relative superposition still more valuable to the agriculturist. Thousands of different beds are known to geologists to occur on various parts of the earth’s surface—each occupying its own unvarying place in the series. Most of these beds also, when they crumble or are worn down, produce soils possessed of some peculiarity by which their general agricultural capabilities are more or less affected,—and these peculiarities may generally be observed in soils formed from rocks of the same age—that is, occupying the same place in the series—in whatever part of the world we find them. Hence if the agricultural geologist be informed that his friend has bought, or is in treaty for a farm or an estate, and that it is situated upon such and such a rock, or geological formation, he can immediately give a very probable opinion in regard to the agricultural value of the soil, whether the property be in England, in Australia, or in New Zealand. If he knows the nature of the climate also, he will be able to estimate with tolerable correctness how far the soil is likely to repay the labours of the practical farmer,—nay, even whether it is likely to suit better for arable land or for pasture, and if for arable, what species of white crops it may be expected to produce most abundantly.

These facts are so very curious, and illustrate so beautifully the value of geological knowledge—if not to A and B, the holders or proprietors of this and that small farm, yet to enlightened agriculturists,—to scientific agriculture in general,—that I shall explain this part of the subject more fully in a separate section. To those who are now embarking in such numbers in quest of new homes in our numerous colonies, who hope to find, if not a more willing, at least a more attainable soil in new countries, no kind of agricultural knowledge can at the outset,—I may say, even through life,—be so valuable as that to which the rudiments of geology will lead them. Those who prepare themselves the best for becoming farmers or proprietors in Canada, in New Zealand, or in wide Australia, yet leave their native land in general without a particle of that preliminary practical knowledge, which would qualify them to say, when they reach the land of their adoption, “On this spot, rather than that,—in this district, rather than that,—will I purchase my allotment, because, though both appear equally inviting, yet I know from the geological structure of the country, that here I shall have the more permanently productive soil; here I am more within reach of the means of agricultural improvement; here, in addition to the riches of the surface, my descendants may hope to derive the means of wealth from mineral riches beneath.” And this oversight has arisen chiefly from the value of such knowledge not being understood—often from the very nature of it being unknown, even to otherwise well instructed practical men. It is not to men well skilled merely in the details of local farming, and who are therefore deservedly considered as authorities and good teachers in regard to local or district practice, that we are to look for an exposition, often not even for a correct appreciation, of those general principles on which a universal system of agriculture must be based—without which principles, indeed, it must ever remain a mere collection of empirical rules, to be studied and laboriously mastered in every new district we go to—as the traveller in foreign lands must acquire a new language every successive frontier he passes. England, the mistress of so many wide and unpeopled lands, over which the dwellings of her adventurous sons are hereafter to be scattered, on which their toil is to be expended, and the glory of their motherland by their exertions to be perpetuated—England should especially encourage all such learning, and the sons of English farmers willingly avail themselves of every opportunity of acquiring it.

SECTION IV.—OF GEOLOGICAL FORMATIONS, AND
THE GENERAL CHARACTERS OF THE SOILS
THAT REST UPON THEM.

The thousands of beds or strata of which I have spoken as lying one over the other in the crust of the globe, have, partly for convenience, and partly in consequence of certain remarkably distinctive characters observed among them, been separated by geologists into three great divisions—the primary, which are the lowest and the oldest; the secondary, which lie over them; and the tertiary, which are uppermost, and have been most recently formed. The strata, in these several divisions, have again been subdivided into groups, called formations. The following table exhibits the names and thicknesses of these formations, and the mineralogical characters of the rocks of which they severally consist.

I. TERTIARY STRATA.

1. The London and Plastic clays, 500 to 900 feet thick, consist of stiff, almost impervious, dark-coloured clays,—chiefly in pasture. The lower beds are mixed with sand, and produce an arable soil, but extensive heaths and wastes rest upon them in Berkshire, Hampshire, and Dorset.

II. SECONDARY STRATA.

2. The Chalk, about 600 feet in thickness, consists in the upper part ([see diagram, No. 3, p. 88]) of a purer chalk with layers of flint; in the lower, of a marly chalk without flints. The soil of the upper chalk is chiefly in sheep-walks, that of the lower chalk is very productive of corn.

3. The Green Sand, 500 feet thick, consists of 150 feet of clay, with about 100 feet of sand above, and 250 feet below it. The upper sand forms a very productive arable soil, and the clay impervious, wet and cold lands chiefly in pasture. The lower sand is generally unproductive.

It is an important agricultural remark, that where the clay (plastic clay) comes in contact with the top of the chalk, an improved soil is produced, and that where the chalk and the green sand mix, extremely fertile patches of country present themselves. ([See pages 86 and 87.])

4. The Wealden formation, nearly 1000 feet thick, consists of 400 feet of sand, covered by 300 of clay, and resting upon 250 of marls and limestones. The clay forms the poor wet pastures of Sussex and Kent. On the sands below the clay rest heaths and brushwood; but where the marls and limestones come to the surface, the land is of better quality, and is susceptible of profitable arable culture.

5. In the Upper Oolite, of 600 feet in thickness, we have a bed of clay (Kimmeridge clay) 500 feet thick, covered by 100 feet of sandy limestones. The clay lands are difficult and expensive to work, and are chiefly in old pasture. The sandy limestone soils above the clay are also poor, but where they rest immediately upon, and are intermixed with the clay, excellent arable land is produced.

6. The Middle Oolite of 500 feet consists also of a clay (Oxford clay) dark-blue, adhesive, and nearly 1000 feet thick, covered by 100 feet of limestones and sandstones. These latter produce good arable land where the lime happens to abound; the clays form close heavy compact soils, most difficult and expensive to work. The extensive pasture lands of Bedford, Huntingdon, Northampton, Lincoln, Wilts, Oxford, and Gloucester, rest chiefly upon this clay, as do also the fenny tracts of Lincoln and Cambridge.

7. The Lower or Bath Oolite, of 500 feet in thickness, consists of many beds of limestone and sandstone, with about 200 feet of clay in the centre of the formation. The soils are very various in quality, according as the sandstone or limestone predominates. The clays are chiefly in pasture,—the rest is more or less productive, easily worked, arable land. In Gloucester, Northampton, Oxford, the east of Leicester, and in Yorkshire, this formation is found to lie immediately beneath the surface, and a little patch of it occurs also on the south-eastern coast of Sutherland.

6. The Lias is an immense deposit of blue clay from 500 to 1000 feet in thickness, which produces cold, blue, unproductive, clay soils. It forms a long stripe of land from the mouth of the Tees, in Yorkshire, to Lyme Regis in Dorset. It is chiefly in old, and often very valuable pasture.

9. The New Red Sandstone, though only 500 feet in thickness, forms the surface of nearly the whole central plain of England, and stretches north through Cheshire to Carlisle and Dumfries. It consists of red sandstones and marls,—the soils on which are easily and cheaply worked, and form some of the richest and most productive arable lands in the island. In whatever part of the world the red soils of this formation have been met with, they have been found to possess in general the same agricultural capabilities.

10. The Magnesian Limestone, from 100 to 500 feet in thickness, forms a stripe of generally poor thin soil from Durham to Nottingham, capable of improvement as arable land by high farming, but bearing naturally a poor pasture, intermingled with sometimes magnificent furze.

11. The Coal Measures, from 300 to 3000 feet thick, consist of beds of sandstones and dark-blue shales (hard clays), intermingled (interstratified) with beds of coal. Where the sands come to the surface, the soil is thin, poor, hungry, sometimes almost worthless. The shales, on the other hand, produce stiff, wet, almost unmanageable clays;—not unworkable, yet expensive to work, and requiring draining, lime, skill, capital, and a zeal for improvement, to be applied to them, before they can be made to yield the remunerating crops of corn they are capable of producing.

12. To the Millstone Grits of 600 feet or upwards in thickness the same remarks apply. They are often only a repetition of the sandstones and shales of the coal measures, forming in many cases soils still more worthless. When the sandstones prevail, large tracts lie naked, or bear a thin and stunted heath; where the shales abound, the naturally difficult soils of the coal shales again recur. These rocks are generally found on the outskirts of our coal-fields.

13. The Mountain Limestone, 800 to 1000 feet thick, is a hard blue limestone rock, separated here and there into distinct beds by layers of sandstones, of sandy slates, or of blue shales like those of the coal measures. The soil upon the limestone is generally thin, but produces a naturally sweet herbage. When the limestone and clay (shale) adjoin each other, arable land occurs, which is naturally productive of oats, yet, when the climate is favourable, capable of being converted into good wheat land. In the north of England a considerable tract of country is covered by these rocks, but in Ireland they form nearly the whole of the interior of the island.

14. The Old Red Sandstone varies in thickness from 500 to 10,000 feet. It possesses many of the valuable agricultural qualities of the new red, consisting, like it, of red sandstones and marls, which crumble down into rich red soils. Such are the soils of Brecknock, Hereford, and part of Monmouth; of part of Berwick and Roxburgh; of Haddington and Lanark; of southern Perth; of either shore of the Moray Firth; and of the county of Sutherland. In Ireland, also, these rocks abound in Tyrone, Fermanagh, and Monaghan; in Waterford, in Mayo, and in Tipperary. In all these places, the soils they form are generally the best in their several neighbourhoods, though here and there,—where the sandstones are harder, more siliceous and impervious to water,—tracts, sometimes extensive, of heath and bog occur.

III.—PRIMARY STRATA.

15. The Upper Silurian system is nearly 4000 feet in thickness, and forms the soils over the lower border counties of Wales. It consists of sandstones and shales, with occasional limestones; but the soils formed from these beds take their character from the general abundance of clay. They are cold, usually unmanageable, muddy clays, with the remarkably inferior agricultural value of which the traveller is immediately struck, as he passes westward off the red sandstones of Hereford on to the upper silurian rocks of Radnor.

16. The Lower Silurian rocks are also nearly 4000 feet in thickness, and in Wales lie to the west of the upper silurian rocks. They consist of about 2500 feet of sandstone, on which, when the surface is not naked, barren heaths alone rest.

Beneath these sandstones lie 1200 feet of sandy and earthy limestones, from the decay of which, as may be seen on the southern edge of Caermarthen, fertile arable lands are produced.

17. The Cambrian System, of many thousand yards in thickness, consists in great part of clay slates, more or less hard, which often weather slowly, and almost always produce either poor and thin soils, or cold, difficultly manageable clays, expensive to work, and requiring high farming to bring them into profitable arable cultivation. Cornwall, western Wales, and the mountains of Cumberland, in England; the high country which stretches from the Lammermuir hills to Portpatrick, in Scotland; the mountains of Tipperary, and a large tract on the extreme south of Ireland,—on its east coast, and far inland from the bay of Dundalk,—are covered by these slate rocks. Patches of rich, well cultivated land occur here and there on this formation, with much also that is improvable; but the greater part of it is usurped by worthless heath and extensive bogs.

18. The Mica Slate and Gneiss systems are of unknown thickness, and consist chiefly of hard and slaty rocks, crumbling slowly, forming poor, thin soils, which rest on an impervious rock, and which, from the height to which this formation generally rises, are rendered more unproductive by an unpropitious climate. They form extensive heathy tracts in Perth and Argyle, and on the north and west of Ireland. Here and there only, in the valleys or sheltered slopes, and by the margins of the lakes, spots of bright green meet the eye, and patches of a willing soil, fertile in corn.

A careful perusal of the preceding sketch of the general agricultural capabilities of the soils formed from the several classes of stratified rocks, will have presented to the reader many illustrations of the facts stated in the preceding section; he will have drawn for himself—to specify a few examples—the following among other conclusions.

1. That some formations, like the new red sandstone, yield a soil almost always productive; others, as the coal measures and millstone grits, a soil almost always naturally unproductive.

2. That good, or better land at least, than generally prevails in a district, may be expected where two formations or two different kinds of rock meet,—as when a limestone and a clay mingle their mutual ruins for the formation of a common soil.

3. That in almost every country extensive tracts of land on certain formations will be found laid down to natural grass, in consequence of the original difficulty and expense of working. Such are the Lias, the Oxford, the Kimmeridge, and the London clays. In raising corn, it is natural that the lands which are easiest and cheapest worked should be first subjected to the plough; it is not till implements are improved, skill increased, capital accumulated, and population presses, that the heavier lands will be rescued from perennial grass, and made to produce that greatly increased amount of food for both man and beast, which they are easily capable of yielding.

The turnip soils of Great Britain are in many districts, it may be, but indifferently farmed; and the state has reason to complain of much individual neglect of known and certain methods of increasing their productiveness; but the next great achievement which British agriculture has to effect, is to subdue the stubborn clays, and to convert them into what many of them are yet destined to become, the richest corn-bearing lands in the kingdom.

CHAPTER VII.

Soils of the Granitic and Trap Rocks.—Accumulations of transported Sands, Gravels, and Clays.—Use of Geological Maps in reference to Agriculture. —Physical characters and Chemical constitution of Soils.—Relation between the nature of the Soil and the kind of Plants that naturally grow upon it.

It was stated, in the preceding lecture, ([see p. 82],) that rocks are divided by geologists into the stratified and the unstratified.[14] The stratified rocks cover by far the largest portion of the globe, and thus form a variety of soils, of which a general description has just been given. The unstratified rocks are of two kinds—the granites and the trap rocks; and as a considerable portion of the area of our island is covered by them, it will be proper shortly to consider the peculiar characters of each, and the differences of the soils produced from them.

SECTION I.—SOILS OF THE GRANITES
AND TRAP ROCKS.

1. The granites consist of a mixture, in different proportions, of three minerals, known by the names of quartz, felspar, and mica. The latter, however, is generally present in such small quantity, that in our general description it may be safely left out of view. Granites, therefore, consist chiefly of quartz and felspar, in proportions which vary very much, but the former, on an average, constitutes perhaps from one-third to one-half of the whole.

Quartz has already been described—([see p. 51])—as the substance of flint, the silica of the chemist. When the granite decays, this portion of it forms a more or less coarse siliceous sand.

Felspar is a white, greenish, or flesh-coloured mineral, often more or less earthy in its appearance, but generally hard and brittle, and sometimes glassy. It is scratched by, and thus is readily distinguished from, quartz. When it decays, it forms an exceedingly fine clay.

A remarkable difference appears thus to exist, in chemical constitution, between these two minerals—a difference which must affect also the soils produced from them. A granite soil, in addition to the siliceous sand, will consist chiefly of silica, alumina, and potash; a hornblende soil, in addition to silica and alumina, of much lime, magnesia, and oxide of iron—of nearly 2½ cwt. of each of these latter for every ton of decayed rock. A hornblende soil, therefore, contains more of those inorganic constituents which the plants require for their healthy sustenance, and therefore will prove more generally productive than a soil of decayed felspar. But when the two are mixed, as in the greenstones, the soil must be still more favourable to vegetable life. The potash and soda, of which the hornblende is nearly destitute, the felspar is able abundantly to supply; while, by the hornblende are yielded lime and magnesia, which are known to exercise a remarkable influence on the progress of vegetation.