CHAPTER IX.
Animal Manures—Their relative value and mode of Action—Difference between Animal and Vegetable Manures—Cause of this difference—Mineral Manures —Nitrates of Potash and Soda—Sulphate of Soda, Gypsum, Chalk, and Quicklime—Chemical action of these Manures—Artificial Manures—Burning and Irrigation of the Soil—Planting and laying down to grass.
Animal Manures—Their relative value and mode of Action—Difference between Animal and Vegetable Manures—Cause of this difference—Mineral Manures —Nitrates of Potash and Soda—Sulphate of Soda, Gypsum, Chalk, and Quicklime—Chemical action of these Manures—Artificial Manures—Burning and Irrigation of the Soil—Planting and laying down to grass.
The animal substances employed as manure consist chiefly of the flesh, blood, bones, horns, and hair of animals, of fish—which in some places are found in sufficient quantity to be laid upon the land—and of the solid and liquid excrements of animals and birds.
Animal substances, in general, act more powerfully as manures than vegetable substances—it is only the seeds of plants which can at all compare with them in efficacy.
Thefleshof animals is rarely used as a manure, except in the case of dead horses, or cattle which cannot be used for food. Fish is chiefly applied in the form of the refuse of the herring and pilchard fisheries, though occasionally such shoals of sprats, herrings, and even mackerel, have been caught on our shores, as to make it necessary to employ them as manure. These recent animal substances are found to be toostrongwhen applied directly to the land; they are generally, therefore, made into a compost, with a large quantity of soil. Five barrels of fish, or fish refuse, made into twenty loads of compost, will be sufficient for an acre. The refuse of fish oils,—of the fat of animals that has been melted for the extraction of the tallow—of skins that have been boiled for the manufacture of glue—horns, hair, wool (woollen rags), and all similar substances, when made into composts, exercise, in proportion to their weight, a much greater influence upon vegetation than any of the more abundant forms of vegetable matter.
Even the bodies of insects are in many parts of the world important manures of the soil. In warm climates, a handful of soil sometimes seems almost half made up of the wings and skeletons of dead insects—the peasant in Hungary and Carinthia occasionally collects asmany as thirty cart-loads of dead marsh flies in a single year;—and in the richer soils of France and England, where worms and other insects abound, the presence of their remains in the soil must also aid its natural productiveness.
Bloodis rarely applied to the land directly—though, like the other parts of animals, it makes an excellent compost. As it comes from the sugar refineries, however, in which, with lime water and animal charcoal, it is employed for the refining of sugar, it has obtained a very extensive employment, especially in the south of France. This animal black, oranimalized charcoal, as it is sometimes called, contains about twenty per cent. of blood, and has risen to such a price in France, that the sugar refiners actually sell it for more than the unmixed blood and animal charcoal originally cost them. This has given rise to the manufacture of artificial mixtures of charcoal, fecal matters, and blood, which are also sold under the name of animalized charcoal. The only disadvantage attending these artificial preparations is, that they are liable to be adulterated, or, for cheapness, prepared in a less efficient manner.
Horn,hair, andwool, depend for their efficacy precisely on the same principles as the blood and flesh of animals. They differ chiefly in this, that they aredry, while blood andflesh contain 80 to 90 per cent. of their weight of water. Hence, a ton of horn shavings, of hair, or of dry woollen rags, ought to enrich the soil as much as ten tons of blood. In consequence, however, of their dryness, the horn and wool decompose much more slowly than the blood. Hence, the effect of soft animal matters is more immediate and apparent, that of hard and dry substances less visible, but continuing for a much longer period of time.
Bones, again, while they resemble horn in being dry, differ from it in containing, besides the animal matter, a large quantity of earthy matter also, and hence they introduce a new agent to aid their effect upon the soil. Thus, the bones of the cow consist of 100 lbs. of
While 100 lbs. of bone-dust, therefore, add to the soil as muchorganicanimal matter as 33 lbs. of horn, or as 300 or 400 lbs. of blood or flesh, they add, at the same time, muchinorganicmatter—lime, magnesia, soda, common salt, and phosphoric acid (in the phosphates),—all of which, as we have seen, must be present in a fertile soil, since the plants require a certain supply of them all at every period of their growth. These substances, like the inorganic matter of plants, may remain in the soil, and may exert a beneficial action upon vegetation after all the organic or gelatinous matter has decayed and disappeared.
From what is above stated, therefore, the reader will gather these general conclusions:
1. That animal substances which, like flesh and blood, contain much water, decay rapidly, and are fitted to operateimmediatelyand powerfully upon vegetation, but are only temporary or evanescent in their action.
2. That when dry, as in horn, hair, and wool, they decompose, and consequently act more slowly, and continue to manifest an influence, it may be, for several seasons.
3. That bones, acting like horn, in so far as their animal matter is concerned, and, like it, for a number of seasons, more or less, according as they have been more or less finely crushed—may ameliorate the soil by their earthy matter for a still longer period—permanently improving the condition and adding to the natural capabilities of the land.
Practical men have long been of opinion that the digestion of food, either animal or vegetable,—the passing of it through the bodies of animals,—enriches its fertilizing power, weight for weight, when added to the land. Hence, in causing animals to eat up as much of the vegetable productions of the farm as possible, it is supposed that not only is so much food saved, but that the value of the remainder in fertilizing the land is greatly increased. In a subsequent section we shall see how far theory serves to throw light upon these opinions. (See Section IV., p. 182 to 186.)
The digested animal substances usually employed as manures are, the urine of the cow and the sheep, the solid excrements of the horse, the cow, the sheep, and the pig, the droppings of pigeons and other birds, and night-soil. The liquid manures act chiefly through the saline substances they hold in solution, while the solid manures contain also insoluble matters, which decay slowly in the soil, and there become useful only after a time. The former, therefore, will influencevegetation more powerfully at first; the action of the latter will be less evident, but will continue to operate for a much longer period.
Urine.—Human urine consists, in 1000 parts, of
A thousand pounds of urine therefore contain 68 lbs. of dry fertilizing matter of the richest quality, worth,at the present rate of selling artificial manures in this country, at least 20s. a cwt. As each person voids almost 1000 lb. of urine in a year, the national waste incurred in this form amounts, at the above valuation, to 12s. a head. And if five tons of farm-yard manure per acre, added year by year, will keep a farm in good heart, four cwt. of the solid matter of urine would probably have an equal effect; or the urine alone discharged into the rivers by a population of 10,000 inhabitants would supply manure to a farm of 1500 acres, yielding a return of 4500 quarters of corn or an equivalent produce of other crops.
The urine of the cow is said to contain less water than that of man, though of course much must depend upon the kind of food with which it isfed. Reckoning, then, the large quantity of liquid manure that is yielded by the cow (2000 or 3000 gallons a year), we may safely estimate the solid matter given off by a healthy animal in this form in twelve months at 1200 to 1500 pounds weight, worth,if it were in the dry state, from £10 to £12 sterling. In theliquidstate, the urine of one cow collected and preserved as it is in Flanders, is valued in that country at about £2 a year. Any practical farmer may calculate for himself, therefore, how much real wealth, taking it even at the Flemish value, is lost in his own farm-yard—how much of the natural means of reproductive industry passes into his drains or evaporates into the air.
This liquid manure is invaluable, when collected in tanks, for watering the manure and compost heaps, and thus hastening their decomposition; but great part of it may also be sprinkled directly upon the fields of grass and upon the young corn, with the best effects. It must, however, be permitted to stand till fermentation commences, and afterwards diluted with a considerable quantity of water, before it will be in the best condition for laying on the land.
Urate.—In order to obtain the virtues of animal urine in a concentrated form, the custom has been adopted of mixing burnt gypsumwith it, in the proportion of 10 lbs. to every 7 gallons, allowing the mixture, occasionally stirred, to stand some time, pouring off the liquid, and drying and crushing the gypsum. This is sold by manure manufacturers under the name ofurate. It never can possess, however, the virtues of the urine, since it does not contain the soluble saline substances, which the gypsum does not carry down with it. Except the gypsum, indeed, 100 lbs. of urate contain no greater weight of saline and organic matter than 10 gallons of urine. If it be true, then, as the manufacturers state, that 3 or 4 cwt. of urate are sufficient manure for an acre, the practical farmer will, I hope, draw the conclusion,—not that it is well worth his while to venture his money in trying a portion of it upon a piece of his land,—but that a far more promising adventure will be to go to some expense in saving his own liquid manure, and, after mixing it with burned gypsum, to lay it abundantly upon all his fields.
Cow and Horse Dung.—So much of the saline, nutritive, and soluble organic matters from the cow pass off in the liquid form, that cow dung is correctly called cold, since it does not readily heat andrun into fermentation. Mixed with other manures, however, or well diffused through the soil, it aids materially in promoting vegetation. The horse being fed generally on less liquid food, and discharging less urine, yields a hotter and richer dung, which, however, answers best also when mixed with other varieties. The dung of the swine is soft andcold, like that of the cow, containing, like it, at least 75 per cent. of water. As this animal lives on more varied food than any other reared for the use of man, the manure obtained from it is also very variable in quality. Applied alone, as a manure to roots, it is said to give them an unpleasant taste, and even to injure the flavour of tobacco. It answers best for hemp, and, it is said, also for hops; but, mixed with other manures, it may be applied to any crop.
Night-soilis probably the most valuable, and yet, in Europe at least, the most disliked and neglected of all the solid animal manures. It varies no doubt in richness with the food of the inhabitants of each district,—chiefly with the quantity of animal food they consume,—but when dry, no other solid manure, weight for weight, can probably be compared with it in general efficacy. It contains much soluble and saline matter, and as it is made up from the constituents of the food weeat, of course it contains most of those elementary substances which are necessary to the growth of the plants on which we principally live.
Attempts have been made to dry this manure also, so as to render it more portable,—to destroy its unpleasant smell, so as to reconcile practical men to a more general use of it,—and by certain chemical additions, to prevent the waste of ammonia and other volatile substances, which are apt to escape and be lost when this and other powerful animal manures begin to putrify through decay. In Paris, Berlin, and other large cities, the night-soil, dried first in the air with or without a mixture of gypsum or lime, then upon drying plates, and finally in stoves, is sold under the name ofpoudrette, and is extensively exported in casks to various parts of the country. In London also it is dried with various mixtures, while in others of our large towns ananimalized charcoalis prepared by mixing and drying night-soil with gypsum and ordinary wood charcoal in fine powder.
The half-burned peat above described (p. 80,) would answer well for such a purpose, while few simple and easily attainable substances would make a better compost with night-soil, and more thoroughly preserve its virtues, than half-dry peat or richvegetable soil, mixed with more or less marl or gypsum. It is impossible to estimate the proportion of waste which this valuable manure undergoes by being allowed to ferment, without mixture, in the open air.
Taffo.—In China it is kneaded into cakes with clay, which are dried in the air, and, under the name of taffo, form an important article of export from all the large cities of the empire.
Pigeons’ Dung.—The dung of all birds is found to possess eminent fertilizing virtues. Some varieties are stronger than others, or more immediate in their action, and all are improved for the use of the farmer by being some time kept, either alone or in compost. In Flanders the manure of one hundred pigeons is considered worth 20s. a year for agricultural purposes.
Guanois the name given by the natives of Peru to the dung of sea-fowl, which in former periods used to be deposited in vast quantities on the rocky shores and isles of the Peruvian coast. The numerous shipping of modern times has disturbed and driven away many of the sea-fowl, so that comparatively little of their recent droppings is now preserved or collected. Ancient heaps of it, however, still exist in many places, more or less covered up with drifted sand, and also more or less decomposed. These are now largely excavated forexportation, not only to different parts of the coast of Peru, as seems to have been the case from the most remote periods, but also to Europe, and especially to England. It is at present sold at 20s. a cwt. in this country, and is capable of entirely replacing farm-yard dung,—that is to say, turnips may be manured successfully with guano alone;—but it has not yet been satisfactorily determined that the English farmer can afford to use it in this way to any extent, at the price now asked for it.
The dung of birds possesses the united virtues of both the liquid and solid excretions of other animals. It contains every part of the food of the bird, with the exception of what is absolutely necessary for the support and for the right discharge of the functions of its own body. It is thus fitted, therefore, to return to the plant a greater number of those substances on which plants live, than either the solid or the fluid excrements of other animals; in other words, to be more nourishing to vegetable growth.
The fertilizing power of animal manures, in general, is dependent, likethat of the soil itself, upon the happy admixture they contain of a great number, if not of all, those substances which are required by plants in the universal vegetation of the globe. Nothing they contain, therefore, is without its share of influence upon their general effects, yet the amount of nitrogen present in each affords the readiest and most simple criterion by which their agricultural value, compared with that of vegetable matters and with that of each other, can be pretty nearly estimated.
In reference to their relative quantities of nitrogen, therefore, they have been arranged in the following order, the number opposite to each representing the weight in pounds which is equivalent to or would produce the same sensible effect upon the soil as 100 lbs. of farm-yard manure.
It is probable that the numbers in this table do not err very widely from the true relative value of these different manures, in so far as theorganicmatter they severally contain is concerned. The reader will bear in mind, however,
1. That the most powerful substances in this table, woollen rags, for example,—2½ lbs. of which are equal in virtue to 100 lbs. of farm-yard manure,—may yet shew less immediate sensible effect upon the crop than an equal weight of sheep’s dung, or even of urine. Such dry substances are long in dissolving and decomposing, and continue to evolve fertilizing matter, after the softer and more fluid manures have spent their force. Thus, while farm-yard manure or rape dust will immediately hasten the growth of turnips, woollen rags will come into operation at a later period, and prolong their growth into the autumn.
2. That besides their general relative value, as represented in theabove table, each of these substances has a further special value not here exhibited, dependent upon the kind and quantity of saline and other inorganic matter which they severally contain. Thus three of dry flesh are equal to five of pigeons’ dung, in so far as theorganicpart is concerned; but the latter contains also a considerable quantity of bone-earth and of saline matter scarcely present at all in the former. Hence pigeons’ dung will benefit vegetation in circumstances where dry flesh would in some degree fail. So the liquid excretions contain much important saline matter not present in the solid excretions,—not present either in such substances as horn, wool, and hair,—and, therefore, each must be capable of exercising an influence upon vegetation peculiar to itself.
Hence the practical farmer sees the reason why no onesimplemanure can long answer on the same land; and why in all ages and countries the habit of employingmixedmanures and artificial composts has been universally diffused.
In what do animal manures differ from vegetable manures,—what is thecause of this difference,—how does the digestion of vegetable matter improve its value as a manure?
1. The characteristic distinction between animal and vegetable manures is this,—that the former contain a much larger proportion of nitrogen than the latter. This will be seen at once, by comparing together the tables given in the two preceding sections, in which the numbers represent the relative agricultural values of certain animal and vegetable substances compared with farm-yard manure. The lowest numbers represent the highest value, and the largest amount of nitrogen, and these low numbers are always opposite to the purest animal substances.
2. In consequence of containing so much nitrogen, animal substances are further distinguished by the rapidity with which, when moist, they putrify or run to decay. During this decay the nitrogen they contain gradually assumes the form of ammonia, which is perceptible by the smell, and which, when proper precautions are not taken, is apt in great part to escape into the air. Hence the loss by fermenting manure too completely,—or without proper precautions to prevent the escape of volatile substances. And as animal manure, when thus over-fermented, or permitted to lose its ammonia into the air, is found much less activeupon vegetation than before; it is reasonably concluded, that to this ammonia, chiefly, their peculiar virtue, when rightly prepared, is in a great measure to be ascribed.
Vegetable substances do not decay so rapidly,—do not emit the odour of ammonia when fermenting,—nor, when prepared in the most careful way, does vegetable manure exhibit the same remarkable action upon vegetable life as is displayed by almost every substance of animal origin.
3. Whence do animal substances derive all this nitrogen? Animals live only upon vegetable productions containing little nitrogen; can they then procure all they require from this source alone? Again, does the act of digestion produce any chemical alteration upon the food of animals, that their excretions should be a better manure,—should be richer in nitrogen than the substances on which they feed? Does theory throw any light upon the opinion generally entertained among practical men upon this point?
These two apparently distinct questions will be explained by a brief reference to one common natural principle.
Animals have two necessary vital functions to perform,—to breathe and to digest. Both are of equal importance to the health and generalwelfare of the animal. The digester (the stomach) receives the food, melts it down, extracts from it what is best suited to its purposes, and conveys it into the blood. The breathers (the lungs) sift the blood thus mixed up with the newly digested food, combine oxygen with it, and extract carbon,—which carbon, in the form of carbonic acid, they discharge by the mouth and nostrils into the air.
Such is a general description of these two great processes,—their effect upon the food that remains in the body and has to be rejected from it, is not difficult to perceive.
Suppose an animal to be full grown. Take a full grown man. All that he eats as food is intended merely to renovate or replenish his system, to restore that which is daily removed from every part of his body by natural causes.In the full grown state, every thing that enters the body must come out of the bodyin one form or another. The first part of the food that escapes is that portion of its carbon that passes off from the lungs during respiration. This quantity varies in different individuals—chiefly according to the quantity of exercise they take. From 5 to 9 ounces a day is the average quantity, though in periods of violent bodily exertion 13 to 15 ounces of carbon are breathed out in the form of carbonic acid.
Suppose a man to eat a pound and a half of bread and a pound of beef in 24 hours, and that he gives off by respiration 8 ounces of carbon (3500 grains) during the same time. Then he has
Our two conclusions, therefore, are clear. The vegetable food, by respiration, is freed from a large portion of its carbon, which is discharged into the air,—nearly the whole of the nitrogen remaining behind. In the food consumed the carbon was to the nitrogen as 9 to 1; in that which remains, after respiration has done its work, the carbon is to the nitrogen in the proportion of only 2 to 1.
It is out of this residue, rich in nitrogen, that the several parts of animal bodies are built up. Hence the reason why they can be formed from food poor in nitrogen, and yet be themselves rich in the same element.
It is this same residue also which, after it has performed its functions within the body, is discharged again in the form of solid and liquid excretions. Hence the greater richness in nitrogen,—the greater fertilizing power of the dung of animals than of the food on which they live.
Two other remarks I shall add for the benefit of the practical man.
1. The manure of the cow, taking it mixed, is not so rich in nitrogen as that of man,—because the cow in the stall, large though it be, and great the bulk of food it consumes, does not give off much more carbon by respiration than an active full grown man. Hence the proportion of carbon in the excretions of this animal is greater than in those of man. The dry manure is richer than the dry food, weight for weight, but not in the same proportion as if the cow respired a quantity of carbon more nearly corresponding to its bulk, when compared with the weight of carbon thrown off from the lungs of man.
2. Since the parts of animals—their blood, muscles, tendons, and the gelatinous portion of the bones—contain much nitrogen, young beasts which are growing, must appropriate to their own use, and work up into flesh and bone, a portion of the nitrogen contained in thenon-respiredpart of their food. But the more they thus appropriate, the less will pass off into the fold-yard; and hence it is natural to suppose that the manure, either liquid or solid, which is prepared where many growing cattle are fed, will not be so rich as that which is yielded by full grown animals. I am not aware how far this deterioration has been observed in practice, but it may with some degree of certainty be expected to take place,—unless by giving a richer food to the young cattle, the difference to the farm-yard be made up.[18]
The general nature and mode of operation of such mineral substances as are capable of acting as manures, will be in some measure understood from what has already been so fully stated in regard to the necessity of inorganic food to living plants, and to the kinds of such food whichthey specially require. A slight notice, therefore, of the more important of these manures now in use will here be sufficient.
1.Nitrates of Potash and Soda.—Saltpetre and nitrate of soda have been deservedly commended for their beneficial action, especially uponyoungvegetation. They are distinguished by imparting to the leaves a beautiful dark green colour, and are applied with advantage to grass and young corn, at the rate of 1 cwt. to ½ cwt. per acre. The nitric acid they contain yields nitrogen to the plant, while potash and soda are also put within reach of its roots, and no doubt serve many beneficial purposes.
Sulphate of Soda, or Glauber’s salt, has lately been recommended in this country for clovers, grasses, and green crops. Mixed with nitrate of soda it produces remarkable crops of potatoes.[19]
Sulphate of Magnesia, or Epsom salts, might also be beneficially applied in agriculture, probably to clovers and corn crops. As it can be had in pure crystals at 10s. a cwt., and in an impure state at a much less price, from the alum works, it might readily be submitted to trial.
Sulphate of Lime, or Gypsum, is in Germany applied to grasslands with great success, over large tracts of country. In the United States it is used for every kind of crop. It is especially adapted to clovers and legumes.
These three substances all afford sulphur to the growing plant, while the lime, soda, and magnesia are themselves in part directly appropriated by it, and in part employed in preparing other kinds of food, and in conveying them into the ascending sap.
Though there can be no question that these and similar substances are really useful to vegetation, yet the intelligent reader will not be surprised to find, or to hear, that this or that mineral substance has not succeeded in benefitting the land in this or that district. If he has already bricks enough at hand, you must carry the builder mortar, or he will be unable to go on with his work: so, if the soil contain gypsum or sulphate of magnesia in sufficient natural abundance, it is at once a needless and a foolish waste to attempt to improve the land by adding more; it is still more foolish to conclude that these same saline compounds are unlikely to reward the patient experimenter in other localities.
Common Salthas undoubtedly, in very many districts, a fertilizing influence upon the soil. The theoretical agriculturist knows that a small quantity of it is absolutely necessary to the healthygrowth of all our cultivated crops, and he will therefore, early try by a preliminary experiment upon one of his fields, whether or not they require the addition of this species of vegetable food. It is in inland and sheltered situations, and on high lands often washed by the rains, that the effect of common salt is likely to be most appreciable. The spray of the sea, borne to great distances by the winds, is in many districts, where prevailing sea winds are known, sufficient to supply an ample annual dressing of common salt to the land.
Kelp.—Among mineral substances kelp ought not properly to be included, since it is the ash left by the burning of sea-weed. It, however, partakes of the nature of mineral substances, and may, therefore, be properly considered in this place. It contains potash, soda, silica, sulphur, chlorine, and several other of the inorganic constituents of plants required by them for food. It is nearly the same also—with the exception of the organic matter which is burned away—with the sea-weed which produces such remarkably beneficial effects upon the soil. In the Western Isles a method is practised of half burning or charring sea-weed, by which it is prevented frommelting together, and is readily obtained in the form of a fine black powder. The use of this variety ought to combine the beneficial action of the ordinary saline constituents of kelp, with the remarkable properties observed in animal and vegetable charcoals.
Wood-ash, among other compounds, contains a portion of commonpearl-ashin an impure form, with sulphate also, andsilicateof potash. These are all valuable in feeding and in preparing the food of plants, and hence the extensive use of wood-ash as a manure in every country where it can readily be procured.
Dutch ashesare the ashes of peat burned for the purpose of being applied to the land. They vary in constitution with the kind of peat from which they have been prepared. They often contain traces of potash and soda, and generally a quantity of gypsum and carbonate of lime, a trace of phosphate of lime, and much siliceous matter. In almost every country where peat abounds, the value of peat ashes as a manure has been more or less generally recognised.
The use of lime is of the greatest importance in practical agriculture.It has been employed, in Europe at least, in one or other of its forms of shells, shell-sand, marl, chalk, limestone, and quicklime, from the most remote periods.
Native limestone, and all the unburned varieties of chalk, shells, &c. consist ofcarbonate of lime(p. 51), more or less pure. When burned in the kiln, the carbonic acid is driven off, and lime, burned lime, or quicklime remains.
Quicklime, when exposed to the air, gradually falls into the state of an exceedingly fine white powder. It will do so more rapidly if water be thrown upon it, when it also heats much, swells, and becomes about one-third heavier than before. After being exposed to the air for some time in this white powdery state, it is found to have again absorbed from the air a portion of carbonic acid, though a very long period generally elapses before it is all reconverted into carbonate. In compost heaps, where much carbonic acid is formed during the fermentation, the conversion of any quicklime that may be mixed with them into carbonate of lime, is much more rapid and complete than in the open air.
Lime, therefore, is laid on the land in two states.
1st, In themildstate—that of carbonate—in marls, in chalk, in shell-sand, &c.
2d, In thecaustic, or quick state, as it comes hot from the kiln, or after it is simply slaked.
Limes are laid on also in a more or less pure form. Marl contains only from 5 to 20 per cent. of carbonate of lime, generally in the state of a very fine powder. Shell-sand consists of a mixture of minute fragments of shells with from 20 to 50 per cent. of siliceous sand. The limestones which are burned are also more or less impure, though, when the impurity is very great, they do not burn well, and are therefore usually rejected.
Some limestones contain much magnesia, by which their agricultural qualities are materially affected. These are known by the name ofmagnesianlimestones. There are few limestones in which a small quantity of magnesia may not be detected, and this minute proportion is likely to be beneficial rather than otherwise; but when it is present to the amount of 10 per cent. or upwards, it appears to have for some time a poisonous influence upon vegetation, if added in the same large doses in which other lime may be safely spread upon the land.
The quantity of lime laid on at a single dressing, and the frequency with which it may be repeated, must depend upon the kind of land, uponthe depth of the soil, and upon the species of culture to which it is subjected. If land be wet, or badly drained, a larger application is necessary to produce the same effect, and it must be more frequently repeated. When the soil is thin, again, a smaller addition will thoroughly impregnate the whole, than where the plough usually descends to the depth of 8 or 10 inches. On old pasture lands, where the tender grasses live in two or three inches of soil only, a feebler dressing, more frequently repeated, appears to be the more reasonable practice, though in reclaiming and laying down lands to grass, a heavy first liming is often indispensable.
In arable culture larger doses are admissible, both because the soil through which the roots penetrate must necessarily be deeper, and because the tendency to sink beyond the reach of the roots is generally counteracted by the frequent turning up of the earth by the plough. Where vegetable matter abounds, much lime may be usefully added, and on stiff clay lands after draining, its good effect is most remarkable. On light land, chiefly because there is neither moisture nor vegetable matter present in equal quantity, very large applications of lime are not so usual, and some prefer adding it to such lands in the shape of composts only.
The largest doses, however, which are applied in practice, alter in a very immaterial degree the chemical constitution of the soil. We have seen that the best soils generally contain a natural proportion of lime, not fixed in quantity, yet scarcely ever wholly wanting. But an ordinary liming, when well mixed up with a deep soil, will rarely amount toone per cent.of its entire weight. It requires about 300 bushels of burned lime per acre to add one per cent. of lime to a soil of twelve inches in depth; if only mixed to a depth of six inches, this quantity would add about two per cent. to the soil.
The most remarkable visible alterations produced by liming are—uponpastures, the greater fineness, closeness, and nutritive character of the grasses—onarable lands, the improvement in the texture and mellowness of stiff clays, the more productive crops and the earlier period at which they ripen.
But these effects gradually diminish year by year, till the land returns again nearly to its original condition. On analyzing the soil, the lime originally added is found to be in great measure, or altogether, gone. In this condition the land must either be limed again, or must be left to produce sickly and un-remunerating crops.
This removal arises from two causes. The rain-water that descends upon the land holds in solution carbonic acid which it has absorbed from the air. But water charged with carbonic acid is capable of dissolving carbonate of lime, and thus year after year the rains slowly remove as they sink to the drains, or run over the surface, a portion of the lime which the soil contains. Acid substances are also formed naturally in the land, by which another portion of the lime is rendered easily soluble in water, and, therefore, readily removable by every shower that falls.
Thechemicaleffects of lime upon the soil are chiefly the following:—
1. When laid upon the land in thecausticstate, the first action of the lime is to combine immediately with every portion of acid matter it may contain, and thus to sweeten the soil. Some of the compounds it thus forms being soluble in water, either enter into the roots and feed the plant,—supplying it at once with lime and with organic matter,—or are washed out by the springs and rains, while other compounds, which are insoluble, remain more permanently in the soil.
2. Another portion decomposes certain saline compounds of iron, manganese, and alumina, which naturally form themselves in the soil,and thus renders them unhurtful to vegetation. A similar action is exerted upon certain compounds of potash and soda, and of ammonia,—if any such are present,—by which these substances are set and placed within the reach of the plant.
3. Its presence in the caustic state further disposes the organic matter of the soil to undergo more rapid decomposition—it being observed, that where lime is present in readiness to combine with the substances produced during the decay of organic matter, that decay, if other circumstances be favourable, will proceed with much greater rapidity. The reader will not fail to recollect, that during this decay many compounds are formed which are of importance in promoting vegetation.
4. Further, quicklime has the advantage of being soluble in cold water, and thus the complete diffusion of it through the soil is aided by the power of water to carry it in solution in every direction.
5. When it has absorbed carbonic acid, and become reconverted into carbonate, the original caustic lime has nochemicalvirtue over chalk, rich shell-sand or marl, or crushed limestone. It has, however, the importantmechanicaladvantage of being in the form of a far finer powder, than any to which we could reduce the limestone byart—in consequence of which it can be more uniformly diffused through the soil, and placed within the reach of every root, and of almost every particle, of vegetable matter that is undergoing decay. I shall mention only three of the important purposes which, in this state ofcarbonate, lime serves upon the land.
1. It directly affords food to the plant, which, as we have seen, languishes where lime is not attainable. It serves also to convey other food to the roots in a state in which it can be made available to vegetable growth.
2. It neutralizes (removes thesourness) of all acid substances as they are formed in the soil, and thus keeps the land in a condition to nourish the tenderest plants. This is one of the important agencies of shell-sand when laid on undrained grass lands—and this effect it produces in common with wood-ashes, and many similar substances.
3. During the decay of organic matter in the soil, it aids and promotes the production of nitric acid,—so influential, as I believe, in the general vegetation of the globe (see page 35). With this acid it combines and formsnitrate of lime—a substance very soluble in water—entering readily, therefore, into the roots of plants, and producing upon their growth effects precisely similar tothose of the now well knownnitrate of soda. The success of frequent ploughings, harrowings, hoeings, and other modes of stirring the land, is partly owing to the facilities which these operations afford to the production of this and other natural nitrates.
The irrigation of the land is, in general, only a more refined method of manuring it. The nature of the process itself, however, is different in different countries, as are also the kind and degree of effect it produces, and the theory by which these effects are to be explained.
In dry and arid climates, where rain rarely falls, the soil may contain all the elements of fertility, and require only water to call them into operation. In such cases, as in the irrigations practised so extensively in eastern countries, and without which, whole provinces in Africa and Southern America would lie waste, it is unnecessary to suppose any other virtue in irrigation than the mere supply of water it affords to the parched and cracking soil.
But in climates such as our own, there are two other beneficialpurposes in reference to the soil, which irrigation may, and one at least of which it always does, serve.
2. The occasional flow ofpurewater over the surface, as in our irrigated meadows, and its descent into the drains, where the drainage is perfect, washes out acid and other noxious substances naturally generated in the soil, and thus purifies and sweetens it. The beneficial effect of such washing will be readily understood in the case of peat lands laid down to water meadow, since, as every one knows, peat soils abound in matters unfavourable to general vegetation, and which are usually in part drawn off by drainage, and in part destroyed by lime and by exposure to the air, before boggy lands can be brought into profitable cultivation.
3. But it seldom happens thatpurewater is employed for the purposes of irrigation. The water of rivers, more generally, is diverted from its course, more or less loaded with mud and other finer particles of matter, which are either gradually filtered from it as it passes over and through the soil, or in the case of floods subside naturally when the waters come to rest. Or in less frequent cases, the drainings of towns, and the waters from common sewers, or from the little streams enriched by them, are turned with benefit upon thefavoured fields. These are evidently cases of gradual and uniform manuring. And even where the water employed is clear and apparently undisturbed by mud, it always contains saline substances grateful to the plant in its search for food, and which it always contrives to extract more or less copiously as the water passes over its leaves or along its roots. Every fresh access of water affords the grass in reality another liquid manuring.
In the refreshment continually afforded to the plant by a plentiful supply of water, in the removal of noxious substances from the soil, or in the frequent additions of enriching food to the land—the efficiency of irrigation, therefore, seems entirely to consist.
A mode of improvement often resorted to is the paring and burning of poor land. The efficacy of burned clay, also, even in superseding manure on good lands, has been highly extolled by some practical men.
1. The effect of paring and burning is easily understood. The matted sods consist of a mixture of much vegetable with a comparatively smallquantity of earthy matter. When these are burned the ash of the plants only is left, intimately mixed with the calcined earth. To strew this mixture over the soil is much the same as to dress it with peat or wood ashes, the beneficial effect of which upon vegetation is almost universally recognised. And the beneficial influence of the ash itself is chiefly due to the ready supply of inorganic food it yields to the seed, and to the effect which the potash and soda it contains exercise either in preparing organic food in the soil, or in assisting its digestion and assimilation in the interior of the plant.
Another part of this process is, that the roots of the weeds and poorer grasses are materially injured by the paring, and that the subsequent dressing of ashes is unfavourable to their further growth.
2. Much greater uncertainty hangs over the alleged virtues of burned clay. That benefits are supposed to have been derived from its use there can be no doubt, though in many cases the better tillage of the land generally prescribed along with the use of burned clay, may have had some share in producing the good results actually experienced during its use.
By the burning, in kilns or otherwise, any organic matter the clay may contain will be consumed, and the texture of the clay itself will be mechanically altered. It will crumble down like a burned brick into a hard friable powder, and will never again cohere into a paste as before the burning. It will, therefore, render clay soils more open, and may thus, when mixed in large quantity, produce a permanent amelioration in the mechanical texture of many stiff wheat soils. It cannot itself undergo any chemical change that is likely so to alter its constitution as to make it a more useful chemical constituent of the soil than before. Any saline matter we may suppose to be set free could be far more cheaply added in the form of a top-dressing to the soil.
Bricks, however, are generally more porous than the clay from which they are formed; burned clay is so also. And all porous substances suck in andcondensemuch air and many vapours in large quantities into their pores. In consequence of this property, porous substances, like charcoal and burned clay, are supposed, when mixed with the soil, to be continually yielding air to decaying vegetable matter on the one hand, and as continually re-absorbing it from the atmosphere on the other, and by this means to be of singular service in supplying thewants of plants in the earlier seasons of their growth. The vapours of nitric acid and of ammonia, which float in the air, they are also supposed to imbibe, and by the beneficial action of the substances believed to be thus conveyed by burned clay into the soil, the fertilizing virtues ascribed to it are attempted to be explained.
It must be confessed, however, that on this point considerable obscurity still rests. It is in some measure doubtful what the true action of charcoal and of burned clay is, both inkindand inquantity. It is the part of science, therefore, to decline offering more than a mere conjecture till the facts to be explained are more fully and satisfactorily demonstrated.
1.Planting.—It has been observed that lands which are unfit for arable culture, and which yield only a trifling rent as natural pasture, are yet in many cases capable of growing profitable plantations, and of being greatly increased in permanent value by the prolonged growth of wood. Not only, however, do all trees not thrive alike on the same soil, but all do not improve the soil on which they grow in an equal degree.
Under the Scotch fir, for example, the pasture is not worth 6d. more per acre than before it was planted—under the beech and spruce, it is worth even less than before, though the spruce affords excellent shelter;—under ash, it gradually acquires an increased value of 2s. or 3s. per acre. In oak copses, it becomes worth 5s. or 6s., but only during the last eight years (of the twenty-four), before it is cut down. But under the larch, after the first thirty years, when the thinnings are all cut, land not worth originally more than 1s. per acre, becomes worth 8s. to 10s. per acre for permanent pasture.[20]
The cause of this improvement is to be found in the nature of the soil, which gradually accumulates beneath the trees by the shedding of their leaves. The shelter from the sun and rain which the foliage affords, prevents the vegetable matter which falls from being so speedily decomposed, or from being so much washed away, and thus permits it to collect in larger quantities in a given time, than where no such cover exists. The more complete the shelter, therefore, the more rapid will the accumulation of soil be in so far as it depends upon this cause.
But the quantity of leaves which annually falls has also much influence upon the extent to which the soil is capable of being improved by any given species of tree, as well as the degree of rapidity with which those leaves, under ordinary circumstances, undergo decay. The broad membranous leaf of the beech and oak decay more quickly than the needle-shaped leaves of the pine tribes, and this circumstance may assist in rendering the larch more valuable as a permanent improver.
We should expect likewise that the quantity and quality of the inorganic matter contained in the leaves,—brought up year by year from the roots, and strewed afterwards uniformly over the surface where the leaves are shed,—would materially affect the value of the soil they form. The leaves of the oak contain about 5 per cent. of saline and earthy matter, and those of the Scotch fir less than 2 per cent.; so that, supposing the actual weight of leaves which falls from each kind of tree to be equal, we should expect a greater depth of soil to be formed in the same time by the oak than by the Scotch fir. I am not aware of any experiments on the quantity of ash left by the leaves of the larch.
The improvement of the land, therefore, by the planting of trees, depends in part upon the quantity oforganicfood which thetrees can extract from the air, and afterwards drop in the form of leaves upon the soil, and in part upon the kind and quantity ofinorganicmatter which the roots can bring up from beneath, and in like manner strew upon the surface. The quantity and quality of the latter will, in a great measure, determine the kind of grasses which will spring up, and the consequent value of the pasture in the feeding of stock. In the larch districts of the Duke of Atholl, the most abundant grasses that spring up are said to be the holcus mollis and the holcus lanatus, (thecreepingand themeadowsoft-grasses.)
2.Laying down to grass.—On this point two facts seem to be pretty generally acknowledged:
First, that land laid down to artificial grasses for one, two, three, or more years, is in some degree rested or recruited, and is fitted for the better production of after-corn crops. Letting it lie a year or two longer in grass, therefore, is one of the received modes of bringing back to a sound condition a soil that has been exhausted by injudicious cropping.
Second, that land thus laid down with artificial grasses deteriorates more or less after two or three years, and only by slow degrees acquires a thick sward of rich and nourishing herbage. Hencethe opinion, that grass-land improves in quality the longer it is permitted to lie,—the unwillingness to plough up old pasture,—and the comparatively high rents which, in some parts of the country, old grass lands are known to yield.
Granting that grass lands do thusgenerallyincrease in value,threeimportant facts must be borne in mind before we attempt to assign the cause of this improvement, or the circumstances under which it is likely to take place for the longest time and to the greatest extent.
1. The value of the grass in any given spot may increase for an indefinite period—but it will never improve beyond a certain extent—it will necessarily be limited, as all other crops are, by the quality of the land. Hence the mere laying down to grass will not makealllandgood, however long it may lie. The extensive commons, heaths, and wastes, which have been in grass from the most remote times, are evidence of this. They have in most cases yielded so poor a herbage as to have been considered unworthy of being enclosed as a permanent pasture.
2. Some grass lands will retain the good condition they thus slowly acquire for a very long period, andwithout manuring, in the same way, and upon the same principle, that some rich corn lands haveyielded successive crops for 100 years without manure. The rich grass lands of England, and especially of Ireland, many of which have been in pasture from time immemorial, without, it is said, receiving any return for all they have yielded, are illustrations of this fact.
3. But that others, if grazed, cropped with sheep or meadowed, will gradually deteriorate, unless some proper supply of manure be given to them,—which required supply must vary with the nature of the soil, and with the kind of treatment to which it has been subjected.
In regard to the acknowledged benefit of laying down to grass, then, two points require consideration,—what form does it assume?—and how is it effected?
1. The improvement takes place by the gradual accumulation of a dark-brown soil on the surface, rich in vegetable matter: and which soil thickens or deepens in proportion to the time which elapses from its being first laid down to grass.
If the soil be very light and sandy, the thickening is sooner arrested; if it be moderately heavy land, the improvement continues for a longer period; and some of the heaviest clays in England are known to bear the richest permanent pastures. On analyzing the soils of the richest of these pastures, whatever be the degree of tenacity of the clays or loams(the subsoils) on which they rest, or their deficiency in vegetable matter,—they are found to be generally characterized by containing from 8 to 12 per cent. of organic, chiefly vegetable matter, from 5 to 10 only of alumina, and from 1 to 6 per cent. of lime.
Thus the soil formed on the surface of all rich old pasture lands is possessed of a remarkable degree of uniformity,—both in physical character and in chemical composition. This uniformity they graduallyacquire, even upon the stiff clays of the Lias and of the Oxford clay, which originally, no doubt, have been,—as many clay lands still are,—left to natural pasture from the difficulty and expense of submitting them to arable culture.
2. But how do they acquire this new character, and why is it the work of so much time? When the young grass throws up its leaves into the air, from which it derives so much of its nourishment, it throws down its roots into the soil in quest of food of another kind. The leaves may be mown or cropped by animals, and carried off the field, but the roots remain in the soil, and, as they die, gradually fill its upper part with vegetable matter. It is not known what average proportion the roots of the natural grasses bear to the leaves; no doubt it variesmuch, both with the kind of grass and with the kind of soil. When wheat is cut down, the quantity of straw left in the field, in the form of stubble and roots, is sometimes greater than the quantity carried off in the sheaf. Upon a grass field two or three tons of hay may be reaped from an acre; and if we suppose only a tenth part of this quantity to die every year in the form of roots or parts of roots, or of excretions from roots, we can easily understand how the vegetable matter in the soil thus gradually accumulating, should at length become very considerable in quantity. In arable land this accumulation is prevented by the constant turning up of the soil, by which the vegetable fibres being exposed to the free access of air and moisture, are made to undergo a more rapid decomposition.
But the roots and leaves of the grasses contain inorganic earthy and saline matter also. Dry hay leaves from an eighth to a tenth part of its weight of ash when burned. Along with the dead vegetable matter of the soil, this inorganic matter accumulates also on the surface, in the form of an exceedingly fine earthy powder; henceonecause of the universal fineness of the surface mould of old grass fields. And the earthy portion of this inorganic matter consists chiefly of silicaand lime, with scarcely a trace of alumina, so that, even on the stiffest clays, a surface soil may be ultimately formed, in which the quantity of alumina will be comparatively small.
But there are still other agencies at work by which the surface of stiff soils is made to undergo a change. As the roots penetrate into the clay, they more or less open up a way into it for the rains. Now the rains in nearly all lands, when they have a passage downwards, have a tendency to carry down the clay along with them. They do so, it has been observed, on sandy and peaty soils, and more quickly when these soils are laid down to grass. Hence the mechanical action of the rains,—slowly in many localities, yet surely,—has a tendency to lighten the soil, by removing a portion of its clay. They constitute one of those natural agencies by which, as elsewhere explained, important differences are ultimately established, almost everywhere, between the surface crop-bearing soil and the subsoil on which it rests.
But further, the heats of summer and the frosts of winter aid this slow alteration. In the extremes of heat and of cold, the soil contracts more than the roots of the grasses do; and similar though less striking differences take place during the changes of temperature experienced inour climate in a single day. When the rain falls on the parched field, or when a thaw comes on, the earth expands, while the roots of the grasses remain nearly fixed; hence the soil rises up among the leaves, mixes with the vegetable matter, and thus assists in the slow accumulation of a rich vegetable mould.
The reader has witnessed in winter how, on a field or a by-way side, the earth rises above the stones, and appears inclined to cover them; he may even have seen in a deserted and undisturbed highway, the stones gradually sinking and disappearing altogether, when the repetition of this alternate contraction and expansion of the soil for a succession of winters has increased in a great degree the effects which follow from a single accession of frosty weather.
So it is in the fields. And if a person skilled in the soils of a given district can make a guess at the time when a given field was laid down to grass, by the depth at which the stones are found beneath the surface, it is because this loosening and expansion of the soil, while the stones remain fixed, tends to throw the latter down by an almost imperceptible quantity every year that passes.
Such movements as these act in opening up the surface-soil, in mixing it with the decaying vegetable matter, and in allowing the slow actionof the rains gradually to give its earthy portion a lighter character. But with these, among other causes, conspire also the action of living animals. Few persons have followed the plough without occasionally observing the vast quantities of earthworms with which some fields seem to be filled. On a close shaven lawn many have noticed the frequent little heaps of earth which these worms during the night have thrown out upon the grass. These and other minute animals are continually at work, especially beneath an undisturbed and grassy sward—and they nightly bring up from a considerable depth, and discharge on the surface, their burden of fine fertilizing loamy earth. Each of these burdens is an actual gain to the rich surface soil, and who can doubt that in the lapse of years, the unseen and unappreciated labours of these insect tribes must both materially improve its quality and increase its depth?
There are natural causes, then, which weknowto be at work, that are sufficient to account for nearly all the facts that have been observed, in regard to the effect of laying lands down to grass. Stiff clays will gradually become lighter on the surface, and if the subsoil be rich in all the kinds of inorganic food which the grasses require,will go on improving for an indefinite period without the aid of manure. Let them, however, be deficient, or let them gradually become exhausted of any one kind of this food, and the grass lands will either gradually deteriorate after they have reached a certain degree of excellence—or they must be supplied with that ingredient—that manure of which they stand in need. It is doubtful if any pasture lands are so naturally rich as to bear to be cropped for centuries without the addition of manure, and at the same time without deterioration.[21]
On soils that are light, again, which naturally contain little clay, the grasses will thrive more rapidly, a thick sward will be sooner formed, but the tendency of the rains to wash out the clay may prevent them from ever attaining that luxuriance which is observed upon the old pastures of the clay lands.
On undrained heaths and commons, and generally on any soil which is deficient in some fertilizing element, neither abundant herbage, nor good crops of any other kind, can be expected to flourish. Laying such lands down, or permitting them to remain in grass, may prepare them for by-and-by yielding one or two average crops of corn, but cannot be expectedaloneto convert them into valuable pasture.
Finally, plough up the old pastures, on the surface of which this light and most favourable soil has been long accumulating—and the heavy soil from beneath will be again mixed up with it—the vegetable matter will disappear rapidly by exposure to the air,—and if again laid down to grass, the slow changes of many years must again be begun through the agency of the same natural causes, before it become capable of again bearing the same rich herbage it was known to nourish while it lay undisturbed.
Many have supposed that by sowing down with thenaturalgrasses, a thick sward may at once be obtained—and on light loamy lands, rich in vegetable matter, this method may, to a certain extent, succeed—but on heavy lands, in which vegetable matter is defective, disappointment will often follow the sowing of the most carefully selected seeds. By the agency of the causes above adverted to—the soil gradually changes, so that it is unfit, when first laid down, to bear those grasses which, ten or twenty years afterwards, will naturally and luxuriantly grow upon it.