CHAPTER IV.
Of the Inorganic Constitution of Plants—Their immediate Source—Their Nature—Quantity of each in certain common Crops.
Of the Inorganic Constitution of Plants—Their immediate Source—Their Nature—Quantity of each in certain common Crops.
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 thereforealong 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 theinorganic foodof 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 acarbonate 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 acarbonate 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 acarbonate 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.Aluminais 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 nameoxide.
8.Oxide of Manganeseis 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 ofsulphuric 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 acidforms with potash asulphate 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 ironor green vitriol. When the sulphate of potash is combined with sulphate of alumina, it forms common alum.
10.Phosphorusis 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 ofphosphorus with oxygen obtained from the air, and are calledphosphoric 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 acidformsphosphateswith potash, soda, lime, and magnesia. When bones are burned, a large quantity of a white earth remains (bone-earth), which is aphosphate 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 thechloridesof 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.
1. The quantity of ash yielded bydifferent plantsis 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 ofpoplar 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 indifferent 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 otherwoodsleave 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 indifferent 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 thevarietyof 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, theGolden KentandFlanders Redwheats 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.
But much also depends upon thequalityas well as upon thequantityof the ash. Plants may leave the same weight of ash when burned, and yet the nature of the two specimens of ash, the kindof 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 ofbeanstraw contain 53½ lbs. of potash, while that ofbarleystraw 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 thesethree 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 wholequantityof 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 ofthesesubstances—as barley and oats will of others—henceonereason 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 ofdifferentkinds of grain may grow where it would greatly injure the soil to take two in succession of thesamekind, 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 oneinorganic 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 thepartof 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:—
Not only are the quantities of the several inorganic substanceskinds 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,onereason 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 yetbe ableto 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 isfavourable, 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.
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.
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 ofthe 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 themverified 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.