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.
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.
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 asolidstate; and as itdoes 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 enterdirectlyinto 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 ofchemical combinationwith other substances; the carbon chiefly in the state ofcarbonic 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.
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 (muriaticacid) be poured upon them, a boiling up oreffervescencewill 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 fermentationof 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 (calledhumusby 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 ofa 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 ishumicacid. But if in this process we use spirit of hartshorn (liquid ammonia), instead of the soda,ulmicacid 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.
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 isproducedduring the burningof 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, ormust 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 aredissolved. Water has the power of thus dissolving numerous other substances in greater or less quantity. Hence, when the rain falls andsinks 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 decomposedartificiallyinto these two gases. The same thing takes placenaturallyin 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 ofall 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 isinvariably 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 ofaquafortis. 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 formsnitrate 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 nitrogenmixedtogether, but when an electric spark is passed through aquantity of air, a certain quantity of the twounitetogether 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 thenitratesabove 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 thunderorlightning 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]
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 servesprincipally 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.