Page images

certained by their ashes after incineration, and constitute a very minute portion ; but, however minute, they are evidently essential to the perfection or fructification of the plant. Besides these there are certain organic acids, which are found in the juices of plants and usually combined with some inorganic bases. The alkaline bases or earths must exist in the soil, or they cannot be found in the plant. In some cases, however, one kind may be substituted for another.

The author discusses at large the doctrine of humus, humin, ulmin, humic acid, apotheme, geine, all referring to one substance, as the food of plants. This matter is generally understood to be a certain brown or carbonaceous substance resulting from vegetable decomposition. Some portions of it are soluble in water or alkalies ; oiber portions are insoluble but by extraordinary means. The common opinion has been that it constituted directly the food of plants, and required only to be dissolved to be taken up by the roots of the plants and assimilated by them. Others have maintained that it requires to be dissolved by the application of alkalies, and combining with them in the form of an acid, it becomes then prepared for the food of plants. Our author wholly denies these positions by showing that so far from humus being extracted from the soil, it is in fact increased by cultivation, as in the case of a forest, the more abundant the growth of wood upon it, the greater the amount of humus in the soil, where the débris of the wood is suffered to remain


the land. " A certain quantity of carbon is taken every year from the forest or meadow in the form of wood or hay ; and, in spite of this, the quantity of carbon in the soil augments; it becomes richer in humus.

“ The opinion that the substance called humus is extracted from the soil by the roots of plants, and that the carbon entering into its composition serves in some form or other to nourish their tissues, is so general and so firmly established, that hitherto any new argument in its favor has been considered unnecessary ; the obvious difference in the growth of plants according to the known abundance or scarcity of humus in the soil, seemed to afford incontestable proof of its correctness. Yet this position, when admitted to a strict examination, is found to be untenable ; and it becomes evident that humus in the form in which it exists in the soil does not yield the smallest nourishînent to plants.” – p. 61. VOL. LIII. No. 112.


[ocr errors]
[ocr errors]

He attempts to prove his position, that the carbon of the plant cannot be derived from the soil, by a calculation of weights and measures. Humic acid, or the humus of the soil, can only be absorbed by the plant in combination with some inorganic bases or metallic oxide. We do not think it important here to give anything more than the results of some of his calculations. He


that upon an average 40,000 square feet of land, Hessian measure, yield annually 2650 lbs. of dry fir wood, which contain 5.6 lbs. Hessian of metallic oxides. Now it is ascertained in what proportion humic acid combines with the metallic oxides, with lime for example. Having determined the metallic oxides existing in such a product, he easily determines the amount of humic acid thus introduced into the trees ; and, allowing humic acid to contain 58 per cent. of carbon, this would correspond only to the production of 91 lbs. Hessian of dry wood. But 2650 lbs. of fir wood are actually produced. These calculations are well worth examining, and, if accurate, it is difficult to deny the inference which follows from them, that the humic acid existing in a soil, supposing all its carbon to be taken up and assimilated, will supply but a very small portion of that which exists in the crop, grown upon the soil.

The same remarks are applied to a crop of wheat. From the known properties of metallic oxides existing in wheat straw the sulphates and chlorides also contained in the asbes of wheat straw not included), it would be found, that the wheat growing on 40,000 square feet Hessian of land would average 1780 lbs. Hessian of straw, independently of the roots and grain, and the composition of this straw is the same as that of woody fibre. Now, according to well-ascertained properties, it could receive but 57) lbs. of humic acid, which would supply with carbon only 85 lbs. Hessian of straw.

Another calculation respects the amount of humic acid which plants can receive through the agency of rain water.

The amount of rain falling in one of the most fertile districts of Germany, during the months of April, May, June, and July, is estimated to be 171 lbs. Hessian upon every square foot of surface, or upon 40,000 square feet Hessian, 700,000 lbs. Hessian of rain water. Now this extent of land averages a product of 2850 lbs. Hessian of corn (wheat); 390 lbs. of humic acid calculated to be absorbed in this case, cannot account for the quantity of carbon contained in the roots and

leaves alone, even if we suppose the whole of the rain water to be absorbed by the plants, whereas a large portion of it must necessarily be lost or pass off in some other form than through the organs of the plants. If these calculations be correct, it is evident that a small portion only of the carbon existing in plants can be derived from the bumus of the soil. Another idea is suggested, viz. that as humus results from the decay of plants, none existed at the time of the creation to form the pabulum of the primitive vegetation. This must have had other sources of supply. Dr. Dana is of opinion that geine or humus is an original creation, coeval with the creation of hydrogen and oxygen and carbon. The conjecture is sufficiently plausible, but it would be idle to advance any opinion on the subject. The only fact which can be said to favor one opinion above the other, is, that the plants found in the earliest coal formation are plants with small roots and expanded foliage, implying that they drew their chief nourishment from the air.

The inquiry which next arises, is, if plants do not derive their carbon, or but a very small portion of it from the soil, whence is it obtained ? This interesting question Liebig discusses at large, and certainly with much ability. The seed itself contains the first supply of nourishment for the roots of the infant germ of the plant. Before it appears above the surface, the humus in the soil quickens and invigorates its growth by the supply of carbonic acid. This supply of carbonic acid is furnished by the accession of atmospheric air from the loosening of the soil, the carbon of the humus combining with the oxygen of the air to produce nourishment for the young plant. When it rises above the surface, and its external organs of nutrition, its stem and its leaves, are fully developed, it ceases to draw nourishment from the earth and obtains all its carbon from the air. It is not a new doctrine that plants absorb carbonic acid from the atmosphere. This fact has been long established ; but it is new that this is the principal source; and the inquiry naturally arises whether the atmosphere, containing, as it does, only a thousandth part of carbonic acid, can furnish in this way a supply of all the carbon which is required by the plant. To this inquiry Liebig replies as before, by making it matter of exact calculation.

“It can be shown, that the atmosphere contains 3,000 billion Hessian lbs. of carbon ; a quantity which amounts to more than the weight of all the plants, and of all the strata of mineral and brown coal, which exist upon the earth. This carbon is therefore more than adequate to all the purposes for which it is required.” — p. 74.

The absorption of carbonic acid from the air, in his opinion is a purely chemical process. Many others have chosen to regard it as a vital operation ; and have considered the leaves as respiratory organs, resembling the lungs of animals. He does not admit the analogy, and thinks that the cause of science is injured by the supposition of a resemblance, where no similitude exists. The absorption of carbonic acid from the air, the assimilation of its carbon, and the return of its oxygen to the air, are chemical processes, effected under the operation of light and heat. Without the aid of chemistry, they are inexplicable ; with it, they become persectly intelligible. The vital action creates nothing. It does not produce carbon, oxygen, or hydrogen; but it puts them into activity ; and they then arrange themselves according to chemical principles; each organ of the plant having its own specific influence in the production of the results.

The author discusses, at large, the nature and action of humus. Humus is merely decayed vegetable substance, whose decay or destruction is effected by the absorption of oxygen from the air. Exclude it from the external air, and the decay would cease ; but would be renewed again as it should be brought in contact with the oxygen of the air. Woody fibre, in a state of decay, consists of carbon and the elements of water. Alkaline substances assist its decay. Humus, however, is not composed exclusively of woody fibre ; other substances are associated with it. We have not the room to follow Liebig in his curious and profound remarks on this subject, and can only give a summary of his views. The constant tendency of humus is to form carbonic acid by the abstraction of oxygen from the air. The stirring of the soil, and opening it to the effects of light and heat and moisture, assist this process, by bringing it in contact with the decaying humus. It forms around itself an atmosphere of carbonic acid, and supplies carbonic acid to the plant in the first period of its growth. The roots of the plants, in the beginning and before their formation, perform the functions of the leaves. The extract from the soil the carbonic acid generated by the humus. When a plant is matured, and when the organs, by which it receives its food from the air, are perfected, the carbonic acid of the soil is no further required. Humus does not afford nourishment to plants, by being taken up into their vessels in an unaltered state ; but only by the supply of carbonic acid, which it generates from the presence of atmospheric air.

Hydrogen is another constituent of plants ; for woody fibre is composed of carbon and the elements of water. Water is decomposed under the power possessed by plants of separating its elements, and of assimilating its hydrogen, and dispensing with that portion of its oxygen not required by the plant in other processes of its growth. Nitrogen is another constituent, found in all plants ; abounding in some, and supposed to form the principal portion of the nutritive properties of some of the cereal grains. The nitrogen of the air cannot enter into combination with any element excepting oxygen. The combination of nitrogen with hydrogen, in the proportion of one volume of nitrogen and three of hydrogen, produces ammonia. It is in the form of ammonia, that plants receive their nitrogen. This ammonia is furnished to the roots of the plants by the decomposition of animal matter in the soil, and to their leaves by the effluvia arising from decayed and decaying animal and vegetable substances. This decay is continually going on, and, together with the excrements of animals, supplies the ammonia contained in the atmosphere. There are, indeed, some natural subterranean sources of ammonia, connected with volcanic action; and ammonia is found in many springs, which, Liebig supposes, derive it wholly from the atmosphere. The principal part of the nitrogen, which is found in plants, is, in his opinion, obtained in the form of ammonia in rain water. Though it appears that it has been discovered by others, that rain water contains animonia, yet it is believed that Liebig has been the first to announce the fact. He goes on to show, by the elements made use of in a former calculation, that by means of the rain falling annually upon 40,000 square feet of soil, the field must receive 80 lbs. of ammonia, or 65 lbs. of nitrogen, which is more nitrogen than is contained in the amount of crops usually produced upon such a surface. The experiments made to ascertain the presence of ammonia in rain water, are decisive, and this interesting fact may be considered as now established. He likewise detected ammonia in the juices of the ma

« ՆախորդըՇարունակել »