The water of the Artesian well at Grenelle, near Paris, according to the analysis of M. Payen, contains, in 100,000 parts: 1. OF THE ORGANIC MATTER of manure and of crops. It is known that the atmosphere and the organic matters diffused through the earth concur simultaneously to maintain the life of plants; but how far each contributes is undetermined. We shall now study the theory of the exhaustion of the soil by culture, and the rotation of crops. When a succession of crops is grown upon fertile land without renewal of manure, the produce gradually diminishes; and after a certain period, if it be grain, the quantity which at the outset was eight or nine times the amount of the seed, will be reduced to three times or even to twice the seed. Thus crops impair the fertility of the soil, and eventually exhaust it. It has been long admitted that different species of plants manifest great diversity in their powers of exhaustion. Certain kinds, indeed, as trefoil and lucerne, far from exhausting it, communicate new vigor. As a general rule, however, every plant may be said to impoverish the soil in which it grows. This impoverishment is always manifest when the plant after maturity is completely removed, but is less sensible when much rubbish is left. Thus, for example, clover, after yielding two crops, which are generally cut as fodder, might still yield a third; this last, however, is generally ploughed into the ground as manure, being buried along with a considerable quantity of roots. This plan of meliorating the soil by the cultivation of trefoil is what is called manuring by smothering; a method practised from a remote period in the south of Europe, and which offers decided advantages in those districts where there is abundance of pasture land. Hence, in smothering trefoil, the soil is amended at the expense of the nutritive matter it contains. Thaer, who endeavored to make theory and practice mutually agree, laid it down as a rule, that the exhaustion occasioned by cropping is proportioned to the amount of nutriment in the crops, estimating the nutritive value according to Einhof's determination. But the above deduction is founded upon error. In fact, to adopt the above principle is tacitly admitting that the whole organic matter of plants originally comes from the soil. This, no doubt, contributes in a certain proportion to the development of plants, but so also do air and water. On the other hand, physiologists, in opposition to the ideas of the school of Thaer, have perhaps exaggerated the material withdrawn from the air. Thus, M. de Saussure reckons that a sun-flower derives from the ground during its growth not more thanth of its weight, supposing the plant dry. The reasoning upon which he formed his conclusion is based, on the one hand, upon a knowledge of the extractive matter of gardenmould; on the other, upon the quantity of water a plant like sunflower may absorb in a given time, to return it again to the air by transpiration.* Little objection could be urged against the above conclusion, did not the experiments of M. Gazzeri tend to prove that roots virtually exercise, by their contact with solid organic matter, an incontestable absorbent action in imparting solubility. I might refer to an observation of M. de Saussure, in which he states that plants grown in garden-mould deprived of its soluble components by repeated washing, reached, nevertheless, perfect maturity, although the produce in seed was less abundant than it might have been. It is most probable that both parties have promulgated extreme opinions. Plants possibly draw from the atmosphere more than agriculturists commonly suppose, and the soil furnishes, independently of saline and earthy substances, a proportion of organic matter larger than certain physiologists admit. There is every reason to believe, from what I could learn respecting guano during my sojourn on the coast of Peru, that the greater part of the azotized principles of plants originates in the ammoniacal salts which exist or are formed in ma nure. In discussing the advantage of one course of crops over another, the question always hinges upon that of exhaustion. Wherever an unlimited supply of dung and of handiwork can be procured, there is no absolute necessity for following any regular system of rotation. Under such favorable circumstances, it is expedient to ascertain what kind of cultivation is, commercially speaking, best suited to the climate and the soil. There is little to fear that by a continued succession of similar crops, the fields will get infested with noxious weeds, because this inconvenience may be obviated by labor. Nor is impoverishment of the soil to be dreaded, since that can be remedied by the purchase of manure. The whole craft of agriculture is reducible to comparison of the probable value of the crop with the cost of manure, labor, &c. Farming of this sort excludes the *Saussure, Recherches Chimiques sur la Végétation, p. 268 † Annales de l'Agriculture Française, No. iii. p. 57 ‡ Saussure, Recherches Chimiques, p. 171. ◊ Annales de Chimie, t. lxv. année 1837. keep and propagation of cattle, and may be strictly regarded more as gardening than as agriculture. But where manure cannot be had from without, things must be reduced to a system; and the amount of produce which it is possible to export each year is fixed within bounds, which cannot be exceeded with impunity. When by judicious cultivation land is rendered fertile, it is necessary, towards securing its fertility, to supply after every succession of crops equal quantities of manure. In considering this in a purely chemical point of view, it may be said that the produce which can be taken away without damaging the fertility of the land, is the organic matter contained in the crops, abstraction made of that present in the manure. Indeed, this latter substance must in some form or other return to the soil to fecundate it anew. It is capital placed in the ground, the interest of which is represented by the commercial value of the produce of all the other agricultural operations. Where lands are extensive, population scattered, and means of communication difficult, there is less necessity for being tied down to systematic cultivation. There is always enough for a scanty population. A field yields grain, and after the harvest is converted for a series of years into meadow-land; such is the pastoral system in all its simplicity. To this primitive state of husbandry may be referred those plantations on cleared land in countries covered with forests. When the trees are felled and burned upon the spot, the soil yields for long and without manure, crops of maize and of wheat of surprising quality, at the cost of the fecundity acquired during ages of repose. But when from increased population the land becomes more valuable, a larger amount of produce is demanded. Imperfect culture would prove inadequate. Accordingly a triennial rotation of crops was very anciently adopted in the north of Europe, consisting as is well known of fallow land frequently ploughed during summer, followed by two years of grain. The fallow land received a certain quantity of manure to repair the exhaustion occasioned by the two crops of grain; hence when this mode of rotation is adopted there should be always sufficient meadow-land to supply manure. Leaving waste one third of the surface has always been held a grave objection against triennial rotation. Hence various attempts have been made to get rid of the summer fallow. Some encouragement was given to these attempts from what occurs in horticulture, where the ground is rendered continually productive.* In certain countries, moreover, tillage is only interrupted by severe weather. On the other hand, it has been long remarked that it is not always beneficial to grow grain during several consecutive years in the same ground, even when it is fertile and manure is abundant, owing to the almost insurmountable difficulty of destroying weeds. The fallow was justly considered the most efficient and economic means of getting rid of these. For this purpose fallow-crops, as they were * Thaer, Agriculture raisonnée. called, were introduced. Peas, beans, vetches, were at first the only plants used as fallow-crops. However, it was soon perceived that the fallow-crops occasioned a very sensible diminution in the produce of corn; to counteract this inconvenience recourse was had to a surcharge of manure; but as this cannot always be obtained, it was necessary either to reduce the cultivated surface or to appropriate a certain amount of meadow. Still the fallow-crops had this advantage, that they enabled the farmer to derive from land a greater amount of produce in a given time without prejudice to the raising of corn. Hence the plan of turn ing the fallow to account was soon generally adopted. The introduction of clover so modified the system of fallow-crops as at one time to induce the belief that the point of perfection had been attained in agriculture.* This was when it was ascertained that trefoil, which had hitherto been only cultivated in small enclosures, might be sown in spring upon corn land, and occupy next year the place of the fallow in the triennial rotation. Trefoil, so far from exhausting the soil, was found to give it new fertility, and the succeeding corn crop yielded a plentiful harvest. It may be easily conceived what advantages were expected in substituting for the unproductive fallow the cultivation of a plant which did not impoverish the land, and furnished a quantity of excellent fodder that served as food for an additional number of cattle. It was even alleged that this plant cleared the fields of weeds. A few years' experience sufficed to show that trefoil did not possess all the advantages attributed to it. On renewing the clover every third year on the same piece of ground it sometimes failed. Schubarth, the most zealous and enlightened advocate for its use, limited the renewal of the artificial meadow at first to the sixth, and eventually to the ninth year; and finding that it did not completely destroy the weeds in corn, he had recourse to hoed-crops for that purpose. The introduction of trefoil has gradually led to the system of alternate rotation of crops generally adopted at present; and moreover, contrary to the anticipations of Schubarth, it may be renewed every four or five years on the same parcel of land. The impossibility of substituting trefoil for the fallow of the triennial rotation was offered as a fresh proof of the principle maintained from time immemorial by agriculturists, namely, that different species of plants should be cultivated in succession on the same land, and that the same species should not recur except at considerable intervals; the earth yielding much finer crops when the same species do not follow in immediate sequence.† Attempts have been made at various times to explain this phenomenon. It was at first thought that different species of vegetables required a particular nutriment; but it was soon perceived to be otherwise, and that the organs of each plant derived the necessary juices from substances which concr in the nutrition of vegetables generally. In effect, plants the most opposite in botanical character and properties, alimentary as well as poisonous, will live and flourish on the same mound of earth, and with the same manure. Moreover these plants reciprocally withdraw nourishment from one another, which could not occur did each species need different elements of nutrition.* When it was taken for granted that the organs of plants elaborate a common nourishment derived from the manure, then vegetables of diverse organizations were supposed endued with the faculty of searching at different depths for the nutritive matter contained in the soil, by reason of a more or less considerable extension and development of their roots. This served to explain how a plant with long and perpendicular roots could, as a sequel to corn, derive benefit from manure situate in the undermost layers of ploughed land. It is possible that an action of this kind may take place under certain circumstances, but the explanation can never be generally received. Another explanation of the necessity for alternate crops is based upon properties assigned to the excretions of the roots, as compared to animal excrements. The excretion of roots, first observed by Brugman in the Viola arvensis,† has been confirmed by the recent observations of M. Macaire. This physiologist obtained the matter exuded from certain plants by keeping their roots in water; but, strange to say, could not discover it in silicious sand in which certain vegetables had been grown. I myself likewise failed in detecting sensible traces of organic matter in sand which had served as soil during several months to wheat and clover; a result which renders the fact of radicular excretion doubtful. The excretion consequent upon immersion in water is perhaps the effect of disease. Be that as it may, upon the assumption of the excretion from roots, Messrs. Von Humboldt and Plenck have explained the cause of the attractions and repulsions of certain plants. § More recently M. de Candolle has reproduced this idea as the basis of a theory of rotation of crops. If it be supposed, in fact, that the excretion from the roots represents vegetable excrements, it may be easily imagined that these excretions once deposited in the soil may be as prejudicial to the plant which produced them as would be the excrement of an animal presented to it as food. On the other hand, by change of species, the plant newly implanted may profit by the excretions of the preceding crop, absorbing them as nourishment. This ingenious hypothesis is deficient in the groundwork, inasmuch as the fact of radicular excretion is not sufficiently established. Again, admitting the excretion, several facts concur to demonstrate that plants may thrive in soil charged with their own excrements. The culture of corn, for example, may proceed uninterruptedly, as we find in the triennial rotation. I have seen in the table-lands * De Candolle, t. i. p. 248. † Ibid. t. ii. p. 1497. De Candolle, t. iii. p. 1474 |