THE FOOD OP VEGETABLKS. 



121 



tliat a change of crop will answer the purpose 

 of the cultivator; for although a soil may be ex- 

 hausted for one sort of grain, it does not neces- 

 sarily follow that it is also exhausted for another. 

 And, accordingly, the practice of the farmer is 

 to sow his crops in rotation, having in the same 

 field a crop, perhaps, of wheat, barley, beans, 

 and tares in succession ; each species selecting in 

 its turn some peculiar nutriment, or requiring, 

 perhaps, a smaller supply than the crop that has 

 preceded it. But even upon the plan of rota- 

 tion, the soil becomes at length exhausted, and 

 the cultivator obliged to have recourse to other 

 means of restoring its fertility. 



In this case, an interval of repose is consider- 

 ably efficacious, as may be seen from the increased 

 fertility of fields that have not been ploughed 

 up for many years, such as those used for pas- 

 ture ; or even from that of the walks and patlis 

 in gardens when they are again broken up. 

 Hence also the practice of fallowing, and of 

 trenching or deep plougliing, which must have 

 nearly the same effect. 



If any one asks how the fertility of a soil is 

 restored by the means now stated, it will be 

 sufficient for the object of the present section to 

 reply that, in the case of draining, the ameliora- 

 tion is effijcted by means of its carrying off all 

 Buch superfluous moisture as maybe lodged in the 

 soil, which is well known to be prejudicial to 

 plants not naturally aquatics, as well as by ren- 

 dering the soil more fiiTn and compact. In the 

 case of burning, the amelioration is effected by 

 means of the decomposition of the vegetable 

 substances contained in the turf, and subjected 

 to the action of the fire, which disperses part 

 also of the superfluous moisture, but leaves a 

 residue of ashes favourable to future vegetation. 

 In the case of the rotation of crops, the fertility 

 is not so much restored as more completely de- 

 veloped and brought into action; because the 

 soil, though exhausted for one species of gi'ain, 

 is yet found to be sufficiently fertile for another, 

 the food necessary to each being different, or re- 

 quired in less abundance. It has also been sup- 

 posed that plants growing long in the same soil, 

 throw out a particular excretion, which is inim- 

 ical to jdants of the same species, though harm- 

 less to those of different families and species. 



In the case of the repose of the soil, the res- 

 tored fertility may be owing to the decay of 

 vegetable substances that are not now carried 

 off in the annual crop, but left to augment the 

 proportion of vegetable mould; or to the accum- 

 ulation of fertilizing particles conveyed to the 

 soil by rains; or to the continued abstraction of 

 oxygen from the atmosphere. In the case of 

 fallows, it is owing undoubtedly to the action of 

 the atmospheric air upon the soil, whether in 

 rendering it more friable, or in hastening the 

 putrefaction of noxious plants ; or, it is owing 



to the abstraction and accumulation of oxygen. 

 In the case of trenching, it is owing to the Id- 

 creased facility with which the roots can now 

 penetrate in the proper depth ; and in the caso 

 o-i deep ploughing, it is owing, as it would ap- 

 pear, to the same cause. 



But it often happens that the soil can no longer 

 bo ameliorated by any of the foregoing means, 

 and in this case there mu.;t bo a direct and actual 

 application made to it of such substances as aro 

 fitted to restore its fertility. And hence the in- 

 dispensable necessity of manures, which consist 

 chiefly of animal and vegetable remains that are 

 burled and finally decomposed in the soil, from 

 which they aro afterwards absorbed by the root 

 of the plant, in a state of solution. 



But as carbon is the principal ingredient fur- 

 nished by manures as contributing to the nour- 

 ishment of the ^lant, and is not itself soluble in 

 water, nor even disengaged by fermentation in a 

 state of purity; under what state of chemical 

 combination is its solution effected ? Is it ef- 

 fected in the state of charcoal? It has been 

 thought, indeed, that carbon in the state of 

 charcoal, is soluble in water; because water from 

 a dunghill, when evaporated, constantly leaves 

 a residuum of charcoal, as was first ascertained 

 by the experiments of Ilassenfi-atz. But there 

 seem to be reasons for doubting the legitimacy 

 of the conclusion that has been drawn from it ; 

 for Senebier found that plants whose roots were 

 immei-sod in water, took up less of the fluid in 

 proportion as it was mixed- with water from a 

 dunghill. Perhaps then the charcoal of water 

 from a dunghill is held merely in suspension, 

 and enters the plant under some other modifica- 

 tion. 



But if carbon is not soluble in water in the 

 state of charcoal, in what other state is it solu- 

 ble. It is soluble in the state of carbonic acid 

 gas. But is this the state in which it actually 

 enters the root? On this subject physiologists 

 have been somewhat divided in opinion. Sene- 

 bier endeavours to prove that carbonic acid gas, 

 dissolved in water, supplies the roots of plants 

 with almost all their carbon, and founds his ar- 

 guments upon the following facts : — In the first 

 place it is known that carbonic acid gas is solu- 

 ble in water; in the second place it is known to 

 be contained in the soil, and generated by the 

 fermentation of the materials composing man- 

 ures ; and in the next place it is known to be 

 beneficial to vegetation when applied artificially 

 to the roots, at least in a certain degree. This 

 is evident from the following experiment of 

 Ruekert, as well as from several experiments of 

 Saussure's, previously related. Ruekert planted 

 two beans in pots of equal dimensions, filled 

 with garden mould; the one was moistened with 

 distilled water, and the other with water im- 

 pregnated with carbonic acid gas. But the lattot 



