THE FARMERS' REGISTER. 



461 



In former times there was mucli dispute as to 

 wnm was Ihe food of plants. Some coiiiemiecl 

 that it was water alone; oihers air. Tull main- 

 taiueil, aiul liis ditsciples ihou^rhi lie li.ui proved, 

 thai finely divided earth was ihe lood ol plaiils, 

 and tliat |)loui;hiiii;; imi^lit serve as a perleci sub- 

 siiute lor manure. D.ivy laid down that vegeta- 

 ble extract, soluble in water, was the Ibod ol |)lanis; 

 and this seecned to be clearly etlaUlithed boih by 

 his reasoning and experiinenls. 'J'liis vei;etab!e 

 exiraci, when alterwards known iiiuicr llie names 

 of humin, liumic acid, and all ilie rombinaiions 

 of tiie hiiter with alkaline earths, &c., was still 

 supposed to furnish directly to the roots much the 

 greater part of the lood o) plants; the balance, 

 not so received, beiiiif ilie iMibona; acid absorbed 

 by the leaves Irom the aimos|)here, and ol which 

 the plant leiains and assinnlates the carbon, and 

 dischartjes its other eonstituent, the oxygen, to 

 puiily lite atmosphere. It is one of the nioii ad- 

 mirable provisions ol nature, that wlule the 

 breathinir ol animals, and the exhalaiions of all 

 decayinii; mailer, furnish carbonic acid to the at- 

 inospbeie, plants are continually absorbing it by 

 iheir leaves, and returning oxygen instead. For 

 Uiough the leaves give out nitrogen gas in some 

 oases, and carbonic acid gas alwuys at night, still 

 the discharge of oxygen gas is very lar the great- 

 est. And thus stood received ofiinions — that plants 

 received pari of their sustenance Irom the atmo- 

 sphere, through their leaves, but a larger part by 

 directly drawing, liom the soil the soluble vege- 

 table (or animal) mailers, there present. 



But Liebig maintains (and it. had indeed been 

 previously maintained by son)e others recently,) 

 that vegetal)le extract, humus, or putrescent 

 manures, whether naturally or artificially applied, 

 and all of which change to or suf)ply humiu and 

 humic acid, do not act directly, in the slightest 

 degree, as the lood of plants ; but only, by com- 

 bining with '.he oxyaeii of the atirrosphere and 

 iormiiig with it carbonic acid, which is received 

 only in the early siage of growth, and in but 

 small quantity, by the roots, and much more 

 abundantly, and then exclusively, by the leaves ol 

 plants afier iney are formed. Now if the au- 

 thor meant that humus was first decomposed, 

 and furnished carbon to the roots of plants by the 

 carbonic acid evolved, ihe practical results would 

 not be materially diHerent. For it still wouKI be, 

 as upon the other supposition, that the greater 

 the quantity of humus, (or manure serving to lorm 

 humus,) ihe itreaier the sup|)ly of carbon to the 

 growing plants. But not so. It would indeed 

 seem that the auihor consideis the supply of car- 

 bon through the roots as so small, as lo permii 

 the inlerence that it is of very little importance. 

 And the main and indeed almost entire supply 

 being from the atmosphere, and lhrou<fh the 

 leaves, it seems also to lollow that the vegetable 

 matters or manure, acting by first evolving car- 

 bonic acid into the atmosphere, would be almost 

 as likely lo give it to plants on the adjacent field, 

 or adjacent farm, as to plants growing immediate- 

 ly above the place of extrication. For, when 

 evolved, and loose above the surface of the eaiih, 

 the carbonic acid gas must be carried off and dif- 

 lused by the slightest breeze, and still more rapid- 

 ly by high winds. Yet we all know that the be- 

 neficial effects of manure will show on, and be 

 limited to plants standing on almost the precise 

 space to which the manure was applied. 



It is only while plants are very young, indeed 

 belbre their leaves are developed, that Liebig 

 supposes the roots lo draw carbonic acid from the 

 t'arth. 



" Humus acts in the same manner in a soil permea- 

 ble to air as in the air itself; it is a continued source 

 ol carbonic acid, which it emits very slowly. An 

 atmosphere oi carbonic acid, formed at the expense 

 of the oxygen ol the air, surrounds pvcry particle of 

 ilt'ca) iiig liumus. The cultivation of land, by lillini' 

 and loosening the soil, causes a free and unobstructed 

 access of air. An atmosphere of carbonic, acid is, 

 therefore, contained in every fertile soil, a.-id is the 

 Ih-t and most important food for the young plunts 

 which grow in it." — (p. 105.) 



" J5y loosening the soil which surrounds young plants, 

 we i-ivor the access of air, and the formation of car- 

 bonic acid; and on the other hand the quantity of 

 their lood is diminished by every difficulty which 

 opposes the lenewal of air. A plant itself etfects 

 this change of air at a certain period of its growth. 

 The carbonic acid, which protects the undecayed 

 humus from fuither change, is absorbed and taken 

 away by the fine fibres of the roots, and by the roots 

 themselves; this is replaced by atmospheric air, by 

 which process the decay is renewed, and a fresh por- 

 tion of carbonic acid formed. A plant at this tune 

 receives its food, both by the roots, and by the organs 

 above ground, and advances rapidly to maturity. 



"When a plant is quite matured, and when the organs, 

 by which it obtains food from the atmosphere, are 

 formed, the carbonic acid of the soil is no further 

 required. 



"Deficiency of moisture in the soil, or its complete 

 dryness, does not now check the growth of a plant, 

 provided it receives from the dew and the atmosphere 

 as much as is requisite for the process of assimilation. 

 During the heat of summer it derives its carbon excha- 

 sively from the atmosphere."— (p. 106.) 



" Substances, containing a large proportion of car- 

 bon, are excreted by the roots and absorbed by the 

 soil. Through the expulsion of these matters unfitted 

 for nutrition, therefore, the soil receives again the 

 greatest part of the carbon, which it had at first yield- 

 ed to the young plants as food, in the form of carbonic 

 acid."— (p. 116.) 



" Plants do not exhaust the carbon of a soil, in the 

 normal condition of their growth ; on the contrary, 

 they add toils quantity. But if it is true that plants 

 give back more carbon to a soil than they lake from 

 it, it is evident that their growth must depend upon 

 the reception of nourishment from the almosiihere. 

 The influence of humus upon vegetation is explained 

 by the foregoing facts, in the most clear and satisfac- 

 tory manner. 



"Humus does not nourish plants, by being taken up 

 I and assimilated in its unaltered state, but by presenting 

 1 a slow and lasting source of carbonic acid, which is 

 j absorbed by the roots, and is the principal nutriment 

 I of young plants at a time when, being destitute of 

 I leaves, they are unable to extract food from the atmo- 

 I sphere." — (p. 116.) 



In a previous passage it is said — 



" The facts which we have stated in the preceding 

 pages prove, that the carbon of plants must be derived 

 exclusively from the atmosphere." — (p. 72.) 



These passages show clearly that the author 

 considers the carbon furnished through the roots 

 (or directly from the soil) as very small. But 

 other passages show that he supposes even 

 the small quantily of carbon so taken up by the 

 roots, to be more than replaced in the soil by sub- 

 sequent excretions of the roots; and thai under 



