728 



FARMERS' REGISTER. 



[No. t2 



beds, since five years were lost in attaining a com- 

 mon degree of fertility. 



The Uieory which we have adopted, is applica- 

 ble, likewise, in the happiest manner, to the ope- 

 ration called marling. JMarl is not a simple mix- 

 ture of silica and alumina with more or less of 

 carbonate of lime. Its base is argillaceous and 

 calcareous silicates ; some mineralogists consider 

 it even as an oryctognosaical species.* It is on 

 this account that plants cannot vegetate in mar! 

 which has been lontr exposed to the air, even 

 when the silica, alumina and lime are in the pro- 

 portions which form good arable land. By expo- 

 sure to the air, carbonic acid destroys the combina- 

 tion which existed between the earths, and it is 

 then, and then only, that marl will enrich the soil. 

 In this case, if the negative element prevails, as 

 in the case of argillaceous marls, it becomes 

 excellent for calcareous soils ; and marls called 

 calcareous are in their turn advantageous for ar- 

 gilio-sandy land. 



It has been remarked, that the alkaline and 

 earthy salts, which, in a certain quantity, injure 

 vegetation, produce a good effect when employed 

 in small doses. Chemists and farmers have 

 soughtto explain this action of saline compounds. 

 Some have thought that certain salts were good 

 for plants, as some are for animals — that salts, and 

 even earths, formed part of the food of vegeta- 

 bles ; others, on the contrary, that they act prin- 

 cipally as stimulants to vegetation. VVithout de- 

 nying that earthy substances may enter into the 

 constitution of a vegetable, to unite and giv^e 

 strength to the parts that are to support the or- 

 gans, like phosphate of lime in the bones of qua- 

 drupeds, I may remark, that with a kw excep- 

 tions, the presence of any salt is not absolutely 

 necessary to vegetation. Thus, for example, bo- 

 rage and lettuce, whose extracts contain much ni- 

 tre when they grow in highly manured soils, do 

 not contain any sensible portion of it when culti- 

 vated without dung. I therefore rather incline to 

 the opinion of physiologists, who think with M. 

 Decandolle, that salts act as excitants or stimu- 

 lants. But, what is the meaning of excitation 7 

 At the present day, science no longer admits of 

 those vague explanations which consist of nothing 

 but words. I understand by excitation, the emi- 

 nent property of conductmg electricity which salts 

 communicate to water. It is in this manner, as it 

 appears to me, that nitrate of potash acts, in the 

 prodigious energy which it gives to vegetation. 

 It is probably in this way that sulphate of lime 

 acts ; that is to say, by rendering the water a bet- 

 ter conductor, though, in this case, the effects ap- 

 pear to me to be complicated, and to be worthy of 

 direct experiment. 



Thus far, for greater simplicity, we have consi- 

 dered lime as free, in speaking of the mixture of 

 silica, alumina, and lime, which constitute a soil : 

 now the lime is in the state of carbonate, but it 

 does not, in that state, cease to be an electro-po- 

 sitive element in relation to silica and alumina. 

 This circumstance allows us to explain an impor- 

 tant vegeto-physiological fact. The carbon in 

 vegetables is produced mostly, if not entirely, by 

 the decomposition of the carbonic acid which they 

 absorb not only from the air, but from the ijround: 

 such is the opinion of the celebrated Decandolle. 



* Brochant's Mineralogy. 



This carbonic acid, furnished by the ground, ap- 

 pears to enter into the vegetable at the moment of 

 its liberation, probably dissolved in the water 

 which the soil contains. It is absorbed by the 

 spongioies of the radicules ; it ascends with the 

 sap, urged forward as by a vis a tergo. But how 

 is this carbonic acid produced? In certain ma- 

 nured soils, and in superficial portions of the earth, 

 penetrated by the air, we may conceive it to be 

 formed by the re-action of oxygen upon the car- 

 bon of organic detritus : but at those great depths 

 which are attained by the roots of oaks and cedars 

 of a hundred years old, how can the carbonic acid 

 be developed ? How cap the oxygen and organic 

 matter penetrate to such depths 1 In our theory 

 there is no difficulty. Carbonic acid comes from 

 the lime, on which the silica and alumina act 

 slowly but conlinuousiy to form silicates.* 



Thus, then, at certain depths, and under influ- 

 ences but little understood, silica would decom- 

 pose carbonate of lime, while at the surface of the 

 earth, and under the influence of exterior agents^ 

 the silicates would be decomposed by carbonic acid 

 produced by the reaction of the oxygen of the air 

 on organic detritus — an admirable and providen- 

 tial rotation, which re-establishes the equilibrium. 

 and incessantly tends to the rejuvenescence of 

 nature. 



The last corollary of my theory — the decompo- 

 sition of silicates by exterior agents, and parti- 

 cularly by carbonic acid, cannot be called in ques- 

 tion. It has been established by M. Becquerel... 

 under circumstances in which the force of cohe- 

 sion might seem to present a serious obsl<K;le. I 

 allude to the decomposition of the feldspar of gra- 

 nite, and the formation of kaolin. The analogy 

 is here so strong that I must render the homager 

 of my first conception to the distinguished acade- 

 mician I have just cited. 



The fact of the decomposition of carbonate of 

 lime by silica in the interior of the earth is equally 

 supported by experiment and observation. And^ 

 first, if, in proceeding to the analysis of a vegeta- 

 ble soil, when the coarser siliceous sand has been 

 separated by agitation and deposition, and the car- 

 bonate of lime has been removed by weak acids;^ 

 we examine the finer terrene substance which has 

 resisted the weak acids, we find that it is not alu- 

 mina, as Chaptal indicates, nor silica, as is stated 

 in various works, but that it consists principally of 

 veritable silicates of lime, of alumina, and of oxide 

 of iron. 



Still, it may be objected that these silicates are 

 anterior to all vegetation; that to prove their re- 

 cent formation and daily production, requires di- 

 rect experiments. These direct experiments are 

 among the objects which I wish to undertake. 

 They require much time. But to prove truth, 

 are we to depend solely upon new experiments 



* Animal manures may contribute to the decompo- 

 sition of silicates, not only by the carbonic acid which 

 they form by absorbinj^ oxygen from the air, but in 

 producing such substances as the tat acids, which 

 have a tendency to unite with lime and to eliminate 

 the silica which is combined with it. M. Easpail^ 

 whose talents we are glad to acknowledge, without 

 sharing in all his scientific opinions, appears to us to 

 have explained the siliceous petrefactions that are 

 found in chalk, in a very happy manner by the action 

 of animals entombed in siliceo-caJcareous beds. 



[Physiol. Vegetate, t. 2. p. 339, 



