CALCAREOUS MANURES— THEORY. ^5 



that the term should be literally understood, and perhaps without attaching 

 to its use any precise meaning whatever. Dundonald only, of all those 

 who have applied ciiemistry to agriculture, has asserted the existence of 

 vegetable acid in soils:* but he has offered no analysis of soils in proof, 

 nor any other evidence to establish the fact ; and his opinion has received 

 no confirmation, nor even the sliglitest notice, from later and more able in- 

 vestigators of the chemical characters of soils. Kirwan and Davy profess 

 to enumerate all the common ingredients of soils ; and it is not intimated 

 by either that vegetable acid is one of them. Even this tacit denial by 

 Davy more strongly opposes the existence of vegetable acid, than it is sup- 

 ported by the opinion of Dundonald, or any of those writers on agricul- 

 ture who have admitted its existence. For it cannot be supposed that so 

 able and profound an investigator would have omitted all reference to an 

 ingredient of soils so general, and therefore so important, as is here asserted, 

 even if its presence had been ever suspected by him, much less known. 

 Grisenthwaite, a late writer on agricultural chemistry, and who has the ad- 

 vantage of knowing the discoveries, and comparing the opinions, of all his 

 predecessors, expressly denies the possibility of any acid existing in soils. 

 His New ' Theory of Agriculture ' contains the following passage : " Chalk 

 has been recommended as a substance calculated to correct the sourness 

 of land. It would surely have been a wise practice to have previously as- 

 certained this existence of acid, and to have determined its nature, in order 

 that it might be effectually removed. The fact really is, that no soil was 

 ever yet found to contain any notable quantity of acid. The acetic and 

 the carbonic are the only two that are likely to be generated by any spon- 

 taneous decomposition of animal or vegetable bodies, and neither of them 

 have any fixity when exposed to the air." Thus, then, my doctrine is de- 

 prived of even the feeble support it might have had from Dundonald's mere 

 opinion, if that opinion had not been contradicted by later and better 

 authority ; and the only support that I can look for, will be in the facts and 

 arguments that I shall be able to adduce. 



I am not prepared to question what Grisenthwaite states as a chemical 

 fact, " that no soil was ever yet found to contain any notable quantity of 

 acid." No soil examined by me for this purpose gave any evidence of the 

 presence of uncombined acid. Still, however, the term acid may be ap- 

 plied with propriety to soils in which growing vegetables continually receive 

 acid from the decomposition of others, (for which no "fixity" is requisite,) or 

 in which acid is present, not free, but combined with some base, by which 

 it is readily yielded to promote, or retard, the growth of plants in contact 

 with it. It will be sufficient for my purpose to show that certain soils con- 

 tain some substance, or possess some quality, which promotes almost ex- 

 clusively the growth of acid plants— that this power is strengthened by 

 adding known vegetable acids to the soil— and is totally removed by the 

 application of calcareous manures, which would necessarily destroy any 

 acid, if it were present. Leaving it to chemists to determine the nature 

 and properties of this substance, I merely contend for its existence and 

 effects ; and the cause of these effects, whatever it may be, for the want of 

 a better name, I shall call acidity. 



The proofs now to be offered in support of the existence of acid and 

 neutral soils, however weak each may be when considered alone, yet, when 

 taken in connexion, will together form a body of evidence not easily to be 

 resisted. 



First proof. — Pine and common sorrel have leaves well known to be acid 



* Dundonald's Connexion of Chemistry and Agriculture. 

 6 



