THIRD AMERICAN EDITION. Vll 



exceedingly small, quite insuflSicient for the supply of 

 all the nitrogen that enters into the vegetable struc- 

 ture. To this it has been replied by Professor Lind- 

 ley, in an elaborate review of Liebig's work, that 

 " the quantity of ammonia given off from thousands 

 of millions of putrefying animals must furnish an 

 abundant, an everlasting source of that principle." 



Important as ammonia, or its nitrogen, is conceived 

 to be to plants, it will be seen that Liebig considers 

 carbon not less so. 



Since the appearance of the former editions of this 

 work, the opinions of American chemists in regard to 

 humus, have become so generally diffused, in the 

 various Agricultural Reports, that it has not been 

 deemed necessary to retain, in this edition, much that 

 was appended to the second. 



Professor Lindley, in speaking of humus, recogni- 

 ses it as " the dark substance which remains when 

 manure is thoroughly rotted, and which colors the 

 soil black, and without going into any technical ex- 

 amination of this product, we may state," he con- 

 tinues, " that it is a substance formed by the decay of 

 plants, and very rich in carbon." He then quotes the 

 expression of Liebig, that this substance, in the form 

 in which it exists in the soil, does not yield nourish- 

 ment to plants, and expresses surprise, that the author ^ 

 should have thought it worth his while to raise such 

 a phantom for the mere pleasure of subduing it. for 

 no one in Great Britain now entertains the opinion, 

 that humus is in itself the food of plants. *^ Every 



