OF PLANTS BY MANURING. 53 



thousandth part. What advantage, however, does 

 the farmer derive by being informed that such or 

 such a quantity of potash, nitrogen, phosphorus, etc. 

 is contained in the compost, if the chemist does not 

 at the same time tell him how these substances oper- 

 ate, and what is their value ? 



On these points, so exceedingly important to prac- 

 tical agriculture, the chemist cannot at present reply 

 with entire and well-grounded certainty ; inasmuch 

 as they are not of a kind to be determined by an in- 

 vestigation which is limited to his study and labora- 

 tory. The conclusive decision in these matters must 

 be sought on the land of the farmer ; the more, 

 therefore, practical men cooperate in these investiga- 

 tions, the more quickly will their many deficiencies 

 be supplied, — the more speedily and surely will the- 

 oretical conjectures be converted into practical cer- 

 tainties. 



Our present knowledge, however, may still be pro- 

 ductive of manifold advantage ; more especially 

 when theory deduces its conclusions, not from scien- 

 tific speculations, but from the practical results of 

 agricultural experience, and in doubtful cases pro- 

 pounds them simply as conjectures. This has been 

 attempted in the subsequent observations. 



The following substances must be regarded as the 

 most important constituents of manure : — 



1. Nitrogen. This article must beyond all ques- 

 tion be considered the most valuable element in all 

 substances employed as manure ; inasmuch as it 

 5* 



