66 ON INCREASING THE GROWTH 



to been named, nitrogen is that upon which, in the 

 generality of cases, the farmer must bestow his chief 

 attention. First, because fields are most in want of 

 this substance ; and secondly, because stable-manure, 

 the agent most generally employed to remedy this 

 want, does not, under its ordinary management, con- 

 tain the quantity required to feed plants so plenti- 

 fully with it that they may furnish that maximum 

 of produce which is, generally speaking, attainable. 

 This attention is the more necessary, because the 

 remaining constituents of manure, as many experi- 

 ments have shown, can only produce their full effect 

 when nitrogen is simultaneously present ; again, be- 

 cause nitrogen is rare and expensive ; and lastly, 

 because it is far more readily lost than the mineral 

 ingredients, inasmuch as it is capable, not merely of 

 running away when liquid, but also of flying away, 

 on account of its conversion into volatile ammoniacal 

 gas through putrefaction and decay. Next to this 

 element, phosphoric acid and the alkalies claim the 

 greatest consideration, because they are the rarest 

 amongst the inorganic nutrients of plants, and are 

 needed in larger quantities for vegetable growth. If 

 we wish to express the relative value of these sub- 

 stances in money, one pound of nitrogen may be 

 valued at about 15, or when converted into ammo- 

 nia at about 16 ; one pound of phosphoric acid, or 

 of the alkalies, at something like 2 ; whilst the same 

 quantity of organic matter, required to form humus, 

 may be taxed at ^^d part of 1. 



