SUPPLY OF AMMONIA FEOM THE AIE. 303 



man's aid the return of a portion of the nitrogen, and in 

 a rotation the whole amount of that substance which 

 has been taken away in the crops ; and further, that 

 the field may be exhausted of every other nutritive sub- 

 stance, however great its store in the ground may be, 

 because they are never spontaneously restored to the 

 soil by nature whereas this can never happen to nitro- 

 gen ; then it is contrary to all the rules of logic in any 

 given case, to ascribe without closer examination the 

 exhaustion of a soil above all other things to a loss of 

 nitrogen. 



We might suppose that, apart from the suggestions 

 of common sense, the palpable advantage which would 

 accrue to the farmer imperatively demands that he 

 should take all possible pains to verify the correctness 

 of this fact, and to discover how much nitrogenous 

 food is annually restored to him by the atmosphere. 

 For when he knows how far upon the whole he may 

 calculate upon this source, he can easily arrange his 

 system of cultivation to make it most profitable to him. 

 If the atmosphere supplies him with the whole amount 

 of nitrogen which he removes from his field by a rota- 

 tion, then he can direct his thoughts to the means of 

 keeping his whole farming operations going in the most 

 effectual manner with the store which he annually col- 

 lects in his manure heap, without spending any money 

 upon nitrogenous food for his plants. If he finds that 

 the atmosphere restores only a portion of that which 

 has been taken away, and he accurately knows what 

 this portion amounts to, then as circumstances require, 

 he can, with judicious economy, supply from other 

 sources what is lacking ; or he may so arrange his sys- 

 tem of cultivation as to make the supply of nitrogen 

 from natural sources cover what is removed in the 

 crops. 



Every advance in an industrial pursuit has a definite 

 standard of value in the price of the products ; and no 

 sensible man would call an alteration in the mode of 

 conducting a business by the name of improvement, 

 unless the price of the products covered the cost of pro- 



