SUPPLY OF A^I^rOXIA FROM THE AIR. " 321 



common sense, that 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 know^s how for 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 rotation, 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 collects 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 

 cii'cumstances require, he can, with judicious economy, 

 supply from other sources what is lacking ; or he may so 

 arrange his system 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 con- 

 ducting a business by the name of improvement, unless 

 the price of the products covered the cost of production. 

 When the price of guano exceeds a certain hmit, so that 

 the crop realised does nOt bear a proper proportion to 

 the outlay of capital and labour, this very circumstance 

 prevents its apphcation. 



From this point of view farmers might long ago have 

 perceived that the question about the necessity of su})ply- 

 ing ammonia to increase the crops of corn, includes 



Y 



