PRINCIPLES OF MANURING. 291 



By common consent, good farmyard manure, is looked 

 upon as the best type of all our manurial applications, and 

 justly so, because it contains within itself all the sub- 

 stances which plants require, and in the right proportions. 

 Not only does good farmyard manure contain all the inor- 

 ganic (mineral) substances the crop requires, but also the 

 organic, which are equally essential to its growth, but 

 which plants can and do, to the greatest extent, obtain 

 from another source the atmosphere. It has been al- 

 ready stated (p. 285) that plants cannot appropriate the 

 nitrogen necessary for their most important organic com- 

 pounds directly from the atmosphere, but derive it from 

 the ammonia always found in the air in minute and 

 variable proportions. To secure this necessary ingredient 

 to the growing crop, in proportions suited to its now 

 (by long cultivation) largely-increased requirements, it is 

 always desirable to secure a due supply of it or its consti- 

 tuents in the manurial compounds we apply to the soil. 

 Experience has shown, and science has confirmed, that 

 different plants (orders) require different proportions of 

 nitrogen in carrying oui? their natural processes, and also 

 that they have different capacities for appropriating it from 

 the atmosphere. It has, therefore, been found in practice 

 desirable with a view, of course, to obtain the largest re- 

 sults to supplement this difference in wants and capacity 

 to obtain, by giving these nitrogenized compounds in the 

 manure in accordance with the requirements of the crop 

 to which they are applied. The leaf-surface of the plant 

 is a good general guide in this case, its capacity to obtain 

 ammonia from the atmosphere being mainly influenced by 

 the amount of breathing surface exposed while our know- 

 ledge that a higher proportion of nitrogen is required for 

 the perfection of the seed than of any other portion of the 

 plant, would show us that crops "cultivated for their seeds" 

 need it more than those cultivated for other purposes. 



