8o 



NITROGENOUS MANURES 



[CHAP. 



potash or phosphoric acid in the soil which makes the 

 nitrogenous compounds the most important manures in 

 practice, though in the formation of this opinion some- 

 thing also must be set down to the fact than an applica- 

 tion of nitrogenous manure always shows itself in the 

 richer green colour and increased vigour of the plant, 

 whereas the effect of phosphatic manures is generally 

 only to be ascertained from the weight of the ripe 

 product like the grain. 



Another result of the amount of mineral reserves in 

 the soil is that crops such as wheat or mangolds, which 

 are chiefly dependent upon an external supply of nitro- 

 gen, give yields that are roughly proportional to the 

 amount of nitrogen supplied as long as it is not large ; 

 there, however, soon comes a point when the law of 

 diminishing returns comes into play and the return 

 for each further addition of nitrogen falls off rapidly. 

 The following table (XX.) taken from the Rothamsted 



Table XX. — Wheat with Increasing Amounts of Nitrogen. 

 Broadbalk, Rothamsted. (Average, 1852-1864.) 



experiments, illustrates this in regard to wheat ; there 

 are five plots each receiving the same phosphoric acid 

 and potash, in excess of the crops' requirements, but 

 the supply of nitrogen increases by regular steps from 

 none to 172 lb. per acre. 



