THE NITROGEN THEORY. 295 



Tlie introduction and application of Peruvian guano and 

 nitrate of soda afforded the so-called nitrogen theory 

 a foundation to rest upon ; no manui'c coidd be com- 

 pai'ed with guano for abundance of nitrogen, while it 

 sui'passed all others in the rapidity and strength of its 

 action. The powerful effect produced by it coincided 

 entirely with the nitrogen theory ; it corresponded with 

 the high amount of nitrogen in the manure, and chemical 

 analysis furnished satisfactory conclusions with regard to 

 the rapidity of its action. The fact that the influence of 

 guano in increasing the crops was generally more rapid 

 than that of other manures containing an equal amount 

 of nitrogen, made it evident that some one of its consti- 

 tuents possessed a peculiar power which was not present 

 in the other manures ", and this constituent was supi)osed 

 to be more conducive than other nitrogenous compoinids 

 to the growth of plants. 



The discovery of this constituent presented no difficulty. 

 Chemical analysis showed that Peruvian guano was very 

 rich in salts of ammonia, and that one-half of its nitro- 

 gen existed in the form of ammonia. But ammonia ^vas 

 already well known as an element of nutrition for plants, 

 and this afforded an easy solution of the rapidity which 

 marked the operation of guano. ' Peruvian guano accord- 

 ingly contained in a concentrated state in tlie ammonia 

 one of the most important nutritive substances for plants, 

 and this nutriment when dispersed in the soil could be 

 directly assimilated by theu' roots. 



From this trnie forward a distinction was drawn be- 

 tween the various kinds of nitrogenous manures, and 

 ' assimilable ' nitrogen was discriminated from that which 

 was termed ' sparingly assimilable.' Assimilable nitrogen 

 was understood to mean ammonia and nitric acid ; but 

 the term ' hard of assimilation ' was applied to other 



