II.] GROWTH PROPORTIONAL TO NITROGEN 29 



most of our field crops is, until some other limiting 

 factor comes into play, proportional to the amount of 

 combined nitrogen they receive, it is necessary to con- 

 clude that they have drawn none from the atmosphere. 

 It is indeed true that the atmosphere does contribute a 

 small amount of nitrogen for the use of the plant under 

 ordinary conditions, because traces of both ammonia 



Table V'I.— Rothamsted Mangolds (1876-1902). 



and nitric acid are found in the air and arc washed 

 out by the falling rain. Table VII. shows the average 

 amount of nitrogen as nitric acid and ammonia brought 

 down in the rain falling at Rothamsted for the thirteen 

 years between ist September 18S8 and 30th August 

 1901, together with the corresponding results obtained 

 at a few other places where observations have been 

 made for any long period. It will be seen that the 

 Rothamsted results are considerably lower than those 

 obtained at the Paris, Copenhagen, or Florence stations, 

 though they do not notably differ, as regards the total 

 amount of nitrogen falling per acre, from those obtained 

 at the two tropical stations in the West Indies. The 

 high results are probably due to the proximity of towns, 

 because the majority of other determinations, not 

 quoted here because they have only been continued for 

 one or two years, agree more nearly with the Rotham- 

 sted figures. It may thus be assumed that ordinary 



