ATMOSPHEKIC AIR AS THE FOOD OF PLANTS. 



95 



ment, Avith no other than tlic atmospheric sup2:>ly of 

 carbon. 



HydrOfi^cn is adequately supplied to crops by water, 

 which e(pially belongs to the Attnos|)here and the Soil, 

 although it enters the plant chiefly fioui the latter. 



Nitrogen exists in immense quantities in tlie atmosphere, 

 and we may regard the latter as the primal source of this 

 element to the organic world. In the at i no -sphere, how- 

 ever, nitrogen exists for the mjst part in the free state, and 

 is, as such, so we must believe from existing evidence, un- 

 assimilable by crops. Its assimilable compounds, ammo- 

 iila and nitric acid, occur in the atmosphere, but in pro- 

 portions so minute, as to have no influence on vegetable 

 growth directly appreciable by the methods of investiga- 

 tion hitherto employed, unless they are collected and con- 

 centrated by rain and dew. 



The subjoined Table gives a summary of the amount 

 of nitrogen annually brought down in rain, snow, etc., 

 upon an acre of surface, a -cording to the determinations 

 hitherto made in England and Prussia. 



Amount of Assimilable Nitrogen annually brought down by 

 THE Atmospheric Waters. 



* From Jan. to Jan. t From Apr. to Apr. % From May to May. 



Direct Atmospheric Supply of Nitrogen Insufficient 

 for C^rops. — To estimate the ailequacy of tliese atmos- 

 pheric supplies of assimilable nitrogen, Ave may compare 

 their amount with the quantity of nitrogen required in the 



