66 THE CULTURE OF FARM CROPS. 



may be that it acts upon the nitrogen left free by the with- 

 drawal of the oxygen from it and so induces its combina- 

 tion with oxygen; and, third; nitric acid is formed in v the 

 atmosphere by the action of electrical discharges by which 

 the oxygen and nitrogen are brought into combination ; in 

 the manner previously mentioned. 



The atmospheric sources of nitric acid are not sufficient 

 to account for the large quantity of it which is found in 

 any ordinary crop. Various experiments have been made 

 for the purpose of ascertaining the amount of combined 

 nitrogen, in either of its forms, which is gathered by plants 

 from the atmosphere. The average of all the determina- 

 tions which have been reached, give the quantity at about 

 10 pounds per acre. But the average consumption by the 

 crops equals 44 Ibs. per acre. So that this atmospheric 

 supply is wholly inadequate for the growth of farm crops. 

 This is one of seeming anomalies of nature, that while no 

 less than nearly 20,000 tons of nitrogen, as it exists in the 

 atmosphere, rest over one acre of surface, a crop cannot 

 procure the small quantity of 44 Ibs. per acre from all this 

 vast atmospheric store. But this is quite consistent with 

 the regular course of natural operations. The elements 

 form combinations, as has been shown of an infinite num- 

 ber and variety, and it is only in these combined forms 

 that they serve their ends. The elementary carbon can 

 provide plants with their supply of carbon, only through 

 its combination with oxygen; and in like manner the ele- 

 mentary nitrogen requires a certain preparation to fit it for 

 assimilation by plants. 



The small quantity of nitric acid which is procured from 

 the atmosphere by the crops, is however sufficient for all the 

 practical needs of the intelligent farmer. He does not de- 

 pend upon the air to supply his crops with this scarce and 

 most valuable nutriment. By a wise course of economical 

 management he accumulates a large amount of organic 

 matter rich in nitrogen, the decay of which he aids by his 

 skillful methods, and so provides an abundant stock of food 

 for his crops. If the atmosphere then contributes a fourth 



