86 



Physics of the Soil. 



such as occur during thunder storms is generally conceded. 

 It is also thought that a part of these combinations may be 

 brought about through the action of ozone upon ammonia. 

 Warington is also of the opinion that the peroxide of hy- 

 drogen in the air causes the conversion of some atmospheric 

 ammonia into nitric acid, and hence that not all the nitric 

 acid brought down by the rains was formed as new ma- 

 terials in the atmosphere from direct union of oxygen and 

 nitrogen gases. 



The amount of nitrogen brought to the soil with the rains 

 seldom equals 5 Ibs. per acre per annum in the open coun- 

 try, as shown by the following table : 



Nitrogen as ammonia and nitric acid, in pounds per acre 

 per annum, in rain. 



I 



N 





FIG. 25 Showing the influence of free-nitropren-fixin? pprms on the growth of 

 peas. The large plants all grew in sand contn in in^ the nitrogen fixing bac- 

 teria, while the small plants grew in soils identically the same except that 

 all bacteria were excluded from them. After Hellriegel 



