THE NITROGEN DERIVED FROM THE AIR. 87 



doing so gives off a portion of the other gases which it had 

 previously taken up and absorbed from the air. But if the 

 water should actually carry to the roots and tuke with it 

 into the circulation of the plants 2 per cent, of its bulk of 

 nitrogen, the whole amount of this nitrogen w r ould be quite 

 inadequate to supply the requirements of a crop. For the 

 whole rain fall in this country, during the season when a 

 crop of hay, wheat, or oats is grown, amounts to about 8 

 inches ; and of this at least one-half is evaporated very soon 

 after it has fallen. If we suppose the quantity left in the 

 soil during this period amounts to 6 inches there would be 

 864 cubic inches of water fall upon a square foot, contain- 

 ing of nitrogen about 17 cubic inches, or about 5 grains in 

 weight. This would give something over 30 Ibs. to the 

 acre of nitrogen carried into the soil. But it would be un- 

 reasonable to suppose that more than one-third of this quan- 

 tity would be carried into the roots and be transpired by 

 the leaves of any growing crop. There would then be 

 about 10 pounds of nitrogen carried into the circulation of 

 the plants, which is only one-sixth part of that which is 

 contained in a crop of hay, and one-eighteenth part of that 

 removed from the soil in a crop of clover. 



This is a rough estimation, but it affords convincing proof 

 that plants cannot depend upon the atmosphere for their 

 supply of this element ; but that they draw their chief sup- 

 ply of it from its combinations with oxygen and hydrogen. 



If it is asked how the first plants grown upon the soil, 

 the origin of vegetable growth upon the earth, gained the 

 nitrogen they required to build up their tissues, it may be 

 replied, that in this case, the earliest plants grown were not 

 of that highly organized character which demanded a large 

 proportion of nitrogen. In the coal beds, which were form- 

 ed of vast deposits of vegetable matter accumulated during 

 lengthened periods of time, are found plants of a far lower 

 character than those grown as farm crops. Mosses, ferns, 

 and semi-aquatic plants made up the larger bulk of them, 

 raid as these died and decayed, the little nitrogen they pos- 

 sessed gradually accumulated in the soil in the mass of de- 



