CYCLE OF NITROGEN 95 



both chemical and biological methods of bringing nitrogen 

 into combination. The difficulty of bringing about these 

 reactions is indicated by the fact that, in spite of the 

 immense amount of free nitrogen, combined nitrogen, un- 

 der existing commercial conditions, costs considerably more 

 than any other food element. 



The nitrogen of the soil. The store of nitrogen in the 

 soil is in the humus, the residue of the organic matter that 

 has undergone decomposition in the soil. The content of 

 average arable soils in nitrogen is from 0.1 to 0.2 per cent., 

 but the nitrogen in humus is not available to the plant, 

 because the humus is insoluble. It is a very fortunate pro- 

 vision of nature that a portion of the nitrogen that has 

 been added to the soil in the organic matter has been 

 thus stored. If it were all immediately available, that 

 \\ -h irh was not used by the growing plant would be quickly 

 leached from the soil in the drainage water. 



Under natural conditions, some nitrogen is lost in the 

 drainage water. As will be seen later, factors are operat- 

 ing in every soil by which combined nitrogen is added to 

 it. Where the entire crop is returned to the land, as is 

 true under uncultivated conditions, the nitrogen content 

 of most soils slowly increases. Thus has accumulated 

 through many thousands of years the present nitrogen sup- 

 ply of our soils. Under cultivation, the removal of the 

 crop and the loss by leaching more than balance the gain, 

 and the soil becomes depleted of nitrogen. There comes a 

 time when nitrogenous fertilizers must be added to main- 

 tain the crop-producing power. This addition may be 

 made in a variety of ways. Besides the plant residues, 

 animal material may be purchased, such as blood and bone 

 meal, fish scrap, wool waste, etc. Guano, the excrement 

 of birds, originally formed an important source of nitro- 

 genous fertilizers. Besides natural plant and animal ma- 



