SOME GENERAL BIOLOGIC PROCESSES 531 



remember that man has learned to use the energy of nitrogen 

 compounds in all his explosives and many other chemicals. This 

 interferes seriously with nature's plan, for the firing of one twelve- 

 inch gun wastes nitrogen enough to raise one hundred bushels of 

 wheat. To repair this loss we are just learning to artificially 

 combine the nitrogen of the air into useful compounds, and replace 

 them in the soil as fertilizers. Unless this is done, the end of the 

 nitrogen supply is in sight, due, as usual, to man's interference in 

 nature's processes. He wastes nitrogen as sewage, chemicals, 

 and explosives, so must do his part in completing the circle or 

 suffer the consequences. 



NITROGEN IN THE SOIL 



Removed by Replaced by 



Life processes Manures 



Decay (some kinds) Decay 



Oxidation of useful forms Bacteria 



Waste of sewage Electrical action 



Industrial uses Artificial processes 



Explosives Fertilizers 



Other Elements. The circles which are followed by the other 

 elements found in plant and animal tissue are not so complicated. 

 Hydrogen comes and goes as water, of which there is a limitless 

 supply in most regions. The sulphur, phosphorous, potassium, 

 and other mineral compounds are usually abundant to begin with, 

 and are not set free by decay, but come back to the soil in usable 

 form. 



If a soil becomes deficient in any of these, they are obtained 

 elsewhere as natural mineral deposits and replaced as artificial 

 fertilizer. In a state of nature this would never be necessary, as 

 the plants would die and decay where they grew and so return their 

 mineral salts to the soil that produced them. It is only when man 

 removes his crops, and uses them elsewhere, that artificial re- 

 placement is necessary. 



