Legumes 



THE NITROGEN CYCLE 



Most plants take nitrogen from the soil, as soluble nitrates. Most animals receive 

 nitrogen from plants or from other animals, as proteins in their food. Nitrogen in 

 the bodies of plants and animals passes on to other living things as food or in the 

 process of decay — which means the feeding of bacteria or fungi. Or it passes into 

 the soil or the air as a result of death and decay. All living things eventually depend 

 upon nitrogen-fixing bacteria, which return to the soil atmospheric nitrogen combined 

 into forms that other living things can use 



islands for hundreds of years. This guano contains nitrogen and other ele- 

 ments usable by plants in food-making, and it has been imported for use as a 

 fertilizer. But the amount of guano is limited and constantly diminishing. 



The nitrogen supply will probably last as long as the present inhabit- 

 ants of the earth are likely to live. But society expects to outlive its indi- 

 vidual members and must look ahead through its statesmen for those not yet 

 born (see illustration above). Two solutions of the "nitrogen problem" 

 have developed in comparatively recent years. One comes from a better 

 understanding of living things; the other is an application of chemical 

 knowledge. 



Rotation of Crops If we grow several crops of grain on a farm, the 

 yield tends to diminish in time because the nitrogen gives out. But we do 

 not have to abandon the farm, nor need we import expensive nitrogen ferti- 



151 



