THE CHEMICAL CYCLE OF LIFE 



63 



->Oxy gen- 



Carbon dioxide- 



A 



^ 



Animals 



FOOD 



Soil Materials-^- 



In the course of the summer the bacteria in the tubercles 

 will take in a large quantity of nitrogen from the air. Part 

 of this they will use in making proteins for immediate con- 

 sumption ; another part will be taken from them by the roots 

 of the plant upon which they grow ; and at the end of the 

 season there will be 

 present in the soil 

 and 071 the soil a 

 great deal more ni- 

 trogen in combined 

 form than there was 

 at the beginning. The 

 crop can be plowed 

 under, and the nitro- 

 gen compounds in 

 the plants will thus 

 be added to the soil. 

 After another season 

 of this kind of crop 

 there will be enough 

 nitrogen added to the 

 soil to support several 

 crops of grain. 



This rotation of 

 crops has been prac- 

 ticed by experienced 

 farmers for many 

 centuries, but it is only within the last thirty or forty years 

 that the significance of rotation has been understood. 



For the chemical solution of the nitrogen problem we are indebted 

 to the Swedish chemist Svante Arrhenius, who worked out a process 

 for making nitrogen combine with other elements under the influence 

 of electric currents. This method is economical only if electricity can be 

 obtained at a low cost, as from waterfalls. To burn fuel for this purpose 

 would cost more than the value of the nitrogen compounds produced. 



Fig. 22. The interrelations of organisms 



The green plants, using water and carbon dioxid and 

 salts from the soil, are the source of all food and the 

 source of much oxygen derived from the decomposition 

 of carbon dioxid (during photosynthesis). The food is 

 used by animals and by lower plants (fungi and bacteria), 

 and in the end the substance of the animals is also used 

 by the fungi and bacteria. The carbon dioxid given off 

 by the animals and by the fungi and bacteria sooner or 

 later finds its way back to the green plants through the 

 air or water. The wastes given off by these organisms 

 also become in time raw material for the food of green 

 plants, through the soil 



