40 THE NITROGEN CYCLE 



is on the credit side then all is well, but this rarely, if ever, 

 happens in Nature, since in practice nitrogen is being restored 

 to the air as fast as it is being taken out. 



In the soil the "plant-available" inorganic nitrogen com- 

 pounds are nitrates or ammonia — ^the nitrates form by far 

 the larger source of nitrogen compounds on which the plants 

 depend. Nitrates are soluble in water, and are thus taken up 

 by the roots; in combination with the carbohydrates they 

 are built up by some wonderful process not yet fully under- 

 stood into the proteins, which are absolutely essential to the 

 life of the higher plants and animals. At present there is a 

 great gap in our knowledge subsequent to the passage of the 

 nitrates from the soil into the plant and the appearance of 

 the complex proteins which form the basis of protoplasm. 

 But, as with the carbohydrates, we see there is a certain 

 circulation or rhythm. The decay of plants and animals 

 restores nitrogen to the air, and so does the action of the 

 denitrifying bacteria. Plants and bacteria which fix nitrogen 

 bring it back into the circle as it is built up into nitrates and 

 ultimately into proteins. Animals get more nitrogen than 

 they want, and the excess is excreted from the body in the 

 form of urea, ammonium salts and uric acid. Whatever 

 excess of nitrogen the plant possesses, and it is not much, is 

 released from the body in the parts which die off at the 

 approach of winter. The decay of both animals and plants is 

 essential to the renewal of life in the world. But for the 

 death and decay of living organisms, accompanied as it is 

 by the breaking up of their complex proteins, life would 

 cease. Were it not for the simple substances which green 

 plants require as food, which are ultimately built up into 

 the organic substances that alone can be digested by 

 animals, the wheel of life could never revolve, and rhythm, 

 save of the blindest of physical forces, would be quite 

 unknown. 



To every thing there is a season, and a time to every purpose undei 



the heaven: a time to be born, and a time to die; a time to plant, and a 



time to pluck up that which is planted;. . .a time to break down, and 



a time to build up. 



Ecclesiastes lu. 1-3, 



