262 NITROGEN AND ITS RELATION TO PLANTS 



fleshy foods and depend upon them for partial sustenance, but 

 the ultimate source of all animal food is plant life, since meat- 

 producing animals live upon plant growth. 



Plants get their food from the air, the soil, and moisture. 

 From the air, the leaves take carbon dioxide and water and 

 transform them into starchy food ; from the soil, the roots take 

 water rich in mineral matters dissolved from the soil. From 

 the substances thus gathered, the plant lives and builds up 

 its structure. 



A food substance necessary to plant life and growth is ni- 

 trogen. Since a vast store of nitrogen exists in the air, it 

 would seem that plants should never lack for this food, but 

 lost plants are unable to make use of the boundless store of 

 Atmospheric nitrogen, because they do not possess the power 

 >f abstracting nitrogen from the air. For this reason, they 

 tave to depend solely upon nitrogenous compounds which 

 /are present in the soil and are soluble in water. The soluble 

 nitrogenous soil compounds are absorbed by roots and are 

 utilized by plants for food. 



246. The Poverty of the Soil. Plant roots are constantly 

 taking nitrogen and its compounds from the soil. If crops 

 which grow from the soil are removed year after year, the soil 

 becomes poorer in nitrogen, and finally possesses too little of it 

 to support vigorous and healthy plant life. The nitrogen of 

 the soil can be restored if we add to it a fertilizer containing 

 nitrogen compounds which are soluble in water. Decayed 

 vegetable matter contains large quantities of nitrogen com- 

 pounds, and hence if decayed vegetation is placed upon soil 

 or is plowed into soil, it acts as a fertilizer, returning to the 

 soil what was taken from it. Since man and all other animals 

 subsist upon plants, their bodies likewise contain nitrogenous 

 substances, and hence manure and waste animal matter is 

 valuable as a fertilizer or soil restorer. 



