2 FERTII.ITV AND FERTILIZER IIIXTS 



Oxygen. — In the free gaseous state, about one-fifth, by bulk, 

 of the atmosphere is made up of this element, mechanically 

 mixed with nitrogen. It is found in enormous quantities in 

 combination with other elements. It constitutes about eight- 

 ninths b}' weight of water and nearly one-half of the earth's 

 crust. All combustion and decay require oxygen. The plant 

 stores up oxygen in combination with other elements and with- 

 out oxygen plants would die. The plant takes in oxygen, from 

 the atmosphere, in combination with carbon, as carbonic acid 

 gas, through the openings on the under sides of the leaves ; the 

 carbon is absorbed and the excess of oxygen given off. Oxygen 

 combines with most other elements forming oxides. It often 

 combines with other elements in varied amounts forming oxides 

 of dift'erent composition which are generally quite stable. The 

 color of soils is often determined by oxides such as iron oxides. 

 The iron oxides influence the moisture condition of soils be- 

 cause of their absorptive qualities and help to oxidize organic 

 substances in the soil. The roots of plants when deprived of air 

 in the soil are able to draw upon iron oxides for oxygen. 



Nitrogen. — About four-fifths of the atmosphere, or about 35,000 



rig. I. — Cowpeas liave the power of gathering nitrogen from the air. 



tons over every acre of land, is made up of nitrogen in the free 



