18 MANUFACTURE OF FERTILIZING MATERIALS 



bined with silicon, carbon, sulphur, and phos- 

 phorus, it forms an essential part of the silicates, 

 carbonates, sulphates, and phosphates, most of 

 which are very abundant and all of which are very 

 widely distributed in the earth's crust. In this 

 form it is exceedingly stable and is rarely set free. 

 With the exception of the oxides of silicon and 

 iron these oxides seldom occur uncombined with 

 the metals as constituents of rocks or soils. The 

 oxides of iron very commonly occur as such in 

 rocks and soils, and play a very important part 

 in organic life. The several oxides of iron very 

 frequently determine the color of soils; as the 

 iron is more or less oxidized, or as it is exposed 

 more or less to access of air, the color of the soil 

 changes. These oxides of iron also play an 

 important part in the absorptive capacities of 

 soils for moisture and other physical conditions of 

 soils, and also in the oxidation of organic matters 

 in the soils. Many organic substances, and even 

 the roots of growing plants when deprived of 

 free access of air, can readily secure oxygen from 

 the iron oxide, thus reducing the iron to a lower 

 form of oxidation, the oxygen being used for the ox- 

 idation of the organic matter or for the needs of the 

 growing plant; while the lower oxide of iron can 

 easily take up the oxygen of the air and again 

 be converted into a higher oxide ready again to 

 give up a part of its oxygen and thus serve as a 

 carrier. 



Silicon never occurs in the free state, but com- 



