60 THE ELEMENTS OF PLANT FOOD 



increases with the proportion of carbon that it contains. 

 Iron containing more than about two per cent of car- 

 bon is cast iron. 



The iron ores as mined are usually in one of two 

 forms, the hematite (Fe 2 O 3 ) having a deep red color 

 of the appearance of red clay, and the magnetite, or 

 magnetic iron ore (Fe 3 O 4 ), which is black in color. 



Various iron compounds are found in the ashes of 

 plants, in the blood of animals, in spring, river, and 

 ocean waters and in all soils. 



Iron is necessary as a plant food, but only a very 

 small amount is required. There is always a sufficient 

 quantity present in the soil. Iron is an aid in the pro- 

 duction of the green coloring matter chlorophyll, 

 without which the plant cannot grow. 



Iron Oxides. Iron unites with O in different pro- 

 portions forming oxides of iron. The various colored 

 brick are made out of clay containing oxides of iron, 

 the colors being developed by burning the clay. The 

 basis of red paint, red ochres, used for barns and other 

 farm buildings is iron oxide. The rusting of iron may 

 be prevented by covering the metal with a coating of 

 paint, varnish, or me'tal. Sheet iron is covered with a 

 coating of tin to make our tin ware. A coating of 

 zinc deposited on iron makes galvanized iron. 



Ferrous Sulphate (FeSO 4 ), sulphate of iron, cop- 

 peras, green vitriol. This compound is used more 

 extensively than any of the other iron salts, being em- 

 ployed in the arts and somewhat as a disinfectant. It 

 has been found to be effective as a destroyer of wild 

 mustard, dandelion, and other noxious weeds when 

 dissolved in water and used as a spray when the parts 

 are tender. 



