28 MANUFACTURE OF FERTILIZING MATERIALS 



most common form is in combination with chlorine 

 as common salt, an important ingredient of sea 

 water. Combined with silica sodium is an im- 

 portant element in many silicates. Sodium, al- 

 though closely related to potassium chemically, 

 cannot in any case be substituted therefor in 

 plant nutrition. While it is certain that plants 

 can thrive without a trace of sodium, it is believed 

 to be helpful in some cases, and its salts may 

 replace those of potassium in so far as osmotic 

 and neutralizing functions are concerned. In 

 combination with nitrogen it forms soda (or Chile) 

 saltpeter which is a valuable fertilizer on account 

 of its content of nitric acid. 



Iron is the most abundant of the heavy metals, 

 and occurs in nature both free and combined with 

 other elements. In the free state it is found 

 only to a limited extent in basaltic rocks and 

 meteorites, but in combination with oxygen it is 

 one of the most widely diffused of metals, and 

 forms the coloring matter of a large number of 

 rocks and minerals. In this form, too, it exists as 

 the valuable ores of iron known as magnetite and 

 hematite. In combination with sulphur it forms 

 the mineral pyrite, Fe$2. The yellow and red 

 colors of soils are due chiefly to iron oxides. Iron 

 salts are essential to the production of chloro- 

 phyll, the coloring matter in leaves, and to its 

 functional activity. Iron is one of the essential 

 constituents of the haemoglobin, or red coloring 

 matter therein, without which oxygen could not 



