26 MANUFACTURE OF FERTILIZING MATERIALS 



dioxide in the mineral dolomite. It is intimately 

 associated with calcium and a trace of it is nearly 

 always found where lime occurs in any consider- 

 able quantity. The bitter taste of sea water and 

 some mineral waters is often due to the presence 

 of salts of magnesia. In combination with silica 

 it forms an essential part of such rocks as serpen- 

 tine, soapstone, and talc. Magnesia is useful, 

 especially in the formation of seeds, but it is not 

 absent from other parts of the plant. It performs 

 its functions best in the presence of calcium com- 

 pounds and in the absence of the latter magnesia 

 salts may easily be injurious. 



Potassium combined with silica is an important 

 element in many silicates as, for instance, ortho- 

 clase, Granite rocks usually contain consider- 

 able quantities of potassium, and on their decom- 

 position this becomes available for plant food. 

 In the form of chloride, potassium is found in 

 small quantities in sea water, and as a nitrate it 

 forms the valuable salt known as niter or salt- 

 peter. Potassium, as is the case with phosphorus, 

 is universally distributed in soils, and forms one 

 of the great essential elements of plant food. 

 Under the form of kainite and other minerals 

 large quantities of potassium are used for fertiliz- 

 ing and for the manufacture of pure salts for com- 

 mercial and pharmaceutical purposes. The or- 

 dinary potassium salts are very soluble and for 

 this reason they cannot accumulate in large quan- 

 tities in soils exposed to heavy rainfall. In the 



