60 SOILS 



to be made into soap. This is one bit of evidence that 

 potassium is present in plants in considerable quantities ; 

 and hence must be present in the soil. 



Common forms in which potassium unites : 



1. With chlorine forming muriate of potash. 



2. With sulphur and oxygen and aluminum forming 

 alum. 



3. With nitrogen and oxygen forming potassium 

 nitrate (saltpeter). 



Potassium is often lacking in soils. It is one of the 

 three elements the others being phosphorus and nitro- 

 gen most often purchased in commercial forms to re- 

 inforce the insufficient quantity in the soil. In the arts 

 and manufactures the potassium compounds are very im- 

 portant, being used in glass making, soap making, in 

 fertilizers, and in many drugs and chemicals. 



Sodium. When isolated from its compounds, this ele- 

 ment is waxy, white, and so readily oxidized that it acts 

 violently upon water, and so to be preserved must be kept 

 under petroleum or some similar liquid. It is present 

 in the soil in sufficient quantities to supply all needs of 

 the plants for it. We know this element as an ingredient 

 of common salt, of sodium bicarbonate, or soda just 

 plain baking soda, of sodium carbonate or sal soda, and 

 of caustic soda. 



Calcium. This element, when united with oxygen, 

 forms lime. It is pale yellow in color when separated 

 from its compounds. The following are its principal 

 compounds : Calcium carbonate or limestone, calcium 

 sulphate or gypsum, calcium fluoride or fluor spar, and 

 calcium phosphate or apatite. 



Magnesium. A light silver-white substance, essential 

 to plants. Commercially we know of it by its com- 

 pounds: Epsom salts as a medicine, talc as a skin pow- 



