THE SOIL AS RELATED TO PLANTS. 91 



iible phosphate is found than when the mineral, 

 or rock phosphate, is thus treated. In this form 

 it is called "dissolved bone-black," and is a val- 

 uable fertilizer. 



Other commercial fertilizers containing phos- 

 phorus, with their comparative values, are given 

 in the table. 



(3) Potassium Compounds. — The potassium 

 in the soil is largely in the form of insoluble sili- 

 cates. The potassium salts of mineral origin 

 used as commercial fertilizers are nearly all ob- 

 tained from German mines ; those most common 

 are the sulphate, muriate, and kainit — a mixture 

 of several salts, as sodium, potassium, and mag- 

 nesium sulphates and muriates. All of these 

 are available for the use of the plant, since they 

 are soluble in water. Pure potassium sulphate 

 contains about 54 per cent, of potassium oxide, 

 but the composition of the commercial article 

 varies, some grades containing not more than 

 30 per cent. The muriate of potassium (KCl) 

 of commerce contains about 52 per cent, of 

 potassium. 



Ashes resulting from burning wood, cotton- 

 seed hulls, and tobacco stems contain from 5 

 to 30 per cent, of potassium carbonate. The 

 amount of potassium carbonate (K^CO,) in ashes 

 depends upon the kind and qualit}' of the wood, 

 the intensity of the heat in burning, and their 

 protection from moisture. Ashes also contain 



