THE SOIL AS RELATED TO PLANTS. 89 



considering its concentrated form, is compara- 

 tively inexpensive. 



(2) PHOSPHOROUS COMPOUNDS. The com- 

 pounds of phosphorus with lime, magnesia, iron, 

 and alumina are widely distributed in the soils, 

 but they are insoluble in water, and hence are 

 so slowly available as to be insufficient to fur- 

 nish the necessary supply for repeated crops. 



Phosphate of lime is the compound used most 

 in the manufacture of commercial fertilizers. 

 The mineral or rock calcium phosphate, or ani- 

 mal phosphates as, bone-black and bone-ash, or 

 animal bone is treated with sulphuric acid 

 (H 2 SO 4 ), in order to render the insoluble tri- 

 calcium phosphate, Ca 3 (PO 4 ) 2 , soluble. The sol- 

 uble phosphate made from the bone-black or 

 bone ash is best, because more of the phosphate 

 may be converted into a soluble form. It makes 

 a fine, dry, easily handled fertilizer. The insol- 

 uble, or tri-calcium phosphate, is treated with 

 sulphuric acid, and a large per cent, of it is 

 rendered soluble by two parts of the lime 

 uniting with the sulphuric acid to form gypsum 

 (2CaSO 4 ). This mixture of gypsum * and the 

 soluble phosphate (mono-calcium phosphate) is 

 sold as a fertilizer under the name of super- 

 phosphate of lime. It is probable also that 

 some of the tri-calcium phosphate loses only 

 one part of the lime and becomes di-calciurn 



* Remsen's Inorganic Chemistry, p. 328. 



