202 SOILS AND FERTILIZERS 



is derived from dissolved phosphate rock, and is in the 

 form of monocalcium phosphate. The citrate-soluble 

 is mainly dicalcium phosphate with variable amounts 

 of iron and aluminum phosphates in easily soluble 

 forms. The insoluble phosphoric acid is tricalcium 

 and other phosphates which are soluble only in strong 

 mineral acids. The insoluble phosphoric acid in fer- 

 tilizers is considered as having but little value. As in 

 the case of nitrogen three fertilizers may have the 

 same total amount of phosphoric acid and yet have 

 entirely different values. 



No. i. No. 2. No. 3. 

 Per cent. Per cent. Per cent. 



Water-soluble phosphoric acid. 8.00 0.25 0.25 



Citrate-soluble " " 1.50 8.00 0.75 



Insoluble 0.50 1.75 9.00 



Total 10.00 10.00 10.00 



No. 3 is of but little value ; the fertilizer contains 

 insoluble phosphate rock or some matarial of the same 

 nature. No i is the most valuable, because it con- 

 tains dissolved phosphate rock or dissolved bone and 

 but little insoluble phosphoric acid. No. 2 is com- 

 posed of such materials as the best grade of basic slag 

 or roasted aluminum phosphate or fine steamed bone. 



274. Potash. The three forms of potash in fer- 

 tilizers are: (i) water-soluble, (2) acid-soluble, and (3) 

 insoluble. Sulphate of potash, kainit, and muriate of 

 potash, are soluble in water and belong to the first 

 class. In some states the fertilizer laws recognize 

 only the water-soluble potash. In the second class 

 are found materials like tobacco stems and other 



