308 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



The calcium sulphate is filtered off, the solution of phosphoric 

 acid concentrated and then mixed with phosphate rock. 



Ca 3 (P0 4 ) 2 + 4 H 3 P0 4 = 3 CaH 4 (P0 4 ) 2 . 



2Ca 3 (P0 4 ) 2 + 2H 3 P0 4 = 3 Ca a H 2 (P0 4 ) 2 . 



The product contains 30 to 45 per cent, available phosphoric 

 acid, depending on the degree of concentration of the acid and 

 the kind of rock used. 



Availability of Phosphatic Fertilizers. The values of different 

 forms of phosphoric acid are compared in the same way as nitro- 

 gen, namely, plants are grown under such conditions that phos- 

 phates are the limiting factor and a comparison made of the 

 amounts of phosphoric acid taken up by the crops. 



The availability of phosphoric materials depends upon other 

 conditions in addition to the form of combination of the material, 

 such as the presence of carbonate of lime or some other sub- 

 stances in the soil, the fineness of the material, the nature of the 

 plant, etc. The effect of these conditions has not been studied to 

 a great extent. 



Potash Materials. Potash is of relatively less importance than 

 nitrogen or phosphoric acid, because potash is more abundant in 

 the soil than either nitrogen or phosphoric acid, and, though 

 larger quantities are removed, the potash is more likely to be re- 

 turned. The potash taken up is most largely in the stems and 

 leaves of plants, that is, the portion of the plant which is gen- 

 erally returned to the soil either directly, or indirectly in manure. 

 When the entire crop is removed, the loss of potash is large. 

 Potash is a very necessary constituent of fertilizers for some soils 

 and some crops. 



The chief commercial potash materials are tobacco stems, wood 

 ashes, and the German potash salts, kainit, muriate of potash, and 

 sulphate of potash. 



Tobacco stems are a by-product from tobacco factories. They 

 contain 6 to 8 per cent, potash, 2 to 2.5 per cent, nitrogen, and 3 

 to 5 per cent, phosphoric acid. They are used largely as an 

 insecticide, but may sometimes be secured for a sufficiently low 

 price to allow their use as a fertilizer. 



