SOILS AND FERTILIZERS. 



379 



exceed the value ; where the difference is great either way— that is, 

 where the price is much higher or much lower than the manurial value 

 — investigation and caution in buying is called for. 



TABLE 6 —Quantities of Feriilizers in pounds per acre to supply a Crop with Potash. 



Phosphoric Acid. — The phosphoric acid supply for fertilizer pur- 

 poses is more varied than that of potash ; its four main sources are: 

 Pirst, bones, either as fresh or spent bone-black ; 2d, phosphatic de- 

 posits like those of South Carolina or the keys of the Caribbean sea ; 

 3d, accumulations at certain places of fossil, chiefly excrementitious 

 material; 4th, Thomas slag, a by-product, recently brought into market, 

 of the smelting of iron ores. In neither of these materials the phos- 

 phoric acid is readily available, and each requires to be ground into an 

 exceedingly fine and impalpable powder, or else to be treated with oil 

 of vitriol to render the phosphoric acid soluble. The first is a simple 

 mechanical process, which makes the product cheaper though less 

 effective than the second, which is a chemical treatment with variable 

 quantities of acid, requiring both knowledge and skill. The prices paid, 

 therefore, per pound of phosphoric acid are in accordance with its 

 degree of solubility, and this, again, depends upon the treatment the 

 raw material has received by the manufacturer; they are about as fol- 

 lows : 



Raw material, ground moderately fine, phosphoric acid 2 cents a pound. 



Raw material, finely pulverized or floated, phosphoric acid 4 cents a pound. 



Superphosphate (raw material treated with acid ), phosphoric acid 6 cents a 

 pound. 



