124 ELEMENTS OF AGRICULTURE 



ground and sold as a fertilizer under the name "floats." 

 There is an increasing amount of floats used, particularly 

 in the central west, where the soils contain considerable 

 amounts of organic matter. But most of the rock is 

 treated with sulfuric acid so as to render it soluble. The 

 product is called acid phosphate, or dissolved rock. In 

 this form it contains about 14 per cent of phosphoric 

 acid, 1 and costs about $14 per ton in New York City. 

 When applied to the soil, it reverts to the original in- 

 soluble form. Being soluble when it is applied, it is 

 distributed in the soil moisture. When it reverts it is 

 deposited on the outside of innumerable soil grains. This 

 gives a larger area exposed to the action of soil water, 

 so that it will dissolve and supply plants faster than it 

 would in the finely ground form. 



126. Bone. Bones are sometimes finely ground to form 

 bone meal, or treated with sulfuric acid to form dissolved 

 bone. They are also used as bone ash, steamed bone meal 

 and bone black. The amount of phosphoric acid varies 

 from 18 to 36 per cent. Good bone meal contains about 

 4 per cent nitrogen and 22 per cent phosphoric acid. 



127. Thomas Slag. This material is a by-product 

 from the manufacture of steel. It is not used to a very 

 great extent in America at present. It is not acid in its 

 nature and so has an advantage over acid phosphate. 

 It is sometimes called "basic slag." 



lr The composition of phosphatic fertilizers is usually given in terms 

 >f phosphoric acid (P ? Os). Such a compound does not exist in fertilizers, 

 jut it furnishes a basis for comparison. Phosphoric acid costs about four 

 and one-half to five cents per pound. The composition of potash fertilizers 

 is also expressed in terms of a substance that does not occur in the fertilizers, 

 potash (K^O). In both cases it would be much more desirable to have the 

 composition expressed in terms of elements, phosphorus (P) and potassium 

 (K). An effort is now being made to change to this system, but it has not 

 beeu generally adopted. 



