Plant Food in the Soil 93 



The fish scrap, which is valued mainly as a source of 

 nitrogen, also has a considerable percentage of phos- 

 phoric acid, and as we have before said, there is a small 

 percentage of phosphoric acid in cotton-seed meal and 

 castor-oil pomace. 



Animal charcoal is made from bones. It is used in the 

 refining of sugar, and after this in the manufacture of 

 fertihzers for the percentage of phosphoric acid it con- 

 tains. It usually contains about 35 per cent, of phos- 

 phoric acid, but as its availability in the soil is slow it is 

 used less now than formerly. Bones gathered on the 

 Pampas of South America are burned and made into 

 what is called bone ash, which may contain 35 per cent, 

 of phosphoric acid. But the chief source of phosphorus 

 of late years is the phosphatic rock found in South Caro- 

 lina, North Carolina, Florida, Tennessee, and others of 

 our Southern States. The rock from different sections 

 varies in its percentage of phosphoric acid, that from 

 Tennessee being generally the richest. In South CaroHna 

 it is dug from the deposits on the coast and is also dredged 

 from the rivers. This rock is often ground to an impal- 

 pable powder spoken of in our first chapter imder the 

 name of " Floats." Chemists formerly attached no value 

 to this because of its insoluble state. But more recent 

 experiments have shown that in the soil it does become 

 gradually available, and its use is increasing because of 

 its greater cheapness as compared with ready dissolved 

 rocks, and also because of the fact that the rock dissolved 

 in sulphuric acid has a tendency to rob the soil of lime 

 carbonate, through the plant taking the phosphorus and 

 releasing the sulphuric acid. This latter at once unites 



