ARTIFICIAL AND CONCENTRATED MANURES. 89 



but little used in this country. They are obtained from 

 the rainless districts of the world, chiefly from the 

 islands bordering the coast of South America and from 

 the West Indies. They are derived from the excrements 

 of birds, and frequently contain considerable organic 

 matter containing nitrogen. 



The Peruvian guano of earlier times was particularly 

 rich in the best forms of nitrogen. The purely phos- 

 phatic guanos are rich in phosphoric acid, and are excel- 

 lent materials; like the iron phosphate, they are not 

 suitable for the manufacture of artificial manures. 



Insolubility of Phosphates. — The phosphates men- 

 tioned constitute what are called "raw materials," and, 

 with the exception of bone, are not largely used directly, 

 or without further treatment to render the phosphoric 

 acid more soluble, and thus more immediately available 

 to plants. As already stated, the phosphoric acid in 

 them becomes food in proportion to the rapidity of de- 

 cay, which is influenced both by the character of the 

 material and the fineness of its division. Eine mate- 

 rials, too, permit of a more even distribution, thus bring- 

 ing more particles of phosphate in contact with the 

 roots of the plants. 



