172 SOILS AND FERTILIZERS 



by the Bessemer process, the phosphorus in the iron is 

 removed as a basic slag. The lime, which is used as a 

 flux, melts and combines with the phosphorus of the 

 ore, forming phosphate of lime. The slag has a varia- 

 ble composition. The process by which the phos- 

 phorus of pig iron is removed-and converted into basic 

 phosphate slag is known as the Thomas process, and 

 the product is sometimes called Thomas' slag. At the 

 present time but little basic slag is produced for fer- 

 tilizer purposes in this country. In Germany and 

 some other European countries large amounts are 

 used. Phosphate slag is ground to a fine powder and 

 is applied directly to the land, without undergoing 

 the sulphuric acid treatment. The phosphoric acid is 

 present mainly in the form of tetracalcium phosphate, 

 (CaO) 4 P,0 5 . 



220. Guano is the Spanish for dung, and is a concen- 

 trated form of nitrogenous and phosphate manure of 

 interest -mainly on account of its historic significance. 

 It is a mixture of sea-fowl droppings, accumulating 

 along the seacoast in sheltered regions, with dead 

 animals and debris, which has undergone fermen- 

 tation. Guano and is concentrated in both nitro- 

 gen and phosphoric acid. The introduction of guano 

 into Europe marked an important period in agri- 

 culture, inasmuch as its use demonstrated the action 

 and importance of concentrated fertilizers. All of the 

 best beds of guano have been exhausted and only a 

 little of the poorer grades are now found on the mar- 

 ket. The best qualities of guano contained from 12 



