VII. 



SPECIAL MANURES 143 



When treated with a small quantity of water a portion of the salt is 

 decomposed, with the formation of dicalcium phosphate as a precipi- 

 tate and free phosphoric acid ; but with larger quantities of water or 

 in the presence of free phosphoric acid, this does not occur. 



Free phosphoric acid, H 3 PO 4 , is a thick semi-solid mass, of specific 

 gravity 1-88, formed by decomposing, say, calcium phosphate with 

 sulphuric acid : 



Ca 8 P 2 8 + 3H 2 SO 4 = 3CaS0 4 + 2H 3 P0 4 . 



It is soluble to practically any extent in water. 



Ferric phosphate, FeP0 4 , and Aluminium phosphate, A1PO 4 , are 

 practically insoluble in water and, unlike tricalcium phosphate and 

 most other phosphates, are not dissolved to any appreciable extent by 

 weak vegetable acids, e.g., acetic acid. Consequently they are not 

 easily available to plants and possess little value as manurial ingredients. 

 Tetracalcium phosphate, Ca 4 P 2 O 9 , is found in the slag produced in 

 the dephosphorisation of cast-iron by the Basic Bessemer or Basic 

 Siemens process. It is practically insoluble in water, but dissolves in 

 many saline solutions. It is therefore available as a plant food. 



The chief varieties of phosphatic manures yet to be described 

 are 



Coprolites. 



Phosphorites, of which there are many varieties. 



Kedonda phosphate. 



Mineral superphosphates. 



Bone superphosphate. 



Basic slag. 



Goppolites are concretionary nodules found in the chalk or other 

 deposits in the South of England and in France ; they are believed to 

 be the fossilised excrements or intestinal deposits of extinct animals 

 which fed upon fish. 



They were formerly of great importance and are still largely used. 

 Their composition varies considerably, the chief constituents being 



Calcium phosphate 50 to 65 per cent. 



Calcium carbonate 20 ,, 25 ,, 



Silica, etc 10 20 



They are sometimes used raw in a finely divided state, but are more 

 frequently converted into "superphosphate". 



Of mineral phosphates or phosphorites there are many varieties, 

 the most important being Norwegian, Canadian, Sombrero, Belgian, 

 Carolina, Florida and Somme. These are occasionally employed in 

 the raw state, but are mainly used in the preparation of "mineral 

 superphosphates ". They all consist essentially of more or less impure 

 apatite, some containing calcium chloride, others calcium fluoride, and 

 some both compounds. In the process of manufacture of superphos- 

 phate the first variety is much preferred, for reasons which will be 

 mentioned shortly. 



Another factor of importance in gauging the value of a mineral 

 phosphate is the proportion of iron and aluminium which it contains. 



