183 



Reverted, reduced, or precipitated phosphoric acid is phosphoric 

 acid, which was once soluble in water, but which, by combination 

 with lime, iron, or aluminium in the soil, has become insoluble 

 again. In that form it is, however, readily assimilated by crops. 



The chief sources of phosphatic manures are : 



Bones are found in commerce broken up as " half-inch," 

 " quarter-inch," and as " bonedust ;" the finer the better, as they 

 act more quickly. Commercial bones are either "raw" or "steamed" 

 and degelatinised. Their respective composition is 



100-0 100-0 



* Equal 3'5 to 4*5 ammonia, t Equal 1*4 ammonia. 



Bonedust and half and quarter-inch have a similar composi- 

 tion to the raw product they are made from. The coarser the bones 

 the heavier should the dressing be. Bonedust is applied at the 

 rate of 3 to 4cwt. to the acre. 



Phosphatic Guanos, such as Abrolhos and Sharks Bay guanos 

 contain 44 to 50 per cent, of phosphate and only \ per cent, of 

 ammonia, the balance being mostly sand and moisture. They can 

 be got in several ton lots at a very reasonable price, and answer 

 well when mixed with a small proportion of a more soluble phos- 

 phate, such as concentrated superphosphate and sulphate of 

 ammonia. A fair dressing would be 3 to 4cwt. to the acre. 



Thomas' Phosphate Powder, or Basic Slag, is another, source of 

 phosphates. It is derived by means of the Thomas G-ilchrist 

 process in making steel from pig iron by smelting it in converters 

 lined with lime ; when the iron is melted the air is blown through, 

 part of the iron and impurities are oxidised, and the phosphoric 

 acid combines with the lime, forming phosphate of lime. This 

 contains 14 to 17 per cent, of phosphoric acid, equal to 30 to 36 

 per cent, of phosphate of lime, and also 40 per cent, lime, and 

 18 per cent, iron oxides, and 6 per cent. sand. It is very cheap 

 in England, where it sells at the factories for 1 a ton, and is 

 retailed at 30s. It is so finely divided that it acts very readily. 



Coprolites and Apatites constitute mineral phosphates of great 

 value, either ground into a fine powder or for making super- 

 phosphates ; they occur in some geological formations under the 

 form of concretions, and consist of the fossil excrements and 

 remains of extinct animals. They vary in richness from 10 to 75 

 per cent, of phosphates. 



Superphosphate of Lime. Either of the raw materials referred 

 to already are ground and treated with sulphuric acid, thereby 



