ACID PHOSPHATE, SUPERPHOSPHATE 22.') 



when similarly mixed with manure, it is, nevertheless, a most 

 excellent phosphatic manure. Its cheapness recommends 

 it, as well as its high content of phosphoric acid. 



The fermenting mixture of rock phosphate and manure 

 has never showed any very greatly increased solubility of 

 phosphoric acid under laboratory conditions of extraction, 

 but field trials have abundantly proved that this mixture 

 is much better than rock phosphate alone or than manure 

 alone, showing that at least under field conditions the phos- 

 phoric acid is rendered sufficiently soluble for the growing 

 plant. 



174. Acid Phosphate, Superphosphate. — This is the best 

 known form of phosphate fertilizer. 



(a) How Made. — To make rock phosphate available it 

 is treated with sulphuric acid which converts the tricalcium 

 phosphate to monocalcium phosphate according to the 

 equation given under Dissolved Bone-Blaek. The finely 

 ground rock phosphate together with the right amount of 

 approximately 65 per cent, sulphuric acid is placed in 

 mixing chambers provided with stirrers. After being 

 thoroughly mixed the material is dumped into "dens" 

 where the reaction is completed. Considerable heat is de- 

 veloped, and the final product solidifies, due to the formation 

 of gypsum, CaS0 4 .2H 2 0. This mass after standing for some 

 time is crushed fine and is readv for use. 



In calculating the amount of acid to use, due regard is 

 paid to the ingredients other than tricalcium phosphate. 

 Calcium carbonate, of course, uses up sulphuric acid and so 

 do the iron and aluminium compounds. Iron compounds, 

 however, interfere with the formation of a good product, 

 if they are present in any considerable quantity — over 4 or 

 5 per cent. The resulting mixture is in bad physical con- 

 dition due to the formation of ferric sulphate and of some- 

 thing like a hydrated, acid iron phosphate. It does not dry 

 sufficiently to pulverize easily. Moreover, insoluble iron and 

 aluminium phosphates are formed, which, of course, are 

 worthless as quick acting fertilizers. 



Acid phosphate runs from 12 to 15 per cent, phosphoric 

 acid with 14 per cent, as the average content of available 

 15 



