58 TROPICAL AGRICULTURE chap. 



Chemistry posed of three equivalents of lime in combination with 

 phosphates, phosphoric acid, and it is called tribasic, or tricalcic phos- 

 phate because the base of the compound — the lime, is treble. 

 It may be thus expressed: — 



Lime. 

 Phosphoric acid 



c Lmie. 

 \ Lime. 

 V Lime. 



If bones be simply broken up and put into the soil, after a 

 time the tribasic phosphate becomes converted in a spar- 

 ingly soluble dibasic phosphate by the action of carbonic 

 acid and water. Thus— 



( Lime. , { f Lime. 



Phosphoric acid Lime. g Phosphoric acid . Water. 



Carbonic acid. 

 Water. 



Lime. ) g \ I Lime. 



% Carbonic acid and Lime. 

 . (or Carbonate of lime.) 



But when sulphuric acid is poured on tribasic phosphate, a 

 perfectly soluble monobasic phosphate^ and sulphate of cal- 

 cium (or gypsum), are formed. The chemical reaction is 

 easily understood from the following equation. 



r Lime. \ ^ \ ( Water. 



Phosphoric acid ( Lime. I S I Phosphoric acid ( Lime. 



[ Lime, f § \ Water. 



Sulphuric acid. ) '^ I Sulphate of calcium. 



This soluble monobasic phosphate is called a Superphos- 

 phate because the proportion of phosphoric acid to the lime 

 is above what it is in the other phosphates. 

 Objection to Reduced Phosphates. — Superphosphates are very soluble, 

 phates. °^ 3.nd it has been found, for this reason, that they are unsuit- 

 able for some soils, as they are too easily washed out by 

 rains, and it has been discovered also that they are too acid 

 for plant food, and, therefore, useless until they have been 

 so acted on by other substances in the soils as to be avail- 

 able for the use of the plant. In calcareous soils the super- 



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