696 



Journal of Agriculture, Victoria. [11 Nov., 191 2. 



Dicaleic phosphate (2('a(), H-iO, P2O5) is insoluble in water, but 

 soluble in wealc organic acids and in citrate of ammonia. It is the 

 form in which the phosphate is found in "reverted" superphosphate. 



Monocalcie phosphate (CaO, 2H-2( >, P2O;,). so named because one unit 

 of phosphoric acid is combined with one unit of lime, is freely soluble 

 in water, and is of innnediate value as plant food. It is the form 

 found in superphosphate, dissolved bones, &c., and corresponds to the 

 "water soluble phosphate" of the manure guarantees. 



The following table summarizes these facts : — 



1. Tricalcic phos-~» 



phate (iiisol- I" 

 uble phosphate ' 



2. Dicalcic phos- 



phate 

 verted 

 phate,' 



ly 



phosphate ") 



3. Monocalcie ^ 



phosphate | 



(water soluble j 

 phosphate) J 



Re-/ 



phos- V 



' slow- / 



soluble \ 



Composition. 



Lime "I Phosphoric 

 Lime c acid 



Lime -> Phosphoric 

 Lime J acid 

 WaterJ 



Lime \ Phosphoric 

 Water \ acid 

 Water > 



Formula. 



3Ca0 PaO- 



or 

 Ca., (POj^j 



2CaO HjjO P}0- 



or 



CajH^PjOs 



Ca0 2HsO PjO, 



or 



CaHiP,0. 



Insoluble in water 

 and citrate of 

 ammonia 



Insoluble in water ; 

 soluble in citrate 

 of ammonia 



Soluble in water . . 



Bones, guano, 

 mineral phos- 

 phate 



Reverted super. 



Superphosphate, 

 dissolved bones 



Note. — in addition, there is a fourth form of phosphate called tetracalcic phosphate, which has the 

 composition — 4CaO P.^O- — which is found in basic slag or Thomas' phosphate. The precise composition 

 of Thomas' phosphate is not known, but it is possible that the phosphoric acid is present in the form 

 of a double silicate and phosphate of lime, while smaller amounts are present as tetracalcic phosphate. 



Superphosphate. 



This is deservedly the most popular and profitable artificial manure 

 used in the wheat areas of Australia at the present time. There are 

 very few soils in the wheat areas proper which will not favorably and 

 profitably respond to applications of super., and in dry seasons many 

 soils fail altogether to produce a crop without soluble phosphates. 



In 1840, Liebig, the great German chemist, suggested that insoluble 

 tricalcic phosphate — the form in which phosphoric acid exists in bones 

 and rock phosphates — could be changed into the soluble monocalcie 

 phosphate by treatment with sulphuric acid. The product obtained 

 as a result of this action is superphosphate. 



The change which takes place may be simply represented thus. 

 When the sulphuric acid acts on the tricalcic phosphate, two parts of 

 lime are abstracted from the latter compound and two parts of water 

 substituted. A mixture of monocalcie phosphate and gypsum is, there- 

 fore, formed, which constitutes the superphosphate of commerce. The 

 action may be represented thus — • 



3CaO.P,0, + iMH.O SO3) = CaO •2H,0 P.,0, -+- 2CaS04 

 (Tricalcic (Surphuric (Monocalcie (Gj'psum) 



phcspliate) acid) phosphate) 



Superphosphate. 



"Reversion" op Superphosphate. 

 The rapidity of the action of superphosphate is due to its great 

 solubility. But under certain circumstances the soluble phosphate in 



