Superphosphates 



every year in England, the phosphates for which 

 come from Florida, Algiers, and Tunis. There are 

 many other sources of mineral phosphates as well, 

 but these give the best results. The favourable 

 action of superphosphates, especially on leguminous 

 plants and on many root crops, is seconded by the 

 presence of gypsum, of which we will describe the 

 effect on page 141. 



Thus nearly the whole of the phosphoric acids m 

 superphosphates are soluble in water, but in some 

 superphosphates, particularly when they contam 

 compounds of iron and aluminium, the phosphate 

 reverts when stored ; that is to say, that though it 

 was monocalcic and soluble in water [CaH4(P04)2] it 

 returns almost to a bicalcic phosphate [Ca2H2(P04)2], 

 which is not soluble in water, but is so in weak 

 acids. 



Perhaps too much importance has been attached 

 to this fact, because in reality this bicalcic phosphate 

 can be regarded as workably assimilable ; besides 

 in the soil the monocalcic phosphate quickly returns 

 to the bicalcic form, or something corresponding. 

 The only advantage the monocalcic presents over 

 the bicalcic is that the former being dissolved by the 

 rain is more easily and completely distributed in the 

 soil, up to the time that it is fixed by the carbonate 

 of lime or the compounds of aluminium or iron 

 in the soil. If rain is slow in coming, however, it 

 will be already fixed and no longer soluble. 



Superphosphate will be decidedly better if it 

 contains much phosphoric acid [P2O5], and it is 

 obviously wise to buy the richest, because, the price 

 being gauged by the imit, transport will be cheaper 



51 



