PHOSPHORUS AND PHOSPHORIC ACID 97 



chloride, and sodium phosphate solutions are added to the liquid to be 

 tested. A white crystalline precipitate shows the presence of magnesium in 

 the solution. 



Phosphates and Phosphatic Manures. — Phosphates occur very abun- 

 dantly in nature. Bones contain phosphate and other salts of calcium (in 

 addition to gelatine and fat), and "bone earth," the white ash left when bones 

 are burnt, chiefly consists of phosphate and carbonate of calcium. Peruvian 

 guano, the product of the partial decay of sea-birds' excrements, chiefly 

 consists of phosphate of calcium mixed, when the guano occurs in rainless 

 districts where the moisture is insufficient for complete putrefaction and 

 nitrification, with phosphate and urate of ammonium. The coprolites found 

 in Suffolk and Cambridgeshire chiefly consist of phosphate of calcium. 

 Canadian appatite is a double phosphate and chloride of calcium. 



The phosphorus required by plants is absorbed from the soil in the 

 form of soluble phosphates, these being slowly formed by the action of the 

 carbonic and other acids of the soil and of the acids in the root sap upon 

 the insoluble phosphates. The soil is, however, frequently deficient in 

 "available," i.e. easily soluble, phosphates, and phosphatic manures are 

 then required. Now tricalcic phosphate as it occurs in bones, coprolites, 

 appatite, and phosphatic guano [i.e. guano containing little or no com- 

 bined nitrogen) is only very slowly dissolved by the weak acids existing in 

 the soil, even when in a fine powder, and it is usual, therefore, previously to 

 convert, into soluble phosphate by sulphuric acid. The "superphosphate 

 of lime " thus produced is a mixture of monocalcic phosphate and gypsum 

 for 



Ca32P04 + 2H2SO4 = CaH42P04 -f 2CaS04. 



The substances produced are known as "dissolved guano," "dissolved 

 bones " or " bone superphosphate," and " mineral superphosphate," accord- 

 ing to their source. 



When the monocalcic phosphate has been used as a manure and has 

 been washed into and diff'used through the soil by rain, it "reverts" into 

 the insoluble dicalcic phosphate owing to the action of the bases of the 

 soil. This being deposited in a fine state of division is easily redissolved, 

 and it is therefore "available" for plant food. The fact that nitrogenous 

 guano contains phosphate of ammonium renders this manure especially 

 valuable, for this soluble phosphate is a neutral salt and therefore does not 

 so easily "revert" in the soil into an insoluble form. "Reversion" is 

 also noticed when superphosphate of lime is kept for some time, for the 

 monocalcic phosphate reacts with unchanged tricalcic phosphate in the 

 manure yielding the intermediate dicalcic phosphate. 



Iron made from Cleveland iron ore contains a large amount of phosphide 



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