IV.] MANUFACTURED PHOSPHATES 133 



be bought on the basis of an analysis. For a long time 

 it was customary in Germany to estimate only the 

 phosphoric acid in the slag which could be dissolved 

 by shaking the material for a specified time with 

 an alkaline solution of ammonium citrate, and to value 

 the slag on such a basis, the idea being that the solvent 

 differentiated between the available tetra-calcium phos- 

 phate which dissolved, and other compounds of phos- 

 phorus and phosphoric acid with iron, which possessed 

 no fertilising value and did not dissolve in the reagent. 

 Ammonium citrate as a solvent has, however, been 

 replaced by a 2 per cent solution of citric acid, and in 

 the United Kingdom the vendor of basic slag is now 

 compelled to give a guarantee of the percentage of 

 phosphoric acid that is dissolved when 5 grms. of the 

 slag are shaken in a litre bottle for half an hour with 

 500 c.c. of a solution containing 10 grms. of citric acid. 

 Though this method gives no absolute separation 

 between the different phosphates in the slag, it affords 

 a sufficiently good practical means of estimating what 

 are easily soluble and therefore of fertilising value. 



A few other manufactured phosphates find their way 

 into commerce, though none of them have much agricul- 

 tural importance as compared with superphosphate and 

 basic slag. Basic superphosphate is a form of precipi- 

 tated calcium phosphate introduced in 1901 by John 

 Hughes and made by mixing ordinary superphosphates 

 with sufficient lime to neutralise all the free acid and 

 convert the superphosphate into di-calcium phosphate, 

 leaving in addition a small proportion of free lime. 

 The material is very finely ground and forms a light 

 white very bulky powder, which remains dry and works 

 readily through any manure-sowing machine. On 

 analysis it shows a little over 12 per cent, of phosphoric 

 acid and about 4 per cent, of free lime, and as the phos- 



