PHOSPHATIC MANURES 235 



phate is soluble in dilute citric acid, and others guarantee 

 the actual precentage of phosphate soluble in that reagent. 

 Such phosphate is probably all basic phosphate and readily 

 available to plants. 



In purchasing superphosphates, only one thing has to 

 be considered the percentage of soluble phosphates. In 

 purchasing basic slag, three things should be taken into 

 account: The percentage of total phosphates, the solu- 

 bility of the phosphate, and the fineness of the sample. 

 The solubility probably depends, to some extent, upon the 

 fineness, but when the former is guaranteed the latter is 

 of secondary importance. 



Artificial Basic Slag. The demand for Thomas' phos- 

 phate has increased so much in recent years that attempts 

 have been made to produce basic phosphates independently 

 of the iron industry. The process consists in fusing 

 apatites and phosphorites with an ordinary slag or flux 

 usually consisting of silica and lime. The product bears 

 a fairly close resemblance to Thomas' phosphate and has 

 been called artificial basic slag. The tricalcic phosphate 

 appears to be converted into a basic form, and thus 

 rendered more available to the plants. Over 90 per cent, 

 of the total phosphate is soluble in ammonium citrate. In 

 some varieties, e.g., the Wiborgh phosphate, potash is 

 introduced in the form of felspar. 



Precipitated Phosphate. Basic slag had been in use 

 for some years before it was realised that the phosphates 

 contained in the spent slag might be useful for agricul- 

 tural purposes. Then it was feared that the large propor- 

 tion of ferrous iron, sulphides, and other substances 

 known to be more or less poisonous to vegetation, might 

 prove injurious to the crops, and numerous processes 

 were invented to obviate this difficulty. One of the most 

 successful of these was to dissolve out the phosphate 

 with dilute acid, and reprecipitate it, chiefly in the form 



