XII.] 



UNIT VALUES 



343 



of different fertiliijers may be entirely altered by the 

 carriage charges. In the instances quoted above the 

 two fertilisers contain nitrogen only and the calculation 

 is consequently of the simplest ; as a rule, however, more 

 than one constituent is present. The unit values of 

 the two have to be obtained by a little adjustment and 

 do not possess quite the arithmetical certainty which 

 characterises the single constituent fertilisers. Among 

 phosphatic fertilisers there are practically only three 

 which contain no other constituent, and in these the 

 unit value of tri-calcium phosphate may be calculated as 

 follows : — 



Table XCVII.— Price of Unit of Phosphate of Lime. 



The unit is dearer in the superphosphates because 

 acid has been employed to render them soluble in water. 



Supposing it is now desired to find the value of the 

 phosphate unit in certain other fertilisers which also 

 contain a little nitrogen, it is necessary to make a 

 deduction from the price of the fertiliser for the 

 nitrogen present, assuming this latter to have approxi- 

 mately the value calculated from such purely nitrogenous 

 fertilisers as the nitrate of soda already quoted. For 

 example, steamed bone flour containing 1-25 per cent, 

 nitrogen and 59 per cent, tri-calcium phosphate is quoted 

 at £df, 7s. 6d. per ton; taking nitrogen at 13s. per unit 

 the 1-25 per cent, would be worth i6s. 3d., which 



