XII.] 



UNIT VALUES 



545 



variations in price ; its unit value is, therefore, the 

 one to be determined after deductions have been made 

 for the phosphates and potash at the rates quoted 

 above. For example, in the fish, meat, and oil cake 

 residues the phosphates are all of much the same order 

 of solubility and may be valued at the same rates, the 

 small amount of potash present also may be neglected, 

 so that the following range of values is obtained for the 

 nitrogen : — 



Table C— Unit Values in Mixed Fertilisers. 



The variation in the price of nitrogen in these very 

 closely related manures is therefore enormous, and the 

 important thing to realise is that these variations do 

 not represent intrinsic differences in value — i.e., greater 

 or less effectiveness in producing crops — but are market 

 variations due to temporary or local fluctuations of 

 supply and demand. As far as anyone knows the 

 nitrogen and phosphoric acid in fish guano are exactly 

 of the same value to the crop as in meat guano, and 

 only a few years ago they were to be obtained much 

 more cheaply in fish guano, the present high price of 

 which is due to a recent falling off in the supply, coupled 

 with a large new demand from Japan. Just in the same 



