120 AGRICULTURE 



In ordinary commercial practice in this 

 country, these and similar nitrogenous 

 manures are generally valued not on the 

 basis of so much per pound of nitrogen, but 

 by what is called the system of units. As 

 illustrating this system we may take the 

 two manures under consideration and adhere 

 to the prices of 10 and 12 per ton re- 

 spectively. To find the value per unit of 

 nitrogen in the case of the nitrate of soda 

 we divide the price per ton (10) by the 

 percentage of nitrogen (15*6), and the result 

 is 12-82 shillings say 12s. lOd. Similarly 

 treating the sulphate of ammonia, we divide 

 12 by 20'6, when we find that the value of 

 a unit of nitrogen is 11*65 shillings say 

 11s. 8d. There is therefore a difference of 

 Is. 2d. per unit of nitrogen in favour of the 

 sulphate of ammonia. Multiplying 20*6 by 

 Is. 2d. we get l, 4s., which, when added to 

 12, gives us the price per ton, 13, 4s., 

 at which we might purchase sulphate of 

 ammonia, and still get as good value as is 

 represented by nitrate of soda at 10 per 

 ton. Or, we get at the same result if we 

 multiply 20-6 by 12s. 10d. = l3, 4s., and 

 from this deduct 12 = 1, 4s., and the 

 latter is the usual way of making the com- 

 parative calculation. On the other hand, 



