CHAP. XII.] VALUATION OF FERTILISERS 341 



since all these materials possess different compositions, 

 a method of valuation must be found which will reduce 

 them to a common basis of the cost of the actual 

 fertilising ingredients alone — i.e.^ of the nitrogen, the 

 phosphoric acid, and the potash respectively. It is 

 possible, for example, to calculate the price per pound 

 of each of these fundamental substances, but the more 

 convenient method is to ascertain the cost of a unit 

 consisting of one-hundredth of a ton ; such unit cost 

 being obtained by dividing the price per ton of the 

 fertiliser by the percentage of the constituent in 

 question. 



It is the custom of merchants to set out the 

 analysis of the various fertilisers in terms of both 

 nitrogen and ammonia, and to express the phosphatic 

 constituents sometimes as phosphoric acid but more 

 commonly as tri-calcium phosphate, and again to give 

 the potassium in terms of potash, whatever may happen 

 to be the form in which the element is actually com- 

 bined. For example, nitrate of soda contains no 

 ammonia, and the statement that a given sample of 

 nitrate of soda contains 19 per cent, of ammonia is only 

 meant to signify that the 15-5 per cent, of nitrogen 

 present is equivalent to 19 per cent, of ammonia. In 

 superphosphate the phosphoric acid is chiefly combined 

 as di-hydrogen calcium phosphate, CaH^PgOg, hence 

 an analysis setting out the presence of 26 per cent, of 

 soluble phosphate or of tri-calcium phosphate rendered 

 soluble, must be read as meaning that it contains 11-9 

 per cent, of phosphoric acid soluble in water, which 

 amount of phosphoric acid would be also contained in 

 26 per cent, of tri-calcium phosphate. Similarly, muriate 

 of potash, which is potassium chloride — KCl — might be 

 described as containing 50 per cent, of potash — K2O — 

 though no true potash or potassium oxide is present ; 



