346 VALUATION AND PURCHASE [chap. 



way, rape dust was formerly almost as cheap a source of 

 nitrogen as nitrate of soda and established itself in the 

 favour of hop growers and other farmers, who have 

 continued to demand it in the face of a falling supply 

 until the price per unit of nitrogen has been forced up 

 to nearly double its former level. 



In Peruvian guano the potash must also be taken 

 into account, and some of the phosphoric acid is water 

 soluble, so that a higher allowance must be made on 

 that account. 



When all these allowances have been made, it will 

 be found that the unit value of either nitrogen or 

 phosphoric acid shows considerable variation in passing 

 from one fertiliser to another, and that the relative 

 position fluctuates from time to time. Even in 

 fertilisers so similar in their use as nitrate of soda 

 and sulphate of ammonia the unit of nitrogen rarely 

 possesses the same price ; sometimes one and some- 

 times the other is the cheaper, the changes being 

 determined by factors of supply and demand outside 

 the fertiliser market, or by the operations of the com- 

 binations controlling the production and sale of each 

 commodity. Furthermore, in comparing the price of 

 the unit of nitrogen generally, it is not as might be 

 expected at its highest in the most active fertilisers 

 such as nitrate of soda, in which form experiments have 

 shown it to be most available to the crop. On the 

 contrary the experience of the market shows that 

 farmers are willing to pay more per unit for nitrogen 

 in organic than in inorganic combinations, thus in- 

 directly bringing into the account the value of the 

 organic matter in maintaining the texture of the soil. 

 The prepossession arising from an old experience of 

 the well-balanced nature of the manure and its safety 

 under almost any conditions also counts in the farmer's 



