XII.] CHOICE OF FERTILISERS 347 



estimation of a fertiliser ; for example, the Peruvian 

 guanos have been favourably known for so long 

 that they always command the highest unit value. 

 Unmixed fertilisers, which require combining by the 

 farmer and so demand more knowledge in their use, 

 are generally the cheaper ; and as a rule, little attention 

 is paid to the imperfect availability of the slow-acting 

 forms of nitrogen, only the shoddies show any such 

 lowering of the unit value as would compensate for 

 their low availability. 



It is impossible, in fact, to reduce all fertilisers to a 

 common basis and choose among them simply accord- 

 ing to the unit value ; the wheat grower who wants a 

 nitrogenous top dressing must choose between nitrate 

 of soda, sulphate of ammonia, and soot, to which 

 nitrate of lime and cyanamide may nowadays be added ; 

 the hop grower requiring an all-round spring fertiliser 

 would not get the quality of growth from a mixture 

 of superphosphate and sulphate of ammonia that 

 is equivalent in nitrogen and phosphoric acid to 

 the guano he usually employs, though a fertiliser 

 made up from the former substances might be per- 

 fectly satisfactory to the grower of barley or Swede 

 turnips. The advantages of the unit system of valua- 

 tion really come in the means of comparison it affords 

 between fertilisers of closely related origin but different 

 composition, as, for example, between the fish and 

 meat guanos in Table C. ; it rarely happens but that a 

 careful enquiry will not reveal on the market some one 

 fertiliser of the desired kind which is considerably 

 cheaper than the rest. 



To this end it is generally more convenient to make 

 a slight change in the form of valuation just described ; 

 instead of calculating out for each manure the unit value 

 of the constituents, we m.ay take a standard series of 



