MANURES 117 



in every sense, be better to report the analysis of a phosphatic 



manure thus : 



Per cent. 

 Total phosphorus pentoxide ; 



Soluble phosphorus pentoxide .... 



Citrate soluble phosphorus pentoxide 



Insoluble phosphorus pentoxide .... 



The meaning of these terms would be precise and definite. 



In assessing the value of a manure from its analysis it is 

 usual to employ what are known as values " per unit " for the 

 three chief fertilising ingredients. These are the commercial 

 values (and therefore liable to fluctuation) of each per cent, 

 per ton, so that really they are the values of 22'4 lb. of the 

 particular ingredient. Thus, suppose nitrate of soda contain- 

 ing 15*75 per cent, of nitrogen be worth £8 per ton, then the 

 value of the nitrogen will be 



8x20 



jTTy^ = 10'2s., or 10s. 2^d. per "unit." 



In sulphate of ammonia at £12 per ton, containing 24*5 per 

 cent, ammonia, equal to 24-5 x — = 20-2 per cent, nitrogen, 

 the value " per unit " of nitrogen will be — 



12 X 20 



2u-2 



= 11-835. = lis. 10c?. 



In a similar way the values " per unit " of phosphorus 

 pentoxide and of potash can be calculated from the market 

 prices of the various manures. In these cases, as with nitro- 

 gen, the values will vary with difierent manures. In most 

 tables of values "per unit," "ammonia" and "phosphates" 

 are the substances valued, but for reasons already given it is 

 preferable to give the figures for nitrogen and phosphorus 

 pentoxide. 



The values "per unit," as already stated, are liable to 



