184 THE ANALYSIS OF MANUKES. VIII. 



Potash in sulphates and mixtures free from chlorine 5 2 



,, ,, chloride ... ... ... ... 4^ 2^ 



while the manurial constituents of foods are valued as 

 follows : 



Organic nitrogen ... 15 cents or 7-|d. per H>. 



Phosphorus pentoxide ... 5 ,, 2^d. ,, 

 Potash ... ... 5 ,, 2^d. 



It must be clearly understood that the above method of 

 valuing manures is based only upon trade prices and in no 

 way depends upon the results of agricultural experience. 



Tables have been constructed giving the relative value of 

 the same manurial constituent when applied to land in various 

 forms as measured by the increase in the crop produced. For 

 example, according to experiments by Wagner in 1886 with 

 wheat, barley, and flax, the effect of the various forms of phos- 

 phatic manures upon plants abundantly supplied with potash 

 and nitrogen are represented by the following numbers": 

 Superphosphate ... ... 100 



Raw guano ... ... ... 30 



Bone meal ... ... ... 10 



Coprolite powder ... ... 9 



Thomas slag, finest ... ... 61 



fine ... ... 58 



,, ,, coarse powder ... 13 



Field experiments with manures are highly valued in agri- 

 culture and undoubtedly furnish valuable, though somewhat 

 empiric, information. In many of these trials the manures are 

 taken at the usual trade valuation, and it is highly probable 

 that in a large number of cases the phosphoric acid of super- 

 phosphates appears to yield better results than it really does, 

 because of the trade custom of ignoring the insoluble phos- 

 phates present in a mineral superphosphate and only reporting 

 that existing in the soluble form. It therefore often happens 

 when a superphosphate containing a stated percentage of 

 " soluble phosphates" is employed in comparison with an 

 equal quantity of phosphates in, say, bones or basic slag, that 

 the " super" plot gets the benefit of the insoluble phosphates 

 in the manure, which may amount to 4 or 5%. 



* v. Thomas Phosphate Powder, by Prof. Wajrner, Darmstadt, 1887. 



