178 



AGRICULTURAL VALUES 



while the manurial constituents of foods are valued as follows : 



Organic nitrogen 

 Phosphorus pentoxide 

 Potash 



15 cents or 7d. per Ib. 

 5 2Jd. 

 5 2Jd. 



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, ac- 

 cording to experiments by Wagner in 1886 with wheat, barley and 

 flax, the effect of the various forms of phosphatic manures upon 

 plants abundantly supplied with potash and nitrogen are represented 

 by the following numbers 1 : 



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 agriculture 

 and undoubtedly furnish valuable, though somewhat empiric, informa- 

 tion. 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 superphosphates appears to yield better 

 results than it really does, because of the trade cus.tom of ignoring the 

 insoluble phosphates 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 per cent. 



1 Vide Thomas Phosphate Powder, by Prof. Wagner, Darmstadt, 1887. 



