III.] AVAILABILITY OF NITROGENOUS MANURES 97 



In the main, however, it will be safer to regard the 

 fertilising effect of both sulphate of ammonia and nitrate 

 of soda as confined to the season of their application ; 

 the only residues they leave behind being due to the 

 increased root and stubble left in the land and the 

 extra nitrogen in the crop, some of which, e.g., the 

 nitrogen in the straw or the roots grown, does remain 

 on the farm as a permanent addition to the stock of 

 fertility. 



Wagner of Darmstadt has made a very extensive 

 series of comparisons of various nitrogenous manures 

 based upon experiments in pots, and from them has 

 compiled the following table, showing the comparative 

 recovery in the crop of 100 of nitrogen supplied in each 

 fertiliser. 



Table XXIX. — Return in Crop for 100 Nitrogen applied and 

 Relative Value when Nitrate of Soda = 100. Wagner. 



These results are, however, based upon the results of 

 pot experiments, which, because of the rapid variations 

 of temperature and the comparative concentration of 

 manures employed, are always somewhat unfair to 

 organic manures, especially to the bulky ones coming 

 at the lower end of the scale. 



Experiments have been carried out of late years at 

 Rothamsted to examine the question from a slightly 

 different point of view by means of field plots. The 



G 



