8 SOURCES OF THE NITROGEN OF VEGETATION 



There remained, however, a number of facts difficult to 

 account for : although laboratory experiments similar to those 

 just described, but resulting in a gain of nitrogen, could be dis- 

 missed as vitiated by the many possible sources of error, yet 

 the statistics of the nitrogen collected by various crops could 

 not be explained in any such fashion. It has already been 

 mentioned that Boussingault made out a balance-sheet of 

 nitrogen supplied in manure and removed in crop during 

 different rotations ; he found that while in a rotation of wheat 

 and fallow alone the wheat contained rather less nitrogen than 

 was applied as manure, yet other rotations in which clover was 

 included, and particularly a five years' continuous cropping 

 with lucerne, gave a large surplus of nitrogen removed over 

 that supplied. Similar evidence was accumulated at Rotham- 

 sted and was made more cogent by the analysis of the soils, 

 which showed not only no decrease but an actual gain of 

 nitrogen during the period when the leguminous crop was 

 producing such large quantities of nitrogenous matter above 

 ground. Thus when the various crops were grown con- 

 tinuously with mineral manures* but without any supply of 

 combined nitrogen, the following average amounts of nitrogen 

 per acre were taken away : — 



Table II. — Average Removal of Nitrogen per acre hy Crops 

 grown continuously vnth Mineral Ifanures only. 



In a comparison of the alternate wheat and fallow plots 

 with the adjacent plots continually under leguminous plants, 



* The term mineral manures will be used throughout for mixtures of the 

 constituents found in the ash of plants, i.e., phosphates, sulphates and chlorides 

 of sodium, potassium, calcium and magnesium, but always excluding nitrogen 

 in any form. 



