QUESTION OF THE SOURCES OF THE NITROGEN OF VEGETATION. 
13 
crop, the increasing amounts of nitrogen yielded in the crops from year to year could 
not be so accounted for; and there would remain the amount of nitrogen in the crop- 
residue itself, still to be provided in addition. In fact, assuming the proportion of 
nitrogen in the crop-residue to that in the removed crop to be as supposed in the 
above illustration, nearly 700 lbs. of nitrogen would have been required for the 
Medicago crop and crop-residue of 1884 ; or if we assume the nitrogen in the residue 
to be only half that in the crop, about 500 lbs. would have been required. Doubtless, 
however, some of the nitrogenous crop-residue would accumulate from year to year. 
The results can leave no doubt that the Trifolium rej^ens, the Melilotus leucantha, 
and the Medicago scctiva, have each taken up much nitrogen from nitric acid within 
the soil. But, at any rate so far as the Medicago is concerned, there is nothing in 
the figures to justify the conclusion that the whole of its nitrogen can have been so 
derived. It is obvious that if nitric acid were the source of the whole there must 
have been a great deal formed by the nitrification of the nitrogen of the subsoil. The 
alternative is—provided the atmosphere be not the source—that the deep-rooted 
plant takes up nitrogen from the subsoil in some other way. 
3. Percentage of Nitrogen in the Surface Soils of the Experimented Plots. 
It has been stated in general terms that, although much more nitrogen had been 
removed from the leguminous crop soils than from the fallow-whear land, for nearly 
30 years prior to the commencement of the experiments with various leguminous 
plants in 1878, yet the leguminous crop surface soil remained rather richer in nitrogen 
than the fallow-wheat soil. It has also been stated that, during the subsequent 
years of experiment with the various leguminous plants, the surface soil had gained 
rather than lost nitrogen. It will be well to give a summary of the actual experi¬ 
mental results relating to these' points. 
In the first place it is to be borne in mind, as Table I. (p. 5) shows, that v,/hilst 
over the 27 years 1851-1877 inclusive, only about 17 lbs. of nitrogen were removed 
per acre per annum in the wheat grown in alternation with fallow, there was, over 
the 29 years, 1849-1877 inclusive, an average of about 32 lbs., or nearly twice as 
much, removed from the adjoining clover plots. 
During the years 1878, 1879, and 1880, the yield of nitrogen in the wheat was 
about the same as the average of the preceding 27 years ; whilst in most of the 
leguminous crops the yield was more than over the preceding 29 years. In the 
autumn of 1880 all the plots were ploughed ^ip, and at the end of March, 1881, before 
resowing, soil samples were taken from five places on the leguminous crop-land, and 
also from five on the portion of the wheat land which w^as then fallow. The samples 
were, in each case, ta.ken to the depth of 3 times 9 inches, or 27 inches in all. 
It has already been stated that no nitrogenous manure had been applied to either 
the fallow-wheat, or the leguminous crop land, for more than 30 years ; but that to 
