QUESTIOJST OF THE SOURCES OE THE NITROGEN OE VEGETA^ITON. 
11 
The determinations of nitiic acid in the soil extracts, the results of which are 
recorded in the table, as well as those given in Table IL, were made in the 
Ilothamsted Laboratory by Mr. D. A. Louis, by Schlcesing’s method, as nitric oxide 
by the reaction with ferrous salts. For each of the twelve depths a mixture of the 
samples from the two holes was prepared, and in each of these mixtures duplicate 
determinations of nitric acid were made. As a control, determinations were also 
made in a mixture of the samples from the 10 lower depths, the third to the twelfth 
inclusive, and the results are given at the foot of the table. 
The first point to remark is, that there was much less nitrogen as nitric acid in the 
Trifolium re^^ons soil in 1885, after the removal of nearly 100 lbs. of nitrogen in the 
crops, than in 1883, when no crop had grown. The deficiency is the greatest in the 
2 upper layers, but it extends to the fifth depth, amounting to that point, which 
represents the range of the direct or indirect action of the superficial roots of the 
plant, to about 61 lbs. Below the range of this action, however, there is even more 
nitrogen as nitric acid in 1885 than 1883 ; due doubtless in part to percolation from 
above during the two preceding seasons without growth, and possibly in part to 
percolation of the nitrifying organisms, and nitrification of the nitrogen of the 
subsoil. 
Let us now turn to the results obtained on the Melilotus leucantha plot. As shown 
in Table III., it is estimated that, in 1882 as much as 145 lbs. of nitrogen was 
removed in the crop, and samples of soil taken that autumn, to the depth of 6 times 
9 inches, or 54 inches in all, showed only 8'4 5 lbs. of nitrogen as nitric acid remaining, 
which was 17’8 lbs. less than was found to the same depth in the Trifolium repens 
plot which had yielded only 74 lbs., or only about hnlf as much in the crop as the 
Melilotus. 
After 1882, however, the produce of the Melilotus declined very much, and in 1885 
the yield of nitrogen in the crop was estimated at only 58 lbs., against 97 lbs. estimated 
to have been removed in the Trifolium repens crop. Under these circumstances the 
Trifolium repens soil shows even rather less nitric acid than the Melilotus soil, in the 
second, third, and fourth depths, which comprise the chief range of action of the 
Trifolium repens roots. At every depth below the fourth, however (except the 12th, 
where the difference is very small), there is notably less nitrogen as nitric acid in the 
Melilotus than in the Trfolium repens soil, the Melilotus having yielded so much 
more in its crops in the preceding years than the Trifolium repens. To the total 
depth of 108 inches there was 24‘69 lbs. less nitric nitrogen remaining in the Melilotus 
than in the Trifohum repens soil. 
Admittedly we cannot know what was the stock of nitric nitrogen in either soil at 
the commencement of the growth of the season. But as during the 8 years 565 lbs. 
of nitrogen were removed in the Melilotus crops, against only 261 lbs. in the Trifolium 
repens, or more than twice as much in the Melilotus as in the Trifoliiim repens, it 
may be supposed that the Melilotus would both leave more nitrogenous crop-residue 
