QUESTION OF THE SOURCES OF THE NITROGEN OF VEGETATION. 
27 
soil itself, and of the channels formed by worms, it is obvious that wherever the 
roots go, water and its contents can follow ; and that, with deep-rooted plants and 
free growth, there will be active movement of water, and there must be of air also, 
in the lower layers of the soil. In our former paper we called attention to the fact 
that in the experiments in 1882, with the greater growth of the Melilotus, there 
remained in the soil less water than in that of the Trifolium repens soil, corresponding 
down to a depth of 54 inches, to a loss of 540 tons per acre, or nearly 5^ inches of 
rain; and again, in 1883, the Vida saliva soil showed down to 108 inches, less water 
than the Trifolium repens soil, in amount corresponding to between 600 and 700 tons 
per acre, or to between 6 and 7 inches of rain. Obviously too, the still deeper rooting, 
and still freer growing Medicago saliva would remove still more water. 
Although much experiment and much calculation have been devoted by several 
investigators to the estimation of the degree of aeration of soils and subsoils of 
different character, the data at command do not justify any very definite conclusions 
on the subject. The results seem to indicate a probable range of aeration from about 
30 to over 50 per cent, of the volume of the soil. But these estimates do not take 
into account the varying amounts of water in the soil or subsoil. In the case of the 
subsoils referred to in this paper, each layer of 9 inches in depth retained from about 
2 to nearly 4 inches of water, the amount varying very much according to the nature 
of the subsoil, and especially according to the amount of growth, and the consequent 
withdrawal of water from below, and its evaporation, chiefly through the plant, but 
partly also from the surface soil. The amount must obviously also vary very much 
according to the character of the season. 
It may here be observed that supposing the subsoil contained at one time, air equal 
to one-third of its volume, this would not sufiice for the nitrification of as much 
nitrogen as was taken up for several years in succession by the Medicago saliva, or 
during two years in the case of red clover on the bean-exhausted land. But the 
nitrogen is not taken up all at once, though most of it will be within a few months 
of the year, during which period there would be the most active withdrawal of water 
from below, and evapojation by the plant and surface soil. The replacement of this 
subsoil water by an equal volume of air would, however, still not sufiice. The 
question obviously arises, how far, or how rapidly, the used up oxygen will be 
replaced, and on this point there is very little experimental evidence to aid us. We 
shall refer to the subject again further on. 
6. Can Rools, by virlue of llieir Acid Sap, attack, and render available, llie olkerivise 
insoluble Nilrogen of ihe Subsoil ? 
Thus, then, although the evidence is clear that the nitrogen of raw clay subsoils, 
which constitutes an enormous store of already combined nitrogen, is susceptible 
of nitrification, provided the organisms are j)resent, and the supply of oxygen is 
E 2 
