THE SOURCES OF THE NITROGEN OF VEGETATION, ETC. 
441 
Nitrogen supplied in the manure ; and when the double amount of the manure was 
employed for barley, over the same series of years, only about 43 per cent, of the sup- 
phed Nitrogen were recovered as increased yield. 
To the statement of these facts it should be added that the Nitrogen (equal in amount 
to, say 60 per cent, of that supplied in the manure) which is not obtained as increased 
yield in the immediate crop does not appear to exist in the soil availably for an imme- 
diately succeeding crop. Thus, when by the use of nitrogenous manures an increased 
yield of Nitrogen has been obtained in the first succeeding wheat-crop, equal in amount 
to about 40 per cent, of the Nitrogen supplied in the manure, the increased yield obtained 
in the second crop, without any further supply, is equal to little more than one-tenth of 
the remainder. 
In connexion with this subject it may be mentioned that, so far as our experiments with 
meadow-grasses at present show, it does not appear that the increased yield of Nitrogen 
in the crop on the use of nitrogenous manures bears a much higher proportion to the 
amount supplied in their case than in that of either wheat or barley. In the case of 
the Leguminous corn-crops, the proportion of the increased yield to the amount supplied 
appears to be even less than in that of the Cereal grains. Eoot-crops, on the other hand, 
would seem to gather up an increase of Nitrogen bearing a larger proportion to the 
quantity directly supplied in the manure. 
On the assumption that the relation of the immediately increased yield of Nitrogen 
to the amount supplied in manure represents really or approximately the proportion of 
the directly supplied Nitrogen which is actually recovered in the immediate crop, the 
following questions seem to suggest themselves ; — 
Is the unrecovered amount of supplied Nitrogen, or at any rate a considerable pro- 
portion of it, drained away and lost 1 
Are the nitrogenous compounds transformed within the soil, and theh Nitrogen, in 
some form, evaporated! 
Does the missing amount for the most part remain in some fixed combination in the 
soil, only to be yielded up, if ever, in the course of a long series of years ! 
Is ammonia itself, or Nitrogen in the free state, or in some other form of combination 
than ammonia, given off from the surface of the growing plant ! Or, lastly. 
When Nitrogen is supplied within the soil for the increased growth of the Grami- 
naceous crop, is there simply an unfavourable distribution of it, considered in relation 
to the distribution of the underground feeders of the crop! — the Leguminous crop, 
which alternates with it, gathering from a more extended range of soil, and leaving a 
residue of assimilable Nitrogen within the range of collection of a next succeeding 
Cereal one ! 
But other and wider questions than those just enumerated present themselves on a 
careful review, as a whole, of the Nitrogen-statistics of field-produce to which attention 
has briefly been directed. For the moment, all may be asked in one — ^namely. What 
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