640 
Rotation oj Crops. 
Turning to the results with the rotation clover grown by 
the mixed manure, calculation shows that in the case of this 
continuously vegetating, unripened, and much higher nitrogen- 
yielding crop, there was very much more of both lime and carbonic 
acid in the ash for 100 of nitrogen assimilated than in the total 
bean-crop. If, however, the whole of the nitrogen of the clover 
crops had been taken up as calcium nitrate it would have 
required nearly twice as much lime as the amount found, pro- 
vided the whole of it remained ; nor would the amounts of 
potash and soda found suffice to make up the balance. Again, 
the amount of carbonic acid found is little more than two-thirds 
as much as would be required to represent organic acid equiva- 
lent to the amount of nitric acid subjected to change. Either, 
therefore, fixed base, partly in combination with organic acid, 
must have been eliminated from the above-ground parts of the 
plant, and passed into the roots, and possibly into the soil, or a 
good deal of the nitrogen must have been taken up in some 
other form than as nitrate ; possibly in part as organic nitrogen 
taken up from the soil by the agency of the acid sap ; or, in 
part as free nitrogen, probably brought into combination under 
the influence of micro-organisms within the nodules found on 
the roots of leguminous plants, the resulting compound being 
either directly available as a source of nitrogen to the host, or 
it may be so only after it has itself suffered change. 
However this may be, considering the very characteristic 
differences in the mineral composition of the different crops of 
rotation according to the amounts of nitrogen they assimilate, 
the fact that undoubtedly the highly nitrogenous Leguminosse 
do take up at any rate a lai-ge proportion of their nitrogen as 
nitrate, and that the greater the amount of nitrogen assimilated 
the more is the ash characterised by containing fixed base, and 
especially lime, in combination with carbonic acid, it seems very 
probable, if not indeed established, that the office of the lime, 
and partly that of the other bases also, is that of carriers of 
nitric acid ; which, when transformed, and the nitrogen assimi- 
lated, leaves the base as a residue, presumably in combination 
with organic acid. Further, the power of these plants to assimi- 
late so very much more nitrogen over a given area than the other 
crops may, at any rate in part, be dependent on their being 
able, by virtue of the range and character of their roots, to 
gather up more nitrogen in the form supposed than the plants 
with which they are alternated. Such a view does not, how- 
ever, exclude the supposition that some of their nitrogen is 
derived in other ways, as above referred to. 
In connection with the foregoing results of direct experi- 
