The Food of our Agricultural Crops. 
81 
comes ; and it has the same tendency to spread over tbe perma- 
nent wheat- and barley-ground. This fact, taken in connection 
with the luxuriant growth of the clover upon the field where 
the bean experiments had been carried on, seems to indicate the 
formation of some compound in the soil of which we are still 
ignorant, but which serves as food for leguminous plants. It is 
evident that this subject opens a wide field for special investiga- 
tion, and it is fortunate that we possess samples of soil taken 
with great care from our various experimental fields at different 
periods, the history of which, in regard to the manures applied 
and the produce removed, has been recorded for a long series of 
years. 
Although I have only referred to the experiments of 
Hellriegel upon the fixation of nitrogen by leguminous plants, 
and the repetition of these experiments by ourselves, numerous 
others upon the same subject have been carried out, in France, 
Germany, and the United States, which appear to prove that 
soils without vegetation fix large quantities of nitrogen ; and 
also that, in those soils where plants not of the leguminous order 
have been grown, still more nitrogen is fixed. As a deduction 
from all these experiments, it has been argued that compensation 
must exist somewhere, otherwise the human race would, sooner 
or later, starve, owing to the exhaustion of the soil. If we 
accepted this idea, we should have to ignore the results of the 
last fifty years' experiments, and go back to the view expressed 
long ago, that plants can obtain all the nitrogen they require 
from the atmosphere, provided they are furnished with sufficient 
mineral food. As far as regards agricultural land under our 
ordinary grain or root crops, we have no evidence to show that 
the soil or the plant derives any appreciable amount of nitrogen 
from the atmosphere, beyond what is furnished by rain or dew. 
Where all artificial supplies of nitrogen in manures have been 
withheld for between forty and fifty years, and an abundance of 
minerals has been furnished, the yield of the crops is now so 
small, and the amount of nitrogen which they carry away bears 
so small a proportion to the quantity stored up in the soil within 
reach of their roots, that any exact comparison of the nitrogen 
carried off in the crops, and washed out of the soil by drainage, 
with the loss of nitrogen in the soil as determined by direct 
analyses, is hardly possible ; but, as far as we can judge, the soil 
has lost about as much as the crop and drains have removed. 
Turning again to the experiments on leguminous crops, we 
have in the garden soil evidence that red clover takes very large 
quantities of nitrogen from the soil — in fact, most probably the 
whole which the crop contains. On the other hand, we have the 
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