That there is a steady increase of nitrogen in the soil of land under
pasture is a fact of universal experience. The older a pasture is the
richer is its soil in nitrogen. The comparison of the analyses of the
soil of arable land with the soil of pastures of different ages shows
this in a striking way.[81] Thus at Rothamsted it was found that while
the amount of nitrogen in an ordinary arable soil was .140 per cent,
that in pastures eight, eighteen, twenty-one, and thirty years old was
respectively .151, .174, .204, and .241 per cent. In the last two
analyses we have a record of the actual gain in nitrogen made by the
same pasture, this being .04 per cent in nine years' time. From these
statistics it may be inferred that the surface-soil of a pasture may
increase at the rate of 50 lb. per acre per annum. A point of great
interest in connection with this subject is the fact that there seems to
be a limit to the accumulation of nitrogen in pastures; for it would
seem that pastures centuries old are not any richer in nitrogen than
those thirty to forty years old.

_Gain of Nitrogen with Leguminous Crops._

Another case where the gain of nitrogen to the surface-soil is very
striking is in that of leguminous crops, such as clover, beans, peas,
&c. This fact has been long recognised--especially with regard to
clover--by farmers, and has been largely instrumental in leading to the
investigation of the "free" nitrogen question. That a soil bearing a
leguminous crop increases in nitrogen at a very striking rate is a
problem that requires to be solved. A partial explanation of the
phenomenon is found in the extraordinary capacity such a crop as clover
has, by means of its multitudinous and ramifying roots, for collecting
nitrogen from the subsoil. This, however, would only account for the
increase in nitrogen to a certain extent. There must be some other
source, and the only other source is the air. That the free nitrogen of
the air is, after all, available for the plant's needs, is a supposition
which has long seemed extremely probable, and which, within the last
few years, has been proved beyond doubt to be a fact in the case of
leguminous plants.

_The Fixation of "Free" Nitrogen._

The method in which these plants are able to make use of the free
nitrogen is still a point requiring much research. So far as the
question is at present investigated, it would seem that the fixation is
effected by means of micro-organis