uperabundant quantity, and it is impossible
satisfactorily to ascertain how much is derived from this source, and
how much from the atmosphere. There are in fact no experiments in which
the effects of a purely mineral soil have been ascertained. The
important and carefully performed researches of Messrs. Lawes and
Gilbert were made upon a soil which had been long under cultivation, and
contained decaying vegetable matters in sufficient abundance to supply
nitrogen to many successive crops, and it would be most unreasonable to
assert that the produce they did obtain by means of mineral manures,
drew the whole of its nitrogen from the air. On the contrary, it may be
fairly assumed that the soil did yield a certain quantity of its
nitrogenous compounds, but to what extent this occurs, it is impossible
to determine. This difficulty is encountered more or less in all the
other experiments, and precludes absolute conclusions. The same fallacy
also besets the arguments of Liebig when he holds that the crop,
increased by means of mineral manures alone, must derive the whole of
the additional quantity of nitrogen which it contains from the air, as
appears to be tacitly assumed throughout the whole discussion. So far
from this being the case, it is just as likely that the mineral matters
should cause the plants to take it from the soil, if it is there, as
from the atmosphere.

Taking a general view of the whole question, it is evident that a
certain amount of vegetation may always be produced by means of mineral
manures, and the quantity obtained is generally much beyond the normal
produce of the soil. But it is still open to doubt whether the largest
possible crop can be thus obtained, although the balance of evidence is
against it, and in favour of the addition of ammonia, and other
nitrogenous and organic substances, to the soil. In actual practice
manures containing nitrogen are more important, and more extensively
applied than any others, and the quantity of that element thus used is
very much larger than is generally supposed. Twenty tons of farm-yard
manure, a quantity commonly applied, and often exceeded on well
cultivated land, contain a sufficiency of organic matters to yield about
2-1/2 cwt. of nitrogen. A complete rotation, according to the six-course
shift, contains almost exactly the same quantity of nitrogen, when we
assume average crops throughout the whole, and it is thus made up.[K]