nt quantity to meet the
requirements of the most abundant crops. Other chemists and vegetable
physiologists again hold that though a certain increase may be obtained
in this way, a point is soon reached beyond which mineral matters will
not cause the plant to absorb more ammonia from the air, although a
further increase may be obtained by the addition of nitrogen in that or
some other available form.

It is admitted on both sides, that all the elements of plant food are
equally essential, and the controversy really lies in determining what
practically limits the crop producible on any soil. The point at issue
may be put in a clear point of view by considering the course of events
on a soil altogether devoid of the elements of plants. If a small
quantity of mineral matters be added to such a soil, it immediately
becomes capable of supporting a certain amount of vegetation, deriving
from the air the organic elements necessary for this purpose, and with
every increase of the former, the air will be laid under a larger
contribution of the latter, to support the increased growth, and this
must proceed until the limit of supply from the atmosphere is reached.
At this point a further supply of mineral matters alone must obviously
be incapable of again increasing the crop, and it would thus be
absolutely necessary to conjoin them with a proportionate quantity of
organic substances. Liebig maintains that this limit is never attained
in practice, but that the air affords ammonia and the other organic
elements in excess of the requirements of the largest crop, while
mineral matters are generally though not invariably present in the soil
in insufficient quantity. Messrs. Lawes and Gilbert, on the other hand,
believe that the soil generally contains an excess of mineral matters,
and that a manure which is to bring out their full effect must contain
ammonia, or some other nitrogenous substance fitted to supplement the
deficient supply afforded by the atmosphere. In short, the question at
issue is, whether there is or is not a sufficiency of atmospheric food
to meet the demands of the largest crop which can practically be
produced.

An absolutely conclusive reply to this question is by no means easy. The
experiments by which it is to be resolved are complicated by the fact,
that all soils capable of supporting anything like a crop, contain not
only the mineral, but the organic elements of its food in large and
generally in greatly s