of importance. Thus, in a dry climate, manures of a
soluble nature will have a better effect than in a wet climate, while
the opposite will be the case with the more slowly acting manures.

_Nature of previous Manuring._

A consideration of equal importance is the previous treatment of the
soil with manure. For example, where a soil has been liberally treated
with farmyard manure, it has been found that mineral manures have a
very inferior effect to that obtained by nitrogenous manure. Lawes and
Gilbert have found this to be strikingly the case in their experiments
on the growth of wheat. In these experiments it was found that the
application of mineral manures was accompanied with little or no benefit
to the crop, whereas very striking results followed the application of
nitrogen. This they attributed to the fact that the supply of mineral
fertilisers in the straw of the farmyard manure is largely in excess of
the supply of nitrogen. The nature of the action of the manure
previously applied is also to be taken into account in determining how
long its influence may probably last. Where, for example, the manure has
been nitrate of soda or sulphate of ammonia, it may be safely concluded
that its direct influence is no longer felt a year after application.
The influence of superphosphate of lime, while scarcely so temporary,
may be said to last only for a comparatively short time.[242] On the
other hand, when the manure applied is of a slow-acting nature, such as
bones or basic slag, its influence will probably be felt for a number of
years.

_Nature of the Crop._

But more important than any of the above-mentioned conditions is the
nature of the crop itself. Our knowledge of the requirements of the
different farm crops is still very imperfect. A very wide experience,
however, of the effect of different manures on different crops, has
conclusively proved that their manurial requirements differ very
considerably. The subject is complicated by other considerations, such
as the nature of the soil, &c.; but notwithstanding this fact, certain
points seem to be pretty well established.

In seeking to understand the respective requirements of the different
crops for different fertilisers, two important considerations must be
borne in mind. These are--(1) _the quantities of the three fertilising
ingredients--nitrogen, phosphoric acid, and potash--which different
crops remove from the soil;_ and (2) _the different power cr