ate of what the value of the unexhausted manures in
reality is. The difficulty arises from the fact that we have not
sufficient data available for guiding us in estimating this value, which
further varies under different conditions. The fertilising ingredients
of a soil are present in the soil for the most part in an inert
condition, from which they are only slowly converted into an available
form.

_Potential Fertility of a Soil._

As indicating the total amount of the more important mineral ingredients
present in a soil, it may be mentioned that it has been calculated, in
the case of a poor sandy soil, _that the amount of potash it contains_
(_provided it were in an available condition_) would be sufficient to
yield three or four average crops of potatoes; of phosphates, nineteen
average crops; and of lime, seventy-three. But then only a very small
amount of this fertilising matter is in a readily available form.

It is for this reason that artificial manures, although added in such
small amounts, exercise so striking an influence in increasing plants'
growth. Their effect, however, is to a large extent only of a temporary
nature; and in attempting to assess the unexhausted value of a manure a
year or two after its application, we must remember this fact.

Some manures are very speedily taken up by plants, and some are very
easily washed out of the soil. Others, again, it would seem highly
probable, have a tendency to become converted into a more or less inert
condition after a while. This remark may be especially applied to the
fertilising constituents (chiefly nitrogen) in farmyard manure.[253] The
whole question, however, is little understood. One or two points may be
drawn attention to. In the first place, it may be safely affirmed that
little direct effect can be expected from such quickly available and
easily soluble forms of nitrogenous manures as nitrate of soda and
sulphate of ammonia a year after application. Potash and phosphates, on
the other hand, may exercise an effect for a considerably longer period;
and what the length of this period may be will depend on their amount
and condition. Thus it is not likely that superphosphate will have much
effect more than two years after application. On the other hand, such
manures as bones, basic slag, and farmyard manure may exert an
appreciable influence for a number of years. How long exactly, it is
wellnigh impossible to say, the rate at which they are appli