c or aluminic phosphate, in which form it is
gradually acted upon by the sap of the plant-roots as required. This
being the case, it may be asked, Why is superphosphate so much more
rapid in its action than insoluble phosphate; or why should we be at the
trouble and expense of dissolving the phosphate if it has to become
insoluble again in the soil? This question is one of very great
importance, for the answer to it furnishes, in our opinion, the key to
the whole phosphate question. When superphosphate is added to the soil,
being soluble in water, it is soon dissolved and carried down by the
rain into its pores, and becomes thoroughly mixed with the
soil-particles. It is thus soon fixed in the soil, beyond the risk of
being washed away. The result is, that the phosphate is obtained in a
state of division infinitely more minute than could ever be obtained by
mechanical grinding, and is, further, most intimately mixed with the
particles of the soil. It is this intimate mixture of the phosphate with
the particles of the soil, and its minute state of division, that
constitute the only reason for rendering superphosphate superior in its
action to even the most finely ground insoluble phosphates. This opinion
is supported by the fact, that although the chemist has imitated nature
in this matter so far as to manufacture precipitated phosphate, he has
failed, as a rule, in getting as favourable results with it as with
superphosphate. Although the mechanical state of division of the
manufactured precipitated phosphate is probably as fine as that
obtained by nature from the superphosphate, it is impossible to obtain
so intimate a mixture with the soil-particles, and hence the results
obtained are different. For these reasons it will be easily seen that
the rate of action of the superphosphate must always be quicker than
that of any other form of phosphatic manure. The phosphate is everywhere
distributed in the soil. The plant-roots are thus furnished with a
continuous supply throughout their growth, and micro-organisms, which
require for their development a supply of this necessary plant-food, are
propagated. A regularity in the plant's growth is thus secured, which is
of great importance. But while admitting this, there are many cases in
which this greater quickness of action does not render soluble phosphate
the most economical form. The nature of the crop, as well as the nature
of the soil, may in many cases be such as to render