the application of
the cheaper insoluble phosphate more economical. It is imperative that
the early growth of some crops be hastened as much as possible by a
ready supply of easily assimilable plant-food, in order to enable them
to successfully sustain the attack of certain pests to which they are
liable to succumb. This, for example, is notably the case with turnips.
In such a case there can be no doubt that the value of soluble phosphate
to the young plants is very great, as it enables them to survive this
critical period.

_Action of Superphosphate sometimes unfavourable._

But even in this case there may be other conditions which render
insoluble phosphate a preferable manure. Such a case is where the soil
is of a very light nature and is deficient in lime. In this case the
acid superphosphate, not having the necessary base to combine with, may
prove even hurtful to the young plants. According to the late Dr
Voelcker, a concentrated superphosphate may produce a smaller crop than
a fertiliser containing only a quarter as much soluble phosphoric acid,
when applied to root-crops on sandy soils, greatly deficient in lime.
Cases such as the above, however, are extremely rare; and we may say
that, in the case of root-crops generally, superphosphate must be
regarded as of special value.

_Application of Superphosphate._

In any case, superphosphate ought to be applied to a soil some time
before it is likely to be assimilated by the plant, in order to allow
neutralisation of its acid character to be fully effected before the
plant's roots come in contact with it. Thus Professor S. W. Johnson, one
of the greatest living American authorities, states it as his opinion
that recent investigations tend to show that soluble and reverted (or
precipitated) phosphates are, upon the whole, about equally valuable as
plant-food, and of nearly equal commercial value. But as Sir John Lawes,
in quoting Professor Johnson to the above effect, remarks, this opinion
is based on an experience of American agriculture, in which country
soluble phosphate is chiefly applied to cereal crops, while in this
country it is chiefly applied to turnips. In the case of cereal crops,
the importance of a speedy early growth is not so great, as we have
already pointed out, as it is in turnips, where the danger to the young
plants from the ravages of the turnip-fly is such that the growth of
even a day or two may make a very considerable difference.