Year: 1874

Every farmer has heard the term super.
phosphate Of lime used many times over, and
yet there are many who may not know ex.
actly what 1t means. 1t is a scientific term,
used first TO designate a class of salts of
phosphoric acid soluble in water, in distinc-
tion from such as are insoluble. 1t has re-
Gently come to be applied as a technical
term to fertilizers which contain phosphoric
acid in form soluble in water.


Phosphoric acid, it must be borne in mind
Is mineral substance that is absolutely es-
sential to the growth of plants. There is
scarcely living plant on the face of the
globe that does not require it as a part OF its
food. And our higher plants, such as are
used for the sustenance Of man, require it in
larger quantities than most others. Our
cereal crops in this country, for instance,
take out from our soils, not less than two
hundred aud seventy-6ve thousand tons of
phosphoric acid every year, which must be
replaced in some form or other, or gradual
exhaustion of this important element must
inevitably take place.


Now some soils contain greater abun.
dance OF phosphorus than others, 10 be sure,
but no matter how large percentage a soil
may contain, if any particular class OF plants
is to be grown upon it repeatedly, and the
crop removed, the necessity of greater sup
plies will be more and moro apparent, and
an artificial application will be needed.


Tne most economical source of this supply
is bones These are composed chiefly of
phosphate Of lime, or lime phosphoric acid,
but in this form the substance is still insolu-
BILE in water. Phosphoric acid is what is
known in chemistry as tri-basc acid, each
atom is capable Of uniting with three atoms
or molecules of a salt as its base, and in
bones this base is always lime that is, the
phosphoric acid in bones is always united
with lime in the proportion Of one atom OF
the former to three of the latter. Now il
we can substitute two Of these three parts Of
lime by water, that is, put two parts Of water
in the place Of two parts of lime, we change
the nature and character Of the whole SUBJ
stance. And this is just what we do when
We treat the bones with sui lphuric acid or the
common oil Of Yitriol Of commerce. When
water takes the place OF two parts Of lime in
the molecular structure Of phosphoric acid
VI puOspudtO VI lluit, tnc SuuStduCS DcCOllcS
easily soluble in water. 1t is converted into
u soluble condition, aud in this form it is
available as the food Of plants, and is called
superphosphate. IL there is an abundance
of sulphuric acid used, the two parts Of lime
which are removed from the combination
with phosphoric acid, will unite with the sur
plus, aud form sulphato of lime, which is
common plaster or gypsum, so that most
true superphosphates contain more or less
plaster.


The difference, then, between phosphate
of lime, as we find it in bones, and super.
phosphate, is that in the one form the phos.
phoric acid with limo is soluble in water,
and in the other it is not. la the first form
it is not available as food for plants, unless
it is dccomposed by the action of sulphuriC
acid the plant cannot get hold of it, till it
goes through the long process Of natural de-
cay. Treatment with sulphuric acid brings
it into a condition to be easily and quickly
available, and it is one of the most important
discoveries Of modern science so far as it,
connection with agricultural ch hemistry is
oncerncd-MaSsachuSettS Ploaghman.