PRESS BULLETIN Ko. 77.
January 2, 1908.
iFlorida Agricullural Experiment Slaiion.
,GROUND PHOSPHATE ROCK AS A SOURCE OF PHOSPHORIC ACID.
BY A. W. BLAIR.
PHOSPHATE ROCK, AND SUPERPHOSPHATE.
Tricalcium phosphate is the chemical name of the phosphate rock of
commerce, before it has been treated with an acid. It is found in Florida, South
'Carolina, Tennessee, and to some extent in other places in the United States.
When this rock is ground very finely and placed on the market without any fur-
ther treatment, it is usually spoken of as ground phosphate rock, or "floats."
In this form it has, by many, been regarded as practically insoluble in the
.soil solutions and therefore not available for plant food. For this reason
the greater portion of the rock that is used is treated with a strong acid-
sulphuric acid-which converts the phosphoric acid into a form that is sol-
uble in water-the monocalcium phosphate. The resulting material is placed
upon the market as "acid phosphate," sometimes called superphosphate, or
superphosphate of lime. Expressed in the form of an equation the reaction
is as follows:
Tricalcium Sulphuric Monocalcium Calcium sulphate
phosphate + acid phosphate + (land plaster)
310 parts 196 parts 234 parts 272 parts
The materials on the right of the sign of equality constitute the "acid
phosphate," or superphosphate of lime.
ACID PHOSPHATE READILY AVAILABLE.
Acid phosphate usually contains from 14 to 16 per cent. of phosphoric
-acid. most of which is regarded as available. But the treatment of rock
phosphate with acid is expensive, and on this account it becomes necessary
to sell the acid pnosphate for considerably more per ton than the ground
rock (floats) would cost, although the latter contains about twice as much
phosphoric acid per ton as is contained in the treated rock (acid phos-
-phate). The phosphoric acid of the treated rock has the advantage of being
largely soluble, and is therefore ,much more readily available than the phos-
phoric acid in the floats.
GROUND PHOSPHATE ROCK SLOWLY AVAILABLE.
However, it is a fact that the phosphoric acid of floats does become
.uioAly available, the availability being somewhat proportional to the fine-
ness of the material and the amount of organic matter in the soil. In a soir
containing much humus it will become available more rapidly than in one
that is deficient in this respect. An acid soil also tends to hasten its avail-
ability, the acid condition of the soil being gradually corrected at the same
time. (It should be borne in mind, however, that this is a rather slow way
of correcting an acid soil, if the correcting of acidity alone is the result toc
be attained.) Experiments have shown that water charged with carbon di-
oxide gas has a solvent effect upon phosphate rock, and since the ground
waters of Florida are more or less charged with this gas (their solvent ef-
fect upon the limestone formations is a proof of it), we might reasonably ex-
pect that some of the phosphoric acid of the phosphate rock would be
made available in this way.
Many experiments have been conducted throughout the United States
which give evidence that the phosphoric acid of floats does become slowly
available, and a few places may be cited-notably Rhode Island and Maryland
-where this ground phosphate rock has given results as good as, if not better
than, the other forms of phosphoric acid.
GROUND PHOSPHATE ROCK A PERMANENT INVESTMENT.
The application of a ton or two of finely ground phosphate rock per
acre might be looked upon as a permanent investment, rather than an im-
mediate fertilizer. It would certainly not be easily lost; but, on the other
hand, would act as a source of phosphoric acid for a number of years. For
tne first two or three years it would be necessary to supplement it with a
more available source of phosphoric acid. After this time it is quite prob-
able that most crops would secure enough phosphoric acid from the grad-
ually decomposing rock.
For quick-growing vegetable crops it would perhaps not be well to rely
to any great extent upon this form of phosphoric acid, but on pineapple
fields and orange groves it should give good results if properly managed.
Here the soil is very sandy and open, and should there be heavy rains soon
after the application is made, much of the soluble plant food that is ap-
plied is liable to be lost by leaching. On the other hand, the floats would be
but slightly affected in this way, and would slowly yield phosphoric acid to the
growing plants for a number of years. After the first two or three years it
is doubtful if it would be necessary to make such an application oftener
than once in three to five years, especially if the soil were kept well sup-
plied with organic matter.
State papers please copy.