The Yale expositor., 1912-08-15
RAW ROCK
PHOSPHATE
By PROF. A. J. PATTEN,
Experiment Station Chemist,
Michigan Agricultural College


phosphoric acid, for fertilizing pur
poses Is found quite extensively
throughout the southern, middle west
ern and western states. 1t has beer
conservatively estimated that il the
present rate of consumption should
be increased three times, there Is now
in sight an amount Of rock phosphate
1900 years. The principal source Of
supply for this section at the present
time Is Tennessee, and good grade
OF rock should be laid down, in car
lots, along the lines of the trunk rail.
roads in the southern part of the state
for $3.50 to $500 per ton.
The grade of rock phosphate gener.
ally found upon the markets contains
about 81 per cent. bone phosphate OF
| lime, which Is equivalent to 20 per
cent. phosphoric acid and 33 per cent
lime. Fineness Is a large factor In
determining the availability of rock
phosphate. The finer the material
the more readily it will become avail
able, consequently this point should be
taken into consideration when pur-
chasing.
To obtain the best results it should
be ground to such f1neness that 95 per
cent. will pass through a 100 mesh
screen. BY far the greater part of the
rock phosphate used for fertilizing
purposes is in the form of acid phos-
phate, dissolved phosphate or super
phosphate). This Is prepared by
treating the rock phosphate with an
equal weight Of sulphuric acid (oil of
vitrioD. This treatment changes the
phosphoric acid from an insoluble to
soluble form and makes it readily
available to the growing plant. The
cost of the phosphoric acid to the con-
sumner is increased by this process to
nearly four times, since the acid phos-
plate sells at retail for nearly double
what the raw phosphate costs and 1t
contains only half as much phosphoric
acid.
In view Of this situation the ques-
ton naturally arises would it not be
more economical to use the slowly
available raw phosphate than to pay
the higher price for the acid phos-
phate. Many of the state experiment
stations have conducted experiments
to compare these two forms of phos-
phoric acid. but the results have been
conflicting. In fact the results obtain
ed at some stationns over a period Of
several years have been so contradic-
tory as to be Of little value. The most
widely advertised experiments with
rock phosphate are those conducted
by DR.. Hopkins in Illinois. These ex-
periments have been in progress dur
ing a period OF II or II years and
have been conducted on the so-called
prairie soils rich in organic matter.
The crops most commonly employed
have been corn, oats, wheat and clo-
yer. The rock phosphate has at all
times been applied IN connection with
liberal amounts OF manure or with
clover turned under. The conclusions
drawn from the results of these expe-
riments are that rock phosphate Is a
more economical source Of phosphor-
ic acid than the treated phosphate.
In Ohio the experiments by Direc-
tor Thomas have been conducted in
much the same manner as the Illinois
experiments. but the results here JUS
tify the conclusion that the acid phos-
phate Is the more economical source
of phosphoric acid.
A few experiments have been con-
Quoted by the Michigan experiment
station, and In no case have the results
favored the rock phosphate, though 1t
should be stated that the experiments
were not conducted under the most ia
vorable conditions.
In reviewing the experiments report
ed from all experiment stations where
experiments have been conducted il
Is impossible to arrive at any definite
conclusion on the subject, consequent
ly letters were written to the direc-
tors Of FL state experiment stations
asking them to state how they advised
their constituent farmers on the sub
ject. Seventeen replied by saying
that they do not recommend the use
of rock phosphate and four recon-
mend its use when accompanied by
liberal applications Of barn yard ma
nure or when green manure Is turned
under. Director Thorne, who is often
quoted as favoring rock phosphate, re-
piled by saying in our experiments
In the use Of raw rock phosphate as
reinforcement of manure, as COM
pared with acid phosphate, we find
very little difference in the net return,
after deducting the cost of treatment,
between the two materials. The diff
ference at present seems to be slight
ly in favor of the acid phosphate. Our
results in the separate use of the two
phosphates as a direct application TO
the land do not encourage the use of
the raw rock"
Although the results from all these
experiments are so contradictory they
have clearly demonstrated one thing
1. e. that 1t is useless to use raw rock
phosphate except as a reinforcement
of manure or upon land that is rich in