Lzunnnl ,

A & M COLLEGE,
CAMPUS.

E125-6M-L180

TEXAS AfiRlCULTIJRAL EXPERIMENT STATION

A. B. CONN ER, DIRECTOR
conuacm STATION, BRAZOS COUNTY, TEXAS

BULLETIN NO. 509 ' JUNE, 1935

DIVISION OF ‘CHEMISTRY

Availability 0f the Phosphoric Acid 0f
Finely-Divided Rock Phosphate

IJBRARY
Agiicultural & Mechanical Conezfi H‘ i @141

 

AGRICULTURAL AND MECHANICAL COLLEGE OF TEXAS
T. 0. WALTON, President

LJA

Administration :

A. B. Conner, M. S., Director

R. E. Karper, M. S., Vice Director
Clarice Mixson, B. A., Secretary

M. P. Holleman, Chief Clerk

D. R. McDonald, Asst. Chief Clerk
Chester Higgs, Executive Assistant
Howard Berry, B. S., Technical Asst.

Chemistry:

G. S. Fraps, Ph. D., Chief; State Chemist
S. E. Asbury, M. S., Chemist

J. F. Fudge, Ph. D., Chemist

E. C. Carlyle, M. S., Asst. Chemist
T. L. Ogier, B. S., Asst. Chemist

A. J. Sterges, M. S., Asst. Chemist
Ray Treichler, M. S., Asst. Chemist
W. H. Walker, Asst. Chemist

Velma Graham, Asst. Chemist
Jeanne F. DeMottier, Asst. Chemist
W. H. Garman, M. S., Asst Chemist
A. R. Kemmerer, Ph. D., Asst. Chemist
A. W. Walde, Ph. D., Asst. Chemist

Horticulture :
S. H. Yarnell, Sc. D., Chief

Range Animal Husbandry:
J. M. Jones, A. M. Chief
B. L. Warwick, Ph. D., Breeding Investiga.
S. P. Davis, W001 and Mohair Specialist
J. H. Jones, B. S., Animal Husbandman
Entomology:
F. L. Thomas, Ph. D., Chief; State
Entomologist
H. J. Reinhard, B. S., Entomologist
R. K. Fletcher, Ph. D., Entomologist
W. L. Owen, Jr., M. S., Entomologist
J. N. Roney, M. S., Entomologist
J. C. Gaines, Jr., M. S., Entomologist
S. E. Jones, M. S., Entomologist
F. F. Bibby, B. S., Entomologist
"R. W. Moreland, B. S., Asst. Entomologist
C. E. Heard, B. S., Chief Inspector
C. J. Burgin, B. S., Foulbrood Inspector
Agronomy: "
E. B. Reynolds, Ph. D., Chief
R. E. Karper, M. S.,-Agronomist
P. C. Mangelsdorf, Sc, D., Agronomist
D. '1‘. Killough, M. S., Agronomist
J. O. Beasley, M. S., Asst. Agronomist
Publications:
A. D. Jackson, Chief

IL J L\IL‘

L)l.fll.'.l'|

Veterinary Science:
‘M. Francis, D. V. M., Chief
H. Schmidt, D. V. M., Veterinarian
"F. P. Mathews, D. V. M., M. S., Veterinarian
Plant Pathology and Physiology:
J. J. Taubenhaus, Ph. D., Chief’
W. N. Ezekiel, Ph. D., Plant Pathologist
L. B. Loring, M. S., Asst. Plant Pathologist
G. E. Altstatt, M. S., Asst. Plant Pathologist
"Glenn Boyd, B. S., Asst. Plant Pathologist
Farm and" Ranch Economics:
L. P. Gabbard, M. S., Chief
W. E. -Paulson, Ph. D., Marketing
C. A. Bonnen, M. S., Farm Management
I**W. R. Nisbet, B. S., Ranch Management
**A. C. Magee, M. S., Farm Management
Rural Home Research:
Jessie Whitacre, Ph. D., Chief
Mary Anna Grimes, M. S., Textiles
Sylvia Cover, Ph. D., Foods
Soil Survey:
"W. T. Carter, B. S., Chief
E. H. Templin, B. S., Soil Surveyor
J. W. Huckabee. B. S.. Soil Surveyor
I. C. Mowery, B. S., Soil Surveyor
Botany:
V. L. Cory, M. S., Acting Chief
Swine Husbandry:
Fred Hale, M. S., Chief
Dairy Husbandry: -
O. C. Copeland, M. S., Dairy Husbandmai:
Poultry Husbandry:
R. M. Sherwood, M. S., Chief
J. R. Couch, M. S., Assoc. Poultry Hush.
Paul D. Sturkie, B. S., Asst. Poultry Hush.
Agricultural Engineering:
H. P. Smith, M. S., Chief
Main Station Farm:
G .T. McNess, Superintendent
Apiculture (San Antonio):
H. B. Parks, B. S., Chief
A. H. Alex, B. S., Queen Breeder
Feed Control Service:
F. D. Fuller, M. S., Chief
James Sullivan, Asst. Chief.
D. Pearce, Secretary
H. Rogers, Feed Inspector
. L. Kirkland, B. S., Feed Inspector
D. Reynolds, Jr., Feed Inspector
A. Moore, Feed Inspector
J. Wilson, B. S., Feed Inspector
. G. Wickes, D. V. M., Feed Inspector
J. K. Francklow, Feed Inspector

‘IIJHTUWWF-tm

SUBSTATIONS

No. 1, Beeville, Bee County:
R. A. Hall, B. S., Superintendent
No. 2. Tyler, Smith County:
P. R. Johnson, M. S., Superintendent

No, 9, Balmorhea, Reeves County:
J. J. Bayles, B. S., Superintendent

No. 10, College Station, Brazos County:
R. M. Sherwood. M. S., In Charge

"B. H. Hendrickson, B. S., Sci. in Soil Erosion L, J. McCall, Farm Superintendent

"R. W. Baird, M. S., Assoc. Agr. Engineer
No. 3, Angleton, Brazoria County:
R. H. Stansel, M. S., Superintendent
H. M. Reed, B. S., Horticulturist
No. 4, Jefferson County:
R. H. Wyche, B. S., Superintendent
"H. M. Beachell, B. S., Junior Agronomist
No. 5, Temple, Bell County:
Henry Dunlavy, M. S., Superintendent
C. H. Rogers, Ph. D., Plant Pathologist
H. E. Rea, B. S., Agronomist
"E. B. Deeter, B. S., Soil Erosion

No. 11, Nacogdoches, Nacogdoches County:
H. F. Morris, M. S. Superintendent
**No. 12, Chillicothe, Hardeman County:
"J. R. Quinby, M. S. Superintendent
"J. C. Stephens, M. A., Asst. Agronomist
No. 14, boners, Sutton-Edwards Counties:
W. H. Dameron, B. S., Superintendent
I. B. Boughton, D. V. M., Veterinarian
W. T. Hardy, D. V. M., Veterinarian
O. L. Carpenter, Shepherd
"O. G. Babcock, B. S., Asst. Entomologist
No. l5, Weslaco, Hidalgo County:

"P. L. Hopkins, B. S., Junior Civil Engineer W_ H_ Friend, B_ S” superintendent

No. 6 Denton, Denton County:

P. B. Dunkle, M. S., Superintendent
"I. M. Atkins, B. S., Junior Agronomist
No. 7, Spur, Dickens County:

R. E. Dickson, B. S., Superintendent

B. C. Langley, M. S., Agronomist
No. 8, Lubbock, Lubbock County:

D. L. Jones, Superintendent

Frank Gaines. Irrig. and Forest Nurs.

Members of Teaching Staff Carrying

G. W. Adriance, Ph. D., Horticulture

S. W. Bilsing, Ph. D., Entomology

D. Sooates, A. E., Agricultural Engineering
A. K. Mackey, M. S., Animal Husbandry
R. G. Reeves, Ph. D., Biology

J. S. Mogford, M. S., Agronomy

F. R. Brison, M. S., Horticulture

‘Dean, School of Veterinary Medicine.

S. W. Clark, B. S., Entomologist
W. J. Bach, M. S., Plant Pathologist
J. F. Wood, B. S., Horticulturist
No. 16, Iowa Park, Wichita County:
C. H. McDowell, B. S.. Superintendent
L. E. Brooks, B. S., Horticulturist
No. 19, Winterhaven, Dimmit County:
E. Mortensen, B. S., Superintendent
"L. R. Hawthorn, M. S., Horticulturist

Cooperative Projects on the Station:

W. R. Horlacher, Ph. D., Genetics

J. H. Knox, M. S., Animal Husbandry
. Darnell, M. A., Dairy Husbandry
. Berry, B. S., Biology

. Stewart, Ph.D., Agronomy

. Little, M. S., Entomology

F‘

A.
R.
R.
V.

{>90

TAs of May 1, 1935

"In cooperation with U. S. Department of Agriculture.
IIn cooperation with Texas Extension Service.
‘In cooperation with State Department of Agriculture.

Soft phosphate with colloidal clay and finely ground rock
phosphate both contain phosphoric acid which has a much lower
availability to plants than the phosphoric acid of superphos-
phate, especially on neutral or basic soils such as generally
prevail in Texas. _ -

The soft phosphate with colloidal clay is a finely-divided phos-
phate of natural occurrence, which is a by-product from min-
ing rock phosphate in Florida. The finely ground rock phosphate
was from Tennessee rock.

The availability of the phosphoric acid in soft phosphate with
colloidal clay in 7 pot experiments was found to vary from 0 to
120 with an average of 40 compared with the phosphoric. acid of
superphosphate as 100. The availability was low on the slightly
basic soils, high on one acid soil, but about the average on some of
the other acid soils.

In 5 pot experiments the availability of the phosphoric acid
of finely-ground rock phosphate was only 40 per cent of that of
superphosphate. lts availability seemed to be lower on neutral or
basic soils than on acid soils.

On some acid soils, the availability of the phosphoric acid
of both soft phosphate with colloidal clay and finely ground rock
phosphate is equal to that of superphosphate, but on other acid
soils the availability is decidedly less than that of superphosphate.

CONTENTS

Page
Soft phosphate with colloidal clay ,,,,,, __; ___________________________________________________________ __ 5
Finely-ground rock phosphate ___________________________________________________________________________ __ 7
Previous work _____ __ -- . 7
Methods for estimating availability ________________________________________________________________ __ 8
Plan of work 8
Description of soils used _________________ .- 10
Results of the work with soft phosphate ............................................................ v10
Additional work with soft phosphate ____________ ..11
Finely-ground rock phosphate  14
Summary ., _________ __ 14

References _  16

BULLETIN NO. 509 JUNE, 1935

AVAILABILITY OF THE PHOSPHORIC ACID OF
FINELY-DIVIDED ROCK PHOSPHATE

G. S. FRAPs

About a hundred years ago, it was discovered that when ground bones
were treated with acids such as sulphuric acid, the treated bones had a
much greater effect upon plant growth than the untreated bones. A little
later it was found that naturally-occurring rock phosphate when ground
and treated with acids likewise had a much greater effect upon plant growth
than the raw rock phosphate. The acid acted upon the bones or phosphate
rock in such a way as to permit the plant roots to take up the phosphoric
acid. The phosphoric acid was changed to what is termed an available
form. Upon this discovery was founded the great fertilizer industry, which
now supplies nitrogen and potash as well as phosphoric acid in available
forms.

There are some deposits of phosphates which are unsuitable for the
manufacture of superphosphate, because they either contain other sub-
stances which interfere with the action of the acid or use up too much
of it, or because they do not contain enough phosphoric acid to permit
of the manufacture of a commercial grade of superphosphate. These
phosphates may be used directly for fertilizing purposes, though it is
generally recognized that the availability of their phosphoric acid is
much lower than that in superphosphate, and that the cost of the fertilizer
may not be recovered in the first season the rock phosphate is applied. Some
agronomists, notably C. G. Hopkins, have advocated for permanent
fertility the use of larger quantities of rock phosphate combined with a
legume rotation to secure nitrogen. The value of rock phosphate has
been considerably studied and many of the results are summarized by
Collings (2).

Claims have been made that phosphate rock which has been very
finely ground, or phosphates which are naturally finely divided,‘ con-
tain phosphoric acid in a highly available condition and approach super-
phosphate in their value (3, 7). The work here reported was plannet"
for the purpose of testing these claims.

Soft Phosphate with Colloidal Clay

This product is‘defined by the Association of Official Agricultural
Chemists (6) as follows:

“A very finely divided low analysis by-product resulting in mining
Florida rock phosphate by a hydraulic process whereby the colloidal
material settles more abundantly at points in artificial ponds and basins
farthest from the washer, Where it is later recovered upon the natural
evaporation of the water.”

Jacob, Hill and Holmes (8) give information regarding this material.
They state that the Florida hard rock phosphate deposits usually contair
a relatively high percentage of finely-divided phosphatic material classi-

6 BULLETIN NO. 509, TEXAS AGRICULTURAL EXPERIMENT STATION

fied under the general term, soft phosphate. Soft phosphate also occurs
in smaller quantities in the Florida pebble phosphate deposits and to a
certain extent as individual deposits in both the hard rock and pebble
phosphate districts. Finely divided phosphates also occur in the Tennessee
brown rock phosphate field and probably, to a certain extent, in the
other phosphate deposits in this country.

Florida soft phosphate (8), usually occurs in the form of more or less
soft, White lumps and when wet exhibits the plastic, sticky characteristics
of clay. It is variable in composition, but Wyatt reports as an average
analysis of 148 samples, 65.15 per cent tricalcium phosphate and 9.2
per cent oxides of iron and aluminaj

During the process of preparing Florida hard rock phosphates for the
market (8), the soft phosphate present in the matrix is washed into
wasteponds Where it settles out along with the clay and other impurities,
the finer particles concentrating at points farthest from the entrance to
the pond. When the ponds become filled with waste material they are
allowed to dry up and the water from the phosphate washers is turned
into new ponds. These “Waste-pond” phosphates, which usually vary in
shade from white to a light straw color, are composed of very fine
particles and when wet they are quite plastic and sticky. Upon drying
t-hey shrink and crack in the manner characteristic of materials con-
taining high percentages of colloid. The air-dried lumps disintegrate
rapidly when placed in water. The air-dry material usually contains
about 40 to 55 per cent tri-calcium phosphate and about 15 to 18 per
cent iron and aluminum oxides. The abandoned waste ponds in the
Florida hard rock phosphate district contain several million tons of this
material. k

Owing to the relatively low content of phosphoric acid and high content
of iron and aluminum, it has not been considered practicable to at-
tempt the conversion of soft phosphate into super-phosphate by treatment
with sulphuric or other acids (8). Waggaman (9), and Matson, have
suggested, however, that owing to the fineness of the particles, it should
prove a valuable phosphate fertilizer material for direct application to
certain types of soil. A small quantity has been produced annually for
this purpose.

Jacob, Hill, and Holmes (8) extracted 60 per cent of colloidal material
from the waste-pond phosphates, 38.1 per cent from the soft phosphate,
and 11.5 per cent from Tennessee rock phosphate ground to pass a 100-
mesh sieve. In the same order the colloidal material contained 18.1, 31.3
and 25.0 per cent of phosphoric acid corresponding to 46.4, 38.1 and 8.5
per cent of the phosphoric acid in the original phosphates.

Sellers of this waste-pond phosphate, or soft phosphate with colloidal
clay, have made exaggerated claims with respect to it, some of which
are as follows (3):

“COLLOIDAL PHOSPHATE has COLLOIDAL availability and is
the only natural phosphate highly available, having the unique physi-
cal property of the colloid. This physical property is as important

 

AVAILABILITY OF THE PHOSPHORIC ACID OF ROCK PHOSPHATE 7

as its chemical property. Colloids regulate the plant’s feeding ability
and determine the soil’s productiveness. They attract, hold, and regu-
late nitrogen, moisture‘ and other elements. They give to the soil
greater capillarity. “COLLOIDAL PHOSPHATE is both electrolytic

and catalytic. It builds into the plant, resistance, vitality and energy. '

It restores to soils their virgin qualities. It gives to the soil many
of the rare elements essential to plant growth.

“It stimulates and increases the growth of bacteria in the soil. It

has high exchange capacity and prevents transference of plant foods‘

in the plant. It contains the elements that are essential in the trans-
position of starches into carbohydrates, as well as the elements that
are essential to the process known as photosynthesis.

“COLLOIDAL PHOSPHATE hastens germination, and brings about
early maturity of the crop, enabling it to reach the market earlier.
It also gives to the fruit firmness, good texture; improving the color
and flavor.”

The product referred to may be considered only as a source ‘of phos-

phoric acid, and the other claims may be disregarded, since there is

practically no evidence to support them.

Finely-Ground Rock Phosphate

Finely ground rock phosphate, sometimes sold as Ruhm Phosphate, or
“Lime phosphate” is made from Tennessee rock phosphate. The dried
phosphate is ground and the fine materials are separated by air currents
which, carefully controlled, carry out the fine particles as fast as they are
powdered, the coarser particles remaining in the mill until they are
ground small enough to float in air.

“Lime Phosphate” as prepared at present ('7), has been claimed to
contain approximately 80% of particles that will pass through a 400
mesh screen. The old standard, pre-war, was 90% through 100 mesh

I Lime Phosphate is much finer than Portland Cement, that part passing

through 400 mesh is finer than flour.”
It is claimed (7) that Experiment Stations Which have made com-
parisons unfavorable to rock phosphate and in favor of acid phosphate

have almost without exception compared acidulated phosphate with rock

phosphate ground by fertilizer factories, almost as coarsely as Lawes
ground it 75 years ago, 99% through a 60 mesh sieve.

It is also claimed (7 ) that new experiments of many States with “Lime
Phosphate” are rapidly showing identical results from both products. We
have not found reports of such experiments, though Ames and Kitsuta

vi (1) found that the phosphoric acid of finely ground phosphate rock was
 more available than the ordinary rock, while not as available as super-
y phosphate.

Previous Work

There is a considerable amount of work which shows that rock phosphate

g is less available to plants than superphosphate, though the relative avail-

8 BULLETIN NO. 509, TEXAS AGRICULTURAL EXPERIMENT STATION

ability depends both on the soil and the plant being grown. No attempt
will be; made here to cite the various reports which have been made re-
garding rock phosphate, since it is not pertinent to the study here reported.
Some are summarized by Collings (2). This Station (4) reported that in
pot experiments on an average the availability of the phosphoric acid of
rock phosphate is 21 per cent of that of superphosphate, though it varied
with different soils.

H. D. Haskins reports (5) the results of a vegetation experiment with
soft phosphate with colloidal clay. The results depended somewhat upon
the quantity of phosphoric acid used. Compared on the yields of dry mat-
ter, with superphosphate as 100, the soft phosphate had an availability of
zero when used in small quantity, 22.0 per cent when used in What was
termed optimum quantity and 62.5 per cent when used in large quantity.
The high result was due partly to a depression in yield of the crop receiv-
ing the high amount of superphosphate. Compared with the phosphoric acid
of superphosphate as 100, on the basis of the phosphoric acid absorbed
by the plant, the availability with the low quantity of phosphate was
26.6, with optimum phosphate 38.4 and with high phosphate 61.5.

Methods for Estimating Availability

There is no chemical method for estimating the availability of rock
phosphate or soft phosphate with colloidal clay. The method of the A. O.
A. C. for citrate-—soluble phosphoric acid is intended only for superphos-
phates or similar phosphates containing _mono-calcium phosphate and di-
calcium phosphate and is not intended for rock phosphate of any kind.
The method for citric acid soluble phosphoric acid is for use with Thomas or
basic slag, which is a by-product from the manufacture of steel. The
method is not intended to ascertain the availability of phosphoric acid in
any other kind of phosphate.

The only available method of comparing the availability of the phos-
phoric acid of rock phosphate, soft phosphate with colloidal clay or mineral
phosphates in general is by means of pot experiments or field experi-
ments with plants. ~

Plan of Work

The comparisons of the availability of the phosphoric acid of -soft
phosphate with colloidal clay and superphosphate were made according
to our usual procedure with pot experiments. The soft phosphate con-
tained 20.09 per cent of total phosphoric acid, the Ruhm phosphate No.
33424 contained 33.51 per cent and the superphosphate F 36120 contained
18.45 per cent of available phosphoric acid. To 5000 gms. of soil in gal-
vanized iron pots, 1 gm. ammonium nitrate and 1 gm. potassium sulphate
were added. The phosphate addition contained 0.1 gm. phosphoric acid
in all soils except No. 31322, which received .08 gm. phosphoric acid.
Water equal to one-half the water capacity of the soil was added. Corn
was planted, thinned to 3 plants in a pot, Watered 3 times a week, har-
vested, dried, weighed, and the phosphoric acid estimated. The avail-

9

AVAILABILITY OF THE PHOSPHORIC ACID OF ROCK PHOSPHATE

Tc i2 3.. .l Hm l. E. w mmo. Eb. 88A >30 3.30.5. 31%
To 3s an.“ Hm. m“: 2. 3A mm w”... m2. Ewoq >30 83.50 23 33m
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2;. mad 2A 3. H: g. 3A m N2. S... >20 @2230 £3 QQQ
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10 BULLETIN NO. 509, TEXAS AGRICULTURAL EXPERIMENT STATION

ability was measured both by the oven-dried weight of the crop and the
phosphoric acid removed by the crop.

Description of Soils Used

The soils used are described as follows. The composition of these

, soils is given in Table 1.

31322. Amarillo silty clay loam, shallow phase, Potter County, 7 to
19 inches deep, dark red silty clay loam, taken from 1% miles N. E. of
Nash.

32644. Lake Charles Clay Loam, 0 to 7 inches deep, Galveston County
surface soil, black clay loam, virgin prairie, flat, poor drainage, few sand
mounds, heavy coarse grass taken from 4 mi. S. of Alto Loma.

32647. Lake Charles very fine sandy loam, 7 to 13 inches deep, Galveston
County, dark gray sandy clay taken, from 5 mi. S. of Alto Loma.

32649. Lake Charles Clay, 0 to 7 inches, Galveston County, virgin prairie,
hog wallow land, sample from flat place between hog wallow humps,
flat—poor drainage, grayish brown clay, taken from 2 mi. E. of League
City.

32650. Lake Charles Clay, 7 to 19 inches deep, Galveston County, dar-k
grayish black clay, from 2 mi. E. of League City.

33125. Moscow fine sandy loam, 3 to 7 inches, Polk County, a gray
loamy fine sand with numerous dark brown to black soft concretions,
taken 6 mi. W. of Corrigan, near the Groveton Road.

33126. Crockett Clay Loam, 0 to 7 inches, Polk County, surface soil
(not cultivated) a dark brownish gray heavy fine sandy loam to light
clay loam, taken 2% mi. E. of Rock Island.

33138. Wilson Clay, 7 to 14 inches, Polk County, a black heavy plastic
clay, taken 3 mi. W. of Moscow on Colita road and 1,é mi. N. (not cultivated).

33140. Gainer clay, 14 to 24 inches, Polk County, virgin sample, a
yellow and gray mottled clay with some soft yellowish concretions, taken
200 yds. N. of East Tempe school.

33782. Wilson Clay, 0 to 7 inches, Collin County, dark gray clay,
taken l/é mi. S. of Climax.

33705. Bell Clay, 0 to 7 inches, Collin County, black clay, taken 2 mi.
S. E. of McKinney.

33708. Trinity Clay, 0 to 7 inches, Collin County, black clay, taken 2
mi. S. W. Climax.

Results of the Work with Soft Phosphate

The soft phosphate with colloidal clay was a sample of a shipment from
Florida to Texas.

A summary is presented in Table 2 and the detailed results are given
in Table 3. The addition of superphosphate is indicated by Pa, of col-
loidal phosphate by Co, and of finely-divided phosphate by Ru. In ‘the
seven tests with soft phosphate, the availability of the phosphoric acid
compared with superphosphate as 100 ranged from 0 to 120 with an
average of 40, when the gain in weight of crop was used. When the

AVAILABILITY OF THE PHOSPHORIC ACID OF ROCK PHOSPHATE 11

recovery of phosphoric acid was used, the availability ranged from 0 to
97, with an average 0f 40. A very wide variation in availability from 0
to 97 or 120, is to be noted. The lowest availability was 0n soil N0.

Table 2. Gain due to finely-divided phosphate compared with gain due to superphosphate

as 100
Relative Relative
Laboratory gain removal of Acidity
number in phosphoric (pH)
crop acid
. . l
Soft phosphate with colloidal clay |
32647 Lake Charles very‘ fine sandy loam 0 0 l 7.6
31322 Amarillo silty clay loam 2 4 7.9
33140 Gainer Clay 24 25 5.3
33125 Moscow fine sandy loam 30 40 5.0
32649 Lake Charles Clay ' 37 58 5.8
33138 Wilson Clay 69 55 5.7
32650 Lake Charles Clay 120 97 5.0
Average 40 40
Finely ground phosphate rock
32647 Lake Charles very fine sandy loam 0 0 7.6
32649 Lake Charles Clay 16 34 5.8
33125 Moscow fine sandy loam 34 49 5.0
32650 Lake Charles Clay 95 95 5.0
l .____ i“.
Average I 36 45

32647, the subsoil of the Amarillo silty clay loam and soil No. 31322, the
Lake Charles very fine sandy loam, while the highest was on soil No.
32650, the subsoil to the Lake Charles clay.

The soils in which the availability is low are slightly basic, as shown
by the pH of 7.60 to 7.81 in Table 1, while the one With a high avail-
ability is acid, having a pH of 5.0. There are, however, two soils with a
similar degree of acidity, pH 5.3 and 5.8, in which the availability is only
from 30 to 37 measured by the crop and 40-58 measured by the phos-
phoric acid removed. The soils in Table 1 and 2 are arranged in order
according to the availability of the phosphoric acid in the soft phosphate,
beginning with the lowest. Aside from the general relation to the acidity
or pH, no other relation can be found to the composition of the soils as
shown in Table 1. The results indicate that the phosphoric acid of soft
phosphate with colloidal clay has an average- availability of about 40
per cent that of superphosphate, while on some acid soil it may have an
availability as high as superphosphate. On neutral or basic soils it
may have little or no value.

Additional Work with Soft Phosphate

Another series of pot experiments was made in which the effect of the
soft phosphate upon the growth of corn and sorghum and the phosphoric
acid removed was ascertained. The pot experiments were conducted by the
method already described, excepting that the complete fertilizer con-
tained one gram of dicalcium phosphate so that there could be no
comparison between the same quantity of phosphoric acid in the soft

12 BULLETIN NO. 509, TEXAS AGRICULTURAL EXPERIMENT STATION

2 2 2w “.5 E2. £2. Mm. mdH 5M M Z N.
g 3H W3 3N .33 E2. g. fimfi o0 M Z m.
l l l l l $2. mm. QNH o0 M Z m
o3 2: Wm _ mam £2. Q»? mm. WNH um M Z w
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5.3m mow
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13

AVAILABILITY OF THE PHOSPHORIC ACID OF ROCK PHOSPHATE

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l l. l l l $8. on. o6 5M M Z M.
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l l l l l $8. .3 w.» c0 M Z m
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14 BULLETIN NO. 509, TEXAS AGRICULTURAL EXPERIMENT STATION

phosphate and in the dicalcium phosphate. The quantity of soft phos-
phate contained .10 grams of phosphoric acid. Two crops were grown,
corn followed by sorghum. The percentage of phosphoric acid recovered
from the soft phosphate by a single crop varied from 0 to 15.3%, the
average being 4.3%. This may be compared with an average recovery
of 43.9% for the phosphoric acid of superphosphate in 21 pot experiment
given in Bulletin 212, and an average recovery of 9.1% for the phos-
phoric acid of rock phosphate. The recovery of the soft phosphate in
these experiments was therefore about half of that in the experiments
given in Bulletin 212.

As with the other experiments cited in the Bulletin, the highest re-
covery of the phosphoric acid from the soft phosphate, was on the acid
soils, while the low recovery was on neutral or slightly alkaline soils. Since
the soils of Texas in general are neutral rather than acid, one would
expect the availability of the phosphoric acid in the soft phosphate to be
low.

Finely-Ground Rock Phosphate

The finely-ground phosphate was a sample furnished by the manu-
facturer, and was at that time called Ruhm phosphate.

In the tests made, the availability of the phosphoric acid of the finely
ground phosphate rock compared with superphosphate as 100 varies from
0 to 95 with an average of 36, when the gain in weight of the crop
is used as a measure and from 0 to 95 with an average of 45 when the
phosphoric acid recovered is used as a measure. The finely-ground rock
phosphate has about the same availability as the soft phosphate with col-
loidal clay. As with the latter, the availability of the Ruhm phosphate
is high in one of the acid soils and low in the neutral or alkaline soils.
However, in another of the acid soils the availability was about the
average. l

The results indicate that the phosphoric acid of finely-divided phos-
phate has an average about 40 per cent of the availability of that of
superphosphate, though on some acid soils it may be nearly equal to super-
phosphate, while on some neutral soils it may have little or no value.

SUMMARY

Soft phosphate with colloidal clay is a finely-divided phosphate which
is a by-product from mining Florida rock phosphate.

The availability of the phosphoric acid in soft phosphate with colloidal
clay in 7 pot experiments was found to vary from 0 to 120 with an
average of 40, compared with the phosphoric acid of superphosphate as
100. The availability was low on the slightly basic soils and high on
one acid soil, although it was not high on some of the acid soils.

The phosphoric acid of soft phosphate with colloidal clay in general has
a much lower availability to plants than the phosphoric acid of superphos-
phate and its availability seems \to be lower on neutral or basic soils,
such as generally prevail in Texas.

15

AVAILABILITY OF THE PHOSPHORIC ACID OF ROCK PHOSPHATE

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16 BULLETIN NO. 509, TEXAS AGRICULTURAL EXPERIMENT STATION

The phosphoric acid of rock phosphate which has been very finely ground
has been claimed to have a high availability, but in five pot experiments
the availability of the phosphoric acid of finely ground rock phosphate
was only about 40 per cent of that of superphosphate. Its availability
seemed to be 10W on neutral or basic soils and high on acid soils.

On some acid soils, the availability of the phosphoric acid of both soft
phosphate with colloidal clay and finely ground rock phosphate is equal
to that of superphosphate, but on other acid soils its availability is de-
cidedly less than that of superphosphate.

The results on the same soils are similar whether the availability is
measured by the relative gain in Weight of the crop or by the quantity of
phosphoric acid taken up by the plant.

REFERENCES

1. Ames, J. W. and Kitsuta, Ky., 1932. Availability of rock phosphate
as indicated by phosphorus assimilation of plants. Jour. Amer. Soc.
Agronomy, 24?:~183.

2. Collings, G."}H., 1934. Commercial Fertilizers. P. Blakeston’s Son
& Co., Inc}, Philadelphia.

3. Colloidal Phosphate Co., 1929. Colloidal phosphate. The Basic Fertilizer
Element (advertising circular).

4. Fraps, G. S.,._1_917. The availability of phosphoric acid in rock phos-
phate, Texas; Agr. Expt. Sta. Bull. 212.

5. Haskins, H.'D., 1929. Fertilizer Control report. Masachusetts Agr.
Expt. Sta.,  51, Control Series.

6. Haskins, H. {D., 1933. Report of the Committee on Definitions of

Terms and Interpretation of Results on Fertilizers and Liming Ma-

_. terials.iJour. A. O. A. C., 17:40.

7. Hoover, R. P., 1929. Radio talk with Farmer Rusk over Station
WENR on Lime Phosphate. Oct. 28, 1929.

8 . Jacob, K. D., Hill, W. L., and Holmes, R. S., 1929. The colloidal na-
ture of some finely divided natural phosphates (sold as colloidal
phosphates). Colloidal Symposium.

l 9. Waggaman, W. H., 1911. A review of the phosphate field of Florida.

U. S. Dept. Agr. Bureau of Soils, Bull. 76.