Bulletin 408 March, 1945
UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION
HAROLD MOWRY, Director
AVAILABILITY OF THE PHOSPHORUS
OF VARIOUS TYPES OF PHOSPHATES
ADDED TO EVERGLADES PEAT LAND
By J. R. NELLER
Single copies free to Florida residents upon request to
AGRICULTURAL EXPERIMENT STATION
Fig. 1.-View of grasses on phosphate source plots June 12, 1936. Phos-
phate has been omitted from the fertilizer for the plots in the foreground
and the response to superphosphate (18 percent POs) shows in the next
2 plots. Dallis grass was growing on the right row of plots, carpet grass
on the left.
BOARD OF CONTROL ECONOMICS, AGRICULTURAL
H. P. Adair, Chairman, Jacksonville C. V. Noble, Ph.D., Agr. Economist1 3
N. B. Jordan, Quincy Zach Savage, M..A., Associates
T. T. Scott Live Oak A. H. Spurlock, M.S.A., Associate
T. Scott Lie Oak Max E. Brunk, M.S., Associate
Thos. W. Bryant, Lakeland
M. L. Mershon, Miami
J. T. Diamond, Secretary, Tallahassee ECONOMICS, HOME
Ouida D. Abbott, Ph.D., Home Econ.1
EXECUTIVE STAFF R. B. French, Ph.D., Biochemist
John J. Tigert, M.A., LL.D., President of the ENTOMOLOGY
H. Harold Hume, D.Sc., Provost for Agricul- J. R. Watson, A.M.. Entomologist1
ture A. N. Tissot, Ph.D., Associates
Harold Mowry, M.S.A., Director H. E. Bratley, M.S.A., Assistant
L. O. Gratz, Ph.D., Asst. Dir., Research
W. M. Fifield, M.S., Asst. Dir., Admin.4 HORTICULTURE
J. Francis Cooper, M.S.A., Editors
Clyde Beale, A.B.J., Assistant Editors G. H. Blackmon, M.S.A., Horticulturist'
Jefferson Thomas, Assistant Editors
Jefferson Thomas, Assistan Editor A. L. Stahl, Ph.D., Asso. Horticulturist
Ida Keeling Cresap, Librarian
Ruby Newhall, Administrative Managers F.S. Jamison, Ph.D., Truck Hort.
K. H. Graham, LL.D., Business Managers R. J. Wilmot, M.S.A., Asst. Hort.
Claranelle Alderman. Accountants R. D. Dickey, M.S.A., Asst. Hort.*
Victor F. Nettles, M.S.A., Asst. Hort.'
J. Carlton Cain, B.S.A., Asst. Hort.'
MAIN STATION, GAINESVILLE Byron E. Janes, Ph.D., Asst. Hort.
F. S. Lagasse, Ph.D., Asso. Hort.2
W. E. Stokes, M.S., Agronomist' PLANT PATHOLOGY
Fred H. Hull, Ph.D., Agronomist
G. E. Ritchey, M.S., Agronomist2 W. B. Tisdale, Ph.D., Plant Pathologist'
G. B. Killinger, Ph.D., Agronomist Phares Decker, Ph.D., Asso. Plant Path.
W. A. Carver, Ph.D., Associate Erdman West, M.S., Mycologist
Roy E. Blaser, M.S., Associate Lillian E. Arnold, M.S., Asst. Botanist
H. C. Harris, Ph.D., Associate
R. W. Bledsoe, Ph.D., Associate SOILS
Fred A. Clark, B.S., Assistant
F. B. Smith, Ph.D., Chemist1 8
ANIMAL INDUSTRY Gaylord M. Volk, M.S., Chemist
J. R. Henderson, M.S.A., Soil Technologist
A. L. Shealy, D.V.M., An. Industrialisti 8 J. R. Neller, Ph.D., Soils Chemist
R. B. Becker, Ph.D., Dairy Husbandman' C. E. Bell, Ph.D., Associate Chemist
E. L. Fouts, Ph.D., Dairy Technologist8 L. H. Rogers, Ph.D., Associate Biochemist'
D. A. Sanders, D.V.M., Veterinarian R. A. Carrigan, B.S., Asso. Biochemist
M. W. Emmel, D.V.M., Veterinarians G. T. Sims, M.S.A., Associate Chemist
L. E. Swanson, D.V.M., Parasitologist4 T. C. Erwin, Assistant Chemist
N. R. Mehrhof, M.Agr., Poultry Husb.s H. W. Winsor, B.S.A., Assistant Chemist
T. R. Freeman, Ph.D., Asso. in Dairy Mfg. Geo. D. Thornton, M.S., Asst. Microbiologist
R. S. Glasscock, Ph.D., An. Husbandman
R. S. Glasscock, Ph.D., An. Husbandman R. E. Caldwell, M.S.A., Asst. Soil Surveyor'
D. J. Smith, B.S.A.o Asst. An. Husb.'
. Dir M..., s t. i Hus. Olaf C. Olson, B.S., Asst. Soil Surveyor
P. T. Dix Arnold, M.S.A., Asst. Dairy Husb.s
G. K. Davis, Ph.D., Animal Nutritionist
C. L. Comar, Ph.D., Asso. Biochemist
L. E. Mull, M.S., Asst. in Dairy Tech.4 i Head of Department.
J. E. Pace, B.S., Asst. An. Husbandman 2 In cooperation with U. S.
S. P. Marshall, M.S., Asst. in An. Nutrition
C. B. Reeves, B.S., Asst. Dairy Tech. I Cooperative, other divisions, U. of F.
Katherine Boney, B.S., Asst. Chem. In Military Service.
Ruth Faulds, A.B., Asst. Biochemist I On leave.
Peggy R. Lockwood, B.S., Asst. in Dairy Mfs.
BRANCH STATIONS SUB-TROPICAL STA., HOMESTEAD
NORTH LORIDA STATION QUINC Ge. D. Ruehle Ph.D., Vice-Director in
NORTH FLOIDA STATION, QCharge
J. D. Warner, M.S., Vice-Director in Charge P. J. Westgate, Ph.D., Asso. Horticulturist
R. R. Kincaid, Ph.D., Plant Pathologist H. I. Borders, M.S., Asso. Plant Path.
V. E. Whitehurst, Jr., B.S.A., Asst. Animal W. CENT. FLA. STA. BROOKSVILLE
Jesse Reeves, Asst. Agron., Tobacco Clement D. Gordon, Ph.D., Asso. Poultry
W. H. Chapman, M.S., Asst. Agron.' Geneticist in Charge2
R. C. Bond, M.S.A., Asst. Agronomist
RANGE CATTLE STA, ONA
Mobile Unit, Monticello W. G. Kirk, Ph.D., Vice-Director in Charge
E. M. Hodges, Ph.D., Asso. Agron., Wauchula
R. W. Wallace, B.S., Associate Agronomist Gilbert A. Tucker, B.S.A., Asst. An. Hush.'
Mobile Unit, Milton STATIONS
Ralph L. Smith, M.S., Associate Agronomist
Mobile Unit, Marianna G. K. Parris, Ph.D., Plant Path. in Charge
R. W. Lipscomb, M.S., Associate Agronomist Plant City
A. N. Brooks, Ph.D., Plant Pathologist
Mobile Unit, Wewahitehka
J. B. White, B.S.A., Associate Agronomist
A. H. Eddins, Ph.D., Plant Pathologist
E. N. McCubbin, Ph.D., Truck Horticulturist
CITRUS STATION, LAKE ALFRED
A. F. Camp, Ph.D., Vice-Director in Charge Monticello
V. C. Jamison, Ph.D., Soils Chemist S. O. Hill, B.S., Asst. Entomologist 4
B. R. Fudge, Ph.D., Associate Chemist A. M. Phillips, B.S., Asst. Entomologist2
W. L. Thompson, B.S., Entomologist
W. W. Lawless, B.S., Asst. Horticulturist' Bradenton
C. R. Stearns, Jr., B.S.A., Asso. Chemist J. R. Beckenbach, Ph.D., Horticulturist in
H. 0. Sterling, B.S., Asst. Horticulturist Charge
T. W. Young, Ph.D., Asso. Horticulturist E. G. Kelsheimer, Ph.D., Entomologist
J. W. Sites, M.S.A., Asso. Horticulturist Dy. B. Creager, Ph.D., Plant Path., Gladiolus
A. L. Harrison, Ph.D., Plant Pathologist
EVERGLADES STA., BELLE GLADE David G. Kelbert, Asst. Plant Pathologist
E. L. Spencer, Ph.D., Soils Chemist
R. V. Allison, Ph.D., Vice-Director in Charge
J. W. Wilson, Sc.D., Entomologist* Sanford
F. D. Stevens, B.S., Sugarcane Agron.
Thomas Bregger, Ph.D., Sugarcane R. W. Ruprecht, Ph.D., Chemist in Charge
Physiologist J. C. Russell, M.S. Asst. Entomologist
G. R. Townsend, Ph.D., Plant Pathologist
R. W. Kidder, M.S., Asst. An. Hush. Laeland
W. T. Forsee, Jr., Ph.D., Asso. Chemist E. S. Ellison, Meteorologist 2 5
B. S. Clayton, B.S.C.E., Drainage Eng.2 Warren 0. Johnson, Meteorologist2
F. S. Andrews, Ph.D., Asso. Truck Hort.'
R. A. Bair, Ph.D1., Asst. Agronomist 1 Head of Department.
E. C. Minnum, M.S., Asst. Truck Hort. 2 In cooperation with U. S.
N. C. Hayslip, B.S.A., Asst. Entomologist 3 Cooperative, other divisions, U. of F.
Earl L. Felix, Ph.D., Asst. Plant Path. In Military Service.
C. L. Serrano, B.S.A., Asst. Chemist 5 On leave.
Fig. 2.-Growth of shallu March 21, 1932. Colloidal phosphate was the
source of phosphorus for the plot in the upper view and superphosphate
(18 percent PO) for the plot in the lower view.
AVAILABILITY OF THE PHOSPHORUS OF VARIOUS
TYPES OF PHOSPHATES ADDED TO
EVERGLADES PEAT LAND
By J. R. NELLER
Everglades peat ...................................... 7 Effect of phosphates on leaf saps.......... 15
Grass crops .......................................... .. 7 Summary and conclusions ........................ 25
Miscellaneous crops ...............-................... 14 Literature cited ...............................--......... 27
In mineral soils the availability to crops of the phosphorus of
various types of phosphates is influenced strongly by the char-
acteristics of the soil-such as its reaction, chemical and adsorp-
tive properties. Availability of phosphorus varies in much the
same way in peat soils but to a lesser degree. Thus Wolkoff
(17.)1 noted that extraction with N/5 nitric acid caused a re-
covery of less phosphorus from phosphates added to mineral
soils than from those added to peats. Doughty (11) measured
the phosphorus fixing power of peats as well as soils high and
low in organic matter and found that organic matter was of
minor importance in the fixation of phosphorus in a form diffi-
cult to dissolve.
It is generally recognized that the problem of correlating the
availability of plant nutrients in a soil of a given type as indi-
cated by extracting solutions with the actual utilization of the
nutrients as shown by the response of plants growing in the
soil is not easy. This correlation is difficult to obtain in the case
of phosphorus, since this element may enter readily into com-
pounds that are either highly insoluble and slowly available
or that become less available in varying degrees after incorpor-
ation with a soil. As pointed out by Buehrer (6) these re-
lationships are probably of a physico-chemical nature. Burd
and Murphy (7) discuss some of the causes of failure to
obtain correlations between plant growth and the acid-extract-
able phosphates of soils. A simple water extract might parallel
plant response more closely were it not that the amounts of
phosphorus thereby obtained from most soils are too small for
comparative purposes. In peat soils, however, there is the pos-
1Italic figures in parentheses refer to "Literature Cited" in the back
of this bulletin.
6 Florida Agricultural Experiment Station
sibility that water extract studies may be of value, as indicated
in some of the following data.
The transformation and fixation of soluble phosphates in soils
is generally sufficient to prevent much loss from leaching, with
the result that quantities of phosphorus tend to accumulate.
This adds to the difficulty of using an extractant method in
measuring the crop requirement of a soil for phosphorus. More-
over, there is good evidence (13) that in Everglades peat some
crops are adversely affected by the applications of too much
soluble phosphate fertilizer. In the Annual Report of the Florida
Station for 1929 Allison states (1) in reference to experiments
with sugarcane that "a reduction of 50 percent in the standard
application of superphosphate was made, since the material
was found to produce rather serious injury to all varieties."
He observed (2) that corn was especially sensitive to the super-
phosphate effect under field conditions but found that it was
not particularly injured in greenhouse trials, even when high
applications of the superphosphates were used. This sensitivity
of some crops creates the possibility that the injudicious use of
soluble phosphates on heavily fertilized crops such as celery
might cause an accumulation that would be harmful to some
Another method of ascertaining the phosphorus needs of a
soil is to analyze the crop or the growing plant for phosphorus
and to correlate the results with composition standards of crops
grown in the same or a similar type of soil that contains suffi-
cient available phosphorus. Alway, Shaw and Methley (3)
carried on an extensive study of this sort with Minnesota peats
and concluded among other things that "definite limiting values
for the phosphate content of a crop, both an upper one, indi-
cating that the phosphate supply of the soil is adequate for
approximately maximum yields, and a lower one, indicating
that heavier phosphate fertilization is practically certain to
cause a great increase in yield, are difficult to place, at least
for peat soils, since values that would apply for seasons or
localities with favorable weather conditions, and consequent
heavy yields, are likely to be so high as to require unprofitably
J heavy fertilization." Some investigators depend largely on crop
response to indicate phosphorus needs. Thus Bartholomew (5)
used sudan grass as a plant index for Arkansas soils and DeTurk
(10) shows how a highly insoluble form such as rock phosphate
and a soluble acid-treated rock phosphate are both useful for
Availability of Phosphorus of Different Phosphates 7
the crops and rotations in Illinois. In Ohio Ames and Kitsuta
(4) found that the acidity of the soil as well as the degree of
fineness of the ground rock phosphate influenced the availability
of its phosphorus.
In the experiments recorded herein various phosphates, some
with lime, were added to Everglades peat and compared by
ascertaining effect upon the peat and upon composition and re-
sponse of crops in field plots. Particular attention was given
to a continuation of a study previously reported (13) on the
nature of the adverse response to superphosphates in some cases.
The phosphate experiments recorded in this bulletin were
conducted at the Everglades Experiment Station in the Ever-
glades peat typical of most of the large area of organic soils of
the Florida Everglades. The physical characteristics of this
peat and of the region in which it is located as well as data on
the climate of the region are recorded in Florida Station Bulletin
378 (9). Waksman and Stevens (16) have studied the organic
composition of this peat and Hammar (12) has reported on its
inorganic composition, while Caldwell (8) made a spectrographic
analysis for various minor elements.
Carpet grass, Axonopus compresses (Swartz) Beauv., was
planted in a series of duplicate plots in which various carriers
of phosphate were used as recorded in Table 1. These supplied
the phosphorus for a 6-12-12 fertilizer mixture in which the
nitrogen calculated as N was derived equally from sodium
nitrate and ammonium sulfate and the potash (K20) from the
sulfate salt (50 percent K20). The mixture was used at the
rate of 1,200 pounds per acre and also contained the sulfates
of copper, manganese and zinc in the amounts of 50, 50 and
121/2 pounds per acre, respectively. The rock phosphate (25.39
percent P205) was ground fine enough to allow 69 percent of
it to pass through a 200-mesh sieve and was used at 4 times
the amount (P205 basis) called for in the 6-12-12 formula, while
the colloidal phosphate (26.4 percent P205) and basic slag (10 per-
cent P205) were used at 2 and 11/2 times, respectively, the rates
for the 18 and 44 percent superphosphates. The lime applica-
tion was at the rate of 2,000 pounds and that of the sulfur at
300 pounds per acre.
8 Florida Agricultural Experiment Station
These materials were applied to the grass plots in May of each
year, except that potash only was used in 1934 and 1937. The
rock phosphate, colloidal phosphate and lime were broadcast
separately and the other carriers of phosphate were incorporated
into the 6-12-12 mixture.
As recorded in Table 1, 4 to 6 cuttings of carpet grass were
made each year. The totals per year calculated on an air-dry
basis are averages of yields from duplicate plots. Although
the yields varied considerably from year to year it may be
noted that the 5-year average was over 6 tons per acre from
the plots that received the more insoluble phosphates in rock
phosphate, basic slag and colloidal phosphate, while it was less
than 6 tons from the plots that received the more soluble super-
phosphates. The highest average yield was from the basic slag
treatment which may have been due to some element other than
phosphorus in the slag, even though these plots were all treated
yearly with the sulfates of copper, manganese and zinc. It
should be stated also that tests with ferrous sulfate have been
negative with various crops, including grasses grown on this
TABLE 1.-AIR-DRY YIELDS OF CARPET GRASS IN TONS PER ACRE PER YEAR
FROM PLOTS FERTILIZED WITH VARIOUS CARRIERS OF PHOSPHATE.
Treatment 1933 1934 1935 1936 1937 Average
None ...................... 4.534 4.025 5.350 4.358 6.680 4.991
Super (18% P20O)- 4.229 4.469 7.625 6.668 5.370 5.672
Super (44% P205).- 4.670 4.588 8.038 7.110 4.955 5.872
Rock ...-........-....-... -- 4.363 6.313 8.550 7.770 4.720 6.343
Basic slag ............ 3.672 5.700 7.900 11.198 5.045 6.703
Colloidal --............... 4.648 5.632 8.050 7.798 4.780 6.182
None plus lime...... 3.285 3.975 4.938 2.710 3.905 3.763
Super (18% P,05)+
lime ....---............... 2.364 4.513 6.825 5.920 4.255 4.775
Super (18% Pz0O)+
sulfur .......-- ...-..... 1.532 4.888 8.563 7.873 5.240 5.619
No. of cuttings .... 4 6 5 6 4
Yields were lower from plots that did not receive phosphates
and were still lower from those that were limed. Lime reduced
the yields also from plots treated with 18 percent superphos-
phate, -the average (Table 1) being less than that from the
nonphosphate plots. The use of sulfur with the 6-12-12 mix-
ture was without effect.
Two of the 1934 and 3 of the 1935 cuttings were analyzed
TABLE 2.-PHOSPHORUS CONTENT OF CARPET GRASS GROWN ON PLOTS FERTILIZED WITH VARIOUS CARRIERS OF PHOSPHATES.
Super Super None Super (18% Super (18%
Year *None (18% (44% Rock Basic Colloi- Plus PAOs) plus PAO0) plus
P205) PLOs) Slag dal Lime Lime Sulfur
Percent Oven-dry Basis
May 17 ........ 0.201 0.350 ....... 0.258 0.292 ........ 0.170 0.288 0.336
July 2 .......... 0.149 0.358 ........ 0.341 0.292 ........ 0.157 0.314 0.332
May 22 ....... 0.144 0.201 0.223 0.288 0.218 0.197 0.192 0.227 ........
July 3 ..-........ 0.162 0.340 0.332 0.284 0.262 0.249 0.328 0.292 0.393
Oct. 21 ......... 0.131 0.288 0.296 0.275 0.258 0.236 0.166 0.258 0.306
Average ........ 0.159 0.307 0.284 0.289 0.264 0.227 0.203 0.276 0.342
___________________________________ __ __ __ ____ _______
10 Florida Agricultural Experiment Station
for phosphorus and the results (Table 2) show that the grass
from the plots from which phosphates were withheld was low
in phosphorus in 1934 and still lower in 1936. An inspection of
the averages shows that the phosphorus content of grass from
the phosphate-treated plots was about double that from those
that received no phosphate except for the colloidal phosphate
treatment. Phosphorus was highest in the grass fertilized with
the 18 percent superphosphate. It is interesting to note that
the averages are almost identical for the 44 percent super-
phosphate and the rock phosphate treatments. Lime reduced
and sulfur increased the phosphorus content of the grass.
Yields of Dallis grass, Paspalum dilatatum Poir, are recorded
for a period of 5 years in Table 3 from duplicate plots that were
fertilized in the same way as recorded above for carpet grass.
Five to 6 cuttings were removed each year and it may be noted
that the total yield varied considerably from year to year.
Averaging yields for the 5 years shows a distinct response to
all types of phosphates and all of about equal amounts, those
from the rock phosphate and basic slag being the highest by
7 to 10 percent. Neither lime nor sulfur was of much effect
on yields. The high yields from all of the plots that received
both phosphate and potash are characteristic of the grass-
producing possibilities of Everglades peat in which a satisfactory
water table is maintained and are in line with other grass
experiments previously reported (14) for this peat. Figure
1 shows the nature of growth and fertilizer response for both
Dallis and carpet grass on these phosphate source plots.
Phosphorus analyses were made of some of the cuttings of
TABLE 3.-AIR-DRY YIELDS IN TONS PER ACRE OF DALLIS GRASS GROWN ON
PLOTS FERTILIZED WITH VARIOUS CARRIERS OF PHOSPHATE.
Treatment 1932 1933 1934 1935 1936 Average
None .................... 3.733 8.854 3.555 5.980 7.240 5.872
Super (18% P205).- 3.818 10.675 5.303 6.385 10.359 7.308
Super (44% P20O)-. 4.288 11.105 5.190 6.390 11.090 7.613
Rock ........................ 4.419 11.817 5.895 6.440 11.578 8.030
Basic slag ....---- --- 3.819 11.423 4.883 7.175 11.586 7.777
Colloidal ............... 3.880 10.612 5.023 6.460 11.148 7.425
None plus lime .-.... 3.507 9.267 3.028 4.830 6.167 5.360
Super (18% P, O).
plus lime ..........- 3.808 10.426 4.663 6.550 10.454 7.180
Super (18% P2Os)-.
plus sulfur .......-. 3.923 7.927 4.380 6.215 12.068 6.903
Number of cuttings 1 5 6 6 5 6
Availability of Phosphorus of Different Phosphates 11
grass from these plots for each of the years 1932, 1934, 1935
and 1936 (Table 4). All phosphate treatments markedly in-
creased the percentage of phosphorus in the grass, the rock
phosphate and basic slag treatments being about equally as
effective as the 18 percent superphosphate. Neither lime nor
sulfur was of appreciable effect. The response to phosphorus
is marked, however, both on yield and on the increased phos-
phorus content of the higher yield. Thus there was an average
annual removal of approximately 44 pounds of phosphorus per
acre in the 7.308 air-dry tons of grass cut from the plots that
received superphosphate while only 21 pounds of phosphorus
was contained in the 5.872 tons in the cuttings from the plots
that received potash but no phosphate. As mentioned above,
the rock phosphate, basic slag and colloidal phosphate applica-
tions were at higher rates than those of the superphosphates.
TABLE 4.-PHOSPHORUS CONTENT OF DALLIS GRASS GROWN ON PLOTS
FERTILIZED WITH VARIOUS CARRIERS OF PHOSPHATE.
Phosphate Percent Oven-dry Basis
Treatment 1932 1934 1935 1936 IAverage
None ................................- .. 0.214 0.166 0.189 0.138 0.177
Super (18% P20) .......--..... 0.273 0.336 0.290 0.308 0.302
Rock --...................................... 0.260 0.284 0.283 0.277 0.276
Basic slag ......................-- ..-- ..... 0.249 0.315 0.258 0.266 0.272
None plus lime ...........-.......... 0.203 0.170 0.198 0.135 0.177
Super (18% PzOs)
plus lime ........---...............--- 0.253 0.343 0.279 0.284 0.290
Super (18% P20s)
plus sulfur ....-...................-- 0.297 0.295 0.306 0.322 0.305
Dallis grass was grown on another series of fertilizer plots
in triplicate on Everglades peat in which an 0-6-12 mixture
was used at the rate of 500 pounds per acre. These treatments
also included a study of the availability of phosphates in differ-
ent carriers, as recorded in Table 5. In these, equal amounts
of phosphate (P205) were used, irrespective of the type of
phosphate, and in addition the phosphate was omitted from the
mixture after the initial application in 1931. The high yields
for that year were due to this fertilizer and to the native plant
food elements in the peat-the Dallis grass was the first crop
removed from this field. It may be noted that the yields were
progressively less during the next 5 years and that the average
yield for the 6-year period was appreciably higher from the
superphosphate treated plots than from those that received the
12 Florida Agricultural Experiment Station
more insoluble forms of phosphates. Table 5 also records the
average yields from the plots that received the 0-6-12 mixture
each year and it may be noted that the 6-year average of 6.781
tons is distinctly higher than that of 4.766 tons where the
superphosphate was omitted after the first year. Parallel ex-
periments (14) showed that the yields would have been increased
by the use of more phosphate and potash but not of nitrogen.
TABLE 5.-AIR-DRY YIELDS PER ACRE IN TONS OF DALLIS GRASS CLIPPED
FROM PLOTS FERTILIZED WITH DIFFERENT TYPES OF PHOSPHATES APPLIED
ONCE ONLY AND EACH YEAR.
Phosphate Omitted After 1931 Added Number
I Year Clip-
Year Super- Super- pings
phos- Rock Colloi- Basic phos-
phate dal Slag phate
1931 9.884 10.521 9.559 9.806 11.499 6
1932 4.765 5.149 3.864 4.776 5.625 5
1933 3.830 3.699 3.419 3.484 6.047 4
1934 3.133 1.911 1.922 1.823 6.589 4
1935 4.467 2.393 2.867 3.492 5.666 5
1936 2.517 1.870 2.195 2.331 5.260 5
Average 4.766 4.259 3.971 4.286 6.781
Omission of phosphate was more marked on the phosphorus
content of the grass (Table 6) than it was on yield, as the per-
centage of phosphorus had decreased by 1936 to less than half
that of 1931 for all types of phosphate fertilizer. In 1931 grass
from the superphosphate plots was appreciably higher in phos-
phorus than from the more insoluble forms but by 1936 there
was little difference. This illustrates the higher rate of avail-
ability of the superphosphate.
TABLE 6.-EFFECT ON PHOSPHORUS CONTENT OF DALLIS GRASS OF
WITHHOLDING PHOSPHATES AFTER AN INITIAL APPLICATION.
Phosphate Omitted After 1931 Added
Crop II Each Year
Year I Colloi- Basic
SSuperphosphate Rock dal Slag Superphosphate
Percent Oven-dry Basis
1931 0.304 0.277 0.253 0.258 0.297
1932 0.215 0.166 0.152 0.167 0.253
1936 0.125 0.114 0.118 0.118 0.249
TABLE 7.-RELATIVE YIELDS OF CROPS GROWN ON PLOTS FERTILIZED WITH VARIOUS TYPES OF PHOSPHATES.
Shallu Sugarcane Corn, Buckwheat Rape Carrots Potatoes Cabbage
Phosphate Cuttings 1932 for 1938, Stalks Straw and
Treatment 1939 and and Ears Grain Dec. April May March c
1940 Mar. July Feb. 1933 1932 1933 1936 1933
Jan. Mar. July 1934 1940
None ................... 100 100 100 100 100 100 100 100 100 100 100 ;
Super (18% P0Os).... 96 69 100 87 102 118 91 228 117 135 127
Super (44% P205).... 129 65 99 76 109 120 105 221 112 144 134
Rock ........................ 88 100 93 95 117 125 119 213 93 128 119
Basic slag .............. 112 94 103 95 122 128 112 236 109 118 140
Colloidal .................. 98 89 99 96 120 106 120 192 93 119 114
None plus lime ........ 106 99 101 98 85 94 52 14 100 83 97
Super (18% P20,)
plus lime ............ 125 101 114 82 99 99 87 215 80 105 132
Super (18% P,05)
plus sulfur .......... 111 43 82 83 98 112 109 236 68 120 123
14 Florida Agricultural Experiment Station
Duplicate plots fertilized in the manner described above for
grasses (Tables 1 and 3) were planted to various other crops.
Yields from some of these were not weighed because of damage
from birds, insects, frosts, etc., and those that were recorded
are given on a relative basis in Table 7.
Of 3 successive cuttings of shallu, Sorghum vulgare roxburghii
(Stapf) Haines, a marked retardation of growth occurred for
the March 19 cutting (Table 7) on plots that received the super-
phosphate (Fig. 2). In the January 14 and July 2 cuttings
there was no consistent response to any form of phosphate. This
is illustrated in Figure 3, which shows that growth was about
the same on plots that received colloidal phosphate as on those
that were treated with superphosphate. It may be noted that
lime overcame the toxic condition exhibited by the second cut-
ting (March, Fig. 2) and that sulfur aggravated it.
Relative averages of yields of 3 crops of sugarcane, Saccharum
officinarum L., are given in Table 7. From these it may be ob-
served that all types of phosphates reduced growth, and particu-
larly the superphosphates. Figure 4 shows that the growth
Fig. 3.-Growth of shallu June 30, 1932. The colloidal phosphate and super-
phosphate (18 percent PO) are at the left and right, respectively.
Availability of Phosphorus of Different Phosphates 15
in the presence of superphosphate (44 percent P205) is less than
that of the adjacent plot that received rock phosphate. In both
1934 and 1940 corn, Zea mays L., responded to phosphates but
the response was somewhat higher for the more insoluble forms,
especially in 1934. Buckwheat, Fagopyrum esculentum Moench.,
responded more to the insoluble forms and rape, Brassica napus
L., responded strongly to all types of phosphates. On the other
hand cabbage, Brassica oleracea L., and potatoes, Solanum
tuberosum L., and particularly carrots, Daucus carota L., grew
best where superphosphates were used. Turnips, Brassica rapa
L., (Figs. 5 and 6) responded strongly to all types of phosphates,
with strongest response to superphosphate and least to colloidal
EFFECT OF PHOSPHATES ON LEAF SAPS
In an attempt to ascertain why some crops respond so differ-
ently than others to different carriers of phosphates a study
was made of the saps expressed from the leaves of the plants.
Samples consisting of about 100 leaves were obtained from the
centers of the plots to avoid border effects. These were taken
in the early forenoon and the saps were immediately obtained
by grinding the fresh material in a burr mill and then subject-
Fig. 4.-Growth of sugarcane in December 1937. The superphosphate (18
percent POs) plot is at the left, the rock phosphate plot at the right.
r IL '
16 Florida Agricultural Experiment Station
Fig. 5.-Turnips photographed March 23, 1934. Upper: Superphosphate
(18 percent P205) plot; lower: Rock phosphate plot.
ing it to a pressure of 16,000 pounds to the square inch. The
sap was centrifuged and portions of the supernatant liquid were
used for the determinations of pH and total acidity. Total
phosphorus was determined by evaporating 5 cc. of the sap and
oxidizing it with magnesium nitrate, after which the colori-
metric molybdenum blue method of analysis was employed. In-
organic phosphorus was determined by a colorimetric analysis
of an aliquot of the sap diluted with distilled water to obtain a
concentration low enough for readings to be made.
Table 8 records the total phosphorus, pH and total acidity of
the saps of the leaves of sugarcane, shallu and corn taken in
June, of rape in December, 1932, and of buckwheat in February,
1933, for which the relative yields from the different phosphate
treatments are given in Table 7. It may be noted that the phos-
phorus and acidity of the saps of all of the crops were increased
Availability of Phosphorus of Different Phosphates 17
by the use of superphosphate. Rock phosphate caused a lesser
increase in phosphorus with no measurable effect on acidity.
Soil treatments with lime tended to decrease and of sulfur to
increase the intake of phosphorus.
A further study was made of the leaf saps of shallu which,
as shown in Table 7 and Figures 2 and 3, was adversely affected
by superphosphate in the period preceding March but not that
preceding July. Figures 7 and 8 give a visual picture of the data
for the sap of the leaves of the March and July harvests. In
both cases the plots are arranged in the order of increasing con-
centrations of phosphorus in the leaf saps. It may be noted
that the concentration increased sharply (Fig.7) in leaves from
plots treated with superphosphate and on which the yields were
reduced (Table 7 and Fig. 2). Leaf saps (Fig. 8) in growth
preceding the July harvest were also highest from the super-
y i '**
Fig. 6.-Turnips photographed March 23, 1934. Upper: Basic slag plot;
lower: Colloidal phosphate plot.
18 Florida Agricultural Experiment Station
phosphate plots but not nearly so much so nor was growth
appreciably retarded (Table 7 and Fig. 3). The acidity values
tended to follow those of total phosphorus but the pH of the
saps remained fairly constant. This high buffering capacity of
the saps of plants in relation to intake of phosphorus is illus-
trated in a previous publication (13).
A special study was likewise made of the leaf saps of buck-
wheat, as this crop was benefited by the use of the more insoluble
carriers of phosphate but not by the soluble superphosphate
(Table 7). Moreover lime caused growth to be reduced by
almost 50 percent. Table 8 shows that the highest concentra-
tions of leaf sap phosphorus were in plants from the plots treated
with superphosphate. Corresponding increase in total acidity
is doubtless caused by the absorbed phosphorus. The adverse
effect of lime on buckwheat (Table 7) was not associated with
the phosphorus concentrations of the saps, as they are lower in
plants from the limed plots than in those from the sulfured plots
and higher than in those from the plots receiving rock phosphate.
The total phosphorus of the entire buckwheat plant (Table 9)
3200 5 I Phosphorus -
S2400 4 7
r 1600 =3 6
2 15 8 4 16
Fig. 7.-Total phosphorus, acidity and pH of leaf saps of shallu March
19, 1932, arranged in order of increasing concentrations of phosphorus.
Phosphate treatments were: No. 2, none; No. 15, superphosphate (18 per-
cent P20O) plus lime; No. 8, colloidal phosphate; No. 4, superphosphate
(18 percent P20); No. 16, superphosphate (18 percent P2Os) plus sulfur.
TABLE 8.-PHOSPHORUS, PH AND ACIDITY OF LEAF SAPS OF SUGARCANE, SHALLU, CORN, RAPE AND BUCKWHEAT Q
FROM PLOTS TO WHICH DIFFERENT TYPES OF PHOSPHATES WERE ADDED.
Sugarcane Shallu Corn Rape Buckwheat 0
Source of 1 AcAcid- Acid- Aci- I Acid-
Phosphate P pH ityH P pH ity2 P pH ity2 P pH ity2 P pH ity2
ppm. cc. ppm. cc. ppm. cc. ppm. cc. ppm. cc.
None ............................. 506 5.46 1.40 940 5.67 2.35 410 5.34 3.35 168 6.40 0.50 231 5.52 1.00
Super (18% P20s) ........ 1030 5.29 2.45 1192 5.59 3.00 822 5.51 5.50 857 6.00 0.75 1046 5.62 2.10
Rock ............................. 655 5.68 1.35 1165 5.68 2.70 1016 5.73 2.90 245 6.30 0.50 404 5.89 1.10
Super (18% P20) t
plus lime ................. 694 5.54 1.50 898 5.60 2.35 681 5.59 3.10 334 6.17 0.70 600 5.71 0.90
Super (18% P,20)
plus sulfur ............ 1144 5.54 2.30 1670 5.66 3.60 1210 5.51 4.85 670 6.07 1.00 953 5.60 1.30
1 Adequate amounts of other plant food elements were added.
S Given as N/10 NaOH. for 5 cc. of sap. co
I Total Phosphorus
[ Inorganic Phosphorus
.2400 4 -- Acidity 7
" .] H pH
1600 >,3 61
S800 2- 5
15 8 2 6 4 5 16
Fig. 8.-Total phosphorus, inorganic phosphorus, acidity and pH of leaf saps of shallu, June 1, 1932, arranged in increasing
order of total phosphorus. Phosphate treatments were: No. 15, superphosphate (18 percent P20s) plus lime; No. 8, colloidal
phosphate; No. 2, no phosphate; No. 6, ground rock phosphate; No. 4, superphosphate (18 percent P205); No. 5, superphos-
phate (44 percent PA0O); No. 16, superphosphate (18 percent P205) plus sulfur.
Availability of Phosphorus of Different Phosphates 21
TABLE 9.-PHOSPHATE, PH AND CONDUCTIVITY FACTORS OF WATER EXTRACTS
OF SOIL AFTER BUCKWHEAT WAS HARVESTED, JANUARY 12, 1932.
Soil Extract 1
Phosphate Total Plant I I Conductivity at
Treatment Phosphorus pH I Phosphorus 240 C. in
| p.p.m. Reciprocal Ohms
None .----................ .. 0.315 5.52 6 1120
Super (18% P20s) 1.289 5.38 153 720
Super (44% P20O) 1.166 5.29 155 1212
Rock .........--........... 0.616 5.36 49 1400
None plus lime...... 0.306 5.91 6 1212
Super (18% P20z)
plus lime ..--..-..... 1.092 5.87 88 844
Super (18% P20O)
plus sulfur ......- 1.389 1 4.38 286 430
1 Phosphorus and conductivity data are based on oven-dry weights of plants and soils.
A soil-water ratio of 1 to 5 was used for the soil extract.
varies with that of the leaf sap and has an interesting relation
to certain characteristics of water extracts of peat from the
surface 6 inches of these phosphate source plots. Table 9 shows
that lime increased the pH of these extracts (1 part soil by
weight to 5 parts distilled water) while the sulfur treatment
reduced the pH to a marked degree. The water-soluble phos-
phorus was increased markedly by the superphosphate treat-
ments, especially when used with sulfur. The rock phosphate
caused a considerable increase also, about 1/3 as much being
obtained. The conductivity measurements were made with an
Ostwald cell and a Wheatstone bridge and indicate an increased
concentration of ions (decreased resistance in ohms, Table 9)
with increased concentration of dissolved phosphorus but the
relationship is not entirely consistent. These experiments with
water extracts are of a preliminary nature and seem to indicate
useful possibilities in a study of the availability of phosphates
added to a soil medium such as that of Everglades peat in which
the added phosphates are not so strongly held or fixed as in
Table 10 shows the phosphorus content of some of the vege-
table crops that were grown on the phosphate source plots.
Carrots (Table 7) and turnips (Fig. 5) require a soluble phos-
phate such as superphosphate and it is thought that lettuce
and broccoli do also, although more crop data are needed on that
point. The superphosphate treatments increased the phos-
22 Florida Agricultural Experiment Station
phorus content of turnips and carrots over that resulting from
the rock phosphate, basic slag and colloidal phosphate treat-
ments, while the differences in effect of these 2 groups of phos-
phates were slight for lettuce and broccoli.
TABLE 10.-PHOSPHORUS CONTENT OF VEGETABLES GROWN ON PLOTS
FERTILIZED WITH VARIOUS CARRIERS OF PHOSPHATE.
Phosphate Turnip Lettuce Broccoli Carrot
Treatment Roots Leaves Buds Roots
Precent Oven-dry Basis
None .............................-....... 0.293 0.550 0.655 0.642
Super (18% P2Os) ............ 0.721 0.760 0.799 0.943
Super (44% P20s) -------. 0.620 0.743 0.777 0.987
Rock ..-......... ............ 0.472 0.817 0.699 0.686
Basic slag ........................... 0.550 0.677 0.751 0.769
Colloidal ...................-........... 0.358 0.633 0.673 0.625
None plus lime ................ ....... 0.463 0.555 0.699
Super (18% P2Os)
plus lime ............ ..-....... 0.598 0.786 0.743 0.900
Super (18% PO) I
plus sulfur .--....-...----. 0.777 0.629 0.795 0.943
Inasmuch as the growth of sugarcane on Everglades peat
appeared to be indifferent to any phosphate fertilizers and inimi-
cal to the soluble or superphosphate forms, Stevens (15) set
up a special series of plots in which superphosphate, basic slag
and rock phosphates were used. He found that there was slight
improvement with basic slag and with rock phosphate but that
superphosphate depressed the yields both in tonnage and in
recoverable sugar. A study was made of the pH, acidity and
phosphorus contents of the stalk juices from these plantings.
Table 11 shows that the pH of the juices ranged around 5.5 and
was not appreciably affected by phosphate treatment. The
acidity was higher, however, and was increased mostly by super-
phosphate followed by basic slag and rock phosphate treatments.
There was no appreciable difference as to whether the fertilizer
was broadcast or applied in the furrow but the small-barrelled
variety Co 281 had a more acid juice than the large-barrelled
CP 2714. These data are for the crop of 1932 for which the con-
centrations of phosphorus are given in Table 12, where it may
be noted that while 250 pounds per acre of superphosphate causes
a measurable increase in phosphorus, over 500 pounds of basic
slag and over 2,000 pounds of rock phosphate are necessary for
a like effect. The variations in phosphorus probably account
TABLE 11.-TOTAL ACIDITY AND PH OF STALK JUICES OF 2 VARIETIES OF SUGARCANE FERTILIZED BY BROADCASTING AND IN
THE PLANTING FURROW WITH VARIOUS AMOUNTS OF DIFFERENT CARRIERS OF PHOSPHATE.
(Crops of 1932 for which see Phosphorus Data in Table 12.)
Phosphate Variety Co 281 Variety CP 2714
per Acre pH __Acidity pH _Acidity _
Broadcast f Furrow Broadcast Furrow | Broadcast I Furrow ] Broadcast I Furrow
Pounds cc. cc. cc. cc. 0
Rock 2,000 .............. 5.42 .... 8.5 ...... 5.47 ...... 5.0
3,000 .............. 5.43 5.47 9.00 11.50 5.45 5.81 5.1 7.5
250 .............. 5.54 5.55 11.0 8.3 5.58 5.53 6.1 6.0
500 .............. 5.53 5.49 11.0 11.5 5.63 5.63 6.5 5.5
1,000 ............ 5.21 5.45 10.5 10.6 5.21 5.46 5.6 6.0 t
2,000 .............. 5.34 5.36 12.6 11.5 5.60 5.35 5.5 5.0
Super (44% P20,)
250 .............. 5.59 5.54 12.5 12.5 5.80 5.71 6.5 8.0
500 .............. 5.22 5.22 14.5 12.0 5.73 5.68 7.0 7.0
1,000 ............. 5.35 5.33 13.5 12.5 5.30 5.35 7.7 9.4
2,000 ............. 5.05 5.26 16.7 15.9 5.26 5.20 8.5 9.0
None ......................... 5.23 5.59 8.0 7.5 5.25 5.47 5.0 4.5
1 Given in cc. of N/10 NaO-H per 100 cc. of juice.
1Given in cc. of N/10 NaOH per 100 cc. of juice. cc
TABLE 12.-PHOSPHORUS IN STALK JUICES AND STALKS OF 2 VARIETIES OF SUGARCANE PLANTED IN 1932 FOLLOWING
APPLICATIONS OF DIFFERENT CARRIERS OF PHOSPHATE BY BROADCASTING AND IN THE PLANTING FURROW.
Total Phosphorus in Stalk Juice in p.p.m. Percent Total Phosphorus of
Stalk in 1932
Treatment Variety Co 281 Variety CP 2714 Variety Co 281 Variety CP 2714
in Pounds I I 2
per Acre 1932 19331 1932 1933' Broad- Fur- Broad- Fur-
Broad- Fur- Broad- Fur- Broad- Fur- Broad- Fur- cast row cast row
cast row cast row cast row cast row I_
500 .......... 278 ...... 107 155 102 ........ ...
2,000 ............. 283 157 157 ...... 149 123 120 0.210 0.211 0.157 0.153
3,000 --..--....... 242 313 142 185 166 197 133 123 ....... ...... ........ .....
500 ....... 313 269 160 122 239 211 161 117 ..............
1,000 ............ 325 304 168 142 236 189 288 113 0.201 0.183 0.162 0.140
2,000 ............. 388 318 257 167 259 218 166 133 0.218 0.185 0.166 0.197
Super (44% P20)
250 .............. 307 314 189 202 223 256 231 180 ........ ...... ........ ........
500 ........... 346 315 249 215 240 294 191 214 0.227 0.205 0.183 0.157
1,000 .......... 446 507 330 231 354 337 318 298 ... ...........
2,000 .......... 594 529 424 280 419 417 328 344 0.323 0.354 0.235 0.249
None .---................... 241 223 129 108 181 159 131 104 0.159 0.175 0.135 0.144
1Potash was reapplied in 1933 but phosphates were not.
Availability of Phosphorus of Different Phosphates 25
for most of those in acidity. Phosphates were not reapplied
for the 1933 crop and while the concentrations were somewhat
less in most cases they varied in about the same manner with
respect to treatment.
Table 12 also gives the total phosphorus content of these cane
stalks as harvested and stripped for sugar extraction. The same
variations obtain as in the case of the phosphorus of the stalk
juices but within narrower limits. The data show that the
average phosphorus content of the stalks was 0.153 percent from
the plots to which no phosphate was added. This would be
about 0.05 percent phosphorus on the green stalk basis for which
the approximate annual crop removal is 40 tons containing 40
pounds of phosphorus. It is rather remarkable that the best
crops of sugarcane are being grown year after year on the same
fields of Everglades peat without the addition of phosphate
fertilizer. Sugarcane being a perennial has an extensive root
system to gather in the available phosphorus and probably ob-
tains some from the soil waters as well as from new zones of
peat by virtue of its annual subsidence of approximately an
inch a year.
SUMMARY AND CONCLUSIONS
Carpet grass was cut 4 to 6 times a year for 5 years from
plots fertilized yearly with a 6-12-12 mixture at the rate of
1,200 pounds per acre. The phosphorus for the different fertil-
izer treatments was supplied as superphosphate, rock phosphate,
basic slag and colloidal phosphate. These last 3 were used at the
rates of 4, 2 and 11/2 times the amounts of phosphorus (P,05)
specified in the 6-12-12 mixture. Lime and sulfur were used
in 2 of the treatments with superphosphate.
The 5-year average of air-dry grass was somewhat less than
6 tons per year for the superphosphate treatments and some-
what over 6 tons for the rock, basic slag and colloidal phosphate
treatments. Lime with the superphosphate depressed yields
while sulfur was without effect. Percentage of phosphorus in
the grass was slightly higher for the superphosphate treatments;'
lime with superphosphate depressed and sulfur with superphos-
phate increased it to some extent.
A comparable 5-year trial with Dallis grass resulted in about
the same effects of phosphates on yields, all of which were
higher than for carpet grass. Lime and sulfur were without
effect on both yields and phosphorus content of Dallis grass.
26 Florida Agricultural Experiment Station
The same sources of phosphates were studied in another series
of Dallis grass plots which were fertilized annually for 6 years
with a 6-6-12 mixture at the rate of 500 pounds per acre. On
some of these plots phosphates were omitted after the first year
for which the yields (6-year average) were highest for the super-
phosphate, somewhat lower for the rock, basic slag and colloidal
phosphate treatments and appreciably lower than the average
yield from plots that received superphosphate each year. Omis-
sion of phosphate in the fertilizer caused the phosphorus con-
tent of the grass to be much reduced for all treatments. In the
sixth year it was 0.125 percent for superphosphate, 0.114 per-
cent for rock phosphate, 0.118 percent for colloidal phosphate
and 0.118 percent for basic slag treatments, these percentages
being less than half those for the cuttings of the first year.
There was no drop in phosphorus content of the grass from plots
treated with superphosphate each year.
Considering growth and phosphorus content, best response was
to superphosphate for carrots, cabbage, potatoes, rape and tur-
nips. Sugarcane and some cuttings of shallu were seriously
reduced in yield in the presence of similar treatments with
superphosphate; corn responded but to a lesser extent than to
rock phosphate and basic slag.
The data of leaf and stem saps of sugarcane, buckwheat, corn,
rape and shallu are given with respect to pH, phosphorus and
acidity in an attempt to ascertain why these crops are of in-
different and in some cases negative response to added available
phosphate. Reaction (pH) was but little affected by any of the
phosphate treatments. Phosphates and acidities were increased
in a variable way for the rock phosphate treatments and to a
high and more uniform peak in the presence of superphosphate.
It appears that there is an upper limit of plant sap concentration
of phosphorus which, because of a direct or indirect reason, be-
comes detrimental to some plants such as sugarcane, corn, and
shallu. In other words, depression in growth of these crops may
be due to phosphates causing elements such copper, manga-
nese and zinc to become less assimilable or available even though
they are included in the soil treatment, as was the case in the
present instance. Other causative factors may be those of a
seasonal character. Thus the second cutting of shallu, growth
of which was retarded in the presence of soluble phosphates,
grew during the shorter day and cooler period of the year.
Availability of Phosphorus of Different Phosphates 27
This is the dry period also when the surface layers of peat are
The experiments show that phosphorus is readily taken up
by crops from superphosphate when added to Everglades peat
as well as from the peat itself, as shown above in the data on
sugarcane. The rather high amounts of water-soluble phos-
phate that were obtained from these superphosphate treatments
indicate that the phosphate remains in a highly available state
when added to this high calcium low moor peat. This concept
is in line with Doughty's (11) data on the lack of influence of
organic matter on phosphate fixation. It may also be an explana-
tion, as discussed above, of why growth of certain crops such as
sugarcane and shallu is retarded in some cases when this peat
is treated with soluble phosphates. Unless care is exercised
there is a possibility, therefore, of excess accumulation of highly
available phosphate in this and similar types of peat, particularly
in the culture of a crop such as celery that is heavily fertilized.
Dr. R. V. Allison established some of the earlier plots and the late
Dr. A. Daane supervised the growing of some of the crops used in these
experiments. Messrs. John Newhouse, L. S. Jones and P. M. McIntyre
assisted in the growing of the crops, in the collection of samples and in
the analytical work.
1. ALLISON, R. V. Soil fertility investigations. Annual Report Fla.
Agr. Exp. Sta. p. 92. 1929.
2. Soil fertility investigations. Annual Report Fla.
Agr. Exp. Sta. p. 126. 1930.
3. ALWAY, F. J., W. M. SHAW and W. J. METHLEY. Phosphoric acid
content of crops grown upon peat soils as an index of the fertilization
received or required. Jour. Agr. Res. 33: 701-40. 1926.
4. AMEs, J. W., and K. KITSUTA. Availability of rock phosphate as
indicated by phosphorus assimilation of plants. Jour. Am. Soc.
Agron. 24: 103-122. 1932.
5. BARTHOLOMEW, R. P. The availability of phosphatic fertilizers. Ark.
Agr. Exp. Sta. Bul. 289. 1933.
6. BUEHRER, T. F. The physico-chemical relationships of soil phosphates.
Ariz. Agr. Exp. Sta. Bul. 42. 1932.
7. BURD, JOHN S., and H. F. MURPHY. The use of chemical data in the
prognosis of phosphate deficiency in soils. Hilgardia 12: 323-340.
28 Florida Agricultural Experiment Station
8. CALDWELL, ROBERT EDWARD. A spectrographic study of certain Ever-*
glades soils with special reference to the growth of sugarcane.
Thesis. Library of U. of Fla. 1941.
9. CLAYTON, B. S., J. R. NELLER and R. V. ALLISON. Water control in
the peat and muck soils of the Flor' IEverglades. Fla. Agr. Exp.
Sta. Bul. 378. 1942.
10. DETURK, E. E. The problem of phosphate fertilizers. Ill. Agr. Exp.
Sta. Bul. 484. 1942.
11. DOUGHTY, J. L. Phosphate fixation in soils, particularly as influenced
by organic matter. Soil Sci. 40: 191-202. 1935.
12. HAMMAR, EDWIN. The chemical composition of Florida Everglades
peat soils, with special reference to their inorganic constituents.
Soil Sci. 28: 1. 1929.
13. NELLER, J. R. Phosphorus content and buffer capacity of plant sap
as related to the physiological effect of phosphorus fertilizers in
fibrous low-moor peat. Jour. Agr. Res. 51: 287-299. 1935.
14. NELLER, J. R., and A. DAANE. Yield and composition of Everglades
grass crops in relation to fertilizer treatment. Fla. Agr. Exp. Sta.
Bul. 338. 1929.
15. STEVENS, F. D. Agronomic studies with sugarcane. Annual Report
Fla. Agr. Exp. Sta. 183-7. 1933.
16. WAKSMAN, SELMAN A., and KENNETH R. STEVENS. Contribution to
the chemical composition of peat. III. Chemical studies of two
Florida peat profiles. Soil Sci. 27: 271-398. 1929.
17. WOLKOFF, M. I. Relative availability of the phosphorus of raw rock
and acid phosphate in soils. Soil Sci. 17: 39-56. 1924.