• TABLE OF CONTENTS
HIDE
 Title Page
 Table of Contents
 Introduction
 Phosphorus rate experiments--bearing...
 Phosphorus rate experiments--young...
 Residual effects of heavy rates...
 Phosphate fertilization and the...
 Soil tests for phosphorus
 Conclusions and recommendation...
 Reference
 Appendix






Title: Phosphorus fertilization of citrus
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 Material Information
Title: Phosphorus fertilization of citrus
Alternate Title: Bulletin 653 ; Florida Agricultural Experiment Station
Physical Description: Book
Language: English
Creator: Spencer, W. F.
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville, Fla.
Publication Date: March, 1963
Copyright Date: 1963
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Bibliographic ID: UF00027106
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
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Table of Contents
    Title Page
        Page 1
    Table of Contents
        Page 2
    Introduction
        Page 3
    Phosphorus rate experiments--bearing trees
        Page 4
        Page 5
        Page 6
    Phosphorus rate experiments--young trees
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
    Residual effects of heavy rates of phosphate and limestone
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
    Phosphate fertilization and the growth of citrus tree feeder roots
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
    Soil tests for phosphorus
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
    Conclusions and recommendations
        Page 38
    Reference
        Page 39
        Page 40
        Page 41
    Appendix
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
Full Text
BULLETIN 653 MARCH 1963


PHOSPHORUS FERTILIZATION

OF CITRUS

W. F. SPENCER







S Fertilizer i
As u, MUH 4tcir* )~




St ........... ti '-,-'F 0
% US AA........ ,g,
r /

UNIVERSITY OF FLORIDA t.,
AGRICULTURAL EXPERIMENT STATIONS
J. R. Beckenbach, Director, GAINESVILLE














TABLE OF CONTENTS
Page

INTRODUCTION ..... ............. ... ........................................... 3

PHOSPHORUS RATE EXPERIMENTS-BEARING TREES ..........--......... ............-. 4
Procedures .---...- --..............--- ---- ....--------- .--- .......-- ........ 4
Results -- ---.......----- -----........ .. ..-..-.--.----... --..... 5

PHOSPHORUS RATE EXPERIMENTS-YOUNG TREES --....-..--........-- ....-- .......... 7
Procedures .........-----------------....---....---...----..----.-- ..-- ........ 7
Results ................... ..... ..... .......---- .. ---..------.-----..............- .. 9

RESIDUAL EFFECTS OF IEAVY RATES OF PHOSPHATE AND LIMESTONE........ 14
Procedures ..............-- ... .. .. --------. --. -----.-- ------.. --.--... -........ 14
Results ---..... ----------..... ......... ...... -..-.. ....... ..----- ....-------- 14

PHOSPHATE FERTILIZATION AND THE GROWTH OF CITRUS TREE
FEEDER ROOTS -....-- .... ----------......-..--...- ---.. ..--- ..-- .-- ..---........- 19
Procedures .....--......- ...-- --..-.......-..-.-...-... ...... ..-- ....--..-----...- 20
Results ........- .... ..- ...-..- ..-....- ---...--....--------- --... ..- ---...- 23
Discussion ...........- .....-------- ...----- ..-..- ..--- ....-- --. ..-.. ..----...--- 30

SOIL TESTS FOR PHOSPHORUS -..-......---------.-..--...--..--..-- ..-- ........... 32
Procedures .-.....-..---..-...-....- ..-..... ---------- -- --- ------- ..--........ 32
Results ..--.......---- -.-------...-...---.. .... ...-- ..- ... .......----------- .. 33

CONCLUSIONS AND RECOMMENDATIONS ........--..-..-- ...--..-----.. --.-..-- .... 38

ACKNOWLEDGMENTS --..-.............-- ... ...--.----------.-. ....-..--..-...... - 39

LITERATURE CITED ...................................... -... .....-.....--.......--- 39

APPENDIX ----.........---- ..-..- ..........- -........ ........ ........- 42









PHOSPHORUS FERTILIZATION
OF CITRUS

W. F. SPENCER 1

INTRODUCTION
Phosphates have been added to citrus fertilizers in Florida
for many years. In only a few of the many experiments con-
ducted in practically all citrus-growing areas of the world has
field-grown citrus shown a positive response to phosphate appli-
cations (1, 2, 7, 12, 23, 33)2. In Florida favorable responses to
phosphate fertilization have been recorded in two experiments-
one conducted on a Davie mucky fine sand near Fort Lauderdale
(33) and the other with young trees on a highly leached Lake-
wood soil in Pasco County (23). Whether or not citrus will re-
spond to added phosphates depends mainly upon the amount of
phosphorus present in the soil that is available for plant growth
and development. Several investigators have shown that ferti-
lizer phosphorus accumulates in soils such as those used in Flor-
ida for citrus (5, 20, 22, 27). Therefore, a suitable soil test for
available phosphorus should indicate when further phosphate
additions are unnecessary.
There is evidence that excess phosphates may have unfavor-
able effects on citrus growth and development. Applications of
phosphates have resulted in changes in fruit quality, inhibited
root growth, and lessened cold hardiness. Lower soluble solids
content of juice was found to be a consequence of high phosphate
fertilization in the field by Young and Forsee (33) and Smith
et al. (25), and in solution culture experiments by Chapman and
Rayner (6). Smith and Reuther (26) found that moderate appli-
cations of superphosphate frequently retarded external color de-
velopment of Valencia oranges and caused them to regreen readi-
ly. Phosphates affect the uptake of various nutrients by citrus
trees (3, 30). Smith (22) first reported that root growth of
citrus trees was decreased by applications of phosphate materials.
Ford (9) found similar effects. Spencer (28, 30) reported that
heavy rates of triple superphosphate applied with and without
limestone to Ruby Red grapefruit trees markedly reduced the
concentration of feeder roots, especially in the surface foot of

SFormerly Associate Soil Chemist, Citrus Experiment Station, now Soil
Scientist, Southwest Branch, Soil and Water Conservation Research Divi-
sion, ARS, USDA, Riverside, California.
2 Numbers in parentheses refer to Literature Cited.








4 Florida Agricultural Experiment Stations

soil. Controlled greenhouse experiments by Spencer (29) dem-
onstrated that phosphorus itself is not toxic to citrus roots and
the depressing effect of superphosphates on root growth is due
to some factor in the phosphate material other than phosphorus.
This bulletin reports on recent research with phosphorus at
the Citrus Station and discusses recommendations, based on ex-
perimental results, for the use of phosphates in citrus fertilizers.
Several field experiments have been conducted to ascertain the
phosphorus needs of bearing trees and of young trees planted on
virgin Lakeland soil, and to relate possible responses to soil tests
for phosphorus. Controlled greenhouse and outdoor pot experi-
ments were also conducted to determine the mechanism of the
depressing effect of superphosphate materials on growth of citrus
tree feeder roots.

PHOSPHORUS RATE EXPERIMENTS-
BEARING TREES
Procedures
Phosphorus rate experiments were established in three bear-
ing Valencia groves on Lakeland fine sand which had not re-
ceived phosphate for approximately five years. Each experiment
consisted of four replications of paired plots receiving 0 or 120
pounds P205 per acre annually in the form of ordinary superphos-
phate (18 percent P205) applied with a fertilizer distributor.
Other nutrients were applied by grower cooperators.
Experiments 1 and 2 were located in 25- to 30-year-old groves
which had not received phosphate fertilizer, at least since 1953
(records not available prior to that time). Each grove received
approximately 200 pounds N and 125 pounds K20 per acre an-
nually and two one-ton applications of dolomite per acre since
the experiments were initiated in 1957. Experiment 3 was lo-
cated in a 30-year-old grove in which phosphorus had last been
applied in February 1952. This grove received approximately
180 pounds N and KzO per acre annually and dolomite as needed
for pH control. All groves received soluble magnesium in the
fertilizer.
Prior to the first phosphate application, soil samples were ob-
tained at various depths from each replication. They were an-
alyzed for "available phosphorus" by three methods and for
total phosphorus. These analyses are reported in the section
on soil testing. Spring flush leaves from non-fruiting twigs were
sampled during July or August, and fruit samples were obtained







Phosphorus Fertilization of Citrus 5

each year for fruit quality studies. Yields were measured when
the fruit was harvested. Soil samples were obtained from the
0- to 6-inch depth of all plots in June 1962, following five years
of differential phosphate treatment. Analyses of these samples
are also reported in the soil testing section.

Results
During the first three years, yields were not significantly
affected by phosphate applications (Table 1). This would in-
dicate that the soil phosphorus levels when sampled in 1957 were
adequate for production of Valencia oranges. In the fourth year
of Experiments 1 and 2, yields were significantly increased by
phosphate. However, the four-year average yields were not sig-
nificantly increased by phosphate applications at any location.
Fruit was picked late during the 1961-62 season (approximately
June 15), which could have been a factor in the yield difference.
Young and Forsee (33) found that dropping of mature fruit was
associated with a lack of available phosphorus, and the later the
date of picking the greater this drop. It should be emphasized
that the yield responses were obtained in the fourth year of the
experiments, but phosphate had not been applied for five years
prior to their initiation.

TABLE 1.-EFFECT OF PHOSPHATE ON YIELD OF VALENCIA ORANGES
IN THREE EXPERIMENTS.

Yield, Boxes/Tree/Year
Experiment it Experiment 2t Experiment 31
Season
0 120 lb. 0 120 lb. 0 120 lb.
P20O POs/A PsOs P 2s/A P2sO P5OJ/A

1958-59 3.47 3.41 6.14 6.33 -
1959-60 7.85 7.63 5.19 5.48 3.78 3.93
1960-61 3.42 3.41 4.46 4.32 2.97 3.09
1961-62 6.24 6.70* 6.11 6.82** 3.58 3.82
Average 5.25 5.29 5.48 5.74 3.44 3.61

Significantly different from the 0 PO0 yield at the 5 % level.
** Significantly different from the 0 P.O, yield at the 1% level.
SEach value is the average yield of 48 trees.
$ Each value is the average yield of 32 trees.

It is thought that these increases in yield due to superphos-
phate applications were due to phosphorus and not to calcium







6 Florida Agricultural Experiment Stations

or sulfur in the superphosphate. The leaf calcium level has been
adequate. Spring flush leaves sampled in August 1961 from the
0 percent P205 plots in Experiments 1, 2, and 3 contained 3.86,
3.40, and 3.47 percent calcium respectively. All groves received
sulfur either in the form of sulfur sprays or in fertilizer mate-
rials containing sulfur each year.
These experiments are being continued. If yields in subse-
quent years follow the same trend as in 1961-62, it would be an
indication that phosphate probably should be applied to bearing
trees at periodic intervals to replace that phosphorus removed
by the crop. This is in accordance with current recommenda-
tions for phosphorus fertilizer (19). Following this procedure
in the groves of Experiments 1 and 2 probably would have re-
sulted in adequate phosphorus levels for maximum production
every year.
The application of 120 pounds of PaO5 per year did not signifi-
cantly affect fruit quality or the phosphorus and calcium con-
tent of leaves at any location (Table 2). Evidently this nominal

TABLE 2.-EFFECT OF PHOSPHATE APPLIED TO VALENCIA ORANGE TREES ON
FRUIT QUALITY, AND LEAF P AND Ca CONTENTS (EXPERIMENTS 1, 2, 3).

P Experiment Number
P,0-
lb/A it 2t 31

Juice content, 0 45.7 46.9 48.1
% 120 45.6 46.8 47.7
Soluble solids, 0 11.93 12.11 11.31
" Brix 120 11.76 11.96 11.28
Acid, % 0 1.04 0.96 1.02
120 1.00 0.94 1.03
Ratio, 0 11.72 12.70 11.08
Brix/Acid 120 11.94 12.86 11.03
Peel thickness, 0 0.36 0.37 0.35
cm. 120 0.36 0.37 0.35
Fruit diameter, 0 2.84 2.90 2.94
inches 120 2.85 2.91 2.93
Leaf phosphorus, 0 0.115 0.128 0.124
"% P 120 0.120 0.131 0.127
Leaf calcium, 0 3.22 3.54 3.47
% Ca 120 3.36 3.68 3.68

t Mean of 4 years' data, 1958 to 1962.
t Mean of 3 years' data, 1959 to 1962.
Statistical analyses indicated that phosphate effects were not significant at any location







Phosphorus Fertilization of Citrus 7

rate of phosphate application was not sufficient to affect fruit
quality. The effect of very heavy rates of phosphorus on quality
of Ruby Red grapefruit is reported in the section on residual
effects of phosphate and limestone.

PHOSPHORUS RATE EXPERIMENTS-
YOUNG TREES
Procedures
Seven young tree experiments were initiated to study phos-
phate effects. Two of these are discussed herein. One (Experi-
ment 4) was designed to evaluate the effect of phosphorus on
tree top and feeder root growth at two levels of nitrogen and
to determine whether the gypsum in superphosphate is benefi-
cial to tree growth. The procedure involved the application of
four rates of phosphate, two rates of nitrogen, and two rates
of calcium to Hamlin orange trees on rough lemon rootstock
growing on previously unfertilized Lakeland fine sand at the
Citrus Experiment Station. The four phosphate rates were 0,
2 and 8 percent P205 in the fertilizer and 200 pounds P205 per
acre broadcast as ordinary superphosphate before the trees were
planted. The nitrogen rates were 4 percent and 8 percent nitro-
gen in the fertilizer. The two calcium variables were no calcium
and calcium as gypsum equal to that in a mixed fertilizer con-
taining 8 percent P205 as ordinary superphosphate. Plots that
received no calcium (2 and 8 percent P205) were fertilized with
phosphoric acid rather than ordinary superphosphate. The phos-
phoric acid was applied as a dilute aqueous solution at the same
time as the other fertilizers.
The experiment was initiated on July 1, 1958, when trees
were planted in the experimental area. The experimental design
was a randomized block with four replications and four-tree plots.
Fertilizer was applied at recommended rates (19). Tehtrees
received two l/2-pound fertilizer applications during 1958. In
subsequent years, each tree received the following yearly rates
of fertilizer applied in four applications annually in approxi-
mately March, April, July, and September: 21/ pounds in 1959,
5 pounds in 1960, and 12 pounds in 1961. Dolomite at the rate
of 1 ton per acre was applied prior to planting.
Trunk diameter six inches above the bud union was measured
each year with a caliper to evaluate treatment effects on tree
growth. Five- to six-month old spring flush leaves were obtained
from non-fruiting twigs for chemical analyses. Fruit yields were








8 Florida Agricultural Experiment Stations

taken in 1961 by counting individual fruits picked from each
tree. Soil samples were obtained during June of 1960 to evalu-
ate phosphorus movement. Feeder root concentration at the
drip of the branches was measured by the auger method (9)
during December 1961.

TABLE 3.-EFFECT OF PHOSPHATE, CALCIUM, AND NITROGEN RATES APPLIED
TO YOUNG HAMLIN ORANGE TREES ON TRUNK DIAMETER, FRUIT PRO-
DUCTION, AND LEAF COMPOSITION (EXPERIMENT 4).

Increase
in Trunk 1961
Diameter, Yield, Leaf Composition
Treatment t 5-29-59 to Fruit/ 2-Year Average, 1960-61
12-1-61 Tree
N P K Ca Mg

cm. % % % % %
PoN1 + Ca 5.0 18 2.94 .198 1.93 2.60 .51
PoN, + Ca 4.6 30 3.24 .171 1.79 2.50 .52
PoN2 Ca 4.6 37 3.11 .179 1.75 2.36 .68
Po Average 4.7 29 3.09 .183 1.82 2.49 .58

PiNi + Ca 4.7 14 2.95 .184 1.91 2.45 .55
P1N, + Ca 5.1 44 3.16 .192 1.69 2.60 .56
PiN2 Ca 5.2 35 3.12 .179 1.68 2.40 .65
P1 Average -.0 31 3.08 .185 1.76 2.48 .59

P2N1 + Ca 4.8 24 2.97 .211 1.80 2.62 .55
PzN2 + Ca 5.2 62 3.04 .207 1.60 2.80 .56
P2N2 Ca $ 4.9 43 3.33 .190 1.70 2.38 .67
P2 Average 5.0 43 3.11 .203 1.66 2.60 .60
PbN1 + Ca 5.0 21 2.98 .210 1.86 2.66 .48
PbN2 + Ca 5.2 45 3.09 .196 1.62 2.84 .48
PbN2 Ca 4.9 43 3.20 .192 1.67 2.42 .58
Pb Average 5.0 37 3.09 .200 1.72 2.64 .51
Statistical
significance
Po vs P N.S. N.S. ** N.S. N.S. N.S.
N1 vs N2 (+ Ca level) N.S. ** ** N.S. ** N.S.
- Ca vs + Ca (N2 level) N.S. N.S. N.S. N.S. N.S. ** **


-Treatments were as follows:
P rates: P, = No phosphate; P, = 2% P2,O in fertilizer; P, = 8% PO, in fertilizer;
Pb = 200 pounds P2O,/A as ordinary superphosphate (OSP) broadcast
before planting.
N rates: N1 = 4% N fertilizer; N2 = 8% N fertilizer.
Ca rates: Ca = No calcium in the fertilizer.
+ Ca = Calcium as gypsum equal to the calcium in fertilizer containing 8%
P,0 as OSP.
f Treatments received phosphoric acid as a source of phosphate.
Statistical significance.
Denotes statistically significant effect at the 5% level.
** IDenotes statistical significance at the 1% level.
N.S. Denotes no significant effect.







Phosphorus Fertilization of Citrus 9

Another phosphate experiment on young trees was conducted
in a commercial grove in Polk County (Experiment 5). Treat-
ments involved in this experiment are detailed in Table 7. The
experimental design was a randomized block with five replica-
tions and six-tree plots. Trunk diameter was measured at the
beginning of the experiment and annually to evaluate growth
effects. Spring flush leaves were sampled each year for chemi-
cal analyses.

Results
The effects of phosphate, calcium, and nitrogen rates on
growth, yield, and leaf composition in Experiment 4 are reported
in Tables 3 and 4. When considering data from both N rates,
growth of trees was significantly increased by the application
of phosphorus.

TABLE 4.-EFFECT OF INDIVIDUAL NUTRIENTS APPLIED WITH ADEQUATE
AMOUNTS OF OTHER NUTRIENTS ON GROWTH AND FRUIT PRODUCTION OF
YOUNG HAMLIN ORANGE TREES (EXPERIMENT 4).

Treatment Increase in
Trunk 1961
Nutrient varied Diameter Yield,
Rate of Nutrients 5-29-59 to Fruit/
not varied Element Rate 12-1-61 Tree
cm.
N2, + Ca Phosphorus No P 4.6 30
+ P 5.1* 50*
+ P, + Ca Nitrogen N1 4.8 20
N, 5.1* 50**
NM, + P Calcium Ca 5.0 40
+ Ca 5.1 50

Statistically significant increase due to the varied nutrient at the 5% level.
** Statistical significance at the 1% level.

Individual nutrient effects on growth and fruit production in
the presence of adequate amounts of other nutrients are shown
in Table 4. The application of phosphorus when adequate nitro-
gen and calcium were applied resulted in increased tree growth
and fruit production. The application of the higher nitrogen
rate with adequate phosphorus and calcium resulted in increased
tree growth and fruit yield. The higher rate of nitrogen was










0
TABLE 5.-EFFECT OF PHOSPHATE, CALCIUM, AND NITROGEN RATES APPLIED TO YOUNG HAMLIN ORANGE TREES ON FEEDER
ROOT CONCENTRATION AT VARIOUS DEPTHS (EXPERIMENT 4).

Root Concentration, gm./sq. ft.

0-6 6-12 12-18 18-24 24-36 36-48Totals
Treatment t in. in. in. in. in. in. 0-12 0-48

PoN1 + Ca ........................ 3.95 4.54 2.82 1.69 5.40 3.50 8.49 21.89
PoN2 + Ca .....---.................. 6.93 3.30 1.24 1.73 5.63 4.14 10.23 22.96
PoN2 Ca ........................ 8.59 4.98 2.59 2.83 5.92 5.50 13.57 30.40
Po All ................................ 6.49 4.27 2.22 2.08 5.65 4.38 10.76 25.09

PIN1 + Ca ....................... 7.38 3.65 0.84 1.66 7.00 8.14 11.03 28.67
P1N1 + Ca ........................ 6.18 4.43 2.01 1.83 7.06 4.76 10.61 25.52
P1N2 Ca ........................ 4.37 2.39 0.80 1.53 5.62 4.90 6.76 19.61
P1 All ................................ 5.98 3.49 1.22 1.67 6.56 5.93 9.47 24.60

P2N1 + Ca .....-.................. 8.38 2.65 1.37 2.43 4.74 3.92 11.02 23.47
P2N2 + Ca ...................... 5.24 4.37 1.11 1.93 5.62 4.29 9.60 22.55
P2N, Ca ....................... 5.04 3.58 2.52 2.94 7.06 5.16 8.62 26.30
P2 All ................................ 6.22 3.53 1.67 2.43 5.81 4.46 9.75 24.11
PbN1 + Ca -----------......... 5.66 3.54 2.35 3.63 7.14 4.06 9.20 26.38
PbN + Ca ...................----. 4.71 3.28 3.25 2.71 6.63 5.34 7.99 26.42
PbN2 Ca ....-...... .....------8.24 2.79 1.69 4.26 6.39 5.03 11.03 28.40
Pb All ............................------- ------ -- 6.20 3.20 2.43 3.53 6.72 4.81 9.40 27.07

Statistical
Significance ........-...- -


t See footnote Table 3 for rates of application.
$ Rates of phosphate, nitrogen or calcium did not significantly affect root concentration.








Phosphorus Fertilization of Citrus 11

approximately equal to that recommended in Bulletin 536A (19).
Calcium in the mixed fertilizer had no effect on either trunk
diameter or yield of fruit.
The phosphate applications increased the phosphorus level
in the leaves. The high nitrogen level resulted in increased
leaf nitrogen, decreased leaf potash, and increased leaf calcium.
The application of calcium in the fertilizer increased leaf cal-
cium and decreased leaf magnesium.
Concentration of feeder roots at various depths to 4 feet was
not affected by phosphate applications (Table 5). Even phos-
phoric acid at rates equivalent to 2 and 8 percent P205 did not
detrimentally affect root growth when applied to this previously
unfertilized soil.
Soil analyses for extractable phosphorus indicated that con-
siderable amounts of fertilizer phosphorus had moved into the
subsoil (Table 6). The soil pH ranged from 5.2 to 5.5 but was
not significantly affected by treatments. Analyses of soil sam-
ples obtained prior to fertilization and their significance are re-
ported in a subsequent section on soil testing.

TABLE 6.-DISTRIBUTION OF PHOSPHORUS IN SOIL UNDER YOUNG TREES
AFTER APPROXIMATELY TWO YEARS' FERTILIZATION WITH DIFFERENTIAL
PHOSPHATE RATES (EXPERIMENT 4).

P Extracted by Bray P1 Test, lb. P/A
0-6 6-12 12-18 0-18
Phosphate Rate in. in. in. in.

None .-.......- ---... ---- ....--.......... 56 56 56 168
2% P20s fertilizer ........--.......---- .... 108 80 55 243
8% P0Os fertilizer .........---................. 204 180 146 530
200 lb. P205/A broadcast ................ 75 64 49 188


The application of phosphate also increased tree growth and
leaf phosphorus content in Experiment 5 (Table 7). Results
with one application annually were comparable to four applica-
tions of phosphate.
Figure 1 shows the trunk diameter of young trees in Experi-
ments 4 and 5 at various dates as affected by phosphate applica-
tions. Differences in growth due to phosphorus mainly occurred
during the first two growing seasons, and growth during the
third year was approximately the same regardless of phosphate








12 Florida Agricultural Experiment Stations

treatment. This is the first reported favorable response to phos-
phorus applications of citrus trees growing on a typical Lakeland
fine sand. Phosphorus responses had previously been reported
on more highly leached soils (23, 33).

TABLE 7.-EFFECT OF PHOSPHATE APPLICATIONS ON GROWTH AND LEAF
COMPOSITION OF YOUNG PINEAPPLE ORANGE TREES ON ROUGH LEMON
RoOTsTOCKS. (EXPERIMENT 5-POLK COUNTY GROVE).

Increase in Trunk
Diameter, 3-59 to P in
Phosphate Rate 12-61 Leaves

cm. %
No POB ............. ..---......... .............. 2.8 0.131
2% P --- .................... -- ...........-....... 3.2 0.141
8% P .......-................-..-...-- .. ....... 3.5 0.154
8% P 05 ..............................----- ....... 3.3 0.150
2% P2zO 1 time/yr. ............-................ 3.3 0.143
8% P2Os, 1 time/yr. .................- ............ 3.2 0.150
Statistical significance:
No P vs P .........-- ...--........----- -. **

t Applied as ammoniated superphosphate in mixed fertilizer. Other treatments received
ordinary superphosphate in same fertilizer. All fertilizers applied 4 times annually.
$ Other fertilizer applied to these plots contained no phosphate.
Statistical symbols:
indicates significance at the 5% level.
"** indicates significance at the 1% level.

A third young tree experiment was conducted on Lakeland
fine sand in a Polk County commercial grove in which phosphate
was being applied as a supplement to the regular fertilizer ap-
plied by the grower. During the first year of the experiment,
the grower inadvertently applied activated sewage sludge as a
source of organic nitrogen. It contains approximately 2.5 per-
cent P205. No response in growth was obtained due to differ-
ential phosphate applications. Evidently the small amount of
phosphorus applied in the sludge (equivalent to a 1 percent P205
fertilizer) was sufficient for growth of the young trees.
Four additional experiments were initiated on flatwoods soils
to evaluate their phosphorus needs. Three of these were se-
verely damaged by freezes and had to be abandoned. The fourth
is being continued.







Phosphorus Fertilization of Citrus 13

All young tree phosphorus experiments, including those on
flatwoods soil, were examined following each period of freezing
temperature for differential freeze effects due to phosphate ap-
plications. No difference in freeze injury was noted due to phos-
phate applications in any of these seven experiments.



7-
P2 N2 +Ca

6 Po N2+Co
EXPERIMENT 4

5


e .83 % P2 05
4 -

z2 5
, I // T / 0% P2 05
< EXPERIMENT 5
P3










0o I I
SPRING SPRING SPRING DECEMBER
1959 1960 1961 1961
DATE

Fig. 1.-Trunk diameter of young trees at various dates as affected
by phosphate applications.







14 Florida Agricultural Experiment Stations


RESIDUAL EFFECTS OF HEAVY RATES OF
PHOSPHATE AND LIMESTONE

Procedures
A field experiment conducted with Ruby Red grapefruit
trees on rough lemon rootstock from 1951 to 1958 indicated that
heavy rates of triple superphosphate applied with or without
limestone markedly reduced the concentration of feeder roots,
especially in the surface foot of soil (28, 30). The heavy rates
of phosphate also decreased tree growth, increased the suscept-
ibility of the trees to cold injury, and affected nutrient uptake
(29). Differential treatments, outlined in Table 8 (Experiment
6), were discontinued after the spring 1958 application of phos-
phate and limestone.
Spring flush leaves from non-fruiting terminals were ob-
tained from the Ruby Red grapefruit trees each summer. Yield
of fruit by individual trees and internal fruit quality, peel thick-
ness, fruit size, and diameter ratios, were measured in fruit
samples obtained at random each year. In October 1961, three
and a half years after the last application of phosphate and
limestone, root concentration at various depths under the drip
of the branches of the Ruby Red grapefruit trees was measured
by the auger method (9).
To help evaluate residual treatment effects, Pineapple orange
trees on rough lemon and sour orange rootstocks were inter-
planted in the plots on May 12, 1959. The trunk diameters of
the interplanted trees were measured annually to evaluate resid-
ual effects on tree growth. A count was made of fruit produced
in 1961. These young trees were removed by pulling in March
1962 in order to provide more growing room for the grapefruit
trees. When the trees were pulled, their root systems were
examined and given a rating from 1 to 10 based on the concen-
tration of feeder roots in the surface foot of soil.

Results
A comparison of Ruby Red grapefruit quality, yield, and
leaf phosphorus contents from treatments which received
phosphate plus limestone with treatments which received lime-
stone only are presented in Table 8. Internal fruit quality was
affected by phosphate applications-the juice content, soluble
solids, and soluble solids to acid ratio were decreased by phos-









TABLE 8.-EFFECTS OF HEAVY RATES OF PHOSPHATE AND LIME ON QUALITY, YIELD, AND LEAF P CONTENT OF RUBY RED
GRAPEFRUIT (EXPERIMENT 6).t

Prior Treatment : Transverse
Juice Soluble Brix/ Peel Average to Longi- Leaf
P20O Limestone Content Solids Acid Acid Thickness Fruit tudinal Yield P
lb/A/yr. lb/A/yr. Diameter Diameter
Boxes/
% o Brix % Ratio cm. in. Ratio Tree %
305 520 46.6 8.21 1.11 7.35 0.56 4.01 1.150 5.24 0.142
1220 2080 45.2 7.78 1.13 6.80 0.54 3.92 1.152 4.46 0.154 C
4880 8320 44.4 7.69 1.13 6.76 0.53 3.94 1.175 4.75 0.163
Average (P + lime) 45.4 7.89 1.12 6.97 0.54 3.95 1.159 4.81 0.153
0 520 49.5 8.38 1.14 7.32 0.60 4.17 1.129 5.53 0.119
0 2080 47.7 8.25 1.13 7.20 0.60 3.96 1.118 5.51 0.114
0 8320 47.1 8.18 1.12 7.29 0.57 3.96 1.115 6.31 0.122
Average (lime only) 48.1 8.27 1.13 7.27 0.59 4.03 1.121 5.79 0.119
"F Values
(treatment means) N.S. ** N.S. **
"F Values
(P + lime vs lime only) ** ** N.S. ** ** N.S. ** ** **


S Each treatment mean is an average of three years' data obtained during 1959 to 1962 from three replications.
t Materials applied from 1951 to 1958. See Appendix Table 3 for extractable soil P and pH.
Statistical symbols:
indicates significance at the 5% level.
"** indicates significance at the 1% level.
N.S. indicates no significant difference.


CA













TABLE 9.-THE RESIDUAL EFFECT OF PHOSPHATE AND LIME APPLICATIONS TO RUBY RED GRAPEFRUIT TREES ON FEEDER ROOT
CONCENTRATION AT VARIOUS DEPTHS, THREE AND A HALF YEARS AFTER THE LAST APPLICATION OF THE MATERIALS (Ex-
PERIMENT 6).

Root Concentration, gm./sq. ft.
Prior Treatment Totals % of
total in
P2Os Limestone 0-6 6-12 12-18 18-24 24-36 36-48 0-12 0-48 surface .
lb/A/yr. lb/A/yr. in. in. in. in. in. in. in. in. 0-12 in.

0 0 1.18 0.83 3.03 4.44 9.07 6.21 2.01 24.76 8.1
305 520 3.45 2.82 1.97 5.78 8.09 4.25 6.27 26.36 23.8
1220 2080 3.22 0.80 0.37 2.54 5.89 4.96 4.02 17.78 22.6
4880 8320 2.06 0.48 0.88 1.89 3.76 2.41 2.54 11.47 22.1
Average P + lime 2.91 1.37 1.07 3.40 5.91 3.87 4.28 18.53 23.0
0 520 6.61 4.21 1.40 6.52 6.92 6.29 10.82 31.95 33.9
0 2080 10.60 5.62 2.53 3.34 7.73 3.86 16.22 33.67 48.2
0 8320 8.08 4.27 1.81 3.80 7.65 4.07 12.35 29.68 41.6
Average lime only 8.43 4.70 1.91 4.55 7.43 4.74 13.13 31.76 41.3
305 0 1.31 1.36 0.68 4.15 6.26 5.14 2.67 18.90 14.1
1220 0 0.18 0.08 0.96 2.87 5.79 3.92 0.26 13.79 1.9
4880 0 1.42 0.17 0.18 0.80 5.07 1.96 1.79 9.61 18.6
Average P only 0.97 0.54 0.61 2.61 5.71 3.67 1.57 14.10 11.1 co







Phosphorus Fertilization of Citrus 17

phate applications. Size of fruit was not affected, but peel thick-
ness was decreased and the diameter ratio was increased by
phosphate. The diameter ratio is the ratio of the transverse to
longitudinal diameter and is a measure of the flatness of the fruit.
Flatter fruit are somewhat more desirable for the fresh fruit
market. In this experiment, phosphorus increased the flatness
of the fruit. The yield of grapefruit was lower in plots which
had received phosphate. This was probably a reflection of great-
er freeze damage in the phosphated plots during the severe win-
ter of 1957-58 (28, 30). It is worthy of note that plots which
received 4 tons of limestone each year from 1951 to 1958 pro-
duced the most fruit. Leaf phosphorus content was consider-
ably increased by the phosphate application.
The concentration of grapefruit tree feeder roots in 1961
was still considerably lower in plots which received phosphate
than in plots which received limestone without phosphate (Table
9). The check plots which received neither phosphate nor lime-
stone had an extremely low pH and exhibited root patterns sim-
ilar to the plots that received phosphate without limestone.
The residual effects of phosphate and lime applications on
growth, yield, and quality of root systems of the young Pine-
apple orange trees are shown in Table 10. The young trees grew
best and produced the most fruit in plots which had received
the medium rate of phosphate plus limestone. The root system
ratings appeared to be related to soil pH. The best root systems
were in plots which had received the high rate of limestone re-
gardless of phosphate treatment. The root-rating results on the
young trees are not in agreement with root distribution measure-
ments made in the same plots by the auger method. This con-
trast will be discussed in the section on feeder root growth.
During the severe winter of 1957-58, trees which were re-
ceiving the high rates of phosphate suffered considerably more
freeze damage than trees not receiving phosphate (28, 30). It
was thought that this effect possibly was due to earlier initiation
of new growth following freeze damage in trees receiving phos-
phate than in trees not receiving phosphate. These trees then
would be more severely damaged by subsequent freezing temper-
atures. With this in mind, the young Pineapple orange trees
were examined during the spring of 1961 to determine whether
or not trees which had received phosphate initiated earlier
growth. On February 13, 1961, each tree was examined and
rated as follows: 0 no new growth; 1 new growth less than 1










TABLE 10.-RESIDUAL EFFECT OF PHOSPHATE AND LIME APPLICATIONS ON GROWTH, YIELD, AND QUALITY OF ROOT SYSTEMS 00
OF YOUNG PINEAPPLE ORANGE TREES ON ROUGH LEMON (R.L.) AND SOUR ORANGE (S.O.) ROOTSTOCKS (EXPERIMENT 6).

Increase in Trunk Quality of Root
Prior Treatment Diameter f Yield, 1961 System t
PO1s Limestone R.L. S.O. R.L. S.O. R.L. S.O. Soil pH
lb/A/yr. lb/A/yr. rootstock rootstock rootstock rootstock rootstock rootstock 0-6 in.

cm. cm. fruit/t fruit/t rating rating
0 0 1.0 0.2 1 0 2.7 3.9 4.6
305 520 2.5 1.3 43 7 5.7 5.8 5.1
1220 2080 3.4 2.2 70 26 8.0 7.6 5.8 ,
4880 8320 2.6 2.0 52 22 9.5 8.6 6.5
Average P + lime 2.9 1.8 55 19 7.8 7.3 5.8

0 520 2.0 1.1 7 5 4.6 5.5 5.5 ta
0 2080 2.6 1.3 25 6 6.7 7.9 6.5
0 8320 2.4 1.6 28 4 9.0 9.0 7.0
Average lime only 2.3 1.4 20 5 6.8 7.5 6.3
305 0 1.8 0.5 24 7 4.8 3.6 4.7
1220 0 2.2 0.8 32 4 4.9 4.5 4.7
4880 0 2.8 1.8 45 11 5.0 4.5 5.1
Average P only 2.3 1.0 35 8 4.9 4.2 4.8
LSD .05 treatment means 0.8 0.7 24 7 1.5 1.9 -
LSD .05 group means 0.4 0.4 14 4 0.9 1.1


t Increase in trunk diameter after three growing seasons-from June 9, 1959, to November 30, 1961.
SRating system from 1 to 10 based on concentration of feeder roots in surface foot of soil. 1 = essentially bare lateral and tap roots; 10 = very
high concentration of feeder roots, almost a solid mass of feeder roots around the tap and lateral roots. Ratings made when trees were removed from
the plots on March 30, 1962.








Phosphorus Fertilization of Citrus 19

inch in length; 2 new growth 1 to 2 inches in length; 3 new
growth greater than 2 inches in length. Trees which received
phosphate had an average rating of 1.2 compared to 0.7 for trees
which received lime only. This difference was statistically sig-
nificant and substantiates the contention that trees receiving
phosphate tend to break dormancy sooner than non-phosphated
trees. It seems logical to conclude that this was probably the
reason for greater freeze damage during 1957-58 to trees in this
experiment which received phosphate.


PHOSPHATE FERTILIZATION AND THE GROWTH
OF CITRUS TREE FEEDER ROOTS

Various workers have reported that applications of super-
phosphates to citrus trees depressed root growth (9, 22, 28, 30).
Hill and Beeson (11) reported that the principal constituents of
superphosphate solutions are phosphoric acid and water. Lind-
say and Stephenson (14, 15, 16, 17) recently reported on the na-
ture of the reactions of monocalcium phosphate in soils. They
found that highly concentrated solutions of extremely low pH
(as low as pH 1.8) occur in zones in the soil around particles of
phosphate materials containing monocalcium phosphate. These
reports suggest that the root injury of citrus trees may be in-

TABLE 11.-RESIDUAL EFFECT OF PHOSPHATE AND LIME APPLICATIONS ON
GROWTH OF ROUGH LEMON SEEDLINGS IN POTS (EXPERIMENT P-I).

Prior Treatment in Yield,
Field (Experiment 6) gm./pot
Amendment Soil pH
POs Limestone in Pot Feeder Total dry at
lb/A/yr. lb/A/yr. Experiment roots matter Harvest

0 2080 None 3.93 30.1 6.4
1220 0 None 3.06 15.6 4.3
1220 2080 None 4.35 49.3 6.1
0 8320 None 2.35 25.3 7.2
4880 0 None 3.26 25.6 4.5
4880 8320 None 5.22 52.1 6.7
4800 0 CaCOs, 2 T/A 4.64 44.6 5.7







20 Florida Agricultural Experiment Stations

directly caused by the acid-forming character of the monocal-
cium phosphate in superphosphates. The acid released when
monocalcium phosphate goes into solution could mobilize copper
in concentrations toxic to citrus roots.

Procedures
Three pot experiments were conducted to determine the mech-
anism of the depressing effect of superphosphates on growth of
citrus tree feeder roots.
Experiment P-I.-In Pot Experiment P-I soil from selected
plots in the Ruby Red grapefruit experiment which received
heavy rates of triple superphosphate and limestone was used to
grow rough lemon seedlings in order to determine whether the
depressing effect on growth of citrus roots still persisted in the
phosphated plots. The seedlings were grown in soils from the
0- to 6-inch depth for a period of approximately six months.
This soil contained approximately 120 ppm copper and 200 ppm
manganese. Treatments were replicated three times. Seedlings
were harvested by dividing into leaves, stems, tap roots, and
feeder roots. All plant parts were dried at 700C., weighed, and
ground for chemical analyses.
Experiment P-II.-Another experiment, designated herein as
Experiment P-II, was conducted outdoors with rough lemon
seedlings growing in 8-inch vitrified sewer tile. This experiment
was designed to determine whether the toxic factor in super-
phosphate was the acid character of the phosphate materials
in conjunction with minor element mobilization in the soil or
toxic constituents in the manufactured product. In general, it
involved the application of phosphate materials and other poten-
tially toxic constituents to seedlings periodically in order to dupli-
cate the field situation where young trees are fertilized fre-
quently. The entire experiment involved 67 treatments repli-
cated three times. Table 12 gives details of treatments which
appear pertinent to this report. In several of the treatments
identical materials were applied to a grove and a virgin soil.
The grove soil contained considerable quantities of manganese
and copper but had not received phosphate fertilizer. The virgin
soil was mixed with calcium carbonate at the rate of 1,000 pounds
per acre and a minor element formulation containing manganese,
zinc, copper, boron, and molybdenum to prevent deficiencies of
these elements.





TABLE 12.-EFFECT OF PHOSPHATES AND OTHER CHEMICALS ON GROWTH AND CHEMICAL COMPOSITION OF ROUGH LEMON SEEDLINGS AND SOIL
PH (EXPERIMENT P-II).
Yield, gm./pot Plant Composition
Soil pH at Harvest P, % Cu, ppm Mn, ppm
Treat- Total -
ment Feeder dry 6- 12- 18- Feeder Feeder Feeder
No. Soil Treatment roots matter 0-6 12 18 24 Leaves roots Leaves roots Leaves roots
1 Gt No phosphate 23 241 5.5 5.1 5.8 5.9 .137 .161 17 960 54 1870
18 V:j: No phosphate 25 207 5.6 5.2 5.6 5.7 .135 .050 10 55 17 630
7 G Ordinary superphosphate (OSP) 29 380 5.2 4.9 5.2 5.5 .182 .163 13 910 57 3100
24 V Ordinary superphosphate (OSP) 21 350 5.1 4.6 5.1 5.2 .182 .183 7 55 20 950
12 G Triple superphosphate (TSP) 20 307 5.3 4.9 5.3 5.6 .168 .171 14 800 38 4280
29 V Triple superphosphate (TSP) 26 339 5.3 4.8 5.3 5.4 .172 .173 5 50 25 1120
3 G Monocalcium phosphate (MCP) 24 322 5.4 5.1 5.5 5.5 .172 .197 11 910 55 2950
20 V Monocalcium phosphate (MCP) 31 308 5.6 5.3 5.5 5.6 .176 .203 6 55 17 920
4 G MCP + Gypsum 23 270 5.2 4.9 5.2 5.4 .183 .160 14 860 58 2300
21 V MCP + Gypsum 25 396 5.1 4.6 5.2 5.3 .170 .170 7 -- 16 1600
5 G Treat. 4 with 10% of P.O. as H.POt 20 275 5.2 5.0 5.1 5.4 .185 .168 13 870 62 3030
22 V Treat. 4 with 10% of P..OO as H.1PO 24 279 5.2 4.8 5.2 5.3 .198 .227 6 -- 28 1070
6 G Treat. 4 with 50% of P.On as HaPO, 16 307 5.2 4.6 5.2 5.5 .180 .150 12 970 44 2720
23 V Treat. 4 with 50% of P.O as HaPOi 29 299 4.9 4.7 5.2 5.3 .180 .243 8 32 1230
17 G MCP* + sulfuric acid 18 253 5.3 4.7 5.1 5.4 .192 .403 12 640 48 2530
34 V MCP* + sulfuric acid 22 284 5.2 4.3 5.2 5.3 .193 .527 7 43 30 2070
13 G Ammoniated superphosphate 9 140 5.2 4.4 4.5 4.8 .185 .127 17 870 126 4210
30 V Ammoniated superphosphate 23 340 5.1 4.2 4.2 4.3 .189 .140 8 70 54 2530
14 G Diammonium phosphate, 18-46-0 7 108 5.0 4.4 4.3 4.6 .210 .167 23 930 107 3800
31 V Diammonium phosphate, 18-46-0 23 299 5.5 4.6 4.3 4.3 .179 .243 9 80 45 3770
15 G Diammonium phosphate, c.p. 6 85 5.2 4.3 4.4 4.9 .223 .170 18 740 85 2700
32 V Diammonium phosphate, c.p. 35 358 5.6 4.8 4.4 4.6 .161 .203 6 70 49 2630
44 V MCP* + 200 lb. Cu*/A 24 284 5.5 5.3 5.5 5.6 .149 .360 8 1200 33 1670
45 V Treat. 44 + OSP 9 112 5.2 4.7 5.2 5.4 .329 .683 22 1370 70 1700
37 V MCP* + 400 Ib. Mn*/A 30 292 5.5 5.3 5.5 5.3 .237 .547 159 8730
38 V Treat. 37 + OSP 28 261 5.0 4.8 5.0 4.9 .257 .767 199 13,670
Statistical significance: LSD .05 11 85 .053 .100 7 145 49 2300
Coefficient of variation, % 27 18 18 18 25 15 43 40
t Grove soil obtained from a plot in Experiment 6 which had received limestone at 500 lb/A/yr. and fertilizer without phosphate from 1951 to 1958. It contained 113 ppm
P, 200 ppm Mn, and 120 ppm Cu, and had a pH of 5.7.
t Virgin soil obtained from an adjacent uncultivated area. It contained 85 ppm P, 12 ppm Mn, 7 ppm Cu. It had a pH of 5.1 when sampled and 5.9 after CaCO, additions.
SBased on analysis of variance of data from all 67 treatments in the experiment.
Indicates materials were mixed in the soil prior to placing in tile; other materials applied periodically to the soil surface.







22 Florida Agricultural Experiment Stations

All phosphate materials and other constituents were applied
at five- to six-week intervals at the rate of 175 pounds P per
acre of soil surface. The other constituents were applied at a
rate equivalent to 175 pounds phosphorus. For example, sul-
furic acid in Treatments 17 and 34 was applied at a rate equiva-
lent to one-half the amount of hydrogen in monocalcium phos-
phate applied at the rate of 175 pounds P per acre. This is
approximately equivalent to ionizable hydrogen in the monocal-
cium phosphate at the pH normally encountered in soils. Nine
applications were made during the course of the experiment. A
nutrient solution containing 0.01 M potassium nitrate, 0.0025 M
ammonium sulfate, and 0.002 M magnesium sulfate was applied
at weekly intervals.
Seedlings were grown for 11 months from November 22,
1960, to October 24, 1961. Leaf samples were obtained for chem-
ical analysis prior to harvesting of the above-ground portion of
the seedlings. Soil was sampled from the 0 to 6, 6 to 12, 12 to 24,
and 24 to 36 inch depth in each tile. Root systems were removed
from the tile. Any unusual characteristics were noted, and pic-
tures were taken of representative root systems from several
treatments which showed treatment differences. The root sys-
tems were divided into feeder roots and remaining below-ground
portion of the plants. All plant parts were dried at 700 C. and
weighed. Leaves and feeder roots were analyzed for phosphorus,
copper, and manganese.
Experiment P-III.-Another greenhouse experiment, herein
designated Experiment P-III, was designed to study the effects
of very high concentrations of phosphate materials on pH and
salt concentrations in the soil solution and the resultant effect
on rough lemon seedlings and emergence of radish plants. The
experiment was an attempt to differentiate between the effects
of salt and acid in the toxicity of very high rates of phosphate
materials reported by Rasmussen and Smith (18). For this
purpose, phosphates were applied at rates from 88 ppm phos-
phorus-equivalent to the medium rate of triple superphosphate
applied each application in Experiment 6-to 2,500 ppm phos-
phorus-the high rate of phosphorus used by Rasmussen and
Smith. Sulfuric acid was applied in one treatment to produce
approximately the same soil pH as 2,500 ppm phosphorus as
triple superphosphate. Soil samples were obtained for pH and
conductivity measurements prior to placing the mixture in 6-inch
clay tile pots. One rough lemon seedling and five radish seeds








Phosphorus Fertilization of Citrus 23

were planted in each pot. Treatments were replicated three
times.
Further treatment details are recorded in Table 13. In Treat-
ment 10, 2,500 ppm P as triple superphosphate was neutralized
to pH 6.0 with calcium hydroxide; whereas, in Treatment 11, it
was neutralized to pH 6.0 with sodium hydroxide. In Treatment
12, the soil was kept moist by surface additions of water. In
Treatment 13, water was added by submerging pots in a con-
tainer of water, which would prevent leaching downward of
the added phosphate. In both cases the soil was sampled and
seedlings planted eight days after mixing. In Treatment 14,
2,500 ppm phosphorus as triple superphosphate was mixed with
soil and deionized water in an Erlenmeyer flask. Conductivity
and pH were measured periodically to determine whether changes
occurred with time.
Copper Mobilization.-A laboratory experiment was conducted
in conjunction with the pot experiments to determine whether
phosphates applied to soils high in copper could solubilize the
copper. Various rates of triple superphosphate, ordinary super-
phosphate, and ammoniated superphosphate were added to a
virgin soil to which 20 ppm copper had been added and to a grove
soil containing 120 ppm copper. Fifty-gram samples of virgin
soil were added to 100 ml of a copper solution containing 1,000
micrograms of copper. After shaking several minutes, the phos-
phate compounds were added in appropriate amounts to each
flask. After occasional shaking for three days, the filtrates were
analyzed for copper by the carbamate procedure. The grove
soil containing 120 ppm copper from fertilizer additions was
treated in the same manner except no additional copper was
added. Very high rates of phosphate materials were used to
simulate the conditions around a particle of phosphate ferti-
lizer in the soil.
To further study copper mobilization, soil samples obtained
from various depths in Field Experiment 6 were analyzed for
copper to determine whether copper distribution was affected
by phosphate and lime applications.

Results
Experiment P-I.-The best growth of rough lemon seedlings
was obtained in soil from plots which had received the high rate
of phosphate and limestone in Field Experiment 6 (Table 11).







24 Florida Agricultural Experiment Stations

Seedlings grew second best in plots using soil obtained from the
medium rate of limestone and phosphate treatments. The poor-
est growth was obtained in soil from plots which had received
the medium and high rates of phosphate without limestone.
These latter soils were highly acid; and the seedlings were ob-
viously affected by copper toxicity, as symptoms of iron chlorosis
were apparent in the leaves and the plants were severely stunted.
When pH was increased with calcium carbonate, seedling growth
was greatly improved. The feeder root yields were very closely
related to the total dry matter production from the seedlings
regardless of treatment.
There did not appear to be any detrimental effects of phos-
phate on root growth in this experiment. Growth was retarded
at low pH levels probably because of toxic levels of copper, and
in the non-phosphated soils, by an apparent shortage of phos-
phorus. This experiment clearly demonstrated that the soil
which received heavy rates of phosphate in combination with
limestone in the field was no longer toxic to the growth of citrus
tree feeder roots.
Experiment P-II.-The effects of phosphate and other chem-
icals on growth and chemical composition of rough lemon seed-
lings and related soil pH of certain selected treatments are re-
ported in Table 12. The major differences in growth were caused
by the application of ammonia-containing phosphate compounds
to the grove soil high in copper and by copper applied preplant
in combination with periodic applications of ordinary superphos-
phate. Detrimental effects of two treatments, ammonia-contain-
ing phosphate compounds applied to soils high in copper and
superphosphate in combination with a high amount of copper
(Treatment 45), were apparent very early in the course of the
experiment.
The ammonia-containing phosphate compounds severely re-
tarded growth in the grove soil but not in the virgin soil. These
compounds resulted in lower soil pH levels, especially in the sub-
soil, than did phosphate compounds not containing ammonia.
A comparison of Treatments 44 and 45 indicated that 200 pounds
copper per acre mixed preplant resulted in normal growth of
seedlings; however, when ordinary superphosphate was applied
periodically in addition to the copper, seedlings were severely
stunted and chlorotic, indicating copper toxicity. These treat-
ments-13, 14, 15, and 45-illustrate the effects of acidifying
materials on mobilization of copper and consequent copper tox-







Phosphorus Fertilization of Citrus 25

city. This is probably the mechanism involved in root damage
in the field experiments following the application of high rates of
superphosphate.
In this experiment, roots were not noticeably damaged due to
periodic applications of non-ammoniated phosphate compounds
to the grove soil. Probably the copper was not sufficiently active,
and most of these phosphate compounds did not greatly affect
the soil pH at the relatively low rate of application. The only
exception was Treatment 6, in which 50 percent of the P2O was
added as phosphoric acid. In this case, feeder root growth was
restricted, especially in the surface foot of soil. Soil salinity
remained at relatively low levels in all treatments throughout
the experiment.
Figure 2 is a composite photograph of root systems showing
treatment differences. In pots which received high amounts of
acid or acid-forming fertilizers applied to a grove soil high in
copper, the surface soils were practically devoid of feeder roots.
It is apparent that similar results could have been obtained with
any other acidifying agent that sufficiently reduced the pH in
the presence of this amount of copper.
The manganese, copper, and phosphorus content of leaves and
roots varied widely with treatment (Table 12). Seedlings grow-
ing in the grove soil which received the ammonia-containing
phosphate compounds were very high in both manganese and
copper. A comparison of Treatments 44, which received copper,
with 45, which received copper plus superphosphate, indicates
that copper is extremely high in both cases, although there was
no root damage in the former treatment. This would indicate
that root analysis for copper probably is not a good guide to use
in ascertaining potential toxic levels of copper. Liebig et al.
(13) studied copper toxicity with orange and lemon cuttings and
reported very little difference in copper content between copper-
injured and healthy roots.
Manganese did not appear to be toxic to citrus roots and did
not depress the growth of citrus seedlings in any treatment. In
Treatment 38, the root manganese level was above 13,000 ppm
with no apparent detrimental effects.
Dry matter production and root growth were not affected by
the application of fluoride or arsenate compounds in this experi-
ment. Apparently these compounds, in concentrations present
in superphosphate materials, do not adversely affect citrus seed-
lings.







26 Florida Agricultural Experiment Stations
























3 3I







6 23 1/7 31 4, 5


Fig. 2.-Root systems from several treatments in Experiment P-II il-
lustrating differences between grove and virgin soils and the effect of OSP
in presence of high soil copper.
13 grove soil + ammoniated super
30 virgin + ammoniated super
14 grove + diammonium phosphate, 18-46-0
31 virgin + diammonium phosphate, 18-46-0
15 grove + diammonium phosphate, c.p.
32 virgin + diammonium phosphate, c.p.
6 grove + MCP + gypsum + HaP04 (50% P2Os)
23 virgin + MCP + gypsum + HPO (50% P05s)
17 grove + MCP + sulfuric acid
34 virgin + MCP + sulfuric acid
44 virgin + 200 lbs. copper
45 virgin + 200 Ibs. copper + OSP







Phosphorus Fertilization of Citrus 27

Experiment P-III.-The effects of high concentrations of
phosphate materials and sulfuric acid on the condition of rough
lemon seedlings, number of radish plants from 15 seeds per
treatment, and the related soil pH and salt concentrations are
shown in Table 13. The condition of the rough lemon seedlings
and the number of radish plants emerging from 15 seeds were
directly related to the concentration of salt in the soil-water
extracts. The addition of 2,500 ppm phosphorus as triple super-
phosphate, ammoniated superphosphate, or ordinary superphos-
phate reduced the soil pH but at the same time greatly increased
the salt concentration in the soil-water extracts. This amount
of phosphorus is equivalent to approximately 11,400 pounds P2,O
or 25,000 pounds triple superphosphate per acre.
The results indicate that acid itself is not toxic to citrus roots
when applied in concentrations which may be present in super-
phosphate. Sulfuric acid added in sufficient quantity to lower
the pH to 3.8 resulted in healthy citrus seedlings and 14 radish
plants from 15 seeds.
A comparison of Treatments 10 and 11, which involved the
neutralization of triple superphosphate with calcium hydroxide
and sodium hydroxide, indicates that it was the salt concentra-
tion of the soil solution which was responsible for the lethal
effects on citrus seedlings and radish plants and not free acid.
The neutralization of triple superphosphate with calcium hydrox-
ide not only increased the pH, but it greatly decreased the salt
concentration of the resultant soil-water extract because of the
precipitation of relatively insoluble calcium phosphate. On the
other hand, neutralization of the triple superphosphate with
sodium hydroxide resulted in the formation of considerable
quantities of soluble sodium phosphates. Therefore, the soil
pH was increased, while the salt concentration remained high.
This resulted in the death of the rough lemon seedlings and the
emergence of only 6 of 15 radish plants.
Watering the pots from the surface leached the salt down-
ward, with a resultant decrease in harmful effects; whereas,
watering the soil by submerging the pots in water resulted in
much less leaching of salts and greater damage to seedlings.
Treatment 14 indicated that soil pH and salt concentration did
not decrease with time when the soil solution was not removed.
The salt effects in this experiment are definitely not the
same effects noted in field experiments where superphosphates









TABLE 13.-EFFECT OF HIGH CONCENTRATIONS OF PHOSPHATE MATERIALS AND ACID ON SOIL PH AND SALT CONCENTRATIONS
AND THE RESULTANT EFFECTS ON ROUGH LEMON SEEDLINGS AND RADISH PLANTS (EXPERIMENT P-III).

Salt Concentration, Condition of No. of
Treatment pH 1:1 ppm in 1:1 Soil: Rough Lemon Radish Plants
No. Treatment' Soil:Water Water Extract t Seedlings t from 15 Seeds

1 Check 5.3 81 Healthy 15
2 TSP, 88 ppm P 5.4 153 Healthy 15
3 TSP, 500 ppm P 4.8 612 Healthy 15
4 ASP, 500 ppm P 4.9 3412 Leaf burn 6
5 OSP, 500 ppm P 4.5 910 Healthy 13 I
6 TSP, 2500 ppm P 3.6 3850 Dead 0
7 ASP, 2500 ppm P 4.6 9360 Dead 0
8 OSP, 2500 ppm P 3.8 3860 Dead 0
9 Sulfuric acid 3.8 394 Healthy 14
10 TSP, 2500 ppm P + Ca(OH)_ 5.9 450 Healthy 15
11 TSP, 2500 ppm P + NaOH 6.6 2363 Dead 6

0-2 in. 2-4 in. 0-2 in. 2-4 in.
depth depth depth depth

12 TSP, 2500 ppm P
top watered 4.9 4.2 43 1341 Leaf burn 12
13 TSP, 2500 ppm P
sub. watered 4.3 4.0 1197 1222 Dead 3
14 TSP, 2500 ppm P
in flask:
0 days 3.7 4433 -
8 days 3.8 4258
16 days 3.8 4140 -


t Based on conductivity measurements.
t Condition of seedlings approximately two months after transplanting into pots. Practically all "dead" seedlings died within one week.
Soil in pots was sampled at the 0-2 and 2-4 inch depth just prior to transplanting of seedlings into them-eight days following mixing.
Abbreviations: TSP-triple superphosphate, 46% P20; ASP-ammoniated superphosphate, 8-15-0; OSP-ordinary superphosphate, 18% P O0.








Phosphorus Fertilization of Citrus 29

reduced root growth of citrus trees. These high salt concentra-
tions are never present in field soils even following fairly heavy
applications of fertilizer.

Copper Mobilization.-The application of phosphate com-
pounds lowered the pH of both the virgin and grove soils and in-
creased the amount of copper in solution (Table 14). The phos-
phate was as effective in mobilizing copper freshly added to the
virgin soil at the rate of 20 ppm as it was in mobilizing the
copper present in the grove soil, even though the concentration
in the latter soil was approximately six times as great. Freshly
applied copper is evidently somewhat more active than that
which accumulates from fertilizer additions over a period of
years.

TABLE 14.-EFFECT OF TRIPLE SUPERPHOSPHATE (TSP), ORDINARY SUPER-
PHOSPHATE (OSP), AND AMMONIATED SUPERPHOSPHATE (ASP) ADDI-
TIONS ON SOIL PH AND SOLUBILIZATION OF COPPER.

Virgin Soil with Grove Soil Containing 120
20 ppm Cu Added ppm Cu from Fertilization
Phosphate Applied
Cu in Cu in
Source Ratet pH solution pH solution

ppm P ppm ppm
None None 5.0 0.05 6.0 0.26
TSP 88 5.0 0.34 5.6 0.25
TSP 352 4.7 0.58 5.2 0.34
TSP 1408 4.3 1.08 4.7 0.68

OSP 88 4.7 0.65 5.5 0.29
OSP 352 4.4 1.51 5.0 0.51
ASP 88 5.0 0.53 5.6 0.17
ASP 352 4.8 0.81 5.2 0.33

t High rates of P were used to simulate conditions in the soil near a phosphate fertilizer
particle; 88 ppm P equals 400 lb. P20,/A-6 inches.

Copper analyses of soil samples from Field Experiment 6 in-
dicate that less copper moved into the subsoil in plots receiving
annual applications of 2,000 pounds limestone per acre than in
plots receiving phosphate with or without limestone (Table 15).
These data suggest the possibility that copper mobilization has
been increased by phosphate.








30 Florida Agricultural Experiment Stations

TABLE 15.-TOTAL COPPER AT VARIOUS DEPTHS AS AFFECTED BY PHOSPHATE
AND LIME TREATMENTS (EXPERIMENT 6).

Prior Treatmentt Copper, lb/A-6 in.
PO2 Limestone 0-6 6-12 12-18 18-24 24-36 36-48
lb/A/yr. lb/A/yr. in. in. in. in. in. in.

0 0 111 25 9 11 9 8
0 2080 125 7 5 2 1 1
1200 2080 136 23 16 11 6 11
1200 0 122 20 9 8 1 8

t Treatment from 1951 to 1958. Copper applied in fertilizer during the same period.
Soil sampled December 1961.

Discussion
The results of the research on phosphate fertilization and the
growth of citrus feeder roots lead to the conclusion that the
reported detrimental effects of phosphate fertilization on root
growth in the field experiments were due to toxicity of copper
mobilized or made more toxic by the acid phosphates. The cop-
per could have been either that present in the soil or that applied
at the same time as the phosphate fertilizers. Monocalcium phos-
phate, the chief constituent of most phosphate materials, forms
phosphoric acid when dissolved in water. This could temporarily
result in sufficiently high levels of copper in the soil solution to
be highly toxic to citrus roots growing in soils containing appre-
ciable amounts of copper.
Results of Experiment P-II showed that copper mobilized by
ordinary superphosphate applications could damage citrus roots
and consequently reduce growth of citrus seedlings. Acidifica-
tion of the high copper grove soil by acid or by ammonia-contain-
ing phosphate compounds resulted in depressed root growth due
to toxicity of copper at the lowered soil pH. In the laboratory,
phosphate compounds, when applied at high rates to simulate
conditions around fertilizer particles, lowered soil pH and in-
creased the concentration of copper in solution. Liebig et al.
(13) found that 0.1 ppm copper was toxic to orange and lemon
cuttings growing in nutrient solutions free of aluminum. Ad-
ditions of very small amounts of aluminum reduced copper tox-
icity in their experiments. These facts point out the extremely
low concentrations of copper necessary for toxicity to citrus,
and suggest the possibility of an aluminum-copper inter-action,







Phosphorus Fertilization of Citrus 31

which may be a factor in Florida citrus groves since phosphate
has an effect on aluminum available.
Data from Experiment P-III showed that the lethal effects
of the very high rates of phosphate application reported by Ras-
mussen and Smith (18) were due to salt toxicity and not to free
acids in the superphosphate. Fertilizer-grade triple super or
ordinary superphosphate usually contain less than 5 percent
free acid. In Experiment P-II the application to a virgin soil
low in copper of phosphoric acid equivalent to 10 and 50 percent
of the P.Oa had no detrimental effect on root growth. However,
when phosphoric acid equivalent to 50 percent of the P205 was
applied to the grove soil containing copper, root growth was de-
creased in the surface foot of soil. The use of phosphoric acid
as the only source of phosphorus to young trees in Experiment
4 with no detrimental effect on root growth further substanti-
ates the fact that free acid itself applied to soils in moderate
amounts is not toxic to citrus roots.
Data on residual effects of phosphate and limestone applica-
tions in Experiment 6 indicated that the concentration of roots
in the surface foot of soil remained lower in the phosphate-
treated plots three and a half years after the last phosphate ap-
plication. However, young trees interplanted in all plots follow-
ing the last application of phosphate produced the best root sys-
tems in plots with high pH levels regardless of past phosphate
treatment. Pot Experiment P-I also showed that soils which
received heavy rates of phosphate in combination with limestone
in Field Experiment 6 were no longer toxic to citrus feeder roots.
This would indicate that the soil toxicity factor is no longer
present in the phosphated plots. The fact that the concentra-
tion of roots is still lower in the Ruby Red grapefruit trees which
received the high rates of phosphate can be accounted for by
injury of the root system during the period of phosphate and cop-
per application. In other words, the present root pattern is a
reflection of the root system established during 1951-58, when
high rates of superphosphate and copper were being applied.
The same mechanism-mobilization and toxicity of copper-
was probably responsible for the reported detrimental effects
of high rates of nitrogen on root development (10) and tree
condition and yields (32). Acidity resulting from nitrogen fer-
tilizers would increase copper in the soil solution in the same
manner as acid phosphates. The results of Experiment P-II,
in which ammonia-containing phosphate compounds greatly re-







32 Florida Agricultural Experiment Stations

duced seedling growth in the presence of high amounts of cop-
per, would suggest that causes of tree deterioration at high
ammonium nitrate rates as reported by Stewart et al. (32)
was due mainly to copper toxicity induced by the low pH as-
sociated with the high rates of ammonium nitrate. Since high
rates of ammonia-containing phosphate compounds applied to
a virgin soil in the pot experiment resulted in healthy seedlings,
it seems logical to conclude that this same high rate of ammonium
nitrate applied to a soil low in copper probably would not cause
tree deterioration.

SOIL TESTS FOR PHOSPHORUS
Several studies (5, 20, 22, 27) have shown that applied phos-
phates accumulate in sandy soils in an available form. Since
the average crop of fruit removes only approximately 20 pounds
of P205 per acre, it is possible to accumulate large reserves of
phosphorus. Soil test correlation work has been aimed at estab-
lishing a level for soil phosphorus above which no response would
be expected from further phosphate additions.

Procedures
Soil in each phosphorus rate experiment was sampled prior
to its initiation. The soil samples were analyzed for "available"
phosphorus by three methods of extraction and for total phos-
phorus by acid digestion of the soil. A large number of soil sam-
ples from other experimental sites, including plots which had
received 0 or 6 percent P205 fertilizer for 15 years, were analyzed
for extractable phosphorus. The tests for available phosphorus
used were two related methods published by Bray and Kurtz
(4), usually referred to as the Bray P1 test and Bray P2 test,
and ammonium acetate acidified with acetic acid to a pH of 4.8
as used by the Florida Soil Testing Laboratory.3 The Bray P1
test utilizes 0.03 N NH4F and 0.025 N HCI as the extracting so-
lution, while the Bray P2 test utilizes 0.03 N NH4F and 0.1
N HC1. The Arnold and Kurtz 4 modification of the Bray P1 and
P2 tests was used with a soil to extractant ratio of 1 to 10, a
shaking time of two minutes, and amino-naphthol sulfonic acid
as the reducing reagent.
SH. L. Breland, Methods of analysis used in soil testing, Fla. Agr. Exp.
Sta. Dept. of Soils, Mimeo Rept. No. 58-1, July 1957.
'C. Y. Arnold and L. T. Kurtz, Photometer method for determining
available phosphorus in soils, Ill. Agric. Exp. Sta. Dept. of Agronomy,
Mimeo AG 1306, 1946.







Phosphorus Fertilization of Citrus 33

In the ammonium acetate method, a soil to extractant ratio
of 1 to 5 was used, and samples were shaken for 30 minutes.
Total phosphorus as described by Bray and Kurtz (4) was de-
termined in nitric-perchloric acid digests of soil samples. De-
tailed procedures for the extractable phosphorus methods are
described in the appendix.

Results
The relationship between soil phosphorus as determined by
four chemical methods and responses to phosphate fertilization
in five field experiments is shown in Table 16. The amount of
phosphorus by the four different methods varied widely. The
acid ammonium acetate extracted much less phosphorus than
the extractants containing NHYF. However, this is not of very
great importance. By all methods phosphorus was lowest in soil
from the young tree experiments which were conducted on pre-
viously unfertilized soil. Experiments 1, 2 and 3 in bearing
Valencia groves indicated no response to added phosphate until
the fourth year of the experiment. It can be concluded, there-
fore, that the soil was sufficiently supplied with phosphorus to
meet the phosphorus requirement of the trees at the time the
experiments were initiated.
The soil test values for phosphorus indicate the level of avail-
able phosphorus above which a response to phosphate fertiliza-
tion would not be expected in any grove. These data are not
sufficient to indicate which soil test most accurately reflects the
amount of phosphorus available to the trees. However, it can be
said with a reasonable degree of assurance that phosphorus test
values above the levels reported in Table 16 for Experiments
1, 2 and 3 would be adequate for optimum production of citrus.
The data indicate that soil test values above 22 pounds phos-
phorus per acre (50 pounds PO25) by acid ammonium acetate,
80 pounds phosphorus (185 pounds P205) by the Bray P1 test,
and 130 pounds phosphorus (300 pounds P5Oj) per acre by the
Bray P2 test would be adequate levels, and soils containing these
amounts would not be expected to respond to phosphate addi-
tions. Soil test values below these levels would not necessarily
indicate that a response will always be obtained from phosphate
fertilizers applied to such soils, and these critical levels may
result in a few groves' being unnecessarily fertilized with phos-
phate. However, this is more desirable than risking a deficiency
of phosphorus. When making fertilizer recommendations for
















""'"'
bll:" it

"' "' "
`" 'L
'"' ""i







Phosphorus Fertilization of Citrus 35

phosphorus on the basis of a soil test, it is necessary to include
a large factor of safety in interpretation of the test (30). This
follows from the fact that the cost of fertilizer saved is only a
small part of the cost of production, and the potential loss in
income is great relative to the small cost of the phosphate ferti-
lizer. With a relatively high income crop such as citrus, fertili-
zation should be at a sufficiently high level that no element is
significantly deficient.
Total phosphorus, undoubtedly, would be related to the
amount of phosphorus accumulated in the soil, and its use as a
soil test method would probably be sufficiently accurate for some
conditions. However, minimum levels of total phosphorus sug-
gested from the data in Table 16 would probably apply only on
sandy soils similar to Lakeland fine sand on which the experi-
ments were conducted. This follows from the fact that phos-
phorus availability is controlled by many factors other than the
amount of total phosphorus in the soil. In contrast, the "avail-
able" phosphorus levels as determined by the extraction pro-
cedures would probably be applicable to all soils planted to citrus
in Florida. The amounts of soil phosphorus in samples from
lower depths in these experiments are reported in Appendix
Table 1.

TABLE 17.-EXTRACTABLE PHOSPHORUS AND CALCIUM AND PH OF SOIL SAM-
PLES OBTAINED FROM THE 0-6 INCH DEPTH OF THE BEARING GROVE EX-
PERIMENTS IN JUNE 1962.

Experiment 1 Experiment 2 Experiment3
0 120 0 120 0 120
P20O P20s POr, PO,5 P2O- P2,0

Extractable P, lb. P/A
Ammonium acetate, pH 4.8 25 30 23 40 31 28
Bray P2 test 68 111 82 172 148 194
Bray P2 test 128 176 139 245 310 306
Extractable Ca, lb. Ca/A
Ammonium acetate, pH 4.8 890 960 730 700 790 770
Ammonium acetate, pH 7.0 590 550 480 500 520 410
Soil pH 6.9 6.8 6.4 6.2 6.6 6.2


Analyses of soil samples obtained from the 0 to 6-inch depth
in Experiments 1, 2 and 3 during June 1962 are reported in
Table 17. The amounts of extractable phosphorus in the no
phosphate plots had not decreased appreciably since they were
first sampled in 1957, with the exception of phosphorus extracted







36 Florida Agricultural Experiment Stations

with the Bray P1 test in Experiments 1 and 2, where a phosphate
response was observed during the 1961-62 season. The applica-
tion of 120 pounds P205 per acre annually for five years increased
the amounts of extractable phosphorus in most cases. The
amounts of extractable calcium and soil pH were not affected
by phosphate applications.
Other information was obtained which may be helpful in
selecting the most useful soil test method for phosphorus in
Florida citrus groves. The relationship between the phosphorus
content of leaves and roots of Ruby Red grapefruit trees in Ex-
periment 6 and the amount of soil phosphorus extracted by the
three methods is reported in Table 18. The amount of phos-
phorus extracted by the Bray P1 test was more highly correlated
with both phosphorus in leaves and in roots than was the phos-
phorus extracted by the other two methods. Amounts of ex-
tractable phosphorus in soil samples from some of these plots
are shown in Appendix Table 2.

TABLE 18.-RELATIONSHIP BETWEEN THE PHOSPHORUS CONTENT OF LEAVES
AND ROOTS AND THE AMOUNT OF SOIL PHOSPHORUS AT THE 0-6 INCH
DEPTH EXTRACTED BY THREE SOIL TEST METHODS (EXPERIMENT 6,
n = 30).
Correlation
Coefficient,
Variables, x vs y r Regression Equation

Bray Pi test vs P in leaves 0.733** y=0.116 + 0.000093X
Bray P2 test vs P in leaves 0.559** y-0.138 + 0.000028X
Ammonium acetate, pH 4.8 vs P in leaves 0.357* y=0.152 + 0.00013X
Bray P1 test vs P in roots 0.920** y=0.066 + 0.00035X
Bray PI test vs P in roots 0.808** y=0.136 + 0.00012X
Ammonium acetate, pH 4.8 vs P in roots 0.667** y=0.173 + 0.00075X

** Correlation significant at the 1% level.
Correlation significant at the 5% level.

Analyses of soil from plots which had received 0 and 6 per-
cent P205 fertilizer for 15 years, in a field experiment herein
designated as Experiment 7, indicated that methods utilizing
NH4F extracting solutions better indicated the amount of phos-
phorus which had been applied than did the acid ammonium ace-
tate extractant (Table 19). With the Bray P1 and P2 tests there
was a sharp distinction between plots which had received no phos-
phate and those which had received a 6 percent P205 fertilizer.
With acid ammonium acetate there was no sharp distinction be-
tween plots which had received phosphorus and those which had








Phosphorus Fertilization of Citrus 37

not. The amounts of extractable phosphorus in soil samples
from lower depths in these plots are recorded in Appendix
Table 3.

TABLE 19.--PHOSPHORU EXTRACTED BY THREE METHODS FROM SOIL OB-
TAINED AT THE 0-6 INCH DEPTH FROM PLOTS WHICH RECEIVED 0 AND 6
PERCENT P205 FERTILIZER FOR 15 YEARS (EXPERIMENT 7).t

Phosphorus Extracted, lb. P/A
% P20s in Fertilizer NHiAc pH 4.8 Bray P1 test Bray P2 test

0 36 80 114
0 15 117 165
0 19 92 145
Average No P2Os 23 96 141
6 43 205 274
6 30 286 396
6 28 216 268
Average 6% P,05 34 236 313

SSee Appendix Table 3 for P extracted from samples obtained at other depths.

Since no marked response was obtained to phosphorus appli-
cations in the bearing grove experiments, it is not possible to
determine, with assurance, which soil test method most accu-
rately predicts the amount of phosphorus available to citrus
trees. Spencer (27) reported that phosphorus accumulated in
sandy citrus soils was available to citrus trees; therefore, any
soil test method for available phosphorus which reflects the
amount of phosphorus applied should be a more suitable soil
test method for available phosphorus than one which does not
do so. Robertson (21) found that the Bray P2 test was most
satisfactory of four methods tested in determining the phos-
phorus status of Red Bay soils. A comprehensive study of soil
test methods on a wide variety of soils and crops by the Soil
Test Work Group of the National Soil Research Committee (8)
indicated that the Bray P1 test was better correlated with the
amount of soil phosphorus available to crops than the other
methods under study. The fact that the Bray methods, uti-
lizing ammonium fluoride, better indicate the amount of phos-
phorus applied to Florida citrus groves and have resulted in
better correlations with response to phosphorus fertilizer (8,
21) would justify consideration of its use on soils from Florida
citrus groves. Further studies of phosphorus test methods for
use on all Florida crops, including citrus, appear to be needed.








38 Florida Agricultural Experiment Stations


CONCLUSIONS AND RECOMMENDATIONS
Phosphorus rate experiments in bearing Valencia orange
groves indicated no response to applied phosphorus during the
first three years. Soil tests on samples from these groves indi-
cate that soil test values above 22, 80, and 130 pounds phosphorus
per acre by the acid ammonium acetate, the Bray P1 and Bray P2
tests, respectively, would probably be adequate amounts of phos-
phorus for growth of citrus. Soils which test higher than these
levels by either of the soil test methods probably do not need
immediate phosphate additions. Soils which test less than these
levels should receive some phosphate in the fertilizer for optimum
citrus production.
Phosphate applications improved growth of young trees grow-
ing on previously unfertilized Lakeland fine sand. No detri-
mental effects on either growth or root concentration occurred
as a result of normal rates of phosphorus applied to the young
trees. Phosphorus, when used at recommended rates, had no
effect on the amount of freeze injury to young trees.
Studies indicated that very heavy rates of phosphorus will
unfavorably affect quality of grapefruit. However, when phos-
phorus was applied at the more normal rate of application of 120
pounds P205 per acre annually in the Valencia experiments, no
detrimental effects of phosphorus on fruit quality were noted.
The growth of rough lemon seedlings in pots and Pineapple
orange trees in the field demonstrated that soil which received
heavy rates of phosphate in combination with limestone in Ex-
periment 6 was no longer toxic to the growth of citrus feeder
roots. Best growth of both seedlings and budded trees occurred
in soil from plots which had received either the high or medium
rate of phosphate in combination with limestone.
The results of the research on phosphate fertilization and
the growth of citrus feeder roots lead to the conclusion that the
reported detrimental effects of phosphorus fertilization on root
growth in field experiments were due to toxicity of copper mo-
bilized or made more toxic by the acid phosphates. The same
mechanism-mobilization and toxicity of copper-was probably
responsible for reported detrimental effects of high rates of ni-
trogen on root development (10) and tree condition and yields
(32).
Copper mobilization and consequent toxicity is not a very im-
portant factor in young trees planted on previously unfertilized







Phosphorus Fertilization of Citrus 39

soils low in copper where phosphates are added in relatively small
amounts. The research reported herein indicates that no detri-
mental effects of phosphates can be expected when they are
applied in the normal manner and at the usual rates of applica-
tion to young trees on previously unfertilized soil. The fact
that responses in growth were obtained to phosphate fertilizers
indicates that phosphorus should be included in fertilizers applied
to young trees. The fact that no response was obtained to phos-
phorus applications on bearing Valencia trees during the first
three years the experiments were conducted further substanti-
ates the earlier finding that phosphorus accumulates in an avail-
able form in these soils and that it is possible to build up the
phosphorus level so that continuous phosphorus applications are
unnecessary.

ACKNOWLEDGMENTS
Appreciation is expressed to the Minute Maid Corporation, Hunt Broth-
ers Cooperative, Eloise Groves Association, and Florence Citrus Growers
Association for permitting the use of their groves for field experiments and
for the helpfulness of their representatives.
Special acknowledgment is due to Louis Muraro and James Kelley for
their help in carrying out the research program described herein.
The work on phosphate fertilization and the growth of citrus tree
feeder roots was supported, in part, by grants in aid from the following
companies:
American Agricultural Chemical Company
American Cyanamid Company
International Minerals and Chemical Corporation
Swift and Company
Tennessee Corporation, U. S. Phosphoric Products Division
Virginia-Carolina Chemical Corporation.


LITERATURE CITED

1. Aldrich, D. G., and J. J. Coony. A field response of citrus to phos-
phorus and potassium fertilization. Amer. Soc. Hort. Sci. Proc. 59:
13-21. 1952.
2. Allwright, W. J. Final report on the fertilizer trials at Rustenburg,
western Transvaal. Citrus Grower 52: 5-19. 1938.
3. Bingham, F. T., and J. P. Martin. Effects of soil phosphorus on growth
and minor element nutrition of citrus. Soil Sci. Soc. Amer. Proc. 20:
382-385. 1956.
4. Bray, R. H., and L. T. Kurtz. Determination of total, organic, and
available forms of phosphorus in soils. Soil Sci. 59: 39-45. 1945.








40 Florida Agricultural Experiment Stations

5. Bryan, O. C. The accumulation and availability of phosphorus in old
citrus grove soils. Soil Sci. 36: 245-259. 1933.
6. Chapman, H. D., and D. S. Rayner. Effect of various maintained levels
of phosphate on the growth, yield, composition, and quality of Wash-
ington navel oranges. Hilgardia 20: 325-357. 1951.
7. Embleton, T. W., J. D. Kirkpatrick, and E. R. Parker. Visible response
of phosphorus-deficient orange trees to phosphatic fertilizers, and sea-
sonal changes in mineral constituents of leaves. Amer. Soc. Hort. Sci.
Proc. 60: 55-64. 1952.
8. Fitts, J. W., J. J. Hanway, L. T. Kardos, W. T. McGeorge, L. A. Dean,
and J. F. Reed. Soil tests compared with field, greenhouse and labora-
tory results. North Carolina Agr. Exp. Sta. Tech. Bul. 121. 1956.
9. Ford, H. W. Root distribution of citrus trees. Fla. Agr. Exp. Sta.
Ann. Rept., p. 207. 1957.
10. Ford, H. W., W. Reuther, and P. F. Smith. Effect of nitrogen on root
development of Valencia orange trees. Amer. Soc. Hort. Sci. Proc.
70: 237-244. 1957.
11. Hill, W. L., and K. C. Beeson. Composition and properties of super-
phosphate. II. Free acid in superphosphate. J. Assoc. Official Agr.
Chem. 18: 244-250. 1935.
12. Innes, R. F. Fertilizer experiments on grapefruit in Jamaica. Trop.
Agr. Trinidad 23: 131-133. 1946.
13. Liebig, G. F., Jr., A. P. Vanselow, and H. D. Chapman. Effects of
aluminum on copper toxicity, as revealed by solution-culture and spec-
trographic studies of citrus. Soil Sci. 53: 341-351. 1942.
14. Lindsay, W. L., and H. F. Stephenson. Nature of the reactions of
monocalcium phosphate monohydrate in soils. I. The solution that
reacts with the soil. Soil Sci. Soc. Amer. Proc. 23: 12-18. 1959.
15. Lindsay, W. L., and H. F. Stephenson. Nature of the reactions of
monocalcium phosphate monohydrate in soils. II. Dissolution and pre-
cipitation reactions involving iron, aluminum, manganese, and calcium.
Soil Sci. Soc. Amer. Proc. 23: 18-22. 1959.
16. Lindsay, W. L., J. R. Lehr, and H. F. Stephenson. Nature of the re-
actions of monocalcium phosphate in soils. III. Studies with metastable
triple-point solution. Soil Sci. Soc. Amer. Proc. 23: 342-345. 1959.
17. Lindsay, W. L., and H. F. Stephenson. Nature of the reactions of
monocalcium phosphate in soils. IV. Repeated reactions with metast-
able triple-point solution. Soil Sci. Soc. Amer. Proc. 23:440-445.
1959.
18. Rasmussen, G. K., and P. F. Smith. Pot studies on the effects of super-
phosphates on the growth of citrus seedlings. Proc. Fla. State Hort.
Soc. 72: 71-74. 1959.
19. Reitz, H. J., C. D. Leonard, I. Stewart, W. F. Spencer, R. C. J. Koo,
E. J. Deszyck, P. F. Smith, and G. K. Rasmussen. Recommended fer-
tilizers and nutritional sprays for citrus. Fla. Agr. Exp. Sta. Bul.
536A. 1959.








Phosphorus Fertilization of Citrus 41

20. Reuther, W. F., F. E. Gardner, P. F. Smith, and W. R. Roy. Phosphate
fertilizer trials with oranges in Florida. I. Effects on yield, growth,
and leaf and soil composition. Amer. Soc. Hort. Sci. Proc. 53: 71-84.
1949.
21. Robertson, W. K. Soil management investigations. Fla. Agr. Exp.
Sta. Ann. Rept., p. 148. 1953.
22. Smith, P. F. Effect of phosphate fertilization on root growth, soil pH,
and chemical constituents at different depths in an acid sandy Florida
soil. Proc. Fla. State Hort. Soc. 69: 25-29. 1956.
23. Smith, P. F., and G. K. Rasmussen. Relation of fertilization to win-
ter injury of citrus trees. Proc. Fla. State Hort. Soc. 71: 170-175.
1958.
24. Smith, P. F., and G. K. Rasmussen. Effect of nitrogen source, rate and
pH on the production and quality of Marsh grapefruit. Proc. Fla. State
Hort. Soc. 74: 32-37. 1961.
25. Smith, P. F., W. Reuther, and F. E. Gardner. Phosphate fertilizer trials
with oranges in Florida. II. Effects on some fruit qualities. Amer.
Soc. Hort. Sci. Proc. 53: 85-90. 1949.
26. Smith, P. F., and W. Reuther. Citrus nutrition: Mineral nutrition of
fruit crops. Horticultural Publications, Rutgers University, New
Brunswick, New Jersey. 1954.
27. Spencer, W. F. Distribution and availability of phosphates added to a
Lakeland fine sand. Soil Sci. Soc. Amer. Proc. 21: 141-144. 1957.
28. Spencer, W. F. The effects of phosphate and lime applications on
growth, root distribution and freeze injury of young grapefruit trees.
Proc. Fla. State Hort. Soc. 71: 106-114. 1958.
29. Spencer, W. F. Phosphorus and the growth of citrus tree feeder roots.
Citrus Magazine 22: 2. 1959.
30. Spencer, W. F. Effects of heavy applications of phosphate and lime on
nutrient uptake, growth, freeze injury, and root distribution of grape-
fruit trees. Soil Sci. 89: 311-318. 1960.
31. Spencer, W. F. Some considerations pertaining to the use of soil anal-
yses in citrus production. Soil and Crop Sci. Soc. Fla. Proc. 20: 374-
381. 1960.
32. Stewart, I., C. D. Leonard, and I. W. Wander. Comparison of nitrogen
rates and sources for Pineapple oranges. Proc. Fla. State Hort. Soc.
74: 75-78. 1961.
33. Young, T. W., and W. T. Forsee, Jr. Fertilizer experiments with citrus
on Davie mucky fine sand. Fla. Agr. Exp. Sta. Bul. 461. 1949.







42 Florida Agricultural Experiment Stations


APPENDIX

Extractable Soil Phosphorus-Methods and Amounts

AMMONIUM ACETATE, pH 4.8
(FLORIDA SOIL TESTING LABORATORY) METHOD

Reagents
1. Extracting solution: ammonium acetate pH 4.8. Add 1,271
ml concentrated acetic acid to 8 to 10 liters of deionized
water and mix. Add 860 ml of concentrated NH4OH, make to
a volume of 18 liters, and mix well. Adjust pH to 4.8.
2. Ammonium molybdate-sulfuric acid solution: Dissolve 25
grams of ammonium molybdate in 200 ml of distilled water
heated to 60 C. Dilute 280 ml of concentrated H2SO, (36N)
to 800 ml (add acid to water). After both solutions have
cooled, add the ammonium molybdate solution slowly with
shaking to the sulfuric acid solution. After cooling, make
up to 1,000 ml with distilled water.
3. Stannous chloride (concentrated): Dissolve 25 g of stannous
chloride SnC12.2H20 in concentrated HC1 and make to a volume
of 100 ml with concentrated HC1.
4. Stannous chloride (dilute): Add 5 ml of concentrated stannous
chloride solution and 5 ml concentrated HC1 to 190 ml of dis-
tilled water. Make fresh each day.

Procedure
1. Weigh out 5 grams of air-dry soil and place in a 125 ml Erlen-
meyer or other suitable extracting flask.
2. Add 25 ml of the ammonium acetate (pH 4.8) extracting so-
lution from an automatic pipette.
3. Shake on a shaker for 30 minutes.
4. Filter through 11 cm filter paper (Whatman No. 5) into a
flask or filter funnel.
5. Transfer 5 ml of the clear filtrate into a 50 ml volumetric
flask or calibrated tube.
6. Add deionized water to make a volume of 25-30 ml.

SH. L. Breland, Methods of analyses used in soil testing, Fla. Agr. Exp.
Sta. Dept. of Soils, Mimeo Rept. No. 58-1, 1957.







Phosphorus Fertilization of Citrus 43

7. Add 5 ml of the ammonium molybdate solution and mix well.
8. Add 0.5 ml of the dilute SnC12 solution and mix.
9. Make to a volume of 50 ml with deionized water and mix.
Let stand 10 minutes before reading the percent transmission
in a colorimeter at 650 m/ wave length.

Preparation of the Standard Curve
1. Add 0, 1, 2, 4, 6, and 8 ml of a 5 ppm P standard solution to
50 ml volumetric flasks or tubes.
2. Add 5 ml NHAc, pH 4.8 to each flask.
3. Proceed to develop color as described above in steps 6 through
9.
BRAY P1 TEST 6
Reagents
1. Ammonium fluoride stock solution (IN): Dilute 37 gm am-
monium fluoride, NHIF, to 1,000 ml with deionized water.
Keep in polyethylene bottle.
2. Approximately 0.5 N HCI: Dilute 41 ml of concentrated HCI
to 1,000 ml with deionized water.
3. P1 extracting solution (0.03 N NH4F, 0.025 N HC1) : Dilute 540
ml of 1 N NH4F and 900 ml of 0.5N HCI to 18 liters with de-
ionized water.
4. Ammonium molybdate-HC1 reagent, boric acid saturated:
Add slowly with stirring a solution of 50 gms of ammonium
molybdate in 425 ml of distilled water to a cold solution of
80 ml distilled water in 850 ml of concentrated HC1. Add 51
grams of boric acid to the mixture.
5. Amino-naphthol-sulfonic acid reagent:
2.5 gm 1-amino-2-naphthol-4-sulfonic acid.
5.0 gm sodium sulfite (Na2SOa).
146.25 gm sodium bisulfite (Meta, Na2S2Os).
Mix these dry materials thoroughly and grind the mixture to
a fine powder with a mortar and pestle. The dry powder will
keep indefinitely.
Dissolve 8.0 grams of the powder mixture in 50 ml of warm
distilled water. If possible, allow this solution to stand over-
" Adapted from C. Y. Arnold and L. T. Kurtz, Photometer method for
determining available phosphorus in soils, Ill. Agr. Exp. Sta., Dept. of
Agronomy, Mimeo AG 1306, 1946 (4).







44 Florida Agricultural Experiment Stations

night before using it. A fresh portion of this solution should
be made up from the dry powder every three weeks.
6. Standard phosphate solution: Dilute 0.4389 gm of dry
K H1PO4 to 1 liter for 100 ppm P. Dilute 20 ml of this solu-
tion to 100 ml for a 20 ppm P standard. Dilute with Reagent
3 in both cases instead of with deionized water.

Procedure
1. Weigh 5 gms of air-dried soil into a 125 ml Erlenmeyer or
other suitable extracting flask.
2. Add 50 ml of P1 extracting solution (reagent 3) from an au-
tomatic pipette.
3. Shake on a shaker for two minutes.
4. Filter through Whatman No. 42 filter paper into 50 ml Erlen-
meyer flasks, filter funnels, or vials. (Set up the filters be-
fore the extracting solution is added to the soil samples.)
5. Transfer a 5 ml aliquot of the filtrate to a test tube. For soils
high in P use a 2 ml aliquot plus 3 ml extracting solution.
6. Add 5 drops of ammonium-molybdate-HC1 reagent and swirl
the tube to mix the reagent with the filtrate.
7. Add 5 drops of amino-naphthol sulfonic acid reagent and mix
immediately. (If the color is being developed in several
tubes at once, the ammonium-molybdate-HC1 reagent may be
added to several tubes before they are mixed; when the amino-
naphthol sulfonic acid reagent is added each tube must be
mixed immediately after the reagent is added.)
8. Fifteen minutes after the amino-naphthol sulfonic acid re-
agent was added to the first sample begin reading the color
in a photometer using a green filter (525 mA wave length).

Preparation of Standard Curve
1. Add 0, 1, 2, and 3 ml of a 20 ppm P standard solution to test
tubes.
2. Add a sufficient amount of P1 extracting solution (reagent 3)
to bring the total volume to 5 ml.
3. Proceed to develop color as described above in steps 6 through
8.







Phosphorus Fertilization of Citrus 45

4. Read in photometer with instrument set on 0 with the 0
standard in the instrument.

BRAY P2 TEST
Reagents
1, 2, 4, 5, and 6 from the Bray P1 test and in addition the P2
extracting solution (0.03 N NHF, 0.1N HC1) as follows: Di-
lute 540 ml of 1 N NH4F and 3600 ml of 0.5N HC1 to 18 liters
with deionized water.
Procedure
Same as for the P1 test except use the P, extracting solution
in step 2 instead of the P1 extracting solution.

Preparation of Standard Curve
Same as for the Pi test except use the P2 extracting solution
in step 2 instead of the P1 extracting solution.
SAdapted from Arnold and Kurtz (4).








46 Florida Agricultural Experiment Stations





APPENDIX TABLE 1.-PHOSPHORUS CONTENT AND PH OF SOIL SAMPLES
OBTAINED AT VARIOUS DEPTHS FROM EXPERIMENTAL GROVES BEFORE THE
EXPERIMENTS WERE INITIATED.

Soil Phosphorus, lb. P/A-6 in.
Extractable Total
Ammonium
Depth, acetate Bray Bray
Experiment Inches pH 4.8 PL test P2 test Soil pH

1 0-6 23 82 121 267 6.3
6-12 14 79 90 191 5.7
12-18 7 38 41 123 5.3
18-24 4 22 24 115 5.1
24-36 3 24 27 124 5.0
36-48 5 24 26 118 4.7
0-48 total 65 301 390 1181 -

2 0-6 25 95 138 332 6.2
6-12 13 86 92 245 5.6
12-18 6 45 45 160 5.2
18-24 4 26 28 138 5.1
24-36 3 26 28 148 4.9
36-48 4 29 32 141 4.7
0-48 total 62 362 423 1453 -

3 0-6 26 110 243 400 6.2
6-12 22 176 174 274 5.5
12-18 18 131 128 204 5.2
18-24 11 76 69 153 5.1
24-36 5 37 32 124 5.3
36-48 4 31 23 102 4.9
0-48 total 95 629 726 1484 -

4 0-6 10 56 78 215 5.6
6-12 6 57 62 198 5.3
12-18 5 49 46 178 5.2
18-24 8 47 51 181 5.2
24-36 5 43 40 166 5.2
36-48 11 40 38 141 5.2
0-48 total 59 374 392 1387 -

5 0-6 2.0 24 23 163 5.2
6-12 .3 28 16 158 5.0
12-18 .6 29 18 153 5.0
18-24 1.0 28 23 176 5.0
24-36 1.5 32 27 189 5.0
36-48 5.0 39 41 217 5.0
0-48 total 16.4 251 216 1462 -








Phosphorus Fertilization of Citrus 47



APPENDIX TABLE 2.-EXTRACTABLE SOIL PHOSPHORUS AND PH AT VARI-
OUS DEPTHS IN PLOTS RECEIVING HEAVY RATES OF PHOSPHATE AND LIME-
STONE (EXPERIMENT 6).t

Extractable P, lb/A-6 in.
Treatment
Ammonium
P2O5 Limestone Depth, acetate Bray Bray
lb/A/yr. lb/A/yr. Inches pH 4.8 P1 test Pt test Soil pH

305 520 0-6 54 450 560 5.4
6-12 295 300 4.7
12-18 115 120 4.7
18-24 68 73 4.7
24-36 40 55 4.7
36-48 30 36 4.7

1220 2080 0-6 146 690 1020 5.8
6-12 480 540 4.9
12-18 380 410 5.0
18-24 260 300 5.0
24-36 170 190 5.1
36-48 77 95 5.0

4880 8320 0-6 626 1470 4520 6.0
6-12 625 750 5.2
12-18 570 600 5.2
18-24 455 510 5.3
24-36 405 450 5.3
36-48 280 350 5.3

0 520 0-6 10 59 80 6.1
6-12 33 39 5.1
12-18 28 35 5.0
18-24 23 31 4.9
24-36 26 33 4.8
36-48 22 30 4.7

0 2080 0-6 17 68 94 6.5
6-12 32 44 5.4
12-18 26 35 5.3
18-24 24 35 5.0
24-36 26 39 4.9
36-48 27 38 4.9

0 8320 0-6 22 68 127 7.1
6-12 38 47 6.0
12-18 38 34 5.6
18-24 24 34 5.6
24-36 29 35 5.2
36-48 29 39 5.3


t Samples obtained October 1957. prior to last application of phosphate and limestone.








APPENDIX TABLE 3.-PHOSPHORUS EXTRACTED BY THREE METHODS FROM SOIL SAMPLES OBTAINED AT VARIOUS DEPTHS
FROM PLOTS WHICH RECEIVED 0 AND 6 PERCENT P2O0 FOR 15 YEARS (EXPERIMENT 7).t+
00
Phosphorus, lb/A-6 in.
Total
Soil Test Plot % P,0e in 0-6 6-12 12-18 18-24 24-36 36-48 0-48
Method No. Fertilizer in. in. in. in. in. in. in.

Bray P1 Test 13 0 80 62 42 28 28 29 326
16 0 117 51 46 42 43 35 412
18 0 92 70 37 28 33 30 353 P
Mean No P2O5 96 61 42 33 35 31 364
14 6 205 236 222 94 39 31 897
15 6 286 253 145 55 41 42 905
17 6 216 272 195 76 50 43 945
Mean 6% P10 236 254 187 75 43 39 916

Bray P2 Test 13 0 114 82 52 34 38 40 435
16 0 165 62 54 55 58 47 546
18 0 145 76 46 36 42 39 465
Mean 0% P,0, 141 73 51 42 46 42 483 ^
14 6 274 294 287 121 54 41 1166
15 6 396 310 203 73 64 61 1232
17 6 268 330 219 89 55 56 1128
Mean 6% P20 313 311 236 94 58 53 1176

NHAc, pH 4.8 13 0 36 19 16 6 3 4 91
16 0 15 4 3 3 3 3 37
18 0 19 7 5 4 3 6 53
Mean 0% P20 23 10 8 4 3 4 59
14 6 43 23 24 26 4 5 134
15 6 30 26 15 4 4 4 92
17 6 28 29 23 8 6 4 108
Mean 6% P10 34 26 21 13 5 4 110

t 788 lb. P/A had been applied to the 6% P20, plots prior to sampling.





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