Technical Bulletin 651 February 1963
for Pastures on
Flatwoods Soils in Florida
University of Florida AGRICULTURAL EXPERIMENT STATIONS
J. R. Beckenbach, Director Gainesville
INTRODUCTION .....-..----... ... .....------- ...-...... ............. 3
METHODS AND MATERIALS .......--... .- .....--------------- ---- ..............
FORAGE PRODUCTION .------------- --.....................---- -............. 6
UPTAKE OF PHOSPHORUS IN FORAGE ..----...........--........ ..---..--......... 7
SOIL DATA ......................... -- ---- --...----.. --- ....... ........----- 10
SUMMARY .--------..........------- .-.........- -------- .... .....-...... 10
CONCLUSIONS .............- .....- ..........-- .--- --. --------- ......-.-----. .. ...... 12
ACKNOWLEDGMENTS -- ---...............--- ...........----- -----...............- 12
LITERATURE CITED ..................... .... ..... --.... ..............---- .............. 12
Comparisons of Phosphorus Fertilizers for
Pastures on Flatwoods Soils in Florida
J. R. NELLER 1
Much of the planted pasture area in Florida is located on
sandy "flatwoods" soils of low native productivity characterized
by low reserves of plant food and by occasional periods of either
too much or too little soil moisture. Fertilizer constitutes a large
part of the maintenance costs of these pastures. For that rea-
son it is important to use the most economical, effective kinds
There are about 15 million acres of flatwoods in Florida of
which about 2 million acres are in planted pastures. The flat-
woods are fairly level areas of acid sandy soils where the native
vegetation consists mostly of palmetto, wiregrass, and thin
stands of pine trees (4).2 These soils require various fertilizer
elements including phosphorus for the production of satisfactory
growth of forage. This bulletin records experiments that were
conducted in a flatwoods area of Leon fine sand in north central
Florida in which a comparison was made of the effectiveness of
ground rock phosphate, triple superphosphate, combinations of
these, and of phosphatic basic slag.
METHODS AND MATERIALS
The experimental plots were 15 x 20 feet in size in six blocks
each containing eight randomized phosphate treatments recorded
in Table 1. They were located on an area of Leon fine sand that
recently had been cleared of native vegetation by removing the
pine trees, bulldozing the palmetto, and removing stumps and
roots. Details and costs of land clearing are described by Reuss
(7). Dolomitic lime at the rate of 2 tons per acre was disked
into the surface 4 inches of soil. The initial fertilizer contained
the sulfates of copper, manganese, and zinc, as well as borax for
boron, at the rates of ~20, 20 5,-ani O pounds per acre, respec-
tively. These materials are deficient in some flatwoods areas
(3) and were included to insure that phosphate response should
not be limited by them.
1 Soils Chemist Emeritus, Florida Agricultural Experiment Stations
2 Numbers in parentheses refer to Literature Cited.
TABLE 1.-AMOUNTS OF PENSACOLA BAHIAGRASS AND LADINO CLOVER CUT AND REMOVED FROM THE PLOTS.
Phosphate Treatments, Lbs. PzOs per Acre
Triple Triple Triple Basic super super Rainfall
Year Rock Rock super super super slag 300 300 in
600 300 15 30 60 47 15 30 LSD* Cuttings Inches
Annual yield x 100 equals lbs. per acre air dry forage-
1953** 80.5 79.1 80.1 87.7 92.0 78.3 82.5 86.2 6.9 5 73.3
1954 96.8 96.2 93.9 96.4 106.9 101.9 105.8 98.6 7.1 4 35.2
1955 105.5 106.6 100.8 115.3 114.3 120.9 110.7 119.0 9.0 4 42.7 tq
1956 96.1 76.6 60.6 70.5 89.1 100.3 79.5 83.0 10.5 4 48.0
1957 116.2 106.7 100.1 108.2 119.7 116.3 115.2 122.1 8.0 4 56.9
1953-57 99.0 93.0 87.1 95.6 104.3 103.6 98.8 101.8
1958*** 95.7 82.5 54.0 83.9 71.5 90.7 84.6 87.6 10.0 4 59.9 R
Least significant difference at 5 percent level.
"** The triple superphosphate and basic slag treatments were made at double rates in 1953.
*** No phosphates were added in 1958 except triple superphosphate at the rate of 120 pounds PO25 per acre to the former super-30 treatment.
Comparisons of Rock Phosphate 5
The phosphate treatments were made at the rates given in
Table 1 and are based on the guaranteed amounts of 30 percent
P205 total for rock phosphate, 8 percent P20, total for basic
slag and 45 percent P205 available for triple superphosphate.
The ground rock phosphate was applied once only when it was
disked into the surface 4 inches of soil at the beginning of the
experiment. The triple superphosphate and basic slag treat-
ments were disked into the soil the first year and then surface
dressed annually for four years at the rates given in the table.
Because of the low phosphorus level in the soil of this virgin
flatwoods area, triple superphosphate and basic slag were applied
at double rates the first year. Annual applications of muriate
of potash (60% K20) and gypsum (28 percent SO4) were made
at the rates of 200 and 170 pounds per acre, respectively. These
amounts are higher than are generally used in pastures because
the forage was cut and removed from the plots. Gypsum was
used to insure a source of sulfur known to be necessary (6) on
the sandy soils of the flatwoods. Triple superphosphate was
used instead of ordinary superphosphate so that the same
amount of gypsum would be present irrespective of variable
rates of phosphate. For the first five years nitrogen was omitted
in the fertilizer program, and its supply was dependent on that
fixed by the clover growing on the plots. Ammonium nitrate
was used the sixth year at the rate of 100 pounds N per acre.
This was done to insure a good growth of grass for the compari-
son of responses to residual phosphates. No phosphates were
added that year except triple superphosphate at the rate of 120
pounds P205 per acre to the treatment which had formerly re-
ceived 30 pounds P205 per acre annually.
The plots were planted to inoculated Ladino clover and Pen-
sacola bahiagrass seed in September 1952. Five cuttings were
removed the first year and four in each of the subsequent years.
Samples were obtained from each cutting for dry matter deter-
minations as obtained by drying the forage to constant weight
at 70C. The samples were then allowed to take up atmospheric
moisture and were weighed. These contained moisture closely
approximating 10 percent, and the forage yields are reported
on that basis. A deficiency of soil moisture generally limited
growth, especially of clover. Hence, overall production was at a
Each year forage samples were obtained for determination of
total phosphorus of cuttings typical of early and late season
6 Florida Agricultural Experiment Stations
growth. Percent total phosphorus for the other two cuttings
per season was interpolated for the calculation of uptake of phos-
phorus in forage removed from the plots. The Fiske and Sub-
barrow method (2) was used to determine total phosphorus in
forage and soil after ashing the samples at 600 degrees C. The
Morgan method (5) was used for determination of soluble soil
phosphorus. Soil samples were obtained in 1953 from the sur-
face 4 inches of each plot one year after application of lime and
fertilizer, and in 1958 after the growing season of that year.
Soil moisture conditions were good in 1953 (Table 1) and
good stands of seedling clover and grass were obtained. The
most growth occurred where triple superphosphate was used
at the highest rate (60 pounds P20s per acre). Growth on
plots of the other phosphate treatments was less, but not to a
In 1954 the most forage was again obtained from the 60
pound P205 per acre treatment, closely followed by forage pro-
duction for the other phosphate treatments.
In 1955 forage from the plots where phosphatic basic slag
was used slightly exceeded yields from plots of the other phos-
phate treatments except where rock phosphate at 300 pounds
P205 per acre had been supplemented with triple superphosphate
at the rate of 30 pounds P205 per acre annually.
Forage production in 1956 was significantly less on plots that
received the least amount of phosphate (15 pounds P205 per
acre as triple superphosphate) than on plots of all other treat-
ments except for the triple superphosphate treatment of 30
pounds P205 per acre. Growth of clover was distinctly less on
the plots of the triple super 15 pound P205 per acre treatment.
The most growth occurred where basic slag had been used, fol-
lowed closely by the rock phosphate treatment at the rate of
600 pounds P205 per acre. Use of rock phosphate at half that
rate resulted in a significant reduction in yields, which in turn
were significantly increased when rock phosphate at the same
rate had been supplemented by triple superphosphate at the rate
of 30 pounds P20O per acre annually. In 1957 this treatment
produced more growth than any of the other treatments.
No phosphates were added in 1958 except triple superphos-
phate at the rate of 120 pounds P205 per acre to the former
triple super-30 treatment. This was done in order to have a
Comparisons of Rock Phosphate 7
well-phosphated treatment with which to compare the residual
phosphates of the other treatments. Growth on the plots where
rock phosphate had been disked into the surface 4 inches six
years previously was significantly greater than for any of the
other treatments except the basic slag and the rock phosphate-
300 plus triple super-30 treatments. The next best production
of forage was on the rock-30 plus triple super-15 treatment fol-
lowed closely by the newly phosphated triple super-30 plus 120
and by the rock-300 treatments. Growth where triple super-
phosphate had been used annually at the rate of 60 pounds P205
per acre was significantly less than for all other treatments ex-
cept for the triple super-15 treatment.
As recorded in Table 1, total rainfall in 1954 was extremely
low with an increase in each of the following years. Forage
production of grass and especially of clover probably depends
more on the distribution of the rainfall over the growing season
than on the overall amount of rainfall. Inspection of the aver-
age production of forage for the five-year period, 1953-1957,
shows the high production for the triple super-60 treatment,
followed in order by the basic slag-47, rock-300 plus triple super-
30, rock-600, rock-300 plus triple super-15, triple super-30, rock
300, and triple super-15 treatments.
UPTAKE OF PHOSPHORUS IN THE FORAGE
Figure 1 portrays a comparison of the relative total amounts
of phosphorus taken up by forage cut from the plots differen-
tially treated with phosphates for the five-year period, 1953-
1957. Whereas the same amounts of forage were removed
from the triple super-60 and phosphatic basic slag-47 treatments
(Table 1), Figure 1 shows that the forage from the plots treated
with basic slag contained only 60 percent as much phosphorus as
that from the triple super-60 plots of which the phosphorus
uptake is given a relative value of 100. In contrast, forage from
the rock-300 plus triple super-30 plots was equal to that from the
triple super-60 plots in both tonnage and content of phosphorus.
The average phosphorus contents of forage from the basic slag,
rock-300 plus triple super-30 and triple super-60 treatments were
0.172, 0.244 and 0.258 percent phosphorus, respectively. In a
study of phosphorus deficiency in cattle, Becker and associates
(1) found that grass from "healthy" and "deficient" ranges
averaged 0.167 and 0.103 percent phosphorus, respectively. For-
age from the plots where rock phosphate was disked into the
8 Florida Agricultural Experiment Stations
surface soil at the rate of 600 pounds P205 per acre, when the
grass and clover seeds were planted, contained about 87 percent
as much phosphorus as forage from the triple super-60 plots.
Use of half as much rock phosphate supplemented with triple
superphosphate annually at the rate of 15 pounds P205 per acre
resulted in about 93 percent as much uptake of phosphorus.
Plots that received triple superphosphate at the rate of 15 pounds
P205 per acre annually produced only half as much phosphorus
in the forage, of which the phosphorus content was 0.171 per-
S 70 -
W 50 -
oO o 40O
PHOSPHATES LBS. P205 PER ACRE
Fig. 1.-Relative amounts of phosphorus taken up in the forage for
1953 to 1957 inclusive on phosphated plots compared with a value of 100
where triple superphosphate was added annually at the rate of 60 pounds
P2Os per acre.
Comparisons of Rock Phosphate 9
cent, as those that received 60 pounds P205 per acre annually.
Figure 1 shows that rock phosphate-300 supplemented with triple
superphosphate compares better with the triple super-60 treat-
ments than twice as much rock phosphate not supplemented with
The newly phosphated treatment of triple superphosphate at
the rate of 120 pounds P205 per acre was given a value of 100
in the comparisons of uptake of residual phosphorus in Figure 2.
The figure shows that the rock-600 treatment resulted in the
highest uptake of phosphorus of all treatments. In comparison
with the triple super-30 plus 120 treatment, phosphorus uptake
in the other treatments was less in the descending order: rock-
3 60 -
S 20 -
S0 a C\0 0 N 0 0
PHOSPHATES LBS. P205 PER ACRE
Fig. 2.-Relative amounts of phosphorus taken up in the forage pro-
duced from residual phosphates in 1958 compared with a value of 100 for
the newly phosphated triple super-30 + 120 treatment.
10 Florida Agricultural Experiment Stations
300 plus triple super-15, rock-300 plus triple super-30, rock-300,
slag-47, triple super-60, and triple super-15.
In 1953, one year after dolomitic lime at the rate of 2 tons
per acre had been disked into the surface 4 inches of soil, the
average pH of the plots was 5.79 (Table 2) as compared with
4.11 before liming. Five years after liming (1958) the pH had
decreased to an average of 5.17. Soil samples of 1958 show the
lime effect of basic slag in increasing pH, and the values for
basic slag were omitted in the averages given above. Soil of
samples taken from the surface 4 inches of these plots before
phosphates were added averaged 87 pounds total P205 per acre.
This amount plus 600 pounds P205 per acre of rock phosphates
is less than the average amount of total phosphate recorded in
Table 2 for the soil samples of 1953. This can be attributed to
variation in soil samples and to the probability that the rock
phosphate contained more P205 than was guaranteed, which is
the amount used in the calculations. The soluble phosphate
values were about the same in 1953 as in 1958.
During 1952 to 1958 an experiment was run in an area of
newly cleared land representative of the Leon fine soils of the
flatwoods of north central Florida. Plots were limed, fertilized,
and planted to Pensacola bahiagrass and inoculated Ladino clover
seed. The phosphate of the fertilizer consisted of eight differ-
ent treatments of rock phosphate, triple superphosphate and basic
slag, and combinations of phosphates. Triple superphosphate
and basic slag were applied at double the rates the first year.
This was done to help insure the establishment of a stand of
clover. Five cuttings were removed the first year and four in
each of the subsequent years. No phosphates were added to
the plots during the last year of the experiment except to one
treatment to serve as a phosphate check with which to compare
the residual effects of the other treatments.
Similar yields of forage were obtained from rock phosphate
at the rate of 600 pounds P205 per acre, disked into the soil when
the plots were planted, as from triple superphosphate applied
yearly at the rate of 60 pounds P205 per acre, and from half as
much rock phosphate supplement with triple superphosphate,
top dressed at the rate of 30 pounds per acre annually.
TABLE 2.-TOTAL AND SOLUBLE PHOSPHATE AND PH VALUES FOR THE LIMED, PHOSPHATED SOILS.
Treatments, Lbs. P20 per Acre
Triple Triple Triple Basic triple triple
Year Rock Rock super super super Slag super super
600 300 15 30* 60 47 15 30
pH values -
1953 5.82 5.81 5.98 5.54 5.71 5.83 5.90 5.80
1958 5.25 5.22 5.46 4.94 5.09 5.62 5.08 5.13 ^
Total phosphate, lbs. PM03 per Acre -
1953 930 433 123 119 131 160 415 389 1
1958 357 163 99 142 110 147 158 224 S
Soluble phosphate, lbs. P20s per Acre I
1953 13 11 11 11 15 12 13 11
1958 11 8 9 12 9 7 11 11
Triple superphosphate at the rate of 120 pounds per acre PO, was applied to the plots of this treatment in 1958.
12 Florida Agricultural Experiment Stations
Since most flatwoods soils are low in sulfur (6) as well as in
native phosphorus, a combination of rock phosphate and ordinary
superphosphate will supply both phosphorus and the sulfur of
gypsum needed to get clover-grass pastures well established.
It is recommended that 1/2 to 1 ton per acre of rock phosphate
be disked into the soil before the pasture is planted and that a
mixed fertilizer that contains ordinary superphosphate be ap-
plied at that time also and annually thereafter.
Basic slag was found to be a satisfactory source of phos-
phorus in comparison with like amounts of P205 in superphos-
phate. But since basic slag contains very little sulfur, it must
be supplemented with gypsum for clover pastures. For that
reason use of a mixed fertilizer that contains the necessary sul-
fate as well as phosphate and potash is to be preferred, provided
the minor element needs of the soils have been satisfied.
Grateful appreciation is extended to the laboratory and field assistants
who helped obtain data during the several years of the experiments. Par-
ticular thanks are due Dr. F. D. Bartlett for his assistance while a gradu-
ate student at the University of Florida.
1. Becker, R. B., W. M. Neal, and A. L. Shealy. Stiffs or sweeny (phos-
phorus deficiency). Univ. of Fla. Agr. Exp. Sta. Bul(264) 1933.
2. Fiske, C. H., and Y. J. Subbarrow. The colorimetric determination
of phosphorus. J. Biol. Chem. 66: 375. 1925.
3. Killinger, G. B., R. E. Blaser, E. M. Hodges, and W. E. Stokes. Minor
elements stimulate pasture plants. Univ. of Fla. Agr. Exp. Sta. Bul.
4. Koger, M., W. G. Blue, G. B. Killinger, R. E. L. Greene, H. C. Harris,
J. M. Myers, A. C. Warnick, and N. Gammon, Jr. Beef production,
soil and forage analyses, and economic return from eight pasture pro-
grams in north central Florida. Univ. of Fla. Agr. Exp. Sta. Bul. 631.
5. Morgan, M. F. Chemical soil diagnosis by the Universal Soil Testing
System. Conn. Agr. Exp. Sta. Bul. 450. 1941.
6. Neller, J. R., G. B. Killinger, D. W. Jones, R. W. Bledsoe, and H. W.
Lundy. Fertilizer should contain a source of sulfur for clover pastures
in many areas of Florida. Univ. of Fla. Agr. Exp. Sta. Cir. S-35.
7. Reuss, L. A. Costs of clearing land and establishing improved pas-
tures in Central Florida. Univ. of Fla. Agr. Exp. Sta. Bul. 600. 1958.