Soil reaction (pH)

Material Information

Soil reaction (pH) some critical factors in its determination, control and significance
Series Title:
Bulletin - Florida Agricultural Experiment Station ; 400
Volk, G. M. ( Gaylord Monroe ), 1908-
Bell, C. E. ( Charles Edward ), b. 1883
Place of Publication:
Gainesville, Fla.
University of Florida Agricultural Experiment Station
Publication Date:
Copyright Date:
Physical Description:
43 p. : charts ; 23 cm.


Subjects / Keywords:
Hydrogen-ion concentration ( lcsh )
Soil acidity ( lcsh )
Liming of soils -- Florida ( lcsh )
bibliography ( marcgt )
non-fiction ( marcgt )


Includes bibliographical references (p. 42-43).
General Note:
Cover title.
Statement of Responsibility:
by G.M. Volk and C.E. Bell.

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
AEN5855 ( ltuf )
18232064 ( oclc )
027118652 ( alephbibnum )


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Full Text

Bulletin 400 August, 1944



Some Critical Factors in Its Determination, Control
and Significance

By G. M. VOLK and C. E. BELL


Single copies free to Florida residents upon request to


H. P. Adair, Chairman, Jacksonville Ouida D. Abbott, Ph.D., Home Econ.'
N. B. Jordan. Quincy Ruth O. Townsend, R.N., Assistant
T. T. Scott, Live Oak R. B. French, Ph.D., Biochemist
Thos. W. Bryant, Lakeland
M. L. Mershon, Miami
J. T. Diamond. Secretary, Tallahassee ENTOMOLOGY

EXECUTIVE STAFF J. R. Watson, A.M., Entomologist'
A. N. Tissot, Ph.D., Associates
John J. Tigert, M.A., LL.D., President of the H. E. Bratley, M.S.A., Assistant
Harold Mowry, M.S.A., Director
L. O. Gratz, Ph.D., Asst. Dir., Research
W. M. Fifield. M.S., Asst. Dir., Admin.' HORTICULTURE
J. Francis Cooper, M.S.A., Editor'
Clyde Beale, A.B.J., Assistant Editor' G. H. Blackmon, M.S.A., Horticulturist'
Jefferson Thomas, Assistant Editors A. L. Stahl, Ph.D., Asso. Horticulturist
Ida Keeling Cresap, Librarian F. S. Jamison, Ph.D., Truck Hort.
Ruby Newhall, Administrative Managers R. J. Wilmot, M.S.A., Asst. Hort.
K. H. Graham, LL.D., Business Managers R. D. Dickey, M.S.A., Asst. Hort.'
Claranelle Alderman, Accountants J. Carlton Cain, B.S.A., Asst. Hort.'
Victor F. Nettles, M.S.A., Asst. Hort.'
MAIN STATION, GAINESVILLE Byron E. Janes, Ph.D., Asst. Hort.
F. S. Lagasse, Ph.D., Asso. Hort.2
AGRONOMY H. M. Sell, Ph.D., Asso. Horticulturist'

W. E. Stokes, M.S., Agronomist'
Fred H. Hull, Ph.D., Agronomist
G. E. Ritchey, M.S., Agronomist2 PLANT PATHOLOGY
W. A. Carver, Ph.D., Associate
Roy E. Blaser, M.S., Associate W. B. Tisdale, Ph.D., Plant Pathologist'1
G. B. Killinger, Ph.D., Agronomist Phares Decker. Ph.D., Asso. Plant Path.
H. C. Harris, Ph.D., Associate Erdman West, M.S., Mycologist
R. W. Bledsoe, Ph.D., Assistant Lillian E. Arnold, M.S., Asst. Botanist
Fred A. Clark, B.S., Assistant


A. L. Shealy, D.V.M., An. Industrialist '
R. B. Becker, Ph.D., Dairy Husbandman' F. B. Smith, Ph.D., Chemisti a
E. L. Fouts, Ph.D., Dairy Technologists Gaylord M. Volk, M.S., Chemist
D. A. Sanders, D.V.M., Veterinarian L. E. Ensminger, Ph.D., Soils Chemist
M. W. Emmel, D.V.M., Veterinarians J. R. Henderson, M.S.A.. Soil Technologist
L. E. Swanson, D.V.M., Parasitologist* J. R. Neller, Ph.D., Soils Chemist
N. R. Mehrhof, M.Agr., Poultry Husb.' C. E. Bell, Ph.D., Associate Chemist
T. R. Freeman, Ph.D., Asso. in Dairy Mfg. L. H. Rogers, Ph.D., Associate Biochemist'
R. S. Glasseock, Ph.D., An. Husbandman R. A. Carrigan, B.S., Asso. Biochemist'
D. J. Smith, B.S.A., Asst. An. Husb.' G. T. Sims, M.S.A., Associate Chemist
P. T. Dix Arnold, M.S.A., Asst. Dairy Husb.' T. C. Erwin, Assistant Chemist
G. K. Davis, Ph.D., Animal Nutritionist
. Davis, Ph.D., Animal Nutritionist H. W. Winsor, B.S.A., Assistant Chemist
C. L. Comar, Ph.D., Asso. Biochemist
L. E. Mull, M.S., Asst. in Dairy Tech.' Geo. D. Thornton, M.S., Asst. Microbiologist
O. K. Moore, M.S., Asst. Poultry Hush.' R. E. Caldwell, M.S.A., Asst. Soil Surveyor'
J. E. Pace, B.S., Asst. An. Husbandman' Olaf C. Olson, B.S., Asst. Soil Surveyor
S. P. Marshall, M.S., Asst. in An. Nutrition

In cooperation with U. S.
C. V. Noble, Ph.D., Agr. Economists 3
C. V. SNoe, Ph.. A Economist 3 Cooperative, other divisions, U. of F.
Zach Savage, M.S.A., Associate
A. H. Spurlock, M.S.A., Associate In Military Service.
Max E. Brunk, M.S., Assistant 5 On leave.

NORTH FLORIDA STATION, QUINCY Clement D. Gordon, Ph.D., Asso. Poultry

J. D. Warner, M.S., Vice-Director in Charge Geneticist in Charge2
R. R. Kincaid, Ph.D., Plant Pathologist ATT STA
V. E. Whitehurst, Jr., B.S.A., Asst. An. Hush.'
W. C. McCormick, B.S.A., Asst. An. Hush.
W. C. M rick, .S.A., Asst. An. Hus. W. G. Kirk, Ph.D., Vice-Director in Charge
Jesse Reeves, Asst. Agron., Tobacco E
Jesse Reeves, Asst. Agron., Tobacco E. M. Hodges, Ph.D., Asso. Agron., Wauchula
W. H. Chapman, M.S., Asst. Agron.4
. C. BCh .S. Asst Agron t Gilbert A. Tucker, B.S.A., Asst. An. Husb.'
R. C. Bond, M.S.A., Asst. Agronomist

Mobile Unit, Monticello
R. W. Wallace, B.S., Associate Agronomist

Mobile Unit, Milton Leesburg
Ralph L. Smith, M.S., Associate Agronomist M. N. Walker, Ph.D., Plant Path. in Charge'

Mobile Unit, Marianna Plant City
R. W. Lipscomb, M.S., Associate Agronomist A. N. Brooks, Ph.D., Plant Pathologist


A. F. Camp, Ph.D., Vice-Director in Charge A.H. Eddins, .D, Plant Pathologist
V. C. Jamison, Ph.D., Soils Chemist E. N. McCubbin, Ph.D., Truck Horticulturist
B. R. Fudge, Ph.D., Associate Chemist Monticello
W. L. Thompson, B.S., Entomologist
W. W. Lawless, B.S., Asst. Horticulturist4 S. 0. Hill, B.S., Asst. Entomologist2 4
C. R. Stearns, Jr., B.S.A., Asso. Chemist A. M. Phillips, B.S., Asst. Entomologist2
H. O. Sterling, B.S., Asst. Horticulturist
T. W. Young, Ph.D., Asso. Horticulturist Bradenton
J. W. Sites, M.S.A., Asso. Horticulturist
J. R. Beckenbach, Ph.D., Horticulturist in
EVERGLADES STA., BELLE GLADE E. G. Kelsheimer, Ph.D., Entomologist
D. B. Creager, Ph.D., Plant Path., Gladiolus
R. V. Allison, Ph.D., Vice-Director in Charge .. reer, Ph.D., Plant Path.,Gdiolus
A. L. Harrison, Ph.D., Plant Pathologist
J. W. Wilson, Sc.D., Entomologist4
J. W. Wion, S.D., Entomologi4 David G. Kelbert, Asst. Plant Pathologist
F. D. Stevens, B.S., Sugarcane Agron.
F.Tho s Bregge h.., Sugarcane A E. L. Spencer, Ph.D., Soils Chemist
Thomas Bregger, Ph.D., Sugarcane
Physiologist Sanford
G. R. Townsend, Ph.D., Plant Pathologist
R. W. Kidder, M.S., Asst. An. Hush. R. W. Ruprecht, Ph.D., Chemist in Charge
W. T. Forsee, Jr., Ph.D., Asso. Chemist J. C. Russell, M.S., Asst. Entomologist5
B. S. Clayton, B.S.C.E., Drainage Eng.2
F. S. Andrews, Ph.D., Asso. Truck Hort.' Lakeland
R. A. Bair, Ph.D., Asst. Agronomist
E. C. Minnum, M.S., Asst. Truck Hort. E S. Ellison, Meteorologist2 5
N. C. Hayslip, B.S.A., Asst. Entomologist Warren O. Johnson, Meteorologist2

SUB-TROPICAL STA., HOMESTEAD 1 Head of Department.
In cooperation with U. S.
Geo. D. Ruehle, Ph.D., Vice-Director in Cooperative, other divisions, U. of F.
P. J. Westgate, Ph.D., Asso. Horticulturist In Military Service.
H. I. Borders, M.S., Asso. Plant Path. 6 On leave.



DETERMINATION OF SOIL PH ...... --------.........- ......... ...... 5

Effect of Air-Drying on Soil pH.................-....... ......- 6

Effect of Soil : Water Ratio ..----...........------....-.--- ..- 6

Effect of Stirring the Suspension ---...--......--- .......---------- 9

Colorimetric Methods as Compared to the Standard Method ..--.-. 13

CONTROL OF SOIL PH --........----- ..---....---- ..------ --...-.-- 14

Factors in the Adjustment of pH ........ ................ .....- 15

Factors in the Maintenance of pH -.... ---------------...----....-- 21

IN FLORIDA SOILS -- ......................... ------------- .-- -..-- 23

pH and Percent Base Saturation -................ .............. .. 23

pH and the Retention of Potassium and Ammonia .................--.. 25

PH AND PHOSPHORUS SOLUBILITY ..........-.....- .....---.------- .... 30

SUMMARY ....-- ----........... ..........----------.. --. -- 36

CONCLUSIONS --.....--...-..---------. ...........--- -----.. 40

LITERATURE CITED .........---...--..--............-----............--- 42


Some Critical Factors in Its Determination, Control
and Significance
By G. M. VOLK and C. E. BELL

The determination of soil reaction is the most widely practiced
of soil tests used in Florida in connection with crop production.
Its most general use is as a reference factor which for any given
soil and crop combination has been found to have a certain sig-
nificance. This interpretation of pH is based on experience
gained by actual crop production on specific soils. It may indi-
cate certain amendment practices which experience has shown
are beneficial.
The more fundamental interpretation given of pH is its specific
role in certain physio-chemical reactions that determine nutrient
retention and solubility in the soil. It is with the latter inter-
pretation that the section on significance in this paper will deal
The potentiometric method using the glass electrode is the
most popular method of determining soil pH, although colori-
metric methods involving the use of indicator dyes are widely
used in field kits.
Carrigan (7)1 proposed a simple, rapid potentiometric method
for Florida conditions. With slight revision it is essentially as
Samples are air-dried and the pH determined potentiometric-
ally by means of the glass electrode on a suspension of 1 volume
of air-dry soil to 2 volumes of water after soaking 40 minutes
for mineral soils or 2 hours for peats2 or mucks. Samples are
stirred by hand at the beginning of the soaking period and im-
mediately before inserting the electrodes. Rinse water on the
electrodes is displaced by rapidly raising and lowering them
several times in the suspension. The reading is taken imme-
diately without further agitation of the suspension.
1 Italic figures in parentheses refer to Literature Cited.
"A dilution of 1 to 10 by weight and 3 hours soaking period was recom-
mended by Carrigan for peat soils.


6 Florida Agricultural Experiment Station

The preceding method will be referred to as the "standard
method" in the following discussion. Unless otherwise stated,
data presented will be those obtained on a suspension made up
of 50 ml. of soil in 100 ml. of water using a 150 ml. beaker and
instrument assembly consisting of Leeds and Northrup thin
glass electrode No. 1199-A and instrument No. 7660. There
are several factors involved in this method which need further
consideration to evaluate data obtained by it.
Effect of Air-Drying on Soil pH.-The convenience of han-
dling dry samples as compared to field-moist samples has led
to the common practice of making pH determinations on air-
dry samples. Barnette (3) found no significant effect of air-
drying on the pH of certain sandy Florida soils. Bailey (1)
concluded from an extensive study of soils of Canada and the
United States that air-drying generally lowered pH.
To obtain more information on the effect of air-drying on the
pH of Florida soils, samples were selected from 7 widely different
types upon each of which 4 different reaction levels had been
established for approximately 1 year. Twenty-eight samples,
1 from each variation of type and treatment, were used. Each
sample was analyzed by the standard method before and after
air-drying. Data and sample descriptions are presented in
Table 1.
The pH was lowered by air-drying in all 28 samples. The
maximum change was 0.41 unit and the average was 0.26 unit.
There appears to be no consistent correlation between soil type
or pH value and depression due to drying.
Variation in partial pressure of carbon dioxide is known to
affect markedly the pH of the soil (33). This is unquestionably
a variable factor in moist sois under field conditions but should
be insignificant after air-drying. Since soil pH should be as
constant as is consistent with real changes in base saturation
and other fundamental criteria of soil condition, it is probably
more desirable to use the air-dry soil method for routine analysis.
However, extended storage of air-dried samples prior to pH
determination should be avoided because it has been noted that
the apparent pH changes during storage periods. pH should
be determined both before and after air-drying in certain studies
where the detailed picture of soil reaction change is desirable.
Effect of Soil: Water Ratio.-Barnette, working with a sandy
Florida soil, also found that the dilution of the soil suspension

Soil Reaction (pH) 7


Soil Type and Ex- pH
change Capacity Sample Depression
in m.e./100 g. No. Before After Due to
Drying Drying Drying
1 5.82 5.62 0.20
Norfolk lfs 2 6.23 5.97 0.26
3.9 3 6.90 6.64 0.26
4 7.36 7.04 0.32
5 5.58 5.43 0.15
Orangeburg fsl 6 6.83 6.44 0.39
6.3 7 7.54 7.15 0.39
8 7.69 7.46 0.23
9 4.54 4.50 0.04
Leon fs 10 5.82 5.50 0.32
7.4 11 7.54 7.16 0.38
12 7.75 7.41 0.34
13 5.62 5.41 0.21
Fellowship fsl 14 7.06 6.65 0.41
11.2 15 7.35 7.02 0.33
16 7.51 7.18 0.33
17 4.64 4.47 0.17
Portsmouth fs 18 5.88 5.49 0.39
19.5 19 7.76 7.58 0.18
20 7.76 7.57 0.19
21 5.16 5.04 0.12
Bayboro fsl 22 6.42 6.11 0.31
33.3 23 7.13 6.80 0.33
24 7.60 7.42 0.18
25 3.86 3.79 0.07
Brighton peat 26 6.97 6.74 0.23
118.3 27 7.20 6.87 0.33
28 7.47 7.28 0.19

Average 0.26

"*Conventional abbreviations: 1-loam or loamy, f-fine, s-sand.-i.e. Ifs-loamy fine
sand, fsl-fine sandy loam. fs-fine sand.

in which the pH was determined had a marked effect on the
reading. A dilution of 1 to 10 of soil to water gave a reading
0.56 unit higher than a 1 to 1 dilution. Others (9, 14) have
reported that the pH continues to decrease as the water in the
ratio is decreased, even down to and below the sticky point or
soil moisture equivalent. However, Davis (11) reported that
pH values determined below a practical limit of the soil moisture
equivalent are unreliable because of residual effect of the solu-
tion in which the electrode was last placed before insertion in
the soil. It is apparently impractical to determine pH in a soil
with a moisture content below that which will give a good film
coverage of the electrode.
The determination of pH at or near the moisture equivalent

8 Florida Agricultural Experiment Station

is questionable as a practical technique for Florida sandy soils
because they do not form fluid pastes that are readily penetrated
by the electrode and they do not carry sufficient water at such
a moisture level to provide a film over the surface of the electrode
bulb. To obtain more specific information on the significance
of the determination at the dilution recommended by the stand-
ard method a series of 28 samples similar to those used in the
previous study were made up just to saturation with water, and
the pH determined with a thick glass electrode. Comparison of
pH values obtained in this manner with those obtained by the
standard method are presented in Table 2.
The standard method gave a higher pH value in 26 out of
28 instances, with a maximum difference of 0.40 unit and an
average difference of 0.14 unit. The saturated sands contained


Soil Type and Ex- Method
change Capacity Sample I Variation
in m.e./100 g. No. A B A-B
Standard Saturated
pH pH Diff.
29 5.59 5.50 0.09
Norfolk sand 30 5.95 5.95 0.00
3.3 31 6.40 6.30 0.10
32 6.24 6.17 0.07
33 5.20 5.07 0.13
Orangeburg fsl 34 5.78 5.55 0.23
6.2 335 6.07 5.93 .0.14
36 6.16 6.05 0.11
37 3.72 3.45 0.27
Leon fs 38 4.35 4.21 0.14
12.7 39 5.05 4.90 0.15
40 4.44 4.28 0.16
41 5.50 5.37 0.13
Fellowship fsl 42 5.87 5.73 0.14
12.3 43 5.75 5.62 0.13
44 6.37 6.23 0.14
45 4.02 3.77 0.25
Portsmouth fs 46 5.78 5.65 0.13
11.1 47 6.29 6.27 0.02
S 48 6.17 6.14 0.03
49 4.93 4.77 0.16
Bayboro fsl 50 5.35 5.12 0.23
18.0 51 5.82 5.75 0.07
S 52 6.02 5.93 0.09
53 3.95 3.55 0.40
Brighton peat 54 5.81 5.55 0.26
118.3 55 6.12 6.04 0.08
56 6.17 6.35 -0.18

Average variation 0.14

Soil Reaction (pH) 9


Soil Type and Ex- Method *
change Capacity Sample A B Difference
in m.e./100 g. No. Standard While A- B
Method Stirring
I pH pH
29 5.62 5.58 0.04
Norfolk sand 30 6.05 6.05 0.00
3.3 31 6.43 6.43 0.00
32 6.36 6.36 0.00
33 5.22 5.22 0.00
Orangeburg fsl 34 5.68 5.68 0.00
6.2 35 6.09 6.09 0.00
36 6.20 6.20 0.00
37 3.57 3.54 0.03
Leon fs 38 4.16 4.16 0.00
12.7 39 4.89 4.89 0.00
40 4.25 4.25 0.00
41 5.38 5.38 0.00
Fellowship fsl 42 5.72 5.74 -0.02
12.3 43 5.75 5.75 0.00
44 6.32 6.32 0.00
45 3.98 3.96 0.02
Portsmouth fs 46 5.96 5.96 0.00
11.1 47 6.40 6.42 -0.02
48 6.47 6.47 0.00
49 4.98 4.98 0.00
Bayboro fsl 50 5.35 5.38 -0.03
18.0 51 5.84 5.88 -0.04
52 6.19 6.19 0.00
S 53 3.61 3.61 0.00
Brighton peat 54 5.79 5.83 -0.04
118.3 55 6.23 6.23 0.00
56 6.26 6.28 -0.02

Average variation 0.01

Using Leeds and Northrup instrument No. 7660 and thin glass electrode No. 1199-A.

on an average 1/3 to 1/4 as much water on the dry weight basis
as was present in the standard method of determination. Un-
questionably, a certain degree of accuracy of field representation
is being sacrificed by making determinations at the higher dilu-
tion. Air-drying has a compensating effect, but there is no
indication that the compensation is consistent with the relative
magnitude of error introduced by dilution. Soil pH should be
determined at lower dilutions where special studies warrant this
Effect of Stirring the Suspension.-Bailey (2) reported a sig-
nificant effect of stirring during reading on the observed pH
of suspensions made up of certain California soils. To determine

10 Florida Agricultural Experiment Station

the effect on Florida soils a series of samples similar to those
used in the effect-of-dilution trials were made up and the pH
determined according to the standard procedure, followed im-
mediately by determination while the suspension was being
agitated at the full speed of the motor-stirrer assembly furnished
with the Leitz Electro-Titrator, using the thin glass electrode
in both instances. Data appear in Table 3.
The maximum variation between the 2 methods was 0.04 unit
and the average variation 0.01 unit. Obviously, the effect of
stirring is negligible as compared to the standard procedure
when a thin glass electrode is used.
The above comparison was repeated using Leeds and Northrup
instrument No. 7662 and thick glass electrode No. 1199-18. With
this assembly the maximum variation was 0.30 unit and the
average variation 0.08 unit. It was observed that maximum
variation in the data usually occurred with soils of low buffer
capacity. Variation of pH values of such soils determined with-
out stirring during reading from those obtained with stirring
was in the direction of what would be the residual effect of the
previous sample as modified by the distilled water rinse inter-
vening. In general, variation in samples with a pH of less than
6.5 was upward unless preceded by a sample of lower pH, in
which instance it was often downward. The greatest upward
variation was noted when a soil of low pH followed a sample
with a pH approaching neutrality. In such instances both the
previous soil and the rinse water were active in the same direc-
tion in building up a residual effect on the potential observed
for the acid soil. In other instances where the soil previously
read was of lower pH, the residual effects of the sample and
wash water were partially compensating.
A specific study of the residual effect of treatment of the
thick electrode on the apparent pH of a soil of low buffer ca-
pacity was made as follows:
Two samples of Norfolk loamy fine sand with pH values of
approximately 4.5 and 6.6 were read intermittently with inter-
vening electrode rinses of distilled water adjusted to pH values
of 4.0 and 7.0 with HC1 and NH40H, respectively. The electrodes
were pumped up and down 4 times in the soil suspension before
reading. Rinsing was accomplished by the usual procedure of
flushing the electrodes with distilled water. Suspensions were
mechanically stirred before but not after immersion of the elec-

Soil Reaction (pH) 11

trodes. The sequence of operations and resultant soil pH values
observed were as follows:

Sequence Treatment Observed pH
1 pH 4.0 rinse
2 pH 4.5 soil 4.50
3 pH 4.0 rinse -
4 pH 6.6 soil 6.00 drift to 6.60
5 pH 7.0 rinse -
6 pH 6.6 soil 6.65
7 pH 7.0 rinse --
8 pH 4.5 soil 4.60 drift to 4.53
9 pH 4.0 rinse
10 pH 4.5 soil 4.50
11 pH 7.0 rinse
12 pH 6.6 soil 6.35 drift to 6.62
13 pH 7.0 rinse --
14 pH 6.6 soil 6.65
15 pH 4.0 rinse -
16 pH 4.5 soil 4.50
17 pH 4.0 rinse --
18 pH 6.6 soil 6.15 drift to 6.66

The total drift in sequences 4 and 18 took place in approxi-
mately 2 minutes, while that in sequence 8 took place in approxi-
mately 1 minute. The drift was very rapid at first and slowed
perceptibly as the final value was approached.
Certain pecularities in the preceding data suggested that
mechanical prestirring of soils of low buffer capacity increased
the dispersion and therefore the buffer capacity to the extent
that it reduced the variation previously noted in data obtained
with and without stirring in the presence of the thick glass
electrode. To evaluate this theory, a new series of samples were
made up according to the standard procedure and mechanically
agitated with the aforementioned motor-stirrer for 30 seconds
before reading but not thereafter. The electrodes were then
pumped in the suspension 4 times and the pH read. Compara-
tive readings by thin electrode No. 1199-A and thick electrode
No. 1199-18 are presented in Table 4.
The maximum variation was 0.07 unit and average variation
only 0.03 unit. It appears that the thick electrode is less re-
sponsive to suspensions of low buffer capacity than the thin
electrode, and that the low buffer capacity soils in question are
at the critical point at which the additional dispersion by the
motor-stirrer is sufficient to overcome the factor causing the
previous variation in observed pH by the 2 electrodes.
It becomes apparent that mechanical stirring either prior to
or during the reading of the pH of low buffer capacity soils by

12 Florida Agricultural Experiment Station


Sample A B Difference
Soil Type No. Thin Thick A B
Electrode Electrode
pH pH

29** 5.69 5.70 -0.01
Norfolk sand 30** 6.19 6.16 0.03
31** 6.36 6.33 0.03
32** 6.36 6.35 0.01
33** 5.25 5.29 -0.04
Orangeburg fsl 34** 5.75 5.74 0.01
35** 6.16 6.14 0.02
36** 6.27 6.22 0.05
57 3.70 3.73 -0.03
Leon fs 38** 4.33 4.29 0.04
39** 5.05 5.07 -0.02
40** 4.39 4.35 0.04
58 5.59 5.54 0.05
Fellowship fsl 42** 5.75 5.70 0.05
43** 5.84 5.83 0.01
44** 6.37 6.37 0.00
45** 4.07 4.00 0.07
Portsmouth fs 46** 6.02 6.02 0.00
47** 6.54 6.51 0.03
48** 6.56 6.50 0.06
49** 4.96 4.92 0.04
Bayboro fsl 50** 5.35 5.30 0.05
51** 5.89 5.87 0.02
_52** 6.17 6.19 -0.02
S 59 3.65 3.62 0.03
Brighton peat 60 6.16 6.15 0.01
55** 6.37 6.43 -0.06
56** 6.29 6.35 -0.06

Average variation 0.03

Mechanically prestirred 1/ minute and pH read without stirring after insertion of
** Same soil sample as used in Table 3 but not identical suspension.

a thick glass electrode is desirable for Florida soils. It is also
apparent that the coarse fraction of soils need not be in suspen-
sion to develop the true potential of the suspension. This is
further borne out by the fact that no significant drift was ever
observed as a result of settling out of coarse separates in the
presence of either the thin or thick glass electrode when the
motor-stirrer was stopped after reading the pH with it in action.
The results obtained do not support the conclusions drawn
by Bailey. It is logical to assume that separates so coarse as
to settle out during the short interval while the reading is made

Soil Reaction (pH) 13

would not have a significant effect on the buffer capacity of the
suspension unless they consisted of aggregates or floccules. The
response to stirring in the presence of thick glass electrodes is
evidently the speeding up of drift to equilibrium and the de-
velopment of a more highly buffered suspension. Soils may
vary considerably in the ease with which they are dispersed
(27). This is undoubtedly a complicating factor in comparing
data obtained on soils from widely different climatic regions.
Colorimetric Methods as Compared to the Standard Method.-
Colorimetric methods using indicator dyes for the estimation of
soil pH have been widely used in connection with field kits de-
signed for qualitative estimation of soil fertility. Carrigan (7),


Soil Type and Standard Hellige Method
Exchange Sample Method B C Variationi Variation
Capacity in No. Analyst Analyst A B A-C
m.e./100 g. A No. 1 No. 2__
SpH pH pH
29 5.59 5.20 5.90 0.39 -0.31
Norfolk sand 30 5.95 5.70 6.30 0.25 -0.35
3.3 31 6.40 6.20 6.50 0.20 -0.10
32 6.24 6.00 6.70 0.24 -0.46
33 5.20 5.00 6.00 0.20 -0.80
Orangeburg fsl 34 5.78 5.70 6.10 0.08 -0.32
6.2 35 6.07 5.90 6.40 0.17 -0.33
36 6.16 6.10 6.40 0.06 -0.24
37 3.72 4.00 4.00 -0.28 -0.28
Leon fs 38 4.35 4.30 4.30 0.05 0.05
12.7 39 5.05 5.00 5.00 0.05 0.05
40 4.44 4.50 4.00 -0.06 0.44
41 5.50 5.50 5.30 0.00 0.20
Fellowship fsl 42 5.87 5.50 6.00 0.37 -0.13
12.3 43 5.75 5.70 6.30 0.05 -0.55
44 6.37 6.20 6.60 0.17 -0.23
S 45 4.02 4.30 4.10 -0.28 -0.08
Portsmouth fs 46 5.78 5.70 6.00 0.08 0.22
11.1 47 6.29 6.20 6.50 0.09 -0.21
48 6.17 6.30 7.00 -0.13 -0.83
49 4.93 4.80 4.90 0.13 0.03
Bayboro fsl 50 5.35 5.10 5.20 0.25 0.15
18.0 51 5.82 5.50 6.00 0.32 -0.18
52 6.02 5.80 6.90 0.22 -0.88
53 3.95 4.20 4.00 -0.25 -0.05
Brighton peat 54 5.81 5.50 6.00 0.31 -0.19
118.3 55 6.12 6.00 6.50 0.12 -0.38
56 6.17 6.00 6.60 0.17 -0.43
variation I I 0.18 0.30

14 Florida Agricultural Experiment Station

working with Florida soils and using the LaMotte-Morgan soil
testing kit, found by comparison to data obtained with the glass
electrode that there was considerable difference in the ability
of 3 different analysts to estimate the soil pH. Allowing plus
or minus 0.4 pH unit as an arbitrary unit of variation from the
standard method, it was found that analyst A was within this
error 99 percent of the time; analyst B, 87 percent of the time;
and analyst C, 88 percent of the time.
The "Hellige" soil test kit method uses a somewhat different
technique in bringing out the indicator color, which it was
thought might improve the readability and accuracy. To deter-
mine this, a series of 28 soils was selected for comparative tests
by 2 analysts. Data are presented in Table 5. Analyst No. 1
had considerable experience with this method and all readings
were within 0.4 unit of the glass electrode value. Analyst No. 2,
who had never used the method before but had considerable
experience with matching of colors, had 75 percent of his read-
ings within 0.4 unit of the glass electrode value.
The colorimetric method of _Hp determination a_ arently is
not to be-recommended for work where errors of over 0.4 unit
are, undesirable, unless limited to use by operators wlho have
had the opportunity to familiarize themselves. with-the- tech-
nique on samples of known p Hand have shown t enessary
color sensitivity for the work in question. The colorimetric
determination of pH by filtration procedures or on high-dilution
suspensions is not to be recommended for Florida soils.

The control of soil pH is feasible and desirable under certain
Florida conditions. It has been found to be particularly im-
portant on sandy soils of low exchange capacity. Sims and
Volk (25) have shown that it plays an important part in the
control of cyanamide toxicity to citrus in Norfolk loamy fine
sand following an application of a carrier of this form of nitro-
gen. Jamison (15), and Volk, Borda and Allison (29) have
presented data to show that appreciable quantities of lime must
be used in connection with citrus fertilization to maintain pH.
Peech (28) has presented data to show that sulfur spray residues
have a significant effect on the soil pH under citrus trees. Volk,
Willson and Blaser (30) have presented data showing the com-
position and growth response of White Dutch clover to lime in-
corporation and resultant pH change in Leon fine sand.

Soil Reaction (pH) 15


i8 Soil Type Exch.Cap.
"m.e./oog. /
A Brighton peat 118.3
B Portsmouth fs 19.5 /
16 C Bayboro fsl 33.5
D Leon fs 7.4
E Fellowship fsl 11.2 /
F Orangeburg fsl 6.3 /
14 G Norfolk Ifs 3.9

1O -

S/ /


4 /




4.0 4.5 5.0 5.5 60 6.5 70 75
soil pH
Fig. 1.-Apparent lime requirement per acre-6-inches, using precipitated
chalk (CaCO3 powd.) in 4-gallon lysimeters under natural precipitation.-
Solid lines are the approximate linear relationship up to pH 6.5. Dotted
lines connect actual experimental points shown by symbols.

Factors in the Adjustment of pH.-The term "lime require-
ment" has been commonly used in the past to denote the amount
of lime necessary to raise the pH of a given soil to a point near
neutrality. However, it is more desirable in view of liming

16 Florida Agricultural Experiment Station

recommendations for Florida conditions to let it signify the
amount of lime necessary to raise the pH a stated amount from
any given point on the pH scale. It is to be expected that soils
of high base exchange capacity will require more lime per incre-
ment of pH change than will soils of low exchange capacity.
The work of Mehlich (22) suggests that the relationship will
not follow a fixed formula for all soils of equal base exchange
capacity. He shows that various soils exhibit characteristics
which are probably the sum effect of specific characteristics of
the various materials which go to make up the exchange complex.
It would be highly desirable to know the basic relationship
between calcium carbonate and pH of typical Florida soils under
conditions that are relatively free from factors such as fineness
of division of the lime, incomplete mixing with the soil and the
effect of oscillation of moisture between the surface and subsoil.
An attempt to obtain this type of information was made by
setting up an experiment which consisted of placing representa-
tive soils in 4-gallon lysimeters and treating them with 4 rates of
precipitated chalk. The lysimeters were set up in quadruplicate
out of doors and buried nearly to ground level. Soils were placed
in the lysimeters by profile to 9 inches depth and the chalk was
incorporated in the surface 6 inches. Suitable outlets were at-
tached for the conduction of drainage to a trench where it was
collected for related studies. The reaction was allowed to take
place under natural precipitation.
Periodic soil pH determinations showed that the maximum
pH was reached within 30 days. From that time on it remained
fairly constant for the following 160 days. Data and soil de-
scriptions are presented in Fig. 1. The 4 lowest values for any
given soil are the averages of determinations made during the
160-day period after maximum pH had been reached following
initial treatment. The values above these were determined 83
days after the second treatment. The data show that a direct
arithmetical ratio between pH and chalk added is approximated
for any given soil between the initial pH and pH 6.5.
The approximate quantities of chalk necessary to bring about
pH changes of 1.0 unit for the various soils in the range below
6.5 are presented in Table 6, column IV. Column I shows ex-
change capacity determined by the ammonium acetate procedure
as used by Peech (24) for Florida soils. Column III is the ex-
pression of these values as CaCO, equivalent per acre 6 inches
on the actual field volume weight. The CaCO3 equivalent is, in

Soil Reaction (pH) 17

Base Ex-
I change IV *
Base II Capacity Lime Re-
Soil Exchange Tons Soil Expressed quirement
Capacity per Acre as CaCOs per 1.0 pH
m.e./100 6 inches Equivalent lbs. per Acre
grams lbs. per Acre 6 inches
6 inches

Norfolk Ifs 3.9 1,040 4,060 3,200
Orangeburg fsl 6.3 951 5,990 5,100
Fellowship fsl 11.2 943 10,560 5,300
Leon fs 7.4 838 6,200 4,100
Bayboro fsl 33.5 420 14,070 9,500
Portsmouth fs 19.5: 615 11,990 15,200
Brighton peat 118.3 13 T3 2700

From Fig. 1.

most instances, 1/2 or less of the lime requirement as estimated
from the requirement per 1 pH unit given in column IV. The
discrepancy probably results primarily from incomplete reaction 5
of the chalk added. This factor is emphasized in a following --
presentation of field plot data. The apparent base exchange J
capacity also may be somewhat in error. The method used by
Peech has been found to be from 7 to 28 percent low as compared
to calcium acetate and barium acetate methods used on these
same soils.3
Theoretically, the rate of reaction of any given materiaLwyith
soil will be influenced by the fineness of grinding, or more
specifically stated, by the surface area of particle. apar.. nit
weight of material. Materials as reactive as precipitated chalk
are not widely used in actual field practice. The materials most
comnmoaly used. in Florida are ground calcic or dolomitic lime-
sto Screen analysis 4 of samples from 2 local sources are as
Standard Ocala Agricultural Lime
Passing 100 mesh -............... 57 percent
Passing 60 mesh .....--......... 69 percent
Passing 20 mesh ............. 91 percent
(CaCO3 equivalent ............... 97.5 percent)
Dolomitic Agricultural Lime
Passing 100 mesh ............. 84 percent
Passing 60 mesh ......-........ 89 percent
Passing 20 mesh ................ 96 percent
(CaCO3 equivalent ..................101.5 percent)
Unreported data by L. E. Ensminger, Fla. Agr. Exp. Sta.
Wet washed through sieves, oven-dry basis.

18 Florida Agricultural Experiment Station

The type of relationship between added increments of the
Ocala limestone listed above and pH was determined by estab-
lishing plots on virgin soils somewhat similar to those used in

Soil Type Exch. Cap.
12 m.e./loo g.
A Brighton peat 118.3
B Leon fs 12.7
C, Porstrouth fs 11.1
D Bayboro fsl 18.0
E Fellowship fsl 12.3
S F Orangeburg fsl 6.2
G Norfolk sand 3.3



4.0 4.5 50 55 6.0 6.5
scil pH
Fig. 2.--Apparent lime requirement, using agricultural limestone incor-
porated in the surface 6 inches of field plots.

Soil Reaction (pH) 19

the lysimeters. Four rates of Ocala lime were incorporated in
the surface 6 inches of soil and pH determinations were made
periodically for 1 year following application. In general, maxi-
mum pH was 21 to 34 weeks after treatment. The
effect of the various rates of limestone on the average soil pH
between the time maximum pH was reached and the end of the
first year are presented in Fig. 2.
It appears that a given amount of agricultural limestone is
relatively less effective than precipitated chalk in changing the
pH as neutrality is approached. This is probably the result of
coarseness in the ground limestone which would tend to reduce
activity as neutrality was approached.
As previously mentioned, apparent field lime requirement is
not in keeping with what would be expected from base exchange
capacity of a given soil. An attempt to determine the reason
for the discrepancy was made as follows:
Titrable acidity was determined on check plots of 4 of the
soils from the field lime requirement plots by a modification 5 of
the Dunn (12) method of laboratory estimation of lime require-
ment. Unreacted CaCO3 was determined at the end of the 12-
month field trial where lime had been added. The data were
plotted against pH and the calculated difference between field
lime requirement and titrable acidity at pH 6.0 compared to un-
reacted lime at this pH. Data appear in Table 7.
I II** Unreacted
Lime Req. by Lime Req. by III CaCO3 After
Soil Type Field Trial, Ca(OH)2 Difference, 12 Months in
Pounds Titration, Col. I Minus Feld Trial,
per Acre Pounds Col. II Pounds
_per Acre per Acre
Fellowship fsl 1,600 700 900 1,170
Portsmouth fs 12,740. 5,040 7,740 9,678
Brighton peat 22,820 12,120 10,700 12,789
Bayboro fsl 5,220 1,750 3,470 2,972

Using Ocala agricultural limestone.
** Using modified Dunn technique.
It is.evident that unreacted CaCO3 is the major factor con-
tributing to the discrepancy between titrable acidity and appar-

Values above the limit of the Dunn method were determined by supple-
mentary addition of CaO. Data are calculated on field volume weight basis.

20 Florida Agricultural Experiment Station

ent lime requirement. The foregoing data bring to attention a
very significant factor which must be considered in estimating
lime requirement using a given material. The completeness of
reaction depends upon the' degree of heterogeneity under which
the reacion-mu.ttake place. This is determined by initial in-


Treatment Soil pH Inches of Equiva-
Soil Type and lbs./acre After Water lent of
Exchange Capacity Treat- Passing Calcium
CaCO3 Sulfur ment Lysi- Lost **
meter** lbs./acre

0 4.65 13.8 59
Leon fs 1,000 4.90 14.6 57
7.4 m.e./100 g. 3,400 5.48 11.4 43
S5,200 5.93 11.7 75

0 0 5.57 9.1 179
Fellowship fsl 1,500 0 5.81 9.1 190
11.2 m.e./100 g. 2,500 0 6.02 8.7 206
3,500 0 6.21 9.0 228
S 0 500 5.24 9.4 754
0 833 5.07 9.1 963
0 1,166 4.89 9.0 1,318

0 4.87 10.8 46
Portsmouth fs i 3,500 5.08 11.9 89
19.5 m.e./100 g. 8,900 5.49 11.8 287
15,000 _5.84 11.5 250

0 i 5.20 7.4 712
Bayboro fsl 2,200 5.45 6.6 700
33.5 m.e./100 g. 4,600 5.71 6.7 771
.____5__e0 8,000 6.04 7.3 788

0 5.63 8.7 139
Orangeburg fsl 1,000 5.84 8.7 146
6.3 m.e./100 g. 1,900 6.00 8.7 173
______e I 3,200 6.23 8.0 188

0 0 5.68 11.5 327
Norfolk Ifs 500 0 5.84 11.2 334
3.9 m.e./100 g. 1,200 0 6.10 10.5 321
2,000 0 6.33 11.9 435
0 400 5.14 10.6 796
_0 666 4.72 11.2 985

0 4.14 9.4 268
Brighton peat 6,000 4.46 10.1 569
118.3 m.e./100 g. 12,000 5.00 11.2 519
18,000 5.48 9.7 620
4-gallon lysimeters filled with soil by profile to 9 inches depth and treatment incor-
porated in top 6 inches.
** Over period of 190 days following treatment.

Soil Reaction (pH) 21

corporation, subsequent tillage and the time element. Labora-
tory methods of estimation of lime requireme-nTare undoubtedly
of help in arriving at a logical value based on titrable acidity,
but such a figure can lead to serious error if the factors deter-
mining heterogeneity of incorporation and consequently the rate
of reaction under various cultural practices are not evaluated
and taken into consideration.
Factors in the Maintenance of pH.-Maintenance of soil pH
after the major initial adjustment is made is particularly- neces-
saryo rtda'-sandy soils of low base exchange--capacity.
Base retention capacity is low and the percent base saturation
is rapidly reduced by plant feeding and the loss of bases by leach-
ing. Loss of bases as bicarbonates or in combination with or-
ganic acids may be significant but is probably secondary in im-
portance to losses due to combination with acid fertilizer residues,
The rate of movement of lime in the soil is brought out by the
following lysimeter trials and lime penetration studies. The
lysimeter studies were carried on in connection with the setup
previously described, supplemented by additional lysimeters
filled with the Norfolk and Fellowship soils treated with sulfur.
The treatments were incorporated in the 0 to 6-inch depth in
quantities given in Table 8. Leachates were collected from the
lysimeters over a period of 190 days following treatment and
analyzed for calcium. Data are presented in Table 8.
The amount of calcium recovered in the drainage was not in
direct proportion to the amount of CaC03 added. Variation was
evidently associated with some characteristic of the soil that is
not readily apparent. The maximum calcium loss from untreated
soil was 712 pounds of CaC03 equivalent per acre from the Bay-
boro soil, while the maximum percent loss of added CaC03 after
deduction of loss from the untreated soil was 5 percent where
6,000 pounds of chalk were added to the Brighton peat.
It will be recalled that the lime was not incorporated in the
bottom 3 inches of soil in the lysimeters. Lime that moved as
bicarbonate could have been absorbed by this horizon in the gnore
aciaoils. Free movement of lime as the bicarbonate would be re-
tarded until the pH of the horizon through which it must pass
approached a point that was in equilibrium with the bicarbonate
at the existing partial CO2 pressure in the soil.
This assumption is supported by rate of lime penetration
trials established in conjunction with the field plot trials pre-
viously described. Treatments consisted of surface applications

22 Florida Agricultural Experiment Station


Increase in pH tWeeks after
Soil pH, Treatment
Soil Type and Sam- Un- Incor-
CaCO3 pling treated Surface Application porated
Equivalent Depth, Check -_ 0-6 in.
Rate per Acre Inches Plots Ocala Ocala
Lime- Dolo- Lime Lime-
stone mite Hydrate stone

0-1 4.06 1.57 158 2.15
1 /-11/2 4.06 0.37 0.43 0.34
Leon fs 1-2 4.07 0.37 0.10 0.17 1.41
6,000 lbs. 2-4 4.12 0.13 0.00 0.04
4-6 4.33 0.20 0.00 0.16

0-1 5.66 1.05 1.01 1.16
2-11/2 5.56 0.56 0.65 0.24
Fellowship fsl 1-2 5.46 0.31 0.38 0.43 0.69
4,025 lbs. 2-4 5.54 0.03 0.14 0.10
4-6 5.59 0.05 0.05 0.02

0-1 4.36 1.84 1.76 2.30
',-11/2 4.33 0.22 0.35 0.24
Portsmouth fs 1-2 4.30 0.21 0.25 0.42 1.72
17,250 lbs. 2-4 4.51 0.00 0.02 0.28
4-6 4.78 0.15 0.35 0.38

0-1 5.07 1.87 1.69 1.57
1/-1/2 5.04 1.30 0.98 1.21
Bayboro fsl 1-2 5.00 0.51 0.55 0.75 1.09
9,200 lbs. 2-4 5.00 0.09 I 0.06 0.16
4-6 5.07 0.04 0.09

0-1 5.94 1.04 0.91 1.19
-1% 5.89 0.65 0.53 0.78
Orangeburg fs] 1-2 5.84 0.31 0.20 0.32 .54
3,680 lbs. 2-4 5.77 0.15 0.00 0.21
4-6 5.77 0.23 0.00 0.18

0-1 5.71 1.32 1.03 1.97
/ -12 5.82 0.47 0.42 0.70
Norfolk sand 1-2 5.92 0.23 0.19 0.83 .42
2,300 lbs. 2-4 5.90 0.04 0.00 0.32
4-6 5.79 0.10 0.08 0.25

0-1 4.26 2.09 2.04 2.50
S-1 % 4.08 0.06 0.23 0.09
Brighton peat 1-2 3.89 0.37 0.41 0.41 2.44
27,800 lbs. 2-4 3.97 0.02 0.07 0.06
4-6 3.73 0.27 0.42 0.21 |
1_ (

Soil Reaction (pH) 23

of calcic limestone, dolomitic limestone and lime hydrate. Rates
of various materials were equal for any given soil on a CaCO.
equivalent basis. However, rates varied for different soils, de-
pending upon estimated lime requirement. Treatments and data
are presented in Table 9.
The penetration cf lime as measured by soil pH change was
remarkably low in atl instances. It was least in soils of low pH,
regardless of otherc.haracteristics. There was no consistent
difference in rate of penetration of the different materials into
the various soils. Penetration below the 2-inch depth during
the period of time involved was negligible in all soils. These
findings are T-n ageenent with those of others (6, 19).
The effect of acidic residues consisting of strong anions is
brought out by the data on the sulfur-treated Fellowship and
Norfolk soils presented in Table 8. The reduction of pH in the
Fellowship soil from 5.57 to 5.24 by the addition of 500 pounds
of sulfur resulted in an crease in the leaching of calcium as
CaC03 equivalent from 179 pounds per acre to 754 pounds per
acre, or 321 percent. Reduction in pH of the Norfolk soil from
5.68 to 5.14 by the addition of .400 pounds of sulfur increased
the leaching from 327 pounds up to 796 pounds, or 143 percent.
Addition of CaCO, to the. Norfolk and Fellowship soils did not
materially increase the loss of calcium from them. It appears
that acidic fertilizer residues would be a major contributing
factor in calcium loss from theseis.

There are certain general relationships .between soil pH, per-
cent base saturation and nutrient retentio0.wi#bj are applicable
to all soils. It is quite universally recognized that base retention
power at a given pH is generally higher in soils of high exchange
capacity than in soils of low exchange capacjty_and that the
amount of bases held in exchangeable form in a given soil is
greater in the slightly acid range than in the irJonA6 id range.
Mehlich has shown that different soils have individual char-
acteristics which may affect the relationship between pH and
percent saturation in different ways. The relationship in the
system may be further complicated by partial saturation of
certain base exchange constituents by organic compounds as
suggested by the findings of Ensminger and Gieseking (13).
pH and Percent Base Saturation.-The importance of main-

24 Florida Agricultural Experiment Station


"S O





"20 -40

4 5 6 7
soil pH
Fig. 3.-Correlation of pH with apparent percent base saturation of 2
Florida soils.

training percent base saturation at as high a level as possible
in keeping with other factors in light sandy soils of Florida has
been suggested by Peech (24). Using an ammonium acetate
extraction, he obtained survey data on 194 surface and subsoil
samples representing 9 Florida soil types which indicated that
there was a general correlation between pH and the average
percent base saturation of all of the soils taken as a group.

Soil Reaction (pH) 25

However, dispersion in his data is such that the theoretical curve
presented would lead to serious error if applied to specific soils.
This is to be expected from the findings of Mehlich, which
showed that the base exchange complex may differ considerably
with different soils.
To obtain further information on the relationship of pH to
apparent percent base saturation determined by ammonium
acetate extraction, samples were taken from the field plots upon
which the reaction levels were established in the previous work
on lime requirement. Total bases were determined by the
method proposed by Bray and Willhite (5) and exchange ca-
pacity by the method used by Peech. The data on the 2 soils
showing the greatest variation are presented graphically in
Fig. 3.
It is evident that the correlation between pH and percent base
saturation determined by simple extraction with ammonium
acetate without proper consideration of the specific nature of
the soil and the ammonium acetate solubility of the free lime
it may contain is of little significance. For example, Portsmouth
fine sand shows a base saturation of approximately 15 percent
at pH 5.3, while Leon fine sand shows a base saturation of ap-
proximately 67 percent at this pH. This differential relationship
is also suggested by the data previously presented in Figs. 1
and 2.
It appears that further investigation is necessary to deter-
mine the significance of pH in terms of base saturation of Flor-
ida soils.
pH and the Retention of Potassium and Ammonia.-Many
Florida soils have reasonably high base exchange capacities.
Preliminary studies indicated that there was a real possibility
that base retention by such soils, even if quite acid, would be
sufficient to justify considerable liberalization in the pH recom-
mendations for them.
A study was conducted with the lysimeter setup previously
described, in an attempt to classify soils in this respect. The
same soils as brought to various pH levels by the lime require-
ment study were used. In addition, lysimeters were set up
using the Norfolk and Fellowship soils treated with sulfur to
(obtain values in the low pH range for these soils. The sulfur
was allowed to react and leaching to be carried on under natural
precipitation of 307 days prior to the base retention trials.
To test the relative retention power at the various pH levels,

26 Florida Agricultural Experiment Station

ammonium nitrate and potassium sulfate at the rates of 570 and
370 pounds per acre, respectively, were applied in 1 liter of water
per lysimeter. Leaching was begun 24 hours later by the addi-
tion of 1/2-inch increments of water at a rate that delivered a
Soil Type .xch ,
Norfolk Ifs 3.9
---*-Orangeburg fsl 6.3
40- -- Leon fs 7.4
S--. Fellowship fsl 11.2
Portsmouth fs 19.5
S...*.. **. Bayboro fsl 33.3
35 -----Brighton Peat 118.3




Fig. 4.-Effect of soil pH on retention of ammonia against leaching.
15- "

0 ....................A-............... ....... _
3 4 5 6 7 8
soil pH
Fig. 4.-Effect of soil pH on retention of ammonia against leaching.
Treatment: 9-inch soil profile in 4-gallon lysimeters treated with NH4NOa
and K=SO4 in solution at rates of 570 and 370 pounds per acre, respectively,
and leached 36 hours to deliver 2% inches of leachate.

Soil Reaction (pH) 27

total of 21/2 inches of leachate in 36 hours. The leachate was
analyzed for ammonia and potassium. Data are presented
graphically in Figs. 4 and 5. Each experimental point represents
the average of data from lysimeters set up in duplicate.
Soil Type Exch.Cap.
m.e./ioo g.
-w-x-x-Norfolk Ifs 3.9
40 ---Leon fs 7.4
---- Fellowship fsl 11.2
----- Portsmouth fs 19.5
................ Bayboro fsl 33.3
35 -----Brighton Peat 118.3
------ Orangeburg fsl 6.3

-0 30 -


E25 -
n \
20 ,
C +

a \



o......... ..... .. ...... 0
3 4 5 6 7 8
soil pH
Fig. 5.-Effect of soil pH on the retention of potassium against leaching.
Treatment: 9-inch soil profile in 4-gallon lysimeters treated with NHWNO0
and KMSO in solution at rates of 570 and 370 pounds per acre, respectively,
and leached 36 hours to deliver 21 inches of leachate.

28 Florida Agricultural Experiment Station

There appears to be a marked positive correlation between soil
pH and retentive power for ammonia and potassium in the acid
range of Norfolk loamy fine sand. Average leaching loss of the
2 bases was 21/2 times as much at pH 5.5 and 41/2 times as much
at 4.0 as it was at 6.8. The retention of bases by the soils of
high exchange capacity is remarkable, even at relatively low
pH levels.
The real significance of leaching data obtained under the arbi-
trary conditions of the preceding type of setup is difficult to
determine because leaching losses are so dependent on cultural
and fertilizer practices and on the intensity and distribution of
rainfall. Unreported work at Gainesville shows that approxi-
mately 16 inches of a total annual precipitation of 43 inches
passed through lysimeters of 4-foot depth filled with Norfolk
loamy fine sand topsoil, under a cultural practice in which the
soil was covered with volunteer vegetation. Other data on the
same lysimeters filled with Norfolk loamy fine sand by profile

40 ,0'

ao 0- \

20- ----


4 5 6 7 8
soil pH
Fig. 6.-Effect of soil pH on relative solubility of surface and subsoil
phosphorus in Norfolk Ifs extracted 1 to 40 with .002N HUSO, buffered
to pH 3.0.

CaCO Exchange Total %
Treatment Depth pH Equiv., % Capacity Phosphorus, Sesquioxides %
No. ppm O.M. m.e./100 g. ppm* as Phosphate** FeOa3**

1 0-6" 4.08 0....... 0.72 2.60 352 1.14 0.094
Sulfur 8-12" 4.08 ..... 0.27 1.75 339 1.37 0.098

2 0-6" 4.17 .......- 0.70 2.52 406 1.21 0.098
Sulfur 8-12" 4.27 ........ 0.24 1.64 298 1.45 0.096

3 0-6" 5.19 ........ 0.65 2.37 373 1.42 0.105
Check 8-12" 5.43 ....... 0.30 1.78 260 1.50 0.106

4 0-6" 6.37 380 0.65 2.35 382 1.37 0.082
Lime 8-12" 6.20 80 0.28 1.75 252 1.39 0.089

5 0-6" 7.12 600 0.67 2.63 407 1.35 0.074
Lime 8-12" 6.63 180 0.26 1.76 272 1.34 0.081

6 0-6" 7.75 2,190 0.67 3.48 407 1.30 0.084
Lime 8-12" 7.30 100 0.28 1.74 265 1.34 0.079

By digestion in hot concentrated HCIO4 until organic matter is destroyed.
** Soluble in hot 2 normal HCL.

30 Florida Agricultural Experiment Station

show that the drainage over short periods was 76 percent higher
from fallow soils than from those carrying a crop of turnips at
field spacing. Transpiration by the turnips when they had at-
tained a heavy top growth during the first 2 weeks of March
exceeded the equivalent of 1 inch of precipitation. Voll.l(32)
and Jones (16) have-also obtained data which emphasize the value
of cover crops in reducing. the leaching of nutrients-from soils
of the Southeast. Theyalue of actively growing,.-egetation in
the reduction of leaching, both by reduction of gravitational
water and by absorption of nutrients into the plant, must not
be underestimated.
There is little doubt that maintaining soil pH at as high a
level as is consistent with other factors would be beneficial in
retarding loss of bases from sandy soils of low exchange ca-
pacity, especially during periods of heavy precipitation imme-
diately after fertilization. The average percent saturation by
a given base under such a practice would undoubtedly be in-
creased. The effect would not be cumulative from year to year
to any great extent in light Florida soils of low exchange ca-
pacity because of their low total capacity for base retention.
This is brought out by the work of Kime (18) on potash residuals
in Norfolk sands.

Relatively little work has been reported on the solubility of
phosphorus in Florida soils. That high quantities of readily
soluble phosphorus do exist is shown by the work of Peech (24).
Unquestionably, soil pH has an important role in the relative
solubility of phosphatic residuals in various extracting solutions
and in phosphorus availability to plants.
Considerable data have been presented to show that phos-
phorus may be retainedj.n the mineral form irlnth in com-
bination with calcium, magnesium, iron and aluminum (8, 10,
17, 20, 23). Thereis also evidence that it is adsorbed in some
manner _hy-aolinitic and montmorillonitic clays, but the actual
significance of retention in this manner is u-fiiest aEle. Dif-
ferentiation between fixation by those materials and by the for-
eign iron and aluminum compounds they may contain is difficult.
A rapid reversion of primary phosphate to some or all of the
more insoluble forms following its application to the soil prob-
ably takes place. Subsequently the fixation continues by recon-
version of the more soluble of these forms to the more insoluble

Soil Reaction (pH) 31

forms. The rate and extent to which this secondary reversion
takes place depend upon the characteristics of the soil. Iron and
aluminum fixation tend to increase as pH drops, especially below
5.5 to 6.0, while calcium and magnesium fixation in various forms
increases as pH rises above this point.
An opportunity to obtain data on the effect of soil pH and
lime on the movement and solubility of phosphorus residuals in
Norfolk loamy fine sand was presented by a series of plant adapt-
ability plots maintained at various pH levels by applications of
lime or sulfur for a period of 17 years in the vicinity of Gaines-
ville. Otherwise moderate uniform fertilization was practiced
during this period. No amendments were added during the year
prior to the following study. Partial analysis of samples from
these plots is presented in Table 10 and Figs. 6, 7 and 8.


0 1
4 5 6 7 8
soil pH
Fig 7.-Effect of pH on the relative solubility of surface and subsoil phos-
phorus in Norfolk ifs extracted 1 to 5 with water. some indication that phosphorus tended to move more
free lyin-the_ extremely acid range than in the neutral range.
This might be attributed, in part at least, to movement in col-
loidal form as a result of the dispersing effect of the Jhigh
ac idity. There was evidence of the effect of acidity on disper-
sion in that the colloids of the top 1/2-inch of surface of soils in
the highly acid range moved down and redeposited at slightly

32 Florida Agricultural Experiment Station


3.0 /

I2/ O

.s /


soil pH

Fig 8.--Effect of soil pH on the relative solubility of surface and subsoil
phosphorus extracted 1 to 5 with CO/-saturated water.

below this horizon, leaving a top cover of relatively light-colored
sand following periods of non-cultivation.
The organic matter levels in the 0 to 6-inch surface and 8 to
12-inch subsoil under the different pH levels are quite uniform.
Exchange capacity shows a slight minimum in the range of pH
5.0Ctoj&.ALin-th4-srface soil. There is a significant differential
accumufation of lime between surface and subsoils where lime
The effect of soil reaction on the solubility of residual phos-

Soil Reaction (pH) 33

phorus of the surface and subsoil at different pH levels is
brought out by Figs. 6, 7 and 8. These data show that there
is a significant difference in relative solubility as determined by
3 different methods:
1. 0.002N H2S04 buffered to pH 3.0 as recommended by
Truog (26), with the exception that the extraction was
made by shaking 1 hour at a dilution of 5:200.
2. Water extraction by shaking 1/ hour at a soil :water
dilution of 1:5.
3. Carbonic-acid extraction made by shaking a soil :water
dilution of 1:5 under 1 atmosphere pressure of C02 for
1/ hour as used by McGeorge (21).

Fig. shows the relative solubility of surface and subsoil
phosphorus in 0.002N H2S04. Equal solubility exists at approxi-
nitely _pH 5.3. Below this point subsoil phosphorus is the more
soluble, while above this point surface-soil phosphorus is the
more soluble. Extrapolation of the curves between experimental
points would probably show a minimum solubility of both surface
and subsoil phosphorus at or below pH 5.0.
Fig. 7 shows the relative water-solubility of surface and sub-
soil phosphorus. In this case solubility is approximately equal
at pH 5.7. Below this point the surface-soil phosphorus is the
more soluble and above this point the subsoil phosphorus is the
more soluble. Surface soil phosphorus reaches a minimum solu-
bility at about 6.3 and subsoil phosphorus at about 4.3.
Fig. 8 shows the relative solubility of surface and subsoil
phosphorus in the carbonic-acid solution. In this instance the
surface and subsoil phosphorus are equally soluble at about pH
5.5. Below this point the surface-soil phosphorus is the more
soluble and above this point the subsoil phosphorus is the more
soluble. Extrapolation of the curves between experimental points
would probably show a minimum solubility of both surface and
subsoil phosphorus between pH 4.3 and 5.0.
It is of importance to note that the order of solubility of sur-
face and subsoil phosphorus in the 0.002N H2S04 is the reverse
of the order of solubility in water or carbonic-acid solution, and
that the points of equal solubility of surface and subsoil phos-
phorus in any given 1 of the 3 reagents is within the range pH
5.3 to 5.7. Above and below this range the differential char-

34 Florida Agricultural Experiment Station



2.0 /

0 ,
0] /

"5 /

acteristics of solubility of surface and subsoil phosphorus result-

ing from differential treatment are apparent.
From the data in Figs. 7 and 8 it appears that water-soluble
and carbonic-acid-soluble phosphorus hold a fairly constant re-
lationship to each other for the surface and subsoil at any given
pH. Fig. 9 shows this relationship as ratio of carbonic-acid-
o /

soluble phosphorus to water-soluble phosphorus. The relation-

cship is remarkable in that surface and subsoil phosphorus are

known to have differenatment characteristics of solubility. This sug-
gests that the solutions react similarly, although at different
relationshipp to each other for the surface and subsoil at any given

soluble phosphorus to water-soluble phosphorus. The relation-
ship is remarkable in that surface and subsoil phosphorus are
known to have different characteristics of solubility. This sug-
gests that the solutions react similarly, although at different
efficiency, as solubilizing agents of soil phosphates. Relative
solubility in carbonic acid and water is approximately equal at
pH 5.6. Above this point the solubility in carbonic acid is con-

Soil Reaction (pH) 35

siderably higher, while below this point the solubility in water
is slightly higher.
Free lime, or the effect of free lime during phosphorus conver-
sion, apparently promotes the relative solubility of phosphorus in
0.002N H2S04, as compared to its effect on solubility in water
or carbonic acid for any given pH where lime was applied. This
is brought out by Figs. 10 and 11. Where free lime is low, car-
bonic acid is relatively far more responsive to pH rise than either
water or H2S04 as a solvent.
The difference between the quantity of sesquioxides in the
surface soil and that in the subsoil in the moderate to highly
acid range probably accounts in part for the relative differential
solubility of phosphorus in water or carbonic acid as compared
to the solubility in the dilute sulfuric acid. It is interesting to
note that the differential is not in iron content but rather in
remaining sesquioxide metals, of which aluminum should be
From the preceding it is obvious that the relationship between

8 %

o 4_

0 T
15 X, ^ /- "-O -8" -
o n 4
o N 4/



4 5 6 7
soil pH
Fig. 10.-Ratio of water-soluble to dilute H2SO.-soluble phosphorus in
Norfolk Ifs at various pH levels.

36 Florida Agricultural Experiment Station

pH and phosphorus solubility in soils of the nature examined is
not a simple one and that interpretation of solubility determined
by arbitrary solvents must take soil pH into consideration.
It is quite commonly accepted that phosphorus fixed by alumi-
num compounds in the soil has a relatively low availability (23),
and that availability of phosphorus usually increases with rise
in pH up to and possibly slightly above 7.0, except when de-
pressed by free lime (4, 31). In the light of the preceding data
it appears that carbonic acid should be the most desirable sol-
vent to use in estimating phosphorus availability in soils cover-
ing a wide range in pH.
Determination of pH.-The effect of air-drying, dilution and
dispersion of the soil sample on apparent pH by the glass elec-

10% /


< 6
0 < / o ___'

a U) (5

"2% -
o \ /

4 5 6 7 8
soil pH
Fig. 11.-Ratio of carbonic-acid-soluble to dilute H2SO4-soluble phosphorus
in Norfolk Ifs at various pH levels.

Soil Reaction (pH) 37

trode was determined for 28 samples representing 4 reaction
levels on each of 7 Florida types. Air-drying lowered pH in all
instances, with a maximum of 0.41 unit and an average of 0.26
unit. Increasing the dilution from saturation up to 1 to 2 by
apparent volume of soil to water resulted in a maximum..increase
of 0.40 unit and an average increase of 0.14 unit. Mechanical
stirring of the 1 to 2 soil suspension during reading with a thin
glass electrode produced a maximum change of only 0.04 unit
and an average change of 0.01 unit as compared to non-stirring
during reading of a suspension prepared by hand stirring prior
to but not after insertion of the electrodes. With low buffer
capacity soils and a thick glass electrode, mechanical stirring
prior to insertion of the electrodes was necessary to eliminate
significant variation between stirring and non-stirring during
reading. The variation referred to with the thick electrode was
observed when soils of low buffer capacity differed significantly
in pH from what would be the residual effect of the soil pre-
viously read as modified by the intervening distilled water rinse,
this despite the fact that the electrode was pumped up and down
4 times in the new suspension to displace rinse water.
Comparative pH determinations on the 28 samples by the
Hellige-Truog colorimetric method and the glass electrode indi-
cated that the personal element was the greatest factor in vari-
atiomnbatween the 2 methods. Differences by an analyst familiar
with the colorimetric method were all within 0.4 unit while those
by an analyst unfamiliar with the method were only 75 percent
within 0.4 unit.
Control of pH.-Lime requirement curves were obtained from
field plot data obtained on 7 Florida soil types treated with Ocala
agricultural limestone. Curves were also obtained for somewhat
similar soils placed in 4-gallon outdoor lysimeters and treated
with precipitated chalk. Lime requirement per pH unit change
in the field plots became progressively higher as pH increased,
while it approximated a straight line relationship up to pH 6.5
in the lysimeters treated with precipitated chalk. Lime require-
ment was much higher than would be expected from the ex-
change capacity of the soils. Lime requirement was also esti-
mated for 4 of the field plot soils by a titration method proposed
by Dunn but modified to actual field volume weight basis. Lime
requirement to raise the pH to 6.0 by the titration method was
1/3 to 1/2 that determined by field trial. Residual CaCO3, deter-

38 Florida Agricultural Experiment Station

mined by analysis at the end of the field trial period, largely ac-
counted for the discrepancy.
The movement of calcium in 7 representative soils was studied
by analysis of drainage from lysimeters. Losses of from 46 to
712 pounds of calcium carbonate equivalent of calcium per acre
took place from untreated 9-inch surface profiles as a result of
passage of from 7.4 to 13.8 inches of natural precipitation
through them. Maximum proportionate loss of applied lime was
301 pounds following a 6,000-pound rate application to Brighton
peat. Application of sulfur to Norfolk and Fellowship soils at
rates of 400 and 500 pounds, respectively, increased calcium loss
by a corresponding 143 and 321 percent. This is equivalent to
327 and 575 pounds, respectively, calculated as calcium carbonate
equivalent. These 2 soils lost 108 and 49 pounds, respectively,
of calcium carbonate equivalent of calcium from applications of
2,000 and 3,500 pounds of lime.
There was no apparent difference in rate of penetration under
field conditions of surface applications of ground calcic limestone,
dolomitic limestone and lime hydrate applied to 7 soils at esti-
mated total lime requirement rates. In 52 weeks the treatments
produced average pH increases of 0.21,' 0.37, 0.29, 0.60, 0.28, 0.42
and 0.40 in the 1 to 2-inch horizon and increases of 0.06, 0.09,
0.10, 0.10, 0.12, 0.12 and 0.05 in the 2 to 4-inch horizon in Leon,
Fellowship, Portsmouth, Bayboro, Orangeburg, Norfolk and
Brighton soils, respectively.
Soil pH and Base Retention.-Percent saturation-pH curves
were plotted from data obtained on Portsmouth and Leon fine
sands from field reaction plots. The Portsmouth soil had a base
saturation of approximately 15 percent at pH 5.3 and the Leon
soil a saturation of 67 percent at the same pH value. Data were
not corrected for free CaC03 dissolved by the ammonium acetate
used for extraction of bases.
Soils from 7 soil types were placed in 4-gallon lysimeters by
profile to 9 inches depth and treated with lime or sulfur to estab-
lish various reaction levels. After coming to equilibrium under
natural precipitation, NH4NOs and K2S04 were applied in 1 liter
of solution per lysimeter at rates of 570 and 370 pounds per acre,
respectively. After 24 hours, leaching was begun by addition of
/2-inch increments of water at a rate that delivered a total of
21/2 inches of leachate in 36 hours. A marked positive correla-
tion between pH and the retention of ammonia and potassium
in the acid range of Norfolk loamy fine sand was found to exist.

Soil Reaction (pH) 39

Loss of the 2 bases was 21/ times as high at pH 5.5 and 41/2
times as high at 4.0 as it was at 6.8. Retention of bases by soils
of high exchange capacity was remarkably high, even at rela-
tively low pH levels.
pH and Phosphorus Solubility.-A study was made of 0 to 6-
inch surface and 8 to 12-inch subsoils of reaction plots on Nor-
folk loamy fine sand which had been held at 6 pH levels ranging
from 4.0 to 7.5 for 17 years by the addition of lime or sulfur,
but otherwise receiving normal fertilization up to 1 year pre-
ceding the study. The pH was approximately the same in the
surface and subsoil for any given plot. Total HClO4-soluble
phosphorus was somewhat variable, but in general sufficiently
uniform to make relative removal by various extractants ac-
ceptable as measures of the effect of pretreatment and pH on
its solubility. Unreacted CaCO3 existed in the surface soil but
not in the subsoil of plots where this material had been used.
There was a differential movement of sesquioxides in the surface
and subsoil in the plots receiving sulfur to maintain low pH
levels. This is assumed to be aluminum compounds, because iron
content was low and uniform.
Where free lime was low or absent, phosphorus solubility in
water or carbonic acid at 1 to 5 suspension increased with rise
inpH. Free lime had a depressing effect on the relative solu-
bility_in these solvents for any given soil pH where applied. At
lowpH values where sulfur had been applied, solubility in these
reagentsinversely correlated with aluminum content. The rela-
tive solubility of surface and subsoil phosphorus was approxi-
mately equal in water and carbonic acid for any given pH.
Above pH 5.6 the solubility in carbonic acid is relatively the
higher -of the 2 while below this pH the solubility in water is
slightlythe higher.
In general, solubility of phosphorus of surface and subsoils
in.O.002N HaSO4-at 1 to 40 dilution was constant between pH
4.0 andL5.3. Above this range solubility increased with rise in
pH. Free lime or previous effect of it increased the solubility
i theA i1ite 04 at any given pH. Where free lime was low
or absent, the rate of solubility increase with rise in pH was
higher for carbonic acid than for dilute H2S04 when the former
was calculated as a percentage of the latter for any given pH.
Differential solubility of surface and subsoil phosphorus of
sulfur-treated plots in dilute H2S04 correlated with aluminum

40 Florida Agricultural Experiment Station

content differential. Higher phosphorus solubility existed where
higher aluminum existed for any given pH.

Air-drying of samples of Florida soils may be expected to
reduce the pH an appreciable amount. Increasing the dilution
of the suspension will usually increase the pH. However, the
error is such that determination of pH on air-dry samples at a
dilution of 1 to 2 by apparent volume of soil to water is accept-
able for general extension service. The above method should
be supplemented by determination at or below saturation and
prior to air-drying for special studies justifying the additional
Samples at 1 to 2 dilution should be mechanically stirred rather
than hand stirred prior to or during determination if other than
a thin glass electrode is used. Stirring during reading is not
necessary, even with Florida soils of low buffer capacity, if they
have been adequately dispersed by mechanical stirring prior to
insertion of the electrodes. Certain thick glass electrodes evi-
dently differ from thin glass electrodes in that they carry'a
residual effect of pretreatment with greater persistence. Soil
separates sufficiently coarse to settle out during reading in the
absence of stirring do not have a significant effect on the buffer
capacity or pH of a suspension.
The personal element is apparently the most significant factor
determining the accuracy of colorimetric methods of estimating
soil pH. The principles involved in the Hellige-Truog method
appear to be sound. The experimental error, exclusive of the
personal element, appears to be in keeping with the degree of
accuracy suggested by the increments of color change presented
on the chart. However, the sensitivity of the method is lower
than that of the glass electrode method under even the best of
The neutralization of titrable acidity is evidently the funda-
mental principle underlying lime requirement of a given soil,
but titrable acidity is not a reliable measure of actual field lime
requirement. The heterogeneity resulting from practical appli-
cation methods does not allow reaction between a soil and given
liming material to approach completion in a reasonable length
of time. Before this can take place other factors which bring
about removal of bases have halted pH rise. Actual field lime

Soil Reaction (pH) 41

requirement to reach a certain pH may be as much as 3 times
the requirement estimated from base exchange phenomena.
Field lime requirement is primarily the resultant of the 5 factors:
titrable acidity, soil pH, liming material, incorporation and till-
age practices, and the time element.
The movement of surface applications of liming material into
acid soils is so slow that appreciable reaction adjustment below
the 2-inch depth will not take place in a reasonable time. The
natural downward movement of calcium in the soil is markedly
increased by strong acid residues such as would result from
fertilization with unbased materials.
Soil pH is not a reliable criterion of percent base saturation
determined by a simple ammonium acetate extraction. The dis-
crepancy is probably the result of solution of free lime by the
ammonium acetate and inherent differences in the exchange com-
plex of different soils.
There is a positive correlation between pH and the ability of
soils of low exchange capacity to retain potassium and ammonia.
pH should not materially affect the residual build-up of potas-
sium in such soils because of their low total base retention ca-
pacity. However, the average percent base saturation with
potassium under a given fertilization program should increase
as a result of retarded leaching following application of potas-
sium carriers. Soils of high exchange capacity have remarkable
retention power for potassium and ammonia even at relatively
low pH levels.
The general solubility of phosphorus residuals in soils of Nor-
folk and similar types of peninsular Florida may be expected
to increase with rise in pH in the general range, pH 5.0 to 8.0,
except where lime complicates the relationship: An increase
in sesquioxides in the range pH 4.0 to 5.0, has a relatively greater
effect on fixation of phosphorus against water or carbonic acid
solubility than against 0.002N H2SO4 solubility.: An increase
in lime content in the range pH 5.5 to 8.0 tends to reduce solu-
bility in water or carbonic acid as compared to its effect on
solubility of phosphorus in the dilute HaS04. A change in ses-
quioxide content or in lime content in the respective pH ranges
referred to above has the same effect on phosphorus solubility
in water that it does on phosphorus solubility in carbonic acid
for any given pH. However, solubility of phosphorus residuals
in carbonic acid increases more rapidly with rise in pH above 5.0

42 Florida Agricultural Experiment Station

than does solubility in water. In the light of the preceding, it
appears that carbonic acid is the best extractant to use in the
estimation of relative availability of phosphorus residuals be-
cause it reacts most nearly in order with plant response to the
effects of pH, lime and sesquioxides on phosphorus availability.

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Soil Reaction (pH) 43

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