Bulletin No. 114.
Soil Reaction as a Basis for Certain
Land Management Practices
STATE OF FLORIDA
DEPARTMENT OF AGRICULTURE
NATHAN MAYO, Commissioner
Reprinted from the Second Proceedings of the
Soil Science Society of Florida (1940)
SOIL REACTION AS A BASIS FOR CERTAIN LAND
R. V. ALLISONJ, Chairman
This symposium on soil reaction has been developed in view of the
rapidly growing interest among a wide variety of growers in the relation of
this factor to the normal growth of plants. It is particularly timely,
and in fact highly desirable just now, since the Agricultural Adjustment
Administration has already begun to seriously explore the possibility of
incorporating appropriate applications of lime or dolomite as a definite
part of its conservation program. The importance of a reasonably
convenient "yardstick" to ascertain the need of a given soil for such
treatment is obvious, therefore, and especially so since workers checking
on compliance of the grower with other aspects of the program would
in many instances be in an excellent position to take appropriate samples
for the purpose of having reaction determinations made on them once
an adequate procedure for taking and submitting these samples is placed
in their hands.
Unfortunate experiences with excesses of lime in the soil caused it
to be regarded as a "poison" by many and it developed quite a bad repu-
tation in consequence of its adverse effect on plant growth under such
circumstances. The high lime content of our marl soils, of the Parkwood
and certain other mineral types and some of our "sweet" or alkaline peats
and mucks that have been even moderately burned, is of course, quite
By the late twenties, however, we had come to understand that the
poor growth of many plants in high lime soil environments is due largely
to the inavailability of certain trace elements, notably manganese and,
in some cases, zinc and boron, that is developed by such highly alkaline
The control we can now exercise over what were formerly recognized
as adverse effects from too much lime might well raise the question of
whether certain qualities in the plant, or the fruit thereof, might not be
improved in one way or another by a high-lime soil environment. Such
a question of course remains for the future. We are more directly con-
cerned at this time with the development of a routine whereby soil samples
and related field data can be collected and the reaction determined in a
systematic way for any assistance this information may lend in directing
A Symposium lhcld b th Soil Scinc h Society of Florida, at llte Tamnpa Terrace
Hotel. Tampa. Florida. April. 1911.
President of the Society and Iflead. Soils Departmeiint, College of Agricultulre,
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 3
a liming program for a particular area. Just what the optimum reaction,
or lime content, of a given soil type shall be for a particular plant remains
to be determined for the most part, especially when various elements of
quality are brought into the picture as suggested above. With the excep-
tion of the so-called acid-loving or lime-loving plants, however, a reaction
range of pH 5.8 to 6.3 is found to satisfy a wide number of species and
varieties quite well.
As opposed to the question of too much lime and too high a reaction
(pH) we must, of course, also keep in mind the dangers of a soil reaction
that is too acid. Under such conditions, many elements, including the
trace elements, tend to go into solution more quickly and, in consequence,
are readily leached out of the soil with the drainage waters. This
condition can be brought about by a variety of causes including a
too continuous use of acidic fertilizers or the use of sulfur (as in
disease and insect control programs) in appreciable quantities. The
corrective measure is, of course, a judicious use of lime.
In all of this it is felt that there is one principle that should be
kept in mind, above all others, namely, that it will always be much
easier and less expensive to put on a little more lime the following year
than to "remove" a portion of that which has already been applied. This
latter, of course, can be accomplished in fact only by the use of supple-
mentary treatments with requisite trace elements or acid-bearing materials
which will tend to counteract the effects of the excess of lime.
It is to be noted in the discussions that follow that the various topics
have been arranged with as much continuity as possible. Thus the sam-
pling procedure itself is reviewed first, followed by that of the routine
of collecting and preparing the samples for shipment to the laboratory
along with the completed field sheets containing the required information
on each sample. This, in turn, is followed by a careful review of the
laboratory procedure used in the determination of the reaction (pH) of
the samples. This phase has been gone into in some detail since in
Florida, perhaps more than in any other state, our growers are interested
in these technical details. Once the values are available, the next natural
step, and of course one of the most important, is to "put them to work."
Under this last phase is discussed the tabulation and return of the data
to the County Agents for use in making recommendations on the various
areas that have been sampled. The propriety and necessity for doing
this is obvious from the fact that the County Agent is personally ac-
quainted with most of the growers in his area and probably has more
complete information than anyone else on the soil and cultural treatment
that has gone before and the condition of the crop or grove that has
For the very obvious reason of his acquaintanceship with the growers
themselves and familiarity with the field and other conditions under which
they are working, it is our hope that eventually all soil samples coming
to the University will pass through the office of the County Agent or at
least be taken and sent in under his instructions.
4 DEPARTMENT OF AGRICULTURE
FACTORS AFFECTING THE SOIL SAMPLING
GAYLORD M. VOLK* AND MICHAEL PEECH*
Proper sampling of soils for chemical or physical analysis is ex.
tremely important. It is often the weakest link in the chain of factors
determining the value of an estimation of soil fertility. Soil samples
must be truly representative of the area from which they are taken and
to which the analytical data will subsequently be applied. Each should
consist of a definite horizon or layer of soil which is fairly uniform
throughout its depth rather than being a mixture of layers of different
textures. Samples should be taken with clean tools, put into clean con-
tainers and submitted promptly for analysis before the changed environ-
ment affects the solubility of nutrients or alters other characteristics
of the soil. It is preferable that they be air-dried before being placed in
containers for shipment.
SOIL SAMPLING TOOLS AND THEIR OPERATION
Numerous types of tools have been designed for the purpose of
taking soil samples. Some are highly specialized for taking deep samples
comparatively free from contamination with the surface soil, while
others are very simple and designed primarily for sampling surface
soils or shallow subsoils. Sampling tools, in general, may be divided into
three classes as follows:
1. Shovel type-shovels, trowels, spades, spoons.
2. Augers-wood bit, cylindrical, post hole, sheathed.
3. Tubes-King (California), open sided, plain cylinder.
Three types of the sampling devices referred to above are shown in
In taking soil samples with the shovel or trowel type of instrument
a clean vertical cut should be made down the side of a hole dug to the
proper depth. The sample should be obtained by making a vertical
cut down the cleaned surface so as to remove a section of uniform
thickness for the entire depth of the layer which the sample is to
The ordinary wood auger (B, Fig. 1) is satisfactory for taking
samples of comparatively moist soil. Dry soils will not stay in the
auger. Cylindrical, post hole, or sheathed augers are more satisfactory
for use in dry soil. In sampling surface soils the auger is sunk to the
desired depth and the entire sample of soil clinging to it upon removal
used to make up the sample. For greater depths the top two inches
of soil brought out by the auger should be discarded and the remaining
soil put into the sample. The depth being sampled can be ascertained
Soil Chemist, Florida Agricultural Experiment Station, Gainesville, and Citrus
Experiment Station, Lake Alfred, respectively.
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 5
by marking the handle of the auger at six-inch intervals. Augers do not
give samples that are as free from contamination when used for subsoil
sampling as do either the shovel or the tube type of instrument. This
is not particularly important when sampling for mechanical analysis, but
is a very important consideration when sampling for nutrients which are
being applied to the surface as soil amendments. Thus in the case of
sandy subsoils, with low exchange capacities, contamination by lime
applied to the surface often leads to misleading results. In a similar
way exposure of unlimed top soils to marl from the subsoil should
be carefully avoided. Further than this, care should be used at all
times to prevent contamination from mixing cloths, dishes, or sieves that
have had previous use for samples differing widely in reaction or
The tube type of instrument is being used more widely at present
than at any time in the past. These instruments are all based on the
principle of forcing a cylinder into the soil by direct vertical pressure.
In almost all types the soil rises in a column as the tube penetrates the
soil. The King or California tube (A, Fig. 1) has a constricted tip to
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6 DEPARTMENT OF AGRICULTURE
cut a soil core slightly smaller than the inside diameter of the tube so
that the soil core will rise with as little friction as possible and will
not compact within the tube. It also has an outside bulge just above the
tip so as to make a hole slightly larger than the outside diameter of the
tube to facilitate withdrawal. The sample is removed by inverting
the tube and the core cut into sections representing the various depths.
Another type of tube (C, Fig. 1) consists of a straight-bore pipe
provided with a narrow opening along the side that permits observation
of the entire core. This slit also prevents the soil from compacting
within the tube. In this way the various sections representing depths to
be sampled are pushed out of the tip of the tube in the reverse order of
depth. This type, usually made of stainless steel, can be designed in a
variety of ways with respect to the opening and cutting edge. For greater
strength, a full cutting band about an inch wide can be left below and
the opening itself made in the form of a long triangle with the base
at the cutting end. A tube of this type is especially convenient for
shallow sampling in certain soils, including loose sands when they are
sufficiently moist for the instrument to retain the sample.
A special type of open-sided tube has a closed point and a window in
the side. The window is covered by a shutter which opens and closes
by a twist of the tube. This tube is used primarily for taking deep
samples that must be exceptionally free from contamination. The tube
is driven into the ground with the shutter closed. A turn in one direction
opens the shutter and continued turning scrapes a sample of soil into
the open window. Reversing the direction of turning closes the shutter
so that the tube can be withdrawn without contamination of the sample.
DETERMINING THE SOIL AREA TO BE REPRESENTED BY SINGLE
OR DUPLICATE SOIL SAMPLES
The first problem in the general sampling of any block of land is
that of dividing it into sub-areas that are uniform with respect to soil
texture, organic matter content, native vegetation, drainage, and past
fertilizer practices. If sub-areas so outlined are large, further arbitrary
division may be advisable. The size of an area to be represented by
one sample is entirely dependent upon the degree of detail desired, and
the purpose for which the sample is taken. In special instances it may be
desirable to sample very small areas such as would be the case where
the analysis of the soil from areas supporting abnormal plant growth
is to be compared with that from the surrounding area supporting normal
growth. The samples should be properly labeled for ready identification
in the laboratory.
SAMPLING OF THE SURFACE SOIL OR PLOW DEPTH
The surface soil generally may be considered as that turned over by
normal tillage operations on plowed lands or as the six-inch surface
layer of soil on unplowed or lightly tilled lands, unless the soil varies
markedly in texture or lime content within this depth such as would be
the case where a shallow surface soil overlies a subsoil of markedly dif-
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 7
ferent texture or where shallow marl can be found within less than six
inches below the surface. Under such conditions the surface soil should
include only the uniform surface layer, and the underlying layer should
be sampled according to instructions given for sampling subsoils.
Each sample of surface soil should consist of a mixture of portions
of soil taken from several locations within the area in question. Portions
of soil from at least ten locations within the area to be sampled are
considered essential to make up a representative sample. The same
amount of soil should be taken from each location, the portions
thoroughly mixed together on a piece of tough paper or clean cloth
or canvas, and approximately a pint of the mixture saved to be submitted
as the surface sample from the area. The sample should receive the
number of the area and the depth to which it was taken: i.e. 0-6" indicat-
ing that the layer occurred between zero and six inches. For greater
accuracy and as a check on the degree to which the first sample repre-
sents the area from which it came it is often desirable to take a second
sample from the same area, but from another set of ten different locations.
The first sample should then receive an additional "A" in its area
number, and the second sample the area number plus a "B": i.e. Area 1,
0-6", A; Area 1, 0-6". B.
Row crop fields should be sampled by taking soil portions from the
row or hill if the land is already bedded or planted. It should cut
through the fertilizer band if determinations of nutrient element levels
are desired but should not intersect the band if reaction only is to be
determined. Special sampling procedures, such as those discussed below
for citrus groves, should be used wherever spray programs such as
those practiced on tree crops are followed. For broadcast crops, pasture
areas, or unbedded or unplanted rowcrop areas, the soil portions going
to make up the sample should be taken at random from the area.
SAMPLING OF THE SUBSOIL
Subsoil samples are seldom taken for reaction measurement except
in particular cases where such are thought to be of special significance.
Where samples are thought to be of interest they are taken by arbitrary
depths such as 6-12" and 12-18" if a fair degree of subsoil uniformity
exists. At least ten individual cores of subsoil of any given depth are
taken and mixed together as in the case of the surface sample. The sub-
soil sample may be conveniently taken simultaneously with the surface
sample. Where distinct horizons of marl, clay, or organic matter occur
they should be sampled individually for laboratory identification and
analysis. Each sample should contain the area number, a location
letter, and the depth from which it came: i.e. Area 1, 6-12", R;
indicating that the sample came from "R" location in area No. 1, and
that the top of the layer began at six inches and that the bottom was at
twelve inches. Extreme care should be taken to prevent contaminating
the subsoil samples with the surface soil, or the surface samples with
the subsoil as, for instance, where the former may be low in exchange
capacity and the latter high in such a component as lime.
DEPARTMENT OF AGRICULTURE
TIME OF SAMPLING
Time of sampling is largely determined by the crop to be grown
and the fertilizing and liming program to be followed. For general
sampling it is preferable to take samples a period of time prior to
fertilizing and planting just sufficient to allow analyses to be com-
pleted and recommendation made. At least six months should elapse
before sampling after an application of soil conditioner such as lime has
been made. Samples may be taken at any time of the year for the pur-
pose of investigating soils upon which abnormal plant growth exists.
It is preferable not to take samples immediately after heavy leaching
rains or during the latter stages of extended dry periods except where
the effect of such a period on soil fertility is being studied. Samples
should not be taken immediately after turning under a green manure
crop. A complete picture of soil reaction fluctuation requires the
sampling of the same area periodically throughout the year.
In checking the effect of an application of any basic material suffi-
cient time should be allowed for the reaction to take place in order to
be assured that the maximum pI-I value has been attained. It should
be remembered, furthermore, that the various basic materials commonly
used to correct soil acidity decompose at different rates. Other factors,
such as initial soil reaction, soil type, cultivation, fertilizer practice,
and seasonal conditions, influence the rate of reaction between the basic
material and the soil. Unless these factors are taken into consideration
and sufficient time is allowed between applications, more basic material
is likely to be applied than is either necessary or safe. In the case of
dolomite, about one year should have elapsed between the time of the
application and the sampling. The best and most convenient time to
take soil samples in citrus groves is perhaps in the early fall preceding
the fertilizer application.
SOIL SAMPLING UNDER CITRUS GROVE CONDITIONS
In view of the fact that the plans of the Agricultural Adjustment
Administration for the liming program referred to above are further
along for citrus growers than for any other group, more emphasis will be
given to sampling under grove conditions. In fact it has been the thought
to work this through in a rather complete manner with the idea of
making of it something of a model to be followed as closely as may
be properly done for other crops later.
Soil Variation Relative to the Distance from the Trunk of the Tree
One of the difficulties encountered in obtaining a representative soil
sample in citrus groves arises from the fact that there exists a consider-
able variation in pH and chemical composition of the soil with respect
to the distances from the trunk of the tree. This is especially true
where basic materials have been applied recently. The soil within the
area of the tree spread is, with but few exceptions, more acid than that
outside the tree spread or in the centers of the checks (center of square
or rectangle formed by four adjacent trees). Usually the pH value of
the soil increases gradually with the increasing distance from the trunk
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 9
to the periphery of the tree but with very little change beyond that
point. In an attempt to arrive at the factors responsible for this soil
variation, an experiment was conducted in collaboration with Mr. W. L.
Thompson, Entomologist of the Citrus Experiment Station, to study
the effect of sulfur sprays incident to pest control on the soil reaction
and chemical composition. In the experimental grove used in this
study trees have been sprayed with different materials about four times
a year over a period of five years. Dolomite was broadcast over the
entire area from trunk to trunk at the rate of 700 pounds per acre
during each of the fourth and fifth years. At the end of the five-year
period soil samples were taken to a depth of 6 inches at the following
distances from the trunk of the tree: 1, 3, 6, 9, 12 and 18 feet. The
soil sample from each of the six locations above consisted of 8 borings
per tree distributed around the tree, and four trees were sampled, making
a total of 32 borings for each composite sample. The analytical data
are listed in Table 1, and the results of pH determinations are plotted in
Figure 1 to show the increase in pH with the increasing distance from
the trunk of the tree.
TABLE 1.-VARIATION IN PH AND CHEMICAL COMPOSITIONS OF NORFOLK FINE SAND,
0-6 INCHES, WITH RESPECT TO THE DISTANCE FROM THE TRUNK OF THE TREE AS
AFFECTED BY SULFUR SPRAYS.
Sulfur Distance Lbs. per Acre-Six-Inches
Sulfur Distance -
Spray from pH Exchange Exchangeable Bases I Phosphorus
Program Trunk1 Capacity" I Acid Water
Ca | Mg K Mn Sol." Sol.
Check 1 4.50 2.40 110 18 23 4.7 42 7
S 3 4.75 2.28 187 22 31 4.8 86 8
6 5.50 2.50 452 63 56 6.7 340 16
9 6.05 1 2.50 535 97 81 6.6 372 18
12 5.85 2.04 387 80 70 6.7 288 18
17.7 5.70 1.94 323 77 76 10.0 198 14
Lime-sulfur 1 4.25 2.04 103 10 19 3.6 42 7
3 4.50 2.04 180 14 23 5.8 102 8
6 5.25 2.22 380 41 51 8.2 314 16
9 6.00 2.54 580 84 82 8.2 405 17
12 6.00 2.30 477 93 82 9.7 362 20
17.7 5.85 1.88 329 75 68 10.0 236 14
Wettable 1 3.65 2.50 26 9 19 3.3 39 10
Sulfur 3 3.75 2.52 71 7 16 3.3 70 15
6 4.55 2.58 232 30 50 10.0 240 22
9 5.45 2.74 484 75 92 9.4 304 20
12 5.70 2.54 470 81 89 11.0 330 21
17.7 5.60 2.50 406 89 94 13.5 266 16
SDistance of 17.7 feet from the trunk represents
radius of tree spread was 8 feet.
centers of checks, the average
2 Exchange capacity in milliequivalents per 100 grams.
S Phosphorus soluble in .002 N HISOI according to method of Truog.
DEPARTMENT OF AGRICULTURE
As shown in Figure 1 the soil under the trees sprayed with wettable
sulfur was more acid than that under the trees sprayed with lime-sulfur,
whereas the soil under the trees sprayed with lime-sulfur was more acid
than that in the check plot. It will be noted, however, that the soil in
the check plot showed the same increase in pH with increasing distance
from the trunk of the tree as that in the lime-sulfur and wettable sulfur
plots. This would indicate that in addition to sulfur sprays there
are other contributing factors responsible for this condition, among
which may be mentioned the method of fertilizer application, pre-
ponderance of roots and consequently greater absorption in the area
within the tree spread, as well as difference in soil moisture conditions
and in the extent of cultivation between the area under the tree and that
outside the tree spread. The amounts of exchangeable calcium, mag-
nesium, potassium, and manganese, and readily soluble phosphorus are
correspondingly reduced with the increasing soil acidity as shown in
Table 1. In sampling grove soils, therefore, for the determination of
pH and other constituents, the practice of compositing soil cores or
borings taken in the area underneath the tree with cores taken outside
the tree spread is to be discouraged as the results obtained on such a
sample are not only difficult to reproduce upon resampling but are also
hard to interpret.
Depth of Sampling
The depth to which the sample should be taken is another important
factor which warrants careful consideration. Experiments conducted at
the Citrus Experiment Station over a period of four years on the effect of
annual applications of varying amounts of different basic materials on
a Norfolk fine sand have shown that while the pH value of the surface
0 CHECK PLOT
e WETTABLE SULFUR
0 2 4 6 8 10 12 14 16 IS
DISTANCE FROM TRUNK OF TREE IN FEET
Figure 2.-Variation of soil reaction with the distance
from the trunk of the tree as affected by sulfur sprays.
The last point, at 17.7 feet, represents the center of the
check; the average radius of tree spread was 8 feet.
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES '11
layer, 0 to 6 inches, has been increased from 5.0 to 7.0, the pH value
of the subsoil, 6 to 12 inches, has not been affected appreciably, indi-
cating little downward movement despite the porous nature of the soil.
It would seem that the surface sample of light sandy soils should not
be taken to a depth greater than 6 inches in consideration of the following
factors: (1) Downward movement of basic materials is slow; (2) effective
depth of cultivation is seldom over four inches; (3) natural change
between the surface soil and the subsoil occurs at five to six inches;
(4) there is a marked difference in exchange capacity as well as in the
exchangeable base content between the surface soil and the subsoil; and
(5) the majority of the fibrous feeder roots are in the surface layer. It
is not advisable to "scrape away two inches of the top soil" before
taking the sample. Subsoil samples, if desired, may be taken from either
6 to 12 or 6 to 18 inches.
Hammock soils are more variable than light sandy soils and conse-
quently greater care must be exercised in selecting areas to be sampled
as well as in compositing the individual cores in order to prevent mixing
the surface soil with the marl layer which lies at varying depths below the
surface layer. For this reason hammock soils should not be sampled
to any arbitrary depth but rather according to soil horizons. However,
where there is no significant change in the soil profiles the surface samples
may be taken to a depth of 10 inches. In either case, the depth of the sur-
face and subsoil sample, if the latter is taken, should be recorded.
Sample A (area outside the tree spread)
By means of a trowel or preferably a tube 1/_ to 1 inch in diameter
take a uniform cross-section core of soil to a depth of 6 inches just out-
side the area of maximum leaf drip (about one foot outside the periphery
of the tree). Ten borings should be made, distributed sufficiently to
represent the area under examination. If the sample is bulky it may
be mixed and a smaller composite sample placed in a manila paper
bag or a pint ice cream container.
Sample B (supplementary sample from the area within the tree
Obtain a composite sample by taking ten borings to a depth of six
inches half way between the leaf drip (periphery of the tree) and the
trunk of the tree. This sample may be taken simultaneously with Sample
A, keeping the borings or cores from these two locations separate.
The results obtained on Sample A should give a good picture of
the soil condition from the standpoints of both the tree and the cover
crop, and can be used accordingly as a guide in applications of basic
materials. Sample A also represents the greater part of the area in
the grove having approximately the same pH value. The relative
proportion and importance of the two areas depends, of course, on the
diameter of the tree spread as shown in Table 2. As stated before, the
soil in the area within the tree spread commonly is considerably more
acid than that outside the tree spread. The pH value of Sample B will
12 DEPARTMENT OF AGRICULTURE
show whether an extremely high acidity has developed in this area and
whether additional basic material should be applied under the tree.
TABLE 2.-PERCENTAGE OF THE TOTAL AREA WITHIN THE TREE SPREAD IN RELATION
TO THE DIAMETER OF TREE SPREAD BASED ON RECTANGULAR PLANTING 25x25 FEET.
Diameter of Tree Spread Percent of the Total Area Within
(feet) the Tree Spread
PREPARATION, DESCRIPTION, AND TRANSPORTATION OF SAMPLES
A pint portion of each sample composite as outlined above should be
placed in a clean ice cream carton or tough paper bag, wrapped securely
and sent to the laboratory. Each sample should carry the sample number
and depth both outside and inside the container. A full description
of the type of land, drainage, crops grown or to be grown, crop condition,
fertilizer used, lime used, and the type and amount of cover crop should
be in a letter or on a form accompanying or mailed at the same time
as the samples. This phase of the procedure will be discussed a little
later in detail.
It must be remembered at all times that the analysis of a soil sample
is only a small part of the information necessary to solve any problem
in soil fertility and that this information can at best represent the
condition only to the extent that the sample represents the area from
which it came.
It is often desirable to take special samples representing soil from
an area upon which abnormal plant growth exists. Such samples should
be taken from the area in question within the extent of the abnormality
in the same manner that other surface or subsoil samples are taken.
It is recommended that duplicate samples be taken from each such
area, as previously suggested for areas where greater accuracy is desir-
able. Samples should also be taken from the surrounding or nearby
areas of normal plant growth for comparison to those of the abnormal
area. The relationship of two samples to each other should be clearly
indicated so that proper comparisons can be made. Samples of plant
material from normal and abnormal areas are often of help in interpret-
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 13
ROUTINE FIELD SAMPLING AND NOTES
ALEC WHITE, ED L. AYERS, AND R. E. NORRIS*
(EDITon's NOTE: In view of the fact that these discussions and those
that follow, exclusive of Mr. Carrington's, were largely extemporaneous,
they will be reported only briefly along the line of the main objective,
namely, developing a routine for taking satisfactory soil samples for
reaction determination and field notes for future reference in connection
with these samples and the resampling it may be desirable to do another
year, and putting these values to work.)
Mr. Alec White was in an excellent position to discuss the field
phase of the problem, since the compliance group of the Agricultural
Adjustment Administration in Hillsborough County had already been
through the experience of taking quite a large number of samples and
sending them to the Experiment Station for testing prior to the time
of the meeting. In this first series, separate composite samples were
taken with the use of 1.8-inch sampling tubes in the middles and from
under the trees, labeled "A" and "B", respectively, and placed in pint
ice cream cartons for shipment. Identifying numbers and other pertinent
data, including date of sampling, initials of sampler, and legal descrip-
tion of area sampled, etc., were written on the bottom of the containers
rather than on the top, for obvious reasons. A red dot, indicating the
general location of the area sampled in a given grove, was also placed
on the official aerial map. The best estimate that could be given of the
time required for this early work was from two to five percent, on the
basis of the total time required for the compliance checkup.
Mr. Ayers discussed his experience in trying to evolve a satisfactory
method of soil sampling over a period of ten or twelve years and told, in
brief detail, of some of the practical benefits that have been returned to
growers in his county, of which he knew personally, as a result of
soil tests of this nature. His discussion was in full agreement with that
of Mr. White as to the practicability of having the compliance men of
the A.A.A. collect the samples and field data. Mr. Ayers particularly
emphasized the great variability in reaction of the soils of Manatee
County, ranging from a pH of 4.0 or less out on the pinelands or in
acid muck pockets to 7.5 or 8.0 for certain of the Parkwood types.
Mr. R. E. Norris presented a tentative outline for the the collection
of field information that he has used with a considerable degree of
success to accompany soil samples taken in Lake County. As indicated
under the discussion of methods of sampling, this first outline was
developed primarily for grove conditions with particular reference to
the conservation program of the Agricultural Adjustment Administration.
Principal emphasis is given in it to the general type of soil, variety of
citrus, kind and amount of cover crop, general condition of the grove
and previous liming treatment, if any. As a result of this and subsequent
discussions the outline shown in Figure 3 was evolved for the collection
of field information in connection with soil sampling under citrus grove
County Agents, Hillsborough, Manatee, and Lake Counties, respectively.
14 DEPARTMENT OF AGRICULTURE
Lab. No. ........................
FIELD SHEET FOR SOIL SAMPLING UNDER CITRUS GROVE CONDITIONS
(This grove inventory is to accompany each soil sample or each pair of
"A" and "B" samples taken from the same grove.)
Date of Sampling ...............................................
Name of Grove Owner County Code No. Farm Serial No.
Address Sample No. Location
Orange ( )
Variety: Grapefruit ( )
Mixed ( )
.................... ( )
(Check the Appropriate Description)
GENERAL GROVE CONDITION: Good ( ), Fair ( ), Poor ( ).
Bronze Leaf: Bad ( ), Medium ( ), Slight ( ), None ( ).
Frenching: Bad ( ), Medium ( ), Slight ( ), None ( ).
KIND OF COVER CROP: None ( ) Clean Cultivated ( ).
Legume ( )...................... Non-legume ( )........................ Mixed ( ).........................
Kind Kind Kind
AMOUNT OF COVER CROP: Heavy ( ), Medium ( ), Light ( ).
KIND OF SOIL: ( ) High pine land ( ) Low marl hammock
( ) High hammock ( ) Low acid hammock
( ) Flat pine land, clay subsoil ( ) Black-jack land
( ) Flat pine land, hardpan subsoil ( ) Scrub land
( ) Flat pine land, sandy subsoil ( ) ................................
PREVIOUS LIME TREATMENT, IF ANY: None ( ) Details not known ( ).
Kind of Material: ( ) Limestone, ( ) Dolomite, ( ) ..........................
Manner of Application: ( ) Full broadcast, ( ) Middles only, ( ) Under trees only.
R ate of A application: ...................................................... ................ ..... ..... ............
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 15
With regard to the recording and packaging of the soil samples, once
they have been collected and air-dried in a protected place, the most
effective container has been found to be a good strong (usually about a
three pound) nail bag such as may be obtained at any hardware store.
The spare top of each bag, upon which has been plainly written the
requisite identifying numbers, is wrapped firmly around the sample,
as shown in Figure 4, where it is also demonstrated how the "A" and
"B" samples are lightly bound together by a rubber band with their field
sheet. Only one field sheet is necessary for each pair of samples, since
they are both taken from the same grove.
Figure 4.-Packaging soil samples, following air-drying, for shipment.
Above: The air-dry sample (about 1/ to :% pint) is placed in a strong, 3-pound
paper nail bag, or its equivalent, and the top folded down neatly and firmly against
the sample after numbering legibly. The field sheet should then be folded to this
size and attached to the sample with a rubber band. Where there is a "B" sample
as well as an "A" sample, the single field sheet for both is lightly bound between
the two samples with the rubber band.
Below: The packed carton shows how neatly and economically well packaged
samples can be prepared for shipment.
A systematic listing of the samples contained in a particular lot or
unit of shipment on a separate sheet will most certainly prove a con-
venience of great importance to the men in the laboratory. As a matter
of fact, care in numbering, packaging, and listing the samples may reduce
the time required to complete and report on a shipment by one half, all
of which will mean a prompter return of the results to the County Agent's
office and to the grower.
The type of list that should accompany each carton or group of
cartons making up a particular shipment is outlined below under Figure
5. An examination of this outline, especially for the manner in which
it is related to the field sheet of Figure 3, will immediately suggest the
great facility it will lend to the handling of the material if it is care-
DEPARTMENT OF AGRICULTURE
Naturally the County Agent will want to keep a copy of the field
sheet as well as the shipping list for future reference.
Code No.: 59048
Date: August 29, 1941
Number Field Sample Number Owner or Operator
430 1 (seed bed) L F. Roper, Winter Garden, Fla.
430 2 (Bumby Farm) Eas L F. Roper, Winter Garden, Fla.
430 3 (Bumby Farm) West L F. Roper, Winter Garden, Fla.
430 4 (Vance Farm) L. F. Roper, Winter Garden, Fla.
638 1A Citrus Grove W. F. Loppacher, Winter Garden, Fla.
638 2 (Plo 1, Veg. land) W. F. Loppacher, Winter Garden, Fa.
638 3 (Plot 2 Veg. land) W. F. Loppacher, Winter Garden, Fla.
638 4 (Plot 3, East side) W. F. Loppacher, Winter Garden, Fa.
638 5 (Plot 3, West side) W. F. Loppacher, Winter Garden, Fla.
638 6 (Plot 4, Veg. land) W. F. Loppacher Winter Garden, Fla.
9060 1A Citrus Grove Matzon Haugaard, Gotha, Fla.
Figure 5.--Sample packing list.
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 17
METHODS OF DETERMINATION OF SOIL pH
RICHARD A. CARRIGAN*
The accuracy with which a pH test represents the area tested is
dependent upon two factors: (a) the faithfulness with which the sample
tested represents the soil area, and (b) the accuracy with which the
test itself is conducted. Methods of obtaining representative and sig-
nificant samples under Florida conditions are described in the preceding
paper by Volk and Peech. It is the purpose of this paper to consider the
commoner methods of conducting the test, to evaluate them in terms of
the accuracy to be expected, and to point out certain difficulties and the
means of overcoming them.
The meaning of the symbol "pH" will not be discussed here, since
good explanations are available elsewhere (1)1. The interpretation of
pH values in terms of soil conditions as affecting plant growth may be
learned by reference to the many publications available which deal with
the growth of specific crops.
Practical methods of determining pH fall into two classes known as
electrometric and colorimetric procedures.
From the theoretical standpoint the electrometric method is the
fundamental means of determining pH and any colorimetric method is
valid only to the extent that it has at some time been calibrated against
the electrometric method and found to give reliable data.
II. THE ELECTROMETRIC METHOD
In the electrometric method the sample to be tested is made a part of
an electric cell or "battery." Matters are so arranged that the voltage
of the resulting cell bears a definite and known relation to the pH of the
Every simple electric cell is made up of three essential parts. These
are the two electrodes, through which the electric current enters and
leaves the cell, and the electrolyte, which is ordinarily a solution sepa-
rating the electrodes. Each electrode, at its surface of contact with the
electrolyte, establishes a definite electrical potential, or voltage. The
individual voltages of the two electrodes combine to produce the overall
voltage of the cell as a whole. The electrodes may be of metal or other
solid material capable of conducting electricity, such as carbon. The
electrolyte may be a solution of an acid, base or salt, or, in the case
of a soil pH test, a mixture of soil and water.
Assistant Chemist, Soils Department, Florida Agricultural Experiment Sttion,
SNumbers in italics refer to literature citations on page 31.
18 DEPARTMENT OF AGRICULTURE
A familiar example of a simple electric cell is the common flashlight
"battery." In this case the electrodes are the zinc can which serves also
as a container for the rest of the cell, and the carbon rod in the center
of the cell. The electrolyte is the black paste soaked with a solution of
ammonium chloride (sal ammoniac) which fills the space between the
In an electrometric pH determination an electric cell is set up as
follows: The solution to be tested becomes the electrolyte, or the
liquid into which the two electrodes are dipped. One electrode is selected
which will yield a voltage that depends alone on the pH of the electro-
lyte which, in this case, is the solution under test. This is referred to as
the indicator electrode. The other electrode, known as the reference
electrode, is chosen to be one that will set up a known voltage which
is always the same, and therefore independent of the pH of the test
liquid. The overall voltage of the cell, which may be determined by
use of a suitable measuring instrument, is the resultant of the two elec-
trode voltages. If the overall voltage, so measured, is corrected for the
constant and known voltage of the reference electrode, it is possible to
find the voltage of the pH-sensitive indicator electrode and from this
to determine the pH of the solution bathing the electrodes by a simple
calculation. With commercial pH meters this calculation is usually
rendered unnecessary either by use of a simple numerical table supplied
with the instrument or by a modification of the instrument itself whereby
direct reading of pH values from a dial is made possible.
THE SOIL SUSPENSION
In running a soil pH test by this method the electrolyte is prepared
by stirring up a portion of the soil to be tested with some water. The
soil does not, of course, dissolve in the water in the strict sense of the
word. The sand particles settle out at once but the finer (colloidal)
particles are said to remain suspended in the water, at least until they
finally do settle out (which, by the way, must not occur before the
completion of the test). The resulting preparation is therefore spoken
of as a soil suspension. Into the soil suspension are dipped the pI-
sensitive indicator electrode and the constant voltage reference electrode.
In this way a complete electric cell is built up which is in all essential
respects a counterpart of an ordinary flashlight "battery." It has,
however, the distinctive characteristic of yielding a voltage that depends
only upon the pH of the soil suspension.
No diagrams will accompany the discussion following because of
the diversity of equipment available. Any operator of a pH- meter can
readily identify the parts described by comparison with his own instru-
ment or by reference to the manufacturer's literature.
The Calomel Electrode
The constant voltage reference electrode most commonly used for all
types of electrometric pH determination is the one known as the saturated
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 19
"calomel electrode." The essential features of this electrode are a small
quantity of mercury in contact with a small volume of a water solution
which is saturated with potassium chloride and also with calomel (mer-
curous chloride). In commercial pH meters the entire calomel electrode
unit is often made up in a compact, easily portable assembly. In
using a calomel electrode, it is necessary to establish electrical connection
from the mercury of the electrode to the voltage terminal of the
measuring instrument. This is done by means of a metal wire leading
from the mercury to the latter terminal. Contact must also be made
between the saturated solution of the calomel electrode and the solution
under test in such a way that the solution of the electrode cannot be
contaminated by the test solution. This is accomplished by having an
intervening solution of saturated potassium chloride through which the
electrical current passes from the calomel electrode to the test solution.
This intervening solution is accordingly known as a "salt bridge." A
common commercial form of "calomel electrode" is made xp integral
with the salt bridge which forms an outer jacket around the calomel
electrode proper. This jacket is ordinarily a glass tube filled with the
saturated potassium chloride solution which communicates through a small
hole with the solution of the calomel electrode in the inner tube of the
unit. Connection with test solution, in which the entire unit is immersed,
may be made through a minute hole in the end of the outer jacket,
through the liquid film wetting the joint between a ground glass sleeve
placed over the end of the outer jacket, or by other means.
Troubles with electrometric equipment are often caused by difficulties
with calomel electrodes and their associated salt bridges. These dis-
turbances are often of such a nature as not to require sending the
entire pH meter back to the factory. They can frequently be avoided
by careful attention on the part of the operator of the equipment and, in
case of trouble not readily corrected by him, the difficulty should be
satisfactorily remedied by purchasing a new calomel electrode unit.
Many of these units are quite small and contain only a small quantity
of liquid, so that they dry out readily. Drying out is often accompanied
by a creeping of the solution over the outer surface of the electrode with
the accompanying deposition of a salt-like residue of potassium chloride.
If severe drying has occurred, the inner calomel electrode proper may
be partially or completely empty of liquid. With a factory-sealed elec-
trode, this condition is difficult if not impossible to correct and may mean
that a new electrode must be obtained. Improved models may obviate
some of this difficulty but with the "dipping" types of electrode, trouble
from this source is more easily prevented than cured simply by keeping
the end of the electrode immersed in water when not in use. Calomel
electrodes of this type are ruined or damaged by disuse as readily as by
misuse. Only by careful attention to the matter of keeping them from
drying out during long periods when they are out of use can they be
preserved for dependable service when they are needed.
Contact from the electrical circuit to the solution in the calomel
electrode must be made only through the body of mercury; otherwise
an unwanted source of voltage will be introduced into the system. This
may occur if the solution in the electrode soaks its way through the
DEPARTMENT OF AGRICULTURE
upper insulating cap on certain types of calomel electrode, thereby making
contact directly to the wire leading from this unit. This condition is
indicated if a deposit of salt-like material is seen to form where this
wire emerges from the top of the electrode. In such a case, the electrode
has completed its useful life and must be replaced.
In re-filling any calomel electrode, only specially purified chemicals
sold for this purpose should be used unless the operator has confidence
in his ability to choose a grade of suitable materials. Contamination
of the electrode solution must be avoided.
The Buffer Reference Electrode
Some commercial quinhydrone pH meters make use of what is known
as a buffer reference electrode instead of the usual calomel electrode.
The buffer reference electrode is freshly made up by the operator him-
self each day the instrument is used and may consist of a simple
platinum (or gold) electrode dipping in a buffer solution (see below)
of known pH which has been saturated with quinhydrone. This arrange-
ment is, in fact, an ordinary quinhydrone electrode assembly (see next
section). The buffer solution in contact with the platinum electrode,
however, is always of constant and known pH and the electrode may,
therefore, be used as a reference electrode since its voltage will then be
constant also. When this system is used, connection from the buffer
reference electrode is often made to the soil suspension in another con-
tainer by a U-shaped glass tube filled with a jelly made from the usual
saturated potassium chloride bridge solution by addition of a sub-
stance known as agar. Another platinum electrode is dipped into the
soil suspension which is saturated with quinhydrone. The latter electrode
then becomes the indicator electrode since it behaves as a quinhydrone
electrode in the solution to be tested, as will be understood from the
next section. The precautions to be observed in using a buffer reference
electrode of the kind described are the same as those specified for the
use of a quinhydrone electrode in any other capacity.
The Quinhydrone Electrode
A useful indicator electrode for determining pH electrometrically is
the "quinhydrone electrode." The actual electrode used with this system
is a piece of platinum or gold, but, in order for it to register a voltage
which is related to the pH, the test solution (or soil suspension) is
saturated with a substance known as quinhydrone, before taking a
reading. Thus, a platinum electrode in contact with a solution saturated
with quinhydrone is called a "quinhydrone electrode." As with any
electrometric method, it is necessary to establish two electrical connec-
tions to the test solution. One of these is made through the pH-sensitive
platinum electrode; the other is made, as usual, through a constant
voltage reference electrode. The latter is ordinarily a calomel electrode
or the buffer reference electrode described in the preceding section.
Advantages of the quinhydrone electrode are (a) that electrical
measuring equipment of moderate cost can be used and (b) that subtle
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 21
sources of error due to failure of electrical insulation are not likely to
cause trouble as with the more elaborate glass electrode instruments.
Disadvantages are (a) that the quinhydrone electrode is not applicable
at pH values above about 8.5, and (b) that with some soils it is impossible
to get stable readings due to an uncontrollable drift in pH caused by the
presence of the quinhydrone itself. The first of these disadvantages is
not serious with most Florida soils. Limited experience indicates that
the second disadvantage may operate with only relatively few of the
soils of the state, but more data are needed on this point. The "quinhy-
drone drift" is recognized by the inability to get a reading that remains
the same long enough for its value to be determined satisfactorily. When
this happens, the closest approach to the correct value will probably be
obtained by taking a reading as quickly as possible after adding quin-
hydrone. In any case, the reading will be uncertain if a drift occurs.
In using the quinhydrone electrode with soils, however, two pre-
cautions are outstanding. (a) Sufficient quinhydrone must be added
and it must then be brought into very thorough contact with the soil
suspension by vigorous agitation so as to saturate the suspension
thoroughly. Quinhydrone is not very readily wet by a water solution.
It tends to "ball up" and float on the surface. This tendency must be
overcome with each sample and may be done conveniently by confining
the mixture of soil suspension and quinhydrone in a corked vial and
shaking violently or by whipping the quinhydrone down into the solution
by vigorous stirring in an open container. Failure to get good admixture
of quinhydrone has led to drifting readings which have been mistaken
for the typical "quinhydrone drift" already mentioned, but in cases when
no drift would have occurred if proper mixing had been achieved.
(b) The platinum or gold electrode must be more than just super-
ficially clean. It is not necessary that the platinum surface be bright,
but it must be free from obscure surface contamination. Treatments
commonly recommended are soaking in warm half and half nitric acid
or in warm chromic acid cleaning solution (the familiar red cleaning
solution of chemical laboratories).
When neither of these treatments suffices to put the electrode into
condition to give accurate results, the procedure of Coons is likely to
be of great help. (2). This involves placing the electrode in a water
solution of sodium bisulfite (5-10%), bringing to a boil, continuing to
boil for about five minutes, and allowing to cool without removing from
the solution. After this the electrode is washed in distilled water. If
the electrode assembly contains mercury, this should be removed before
the treatment and replaced afterwards.
Neither a quinhydrone nor any other electrode should be used until
it has been determined to be giving accurate results. To check on
the performance of an electrode, the pH of one or more "buffer solutions"
of accurately known pH should be measured with the electrode. It is
important that this be done every day the instrument is used. A buffer
solution is a solution which tends to maintain its pH accurately despite
the action of influences (such as contamination, aging, dilution, etc.)
which act to change it. Standard buffer solutions of accurately known
pH can be purchased from the dealers in pH equipment for checking
DEPARTMENT OF AGRICULTURE
performance of pH meters. Although a buffer solution tends to main-
tain a constant pH, it can deteriorate if not preserved carefully. A
solution should be regarded with suspicion if it shows evidence of con-
tamination or invasion by microorganisms. After a portion of buffer
solution has been used to check a quinhydrone electrode, it must be
discarded, since the quinhydrone soon oxidizes and renders the solution
unfit for use.
The Glass Electrode
The glass electrode is tending to supplant the quinhydrone electrode
in soils work due to the aforementioned characteristic of the latter elec-
trode of giving drifting and erroneous readings with occasional soils. The
glass electrode is another pH-sensitive "indicator" electrode, which, when
properly used, will give correct readings even with soils that would give
drifting readings with a quinhydrone electrode. In using a glass elec-
trode, no quinhydrone is added to the sample. The glass electrode is not
limited to pH values under 8.5 as is the quinhydrone electrode. As
with the quinhydrone system, the glass electrode is ordinarily used in
conjunction with a calomel reference electrode.
It is now generally believed that the glass electrode is capable of
giving correct pH readings with all soils and its use is now widely
accepted as being the best practice in soil pH work. Before it could
be put to practical use however, it was necessary to develop devices
suitable for measuring the voltages generated by it. Ordinarily we
think of glass as being a non-conductor of electricity, so it seems at
first sight that it would be impossible to construct an electrode from it.
As a matter of fact, no solid substance is a perfect insulator. Even
glass will permit the passage of minute currents of electricity. The
currents that do flow through a glass electrode are, however, exceedingly
small, so small, in fact, that ordinary electrical measuring instruments
are not able to register them. Thus, the electrical measuring equipment
ordinarily used with a quinhydrone electrode would be inoperative with
a glass electrode. In order to make the electrical circuit sensitive
enough, some means of amplifying the minute electrical current must be-
employed. This is accomplished by means of one or more vacuum tubes
("radio" tubes) acting in the measuring circuit. In this way it is
possible to get a current large enough to actuate the indicating needle
of the galvanometer or milliammeter from which readings are taken.
The necessary vacuum tubes are incorporated in all of the commercial
sets offered for sale for use in determining pH with the glass electrode.
Electrical Leakage Troubles with the Glass Electrode
Although certain difficulties inherent in the quinhydrone electrode
are obviated by use of the glass electrode, the extreme sensitivity required
for this type of test often leads to trouble of another sort. This arises
from the fact that when measurements are made upon any system having
such high electrical resistance as the glass electrode, the electrical insula-
tion employed in certain parts of the apparatus must be unusually good.
In an electrical measuring circuit, minute currents may flow over the
surfaces of the various insulators. When a system such as a glass
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 23
electrode and accompanying calomel electrode is being measured, where
the current that must be detected is exceedingly small, it is possible for
these surface currents to be of a magnitude comparable with the current
through the glass electrode itself. The unwanted leakage currents, as
they are called (since they are in the nature of leakage of electricity over
the insulators), often get into the measuring system of the pH meter and
cause fictitious results to be obtained. Thus if, in addition to the
current through the glass electrode, there is also another current affecting
the readings, the latter current will cause incorrect values to be registered.
In running glass electrode pH measurements it is necessary to know
where these electrical leakages are likely to occur, at least if irksome
difficulties with attendant delays are to be avoided. Those parts of the
instrument at which leakage may be expected to occur are: (a) across
any insulator which serves the function of supporting the wires leading
from the glass and calomel electrodes to the measuring instrument proper
(some exceptions, depending on the type of circuit employed), (b) over
the outside of the glass electrode, in a path leading from the terminal at
the top of this unit down to the solution under test, (c) over the support-
ing members which hold the glass and calomel electrodes in place during
the reading, (d) over the support which holds the sample cup, or beaker,
in place during measurements, and (e) over the outside of the glass
envelope of the vacuum tube ("radio" tube) which is connected to the
wire leading from the glass or calomel electrode. Any well-designed
glass electrode pH meter will be provided with good insulation at all
of these points; but the leakage is most likely to occur over the surface
of an insulator and not through its body. Surface leakage of this kind is
ordinarily the result of some form of contamination of the surface. This
may result from the presence of an imperceptible film of moisture which
may deposit at times of high atmospheric humidity, or it may be caused
by accumulations of dust, laboratory fumes, or other foreign material.
With these facts in mind, it is possible to avoid much of the trouble
commonly encountered in glass electrode work. A proper appreciation
of the degree of cleanliness required is not always obtained by reading
the instructions supplied by the manufacturer of the instrument. It is
perhaps no more than a slight exaggeration to say that the operator of
glass electrode equipment should be as careful of the condition of the
insulators of his set as a surgeon is of the sterility of his instruments.
To this end, the pH meter should not be stored in a laboratory where
fumes are frequent; dust must not be allowed to gather on the insulators;
solutions must not be spilled on them, and they must be kept absolutely
free of the ever-present potassium chloride (the white salt-like material
in the calomel electrode). It is preferable not to touch an insulator
with bare fingers any more than is absolutely necessary. In short, the
strictest cleanliness must be observed in connection with all parts of the
instrument, especially those external parts associated with the electrodes.
Regarding errors due to electrical leakage, an important consideration
must be kept clearly in mind. This is the fact that errors due to this
cause are not constant but vary in a way that depends on atmospheric
humidity, the extent and kind of contamination and, what is most im-
portant, on the pH of the solution being tested. These facts lead to the
24 DEPARTMENT OF AGRICULTURE
important conclusion that it is not possible to allow for leakage errors
by establishing a constant correction to be applied alike to all determina-
tions run on the day the correction is determined. To place faith in a
"correction factor" determined by running a standard buffer solution of
known pH is to invite hidden errors which must necessarily be over-
looked in the very nature of the case. The instrument is either function-
ing properly or it is not. The only uniformly safe procedure is to check
the performance of the meter every day it is used. If it is not giving the
required accuracy for the purpose at hand, steps should be taken to
remove the cause of the trouble and not to "correct" all readings on the
basis of the readings taken on the standard buffer solutions.
Checking the Performance of a Glass Electrode pH Meter
With thee -facts in mind, it is appropriate to consider what steps need
to be taken in order to determine beyond a reasonable doubt whether a
glass electrode pH meter is working properly before proceeding to take
readings. If the batteries in the meter have run down, if a switch has
failed, if there is a loose connection in the circuit, or if a vacuum tube
has "gone dead," in all probability the difficulty will be immediately
recognized by the obvious failure of the instrument to respond in the
expected manner to the manipulations specified in the instructions.
Troubles of this nature are therefore not likely to pass unnoticed. With
leakage errors, however, the case is different. With certain types of
equipment it is entirely possible for the operator to follow all instructions
of the manufacturer and yet be unaware of hidden errors due to this
cause. No one assumes that a clock will keep correct time simply
because it runs on being wound. There is no more reason to assume
that a pH meter is giving trouble-free service merely because it responds
as expected to the adjusting operations preliminary to taking the actual
pH readings. Just as the owner of a clock checks its performance against
standard time signals, so the pH operator may determine whether his
instrument is giving correct values by equally simple means. The test
is made by using the meter to measure the pH of two buffer solutions
of accurately known pH. These buffer solutions should have pH values
lying near the pH range of the samples to be run; they must also differ
in pH from one another, preferably by about three pH units. Suitable
buffers for soils work may have pH values of 4.0 and 7.0 (or 8.0).
If the instrument is functioning properly, correct readings will be obtained
on both buffers.
The recommendation just given for checking the performance of a
glass electrode pH meter differs in an important respect from instructions
often given for this purpose. Some instrument manufacturers commonly
supply a single buffer solution with each instrument sold, specifying
that before running tests on each day the instrument is to be used, it should
be checked against this buffer. If the correct reading is not obtained,
an adjustment is made on the instrument by means of convenient manipu-
lation of a "zero adjustor" which is intended to provide correction to
allow for the asymmetry potential of the glass electrode. This step can
do no more than correct for the asymmetry potential. (The asymmetry
potential is an extraneous voltage inherent in each individual glass
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 25
electrode; it varies only very slowly and may therefore be legitimately
corrected for by this procedure.) Since this adjustment against a single
buffer merely corrects for the asymmetry potential, it cannot be expected
to reveal at the same time whether the instrument is performing as it
should. In order to determine this, it is now necessary to test the instru-
ment by measuring the pH of another buffer of different pH, having used
the first buffer solely for establishing the necessary correction. If a
leakage error exists, the test with the second buffer will reveal it. If
the second buffer were not used, however, leakage errors of serious
magnitude might pass unnoticed; the instrument might then give correct
readings only at the pH of the first buffer, since it was made to do so by
the procedure of "zero adjustment." If the meter is working in good
order, the reading with the second buffer will accord with the known pH
of that solution.
Too much emphasis cannot be placed on the necessity of frequent
checking against two buffers, as outlined in the preceding paragraphs.
It is well to do this before and after running samples every day the
instrument is in use, especially if it is not used regularly.
Before leaving the subject of electrical test instruments, it is well
to call attention to the necessity of keeping all electrical contacts in
good condition. Workers without special training in the use of electrical
equipment are prone to overlook this important detail. Whenever the
instrument behaves in an erratic manner or when its indications are
affected by slight vibration, a poor contact should be looked for. All
permanent connections within the body of the meter are soldered. Certain
connections, however, must occasionally be manipulated by the operator
of the instrument. Such are the battery connections and the points of
contact between the glass and calomel electrode lead wires and the
measuring instrument itself. Metal contacts at all these points must be
kept clean and bright where metal touches metal and, by all means,
screwed contacts must be made tight. Failure to observe these precautions
is a common source of easily remedied trouble.
A pH meter should at all times be kept in a dry place.
Preparation of the Soil Suspension and Conduct of the Test
The preparation of the soil suspension for an electrometric pH
determination is a relatively simple matter. There are certain pre-
cautions, though, that should be kept in mind.
It is desirable for soil samples to be dried by exposure to the air
at ordinary temperatures as soon as possible after sampling, particularly
so with samples containing much organic matter, lest the activities of
microorganism alter the pH before the test is run. Too small a sample of
the air-dry soil should not be taken for preparing the soil suspension:
otherwise the portion selected is likely not to represent the average com-
position of the whole sample brought in from the field. Some pH
meters require only about a thimble full of soil suspension for a test.
With such equipment it is desirable to prepare a larger quantity of soil
suspension than actually needed in order that a reasonably large portion
of soil will be used for the test.
26 DEPARTMENT OF AGRICULTURE
The procedure in use in the Department of Soils of the Florida
Agricultural Experiment Station will be described. The conditions chosen
are the result of an effort to use the least possible quantity of water for
a given amount of soil compatible with the nature of the equipment in
use. A 50 cc. beaker of air-dry soil, which has previously been mixed
and screened through a sieve (having holes of diameter 2 mm.) is
measured into a 150 cc. beaker and 100 cc. of distilled water added.
The water and soil are stirred well with a heavy glass rod and the
suspension is allowed to stand for from one to two hours. At the end
of this time the suspension is vigorously stirred again and the glass and
calomel electrodes are immersed in it at once and a reading taken. With
peat and muck soils 10 parts of water are taken to one of soil (by
weight) and allowed to soak three hours. The extra time of soaking
is intended to allow for the tendency of these soils to resist wetting
after they are once dried out.
Even if the smaller type of sample cup on certain pH meters is used
it would be better to prepare the soil suspension on a larger scale, as
just described. Then, after soaking an hour or two, the soil suspension
may be stirred and a small portion of the turbid suspension transferred
to the sample cup for the reading. In doing this one precaution must
be observed. It does no harm for the coarse sand particles to settle out
and be left in the original container, but the finer colloidal material
(clay particles and very finely divided organic matter) must not have
settled appreciably before the transfer is made. The colloidal material
must remain in suspension since these fine particles must actually be in
contact with the glass (or quinhydrone) electrode at the moment the
reading is taken. This is why, in any case, the suspension is always
stirred immediately preceding the taking of the readings.
As pointed out earlier, potassium chloride from the "salt bridge"
associated with the calomel electrode must not contaminate the soil sus-
pension before the reading is taken. Such contamination leads to low
results with acid soils.
If the individual worker prefers to use samples in the field-moist
condition, he should recognize the difficulty of getting adequate mixing of
soils while damp. This may, of course, be possible when suitable pre-
cautions are taken to insure that a representative sample is used in the
Care should be taken to see that portions of powdery marl samples
do not remain on the sieve in a cake-like deposit to contaminate later
samples of poorly buffered sandy soils and so affect their pH values
III. THE COLORIMETRIC METHOD
Colorimetric methods find favor where the expense of electrometric
equipment cannot be justified because of the small number of samples
to be run. The colorimetric technique has the advantage that it requires
only inexpensive and simple apparatus which is light in weight and
which occupies but little space. Moreover, the procedure is quite
simple. Disadvantages are (a) that the readings taken depend on the
personal judgment of the observer, (b) that the procedure used is not
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 27
susceptible of the degree of precision attainable with electrometric meth-
ods, and (c) that in some colorimetric procedures inconveniently small
samples are necessary.
In the colorimetric method, pH-sensitive dyes are used which alter
their colors on being brought in contact with the material tested in such a
way that the resulting color of the dye is dependent on the pH of the
sample. Such dyes are known as indicators. The pH of the sample is
determined by matching the color assumed by the dye against a series of
color standards, each color standard corresponding to a definite pH
Several portable kits for carrying out colorimetric pH tests on
soil are available commercially. In order to determine the accuracy
obtainable with one of these kits, a series of 155 samples of soil from
the sandy citrus areas of Florida were subjected to the colorimetric
test. Glass electrode pH determinations had been run on the samples and
are considered to give the correct values, against which the colorimetric
values may be compared to determine their accuracy. Each sample
was tested colorimetrically by three different operators, all of whom
worked independently of one another and were in ignorance of the
correct values as obtained on the glass electrode. No effort was made
to train the operators to check one another. Each operator was allowed
to develop his own judgment, subject only to a reasonable amount of
The colorimetric method selected for study was that provided in the
LaMotte-Morgan Soil Testing Set. This equipment, together with fresh
indicator solutions and new color charts, was obtained from the LaMotte
Chemical Products Company, Baltimore, Maryland. The LaMotte-
Morgan procedure calls for allowing several drops of the indicator
solution to percolate through a small quantity of soil retained in a de-
pression on the slanting side of a special porcelain block. After passing
through the soil, the indicator solution passes down a shallow trough
to another depression lower down on the block, where the color of the
indicator is compared with a standard color chart.
The results of the comparative study are presented in the accompany-
ing table. Before tabulating, all errors were rounded off to the nearest
0.1 pH unit. The three operators are designated as Observers "A", "B",
and "C". The table shows what percentage of each observer's readings
fell within a given range of error. Thus, reference to the table reveals
that 62 percent of the tests by Observer "A" were within 0.1 pH of
the truth; 92 percent of this observer's tests were within 0.3 pH of the
correct value; or, to state it differently, 8 percent of his values were
at least 0.4 pH in error. The data of Observer "C" were somewhat less
accurate. The latter observer obtained values within 0.1 pH unit of the
correct value in only 23 percent of hIis tests. Seventy-six percent of
his values were within 0.3 pH of the truth, so that 24 percent of his
tests were at least 0.4 pH off, either too high or too low. Judged on this
basis, Observer "B" attained an accuracy intermediate between that of
the other two observers, except for the fact that he registered a few more
very large errors than the other two.
28 DEPARTMENT OF AGRICULTURE
COMPARISON OF LAMOTTE-MORGAN COLORIMETIIC DETERMINATIONS WITH
GLASS ELECTRODE DATA.
The figures in all but the last two columns show what per cent of an observer's
readings agreed with the glass electrode values within the limit of error shown at
the head of each column. The last two columns show what per cent of each observer's
readings were too low and too high respectively.
S I Per Cent Per Cent
Error, p Units 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Low High
Enr______ _B __ Readings Readings
Observer A .......29 62 81 92 99 9999 99 100 57 14
Observer B ...-... 10 34 59 76 87 91 93 95 98 35 55
Observer C ........ 5 23 50 76 88 96 99 99 100 92 3
Further interesting comparisons between the three observers are shown
in the last two columns of the table, in which are given for each observer
his percentage of low results and of high results, respectively. Thus it
is seen that Observer "A" obtained values which were too low by at
least 0.1 pH unit in 57 percent of his readings, while only 14 percent
of his readings were too high. Observer "C" had an even more pro-
nounced tendency toward low results. Observer "B", on the other hand,
showed only a moderate trend toward high results. In other words, any
single determination by Observer "B" is almost as likely to be too low
as to be too high. When it is considered that Observer "B" reported
values which were at least 0.4 pH unit in error for 24 percent of his
tests, it is obvious that the interpretation of his data under actual
routine conditions would be rendered somewhat uncertain. About one
out of each four of his tests would be at least 0.4 pH unit from the
true value and there would be no way of knowing whether the reported
value were too high or too low in a given case. Thus, for any given
reported reading, the true value for the sample might lie anywhere
within a range of pH- so wide that a recommendation based on the
reading might be significantly in error.
The preceding conclusion applies, of course, to Observer "B". The
other two observers reported data of considerably more value in that
Observer "A" attained a higher degree of accuracy, and both Observers
"A" and "C" were more consistent. That is to say that the errors of the
latter two observers were usually in the same direction, so that there
would be less uncertainty regarding the range within which the true pH
value corresponding to a given reading might lie, as contrasted with
the tendency of Observer "B" to obtain readings which were about as
likely to be high as to be low.
Before attempting to arrive at an interpretation of the data presented
in the table, it is first necessary to have some criterion for deciding what
would constitute satisfactory performance for the colorimetric method
in terms of its ability to reproduce results obtained by the glass electrode.
It is sometimes argued that comparatively rough determinations are
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 29
adequate, on the basis that the sampling error is relatively great and
that there is no need for the test to be more accurate than the sampling
procedure. To a certain extent, this reasoning in logical. If the field
sample in any given case represented the area sampled with an accuracy
of no more than two or three tenths of a pH unit, then there would be
no need of insisting on having a test method which could yield readings
accurate to within two or three hundredths of a pH unit. On the other
hand, if the test itself is subject to errors as great as, or greater than
those of the sampling procedure in the case cited, then there is danger
that in a considerable number of tests the error of sampling will combine
with the error of the test method to produce an overall error too great
for safety. The truth of this statement is especially evident when it is
remembered that for any given test it will not be known whether the
value obtained is too high or too low. In order for a pH test method
to be considered completely satisfactory, it would not seem unreasonable,
therefore, to require that it should be capable of yielding readings which
are in no case more than 0.2 pH unit from the true value for the small
portion of soil actually tested. The choice of 0.2 pH for this limit is,
of course, quite arbitrary, since the accuracy needed is dependent to
some extent on the use to which the test is to be put. It should not
be concluded that any test of somewhat lesser accuracy should be dis-
carded, since the question of expense is sure to be a determining factor
where only a small number of samples are to be run. In cases where
only the less expensive colorimetric methods can be afforded, more
samples should be run from each area in order to achieve at least a
partial compensation for the lower accuracy obtainable on a single
sample. Larger organizations and testing agencies would do well, how-
ever, to make use of the more reliable electrical methods.
Consideration of the data in the table shows that the particular
colorimetric method studied is at best but a substitute for the glass
electrode, although it will find, however, a certain field of usefulness. It
is at once evident that different operators will use the method with
varying degrees of success. For reasons already explained, Observer "B"
reported readings which were quite unsatisfactory. The data of Observer
"C" were not particularly satisfactory. An operator with the skill of
Observer "A" could, however, obtain useful data, even though there
would be much to be desired from the standpoint of accuracy. Thus
81 percent of this observer's readings were within 0.2 pH unit of the
correct value or, stated differently, about one reading out of every five
was outside of the limit of accuracy which we have tentatively established.
No operator should, however, take it for granted that he will be able
to equal the performance of Observer "A". Experience with other
users of the test who have sent samples to the laboratory for checking
their colorimetric results against glass electrode determinations has in-
dicated that Observer "A" may have been exceptionally skilled in using
the test. Moreover, the data reported here were obtained under favorable
laboratory conditions and cannot be taken to represent what might be
expected in the field where workers, no matter how conscientious, would
be under certain disadvantages. These disadvantages are: (a) limited
supply of distilled water for washing apparatus, (b) difficulty of pro-
DEPARTMENT OF AGRICULTURE
testing color charts and indicator solutions from premature fading and
contamination and (c) the serious difficulty of obtaining samples in
an air-dry condition so that they may be put through the thorough
process of mixing necessary to insure that the small portion of soil used
in a color test will be representative of the entire sample withdrawn from
the field. The conclusion to be drawn from the evidence submitted
would seem to be that before any worker undertake to use the colorimetric
method he should determine what accuracy he is capable of attaining
by comparing a large number (at least twenty-five) of his test results
with glass electrode determinations on a variety of soils. He would
thus know whether he has the inherent ability to make the necessary
color evaluations with the precision required for attaining accuracy com-
parable with that obtained by Observer "A". Experience is doubtless of
some help in this connection. Conditions peculiar to the individual
operator's work, such as suitability of water supply, would also be
evaluated by such a comparison, so that if anything were wrong it
would be easily possible for him to recognize the difficulties and limita-
tions of his procedure. A procedure that has been found particularly
desirable in cases where workers must use colorimetric methods is to
have periodic check-ups by comparing data with the glass electrode on
at least a dozen samples about every three months. Only in this way
can each operator maintain a constant check on the accuracy he is
The conclusions arrived at in the preceding are derived entirely from
experimental work on the LaMotte-Morgan method. The soils used
came entirely from citrus areas. Experience indicates, however, that the
recommendation given above can well be applied to colorimetric methods
Regarding the use of colorimetric methods, a few precautions are
worth mentioning. Filtration of soil-water suspensions through filter
paper preparatory to determining pH on the water extract as a means of
arriving at the pH of the soil is a procedure that can lead to errors
approaching as much as an entire pH unit.
With any colorimetric method it is necessary to have a clear under-
standing of the fact that any indicator is capable of giving indications of
pH only with a sample the pH of which falls within the pH range over
which the indicator in question changes color. Thus, if phenol red is
being used, it must be realized that this indicator gives a yellow color at
pH about 6.6 and also at all pH values below this point. In other words,
the color of this indicator reaches almost its maximum degree of yellow-
ness at this pH and its color will not change much more on going below
pH 6.6. Accordingly, if a reading of about pH 6.6 is obtained with this
indicator, the operator must not fail to check the determination by
use of another indicator, the pH range of which overlaps the range of
phenol red on the low pH side of the latter indicator, for example
bromthymol blue which covers the range 6.0 to 7.6. Failure to check
with another indicator in such a circumstance is almost sure to lead to
occasional, and probably frequent, errors of extreme magnitude. As a
matter of fact, whenever possible, each sample should be checked with
two different indicators. Such a procedure will, in fact, do much
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 31
toward making the operator realize the limitations of the procedure he is
Great care needs to be taken to preserve color charts and indicator
solutions in good condition. Color charts should not be exposed to light
more than necessary and must not be allowed to become wet or soiled.
New charts should be provided when old ones lose their original appear-
ance. Indicator solutions and charts must be supplied by the same
manufacturer for use with each other.
Color comparisons are best made in daylight. A bench indoors,
facing a north window, is excellent.
Apparatus must be cleaned carefully between each determination,
with final rinsing with distilled water. Traces of soap and other clean-
ing materials which might affect readings must not be allowed to remain
in the apparatus.
In comparing colorimetric readings with electrometric data, the oper-
ator should always make his colorimetric readings before referring to the
electrometric values. Otherwise the most conscientious worker will prob-
ably be under the almost unavoidable psychological disadvantage of
being unable to exercise his judgment of color independently of his
prior knowledge of the correct result.
The author is indebted to the National Youth Administration for supplying part
of the labor used in conducting the study of the colorimetric test.
The series of soil samples used in the colorimetric study were kindly supplied
by Dr. Michael Peech, together with the glass electrode data.
1. BYERS, H. G., ANDERSON, M. S., and BRADFIELD, R. General chemistry of the soil.
U. S. Dept. of Agriculture Yearbook, "Soils and Men." Page 923 ff., (1938).
2. Coons, C. C. Continuous measurement of pH with quinhydrone electrodes. Ind.
and Eng. Chemistry, Anal. Ed. 3, 402, (1931).
DEPARTMENT OF AGRICULTURE
METHOD OF REPORTING RESULTS TO THE COUNTY
AGENT AND TO THE GROWER
E. F. DEBUSK* AND L. H. ROGERS*
Once the soil samples have been taken and the reaction values
determined, it was not deemed advisable by Messrs. DeBusk and Rogers to
undertake to make detailed recommendations from the Experiment Station,
especially in view of the large number of samples that may be involved
and the extremely limited amount of help that is available to take care
of this type of work. Accordingly, it was decided that a form should
be developed to report these values back to the County Agent for
distribution to the grower with any comments or recommendations that
the Agent may care to include on the basis of his personal knowledge
of the property, over and above the information shown on the field
Farm Serial No.1 Field Sample No.' pH Laboratory Number
1547 A 5.68 I 394
1547 I B [ 5.47 395
1572 1 lA | 6.12 [ 364
1572 1 113 5.75 | 365
1572 2A I 5.58 358
1572 1 2B 5.49 359
1573 A 6.05 374
1573 B I 5.93 375
1576 A 5.81 1 372
1576 B I 5.31 1 375
1579 I A 5.78 I 333
1579 B 5.59 I 334
1591 A 5.30 1 321
1591 B 5.77 | 322
1616 | A 5.61 1 352
1616 B 4.77 353
16488 l1A 5.68 688
1648 1B 5.78 389
1648 2A 6.09 335
1648 | 2B 5.98 I 336
1648 3A I 5.73 | 237
1648 3B | 5.10 I 238
1666 1 A 5.10 I 323
1666 I B 6.16 324
Date: December 11. 1940 T. rlr. t,,, C onu N r 1 ;
for reporting reaction values to the county.
for reporting reaction values to the county.
Citriculturist, Agricultural Extension Service, and Associate Biochemist, Soils
Department, Agricultural Experiment Station, respectively, Gainesville.
SSe also Figure 3.
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 33
Inasmuch as the County Agent will doubtless keep a copy of the
field sheets in his office, it was thought sufficient for these report forms
simply to list the reaction values according to the consecutive laboratory
numbers given them when they arrive at the Experiment Station along
with their county serial number, and their subordinate field numbers,
which will serve to identify them in the records of the County Agent's office
as to farm, field or grove, and operator. The laboratory number serves
for ready reference to the samples of each county for whatever period
they are held in storage. These numbers are also useful in affording a
ready record of the number of samples that have been sent in from a
particular county and in otherwise facilitating their handling. Such a
tabulation form as might prove useful in sending the reaction information
out to the county offices is to be found in Figure 6, where a typical set of
values are presented exactly as they were reported back to the County
On the basis of these values the County Agent is in a position to
arrange them on cards for each individual field or area sampled in such
a way as to bring together the reaction values of each for a succession of
seasons. Such a card should also provide a place for date, rate, and
manner of soil treatment with liming materials. The advantage of such
a record is obvious in following the effect of the treatments that are
being made from season to season. Such a study is in line, furthermore,
with the keynote of conservatism in the application of liming materials
that was sounded above, especially in the avoidance of too heavy treat-
ments or overliming.
DEPARTMENT OF AGRICULTURE
H. G. CLAYTON, Leader
W. E. STOKES
A. F. CAMP
R. P. TiORNTON
Mr. H. G. Clayton opened the discussion by very briefly outlining the
prospective liming program of the Agricultural Adjustment Administra-
tion in the State. He not only touched on the financial manner in which
it would operate but also emphasized the real need for carefully de-
termined reaction values on soil samples that had been taken with equal
care as a basis for liming recommendations. He then called on Messrs.
Thornton, Stokes, and Camp to assist in leading the discussion. Doctor
Camp was out of the room for a time but returned to take part in the
discussion that followed.
In his brief discussion of the subject, Prof. Stokes emphasized the
need for recognizing other factors than soil reaction as well as the
limited amount of information available on the lime requirements of
agronomic crops. He expressed himself as strongly in favor of bringing
all available data and opinion, based on experience, to bear on the
question from every angle and urged this phase of the problem upon the
attention of the group. The strong plea for conservatism in the use
of reaction values as a basis for determining the amount of lime to
apply was well supported by the comments from the floor that followed,
which left little doubt, however, that carefully determined reaction values
could and should be of much greater value than personal opinion.
Mr. R. P. Thornton briefly reviewed his several years of experience in
the sampling and analysis of soils and supported in a quite definite
manner the need for a well established routine for taking, labeling, and
preparing soil samples for shipment to the laboratory as a basis for the
work to follow. He also commented on the discussion of methods by
Mr. Carrigan and recommended the use of the glass electrode for reaction
(pH) determinations wherever it was at all feasible to do so.
The extensive soil reaction survey developed in South Carolina a
few years ago by Dr. H. P. Cooper, now Director of the State Experiment
Station at Clemson, was brought to the attention of the group by Prof.
E. L. Lord. He quoted the survey, which involved many hundreds of
thousands of samples of soil-one from each field of each farm of the
state, in fact-as showing 40 percent to be more acid than pH 5.5. Mr.
Lord declared a similar condition prevailed in practically all virgin soils
in Florida. Such a reaction he regards as definitely unsatisfactory, calling
attention, at the same time, to the difficulty of maintaining an adequate
supply of lime in the soil against normal leaching losses. In connection
with the above survey Mr. Gray Singleton of the Federal Land Bank,
Columbia, South Carolina, pointed out that prior to this study in South
SOIL REACTION BASIS FOR CERTAIN LAND PRACTICES 35
Carolina the average yield of lint cotton was 214 pounds per acre, whereas
by 1939 it had risen to 368 pounds per acre. While it was not inferred
that a number of other factors were not closely involved with this improve-
ment in yield, the thought was expressed that the large increase in the
use of dolomite following the survey exercised a considerable influence.
This relationship was emphasized by the fact that the characteristic
chlorosis of cotton foliage, due to magnesium deficiency, has almost dis-
appeared from South Carolina cotton fields since the law was passed
requiring that dolomite be used as the filler for fertilizer.
The value of a reasonably accurate knowledge of soil reaction (pH)
in strawberry culture was well brought out by Dr. A. N. Brooks, in
charge of the Strawberry Laboratory at Plant City. As the result of
several years of work with the soils of eastern Hillsborough County he
has found that a pH of 5.5 is about optimum for this crop. The very
definite dangers of overliming were referred to briefly and reference was
made to the need for manganese and for the use of acidic fertilizers or
other acidulating materials under such circumstances.
Mr. Robert Edsall of Wabasso raised a question regarding the effect
of the moisture content of the soil at the time of sampling on the reaction
value that it shows, with the comment that he has observed an undried
field sample reading as high as pH 7.5 drop as low as pH 6.8 upon
being tested after air drying. On the other hand, he has observed soils
showing a pH as low as 5.2 in their undried state to increase in this
value upon being dried. In response to this inquiry, Dr. Peech said
he had not found appreciable variation in reaction as the result of air
drying light sandy soils. Edsall commented that only soils with high
replaceable calcium show this effect. The question was then raised
whether, for instance, a soil should be limed that showed a pH of
6.0 when moist and of 5.6 when air dried. The principal comment on
this inquiry was to the effect that working at field moisture content does
not give a permanent point of reference. Neither does air drying bring
about a permanent change in the soil. In other words, the reaction (pH)
of most soils must fluctuate considerably in the field under normal con-
ditions. An instance was made of the immediate rise in the pH of the
soil after adding organic fertilizers. Mr. Edsall then asked whether it
might be possible to develop a factor for converting reaction values from
one basis (air dry samples) to the other undriedd samples).
In response to the above question, Dr. A. F. Camp referred to an
experience he had several years ago with a soil sample from a blue-
berry area. The sample was brought to the laboratory in a tightly closed
container. The reaction was found to rise a full pH (from 4.5 to 5.5)
in a couple of days. Furthermore, no stable e.m.f. could be obtained with
the hydrogen electrode until all ammonia was removed by aeration. Such
experiences are not uncommon and suggest the advisability of air drying
the samples in an airy, well-protected place as soon after taking as
Mr. A. M. Hill, Jr., of Vero Beach, asked on what basis liming recom-
mendations should be made after determining the pH value of the soil,
stating that the National Lime Association has a simple procedure for
determining lime requirement. Dr. Camp stated that lime requirement
36 DEPARTMENT OF AGRICULTURE
depends on the texture of the soil as well as its pH and noted that
while this value can be: determined from a knowledge of the reaction and
exchange capacity, the maintenance of a given reaction in the soil is
made difficult in practice because of variations in the spray program
(under citrus grove conditions). Inasmuch as dolomite has been found
slower in its action and less likely to result in overlining effects, it
was referred to as a conservative material to use where there is any
In response to a further question raised regarding the danger of
applying too much lime to sandy soils of low pH, Dr. Peech referred to
certain experiments in progress and expressed the opinion that fairly
accurate recommendations can be made if the reaction of the soil and
the records of the fertilizer and spray programs are available for a
On the whole, it appeared that the most urgent need is the develop-
ment and adoption of a standard procedure and close adherence to it so
that plant performance in the field can be calibrated against it. In other
words, if we undertake to check variable plant responses against variable
results derived from using different methods, it will take a long time to
Earlier in the discussion, Mr. H. G. Clayton called attention to the
fact that the Agricultural Adjustment Administration has the organization
and routine machinery available for carrying out a liming prograni if
the need for lime can be determined. He also asked for a motion to
appoint a committee to develop a method of procedure for getting soil
reaction determinations made on a sufficiently comprehensive scale to be
of practical assistance. This motion was made and seconded at the
close of the general discussion and carried.