The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
site maintained by the Florida
Cooperative Extension Service.
Copyright 2005, Board of Trustees, University
Soil Acidity and Alkalinity, Lime and Liming1
E. A. Hanlon2
Importance of Soil pH
Because of the influence of pH on availability of
plant nutrients, the pH of a soil is probably its most
important single chemical characteristic. When soil
pH is too low or too high for optimum plant growth,
the pH can be raised by applying a liming material or
lowered by applying an acid-forming material.
Determining soil pH is the first step in any
pH of Florida Soils
Most Florida soils are acid, with pH's ranging
between 4.0 and 6.5. The most acid are unlimed
flatwoods soils, having pH's of 3.0 5.0. However,
there are many soils in Florida that contain naturally
occurring calcium carbonate and have pH values
above 7.0. These soils generally are found in the
vicinity of the coast or in the marl soil areas of
Terms frequently used to dLscibL the
approximate soil pH ranges are indicated in Table 1.
SOURCES OF SOIL ACIDITY
Soils in humid regions naturally become more
acid with time due to leaching of the basic cations,
primarily calcium, magnesium, potassium, and
sodium. In addition, as the basic cations are taken up
by plants they are replaced with hydrogen,
contributing further to soil acidity. Decomposition of
organic matter also contributes to the natural
tendency for soils to become more acid with time.
Table 1. Soil pH Descriptions
strongly acid < 5.0
moderately acid 5.0 6.0
slightly acid 6.0 6.5
near neutral 6.5 7.5
slightly basic 7.5 8.0
moderately basic 8.0 8.5
strongly basic > 8.5
Many fertilizers contain ammonium nitrogen
which releases hydrogen when nitrified in the soil.
Application of significant amounts of such fertilizer
will lower the soil pH.
Resistance to change in pH is referred to as
buffer capacity. The buffer capacity of soil is directly
related to the soil's cation exchange capacity. Sandy
soils tend to have small buffer capacities and clayey
soils or those high in organic matter have large buffer
If just enough liming material were added to
neutralize the acid in the soil solution, more acid
would move from the exchange complex to the soil
solution. As a consequence, the pH rise would be
negligibly small and would remain negligibly small
until enough lime were added to appreciably deplete
the exchangeable (reserve) acidity.
1. This document was published December 1992 as RF-AA002, Florida Cooperative Extension Service. For more information, contact your
county Cooperative Extension Service office.
2. Extension Soil Scientist, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville.
The Institute of Food and Agricultural Sciences is an Equal Opportunity/Affirmative Action Employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, or national origin.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean
Soil Acidity and Alkalinity, Lime and Liming
This buffering effect is equally important in
preventing a rapid lowering of soil pH. For example,
H ions are produced by soil biochemical changes and
the addition of certain fertilizers. This increases the
number of H ions in the soil solution. Almost
immediately, however, this acidity becomes adsorbed
on soil cation exchange sites, again resulting in little
change in the soil solution pH. These two examples
show that the basis of buffer capacity lies in the
adsorbed cations of the exchange complex.
INFLUENCE OF pH ON PLANT
Plants differ in the pH range over which they may
grow. This is due primarily to the effect of pH on the
availability of various plant nutrients in the soil at
different pH levels. In strongly acid mineral soils (pH
below 5.0), aluminum, iron, zinc, copper, and
manganese availability increases and these ions may
reach levels that are toxic to plants. Strongly acid
soils are usually depleted of the nutrient
bases--calcium, magnesium, and potassium--so that
plants may be unable to obtain the quantities of these
nutrients necessary for proper growth. Also, reduced
availability of phosphorus and molybdenum occurs at
low soil pH.
Soil pH is so important in plant nutrition that its
influence on each of the crop nutrients will be
discussed briefly below.
The effect of pH on nitrogen availability is an
indirect one. N availability in soil is closely associated
with the activity of microorganisms which carry out
nitrogen transformations and decompose organic
matter. The most favorable pH range for these
microorganisms is 6.0 to 8.0.
The availability of phosphorus is quite complex
and to a large extent reduces the pH range in which
crops can be economically produced. At pH levels
below 5.5, insoluble iron and aluminum phosphates
form and the phosphorus is essentially unavailable to
plants. At pH values above 7.5 the formation of
insoluble tricalcium phosphate becomes an important
factor in reducing the availability of soil phosphorus.
The greatest availability of soil phosphorus in mineral
soils is between pH 5.5 and 6.5.
The availability of potassium is less influenced by
soil pH than that of most of the other elements.
It is incorrect to say that pH influences calcium
availability, because it is primarily the calcium
concentration in the soil that influences pH. When
the soil exchange capacity is largely (e.g. 90%)
saturated with bases (calcium generally being the
most abundant), the pH will usually be high (e.g. 6.5)
and, when the exchangeable Ca is low (e.g. 50%) the
pH will usually be low (e.g. 4.5). Calcium deficiency
is highly unlikely if soil pH is above 5.5, either
naturally or after correction with lime.
Please note that the Mehlich I (double acid)
extractant removes more than just exchangeable Ca
from the soil. Since calcium phosphates are also
solubilized it is possible to have a high Ca soil test
and a relatively low pH. Any limestone particles will
also be solubilized by the extracting acid.
Magnesium, like Ca, is one of the basic ions
whose concentration influences the soil pH. When the
soil is acid it is usually because the bases have been
leached out, and Mg will usually be low. Situations
occur, however, where pH is adequate for good plant
growth but the Mg content of the soil is low. In such
cases fertilizer Mg should be applied in some form
other than dolomite to avoid the adverse effect of
over-liming on other nutrients.
Like nitrogen, the availability of sulfur is very
much related to the activities of soil microorganisms.
If the pH is favorable for the microorganisms it will
also be favorable for sulfur availability. Sulfur is very
mobile in soil and tends to leach quite readily. Many
of the bases (calcium, magnesium, potassium) leach
out of the soil as sulfates.
Iron is quite soluble in an acid soil and becomes
increasingly less soluble as the pH increases. When
the soil pH is much above 6.2, the solubility of iron is
seriously reduced and certain plants may suffer Fe
Soil Acidity and Alkalinity, Lime and Liming
deficiency. If the soil becomes too acid and is high in
phosphorus, the iron combines with phosphorus to
form insoluble iron phosphate.
The availability of manganese is very much like
that of iron. As soil pH increases the availability of
manganese decreases. Florida soils are generally low
in Mn and many crops in Florida show Mn deficiency
above pH 6.2. In other areas of the country, Mn may
be toxic in strongly acid soils.
Zinc availability also decreases with increasing soil
pH. Usually a pH above 6.5 is too high for soils low
in zinc, especially for crops with a high zinc
requirement. Crop responses to soil applications of
zinc have been measured when the pH ranged from
6.0 to 6.5. Where zinc deficiency is a problem and
the pH of the soil is high, most crops respond to
nutritional sprays containing zinc.
Like zinc, copper is quite available at acid pHs,
becomes increasingly less available as the pH rises,
and is almost unavailable above pH 7.5. In vegetable
fields and citrus groves where copper sprays have
been used year after year, the copper levels have built
up to toxic concentrations and copper toxicity is
avoided by liming the soil to a pH between 6.5 and
Boron is most available between pH's of 5.0 and
7.0. Above 7.0 the availability decreases very rapidly
until a pH of about 8.5. Above 8.5 the availability
again increases. Boron is needed in such small
quantities that there is a narrow margin between a
concentration in the soil that is deficient and one that
Molybdenum is the only micronutrient that is
more soluble at higher than at lower pH levels.
Liming often concurrently solves molybdenum
deficiencies. Molybdenum may become toxic in some
MEASURING SOIL pH
The pH test is an important diagnostic
determination made on the soil. It is used in making
liming recommendations and to aid in evaluating the
availability of various plant nutrients. Soil pH is most
frequently measured in a soil and water mixture. A
very common procedure is to mix two volumes of
water with one volume of soil, allow the mixture to
equilibrate for about 30 minutes, stir, and determine
the pH of the suspension with a pH meter. This is the
procedure used by the IFAS Extension Soil Testing
The use of a glass hydrogen-sensitive electrode
and potentiometer (pH meter) is the most common
means of determining the pH of the soil. The pH
value obtained for the soil and water mixture is read
directly from the dial or electronic readout. The
meter should be standardized using two standard
buffers, preferably pH 7.0 and 4.0 and checked with
a pH 10 buffer. It should be standardized each time
the meter is used and every hour when in continuous
operation. If the instrument is set to read pH 7.0, the
pH 4.0 buffer should read 4.0. If the pH 4.0 buffer
cannot be made to read 4.0 by adjusting the
temperature compensator, then the buffers should be
checked with another instrument or new ones should
be made up. If the buffers still do not check, look for
trouble in the electrodes. Consult the operating
manual for procedures for checking the operation of
the instrument itself. The electrodes should be kept
moist, with the tips in distilled water or pH 7.0 buffer
at all times when not in use. The KC1 crystals in the
reference electrode also need to be maintained at the
recommended level. Diffusion of salt through the salt
bridge to the less concentrated samples causes
gradual depletion of the saturated KC1 as indicated in
the operating manual.
The Extension Soil Testing Laboratory will be
happy to assist county labs in diagnosing problems
with their pH meters.
Check the pH of a standard soil along with any
Soil Acidity and Alkalinity, Lime and Liming
Indicator dyes have been used extensively in some
laboratories and in kits used for rapid soil testing in
the field. They have definite limitations. If indicators
are used, a standard soil should be used to check the
indicators everytime a pH is determined on an
unknown soil. Various salts in the soils may alter the
color of the indicator dye, thus giving erroneous pH
A major problem with the use of indicators for
determining pH of Florida soils is that organic matter
tends to mask the dye color. Indicator dyes for
determining pH should be used with caution.
Under the best circumstances, indicator dyes
estimate soil pH only to the nearest 0.5 pH unit. This
is adequate for determining the general range of soil
pH, but should not be used for serious management
LIME AND LIMING
Pre-Requisites of Soil Liming Materials
A material must have three fundamental
characteristics in order to be considered suitable for
neutralizing the acidity of agricultural lands.
It must remove acidity from the soil system.
It must not add harmful ions to the soil.
It must be plentiful and relatively inexpensive.
The carbonates, oxides, and hydroxides of calcium
and magnesium are the most commonly used
agricultural liming materials because they fulfill three
prerequisites. Examples of materials which are not or
can not be used for liming agricultural lands because
they fail one or more of the prerequisites include:
calcium sulfate (gypsum), produces no net
change in acidity of the soil system results.
-sodium carbonate would add large quantities
of sodium ion; sodium causes loss of nutrient
cations and can also have detrimental effects
on soil physical properties of fine-textured
-potassium carbonate fills the first two, but is
too expensive to use as an agricultural liming
Liming Materials Used in Florida
Calcitic and dolomitic limestone are by far the
most commonly used agricultural liming materials in
Florida, because they are the cheapest and most
abundant materials available for this purpose. These
materials are taken from quarries, ground to a
powder, and shipped by rail, barge, or truck. Without
any further pIw' \ssin', they meet the fundamental
criteria discussed above. Transportation is a major
portion of the cost of these liming materials.
Many other materials may be used for liming
agricultural soils, provided that they meet the
fundamental criteria discussed above.
Some that have been used, and a few comments
about each, are listed below:
hydrated lime: fast acting calcium hydroxide;
considerably more expensive than agricultural
-burnt lime: fast acting, mainly calcium oxide,
caustic to handle
marl: variable in carbonate and moisture
shells: oyster shells or other sea shells are
composed mostly of calcium carbonate, must
be finely ground to be good liming materials
industrial by-products: slags and refuse limes
from paper mills and water treatment plants
are examples, frequently must be dried before
useable, must verify that harmful impurities
are not present
Importance of Particle Size
The finer the agricultural lime particle, the higher
is its acid-neutralizing efficiency. A material that is
finely ground has more surface area, comes in contact
with more soil, and thus raises the soil pH more
quickly than the same material coarsely ground. Data
shown in Table 2 illustrate this effect.
Soil Acidity and Alkalinity, Lime and Liming
By Florida law, agricultural limestones must be
ground so that:
1. Not less than 90% passes an 8 mesh sieve.
2. Not less than 80% passes a 20 mesh sieve.
3. Not less than 50% passes a 50 mesh sieve.
"Mesh" refers to the number of openings per
linear inch in the screens used for analyzing particle
size. A 60 mesh sieve has 60 openings per linear
inch, or 3600 openings per square inch. Particles
passing through a 60 mesh sieve are smaller than
0.0098 inch in diameter.
Factors Affecting Lime Requirement
Soil Buffering Capacity
Soils with the same pH but different buffering
capacities require different amounts of lime to reach
the desired pH level. Chemical determination of
buffering capacity is part of the Adams-Evans Lime
Requirement Test used by the IFAS Extension Soil
Crop to be Produced
The pH level to which a soil should be limed
depends on the requirement of the specific crop to be
produced. See Circular 817 for target pH levels of
Table 2. Increase in soil pH with three size fractions
of dolomitic limestone. (Initial soil pH was 5.0.)
PARTICLE SIZE (MESH)
APP ICATION 8 to 20 40 to 60 60 to 100
1 5.0 5.4 5.4
12 5.6 6.0 6.2
24 5.9 6.3 6.4
36 6.3 6.5 6.6
Fertilizer salts generally lower the measured soil
pH as soon as they are added to the soil. As soon as
the fertilizer disipates, the measured pH returns to
normal. The net acid-forming potential of various
fertilizer materials is shown in the section "Fertilizers
& Fertilization". In a good liming program, fertilizer
applications should be taken into consideration.
Availability, economy, magnesium content,
neutralizing value, and fineness should all be
considered in selection of a liming material.
The decision of whether to use dolomitic or
calcitic ("hi cal") lime should be based primarily on
cost of the material to the producer. When both lime
and Mg are needed, dolomite can serve as the liming
material and also as a source of the nutrient Mg.
However, if the cost of dolomite is significantly higher
than that of calcite, the producer should consider the
alternative of applying calcite as the liming material
and Mg in the fertilizer. Application of dolomite as a
source of Mg without regard to the liming effect can
lead to other nutritional problems in soils with pH
above 6.3 or so.
Frequently, producers may have access to
by-product materials which can serve very well for
liming agricultural land if the nature of the material
is understood and proper precautions are followed.
Lime from municipal water treatment plants is an
example. Some suggestions about its handling and use
-As received from the water treatment plant,
the lime usually has the consistency of a thick
paste. Pile and allow to dry before attempting
-Turn with a front end loader to promote
drying. Spread before completely dry and on
a calm day, to minimize drift.
-Use about 80% as much material as you
would agricultural limestone. It will react
quickly due to its fineness and thus carries
more potential for overliming if not properly
Soil Acidity and Alkalinity, Lime and Liming
Materials sold as aglime are covered by the
Florida Commercial Fertilizer Law and must meet
specifications of fineness of grind, carbonate
equivalence, and Mg content (in the case of
dolomite). This affords some consumer protection.
Lime by-products are not covered by the law and the
consumer must realize more personal responsibility
when dealing with such products.
Liming is probably the most important soil
fertility practice on strongly acid mineral soils.
However, many field crops in Florida produce just as
well on moderately acid as on slightly acid soil.
Much has been written about the "optimum pH
range" for crops. The most commonly quoted is 5.5 to
6.5. This has resulted in a commonly-held belief that
"most" crops produce better at 6.5 than at 5.5. If read
carefully, the statement says that a pH of 5.6 is just as
"optimum" for a crop as is a pH of 6.4. Liming when
no positive crop response is obtained is not a good
use of resources.
Frequency and Placement
The frequency of liming can best be determined
through a combination of soil testing every two to
four years and good liming and fertilization records.
The lime requirement is calculated on the basis of a
6-inch plow layer of soil. Therefore, where practical,
the applied lime should be mixed with the soil by
Lime Requirement Determination
The Adams-Evans (A-E) Lime Requirement Test
used by the IFAS Extension Soil Testing Lab involves
shaking a known amount of soil with a given quantity
of buffer solution. The solution is composed of
several chemical reagents and is buffered at pH 8.00.
After a period of time, the pH of the soil-buffer
mixture is determined. The amount of soil acidity is
calculated from the pH depression of the original
buffer solution by the soil. The lime required to raise
the soil pH to a desired value is then estimated using
relationships associated with the method. The liming
recommendation, rounded to the nearest half ton, is
reported by the IFAS Extension Soil Testing
Laboratory as part of the Soil Test Report and
Standard Fertilizer Recommendations.