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
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):FIDA COOPERATIVE EXTENSION SERVICE
UTE OF FOOD AND AGRICULTURAL SCIENCES
UNIVERSITYY OF FLORIDA, GAINESVILLE
SOIL REACTION (pH)
By J. NeSmith & E. W. McElwee
What is Soil pH?
Soil pH is the measure of acidity (sourness) or
alkalinity (sweetness) of a soil. Though derived by
a complicated technical formula, a simple numeri-
cal scale is used to express pH. The scale goes from
0.0 to 14.0, with 0.0 being most acid and 14.0 be-
ing most alkaline. The halfway value on the scale,
7.0, is neutral, i.e., neither acid nor alkaline (see pH
scale below). Soil acidity increases as pH values
decrease from 7.0 to 0.0 and soil alkalinity in-
creases as pH values increase from 7.0 to 14.0.
The pH values for several common substances
are listed to help explain the pH scale.
4 Lemon Juice pH 2.4
j Orange Juice pH 3.7 Acid
Sour Milk pH 4.7
Fresh Milk pH 6.7
Pure Water pH 7.0 Neutral
I I Human Blood pH 7.3
7 3 Sea Water pH 7.9 Alkaline
S Soap Solution pH 9.3
Why is pH Important?
Soil pH is important because it influences
several soil factors that affect plant growth; such
as: (1) soil bacteria, (2) nutrient leaching, (3)
toxic elements, (4) nutrient availability, and (5)
soil structure. The activity of bacteria that change
and release nitrogen from organic matter and cer-
tain fertilizer materials are particularly affected by
soil pH. These organisms operate best in the opti-
mum pH range of 5.5 to 7.0. Plant nutrients leach
or wash out of the soil much more rapidly at pH
values below 5.0 than from soils with values be-
tween 5.0 and 7.5. In certain soils, when the pH
drops below 5.0, aluminum may become toxic to
plant growth. Soil pH also affects the availability
of plant nutrients. Plant nutrients are generally
most available to plants in the pH range 5.5 to 7.0.
The Nutrient Availability Chart shows this effect.
PH affects the structure of the soil, especially in
soils containing appreciable amounts of clay. In
the optimum pH range 5.5 to 7.0 clay soils are
granular and are easily worked, whereas if the soil
pH is either extremely acid or extremely alkaline,
clays tend to become sticky and hard to cultivate.
A pH determination will tell whether
your soil is within range that produces good
plant growth or whether it will need to be
treated to adjust the pH level. For most
plants, the optimum pH range is from 5.5 to
7.0, but some plants will grow in more acid
soil and some at a more alkaline level.
PH is not an indication of fertility, but
as pointed out above, it does affect the availa-
bility of fertilizer nutrients. A soil may con-
tain adequate nutrients yet growth may be
limited by a very unfavorable pH. Likewise,
builder's sand, which is virtually devoid of
nutrients, may have an optimum pH for plant
How to Correct pH
Normally, lime or dolomite is used to in-
crease the pH, or "sweeten" the soil. Lime
contains mainly calcium carbonate and dolo-
mite contains both calcium carbonate and
magnesium carbonate. Ground limestone and
dolomite are less likely to "burn" plant roots
than hydrated lime and is therefore recom-
mended for home use. The amount of these
materials necessary to change the pH will de-
pend on the soil. The greater the amount of
organic matter or clay in a soil, the more lime
or dolomite required to change the pH.
Table I shows how the amount of liming
material necessary to change the pH varies
with clay and organic matter content.
If a soil is too alkaline, determine if it is
due to a soil characteristic or lime application.
It is quite difficult, if not impossible, to
change appreciably the pH of naturally alka-
line soils by use of sulfur, ammonium sulfate,
or similar acid-forming materials. If a high pH
is due to applied lime or other alkaline addi-
tives, sulfur, ammonium sulfate, or similar
acid-forming materials can be applied.
To decrease soil pH use super-fine dust-
ing or wettable sulfur. It takes approximately
1/3 the amount of wettable or super-fine sul-
fur to decrease pH one unit as it does ground
limestone or dolomite to raise soil pH. In
other words, divide the amounts listed for a
particular soil type and size of area by 3 to
determine the amount of wettable or super-
fine dusting sulfur needed to acidify the soil
one pH unit. Not more than 1/2 ounces of
sulfur per 10 square feet, one pound per 100
square feet, or 10 pounds per 1000 square
feet, should be used in one application. Re-
peat applications of sulfur should not be
made more often than once every eight weeks.
Remember that in the soil sulfur oxidizes and
mixes with water to form a strong acid that
can burn the roots of plants--use with caution.
Consult your County Extension Agent or
garden supply dealer before using sulfur for
Approximate pounds of ground limestone or dolomite lime
required to increase soil one unit of pH.*
Soil Content of Sod Coentn of
l Orgni Matter Low Organic Matte Modr.,te
Lb Pr Lb Per Lb Per Lb Per Lb Per LbsPer
010SqFt O SqFt 1000SFt 10 Ft 100 Ft 1000 SFt
Sand 0.4 3.7 37 0.7 7.0 70
SLomy Snd 0.5 4.7 47 0.8 8.4 84
SSandyLonam 0.6 60 60 0.9 9.3 93
SandClay Loam 09 9.3 93 110 10.9 109
*Your County Extension Agent or Garden Supply Dealer should be
consulted as to the kind and amount in your particular area.
Approximate areas listed are useful in estimating liming needs:
(1) For individual plants. For example, a shrub with a spread of
of about 3 feet covers approximately 10 square feet of area.
(2) Flower and shrub beds are commonly measured in hundreds
of square feet. For example, a 5 ft. x 20 ft. strip covers 100
square feet or a row 33 ft. long covers 100 square feet.
(3) Lawns are commonly measured in thousands of square feet.
For example, a strip of lawn, 10 ft. x 100 ft., covers 1000
NUTRIENT AVAILABILITY CHART
Note: Width of Bar indicates relative availability
Medium 6 Slghtly
PHOSPHA TES .
- ,---. I
A Medium 9
CALCIUM PHOSPHA TE ::
..... ..::':'''" ... .. .. ..-v v ------." ''''': ......iiii~iiii~i ii iiiiiiii iii: i ii iiii i! i ~iiiiiiiiiiii~ ii
a I I I __________ -
.. .. ,~- ~ .- ~ .- -- ....... ... ......... .......I
............- -- - """ '' --- --
.-.- .-.-.- -,- --,-,,-,- -, ,
W M .W O..
_- -9. _. ..
rl L. ~~
DESIRABLE PH RANGES FOR COMMON GARDEN AND LANDSCAPE PLANTS
Moderately to Moderately Slightly
Strongly Acid Acid Acid
oH 5.5-5.9 oH 6.0-6.4 pH 6.5-6.9
Garden Flowers and Bulbs
(also to pH 7.5)
(also to pH 7.5)
Below oH 5.4
What, exactly, is IFAS?
IFAS is a complex of scientific disciplines devel
three-pronged statewide approach to support F-
agriculture. Scientists conduct basic and applied re
to form one arm of IFAS the Agricultural Expe.
Stations. Other scientists expert in helping p,
adjust to change concentrate their efforts on ge:
scientific methods and new knowledge used by the
cultural industry in the most effective manner pos.
They make up the Cooperative Extension Service.
other scientists and educators are responsible for d
oping the trained manpower for Florida's agricu
industry through the teaching programs of the Colli
Agriculture and School of Forestry.
How can I find out more about I FAS programs?
Visit the Extension agent's office in your county o,
Research Center in your area. And, when you a?
Gainesville, visit your University of Florida whert
headquarters of IFAS is located.
This public document was promulgated at
cost of $762.50, or 8.05 cents per copy, to info
nurserymen and their employees about pr-
gation of woody ornamentals by cutti&
COOPERATIVE EXTENSION SERVICE, UNI-
VERSITY OF FLORIDA, INSTITUTE OF FOOD
AND AGRICULTURAL SCIENCES, K. R. IL
Tefertlller, director, In cooperation with the
United States Department of Agriculture, publishes
this Information to further the purpose of the May
8 and June 30, 1914 Acts of Congress;and is authorized to p,
research, educational Information and other services only to
duals and institutions that function without regard to race,,.
sex or national origin. Single copies of Extension publics
(excluding 4-H and Youth publications) are available free to FI
residents from County Extension Offices. Information on bulk
or copies for out-of-state purchasers is available from C. M. h
Publications Distribution Center, IFAS Building 664, Univer
Florida, Gainesville, Florida 32611. Before publicizing this p;
tlon, editors should contact this address to determine availa
I nierit o Ford