APPENDIX 16
METHODS FOR DETERMINING LIME REQUIREMENTS OF ACID SOILS
(REPRODUCED WITH PERMISSION FROM CHAPMAN AND PRATT, 1961)
Lime requirement of acid soils
Many procedures have been developed for measuring the lime
requirement of soils, defined as the amount of lime needed to
bring the pH value from its present value to any given pH value.
Two methods are described here.
The first method is the most
reliable, but requires more time and equipment, and involves a
direct titration with calcium hydroxide.
The second method,
developed by Shoemaker (1959), depends on the depression in pH of
a buffer solution when soil is added.
It is rapid and involves a
greater error, but can be used in the lime requirement estimation
of large numbers of samples in relatively little time.
Calcium Hydroxide Titration
Reagents - Calcium hydroxide solution.
Add 1 g of calcium oxide
or 1.5 g of calcium hydroxide per l of carbon dioxide-free water
used.
Mix and let stand protected from air until the excess has
settled.
Siphon off the solution.
Store in a bottle protected
from the carbon dioxide of the air.
Procedure - Place 10 g of acid soil in each of seven 100 ml
beakers and add 0, 5, 15, 20, 30, 40, and 50 ml of calcium
hydroxide solution to beakers 1 through 7 respectively.
Add
sufficient water to make each sample to a soil to water ratio of
1:5.
Let stand for 3 days and determine the pH value of the
soil-water suspension.
Plot the pH against the milliequivalents
(me) of calcium added per 100 g of soil and determine the amount
of lime needed to bring the pH to the desired level.
One me of
calcium per 100 g is equal to 100 pounds of lime per acre,
assuming the lime is mixed with 2,000,000 pounds of soil.
Remarks: - This method can be used if only a few samples are to
be analyzed.
If, however, there are large numbers, the space and
time limitations become too great and the faster method described
in the next section can be used.
Three days are required for the reaction of calcium hydroxide
with acid soil to come to an approximate equilibrium.
Actually,
about 97 percent of the reaction is complete in this time and the
true equilibrium is attained after many days.
Buffer Method
Reagents: - Buffer solution.
Dissolve 1.8 g of p-nitrophenol,
2.5 ml of triethanolamine, 3.0 g of potassium chromate, 2.0 g of
Ca(OAc)2 2H20, and 40.0 g of CaCl2.2H20 in approximately 800 ml of
distilled water.
Adjust the pH to 7.50 using hydrochloric acid
or sodium hydroxide solutions, and dilute to 1 l.
Best results
are obtained if 10-20 l are prepared at one time.
If protected
from carbon dioxide, this reagent will remain stable for 6 months
or more.
When titrated with hydrochloric acid, 50 ml of buffer
should require 2.6-2.7 me to bring the pH to 3.5 and the
titration curve should be a straight line between pH 7.5 and 3.5.
Procedure: - Weigh 10.0 g of soil and transfer to a 125 ml
Erlenmeyer flask.
min.
Add 20 ml of buffer solution and shake for 10
Transfer to a 50 ml beaker and use a pH meter to determine
the pH value.
The lime requirement is proportional to the
depression in pH of the buffer.
The lime requirement can be
determined from the data in Table A.11, or the data in Table A.11
can be plotted and the lime requirement obtained by reading from
the pH vs. lime requirement line.
If the pH of the soil-buffer suspension is greater than
approximately 6.5, as is found with some highly acid, sandy
soils, repeat the procedure using 50 g of soil and 20 ml of
buffer, then divide the obtained lime requirement by 5.
This
modification gives better accuracy for poorly buffered soils of
low lime requirement.
The answer is obtained in terms of tons of pure calcium carbonate
per 2,000,000 pounds of soil to bring the pH to 6.5.
Appropriate
corrections must be made for variations in depth of mixing of
lime or in bulk density of soils.
A 6.5-inch depth of soil over
an acre in area will have 2,000,000 pounds of dry soil if the
bulk density is 1.35.
Table A.11.
Lime requirement scale for buffer method.
Soil Buffer
pH
Lime
Requirement
Soil Buffer
pH
tons CaCO3*
Lime
Requirement
tons CaCO3
6.7
1.6
5.7
7.6
6.6
2.2
5.6
8.2
6.5
2.8
5.5
8.9
6.4
3.4
5.4
9.5
6.3
4.0
5.3
10.1
6.2
4.5
5.2
11.0
6.1
5.2
5.1
11.7
6.0
5.8
5.0
12.4
5.9
6.4
4.9
13.2
5.8
7.0
4.8
14.0
*Tons of pure calcium carbonate per 2,000,000 pounds of soil or
per acre if it is mixed with 6.5 inches of soil having a bulk
density of 1.35.