Soil pH and buffer pH methodology:
water and SMP/Sikora buffer reaction time
with soil and a laboratory survey
1. Soil pH and SMP Buffer pH methodologies
a. Time for SMP buffer to set in soil-water slurry
b. Conditions on LabFit for soil pH and Sikora
buffer pH
2. Laboratory survey on pH and lime recommendations
prepared for SERA-6 conference
June 4-5, 2007
F.J. Sikora
University of Kentucky
Variations in methods for determining 1:1 soil:water pH
Stirring (or
mixing) time
after adding
water
Standing
time before
determining
pH
Stirring during/before pH
measurement
Reference
pub
Quantity of
soil
SPAC, Inc.
4.25 cm3, 5 g,
or 10 cm3
Stir for 5 sec
10 min
stir during pH measurement
Southern
10 cm3
Mix for 5 sec
30 min
stir soil suspension by
swirling the cup slightly just
prior to reading pH
Northeastern
Scoop 5 cm3
or weigh 5 g
Stir
vigorously
for 15 sec
30 min
place electrodes in the
slurry, swirl carefully and
read pH immediately
North
Central
4.25 cm3
(5 g scoop)
Stir
vigorously
for 5 sec
10 min
place electrodes in the
slurry, swirl carefully and
read pH immediately
Western
5  0.1 g
Stir
thoroughly
for 5 sec
15 min
place electrodes in the
slurry, swirl carefully and
read pH immediately
Variations in SMP methods for determining buffer pH
Volume of buffer
added to soil pH
slurry and shaking
time
Oscillations/min
Standing time
before
determining
pH
Reference
pub
Quantity of soil
SPAC, Inc.
4.25 cm3 or 5 g
10 mL buffer, shake
for 10 min
(or 15 min)
>180
30 min
(or 15 min)
Northeastern
Scoop 5 cm3 or
weigh 5 g
10 mL buffer, shake
for 10 min
(or 15 min)
250
30 min
(or 15 min)
North Central
4.25 cm3
(5 g scoop)
10 mL buffer, shake
for 10 min
(or 15 min)
250
30 min
(or 15 min)
Western
5  0.1 g
10 mL buffer, stir
every 5 minutes for
20 min
Southern
20 min
excerpt from
Mclean, Eckert, Reddy, and Trierweiler. 1978. An improved SMP
soil lime requirement method incorporating double-buffer and
quick-test features. SSSAJ 42:311-316.
1975 North Central publication
on soil test methods.
Table in the current North Central region publication on soil test methods.
SMP Buffer pH
Lexington Quality Control Sample 4108
SMP buffer
7.0
Sikora buffer
6.9
+3s
6.8
Buffer pH
+2s
6.7
mean
6.6
6.5
-2s
-3s
6.4
6.3
0
50
100 150 200 250 300 350 400 450 500 550
Days after Jan 1, 2005
LEXINGTON LAB
PRINCETON LAB
buf
buf
LabFit method with Sikora buffer
Method for adding
water to soil
stirring time for
buffer in soil
time buffer set in soil
prior to measurement
Each sample stirred with stir rod
and let set for 30 min.
0.33 min.
±0.11
37 min.
±4
1.5 hrs
for 3
trays
(n=120)
Soil pH
Buffer pH
instrument
LabFit
pH,
Buffer
SikoraKY
buffer
pH,
new instr
water
pH, new
instr
instrument
LabFit
Soil pH,
10
y = 0.9613x + 0.1576
R2 = 0.9763
9
8
7
6
5
5,10
4
4
5
6
7
8
9
8
y = 1.0486x - 0.4033
R2 = 0.9607
7.5
7
6.5
6
5,10
5.5
10
5
5
Soil pH,
median
waterNAPT
pH, old way
5.5
6
6.5
7
7.5
SMP Buffer
pH,pH,
NAPT
SMP buffer
old waymedian
1:1 line
regression line
8
Soil pH
Each sample stirred with stir rod
for 5 sec and let set for 30 min.
Tray swirled for 5 sec. and let
set for 30 min.
10
y = 0.9613x + 0.1576
R2 = 0.9763
9
instrument
LabFit
Soil pH,
water
pH, new
instr
water
pH, new
instr
instrument
LabFit
Soil pH,
10
8
7
6
5
5,10
4
y = 0.8927x + 0.6065
R2 = 0.9647
9
8
7
6
5,10 tray swilred
5
4
4
5
6
7
8
9
10
4
Soil pH,
NAPT
water
pH, old median
way
5
6
7
8
Soil pH,
NAPT
water
pH, old median
way
1:1 line
regression line
9
10
A Mid-Atlantic Soil Testing and Plant
Analysis Work Group survey on methods for
soil pH, buffer pH and lime recommendations
Objectives are to assess:
1. variation in lab methods being used.
2. effect of method variation on intra- and
interlaboratory error.
3. variation in how lime recommendations are
made.
2006019
2006009
2005019
2005009
2004119
2004109
2003119
2003109
2002119
2002109
_01119
_01109
_00119
_00109
99119
99109
98123
98113
Soil pH MAD values in the NAPT program
0.25
0.2
0.15
0.1
0.05
0
2006019
2006009
2005019
2005009
2004119
2004109
2003119
2003109
2002119
2002109
_01119
_01109
_00119
_00109
99119
99109
98123
98113
SMP Buffer pH MAD values in the NAPT program
0.25
0.2
0.15
0.1
0.05
0
extra slides
3.5
y = -0.837 x + 7.00
Coefficient of Variation (%)
3.0
2
r = 0.658
2.5
2.0
1.5
1.0
y = -0.803x + 6.347
R2 = 0.691
0.5
0.0
5
5.5
6
6.5
SMP buffer pH
7
7.5
8
3.5
y = -0.1749x + 2.6093
R2 = 0.1805
Coefficient of Variation (%)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
4
5
6
7
water pH
8
9
10
Variations in methods for Mehlich III extractable nutrients
Reference
pub
Quantity of soil
Volume of Mehlich III
Mixing time and
method
SPAC, Inc.
5 cm3
50 mL
5 min on
reciprocating shaker
Southern
2.5 cm3
25 mL
5 min on
reciprocating shaker
Northeastern
Scoop 2.5 cm3
or 2.5 g
(references orig.
method on 10:1
ratio based on vol.)
25 mL
5 min on
reciprocating shaker
at 200 epm
North
Central
1.7 cm3
(2 g scoop)
20 mL
5 min on shaker at
200 or more epm
between 24 to 27oC
Western
2  0.05 g
20 mL
5 min on
reciprocating shaker