Soil Testing Methods: How they
affect fertilizer and lime
recommendations
20th annual Kentuckiana Crop Production Seminar
Dec. 9-10, 2003
Executive Inn – Owensboro, Kentucky
F.J. Sikora, Ph.D.
University of Kentucky
Nutrient recommendations
Soil
acidity
Interpretation
of lab results
Available plant
nutrients
Nutrient recommendations
Soil
acidity
Interpretation
of lab results
Available plant
nutrients
Interpretation
of lab results
Available plant
nutrients
First stool leg
Soil acidity
Available plant nutrients
Interpretation of lab results
Target pH
Most agronomic crops
6.4
Tobacco
6.6
Alfalfa
6.8
Soil pH
Buffer pH
Water pH
They measure……
Active acidity
Residual acidity
pH electrode
K+
-
H+
Mg2+
Ca2+
-
H+
H+
H+
K+
-
H+
K+
H+
H+
Mg2+
H+
Na+
Ca2+
-
Na+
K+
Mg2+
-
Mg2+
K+
H+
Ca2+
Ca2+
Mg2+
H+
K+
10 g soil + 10 ml water
Soil pH measures Active acidity
Ca2+
+
soil particle
H
+
+ H
- B- - B
B
B
- B
-
B B
H
+
+
H
+
H
+
H
H
+
H
+
+
H
H
soil particle
+
H
H
H
+
+
pHelctrod
SMP Buffer
Active and Residual acidity
pH of buffer
alone
7.5
x
6.8
Soil Buffer pH
6.4
Soil Buffer pH
Acidity (meq/100g) = 6.2 x (7.5 – Buffer pH)
y
Initial pH
UK standard
7.5
SMP buffer
6.6
Mehlich buffer
8.0
Adam and Evans buffer
7.0
Woodruff buffer
Second stool leg
Soil acidity
Available plant nutrients
Interpretation of lab results
Phosphorus
Potassium
P
K
They measure……
Concentration of P and K in soil that is
related to plant-available P and K
Common soil extractants
Bray I
Bray II (for P)
Mehlich I
Mehlich III UK standard
Ammonium Acetate (for K)
SOIL
Mehlich I =
Mixture of
2 acids
whole pie is total P
SOIL
Mehlich III
Mixture of
1 acid
+ fluoride
+ salt
+ chelate
+ pH buffer
whole pie is total P
Comparison of 6 extractants for soil test P
Extractant
P (lbs/acre)
Bray I
112
Bray II
212
Mehlich I
46
Mehlich III
90
NAPT sample 2003-110: Rozetta silt loam from
Bureau Co., IL. Soil pH = 6.5, CEC = 8.3
Comparison of 6 extractants for soil test K
Extractant
K (lbs/acre)
Bray I
272
Mehlich I
250
Mehlich III
332
Am. Acet.
330
NAPT sample 2003-110: Rozetta silt loam from
Bureau Co., IL. Soil pH = 6.5, CEC = 8.3
Variation in how 45 Laboratories in North
America report Mehlich III Soil Test P and K
 20 labs report as mg/kg
 15 labs report as lbs/acre
 1 lab reports as lbs/acre-6 inches
 2 labs report as lbs/ppm
 2 labs report as ppm/lbs/acre
 1 lab repors an index
 3 labs reports as kg/ha
 1 lab reports as mg/L
Source - NAPT, 2nd quarter 2000
UK standard for P and K
Mehlich III in lbs/acre
mg/kg x 2 = lbs/acre
Third stool leg
Soil acidity
Available plant nutrients
Interpretation of lab results
Relative Sources of Nutrients
Soil Test
Level
Sufficiency
Level
soil
High
V.Low
fert.
soil
Med
Low
100%
soil
fertilizer
soil
fertilizer
Nutrients
available
from soil
External
nutrients
required
70-100%
50-70%
<50%
Agronomic Threshold
110
100
90
80
High
Med
70
60
Low
50
40 0
V.low
5
30
60
1
Soil Test P (lbs/acre)
Soil Test P (lbs/acre)
3 philosophies with a high soil test
maintenance
Soil test P or K
Agronomic Threshold
Time (years)
Dollars/acre
Fertilizer costs for Corn Production
90
80
70
60
50
40
30
UT lab
lab A
lab B
20
10
0
Lincoln Co
Lawrence Co.
* Data from H.J. Savoy, Univ. of Tenn.
Corn Yields
175
Bushels/acre
150
125
100
UT lab
lab A
lab B
75
50
25
0
Lincoln Co
Lawrence Co.
* Data from H.J. Savoy, Univ. of Tenn.