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Soil Testing Methods: How they
affect fertilizer and limerecommendations
F.J. Sikora, Ph.D.
University of Kentucky
20thannual Kentuckiana Crop Production SeminarDec. 9-10, 2003
Executive Inn Owensboro, Kentucky
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Nutrient recommendations
Soil
acidity
Available plant
nutrients
Interpretation
of lab results
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Nutrient recommendations
Soil
acidity
Available plant
nutrients
Interpretation
of lab results
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Most agronomic crops 6.4
Tobacco 6.6
Alfalfa 6.8
Target pH
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Mg2+
K+
H+
Ca2+
Ca2+
Mg2+
K+
Ca2+
Na+
K+
Na+H+
Ca2+
K
+
Mg2+
Mg2+
K
+
H+
H+
H+
Ca2+
Mg2+
K+
H+
Soil pH measures Active acidity
10 g soil + 10 ml water
pH electrode
H+
H+
H+
H+
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H+
H
+
H
+
H
+
H+
H+
H
+
H
+
H+
soil
particle
B
-
B
-
B-B
-B
-
B
-
B
-
B
-
H
+
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H
+
H
+
H+
soil
particle
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pH of bufferalone 7.5
Soil Buffer pH6.8
Soil Buffer pH6.4
Active and Residual acidity
x
y
Acidity (meq/100g) = 6.2 x (7.5
Buffer pH)
SMP Buffer
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SMP buffer
Mehlich buffer
Adam and Evans buffer
Initial pH
7.5
6.6
8.0
Woodruff buffer7.0
UK standard
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Common soil extractants
Bray I
Mehlich I
Ammonium Acetate (for K)
Bray II (for P)
Mehlich III UK standard
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Mehlich I =
Mixture of
2 acids
whole pie is total P
SOIL
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Mehlich III
whole pie is total P
SOIL
Mixture of
1 acid
+ fluoride
+ salt
+ chelate
+ pH buffer
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Extractant K (lbs/acre)
Bray I 272
Mehlich I 250
Mehlich III 332
Am. Acet. 330
Comparison of 6 extractants for soil test K
NAPT sample 2003-110: Rozetta silt loam from
Bureau Co., IL. Soil pH = 6.5, CEC = 8.3
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Variation in how 45 Laboratories in North
America report Mehlich III Soil Test P and K
Source - NAPT, 2nd quarter 2000
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
20 labs report as mg/kg
15 labs report as lbs/acre
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Mehlich III in lbs/acre
UK standard for P and K
mg/kgx 2 = lbs/acre
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Soil acidity
Available plant nutrients
Interpretation of lab results
Third stool leg
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High
Med
Low
V.Low
Soil Test
Level
100%
70-100%
50-70%
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0 1
Soil Test P (lbs/acre)
40
50
60
70
80
90
100
110
60305
Soil Test P (lbs/acre)
Low
MedHigh
V.low
Agronomic Threshold
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0
10
20
30
40
50
60
70
80
90
Dollars/
acre
Lincoln Co Lawrence Co.
UT lablab A
lab B
Fertilizer costs for Corn Production
* Data from H.J. Savoy, Univ. of Tenn.
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0
25
50
75
100
125
150
175
Bushels/
acre
Lincoln Co Lawrence Co.
UT lablab A
lab B
Corn Yields
* Data from H J Savoy Univ of Tenn
