soil testing methods: how they affect fertilizer and lime recommendations f.j. sikora, ph.d....
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Soil Testing Methods: How they affect fertilizer and lime
recommendations
F.J. Sikora, Ph.D.University of Kentucky
20th annual Kentuckiana Crop Production SeminarDec. 9-10, 2003
Executive Inn – Owensboro, Kentucky
Nutrient recommendations
Soil acidity
Available plant nutrients
Interpretation of lab results
Nutrient recommendations
Soil acidity
Available plant nutrients
Interpretation of lab results
Nutrient re
commendations
Available plant nutrients
Interpretation of lab results
Soil acidity
Available plant nutrients
Interpretation of lab results
First stool leg
Most agronomic crops 6.4
Tobacco 6.6
Alfalfa 6.8
Target pH
Water pH
Buffer pHSoil pH
Active acidity Residual acidity
They measure……
-
-
-
-
-
-
-
-
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+
H+
H+ H+
H+
H+
H+
H+H+
H+
soil
part
icle
B-
B-B-B-
B- B-B-B-
H+
H+
H+
H+
soil
part
icle
pH e
lect
rode
pH of buffer alone
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
SMP buffer
Mehlich buffer
Adam and Evans buffer
Initial pH
7.5
6.6
8.0
Woodruff buffer7.0
UK standard
Soil acidity
Available plant nutrients
Interpretation of lab results
Second stool leg
P
PotassiumPhosphorus
Concentration of P and K in soil that is related to plant-available P and K
They measure……
K
Common soil extractants
Bray I
Mehlich I
Ammonium Acetate (for K)
Bray II (for P)
Mehlich III UK standard
Mehlich I =
Mixture of2 acids
whole pie is total P
SOIL
Mehlich III
whole pie is total P
SOIL
Mixture of1 acid+ fluoride+ salt+ chelate+ pH buffer
Extractant P (lbs/acre)
Bray I 112
Bray II 212
Mehlich I 46
Mehlich III 90
Comparison of 6 extractants for soil test P
NAPT sample 2003-110: Rozetta silt loam from Bureau Co., IL. Soil pH = 6.5, CEC = 8.3
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
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
Mehlich III in lbs/acre
UK standard for P and K
mg/kg x 2 = lbs/acre
Soil acidity
Available plant nutrients
Interpretation of lab results
Third stool leg
High
Med
Low
V.Low
Soil Test Level
100%
70-100%
50-70%
<50%
Sufficiency Level
Nutrients available from soil
External nutrients required
soil
soil
soil
soil
fertilizer
fertilizer
fert.
Relative Sources of Nutrients
0 1
Soil Test P (lbs/acre)40
50
60
70
80
90
100
110
Yie
ld (
% o
f m
axim
um
)
60305
Soil Test P (lbs/acre)
Low
MedHigh
V.low
Agronomic Threshold
3 philosophies with a high soil test
Time (years)
So
il t
est
P o
r K
Agronomic Threshold
maintenance
sufficiency
buildup
0
10
20
30
40
50
60
70
80
90
Dol
lars
/acr
e
Lincoln Co Lawrence Co.
UT lablab Alab B
Fertilizer costs for Corn Production
* Data from H.J. Savoy, Univ. of Tenn.
0
25
50
75
100
125
150
175
Bus
hels
/acr
e
Lincoln Co Lawrence Co.
UT lablab Alab B
Corn Yields
* Data from H.J. Savoy, Univ. of Tenn.
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