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Soil Sampling and Nutrient Additions for Crop Production
Edwin RitcheyExtension Soil Specialist
UK-REC
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2
Soil Sampling
• Indicates the current nutrient status of soil
• A guide for nutrient additions
• Diagnose nutrient deficiencies or toxicities
• Saves money• Cost is small compared
to over/under fertilization
Photo, Josh McGrath
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Random Soil Sampling
• Want a representative sample (AFS=2,000,000 lb)• Uniform areas - based on drainage, cropping history,
erosion, past management • e.g. feeding areas (manure), fencerows, etc.
• Collect at least 10 cores, more for larger areas• The more samples (cores) collected, the closer to the
true field value obtained• Every two years, or so…• Fall or Spring?• Depth?
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4
MANURE SAMPLING & TESTING
Recommendations are only as good as the sample they are based on
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Determining Nutrients Present
H
H
H
H
_ _ _
Na+
Al3+
Mg2+
K+
Ca2+
_ _
_ _
_ _
soil pH 5 Mehlich 3
NH4+
NH4+NH4
+
NH4+
NH4+NH4
+
NH4+
NH4+NH4
+
NH4+
NH4+NH4
+
NH4+
Base cations analyzed in leachate
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How much fertilizer do I add?
• Soil Test (UK, Waters, A&L)• Considerations for soil lab
– Must be easily repeatable– Relatively quick– Must be “equivalent” to what is available to plant
• Correlation• Calibration
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Nutrient availability to plants
Readily available
Slowly available
Soil solution
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0 1Soil Test P (lbs/acre)
40
50
60
70
80
90
100
110
Y ie l
d (%
of m
axim
um)
60305
Soil Test P (lbs/acre)
Correlation: Is a soil test a good predictor of nutrient ……………………..availability?
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0 1Soil Test P (lbs/acre)
40
50
60
70
80
90
100
110
Y ie l
d (%
of m
axim
um)
60305
Soil Test P (lbs/acre)
Low
Med
High
V.low
Calibration: How much fertilizer is needed below threshold?
Arrows represent the amount of fertilizer needed to achieve maximum yield
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Response Curve
0 25 50 75 100 125 150 175 2000
20
40
60
80
100
120
f(x) = − 0.00384761904761905 x² + 0.974285714285715 x + 39.4761904761905R² = 0.998301113063855
Fertilizer Applied
Whe
at Y
ield
(Bu/
A)
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0 20 40 60 80 100 120 140 16040
50
60
70
80
90
100
f(x) = − 0.00285714285714286 x + 85.2142857142857R² = 0.0357142857142827
Fertilizer Rate
Whe
at y
ield
(bu/
A)
No Response to Fertilizer
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UK Soil Test Levels
• Low testing soils, expect a yield response to added fertilizers and large amounts are added
• Medium testing soils, yield response is small to none, add fertilizer to maintenance levels
• High testing soils, no yield response expected, no fertilizer recommended
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Time (years)
Soil
test
P o
r K
9 lb/ac P2O5 remaining in soil increases soil test 1 lb/ac
4 lb/ac K2O remaining in soil increases soil test 1 lb/ac
Agronomic Threshold
Sufficiency
2 Philosophies with a Low Soil Test
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2 Philosophies with a Low Soil Test
Time (years)
Soil
test
P o
r K
Agronomic Threshold
Rapid Build-up
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Time (years)
Soil
test
P o
r K
Agronomic Threshold
maintenance
sufficiency
buildup
3 Philosophies with a High Soil Test
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Differences in Soil Labs
• Many labs use Mehlich III extractions– UK, Waters, A&L, etc– Test for pH, Bu pH, K, P, Ca, Mg, Zn
• “Similar” nutrient results from all labs • Differ some in philosophy (so do people)• Sufficiency, Maintenance, Build-Up
– Micronutrients, Soil Cation Balance
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Soil Test Labs
• One soil sample mixed and submitted to– UK: P = 84 lbs/A, K = 234 lbs/A– Waters: P = 79 lbs lbs/A; K = 243 lbs/A– A&L: P = 47 ppm (94 lbs/A); K = 135 ppm (270 lbs/A)
• Recommendations for corn– UK: P2O5 = 0 lbs/A; K2O = 40 lbs/A
– Waters: P2O5 = 85 lbs/A; K2O = 125 lbs/A
– A&L: P2O5 = 60 lbs/A; K2O = 80 lbs/A
• Differ some, more philosophy than test
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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
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Profit Probability
100%
90%
60%
15%
V.low Low Medium High
Levels of Soil Fertility
Pro
bab
ility
of
Pro
fita
ble
Cro
p R
esp
on
se t
o F
erti
lizer
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University of Kentucky AGR-1
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K deficiency - yellowing of leaf margins of older soybean leaves. WHY???K deficiency - yellowing of leaf margins of older soybean leaves. WHY???
Photo: Greg Schwab
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What is CEC and How Does CEC Influence Fertilizer Rates in KY?
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Cation Exchange Capacity
Ca++
Plant Root
H+
K+H+
K+
-----
- - --
-
-
-
- - --
H+NH4+Ca++
Ca++
H+
H+
K+
K+
Mg++
Mg++
Ca++
Ca++Ca++
Ca++
Mg++
Mg++
Mg++
H+H+ H+
K+
K+
H+
K+
Ca++
Ca++
NH4+
NH4+
NH4+
Clay
ClayClay
Clay
Soil Solution
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Determining Nutrients Present
H
H
H
H
_ _ _
Na+
Al3+
Mg2+
K+
Ca2+
_ _
_ _
_ _
soil pH 5 Mehlich 3
NH4+
NH4+NH4
+
NH4+
NH4+NH4
+
NH4+
NH4+NH4
+
NH4+
NH4+NH4
+
NH4+
Base cations analyzed in leachate
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Calculated CEC
Measured CEC
Number of marbles are estimated
Number of marbles are counted
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0 5 10 15 20 25 300
5
10
15
20
25
30
Calculated CEC (meq / 100 g)
Mea
sure
d CE
C (m
eq /
100
g)
557 Kentucky soils
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Uncertainty in Calculated CEC
For CEC less than 10 meq/100 g: ± 2 meq/100 g
For CEC between 10 and 20 meq/100 g: ± 3 meq/100 g
7 meq/100 g 5 to 9 meq/100 g
15 meq/100 g 12 to 18 meq/100 g
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What does CEC and cation saturation tell you?
Soil texture (sandy vs clayey soil)
% base saturation as another indicator of soil acidity
Ability to Buffer Change
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Indiana soils
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7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 270
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
CEC (meq/100 g)
Num
ber o
f sam
ples
Ag samples with soil pH <= 7 inAll counties
Year = 2011Total number = 28,658
14 % >181 % <10
Frank Sikora, 2012
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8 9 10 11 12 13 14 15 16 17 18 19 20 21 220
50
100
150
200
250
300
350
400
450
CEC (meq/100 g)
Num
ber o
f sam
ples
Ag samples with soil pH <= 7 inBreckinridge, Daviess, Hancock, Henderson, McLean, Ohio, Webster
Year = 2011Total number = 1915
4 % >181 % <10
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Why Do We Lime?
• Proper pH is the foundation of a good soil fertility program – BIG BANG FOR YOUR BUCK
• Adjust soil pH (neutralize acidity)• Soil pH influences:
– Nutrient Availability– Root Growth– Herbicide Activity
• Soil pH can limit crop yield w/out noticing
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Corn Alfalfa THE MASTER
VARIABLE
The effect of soil pH on the activity of soil micro-organisms, availability of plant nutrients, and occurrence of toxic elements
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Effect of pH on Root Growth
Soil pH6.6 6.2 4.7
Penn State
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Effect of Soil pH on Soil Tests of Nutrients
1. Availability of many nutrients is changed by changing soil pH.
2. Maintaining proper pH will maximize nutrient availability and reduce fertilizer need.
3. Soil tests will rise faster and reduce slower if pH is properly maintained.
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Sources of Soil Acidity
1. Kentucky soils have a naturally low pH due to high rainfall and the leaching of calcium from the soil.
2. Main source of acidity in modern farming is nitrogen
- Most common nitrogen sources lower pH Urea, Anhydrous Ammonia, liquid nitrogen, and ammonium nitrate.
NH4+ + O2- NO3
- + H2O + 2H+
3. In general, 3 lbs of ag lime are needed to neutralize acidity from one pound of nitrogen
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Sources used to neutralize acidity
• Lime: Calcitic: CaCO3 or dolomitic: CaMg(CO3)2
– CaCO3 + 2H+ H2CO3 H2O + CO2
• Quick lime: CaO• Hydrated lime: Ca(OH)2
• Pelletize Lime: CaCO3
• Waste products: biosolids, paper waste, shells• Gypsum: CaSO4
– CaSO4 + 2H20 + 2H+ Ca2+ + SO42- + 2H2O + 2H+
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Liming soil
H
H
H
H
_ _ _
Na+
Mg2+
K+
Ca2+
_ _
_ _
_ _
soil pH 5
CaCO3
Ca2+
CO32-
H2CO3
H2O and CO2
_
_
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Liming soil
H
H
H
H
_ _ _
Na+
Mg2+
K+
Ca2+
_ _
_ _
_ _
soil pH 6
CaCO3
Ca2+ Ca2+
_
_
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Summary
• Soil testing is the foundation – DON’T GUESS, SOIL TEST
• Labs differ little in results, more in recs• CEC is important, but don’t worry about it• Yield responses are typically not seen when
soil fertility is adequate• Mind the fundamentals • pH management – basis of good fertility
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Questions or Comments
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Manganese Deficiency Symptoms
Interveinal chlorosis
Photo Greg Schwab
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Soil pH Affect on Herbicide Activity1. The effectiveness of several
herbicides is influenced by soil pH.
2. Herbicides in Triazine family (Atrazine and Princep) are widely used with corn.
3. The effectiveness of weed control of Triazines is reduced below a pH of 6.0 and greatly reduced at 5.5 pH.
4. Extends persistence of many Sulfonylurea herbicides above pH 7 and may cause carry over problems.