Title: Soil Test Methods: Principles and Practices
1Soil Test MethodsPrinciples and Practices
Ann Wolf Ag Analytical Services Lab Penn State
University
2Todays talk
- Soil testing-basic principles
- Commonly used soil tests and how they differ
- Why do test results among labs differ?
- Quality control and quality services
- Selecting a lab to meet your testing needs
3Basics of a good soil test
- Rapid, cost-effective and relatively easy to
perform - Reproducible results
- Correlated to crop response
- Environmentally friendly
4Soil Tests
- pH and acidity
- Phosphorus
- Potassium, calcium, magnesium
- CEC
- Trace elements
- Nitrogen
5pH and Lime Requirement
pH and acidity (lime requirement)
6Soil pH and Soil Acidity
Soil solution pH
Reserve Acidity
7Soil pH Measurement
- Soil pH is measure of H activity in the soil
solution. - Determined by mixing soil with water and
measuring with pH electrode - Results should not vary significantly among labs
8Soils with same pH can have different levels of
reserve acidity and different lime requirements
9Soil Acidity Measurement
- A buffer solution is added to the soil to react
with the soil acidity. The drop in pH of the
buffer is directly proportional to soil acidity
present. - The test must be calibrated on soils from region
to be meaningful.
10Soil Acidity/Lime requirement
- Common buffers
- SMP Buffer
- Most common buffer in Mid-Atlantic/North Central
US - Contains chromium Hazard to lab technicians
all waste collected for hazardous landfill - Mehlich Buffer
- Only buffer that contains no hazardous components
- Adams-Evans Buffer (Primarily for Southeast
Soils) - Recommended for Pennsylvania soils
11Calibration of Buffer Tests PASoils
SMP Buffer
Mehlich Buffer
- Mehlich Buffer
- better predictor of LR than SMP
- Safer in the Lab
- No hazardous waste
- PSU switched from SMP to Mehlich Buffer, July 2004
12Soil Acidity/Lime requirement
- Other methods used
- Estimate from pH/base saturation
- Hand Texture (estimate of clay)
13PHOSPHORUS
PHOSPHORUS SOIL TESTS
14Phosphorus Soil Tests
15Soil Test P Response Curve
Mehlich 3. Bray P (PA)
50
30
Mehlich 1 P (Md)
50
25
Morgan P (NY)
20
5
Above Optimum
Optimum
Low
Yield
Soil Test Level (ppm)
16Effect of instrumentation on P measurement
ICP vs Color for Mehlich 3 P
ICP
P
P
P
P
Color
P
Soil Extract
17Instrumentation effecton Mehlich 3 P measurement
18Cations and Cation Exchange Capacity
19Cations K, Ca, Mg
K
-
Ca
-
-
Mg
Mg
-
-
-
-
-
K
Cations are held on negatively charged sites on
soil particles
-
-
Ca
Soil Colloid
-
-
Mg
-
K
-
-
-
-
Mg
-
-
-
K
Ca
Ca
20Soil Tests for Cations
Replace cations from exchange sites with counter
ion
Extracting solution
K
Ca
-
-
Mg
-
Mg
-
-
-
-
-
K
NH4
H
-
H
-
Ca
Soil Colloid
NH4
-
-
NH4
Mg
-
K
NH4
NH4
-
-
-
-
Mg
H
-
-
-
H
K
Ca
Ca
21Soil Tests for Cations
Measure cations in soil extract ICP, AA, Color
NH4
Soil Extract
H
-
-
NH4
-
NH4
-
-
-
-
Mg
K
Ca
-
NH4
-
K
K
-
H
Soil Colloid
-
Ca
Mg
-
Ca
NH4
-
H
NH4
Mg
-
-
NH4
-
-
-
H
-
-
K
NH4
H
Ca
Mg
H
NH4
H
22Soil Tests for K, Ca, Mg
- Mehlich 3 (H, NH4)
- Current PA test
- Ammonium Acetate pH 7 (NH4)
- PA test before 1991
- Mehlich 1 (H)
- Morgan/Mod Morgan (Na or NH4 )
In general, tests extract similar quantities of
Ca and Mg , but extracts that contain NH4
will extract more K
23Cation exchange capacity (CEC)
- Measure of negatively charged exchange sites for
cations on soils (meq/100 g) - Determined by summation
24CEC by Summation
K
H
-
-
Ca
Mg
Mg
-
Ca
-
-
-
-
-
-
-
H
-
-
K
Basic Soil
Acidic Soil
-
-
-
-
Ca
Ca
-
H
-
-
-
-
-
-
-
-
-
Mg
H
-
-
-
-
Ca
H
H
Ca
K
K
Ca
CEC KCaMg
CEC Acidity KCaMg
25Potential errors with CEC by summation
K
Mg
Ca
-
Soil Extract
-
Mg
-
-
-
K
-
-
-
Mg
K
-
CaCO3
Ca
-
Ca
Ca
Soil with Free CaCO3
-
Ca
-
Ca
Mg
Ca
Ca
-
K
CaCO3
CaCO3
Ca
-
Mg
-
-
-
Ca
Mg
-
-
-
Ca
K
Ca
K
Ca
Ca non-exchangeable Ca dissolved directly
from calcium carbonate minerals
26Errors in CEC measurements by summation
27Trace Elements
28Trace Elements
- Trace element deficiencies are uncommon in PA
soils making reliable interpretation and
recommendations on basis of soil tests difficult. - Plant Analysis should be used in conjunction with
soil testing to evaluate deficiencies and help
guide fertilizer applications.
29Mehlich 3 Cu distribution in PA soils
Concern with high levels from
dairy foot baths
0.5 Normal Range 15.0
30Mehlich 3 Zn distribution in PA soils
Occasionally deficient in soils with high pH and
high P from fertilizer (not manure) additions
1.0 Normal Range 80.0
31Mehlich 3 S distribution in PA soils
Not currently a common deficiency but could be in
the future if acid rain is reduced
10 Normal Range 75
32NITROGEN
OSU Extension fact sheet
33Nitrogen
- Nitogen (N) is one of the most important
nutrients required for optimum plant growth BUT - Given the complexity of the nitrogen soil cycle,
no soil test can accurately predict nitrogen
availability in soils over the growing season.
34Tests for improving N management for corn
35Reporting Units and element forms
P
P2O5
ppm
K2O
Mg/kg
Meq/100 g
K
sat
Lb/A
36Units ppm vs mg/kg vs lb/A
1 ppm part per million 1 part/1 million 1
mg/kg
lb/A 1 lb/2 million lbs of soil part/2
million
37Elemental vs Oxide forms
38Quality Control
- A good laboratory quality control program is
essential for producing results that are accurate
and reproducible.
39Quality Control PSU example
- 1 QC soil sample at start of day
- 1 QC soil sample every 30 unknowns
40Quality Control PSU example
Instrument calibration checks every 10-15 samples
41Quality Control PSU example
Instrument calibration checks every 10-15 samples
Participation in North American Proficiency
Testing Program for soils
42Quality Services
- Rapid turn-around time
- Results available electronically
- Re-runs on questionable results
- Prompt and accurate response to questions and
problems - Supplemental materials to help use the results
paper and web - Complete information on methods
- All recommendation tables readily available
- Confidentiality of results
43Are you using the right lab?
- Is it performing tests appropriate for PA soils?
- SMP/Mehlich Buffer for acidity
- Mehlich 3 for phosphorus and cations
- Summation of cations and acidity for CEC
- Have the tests it uses been calibrated for PA
soils and growing conditions?
44Are you using the right lab?
- Are units and test methods clearly specified on
reports? - Is it following appropriate quality control
procedures and does laboratory staff have
adequate background in soils/soil testing. - Does it provide the quality services you need?
45The End