Transcript Soil Testing for P & K - International Plant Nutrition

Soil Testing for
Phosphorus and Potassium
Routine Soil Testing goals
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Rapid
Affordable
Predictive
Reproducible
Widely applicable
Track changes in fertility
Develop nutrient
management plan
Soil Testing basics
Soil testing starts
with collecting a
good sample
Soil testing is not useful
without meaningful samples
Taking a Good Soil Sample
• Decide on sampling
equipment, soil depth, number
of samples, and location
• Have a clean plastic pail for
mixing the individual cores
• A light coat on the interior
of the sampling probe of a
spray lubricant (such as WD 40)
can help with removal of the sample
• A field map or GPS unit to record where the samples came
from
• Clearly labeled soil bags or boxes for sending to the
laboratory
Taking a Good Soil Sample
• Divide the field or management
area into areas depending on
topography, soils, management
history. A soil map will be helpful
for this
• Take 15 to 20 individual soil cores
and mix well into one composite
sample to be analyzed by the
laboratory
• Clearly label the sample container and completely fill
out the information sheet from the soil testing lab, so
proper recommendations can be made for the specific
field area and the crop to be grown
Where to Avoid Sampling
• Field borders, especially if close
to a gravel road with crushed
limestone
• Where there have been brush
piles, straw or haystacks, manure
piles, lime piles, etc. in the field
• Trouble spots, such as due to
erosion or salinity, unless
sampled separately
• Old fertilizer bands in row crops
• Injection knife tracks
• Old fence rows, roads, or buildings
• Animal excretion or congregating spots
Sampling Depth
General recommendations are based on previous crop, tillage
system, and fertilization practices. Follow the recommendations of
your soil testing laboratory:
• Plow/ Disc/ Chisel tillage:
– Most frequently sampled to depth of 6 in., but may be
– 7, 8, or 12 in. in some areas (depending on cropping system)
• Ridge-tillage
– Sampled to a depth of 6 in., taken 6 in. from the row
• No-tillage or minimum tillage:
– Sampled to a depth of 4 in., but may be 3 or 6 in.
– (sometimes the surface 1 or 2 in. is sampled for soil pH)
• Established pasture and turf:
– Sampled to depth of 3 or 4 in. Remove heavy thatch before
sampling
General Recommendations
for Depth of Sampling
No-till
(2 samples)
Turf
Forest
Pre-Sidedress
Nitrate Test
Plowed
Ridgetill
6 inches
Residual
Nitrate
6 inches
8 in.
16 in.
24 in.
32 in.
Depth and Location of
Cores Impact Variability
4 in.
330 – 580 ppm K
Depth in.
8 in.
160 – 580 ppm K
0
440
440
580
440
330
330
160
6
10
Robbins and Voss, 1991 (IA)
160
37 in.
37 in.
Sampling in Ridge-Till Systems
with Residual Fertilizer Bands
• Sample to 6 in. depth;
6 in. from the row
• Avoid high P and K zones that
may have been band-applied
near the row
6 in.
6 in.
Sampling Soils with
Banded Fertilizer
• In soils with residual fertilizer bands…
– The general recommendation is to double the number of cores in a
composite sample sent to the lab to get a representative analysis
• If the location of the P fertilizer band is known:
– 30-in. row spacing: Sample once in-the-band for every 20
between-the-band samples (1:20 ratio)
– 12-in. row spacing: Sample once in the band for every eight
between-the-band samples (1:8 ratio)
• If the location of the P fertilizer band is unknown:
– If <20 subsamples (cores) are taken, paired sampling in the field
consisting of:
• 1. A completely random set of samples and
• 2. A second set of samples, collected at half the fertilizer band spacing,
perpendicular to the row
The greatest deviation from the "true" P soil test occurs when inadequate
sampling includes rather than excludes the band
Soil Sampling Orchards
• Leaf sampling is usually
more accurate than soil
analysis to monitor nutrient
status of perennial crops,
but soil testing still
provides useful information
• Before planting, obtain a
soil map and take samples
according to soil type and
field characteristics
Soil Sampling Orchards
Irrigated Orchards:
• Sample in area wetted by irrigation
• Take soil cores under the drip-line for
sprinkler or basin irrigation
• With drip or micro-sprinkler irrigation, take
cores 1/2 to 2/3 of the way out from emitter
towards wetted edge
Non-irrigated Orchards:
• Sample in active rooting area
• Take multiple cores around drip-line for a
composite soil sample
• One-foot depth is generally adequate
How Many Cores are Needed?
Percent of values falling
into the mean range
100
80
60
40
20
0
0
2
4
6
8
10
Number of sample cores
• One core is not adequate to represent field variability!
• The suggested number of cores depends on the degree of field variability
• Taking 5 to 8 cores may be adequate, but 15-20 cores may be required to
get a representative sample
Franzen and Berglund , 1997
“More” Cores Improves
Precision and Accuracy
True average
40
Frequency (50 total)
35
30
5 cores per sample
25
17
20
20
15
7
10
5
0
40
35
30
25
20
15
10
5
0
0
3
2
1
34
20 cores per sample
8
0
0
10
20
30
8
40
50
0
0
60
70
Soil test P category upper limit, ppm
Soil Sampling Equipment
Sampling Tools:
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Shovel:
Use clean tools
Sample from the proper depth and location
Place samples in clean bucket for mixing
Soil Sampling Equipment
Sampling Tools:
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Shovel:
Use clean tools
Sample from the proper depth and location
Place samples in clean bucket for mixing
Push probe:
– One-inch diameter tube is most common
– Convenient to use in soils without stones
– Easy to clean tube and sample to a
consistent depth
Soil Sampling Equipment
Sampling Tools:
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Shovel:
Use clean tools
Sample from the proper depth and location
Place samples in clean bucket for mixing
Push probe:
– One-inch diameter tube is most common
– Convenient to use in soils without stones
– Easy to clean tube and sample to a
consistent depth
Auger:
– More convenient in rocky, wet, and hard soils
– Easier to sample to deeper depths
– A variety of tips and designs are available for
different soil textures
Equipment-Mounted
Sampling Equipment
Tractor-mounted
Truck-mounted
ATV-mounted
Sample Handling
and Shipping
• Once the individual cores have been collected
in a bucket, break the lumps, remove stones,
and mix well
• Mix the soil completely and fill the sample box
or bag to the “full mark” (usually one to two
cups of soil)
• Avoid taking wet soil samples, but allow to air
dry if the samples are too wet for shipping
• Carefully label each sample container and
make careful description on a field sketch or
field notes of where the samples were taken
• Accurately complete the field information sheet
requested by the lab in order to get the most
accurate recommendations possible
Practical Sampling
Equipment Considerations
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Probes and shovels do not work well in rocky soils
Bucket augers may work best in sandy-textured soils
Special tools are sometimes used in sampling turf
Use a tool that permits sampling to a consistent,
accurate depth
Time and Frequency of
Sample Collection
What season of the year?
• Seasonal variability does exist
– But more for soil pH than
for P or K
• If possible, sample at the same
time of year to reduce variability
• If not possible to sample at the
same time, the soil analysis will
still be useful for making nutrient
decisions and tracking trends
Natural and Man-Made
Variability Impacts Soil
Productivity
Treeline and
end-row compaction
Topsoil variation
bu/A
Low soil P and K
End-row
compaction
Soil pH
Historically
managed less
intensively
Greater weed pressure
Nearly flat, ponding
Corn Grain Yield
Map Courtesy of Kitchen, USDA - ARS
When to Take Directed
Soil Samples?
Suspected Field Variability
Soil P Concentrations
Indirect Indicators:
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Topography
Aerial photos
Soils map
Yield map
Soil EC
Direct Indicators:
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Cropping history
Fertilizer history
Manure history
Old homesteads
Old feedlots
Map Courtesy of R. Koenig, Washington State University
1-2 mg P/kg
3-4
5-6
7-8
9-10
11-18
Sampling by Soil Type May Be
Best Choice for Some Fields
• In highly variable landscapes,
sampling by soil type (zone) is
superior to a random sampling
scheme
• Sampling by soil type and
landscape position is
frequently the best way to get
accurate information on the
fertility status of a field
Zone or Grid Sampling?
• Zone sampling (Stratified sampling)
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Uses farmer knowledge of field variation
Excellent if location of variation known
Use if there are regular or repeating patterns
Good for large sampling areas (> 5 acres)
• Grid sampling
– Will help locate unknown sources of variation
– Easy to manage fertilizer with field maps
– Can increase knowledge of the field
Grid Sampling: Different
Approaches
• Composite (or cell) system
x x
– More robust for large grid size
– Requires more effort
– Less variable grid
x x x
x
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x x
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x x
x
x x
x
x x
x
x x
x
composite grid
• Grid point system
x xxx
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x xxx
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x xxx
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x xxx
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x xxx
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x xxx
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x xxx
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x xxx
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x xxx
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x xxx
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point
– Assumes sampled areas can predict
unsampled areas
– Difficult on narrow fields
– Superior if you can afford a small grid size
Grid or Zone-Based Soil
Sampling
• Easiest with the assistance of a
GPS monitoring system to record
sampling sites
• Cores are collected and
composited surrounding the
pre-determined sampling site
• Results from the soil analysis are
processed with GIS data-handling
software to make field maps
More Intensive Sampling
Results in Better Data
…but More Expense Too
- - - - - Soil P concentrations - - - - sampled on 2.5-acre grid
Set1
Set2
5
Map Courtesy of Kitchen, USDA - ARS
24 43 62 81 100
lb/A
Set3
sampled on 0.15-acre grid
Examples of Sampling
Strategies
• Field Composite
– Sampling representing the mean
concentration of the field
• Stratified/Zone Composite
– Separate samples based on
known or expected field variability
• Grid
– Samples taken based on predetermined pattern and spacing
Choosing a Soil Sampling
Strategy
• First consider the sources and degree of field variability
(both natural and man-made)
• Whole field (random) sampling most appropriate when:
– the existing fertility is high and/or variability is low
• Field-zone sampling (by soil type or landscape position)
may be most appropriate when:
– location of variation is known
– sampling areas are large
– resources are limited
• Grid sampling may be most appropriate when the
location of variation is unknown and future management
can address the spatial variability
What is the Right Soil
Extractant?
• Chemical solutions are added to soil samples that
mimic root and soil processes- estimating both current
and future nutrient availability
• The nutrient extracting solution should simulate the
natural processes found in different types of soils
• Some extractants and methods are better suited for
particular soils and the lab results must be calibrated
with local field research
Nutrient Extraction
Process: Theory
• Measure what is currently available and predict
what will soon become available to the plant
• Not a prediction of the total quantity of nutrients
in the soil
Extractant
Provides an
“Index of Availability”
Phosphate
Soil Surface
Selecting a Soil P
Extractant
• The extracting solution should remove plant-available P
from the soil through at least one of these reactions:
1. Dissolving action of acid
2. Anion replacement to enhance P desorption
3. Complex the cations that bind P
4. Hydrolysis of cations that bind P
Selecting a Soil Extractant
• Select a soil extracting solution that has been previously calibrated
for the soils in a specific region. Commonly used extracts include:
Phosphorus
Potassium
Bray 1
Mehlich 1
Mehlich 3
Modified Kelowna
Modified Morgan
Sodium-Bicarbonate (Olsen)
Ammonium Acetate
Modified Morgan
Sodium Acetate
Mehlich 1 or Mehlich 3
Extraction
Analysis
Ion Exchange Membranes
and Resins
H+
H+
Ca2+
H+
Mg2+
H+
K+
H+
HCO-3
HCO-3
NO3SO42-
HCO-3
HPO42HCO-3
HCO-3
• Membranes are designed
to simulate a plant root by
attracting anions (on
cation resin) or cations
(on anion resin)
• Exchange membranes
estimate nutrient
availability without soil
disturbance
• Sequential measurement
can provide an estimate
of the nutrient availability
rate
Choose a Well-Established
Soil Testing Lab
that Uses Appropriate
Techniques and Participates in
a Quality-Assurance Program
Steps to Sampling Success
• Good Field Sampling is the First Step
• Accurate Chemical Analysis is the Second Step
• Data Interpretation is the Third Step
– Analytical accuracy is essential… but of little value in the field
without relating these lab numbers to actual crop response
– Are the fertilizer response predictions accurate for your soil
types, crops, and management practices?
Two Essential Parts of a
Soil Test Report
Recommendations
Analytical results
Examples of Relationships
between:
• Soil Test Values
• Crop Response
• P and K Fertilizer Recommendations
100
Little or No
Response Probability
40
Medium Response
Probability
60
20
No Response Expected
80
Very High
Response Probability
Percent of Maximum
Crop Response
Fertilizer Requirement
0
0
20
40
60
Soil Test Index of Nutrient Availability
80
100
Sampling pastures and
Fields Receiving manure
• Accurate assessment of nutrients
in fields receiving animal waste is
important for nutrient
management planning
• Highly variable fertility levels
across the field make it difficult to
collect an accurate soil sample
• Careful soil sampling allows
better decisions to be made and
efficient use of essential plant
nutrients
Sampling manure-Amended
Soils
• Poor estimates of soil nutrient
status makes it difficult to have an
accurate nutrient management
plan:
– Poor agronomic results
– Unwanted environmental impacts
• Non-uniform manure application
makes it difficult to get a field
“average” of nutrient content
• A large number of cores is
necessary to represent both high
soil test areas and low soil test
areas
Sampling Pastures
• Avoid sampling in areas that are
not representative of the area –
consider that animal activities are
a huge source of variation such as
around feeders, water, shade trees
• Avoid sampling near fresh manure
piles or recent urine spots since
they may not be representative of the field
• Use a random zig-zag pattern to collect 15 to 20
individual cores for each field (less than 20 acres)
• Remove plant and manure debris, break the cores, and
thoroughly mix the samples before submitting for
analysis
Sampling pastures:
Guidelines
• Divide fields into
smaller management
zones (usually less
than 20 acres)
• Avoid sampling
adjacent to roads,
fence lines and
congregation spots
• Take at least 15 – 20
cores at random
points along a zigzag pattern
Summary
• Before sampling, decide on the
purpose of soil testing and how the
information will be used
• Choose an appropriate sampling
strategy for your individual situation
• Take appropriate number of cores,
using appropriate equipment to get
accurate results
• Thoroughly mix the cores and send
samples to a well-respected
laboratory that uses appropriate
analytical techniques for your situation
• Review the results and
recommendations to verify that they fit
with your field experience
International Plant Nutrition Institute (IPNI)
655 Engineering Drive, Suite 110
Norcross, GA 30092-2837
Phone: 770-447-0335; Fax: 770-448-0439
Website: www.ipni.net
Reference: 06128