Soil Testing for Phosphorus and Potassium

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Soil Testing for Phosphorus and Potassium
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Routine Soil Testing goals

• Rapid
• Affordable
• Predictive
• Reproducible
• Widely applicable
• Track changes in fertility
• Develop nutrientmanagement plan

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Soil Testing basics
Soil testing starts with collecting a good sample
Soil testing is not useful without meaningful
samples
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Taking a Good Soil Sample

• Decide on samplingequipment, soil depth,
  numberof samples, and location
• Have a clean plastic pail formixing the
  individual cores
• A light coat on the interiorof the sampling
  probe of aspray 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

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Taking a Good Soil Sample

• Divide the field or managementarea into areas
  depending ontopography, soils,
  managementhistory. A soil map will be
  helpfulfor this
• Take 15 to 20 individual soil coresand mix well
  into one compositesample to be analyzed by
  thelaboratory
• 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

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Where to Avoid Sampling

• Field borders, especially if closeto a gravel
  road with crushedlimestone
• Where there have been brushpiles, straw or
  haystacks, manurepiles, lime piles, etc. in the
  field
• Trouble spots, such as due toerosion or
  salinity, unlesssampled separately
• Old fertilizer bands in row crops
• Injection knife tracks
• Old fence rows, roads, or buildings
• Animal excretion or congregating spots

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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

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General Recommendations for Depth of Sampling
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Depth and Location of Cores Impact Variability
0
580
440
440
440
330
Depth in.
330
160
6
160
10
37 in.
37 in.
Robbins and Voss, 1991 (IA)
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Sampling in Ridge-Till Systemswith Residual
Fertilizer Bands

• Sample to 6 in. depth6 in. from the row
• Avoid high P and K zones thatmay have been
  band-appliednear the row

6 in.
6 in.
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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 20between-the-band samples (120 ratio)
• 12-in. row spacing Sample once in the band for
  every eight between-the-band samples (18 ratio)
• If the location of the P fertilizer band is
  unknown
• If lt20 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

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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

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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

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How Many Cores are Needed?

• 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
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More Cores Improves Precision and Accuracy
Frequency (50 total)
Soil test P category upper limit, ppm
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Soil Sampling Equipment

• Sampling Tools
• Shovel
• Use clean tools
• Sample from the proper depth and location
• Place samples in clean bucket for mixing

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Soil Sampling Equipment

• Sampling Tools
• 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 aconsistent
  depth

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Soil Sampling Equipment

• Sampling Tools
• 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 aconsistent
  depth
• Auger
• More convenient in rocky, wet, and hard soils
• Easier to sample to deeper depths
• A variety of tips and designs are available
  fordifferent soil textures

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Equipment-Mounted Sampling Equipment
Tractor-mounted
Truck-mounted
ATV-mounted
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Sample Handlingand 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

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Practical Sampling Equipment Considerations

• 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

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Time and Frequency of Sample Collection

• Seasonal variability does exist
• But more for soil pH thanfor 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

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Natural and Man-Made Variability Impacts Soil
Productivity
Map Courtesy of Kitchen, USDA - ARS
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When to Take Directed Soil Samples?

• Suspected Field Variability
• Indirect Indicators
• Topography
• Aerial photos
• Soils map
• Yield map
• Soil EC
• Direct Indicators
• Cropping history
• Fertilizer history
• Manure history
• Old homesteads
• Old feedlots

Map Courtesy of R. Koenig, Washington State
University
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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

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Zone or Grid Sampling?

• Zone sampling (Stratified sampling)
• 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 (gt 5 acres)
• Grid sampling
• Will help locate unknown sources of variation
• Easy to manage fertilizer with field maps
• Can increase knowledge of the field

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Grid Sampling Different Approaches

• Composite (or cell) system
• More robust for large grid size
• Requires more effort
• Less variable grid
• Grid point system
• Assumes sampled areas can predictunsampled areas
• Difficult on narrow fields
• Superior if you can afford a small grid size

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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

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More Intensive Sampling Results in Better
Databut More Expense Too
- - - - - Soil P concentrations - - - - -
Map Courtesy of Kitchen, USDA - ARS
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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 pre-determined pattern and
  spacing

Stratified composite
Field composite
Grid
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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

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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

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Nutrient Extraction Process Theory

• Measure what is currently available and
  predictwhat will soon become available to the
  plant
• Not a prediction of the total quantity of
  nutrientsin the soil

Provides an Index of Availability
Extractant
Phosphate
Soil Surface
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Selecting a Soil P Extractant

• The extracting solution should remove
  plant-available P from the soil through at least
  one of these reactions
• Dissolving action of acid
• Anion replacement to enhance P desorption
• Complex the cations that bind P
• Hydrolysis of cations that bind P

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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

• Potassium
• Ammonium Acetate
• Modified Morgan
• Sodium Acetate
• Mehlich 1 or Mehlich 3

Phosphorus Bray 1 Mehlich 1 Mehlich 3 Modified
Kelowna Modified Morgan Sodium-Bicarbonate (Olsen)
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Ion Exchange Membranes and Resins

• 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

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Choose a Well-Established Soil Testing Labthat
Uses Appropriate Techniques and Participates in a
Quality-Assurance Program
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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?

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Two Essential Parts of a Soil Test Report
Recommendations
Analytical results
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Examples of Relationships between

• Soil Test Values
• Crop Response
• P and K Fertilizer Recommendations

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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

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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

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Sampling Pastures

• Avoid sampling in areas that arenot
  representative of the area consider that
  animal activities area huge source of variation
  such asaround feeders, water, shade trees
• Avoid sampling near fresh manurepiles or recent
  urine spots sincethey 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

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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 zig-zag pattern

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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

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International Plant Nutrition Institute (IPNI)
655 Engineering Drive, Suite 110Norcross, GA
30092-2837Phone [phone] Fax
770-448-0439Website www.ipni.net
Reference 06128