Sample Handling and Soil Test Potassium

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Sample Handling and Soil Test Potassium

• George Rehm
• Dept. of Soil, Water, and Climate

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Major Points

• Consultant observations
• Basics of potassium in soils
• Drying time and temperature

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Past Research

• Dry conditions affect soil tests for K
• Decrease if soil test values are high
• Increase if soil test values are low

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Consultant ObservationsK Soil Tests
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Consultant ObservationsK Soil Tests
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Change in Soil Test KResearch Sites
0-0-60 applied between Spring (yr1) and Fall
(yr1) at a rate of 200 lb./acre
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Potassium in Soils
Unavailable K 90-98 of Total
weathering
Slowly Available K 1-10 of Total
Readily Available K 1-2 of Total
Non Exchangeable K
Exchangeable K
Solution K
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90
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Soil Test K

• Currently used ammonium acetate procedure
  measures total of exchangeable and solution K

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Clay Minerals Play a Role

• Kaolonite (not a factor)
• Illite (fixing, slow release)
• Montmorillonite
• Expand and contract with drying
• Dominate in Minnesota (cracks)
• Trap K when dry

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Illite
Non Exchangeable K
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K
Montmorillonite
K
K
K
Exchangeable and fixed in dry soils
K
K
K
K
K
K
K
K
K
K
K
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There is Reported Variability in Results of Split
Samples Sent to More Than One Laboratory
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Possible Explanations

• Variability in sample collection
• Drying temperature and time
• Extraction procedure

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Temperature and Drying Time Affect Soil Test K
Source Gelderman, South Dakota
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Standard Analytical Procedures

• Sieve soil through 2 mm screen
• Scoop with standard 1.7 cc scoop
• Place soil in 50-ml flask
• Add 1 N ammonium acetate
• Shake for 5 minutes
• Filter and measure K

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Drying Procedure (NCR-13)

• Place sample in paper bags or cardboard box
• Dry in cabinet equipped with exhaust fan
• If heat is necessary, temperature inside cabinet
  should not be high than 104ºF

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Standard Handling Procedure is Necessary

• Fertilizer recommendations from Land Grant
  Universities based on results of analysis using
  the standard procedures.