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Soil Phosphorus Tests in the North-Central Region

Soil Phosphorus Tests in the North-Central Region. Antonio Mallarino Iowa State University. Basics of Soil Testing. A relative measurement of nutrient sufficiency for plants. Tests measure an amount of nutrient that is proportional to the amount actual available for plants.

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Soil Phosphorus Tests in the North-Central Region

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  1. Soil Phosphorus Tests in the North-Central Region Antonio Mallarino Iowa State University

  2. Basics of Soil Testing • A relative measurement of nutrient sufficiency for plants. • Tests measure an amount of nutrient that is proportional to the amount actual available for plants. • Many P tests are used and measure different amounts of P. The amount measured has no meaning by itself and cannot by used without calibration.

  3. Field Correlation/Calibration • Gives a meaning to a soil test result. • Establish critical concentration ranges for different crops and soils. • Establish probability of response for different test results. • Determine fertilization rates when test is below a critical range: interpretation classes or continuous formulas.

  4. Soil Testing for P • Bray-1 and Olsen have been extensively used across the North-Central region. • Bray-1 underestimates plant-available P in many calcareous soils. • Both tests have been used with a colorimetric determination method, intensity of a blue color. • Colorimetric methods measure orthophosphate P.

  5. Common Extractive Solutions • Bray-1: HCl + NH4F • Olsen or bicarbonate: NaHCO3 (pH 8.5). • Mehlich-3: CH3COOH + HNO3 + NH4F + EDTA NH4NO3 + EDTA

  6. Iowa Field Calibrations: Bray & Olsen Bray-1 Olsen

  7. The Mehlich-3 Extractant • M3 extractant began to be used in the NC Region for P, K, Ca, Mg, and some micronutrients during the late 1990s. • The NCR-13 committee included the M3 among procedures for P, K, Ca, and Mg. • Iowa published field calibrations for M3-K and the M3-P colorimetric in 1999. • M3- P is similar to Bray-1 P, except in some high-pH soils.

  8. Bray-1 Vs M3 and Soil pH

  9. Bray-1 and M3 in Manured Soils

  10. D. Leikam Kansas: M3 vs Bray

  11. Wisconsin: M3 vs Bray L. Bundy

  12. Iowa Field Calibrations: Bray and M3 Bray-1 M3 Colorimetric

  13. Now Comes the ICP! • Some labs began using ICP (inductively coupled plasma) for the M3 test. • ICP measures uses a very hot flame that breaks down all compounds. • Great confusion! ICP measures more P than the colorimetric method whatever extractant is used. • But problem with M3 because it is used for several nutrients.

  14. ICP Measures Organic P - Minnesota

  15. ICP Measures Organic P - Iowa

  16. M3-ICP vs M3-Colorimetric or Bray

  17. D. Leikam Kansas: M3 ICP vs Colorimetric

  18. Iowa: M3-ICP vs M3-Colorimetric Relative Absolute

  19. D. Leikam Kansas: M3 Colorimetric/ICP Ratio

  20. MN: M3-ICP vs M3-Colorimetric

  21. Iowa: M3 ICP & colorimetric and OM Relative Absolute But for this data set, high OM sites also had higher pH and lower P by any test.

  22. MN: M3 ICP & colorimetric and OM

  23. Iowa: M3 ICP & colorimetric and pH Relative Absolute But for this data set, high pH sites also had higher OM and lower P by any test.

  24. MN: M3 ICP & colorimetric and pH

  25. NCR-13 M3 ICP-Colorimetric Work • ICP extra P seems to be derived from soluble organic P compounds. • IA, MN, MO: No relationship with soil organic matter or manure history. • Not clear results for pH. Sometimes ICP - Colorimetric difference is smaller for high-pH soils, but often have less P. • Many public and private labs: P from ICP correlates well with colorimetric P but there is no reliable, constant factor.

  26. Iowa M3 ICP Field Calibrations

  27. Iowa M3 ICP Field Calibrations

  28. Critical Soil P Concentrations Model Bray-1 M3-Col M3-ICP ------------------- ppm P ------------------ Cate-Nelson 13 16 20 Lin-Plateau 20 21 32 Quad-Plateau 26 28 43

  29. Iowa M3 ICP & Color Interpretations Colorimetric ICP

  30. Iowa Soil-Test P Interpretations Maintenanceadjust basedon removal

  31. Four States Bray-1 P Classes Class IA IL MN NE WI - --------------------------- ppm ------------------------------ VL 0-8 na 0-5 0-5 0-7 L 9-15 na 6-11 6-15 8-12 O, M 16-20 22-32 12-15 16-24 13-18 H 21-30 32+ 16-20 25-30 19-28 NE, 8-inch sampling depth; others 6 to 7 inches Optimum, Medium: IA, IL, WI apply maintenance MN, NE starter or < maintenance

  32. EnvironmentalSoil Phosphorus Tests

  33. Dissolved P in Runoff

  34. P in Tile Drainage

  35. Soil P Testing in the NC Region • No problem with Bray and Olsen tests in the NC Region. Known high pH issue. • M3-COL is similar to Bray in acid or neutral soils, better in IA high-pH soils. Some different results in WI. • M3-COL and M3-ICP are different soil tests!! Getting M3-COL or Bray from M3-ICP is a very risky business. • Base interpretations on calibrations! • Used in P indices for NMPs or MMPs.

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