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. 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.

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.

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.

Common Extractive Solutions Bray-1: HCl + NH 4 F Olsen or bicarbonate: NaHCO 3 (pH 8.5). Mehlich-3:CH 3 COOH + HNO 3 + NH 4 F + EDTA NH 4 NO 3 + EDTA

Iowa Field Calibrations: Bray & Olsen Bray-1Olsen

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 M3- P is similar to Bray-1 P, except in some high-pH soils.

Bray-1 Vs M3 and Soil pH

Bray-1 and M3 in Manured Soils

Kansas: M3 vs Bray D. Leikam

Wisconsin: M3 vs Bray L. Bundy

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

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.

ICP Measures Organic P - Minnesota

ICP Measures Organic P - Iowa

M3-ICP vs M3-Colorimetric or Bray

Kansas: M3 ICP vs Colorimetric D. Leikam

Iowa: M3-ICP vs M3-Colorimetric Relative Absolute

Kansas: M3 Colorimetric/ICP Ratio D. Leikam

MN: M3-ICP vs M3-Colorimetric

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.

MN: M3 ICP & colorimetric and OM

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.

MN: M3 ICP & colorimetric and pH

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.

Iowa M3 ICP Field Calibrations

Critical Soil P Concentrations Model Bray-1M3-Col M3-ICP ppm P Cate-Nelson Lin-Plateau Quad-Plateau

Iowa M3 ICP & Color Interpretations ColorimetricICP

Maintenance adjust based on removal Iowa Soil-Test P Interpretations

Four States Bray-1 P Classes Class IA ILMN NE WI ppm VL0-8 na L 9-15 na O, M H NE, 8-inch sampling depth; others 6 to 7 inches Optimum, Medium: IA, IL, WI apply maintenance MN, NE starter or < maintenance

Environmental Soil Phosphorus Tests

Dissolved P in Runoff

P in Tile Drainage

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.