Do You Know What Soil Test Method Your Laboratory Used?

Soil testing is an essential tool for managing phosphorus (P), potassium (K) and other nutrients. In Iowa and many states, the units reported in a soil test are expressed in parts per million (ppm) of the elements (not oxides P₂O₅ or K₂O, as in fertilizers) of dry or moist soil depending on the test. Different test methods can be used for a specific nutrient. Very importantly, the nutrient concentration measured by different tests for a specific nutrient may differ. This is because soil tests estimate probable nutrient sufficiency for crops and probable response to fertilization and measure an amount that is related to the amount crops may extract during the growing season. This is a fundamental reason for soil-test calibration with yield response; to provide a yardstick specific for each test. Both the extractant chemical solution and the procedure used to measure the extracted nutrient define a soil test. Therefore, soil-test laboratories should clearly indicate what method is used for the test results provided.

Iowa State University (ISU) provides interpretations for four P tests, four K tests, and two tests for the micronutrient zinc (Zn) based on numerous field trials in Extension Publication PM 1688, which was last updated in November 2023.

Phosphorus (P) Soil Tests

A key issue for P tests is that the extracted P can be measured by the traditional colorimetric procedure that measures orthophosphate P or, more recently, by inductively coupled plasma (ICP) that measures all dissolved P forms. The ICP analytical method measures more P. Most laboratories use a colorimetric measurement for the methods Bray-1 and Olsen but for the Mehlich-3 extractant some measure extracted P colorimetrically or by ICP.  The reason many labs use the Mehlich-3-ICP is that the same extracting solution and ICP can be used to measure by K, calcium (Ca), magnesium (Mg), sodium (Na) and some micronutrients. Therefore, the P tests supported by ISU (in publication PM 1688 and with research) are:

• Bray-1 with colorimetric measurement of extracted P
• Olsen with colorimetric measurement of extracted P
• Mehlich-3 with colorimetric measurement of extracted P
• Mehlich-3 with ICP measurement of extracted P

All four tests are appropriate for Iowa soils, except the Bray-1 should not be used in calcareous soils (pH 7.3 of higher). So watch out, because some laboratories provide test results for the Mehlich-3 without clearly stating which method was used, the colorimetric or the ICP. Moreover, some labs analyze P with the Mehlich-3 ICP but report results as Mehlich-3 colorimetric values using some transformation factor each lab had determined by preliminary analyses of many samples using both procedures.

Potassium (K) Soil Tests

There are two key issues for K tests: 

1. Labs can use the ammonium-acetate extractant or the Mehlich-3 extractant and in the last decade the majority of labs measured extracted K by ICP. Research has shown that these two K tests measure comparable K amounts. Again, many labs use the Mehlich-3 ICP K test because they can measure P and some micronutrients at the same time.
2. In 2013 ISU began providing interpretations for the traditional K test on dried soil samples and also a new method based on field-moist or slurry soil samples because the moist test is much more reliable mainly in soils with moderately poor to very poor drainage. A key issue is that the moist test can extract less, similar or more K than the dry test depending on the soil and drainage, and no simple factor can be used to transform results of one to the other.

Therefore, the K tests supported by ISU (in publication PM 1688 and with research) are:

• Dry K test using the ammonium acetate or Mehlich-3 extractants (similar interpretations)
• Moist K test using the ammonium acetate or Mehlich-3 extractants (similar interpretations)

Zinc Soil Tests

The key issue for Zn micronutrient tests is that labs can use the DTPA traditional test or the Mehlich-3 test. Of course, many labs have switched to the Mehlich-3 Zn test because they can also measure P and K in the same extracted solution. A key issue is that the Mehlich-3 extractant measures more Zn than the DTPA test. While the Mehlich-3 extracts more Zn, the ISU calibrations take that into account and should be used appropriately. Therefore, the Zn tests supported by ISU (in publication PM 1688 and with research) are:

• DTPA Zn test
• Mehlich-3 Zn test

In summary:

• Labs need to clearly specify what tests they are using, and you need to know!
• Contact the lab for clarification if there are any questions about which test is used.
• Does the lab clearly display or report the method used?
• P results as Mehlich-3 should indicate if they are colorimetric or ICP values.
• K results as either ammonium acetate or Mehlich-3 tests should also indicate if the extraction method is on dry or moist samples.
• Zn results should clearly specify if they are analyzed with either DTPA or Mehlich-3 methods.

Additional useful resources:

• Iowa phosphorus and potassium soil-test interpretations were updated in 2023: changes and reasons. Mallarino 2023. p. 158-163. In 53th Annual North-Central Extension-Industry Soil Fertility Conference Proceedings. 15-16 November 2023. Des Moines, Iowa.
• Iowa State University Extension Soil Fertility website.