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What is the best soil test option for phosphorus?

soil test p phosphorus fertilizer

By: Dan Kaiser, Extension nutrient management specialist

A soil test for phosphorus represents an index of phosphorus (P) availability to crops. A soil test needs to correlate to crop response in order to provide a useful index of crop response and any extraction solutions can be used as long as they correlate to crop response.

While there are many tests out there, researchers at land grant universities like the University of Minnesota have spent a lot of time and effort determining which tests are the best for soils within a given state. However, increases in the amount of soil samples being collected have led to more samples being sent across state borders, which has led to more questions about specific soil tests which may be run by a lab in one state but not suggested for use in another.

Bray-P1 and Olsen soil tests

The two tests suggested for determining phosphorus sufficiency in Minnesota are the Bray-P1 and Olsen soil tests. The two tests differ in their extraction solutions.

The Bray-P1 test extracts P with acids and has been a popular test for over 50 years as data continue to show the ability of the Bray-P1 test to predict crop yield response to P. The Bray-P1 test works well for most soils that are slightly alkaline to highly acidic (pH of 7.4 or less).

The Olsen test extracts P using sodium bicarbonate and is the best test to use for situations where soil pH is 7.4 or greater. The Olsen test can be used on more acidic soils in situations where pH is 6.0 or higher. Many labs using the Bray-P1 or Olsen tests will run the Olsen test at a certain pH automatically, which makes it easier as you do not have to decide which test to use before you submit samples.

Mehlich-3 soil test 

The Mehlich-3 extraction solution is becoming more popular with soil testing labs as it is a single extractant which can be used to extract P, potassium (K), and micronutrients. A single test is advantageous due to the lower cost of soil testing and speeds up analysis in labs because they can use fewer analytical procedures. There are no current guidelines for the use of the Mehlich-3 extraction solution in Minnesota. While research is ongoing, there have been a few issues noted based on research in Minnesota.

Table 1. Summary of soil test information from a set of P fertilizer trials conducted in 2020. The letters in parentheses represent the soil test classification (Very Low, Low, Medium, High, or Very High). The Mehlich-3 colorimetric column is assessed using the Bray-P1 classifications. No classification is available for the Mehlich-3 ICP. Data shows a much greater extraction of P for the Mehlich-3 test at Benson, Crookston, and Stewart. (CCE = calcium carbonate equivalency)
 Location Bray-P1   Olsen Mehlich-3 (colormetric) Mehlich-3 (ICP) Soil pH CCE
 Benson  11 ppm (M)  9 ppm (M)  34 ppm (VH)  41 ppm  7.9  2.5%
 Crookston  4 ppm (VL)  3 ppm (VL)  18 ppm (H)  26 ppm  8.1  2.1%
 Lamberton  8 ppm (L)  15 ppm (H)  15 ppm (M)  24 ppm  5.1  0.3%
If Mehlich-3 P results are determined colorimetrically, then the Bray guidelines could be used for Mehlich-3 results for most soils. This is an important point. When soils are extracted with Mehlich-3 solution, organic acids and organic P are also extracted. Many labs will determine P in the extraction solution using an ICP. The specifics for this acronym are not important for this discussion but an ICP analysis will measure all the P extracted, including organic P, which gives a higher result compared to colorimetric analysis. Since the result is greater, you cannot use the U of M guidelines for the Bray-P1 test if the Mehlich-3 P is determined with ICP. Mehlich-3 P determined with ICP needs a separate set of correlations, which are not available for soils in Minnesota. 

The Mehlich-3 extraction solution is supposed to work across a range of acidic and alkaline soils. However, I have come across a few soils where the Mehlich-3 test will extract greater amounts of P. An example of this is several soils we have collected for our current AFREC-funded fall versus spring application study. The over-extraction of P is concerning as it would result in less P suggested, which may result in yield loss. The Olsen test is the best option for high pH soils and issues noted with the Mehlich-3 test have resulted in the test not being adopted for Minnesota.

Research in the Northcentral region has also shown a poor agreement between the Mehlich-3 test and the DTPA soil test when it comes to the extraction of micronutrients. There is a relationship between the two tests when it comes to zinc, but the Mehlich-3 test generally extracts more zinc, so a separate correlation and calibration procedure is needed. Fertilizer suggestions for micronutrients using the DTPA soil test should not be used interchangeably with the Mehlich-3 test.

Cheaper is not always the better option

If you are interested in testing soil for phosphorus, remember that cheaper is not always the better option. I have had questions in the past about interpretations of soil tests not suggested for use in Minnesota, such as the Morgan and Mehlich-1 tests which are used in the eastern United States. Different extraction solutions will pull phosphorus out of different pools in the soil, so while some tests correlate well to one another, some do not. In the case of the Morgan and Mehlich-1 tests, I had no way to correct those tests to Bray-P1 or Olsen P results.

There is a chance a lab may be analyzing soils using one test, such as the Mehlich-3, and then using an equation to report data using a different soil test if the Bray-P1 or Olsen tests are not a commonly used test in their area. While this is cost effective for the lab, it could be problematic in some situations. Using labs that operate closer to home or routinely use suggested tests for your soils is a good option, and so is requesting soil test options in your growing area to give you the best information possible to make fertilizer management decisions using tests that are suggested for the region.

Thanks to AFREC for funding this study. 


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