Is 'fixed' P lost? 4 things to consider

By: Dan Kaiser, Extension nutrient management specialist

Recent posts on social media brought to mind a few things related to what happens to phosphorus (P) fertilizer when it’s applied to soil. With MAP and DAP prices still relatively high, what concerns, if any, should farmers have about how much value they are getting? Here are a few things that you should know about what happens to P when it is applied to the soil.

1. You will never get 100% utilization of the fertilizer you apply the first year after application.

Us university types will typically tell you that, at maximum, you might get about 65% of what you apply taken up by the plant in the first year of application. While this might cause some concern about where the other 35% is going, remember that the plant needs a specific amount P to achieve optimal yield. If fertilizer supplies 50% of the crop’s needs for example, then the deficit needs to come from somewhere, and that somewhere is the soil.

Where did the P in the soil come from? Some of the P may be from mineral sources but much of what the crop is taking up comes from the other 50% of your fertilizer that was not taken up by the crop. This is because the fertilizer reacted with the soil. Researchers like Antonio Mallarino at Iowa State University refer to P retention rather than fixation. Just because you do not see it does not mean it is not there.

Will any of the P be lost through leaching or runoff losses? That depends on several factors but, for most growers managing soil tests near optimal levels for crop production, the potential for P loss is low. In the end, if we could track P uptake more closely, efficiencies of a specific fertilizer application may be much greater than 65% after factoring in P uptake ten years post-application.

2. Can my soil reduce the availability of P from fertilizer?

It is true that P is highly reactive with some metals in the soil and can form compounds with calcium at high pH and iron and aluminum at low pH when these elements are in abundance. This can happen relatively quickly, but the question is whether any P is lost. Technically, P is not lost if it reacts with calcium, but the solubility of different calcium-bound phosphates can reduce the availability of P the year it is applied. I have seen this when looking at June soil test P values for fall versus spring applied MAP. For soils with pH 7.5 or greater and high carbonates, there were instances where soil tests in June did not reflect the rates of P applied in the fall. So, essentially, the P had reacted with calcium and was not being picked up by the soil test, and corn yield was significantly greater when P was applied in the spring (click here to see the research). Under these circumstances, timing the P closer to when the plant needs it (spring application or starter) seemed to be a better option for corn. Soybean did not care when the P was applied in the fall versus spring but there seemed to be some yield advantage to applying P ahead of soybean for high pH soils. So yes, “tie up” or “fixation” is possible in some soils, but, as I talked about in point #1, some of the “fixed” P may become available in future years.

3. So, how do I manage P to make sure I am getting the most out of what I am applying?

The big thing here is to try to get out of the mindset that there will be a crop failure if you do not apply P. Some may say this is crazy but consider the fact that, as soil tests increase, most if not all of the P that is applied in fertilizer is just replenishing what the crop is taking out of the soil. A high soil test means there is a high probability that the soil can supply all the given P for a crop (see Table 9 from the U of M’s corn fertilizer guidelines for phosphorus).

 

The P soil test predicts the probability that fertilizer is needed. To combat high fertilizer prices, the key is to put the fertilizer where it is needed and reduce or avoid applications where there is little to no probability that the fertilizer applied is required by the crop to produce a portion of the crop’s yield. Consequences of not applying P in the long-term are that soil tests will decline. Most data shows that the Bray-P1 test will decline by an average of 1 ppm per year when no P is applied. So, eventually there will be a point when more fertilizer is needed but, in most cases, P applications will more than pay for themselves even at higher fertilizer costs when soil tests get into more of a responsive range.

4. Finally, I have seen some people saying that the soil tests that we suggest are old outdated and should be replaced by newer tests.

This is hogwash. A soil test is only useful if it can predict the response of a crop to a specific nutrient. I have data through 2024 using the Bray-P1 and Olsen tests in Minnesota showing that they still have the same ability to predict crop response to P now as they did 60 years ago. Some people are promoting tests like the Haney H3A test or the Mehlich-3 test because they use organic acids similar to what are released by the roots in the case of the H3A test or that they are better because they are newer. I have compared the H3A test to the Bray-P1 and Olsen tests in Minnesota and they all correlate with each other, meaning that an increase in one will lead to a direct increase in the others. They may not increase at the same rate (they extract from different pools of P). I can use an equation and convert from one test to another with some relative certainty. So, the H3A is not better than the Bray-P1 and Olsen tests, and the use of organic acids has been studied before but tests like the Bray-P1 and Olsen tests were better at predicting P responses across many locations.

Relationship between the Haney H3A test and the Bray-P1 or Olsen P tests from soils collected at various Minnesota locations that have a soil water pH of less than 7.5.

Why do we still use the older tests? Because we have more data supporting their use and they are tests that can be relatively quick and easy. The same goes for the Mehlich-3. Those pushing for the Mehlich-3 test replacing other P t test are doing so as it is a one stop extraction for multiple elements so the test simplifies testing if you are a soil testing lab. I have found issues with the Mehlich-3 test failing in high pH soils in western Minnesota, so it can have issues like the Bray-P1 test on high pH soils. For those saying the H3A is better, it would be nice to see the data, as all the data I have says that there is no advantage to the H3A over the tests we currently suggest for P in Minnesota.

To wrap up, there are also those out there selling products that are supposed to aid in making P more available from fertilizer or from the soil. The consensus from independent researchers in the Upper Midwest is that there is no miracle product out there. I have tested a few products and have never found anything that works. My thoughts are that the few dollars people look to invest in these products would go further when spent on other inputs in an operation. It is essentially like buying insurance that you will never use.

The best thing to do when commodity prices are low is to go back to the basics and work within your fertilizer program to cut costs where inputs are not needed and avoid extra purchases that have a low probability of increasing yield.

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