P and K “fixation” in the soil: What you need to know

By: Dan Kaiser, Extension nutrient management specialist

One word that often comes up when discussing phosphorus (P) and potassium (K) fertilizer management is “fixation.” What is nutrient fixation and what should farmers understand about it?

Fixation generally refers to the tendency for nutrients to become “tied up” in the soil and therefore less available to the crop. This happens when nutrients react with the soil, or ions in the soil solution, forming compounds or holding onto nutrients very tightly.

Nutrient fixation is often misunderstood to mean that once a nutrient is “fixed,” it is lost forever and will never be available to the plant. While we know that reactions occur in the soil which can bind elements, the process of fixation is more about “retention.” The so-called “fixed” nutrients may become available at some point, but it can take time.

“Pools” of nutrient availability in the soil

One way to think about nutrients in the soil is that they are in “pools” that vary in how available nutrients are to crops.

When it comes to nutrient availability in the soil, you may hear about something called nutrient “lability.” The word “labile” is an adjective used to describe something that is easily changed. Nutrients are grouped into pools of:

1. Labile nutrients
2. Moderately labile nutrients
3. Non-labile nutrients

These conceptual pools of nutrients are not mutually exclusive. They will change over time based on the amount of nutrients in each pool, but the rate of change will vary, with changes occurring more rapidly in the labile pool and very slowly in the non-labile pool. The point here is that “non-labile” does not necessarily mean the nutrients will not be available to the crop at some point.

Why P fixation is different than K and ammonium fixation

Normally, when we talk about fixation, we are talking about K and ammonium. These ions can fit in pockets on the outside layers of 2:1 clay that is present in most Minnesota soils. As clays shrink and swell, it can affect the release of K to the soil solution. In this case, what we think of as “fixation” is K or ammonium being trapped inside clay layers.

P, on the other hand, is highly reactive with other ions in the soil and can form compounds of varying solubility, with calcium in high pH soils and iron and aluminum in low pH soil. P is not "fixed' in the same sense as K and ammonium. Phosphorus forms compounds in the soil with calcium, iron, and aluminum that can be very insoluble, making P in the soil less available for crop uptake.

Research in Iowa and Minnesota

While people normally think nutrient fixation means that the nutrients are not available to the crop, some recent research in Iowa indicates that this may not be true. Researchers from Iowa State University has found instances where potassium fertilizer does not increase soil test K on some soils but corn or soybean grain yield is increased. However, the amount of “fixed” K in the soil increased in relationship to the amount of K fertilizer applied even though the extractable K was not impacted. Since the crop responded to K, that means that the fixed fraction may not be as “fixed” as believed and may be a bigger part of the annual K cycling to crops.

One of our current projects, funded by the Minnesota Corn Research and Promotion Council, is looking more into fixed potassium in Minnesota and see how it may be impacting crop yields here. This work is in the early stages as we are looking into interactions between applied nitrogen fertilizer and potassium and how that affects fixed potassium and ammonium in the soil. We are also in the process of developing better options for determining available K for crops with this project, as well as other research focused on better understanding K dynamics in the soil.

Lastly, I have been asked about whether biological products can “unlock” fixed nutrients. The simple answer is that there is nothing out there that can be considered a silver bullet that would allow us to not apply fertilizer. Soils that are low in available nutrients will need fertilizer or manure application to produce a profitable corn crop.

In summary

While some of this information may be technical, the main takeaway is that “retention” is a better word for what we normally talk about as nutrient fixation, as some of the so called “fixed” nutrients may be an important pool of nutrients available to a crop on a given year. However, research has not shown that accounting for some of the larger pools of nutrients in the soil is better than the routine soil tests suggested for use in determining nutrient availability for crops. Routine soil tests are still the best way to determine how much P and K are available to the crop at a given point in time.

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