High fertilizer prices can lead some farmers to prioritize specific fertilizer applications based on their experience in fields. When it comes to phosphorus (P) and potassium (K), most growers seem to prioritize P even though soil test results may show a greater need for K.
We know that P is important for early plant development, especially in corn. Starter fertilizer research has consistently shown that the addition of P early in the growing season can boost early plant growth. In most cases, however, the increased growth does not translate into greater yield.
The effect of P on yield is solely dependent on the ability of the soil to supply the nutrient to the crop. If soils test medium or lower in P, there is a greater likelihood that P will increase yield.
Interpreting P and K soil test results
When we look at all our data, it is much easier to predict where crops will need P compared to K. When I look at a P soil test, I have more confidence in what the values mean. K soil test results are more variable. A Very High K soil test for one soil type may not be Very High for another.
In addition, dry soils can affect K soil test values. We know from past research that K soil test values vary over the course of the year while P soil test values are more consistent over time. K soil test values can differ based on whether the samples are collected in the early fall versus the late fall or the following spring.
This impact may be less pronounced when sampling following soybean or another crop where the residue releases K more quickly. Remember, a large amount of K is held in the plant and released to the soil through residue decomposition and leaching of water through the residue, and both processes are slowed in dry years.
At face value, the K cycle is more simplistic than the P cycle since K is not a part of organic molecules. However, the impact of clay type on K fixation can also impact the amount of K extracted by common soil test procedures.
Some people think that “fixation” means that the K is strongly held in the soil and not available to plants. “Retention” is probably a better term than “fixation” in this case as recent research has shown that a portion of the so-called “fixed” K may be available for plants. If you have ever noticed that’s it’s harder to build K soil test values on high clay soils, that could be because the K you’re applying is moving into the “fixed” form.
Ongoing K research project
One of our ongoing research projects is looking into K fixation and retention in Minnesota soils. The project, funded through corn checkoff dollars from the Minnesota Corn Research and Promotion Council, is the first step in the process of developing a revised set of K guidelines for fertilizing corn in Minnesota. Part of this work is focused on clay mineralogy to determine whether different critical levels should be maintained based on soil clay content.
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Some of the reasoning behind differing critical levels is due to differences in cation exchange capacity (CEC) of soils. Soil CEC is the ability of the soil to retain available cations, which include K. Sandy soils, which are low in clay, have a low CEC, so building these soils to the same critical level may not make sense as K can leach in these soils.
Our corn and soybean fertilizer guidelines for K in Minnesota were updated in the last few years to account for high clay soils potentially requiring a higher critical K level. Sands and silt loam soils may have lower critical levels, which is one of the main focuses of my current studies.
If you are a corn grower in Minnesota, how should you prioritize K?
For most of central and western Minnesota, we know that the likelihood of a response to K drops off substantially when soil tests reach around 200 ppm. If your soil test is above 200 ppm, you can significantly reduce your K rate or even skip an application.
This is true for soybean as well as corn. An average-to-high yielding soybean crop will remove more K annually than an average-to-above average corn crop. That does not mean that soybean requires a higher level of K in the soil but it can lead to greater pressure on soil K supply, decreasing soil K more rapidly over time.
Historically, the critical level for K was near 160 ppm, which I think works better for silt loam soils in the southeast and far southwest parts of the state. Sandy soils are where we have additional questions as some data suggests building K soil tests to 160 ppm may be difficult and unnecessary. However, this may depend on whether a sandy soil is irrigated or not. Stay tuned for additional information on this as we wrap up some of our current research.
Right now, the best tool growers have to manage both P and K is a soil test. Recent soil test results make it easier to figure out where nutrients are most needed. We know the K soil test is a bit of a challenge but the best thing you can do is to try to take samples from the same fields at similar times of the year. If you do have soils high in clay that are difficult to build, I would be interested in hearing about it and potentially collecting samples for some lab work (email: email@example.com).
I have not encountered these types of soils in my current study, but they tend to be soils that are very poorly drained, and while they may have low soil test K values, they may or may not need supplemental K. That’s one of the things we are hoping to figure out.