Base saturation and cation ratios: Can you use them to better manage nutrients like potassium?

By: Dan Kaiser, Extension nutrient management specialist

Travelling around Minnesota the last few years, it has become apparent that the concept of “base saturation” is alive and well across the state. I recall one person commenting that they have moved on to a more advanced fertilization strategy by looking at cation ratios for managing crop nutrients, which inherently comes back to base saturation. I figured it would be a good time to lay out what base saturation is and what the current research says about it when it comes to managing crop inputs.

What is base saturation?

The idea of base saturation and cation ratios are not new. Some of the earliest research dates to the 1940s and 1950s, so this is not a new concept. We know that the cation exchange capacity (CEC) is the capacity of the soil to retain positively charged ions (cations), which can be considered bases. The cations include potassium (K), calcium, magnesium, and sodium, as well as hydrogen and aluminum.

The theory is that there is an “optimal” concentration of these nutrients that needs to be maintained to maximize crop yield. However, almost every scientist focused on crop nutrition does not use the concept of cation saturation ratios. This is because the concentration of cations on the CEC is not fixed and can change based on nutrient applications.

Potassium (K) and base saturation

Potassium tends to be the major nutrient that is discussed when it comes to cation saturation ratios. For potassium, the data shows that maintaining an optimal soil test can achieve high yield potential without focusing on the amount of the CEC occupied with potassium. There are a couple issues with using cation saturation ratios:

#1 Achieving an “optimal” K base saturation ratio can be difficult on soils high in clay

Clay content tends to be one of the largest factors impacting the K base saturation ratio. I have heard claims that a response to K is highly likely when the K base saturation ratio is less than 4%, which to me is extremely high. So, let’s investigate this a bit and what it takes to get a K base saturation ratio of 4%.



The figure above shows the soil test needed to get to a base saturation of 4% for various CEC values, which are measured in milliequivalents (meq) per one hundred grams of soil. A milliequivalent is a measure of charge and is based on the charge of an ion, which in the case of potassium is plus one. If we look at a Hubbard loamy sand from near Becker, Minnesota with a CEC of around 5, it would take a soil test K concentration of 78 ppm to get to 4% K base saturation. Compare that to a Normania loam soil from Lamberton, Minnesota with a CEC of 25, which needs a soil test K concentration of 391 ppm to get to 4% K base saturation. Therefore, it is a lot easier to get to 4% on the sandier soil versus a loam soil, and I think the major reason many people are recommending higher K base saturations is that they may be looking at yield achieved in, say, a sandy irrigated soil compared to a higher clay non-irrigated soil and claiming that the K base saturation ratio being different is the reason for the yield difference. This would be possible if the only factor influencing the maximum yield is K, but that is not likely the case.

#2 Soil test labs that use the cation summation method can overestimate cation exchange capacity (CEC)

The second issue with base saturation ratios that concerns me is that most labs use what is called the “cation summation method” for determining CEC. The summation method estimates the CEC based on extractable potassium, calcium, magnesium, sodium, aluminum, and hydrogen. There are formulas by which the Mehilch-III or other tests can be used to give an estimate of the CEC. However, the summation method can overestimate CEC on high pH soils with high concentrations of free calcium if steps are not taken to modify the extraction solution used. So, in some cases, the CEC reported is not accurate. If you are in Minnesota and your reported CEC is in the mid 30s or higher and you have a high soil pH, the CEC probably is not accurate. In this case, if you get a reported K base saturation ratio, it is probably low. Be aware of this when looking at your soil test reports.

Current potassium research

What does the data tell us about K base saturation ratio? Is it more accurate than just using a soil test? I am going to use some corn data collected from a current AFREC study that compares four rates of potassium applied in the fall or spring ahead of corn. I am not going to discuss results for the time of application, as we have found no difference between fall or spring application of K when it comes to increasing the yield of corn. I am including a figure below that shows the range is soil test values for soil samples taken in the middle of June and the calculated K base saturation ratios for field moist soil samples that were not dried prior to extracting K. I have color coded the data for different ranges of CEC which roughly equate to sandy soils (gray, CEC ≤ 10), silt loam soils (maroon, CEC 10-20), and clay soils (gold, CEC ≥20).



One thing that is apparent is that the June soil test K is related to the yield potential of corn. The figure below shows that yield was generally less when June field moist soil test K was around 80 ppm or less. I am not showing the relationship between relative yield and air-dried K, which would have a higher critical level. Be aware that the interpretations for K determined on field moist samples are different for dried samples.



Was there a better relationship between relative yield potential for corn and the K base saturation ratio? No, and since the K base saturation ratio is calculated using the K soil test value, it is highly likely that there will be a relationship between K base saturation ratio and relative corn grain yield. So, it would not improve our corn fertilizer guidelines to consider K base saturation. I looked at the K base saturation ratio where yield was maximized and it was closer to 1 for both field moist and air-dried samples and not 2% or 4% like what some people are telling farmers. I also looked at the maximum yield produced and there was no relationship between maximum yield potential and K base saturation. The fact is, out of the 12 corn trials in our study, 11 of the trials had a K base saturation of 2.0% or less, yet only about one-third of those trials responded to K.

In the end, K base saturation is just another way to look at your soil test and provides no better information than the soil test alone. It is better to target optimal soil test concentrations instead of trying to change ratios that are difficult to change, particularly in high clay soils where our data suggests a very low probability of response to K.



Some may ask about the ratios of potassium to magnesium. That is a topic for another day. We know that uptake of nutrients can be rotated but getting to optimal ratios is difficult and may need very large and unnecessary applications of fertilizer to achieve. What seems to be more critical is maintaining soil tests near 200 ppm K for higher clay soils and for soils with lower CEC 100 to 160 ppm may be enough.

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