AGVISE Laboratories tests over 300,000 soil samples each year. We test thousands of samples for CEC and calculate the base saturation for many customers. AGVISE has 32 years experience in soil analysis and participates in the North American Proficiency Testing program (150 soil testing labs from across North America).

Why Is CEC Important?

CEC (Cation Exchange Capacity) is an important property of the soils on your farm. Soil particles have a negative charge, which allows the soil to hold cations. The cations of interest are potassium, calcium, magnesium and sodium. These cations have a positive charge and are held to the soil particles, somewhat like a magnet would attract iron nails. This attraction is a good thing because it prevents the cations from being leached away with excessive rainfall. Soil organic matter also has a negative charge and holds cations and is part of the total CEC of the soil.

The CEC of a soil is a permanent characteristic and is directly related to soil texture. The higher the CEC of your soil, the higher the soil clay content. If you know the CEC of your soil from a soil test, you have a good idea what the soil texture is. In general, a low CEC soil has a coarse texture like loamy sand and a high CEC soil has a fine texture such as a clay loam (see the table below). This is important information when deciding things like which fields can have fall nitrogen applied to them and which fields should have nitrogen applied in the spring.

What about Cation Ratios like Calcium to Magnesium?

Soil testing laboratories calculate the CEC of a soil by adding up all of the cations test levels in your soil sample (potassium, calcium, magnesium and sodium). The laboratory also calculates what percentage of the total cations is made up by each individual cation. For example Calcium might make up 65% of the total cations with potassium, magnesium and sodium making up the other 35% of the cations. In the 1940's there was some research on alfalfa that suggested an "ideal soil" needed to have a specific percentage of each cation to achieve high yields. In the late 1950's, there was other research that suggested a "range" for each cation was "OK" to achieve high yields. In the 1970's, with much better instrumentation, research showed that the percent of each cation or the ratio of one cation to the other was not important for achieving high yields. In fact the research showed that the ratio of one cation to the other varied widely in both high and low yielding situations (see table below). With that in mind, it was very obvious that the ratio of one cation to another was not the reason for low or high yields.

Ohio State University

University of Wisconsin

Many research studies from the 80’s to the present have confirmed what was discovered in the 70’s. The most important thing is to have a sufficient level of each cation in the soil, not the ratio of one to the other. The critical level for these cations is shown below.

Does Potassium Need to be a Certain % of the Total Cations?

In the western U.S. and Canada, many soils have a high CEC, pH higher than 7.3 and a potassium test level higher than 150 ppm. On these soils, the calcium levels are usually high as well. Calcium can make up more than 80% of all the cations held on these soils. The remaining 20% of the cations is made up of potassium, magnesium and sodium. It is very common for potassium to only be 1-3% of the remaining cations held on this type of soil. Some people would tell you that more potassium fertilizer is needed to achieve high yields in this situation because the percent potassium is too low compared to the total cations. Research has shown many times that adding extra potassium fertilizer to increase the percent potassium held on the CEC does not increase yields. No more potassium fertilizer is needed because the total CEC of these soils is so high that even though a small percentage of the cations are potassium, there is still plenty of potassium to achieve high yields. In other words, if you have a low CEC soil (small pie) and a high CEC soil (big pie), the potassium the crop gets from the narrow slice of the big pie is the same or more than from a big slice of a smaller pie. The percentage is not important.

The important thing is that the potassium soil test level is higher than 150 ppm. If the potassium sol test is lower than 150 ppm, then you will need to apply potassium fertilizer to achieve high yields. The potassium fertilizer is applied to take care of crop needs, not to try and change the cation ratio in the soil. It does not matter that potassium is a low percentage of the total cations. It is most important to have soil test level for each cation test higher then the critical level established by university and industry research.

Important Things To Remember

1. Soil CEC (Cation Exchange Capacity) is important and is directly related to soil texture. Knowing the CEC is very important for decisions like nitrogen management.
2. The ratio of one cation to the other is not important as long as the soil test level for each cation is above the critical soil level needed for high crop yield.
3. Soil testing will show you if the soil test level for each cation is above the critical level Potassium 150 ppm, Magnesium 100 ppm and Calcium - lime requirement by buffer pH determination.
4. Applying extra potassium fertilizer to try and change cation ratios in the soil is expensive and does not increase yield. 50 lb/a of potash fertilizer (0-0-60) used to cost about $4.00/acre and now it costs about $20.00/acre. Why spend money on extra potassium fertilizer that will not increase yield?

Please contact your AGVISE Soil Scientist or Agronomists if you have any questions.