Cation Exchange Capacity (CEC) is a measure of a soils ability to hold exchangeable cations. The CEC is impacted by many factors like soil type & organic matter. Generally speaking sandy soils have a lower CEC and clay soils have a high CEC.
In the lab there are two ways we can measure CEC:
Estimated CEC – This involves adding up the extracted base cations (K, Mg, Ca, Na) from a traditional extractant (Ammonium Acetate or Mehlich III) to derive a CEC number.
Displacement CEC – Involves washing the soil to remove existing cations and using an ‘index ion’ to determine the amount of exchange sites.
Are there differences between the two Measurements?
We analyzed a set of 667 samples by both methods to look at the differences at different pH values. Here is what we found:
Where the values from the two methods tend to diverge is at a pH of 7.3 or higher. At pH less than 7.3 there were still some samples with differing results, but not to the same degree as the samples that were over 7.3 pH. Extracting solutions like Mehlich III and Ammonium Acetate can overestimate plant available calcium (and magnesium to a lesser extent) that contain calcium and magnesium carbonate.
The data shows that the difference and the variance between the estimated and displacement CEC are quite large and become larger as you look at higher pH ranges.
What can we do with this information?
-Having a displacement CEC value in high pH soils can help us better understand a soils ability to provide nutrients to the crop.
-Understand which soils may contain calcium and magnesium carbonates.
-Properly characterize soil Cation Exchange Capacity, even in high pH soils.
Honeyland Ag Services complete soil test provides both estimated and displacement CEC. Click here for more information on our soil test.
In 2017 we conducted a trial using Honeyland Ag Services Staygreen program, variable rate application technology and “I” drops on a highboy sprayer. The goal was to evaluate if there would be any benefit to this approach vs the grower’s normal flat rate.
We went to the field when the corn was approximately V9-10 and collected samples of both soil and whole plants.
Based on the analysis we observed that the nitrogen was much more plentiful in the low areas of the field and the crop demand (determined by whole plant analysis) was relatively even throughout the field.
We created a VRT prescription that varied based on the topography of the field, crop demand & soil nitrogen supply. separating the field into knolls, middle ground and valleys. The area that received VRT application was just over 5 acres.
The VRT nitrogen was applied in four (summarized as two) test strips and the rest of the field was applied at the grower’s flat rate. Comparing the VRT strips to the swaths directly beside them the results were as follows:
Farmers No VRT Rate
Honeyland VRT Rate
By placing nitrogen where it was needed using a variable rate prescription we reduced nitrogen use by 7.4 gallons and increased the yield by 5 bushels. This represents a total gain of approximately $30/acre.
The Right Rate of Nitrogen: A Simple Way to Adjust N Rates for Corn Crop Potential
After compiling research from the past two years we know the amount of N that a crop needs is just as variable as the nitrogen in the ground. A field with high yield potential (230+ bu) will need 300+ lbs of N to meet it’s potential. A 200 bushel crop of corn requires about 220 lbs. The key to not running out of “gas” is to know how much you need to get to your destination!
A good rule of thumb is that corn will have 50% of its N out of the ground by V10. If you have 100 lbs removed at V10 your crop will remove approximately 200 lbs. Last year the best performing field we measured removed 170 lbs by V10!
How do we measure crop potential? By measuring the amount of N removed from the ground.
When should you measure crop potential? We can assess how much N your crop will need starting at knee high corn through to tassel and you can use this information to adjust your late N application.