By William McKibben
Excerpted from The Art of Balancing Soil Nutrients, published by Acres U.S.A. This has been reprinted with permission from the publisher.
This chapter will deal with the topic of balancing soils that have a total exchange capacity of less than 10. This cutoff number is somewhat arbitrary, but it was chosen so that this chapter generally will deal with sandy soils. This would include artificially created soils used in golf course greens and tees, landscape and garden soils, as well as naturally occurring agricultural soils. Clay soils in the southern part of the United States where they are mostly kaolinite types that have been highly weathered and have low exchange could also be included in this section.
Looking at a soil, it is not as easy as you might think to decide if the soil falls into this low exchange capacity classification. The total exchange capacity (TEC) or cation exchange capacity (CEC) as they appear on a standard soil test are the result of the summation of cations collected from the extracting solution. The main difference between TEC and CEC is basically that the TEC measures the sodium cations and the CEC does not. The assumption is that all these cations are extracted from the soil colloid. This may or may not be the case. Some of these cations could be the result of soil fertilizer being dissolved or lime breaking down in the extracting solutions.
Be warned that the low pH extracting solution method such as used by Mehlich III is capable of dissolving lime-based or calcareous sands and exaggerating the exchange capacity. The exchange capacities shown in Figure 5 would rival a muck soil or one of the heaviest clay soils in the country, not a golf course green. An extracting solution that is pH neutral such as ammonium acetate will not dissolve the lime in soils so dramatically, however numbers may still be exaggerated.
There is only one way to find out the exact exchange capacity of your soil. This is done in the laboratory by saturating the soil sample with sodium, barium or ammonium and then flushing the sample with an alcohol to remove any cations not attached to the soil colloids. The above-mentioned cations contained in the saturated soil are then removed from the soil colloids by the extracting solution. The amount of cations, which are attached to the colloids, will produce the actual exchange capacity.
Most people who have worked with their land for any length of time can tell if their standard soil tests have exaggerated exchange capacities. Exaggerated exchange capacities that are the result of dissolving lime in the soil do not normally affect the extraction of other cations and trace elements. The problem arises when labs set recommended or desired levels for cations based on the exchange capacity. For example, the desired amounts of potassium and magnesium could be overstated as shown by the soils in Figure 4. Anytime that the pH is much above 7, except when it is the result of sodium levels, the exchange capacities could be elevated. It is for these reasons that I find little benefit in using the standard test on calcareous sands—especially when using low pH extracting solutions. The paste test, which will be covered later in detail, is my choice for analysis in these situations.
At the other end of the pH scale, exchange capacities may also be elevated due to a high concentration of hydrogen cations. Exchange capacities are measured in milliequivalents (mEq). Without getting into the specifics here, it is widely accepted that:
1.0 mEq of hydrogen = 20 pounds of hydrogen in an acre furrow slice (AFS)
1.0 mEq of calcium = 400 pounds of calcium in an acre furrow slice
1.0 mEq of magnesium = 240 pounds of magnesium in an acre furrow slice
1.0 mEq of potassium = 780 pounds of potassium in an acre furrow slice
1.0 mEq of sodium = 460 pounds of sodium in an acre furrow slice
When you look at the above numbers, it is easy to see that when adjusting the balance in the soil, the milliequivalents (mEq) of the exchange capacity will constantly be changing. By replacing a single positively charged hydrogen cation on the colloid with a double positively charged cation of calcium, the exchange capacity is going to go down. The colloidal charge of the soil is basically fixed and the overall electrical balance will be maintained at the expense of the number of cations found in solution. As plants take up nutrients such as calcium and magnesium, the plant exchanges hydrogen ions in order to maintain an electrical balance in both the plant as well as the soil. Figure 6 is a comparison of the same field before and after liming has occurred. Notice the drop in the exchange capacity from one year to the next year. This was primarily due to the lime that was applied during the fall of 2006. The amount of calcium and magnesium applied was doubled (an increase of 5,394 to 10,647 lbs. of calcium and 506 to 1,277 lbs. of magnesium) but the total exchange capacity (TEC) went down because of the amount of exchangeable hydrogen that was lost. Exchange capacities are not static and should fluctuate as soils balance. If you notice that the exchange varies on your test results, first take note of any change in the cation amounts and then the pH values.
Clay and organic matter provide the bulk of the exchange sites that are used for holding cations. Pure vermiculite-type clay can contribute upwards of 150 CEC mEq per 100 grams whereas kaolinite clay has a CEC around 5 mEq per 100 grams. Humus, depending upon its quality, can contribute from 100 to >300 mEq per 100 grams of CEC.
About the Author:
William “Crop Doc” McKibben is an Ohio-based consultant specializing in soil fertility balancing and managing crop yields, as well as livestock nutrition. He holds a master’s in soil science from Ohio State and has worked as an agronomist in the Midwest for more than 30 years, much of that with Brookside Laboratories.
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