Gypsum Magic – Part 3

Changing Cation Ratios, Soil pH and CEC

Gypsum is still a hot topic in many areas and the calls keep coming. Recently we have received several calls asking if there is an optimum ratio of cations (Ca, Mg, Na and K), which are held on the soil. The concept of an optimum ratio of cations originated from research in New Jersey in the 1920’s and 30’s. All of this research was done on acid soils (pH less than 7.0). The original research from the 1920’s showed that an ideal soil for alfalfa production should have the following percentages of cations: calcium 65%, magnesium 10%, potassium 5% and hydrogen 20%. In the 1960’s other researchers determined that the ideal soil should have a range of cations such as: 60-80% calcium, 10-20% magnesium and 2-5% potassium and no hydrogen needed. In the 1970’s and 80’s as research techniques and laboratory instrumentation improved, soil fertility researchers determined that there actually was no perfect ratio. Example: soil #1 has 3000 ppm Ca and 1000 ppm Mg, soil #2 has 300 ppm Ca and 100 ppm Mg. Both soils have a ratio of calcium to magnesium of 3 to 1, but soil #2 has a very low level of magnesium. The cation ratio concept would have identified this ratio as OK and missed the fact that the magnesium level was low in the soil. Research from Wisconsin and many other Universities has shown that the ratio of calcium and magnesium can vary across a very wide range in the soil while still producing high yields (Figure 1). gypsum3a

Even though many studies have shown that there is no perfect ratio for cations, some people insist on trying to change the ratio of cations held on the soil with products like gypsum (calcium sulfate). AGVISE recently conducted a laboratory project looking at the affect gypsum has on soil test levels of calcium, soluble salts, cation ratios, soil pH and CEC. The results of the laboratory project are shown in figures 2 & 3. Some people who sell gypsum claim that it lowers the soil pH dramatically. It is apparent that this is not true, even at rates as high as 36000 lb/a gypsum the soil pH is about the same as the check.

As the rate of gypsum is increased you can see the test level for calcium increases while the level of magnesium, potassium and sodium remain the same (Figure 2). gypsum3b When gypsum (calcium sulfate) is applied to the soil, it dissolves in the soil solution. Some of the calcium becomes attached to soil particles as part of the cation exchange capacity of the soil. The remaining gypsum stays in the soil solution as dissolved calcium sulfate salt. The soil testing method used by all commercial and University soil testing laboratories picks up the calcium that is held on the soil, as well as the calcium that is dissolved in the soil solution as soluble salts. The calcium in the soil solution is “NOT” held on the soil, and should not be included because it is not held on the soil, but it is included in the common method used by all soil testing laboratories. Because this soil testing method includes the calcium from the soil solution, the test values reported are inflated on the high side. You can see this inflation occurring as the rate of gypsum increases in Figure 2. The calcium test value goes up, but the soil is not holding more calcium, the test is just including the calcium dissolved in the salts in the soil solution. You can see this is true because the salt level increases as the rate of gypsum increases.

The base saturation value for a soil is a calculation that determines the percent each cation makes up of the total cations in the soil. When the percent base saturation for a soil is calculated, the ppm value for calcium, magnesium, potassium and sodium are used in the calculation. Since the calcium ppm level keeps increasing as more gypsum is applied, calcium becomes a larger percentage of the total cations. We know that the soil is not holding more calcium, we are just measuring the increasing amount of calcium in the salts of the soil solution.

The Cation Exchange Capacity (CEC) of a soil is the ability of a soil to hold the cations calcium, magnesium, potassium and sodium. The CEC of a soil is a permanent feature based primarily on soil texture, clay content and organic matter. When gypsum is applied to the soil it does not actually change the CEC of the soil, but it does change the calcium test value determined in the laboratory, which is used to calculate the CEC value for the soil. Because an inflated calcium value is used to calculate the CEC of the soil, the calculated CEC goes up as the gypsum rate increases (Figure 3). gypsum3c These CEC values are erroneous due to the error caused by including calcium from the salts in the soil solution. The correct CEC of this soil, determined by a special laboratory method that does not include calcium from the salts in the soil solution is 18 meq. The routine method, used by all commercial soil testing labs, did a good job of determining the CEC to be 17, until higher rates of gypsum were applied. That means that you can change the calculated CEC of the soil by adding gypsum, but you are not really changing the ability of the soil to hold cations. If you have any questions on how gypsum affects soil properties, please call and talk with one of our technical support staff.

Facts learned from past field research and AGVISE laboratory project:
  1. You can achieve high yields on soils with a wide range of cation ratios.
  2. You can apply enough gypsum to a soil to change the laboratory test results for calcium, but this does not actually change the CEC of the soil or the amount of each cations actually held on the soil.
  3. Even low rates of gypsum over the long term on a poorly drained soil will increase the salt level of the soil. As the soil salt level increases, crop yields will decrease over time.
  4. Gypsum does not decrease or increase the soil pH of productive soils
  5. It is most important to know the level of each nutrient in the soil. If a nutrient tests in the deficient range, it needs to be applied. The concept of balancing cations is not supported by the facts of the real world.