Gypsum (calcium sulfate) is a soil amendment that is used to reclaim soils that have high levels of sodium. High levels of sodium in the soil causes soil productivity to decline rapidly. When the sodium level becomes too high in a soil, the soil structure is destroyed, water infiltration and percolation are limited, crusting is severe, and the soil becomes dark and very hard when dry. When gypsum is applied to a sodic soil, the calcium from the gypsum replaces the sodium, which is held on the soil particles. The sodium that was knocked off the soil particles must be leeched through the soil profile before the physical properties can improve. The calcium from the gypsum improves the soil structure over time and greatly increases water infiltration into the soil. Gypsum is a well recognized soil amendment for sodic soils around the world. The rate of gypsum normally required is tons per acre. Gypsum is not used to lower the soil pH, it is used to reclaim sodium affected soils.
Recently, we have received a number of calls asking if gypsum can be used to increase yields on high pH, salty soils in the Midwest. It has also been suggested that the base saturation ratio of many soils in the Midwest is “out of whack”, and this “problem” can be fixed by adding a few hundred pounds of gypsum. To understand this situation better, you have to first understand how the level of potassium, calcium, magnesium and sodium are determined in the laboratory. The ammonium acetate method of extraction is used by all laboratories in the Midwest to determine these nutrients. This method works very well to determine the level of potassium, calcium, magnesium and sodium held on the soil particles when the soil pH is 7.0 or lower, and the salt value less than 0.5. The level of potassium, calcium, magnesium and sodium from this test is used to calculate the cation exchange capacity and the base saturation value of a soil. If a soil has a pH higher than 7.0, and a salt level higher than 0.5 the normal testing procedure has some problems. Most soils with a pH higher than 7.0, have calcium and magnesium carbonate in the soil. Some of this carbonate is dissolved in the testing procedure and is included as part of the calcium and magnesium held on the soil. The problem is the calcium and magnesium dissolved from carbonate is not held by the soil, it comes from dissolving part of a rock in the soil (carbonate). Because of the carbonate, the calcium and magnesium test results on these soils is inflated on the high side.
Soils with a salt level higher than 0.5 also cause problems for determining the correct level of calcium and magnesium. Many salts in the soil solution contain calcium and magnesium. The calcium and magnesium contained in these soluble salts is already dissolved in the soil solution, and is not held on the soil particles. Any calcium and magnesium contained in the soil solution will be included in the test results for calcium and magnesium. The test values for calcium and magnesium on soils with a salt level higher than 0.5 will be inflated on the high side because of the error caused by calcium and magnesium from the salts being included in the test results.
Since the test values for calcium and magnesium are inflated due to problems caused by carbonates and salts, it is not possible to accurately determine the ratio of calcium or magnesium to any other nutrient using the ammonium acetate method. The whole concept of balancing nutrient levels to get to some perfect ratio is not supported by research. There is a lot of University research that shows making fertilizer decisions based on a ratio of calcium or magnesium to other nutrients does not increase yields. The approach that works is making sure you supply enough of each nutrient to supply the crops’s needs (forget the ratios).
So what does it mean when you have a soil that has a high or very high level of calcium and magnesium on the report? If the soil has a pH higher than 7.0, and the salt is higher than 0.5, it means there are carbonates and salts in the soil. The high salt level is the result of poor drainage. Improving the open drainage and tiling will fix the salt problem in the long term. The high level of carbonate in the soil means that you will want to place your phosphorus fertilizer in a band for efficiency and choose your soybean variety carefully. It does not mean that applying a few hundred pounds of gypsum will increase yields. Some of the most productive soils in the world (The Red River Valley) have a high pH with a low salt level. That should tell you that a high soil pH alone does not limit yields. Gypsum is a recognized amendment for sodium affected soils around the world. It will not lower soil pH or increase yields if sodium is not a problem. If you are considering applying gypsum to your fields, just to see if we know what we are talking about, you may want to test for gypsum first. Many soils already contain gypsum in this region. We are currently working on a routine soil test for gypsum. Please give our technical support staff a call if you have any questions.