Calcium sulfate (or gypsum) is a soluble inorganic compound. Gypsum has been used for many years to improve aggregation and inhibit or overcome dispersion in sodic soils. One of the many misnomers about calcium sulfate, or gypsum, is that it can effect soil pH. That misnomer arises from the fact that limestone, another calcium-containing mineral (calcium carbonate) can raise pH so ‘naturally’ that it’s assumed gypsum should either raise or even lower pH. But neither is true.
The key to understanding why calcium carbonate or lime tone changes pH, but calcium sulfate or gypsum does not, begins with an understanding of pH. Soil pH is the relative content of hydrogen (-H) and hydroxyl (-OH) protons. At a pH of 7 (0. 0000001 Molar), there is an equal amount of both and the pH is neutral. At a pH of 6 the H concentration is 0.000001 Molar and at a pH 8 has an H concentration of 0.00000001 Molar. A one pH unit change means a ten-fold (10X) difference in H concentration.
To raise soil pH means decreasing the H concentration in the soil. And to accomplish that takes primarily carbonate – but calcium plays an important role as well.
Lime increases pH by changing some of the hydrogen ions into water and carbon dioxide. A calcium ion from the lime replaces two H ions on the cation exchange complex. The carbonate (CO3) reacts with water to form bicarbonate (HCO3-). The bicarbonate further reacts with H to form H2O and CO2. The pH increases because the H+ concentration has been reduced.
Gypsum will not reduce soil pH. Calcium in the gypsum will displace H but the H remains in the soil solution so there is no change in soil pH due to gypsum per se.
In very acid sub soils (pH 5.0 or less) aluminum becomes more soluble and is toxic to roots tips. Calcium leaches into the subsoil and displaces some of the Al so it leaches below. The more calcium that moves down through the subsoil, the more aluminum is displaced. Calcium sulfate or gypsum is better than calcium carbonate for displacing aluminum in the sub soils due to its solubility. However gypsum will not change the pH of the subsoil. Gypsum is an excellent source of sulfur for plant nutrition and improving crop yield.
Author: Dr. Daniel Davidson
Dr. Daniel Davidson – EcoGEM Agronomist. Dr. Daniel Davidson is a nationally recognized agronomist. He served most recently as Director of Strategic Research for the Illinois Soybean Association. Dr. Davidson has also served in various capacities at GEOSYS, Cargill, Agri Business Group and Agri Growth, Inc. He holds a Ph.D. in Agronomy from Washington State University and an MS in Agronomy from the University of Missouri.
Dr. Davidson posts articles on soil health and management related subjects. If you have suggestions for topics or questions, feel free to contact him at firstname.lastname@example.org or call 402-649-5919.