Grinding limestone finer improves its solubility and speeds up reaction with soil. Hydrogen attached to the clay is exchanged with calcium from the soluble limestone. The hydrogen is then in the soil solution and reacts with the carbonate, also from the soluble limestone, to form carbonic acid. Carbonic acid is not stable in soils and further breaks down to carbon dioxide gas and water. The hydrogen has then been converted from an acid ion on the clays to a neutral molecule of water.
Many people think calcium is the key to reducing acidity.
However, without the carbonate, the soil would still contain the acid hydrogen ion and pH would not change. Gypsum will not reduce soil acidity. Calcium from soluble gypsum does replace hydrogen on the clays. The hydrogen then reacts with the sulfate in soil solution to form sulfuric acid. Sulfuric acid is stable in the soil and remains in the system, thus there is no change in pH. The acid hydrogen ion remains as an acid hydrogen ion.