Microorganisms are key to breaking down the rock mineral fertilizers to usable form for the plant. Sulfur Apply in late spring or summer to lower pH of alkaline soils. Sulfur is mined in the western U.S., but most is a by-product of oil refining. Epsom Salt Apply at planting time or during the growing season to supply magnesium and sulfate. It's fast-acting, but benefits last 1 or 2 growing seasons.
Epsom salt is mined in Europe, but most that is available to gardeners is manufactured.
Dolomitic Limestone Apply in fall to raise pH of acidic soils that contain insufficient magnesium. Dolomitic limestone is mined around the world. Gypsum Apply in fall or spring to supply calcium and sulfate, or to improve soils that contain excess sodium. Benefits last 1 to 2 years. Gypsum is mined primarily in the western U.S. Pelletized Calcitic Limestone Apply in fall to raise pH of acidic soils that contain adequate magnesium.