Gypsum vs. Lime: Soil Chemistry for Gardeners

Gardeners often wonder if leftover drywall (gypsum, calcium sulfate) can substitute for garden lime (calcium carbonate) to raise soil pH. Although both contain calcium, their chemical behavior differs. Lime's carbonate reacts with hydrogen ions in the soil, forming carbon dioxide and water that leave the soil, reducing acidity. Gypsum's sulfate does not remove hydrogen ions, so it doesn't change pH.

However, gypsum does help clay soils—but only if they are high in sodium. In sodic soils, gypsum replaces sodium with calcium, improving structure. Most Georgia clays are low in sodium, so gypsum won't soften them. Adding sodium plus gypsum to create this effect is not recommended.

Using Gypsum for Blossom End Rot

Gypsum is a good calcium source for preventing blossom end rot in tomatoes, peppers, and squash. This disorder stems from calcium deficiency in developing fruit. Burying a piece of drywall beside each tomato plant at planting supplies calcium slowly—just a couple of tablespoons per plant is enough.

Hydrangea Color and Soil pH

Hydrangea flowers turn blue in acidic soil (available aluminum) and pink in alkaline soil. If your 'Forever Pink' hydrangeas are turning purple, your soil has become more acidic. Raise pH quickly by drenching the root zone with a solution of 1 cup hydrated lime in 5 gallons of water (3–5 gallons per plant). For long-term maintenance, scatter 1 cup garden lime around each plant annually.

These examples show why understanding soil chemistry pays off—maybe it'll keep teenagers awake in science class!