Calcined clay is clay that has been heated to high temperatures, typically 1,110 to 1,470°F (600 to 800°C), to permanently alter its physical and chemical properties. The heating process removes chemically bound water, hardens the particles, and creates a stable, porous material that absorbs and retains water while maintaining air-filled pore space. Unlike expanded clay, calcined clay does not puff or become lightweight during firing.
In horticulture, calcined clay is commonly used as a soil amendment or growing medium to improve drainage, aeration, and moisture retention. Because the particles are hard and resist decomposition, they help maintain soil structure over time. Most calcined clays have a moderate cation exchange capacity (CEC), allowing them to retain nutrients more effectively than materials such as perlite or sand.
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