Ultramafic refers to rocks or soils that contain very high levels of magnesium and iron, and very low levels of silica. These materials originate from the Earth’s mantle and commonly include rock types such as peridotite and serpentinite.
In gardening and plant ecology, ultramafic soils are chemically extreme. They tend to be low in essential nutrients like calcium, nitrogen, and phosphorus, while often containing elevated levels of heavy metals such as nickel and chromium. This combination creates harsh growing conditions that limit plant growth and favor specialized, slow-growing species adapted to nutrient stress and metal tolerance.
Ultramafic soils are also typically shallow, rocky, and fast-draining, which further reduces water and nutrient availability. In cultivation, plants native to these soils often perform poorly in rich, conventional garden mixes, as they are adapted to scarcity rather than abundance.
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