The Dirt on Dolomite: Using Dolomite in Organic Gardens

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Photo courtesy of qmnonic at

Organic gardeners place a high value on dolomite, both for its capacity to “sweeten” soil by lowering its acidity and for its tendency to act as a secondary fertilizer, especially for tomatoes and related plants. But even many veteran horticulturalists have little idea what dolomite actually is, except in the general sense that it’s a mineral.

In fact, dolomite starts out as a deposit that builds up on the bottom of saline bodies of water as they evaporate. Later, it may become compressed into stone by overlying sediments. While dolomite rarely develops in modern environments, vast deposits can be found all over the world in old layers of rock.

Chemically, dolomite is a form of calcium magnesium carbonate. The magnesium, which makes up about 40 percent of the mineral, is slowly released from the crystals as it breaks down; therefore, it’s an excellent source of a trace mineral all plants require. This is why the mineral form is so often used in fertilizers. In addition, the carbonates in dolomite increase the pH of any soil it’s added to, thus decreasing the acidity.

Given its high magnesium content, dolomite stone is also a significant magnesium ore. In addition, it’s a common source of concrete aggregate, along with the closely-related dolomitic limestone.