Dolomite is a form of limestone, rich in approximately equal parts of magnesium carbonate and calcium carbonate. Dolomite limestone contains about five times as much magnesium and five eighths as much calcium as ordinary limestone. Dolomite also contains small amounts of chlorine, phosphorus, and potassium, in addition to more than 20 other trace elements.
 

It provides valuable nutrients to plants while also correcting the pH of the soil.