The word “lime” properly belongs to calcium oxide, but it is often broadly used to mean a family of calcium minerals. It may refer to calcium carbonate, calcium oxide, or calcium hydroxide.
Calcium carbonate (CaCO3)
Calcium carbonate is found in nature in many forms. It is most commonly found as limestone, marble, chalk, or marl, which forms the bedrock that underlies many parts of the world. Many marine and land animals form their bones and shells from calcium carbonate. Organic forms of calcium carbonate are commercially available as ground oyster shells or bone meal.
Limestone that has been ground into a fine flour is an excellent soil amendment for making soil more alkaline (less acid). Dolomitic limestone is a related mineral that contains magnesium carbonate as well as calcium carbonate. This is useful in soils that are deficient in magnesium.
The effect of ground limestone on the soil depends on how finely it has been ground. To be effective, it should feel like a gritty flour. If it looks like coarse sand or gravel, the particle sizes are too large for it to be effective.
Calcium Oxide (CaO)
When calcium carbonate is burned, it forms calcium oxide, known as lime or quicklime. Although a good source of calcium, lime is caustic and highly reactive with water. It is used to make plaster, mortar, and Portland cement. It should not be used as a soil amendment.
Calcium Hydroxide (Ca(OH)2)
Lime that has been treated with water is called slaked lime or hydrated lime. It is no longer caustic or highly reactive, and is often sold for agricultural use.