According to the Kentucky Geological Survey, limestone covers 25 percent of the land surface in central, south-central, and southwestern Kentucky. However, only the Inner Bluegrass region consisting of Bourbon, Fayette, Jessamine, and Woodford counties is famous for racehorses. Why?

It has to do with diet. Racehorses need strong bones. If racehorse bones are not strong enough, the impact from running at top race speed damages the bones, causing injury to the horse. Making bones strong requires both calcium and phosphorus in the diet.

All animals get the nutrients they need from the food they eat. Horses eat plants that get their nutrients from the air and the ground. The calcium and phosphorus that make racehorse bones strong come from limestone in the ground beneath the grass they eat.

Limestone is rich in calcium because the main mineral in limestone is calcite (calcium carbonate, CaCO₃). When calcite dissolves in water, the compound separates into calcium ions and carbonate ions, which can be absorbed and used independently from each other.

As rainwater seeps through limestone, water dissolves a tiny amount of the minerals from the rock. Over time, this causes sinkholes and caves. Meanwhile, mineral-laden water traveling underground and resurfacing at springs waters the soil and the plants growing in that soil. This is how limestone is transferred to plants.

What makes limestone in the Inner Bluegrass region special is its unusually high phosphorus content, while limestone in other parts of the state and most parts of the world lack this element. Because of this, ground water and surface water there have unusually high amounts of dissolved phosphorus in addition to calcium. Grass in that region absorbs more natural phosphorus than grass grown in other parts of the state, and horses grazing this phosphorus-rich grass develop lighter, stronger bones than horses without the extra phosphorus in their diet. Thus horses bred and raised in central Kentucky developed a natural advantage over other racehorses during the infancy of the racehorse industry.

Today, phosphorus can be added to any horse’s diet directly, similar to the way vitamins and minerals are added to breakfast cereals for people, or indirectly, by adding phosphorus-rich fertilizer to the soil. Yet, the bluegrass region of central Kentucky is still known as one of the best locations in the world to raise horses.

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