The 11.5-hectare vineyard of Petrus is located on a plateau in the eastern portion of Pomerol; 100% is planted with merlot grapes. The soil consists of a high percentage of iron-rich clay.
The estate was among the first in Bordeaux to implement green-harvesting as a way to lower crop yields and raise the quality of the remaining grapes. The yield is among the lowest in Bordeaux partly through éclaircissage or green-harvesting to concentrate the power and quality of the remaining crop (eliminating up to 50% of the crop in certain years).
What makes the terroir of Petrus so unique? To start, the clay soils of Petrus are at least 40 million years old. The thick gravel on the surrounding plateau is only 1 million years old. There are two layers of clay at Petrus. The topsoil of dark clay is 60 to 80 centimeters thick. But it’s the unique subsoil that is not found in any other vineyard. The soil is packed with very, dense, deep, dark blue clay. The clay is so hard, that the roots cannot penetrate. The clay is smectite. When this type of clay absorbs water, it becomes impermeable. The water molecules are able to penetrate the interlayer molecular spaces of the clay. When the dry summer months arrive, the vines are still able to feed on the much needed moisture.
The majority blue clay on the Pomerol plateau is about 15 hectares in total. Petrus is unique as its entire 11.5-hectare vineyard is right on top of the clay. Neighboring vineyards only have a portion of blue clay in their soil.