The Soils of Carneros

Carneros soils tend to be dense, shallow (approximately three feet deep), high in clay content, and of low to moderate fertility. These soils impact the vine's vigor by restricting development of the root system, providing just enough nutrients and water to sustain growth without excess development.

Subsoils also vary in Carneros. Each of the different subsoils substantially changes the environment of a grapevine's roots, and affect the composition of the fruit. Thus it is no surprise to find diversity in Carneros wines.

The two predominant soil types present in Carneros are the Haire and the Diablo series. Both are relatively young, formed during the ice ages from deposits of soil-forming materials left by the receding waters of the San Pablo Bay.

The yellowish-brown, alkaline Haire soils are derived from shallow tidal waters that contained abundant organisms and salt marsh vegetation. Signs of this life are visible today in the form of fossils scattered throughout the soil. As the fossils decay (most often in the form of sea shells) they add calcium to the soils which is beneficial to grape vines. These soils typically have a pH of 7-8 as well as a high clay content that provide high water holding capacity.

The more acidic, black Diablo soils are derived from deeper marsh and bay sediments where little oxygen was present, yielding few organisms and no fossils. These soils are high in manganese, an essential nutrient for grape vines that help plants form chlorophyll. Although less permeable, Diablo soils have a high clay content and also high water holding capacity.

Haire and Diablo soils tend to alternate in a soil profile, suggesting that the grape vine adapts to a different chemical environment as the roots grow from one layer to another.

Interspersed among the Haire and Diablo soils are less frequent occurrences of soils formed by sediments washed through the region by creeks and rivers. Classified as Cole silt loam, Zamora silt loam and Yolo loam, they account for approximately 10% of Carneros soils. They are coarse in texture and loamier. These river sediments are derived from volcanic, metamorphic and sedimentary rocks. Compared to the Haire and Diablo soils, they have a more neutral pH and higher levels of organic matter, allowing for greater vigor in the vineyard.

Carneros subsoils vary with their location. While the lower areas of the region and those closer to the bay may have clay hardpan beneath the topsoil, Carneros' rolling hills feature various rock formations in the subsoil. The most common rock formation is a fractured shale, but one also finds decomposed volcanic basalt, sandstone, volcanic ash, and even coarse gravel. In general, the subsoil rock formations are most exposed on the west faces and hilltops in Carneros, where storms from the Pacific have most eroded topsoil over the millennia.