Terra rosa is the name of a certain type of residual soil, and is also known as terra rossa, which is the correct spelling of the Italian term for red soil. Deposits of this soil can vary from orange to reddish brown and various shades of red in color. Terra rosa is mainly found in areas where the underlying bedrock consists of limestone, and is created when limestone weathers and erodes, producing a mix of clay and sand that contains iron oxide, giving the soil its color. This type of red soil is found in regions all over the world, including around the Adriatic Sea, North Africa, La Mancha in Spain, Coonawarra in Australia, and the Judean Hills in Israel. Around the Mediterranean, this red soil has been known since ancient times to be very suitable for agriculture, especially wine production.
In pedology, or soil science, terra rosa is classified as a chromic luvisol. The various deposits of terra rosa soil around the world were created over millions of years, as limestone rich in iron oxides eroded. Scientists believe that frequent changes in climate during this geological period, especially heavy rains associated with these changes, helped break down the limestone, turning it into terra rosa soil.
Most regions with terra rosa soil have a Mediterranean climate, and are karst areas, a geological term referring to areas where the underlying bedrock is carbonate rock, usually limestone or dolomite, that is prone to erosion. Limestone bedrock is often characterized by being relatively porous, and the formation of caves and other subterranean hollows is a common feature in karst areas. Such areas sometimes lack significant sources of surface water, such as lakes and rivers, because rain water seeps through the bedrock rather than collects on top of it. Instead, groundwater often collects underground in large aquifers, which are layers of wet, subterranean rock and sediment.
Terra rosa provides good drainage, especially compared to many other clay soils, and the well-drained nature of the underlying bedrock contributes to the soil being able to retain moisture without becoming waterlogged. These characteristics, and the fertile chemical composition of the soil, help make it well-suited to agriculture. Vineyards are a common form of agriculture in some areas with terra rosa soil. One example of such an area is Coonawarra in Australia. The high quality of the wines from this region is said to be partly due to the characteristics of its red soil.