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The Tasmanian branch selected a Ferrosol as their State Soil.
Ferrosols are deep, well structured soils with a red or red-brown colour. In Tasmania they have formed mainly from the weathering of basalt, a volcanic rock extruded as lava by numerous small volcanoes in northern Tasmania some 10-50 million years ago. This means that Ferrosols are relatively old soils in Tasmania. As a result the basalt has had quite a long time to weather, which explains why these soils are so deep, with usually more than 1 m to unweathered rock.
In many places profiles are several meters deep. The long period of weathering also helps to explain the red colour of Ferrosols. Basalt is a rock that is rich in iron and as the basalt weathers, the iron it contains is oxidised, in the same way that roofing iron rusts when left exposed to the elements.
The name Ferrosol comes from “ferrum”, the Latin term for iron. To be classed as a Ferrosol, a soil has to contain at least 5% free iron oxides. Another name for Ferrosols is krasnozem, which in Russian means “red soil” or “red land”. Some of Tasmania’s Ferrosols in the colder, wetter, more elevated, inland regions around Tewkesbury and Ridgley are not as red as those closer to the coast.
The cooler and wetter climate keeps more of the iron in the form of goethite, which has a yellower hue than the redder haematite found in coastal Ferrosols. Red to brown, acid, strongly structured clay soils (50-70% clay) ranging in depth from less than 1 m to over 7 m. Their clay mineralogy is dominated by kaolin and iron and aluminium oxides, and this ensures that the soils have variable charge properties with low cation exchange capacity and usually a significant anion exchange capacity. Free iron oxide contents range from about 7 to 18% Fe.