Further south than the gray-brown podzolic soils, in a zone of increasingly warmer climate but with equally abundant precipitation, lays a great area of red-yellow podzolic soils. These soils occupy the southern United States from Texas to the Atlantic, and coincide fairly well with the extent of the humid subtropical climate. A similar geographic relation holds in Japan. While in southern Brazil and southeastern Paraguay is also a substantial area of these soils at comparable latitudes. Smaller coast red-yellow podzolic soils are found in South Africa, Australia, and New Zealand.
The red-yellow soils are of the podzolic type and show the same characteristic leaching of the 2 horizons. Warm summers and mild winters favor bacterial action. Humus content is low. Thus, both podzolization and laterization act in concert as pedogenic processes. The typical red and yellow colours are a staining in the form of hydroxides of iron. The yellow soils are the more strongly leached of the two and are found on sandy coastal plain belts. Aluminium hydroxides are also abundant in these soils, a condition typical of tropical soils in warm humid regions.
Deciduous forest was the natural vegetation of the northern part of the red soil belt in the United States. Soils of these southern states, though low in plant nutrients, respond well to fertilizers and have been important producers of tobacco, cotton, peanuts, soybeans, corn, sweet potatoes, cowpeas, and many other crops. Yellow soils, owing to their strong leaching, support forests of longleaf, loblolly, and slash pine.
In the other world regions of red-yellow podzolic soils noted above, natural vegetation is dominantly rainforest, of both tropical and temperate classes.
A great soil group related to the red podzolic soils are the terra rosa of Mediterranean lands. This red soil, poor in humus, is rich in sesquioxide of iron (Fe2O3) to which it largely owes its colour. The origin of terra rosa is the source of much speculation. Perhaps it represents a soil that was. once much richer in humus, but which has lost that humus because of the destruction of forest cover by man and his grazing animals over the course of many centuries. In some areas of its occurrence, terra rosa is associated with limestone as the parent matter, suggesting that its properties are inherited from a calcareous subsoil.
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